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1. BULE HORA UNIVERSITY
COLLEGE OF NATURAL AND COMPUTATIONAL SCIENCE
DEPARTMENT OF GEOLOGY
PROGRAM REGULAR POST-GRADUATE IN MINING GEOLOGY
ASSIGNMENT OF ADVANCED PETROLOGY (GEOL.606)
By; Gemechu Birbirsa Roba.
TO Dr. Melese (Ph.D.)

2. Assignment 1
1) Discusses in detail the petrological and geochemical (major element, trace
element and isotopic) characteristics of;
a) Plateau volcanic rocks of Ethiopia (20 ppt)
b) Rift volcanic rocks of Ethiopia (20 ppt)
c) Intrusive igneous rocks of Ethiopia (15 ppt)
2) Discusses the geodynamic evolution intrusive and volcanic rocks of Ethiopia (30
ppt)
3) Discusses in detail the petrological and geochemical (major element, trace
element and isotopic) characteristics of (50 ppt).
1 Mid-Ocean Ridges
2. Intra-continental Rifts
3. Island Arcs
4. Active Continental Margins 5. Back-Arc Basins 7. kimberlites, carbonatites,
anorthosites

3. ANSWER A
Plateau volcanic rocks of Ethiopia (20 ppt)
INTRODUCTION
• Ethiopian volcanism has been strictly controlled in space and time by the
development of the Ethiopian swell and its traversing rift-valley system.
Contemporaneous volcanism in the rift and on the plateaus shows some
interesting differences. than do the rift basalts, although minor occurrences of
tholeiite are restricted to the plateaus.
• Peralkaline intermediate lavas, usually strongly undersaturated, form notable
associations on the plateaus, but in the rift they are subordinate to silicic lavas
and are definitely more calcic. silicic lavas have comenditic affinity, whereas those
of the rift have pantelleritic affinity. Although these distinctions are usually
transitional rather than abrupt, they point to fundamental differences in magma
genesis under the volcanic rift floor and under the sialic plateau
• The Cainozoic volcanics of Ethiopia and Yemen were erupted during development
of the Arabo-Ethiopian swell and its traversing rift valleys. They form what is
perhaps the world's largest alkaline volcanic province, though calc-alkaline lavas
(e. g., tholeiitic basalts) are not unknown. The province is notably situated on a
triple junction of the world ridge-rift system, where the Gulf of Aden, Red Sea,
and African rift structures converge at Afar.

4. • Contrasting with the thick sialic crust of the continental plates [Niazi, 19681.
However, the occurrence of Afar and its bordering sialic horsts within the triple
junction is an unexplained paradox in the simple theory of plate separation of
Arabia from Africa.
• Some workers have considered Afar to be merely subsided sialic crust once
contiguous with the bordering Ethiopian and Somalian plateaus [e. g. , Gortani,
19501. More recently, the concept that the Afar depression represents exposed
oceanic crust has become widely accepted because it fits with the oceanographic
data [Bullard, 19691 .
• Other workers point to the problem of the existence of the Danakil and Aisha
horsts within the triple junction, and the implications from structural geology
that fragmented blocks of sialic crust form an appreciable proportion of the Afar
floor under the cover of young lavas and sediments [Mohr, 1967; Crass and
Gibson, 19691. imposed of young oceanic crust.
• As a contribution toward resolving this controversial problem, an attempt is
made here to examine and contrast the volcanic chemistry of the plateau and rift
subprovinces of the Ethiopian region. tinctions can then be discussed int e r m s
of presumed differences in crustal and subcrustal nature of the two
subprovinces, though a major handicap remains our lack of understanding of the
petrogenesis of silicic magma in a rift environment [Sigurdsson, 1967; McBirney,
19691) .

5. AGE AND DISTRIBUTION OF THE ETHIOPIAN VOLCANIC
The sequence of major volcanism can be summarized as follows;
4 Aden Series
a) Fissure basalts and caldera silicics in rift. Rare basaltic cinder-cone activity on
plateau (Uppe r Pleistocene- Holocene).
B) Basalts, tephrites, phonolites on the plateaus, especially in the Tana rift. strong
silicic volcanism in main Ethiopian rift and Afar (Middle Pleistocene).
C ) Extensive panteller itic ignimbrites in southcentral Ethiopia, and possibly
contemporaneous silicics in Afar. Peralkaline intermediate lavas of re s tricted
extent on the plateaus. Rather minor basalt and nephelinitic activity, particularly
on the Somalian plateau (Plioceneearly Pleistocene). Minor basaltic
3. Afar Series
Flood basalts and mugearitic derivatives, with minor end-phase silicic volcanism, in
central and eastern Afar (late Miocene? -Pliocene)

6. Shield Group
• Central-type flood alkali olivine basalts and hawaiites on plateaus. Occasional
end-phase comenditic lavas and stocks. Abundant minor intrusive activity, with
dike swarms of both basaltic and silicic composition (Lower MiocenePliocene).
1 Trap Series
• Fissure flood basalts forming a very extensive cover on the Arabo-Ethiopian
swell. bas alts predominate. unconformities are too localized to be used for
subdividing this thick series on a regional basis. Silicic lavas very rare. under the
margins of the present rift floor, but its extent under Afar is debatable (Eocene-
Oligocene). Alkali olivine Some intraformational The Trap Series plunge

7. • In the Ethiopian Plateau, the Oligocene magmatic activity is ended with the
build-up of alkaline central type shield volcanism, for example, 23 Ma Choke and
Guguftu (Kieffer et al., 2004) and 11.2-7.8 Ma Wollega shield volcano (Berhe et
al., 1987) in the northwestern plateau. Compositionally, the shield volcanoes are
bimodal. Recent magmatic activities were mainly concentrated in the Main
Ethiopian Rift (MER) that bisecting the Ethiopian Plateau into northwestern and
southeastern sectors.
• The magmatism in MER is characterized by temporally and spatially variable
fissural and central basalts and started in the early Miocene (20-21 Ma;
WoldeGabriel et al., 1990; Ebinger). These episodic magmatisms (Girdler, 1983;
WoldeGabriel et al., 1990) were distributed into structurally distinct Central
(CMER; 5-3 Ma) and Southern (SMER; 18-15 Ma) segments of MER (Hart et al.,
1989; WoldeGabriel et al., 1990).
• The volcanic rocks in the MER are bimodal in nature (basalt-rhyolite) with clear
absence of intermediate rocks (Ayalew, 2000). The rift volcanic rocks are
transitional to alkaline in composition and show peralkaline character for the
more evolved volcanic rocks (Peccerillo et al., 1995, Trua et al., 1999; Ayalew,
2000).

8. Chemistry of Plateau volcanic rocks of Ethiopia
• Nature of the Ethio-Yemen volcanics. The alkaline nature of the EthioYemen
volcanics has long been recognized [Manasse, 1909; Raisin, 1903; Weber, 1906;
Roman, 19261, and comparisons effected with the volcanics of the East African
rift system farther south [Prior, 1903; Shukri and Basta, 19551.
• awareness of the role of oceanic rifts in crustal-plate tectonics naturally lead to a
reexamination of all available data on the Ethio-Yemen volcanics in relation to
their tectonic setting: for example, an examination of the basalts of the axial zone
of the rifts for tholeiitic tendencies.
• McBirney and Gass [1967] have demonstrated a pattern of chemical variation of
oceanic-island silicic lavas according to their position of extrusion on the mid-
oceanic ridges.
• analogous to oceanic ridges [Baker and Mohr, 19701 , and their silicic lavas thus
reexamined from this aspect. The recent success of the hypothesis of sea-floor
spreading.

9. • The classification of individual rocks has been according to limiting oxide
parameters as follows: for "alkaline basalts," Si0 < 50%, MgO < lo%, CaO < 1476,
and Na 0 t K 0 > 37'0; for "subalkaline basalts, I' 4770 < Si02 < 537'0~ CaO <
147'0, and Na,O t K,O < 3%; for "intermediate volcanics," 5570 < SiO, < 67%; for 2
2 2 b. L. "sodic rhyolites, Si02 > 677'0 and Na20 < K20. Si02 > 677'0 and Na20 This
method of crude classification L. > K 0; and for "potassic rhyolites.
• This method of crude classification L. > K 0; and for "potassic rhyolites, I' 2 is
certainly not ideal but is necessitated by the lack or paucity of petrographic
description for many of the published rock analyses. In particular, the distinction
made here between alkaline and subalkaline basalt is arbitrarily based on Yoder
and Tilley's [ 19621 boundary condition for alkali basalt- tholeiite of 37'0 total
alkalis; in fact,
• A recently described olivine tholeiite from the Ethiopian plateau contains more
than this amount [LeBas and Mohr, 19701. Nevertheless, the essential nature of
the Ethio-Yemen volcanism is brought out, and its alkaline nature emphasized in
comparison with other continental flood- basalt occurrences. For such a
comparison, the reader is referred to Mohr [1963].

10. METHODS AND LIMITATIONS OF CHEMICAL COMPARISONS
The strong suggestion of distinctive chemistry between rift and plateau volcanics in
Ethiopia and Yemen is shown: for the basalts in a more undersaturated and alkali-
rich character on the plateau;
for the intermediates in a less saturated, less calcic, less femic, and more
peralkaline character on the plateau; and for the silicics in a more oversaturated,
less sodic character on the plateau.
volcanics: in the basalts and intermediates, this is largely due to potash enrichment
for the plateau (more probable than a potash depletion in a rift environment), but
in the silicics it is due to soda enrichment [Mohr, 197Obl. In all three classes, Na/K
is higher in the rift .

11. • On the basis of crystal fractionation processes, it is conceivable that the plateau
intermediates could be linked with the rare melanephelinites of the plateau,
though field associations are even rarer.
• silicics are products of strong fractionation of plateau basaltic magma, then they
have not passed through a preserved intermediate stage; plateau intermediates
are too peralkaline and salic to provide such a link.
• But if the plateau It is easier to conceive a continuous line of descent for the rift
volcanics. Rift basalts, trachybasalts, and mugearites are found in intimate field
assocciations, as are trachyrhyolites and rhyolites, but there is still strong
evidence for a hiatus in the intermediate range [Tazieff et al., 1970; Barberi - et
al., 19701)

12. Major elements
• Major element data for Kella volcanic rocks are the MgO contents for Oligocene
basalts are ranging from 5.29-6.11 wt. %, for Miocene and Quaternary basalts
are between 8.21-8.86 and 7.4-8.45 wt. %, respectively. Relatively Miocene
basalts are more magnesium rich than the Oligocene and Quaternary basalts.
• In the total Alkali-Silica (TAS) classification diagram of Le Bas et al. (1986), all
Kella samples are basalts in composition and subdivided into alkaline and
tholeiite series . Oligocene basalts are tholeiite series and plot with Low-Ti
basalts of Pik et al. (1998, 1999) while the Miocene and Quaternary basalts are
alkaline series and plot with High-Ti basalts (Pik et al., 1998, 1999) and NMER
western rift wall basalts (Ayalew et al., 2018).
• The variations of Kella basaltic lavas are clearly demonstrated in their CIPW
normative. the Oligocene tholeiitic basalts are quartz normative whereas
Miocene and Quaternary alkali basalts are olivine and nepheline normative

13. • Selected major element versus MgO (wt. %) plots are shown the Kella basaltic
rocks show two distinct trends:
1) for the Miocene and Quaternary alkali basalts and
2) for the Oligocene tholeiitic basalts.
• In all groups, TiO2, Na2O, K2O and P2O5, show negative correlations with MgO.
Al2O3 is positively correlated with MgO in the Oligocene tholeiite basalts and
negatively correlated in the Miocene and Quaternary alkali basalts .
• CaO shows positive trends for the Oligocene tholeiitic basalts, as well as the
Miocene and the Quaternary alkali basalts . Fe2O3 is almost constant in Miocene
and Quaternary alkali basalts with MgO > 6.11 wt. %, but in the Oligocene
tholeiitic basalts Fe2O3 is negatively correlated with MgO.

14. Trace elements
• Trace element data of the Kella basaltic rocks are show the compatible element Ni is
positively correlated with MgO in all groups . while negative correlation is observed for
Oligocene tholeiite ( show slight positive correlations with MgO for the Miocene and
Quaternary alkali basalts,
• while negatively correlated in Oligocene tholeiite basalts ( Nb, Zr, Rb, Ba and Sr versus
MgO show negative correlation in all basaltic groups . In the Nb, Rb, Ba and Sr two
distinct negative trends one representing the Miocene and Quaternary alkali basalts
and the other one representing the Oligocene tholeiitic basalts.
• The general patterns in all groups show a close similarity to oceanic island basalts (OIB-
type) than that of a typical normal mid-oceanic basalt (N-MORB) (Sun and McDonough,
1989). Even though all groups show a general OIB pattern, there are some different
features between the Oligocene tholeiitic basalts and the other groups.
• The Miocene and Quaternary alkali basalts plotted closer to OIB pattern but the
Oligocene tholeiitic basalts are relatively lower than the average OIB. Moreover, the
Miocene and Quaternary Alkali groups show slight enrichment in HFSE (Nb and Ta)
compared to the LREE (La and Ce), and LILE (except Ba) which is a typical characteristic
feature in Oceanic Island basalts observed.

15. • In the other hand, HFSE (Nb and Ta) in the Oligocene tholeiitic basalts show no
significant enrichment relative to LREE (La and Ce) and LILE. In general, the
Miocene and Quaternary Alkali basaltic groups are characterized by relatively
higher HFSE/LREE and HFSE/LILE ratios compared with the Oligocene tholeiitic
group.
• The Miocene and Quaternary alkali basalts are characterized by slight positive
anomalies of Ba than the Oligocene tholeiitic basalts.
• Chondrite-normalized rare earth element (REE). The rare earth element patterns
of alkaline basalts show enrichment of light rare earth element (LREE) relative to
heavy rare earth elements (HREE) analogues to typical ocean island basalt (OIB)
like pattern (Sun and McDonough, 1989). The tholeiite basalts show slight
enrichment in heavy rare earth elements (HREE) than the alkaline basalts
• The MREE to HREE patterns of Oligocene tholeiitic basalts are analogues to
typical Mid Oceanic basalt (MORB) like pattern (Sun and McDonough, 1989). The
Oligocene tholeiitic basalts have lower ratios of LREE/HREE ((La/Yb)N = 4.04-
4.90), LREE/MREE ((La/Sm)N = 1.88-2.13) and MREE/HREE ((Tb/Yb)N = 1.44 -
1.47, (Dy/Yb)N = 1.40-1.41) as compared to the other two groups. .

16. • The Miocene and Quaternary alkali basalts, in the other hand, show higher
ratios of LREE/HREE ((La/Yb)N = 7.45- 9.56 and 9.40-9.76, respectively),
LREE/MREE ((La/Sm)N=2.66-2.88 and 2.70-2.97, respectively) and MREE/HREE
((Tb/Yb)N= 1.70-1.80 and 1.66-1.81; (Dy/Yb)N=1.45-1.61 and 1.46-1.61,
respectively) relative to the tholeiitic basalts .
• The geochemical variations of basaltic samples from Kella area exhibit two
compositionally distinct basaltic groups. The Oligocene tholeiitic basalts display
low MgO (5.29-6.11 wt.%), TiO2 (2.15-2.47 wt.%), P2O5 (0.28-0.34 wt.%), and
high ratio of CaO/Al2O3 (0.68-0.72) and La/Nb (0.89-1.10).
• Whereas Quaternary and Miocene alkaline basalts display high MgO (7.40-8.86
wt.%), TiO2 (2.4-2.53 wt.%), P2O5 (0.44-0.52 wt.%) and low ratio of CaO/Al2O3
(0.62-0.66) and La/Nb (0.71-0.76). The contrasting incompatible element ratios
(e.g., K/Nb, La/Nb, Rb/Zr and Zr/Nb) between tholeiite and alkaline basalts
reflect differences in their mantle sources.

17. Geochemical variations;
• The geochemical variation of basaltic samples from Kella area exhibit two
compositionally distinct basaltic groups. The Oligocene tholeiitic basalts display
low MgO (5.29-6.11 wt.%), TiO2 (2.15-2.47 wt.%), P2O5 (0.28-0.34 wt.%), and
high ratio of CaO/Al2O3 (0.68-0.72) and La/Nb (0.89-1.10). Whereas Quaternary
and Miocene alkaline basalts display high MgO (7.40-8.86 wt.%), TiO2 (2.4-2.53
wt.%), P2O5 (0.44-0.52 wt.%) and low ratio of CaO/Al2O3 (0.62-0.66) and La/Nb
(0.71-0.76).
• Major and trace element variations, therefore, reflect the involvement of two
geochemically distinct mantle sources in the petrogenesis of Kella basaltic lavas:
i) the Oligocene tholeiite basaltic melts derived from enriched asthenosphere
mantle source (E-MORB) and ii) the Miocene and Quaternary alkali basaltic melts
show a close similarity with ocean island basalts (OIBs) geochemistry, and this
end member ascribed to the arrival of Afar plume head.
• The geochemical modeling reveals that the Oligocene tholeiite basaltic
melts produced by an equilibrium melting with 3-5 % degree of partial
melting in spinel lherzolite mantle source, whereas the alkali basalts
were produced with ~2% degree of partial melting within spinel-garnet
lherzolite transition zone mantle sources.

18. Isotopic
• Whole rock geochemical and isotopic studies in the Ethiopian continental flood
basalts and the magmatism in Main Ethiopian Rift show the involvement of
compositionally heterogenous Afar mantle plume, asthenospheric mantle and
lithospheric mantle in their generation (e.g., Hart et al., 1989; Marty et al., 1996;
Stewart and Rogers, 1996; Pik et al., 1998, 1999; Kieffer et al., 2004; Meshesha
and Shinjo, 2007, 2008: Beccaluva et al., 2009, 2011; Natali et al., 2016)
• The contributions of from distinct mantle end-member components [intrinsic
Depleted Mantle (DM), Enriched Mantle (EM I & II), High-µ (HIMU) and Primitive
Mantle (PREMA)] in different proportions and their interaction with
subcontinental lithospheric and asthenosphere mantle for the genesis of
magmas in the EARS have been suggested by many authors (Stewart and Rogers,
1996; Pik et al., 1998, 1999, Kieffer et al., 2004.

19. • The contributions of from distinct mantle end-member components [intrinsic
Depleted Mantle (DM), Enriched Mantle (EM I & II), High-µ (HIMU) and Primitive
Mantle (PREMA)] in different proportions and their interaction with
subcontinental lithospheric and asthenosphere mantle for the genesis of
magmas.
• Nonetheless, the involvement of lithospheric component having high
87Sr/86Sri, and elevated 207Pb/204Pbi and 208Pb/204Pbi for a given
206Pb/204Pbi (Pik et al., 1998, 1999; Shinjo et al., 2010; Feyissa et al., 2017)
and/or continental crustal material characterized by higher incompatible trace
element ratios (e.g., Ba/Nb and Rb/Nb), 87Sr/86Sr and non-radiogenic Pb
isotopes (Baker et al., 2000; Meshesha and Shinjo, 2007, 2010) is still disputed.
• In order to investigate the involvements of variable mantle sources in the
genesis of basalts the Kella area (western rift wall, Fig. 1b) is of a great interest
because it represents both pre- and post-rift
• Source .
• ETHIOPIAN RIFT AND PLATEAUS: SOME VOLCANIC PETROCHEMICAL
DIFFERENCES

20. Answer B
Rift volcanic rocks of Ethiopia (20 ppt)
Introduction
• The East African Rift System (EARS) is a more than 3000 km long system of
depressions flanked by broad uplifted plateaus. A long record of volcanism in
EARS provides invaluable constraints on past and present processes, as well as
the various depth levels of magma generation and storage.
• the outline volcanic processes along the length of the EARS from pre-rift setting
through rift initiation to continental breakup.
• Ethiopian volcanism has been strictly controlled in space and time by the
development of the Ethiopian swell and its traversing rift-valley system.
Contemporaneous volcanism in the rift and on the plateaus shows some
interesting differences.

21. • The volcanic evolution of the EARS reveals consistent patterns in the distribution,
volume, compositions and sources of volcanic products allowing its subdivision
into volcanic provinces.
• EARS extends from the Red Sea in the north to Mozambique and beyond. The
system is characterized by regional topographic uplift, the Ethiopian dome, the
Kenyan dome. The principal rift sectors include the Ethiopian, Eastern and
Western rift valleys.
• The Ethiopian and Kenyan branches of the rift are the site of substantially greater
volcanism than is observed at the Western rift.

22. Volcanic Provinces and distribution
• Within Ethiopia, we distinguish three separate volcanic subprovinces: pre-rift
plateau, Afar rift and the Ethiopian rift.
• The northern Kenya rift comprises the Turkana, Huri Hills and Marsabit regions,
as well as the Quaternary central volcanic complexes of Emuruangogolak,
Namurunu, Silali, Paka and Korosi Volcanic complexes in central and southern
Kenya rift include the within-rift centres Menengai, Eburru, Naivasha, Longonot,
Suswa and Lenderut as well as off-rift volcanic field at Chyulu Hills In the Western
rift, eruptive activity is restricted to four spatially distinct sub-provinces along the
rift axis.
• From north to south these are: the Toro-Ankole region in western Uganda, the
Virunga and Kivu sub-provinces along the border of the Democratic Republic of
the Congo with Uganda, Rwanda and Burundi and the Rungwe volcanic field in
southwestern Tanzania. The Rungwe Volcanic Province has been a long-lived
volcanic area in the past 9 Ma and is developed at a triple junction of the EARS.

23. • The Oligocene flood basalt province, within Ethiopia and Eritrea, covers an area
of about 600,000 km2 , with an estimated total volume of about 350,000 km3 .
The thickness of this lava pile varies, reaching up to 2000 m in northwest Ethiopia
and thinning to ~500 m towards both north and south.
• Well-developed polygenetic calderas are situated in the center of the Main
Ethiopian Rift axis. Recent basalts are erupted from within fissures adjacent to
and distal from these central volcanoes. Quaternary volcanism covers about two-
thirds of the Afar Depression at the northern end of the Triangle.
• The axial part of Afar is characterized by discrete active magmatic segments
(axial ranges) that are considered nascent oceanic ridges in stretched continental
lithosphere

24. Timing and duration of volcanism
• The earliest recorded volcanic activity in the EARS region took place 40-
45 Ma in southernmost Ethiopia/northern Turkana depression. Then,
flood basalts, forming the Ethiopian and Eritrean plateaus erupted
apparently in short time interval (<5ma) with the greatest eruption
rates during 31 to 28 Ma.
• This event was followed by shield-volcano-building episodes, (23 to 11
Ma) on the northwestern and southeastern Ethiopian plateaus.
• The magmatic activity in the Main Ethiopian rift was episodic rather
than continuous with lesser amount of magma relative to the ~30 Ma
flood basalts.
• In southwestern Ethiopia volcanism started in theearly Miocene around
20–21 Ma and reached the central and northern portions of the Main
Ethiopian rift ~11 Ma. This was followed by the eruption of voluminous
ignimbrites in the Central MER during the Pliocene (5–3 Ma).

25. • Magmatic activities in the broad rift zone of Turkana Depression are episodic.
Three discrete volcanic episodes have been described as;
(1) prerift late Eocene–early Oligocene magmatism (32–40 Ma) in the south central
portion of the rift (Kajong area and Mount Porr),
(2) more voluminous synrift magmatism at 26–16 Ma in the western and eastern
part of the rift (Lodwar and Jarigole area), and
(3) Plio-Pleistocene to present eruption of axially aligned composite volcanic
centers. Since the earliest volcanism in the Turkana region, the onset of magmatism
has also migrated southward through central and southern Kenya, finally reaching
northern Tanzania some 5–8 Ma.

26. Duration of volcanism in various sectors of EARS may be summarized as follows;
In Ethiopia,
In southwestern Ethiopia: 19-17 Ma;
In southern Afar, volcanic events occurred at 14–11;
In the northern Kenyan rift
Basalts and rhyolites were emplaced at 33–25 Ma;
Then nephelinites and phonolites at 26–20 Ma; 15 Ma
Beginning of the Pliocene, trachytic, phonolitic, nephelinitic rocks, and
basaltic volcanism were accompanied with some rhyolitic activity

27. In the central Kenyan rift,
• Flood basalts erupted between 20 and 16 Ma,
• Large volumes of trachyte and phonolite between 5 and 2 Ma;
• Carbonatite and nepheline-phonolite volcanoes around 1.2 Ma
In the Western rift branch
• In the north (Ruwenzori), volcanism began at 12 Ma
• In the Virunga, massif fissure volcanism started between 11 and 9 Ma,
• Large Pliocene-Pleistocene central volcanoes were formed, o In the Rungwe
province activity began at 8.6 Ma

28. Geochemical
• Lavas erupted along the EARS display a wide range of geochemical compositions
that reflect heterogeneity in both magmatic processes and mantle sources. The
Ethiopian Oligocene flood basalts are primarily tholeiitic-transitional in their
petrology.
• The southwest Ethiopia volcanic province is characterized by episodic volcanism
that becomes increasingly silica-saturated through time, ranging from essentially
tholeiitic compositions 40-45 Ma to nephelinenormative basalts since ~19 Ma.
• These features are similar to those displayed by contemporaneous lavas from
the Turkana region which is located immediately to the south with evolved
products encompassing primarily rhyolites and trachytes.
• The northern Kenya rift mafic lavas associated with central volcanoes are
transitionaltholeiitic basalts; the corresponding felsic lavas are dominantly
trachytic in composition.

29. • Volcanic complexes in the axial part of the central and southern Kenya rift are
dominantly trachytic to pantelleritic with little to no exposure of basalts.
• Where present the basalts are dominantly transitional-tholeiitic, although the
Chyulu Hills lavas are markedly silica-undersaturated.
• The lavas in the Western rift are characterized generally by silica-undersatura
mafic volcanoes the products of which include ultrapotassic, hypersodic and
carbonatitic compositions ted

30. Major elements
Major element variations in African Rift mafic lavas. Within-suite variations in
total alkalis against silica define consistent regional patterns. the alkaline-tholeiitic
division of Macdonald and Katsura (1964).
Trend 1:
Silica-saturated lavas from Naivasha in the southern Kenya Rift (SKR) overlap the
field of quartz- and hypersthene-normative 30 Ma Ethiopian flood basalts
(shaded).
Trend 2:
Lavas from the northern Kenya Rift (NKR; Turkana, Emuruangogolak, Silali, Huri
Hills) as well as basalts from the Main Ethiopian Rift (MER) define a transitional-
tholeiitic sequence.
Trend 3:
The most highly silicaundersaturated samples are found in the Western Rift (WR;
e.g., Rungwe, Muhavura) and southern Kenya (Chyulu Hills). (b) Variations in P2O5
against MgO show similar regional trends. Tholeiitic basalts from the southern
Kenya Rift (SKR: Ol Tepesi, Singaraini, Kirikiti) have the lowest overall incompatible
element contents.

31. • Transitional tholeiites from Turkana, Huri Hills and the northern Kenya Rift (NKR)
and Main Ethiopian Rift (MER) have higher incompatible element abundances,
and silica-undersaturated lavas from Chyulu Hills and the Western Rift (WR; here
represented by the Rungwe province) have the highest incompatible element
contents. Source: Furman 200.
• The EARS magmas are widely variable from alkaline to hyperalkaline, with widely
varying geochemical and isotopic compositions. Extensive polybaric fractionation
within the upper mantle and lower crust accompanied by crustal assimilation.
Fractionation generated a wide range of mugearitic, trachytic, peralkaline
rhyolites and phonolitic magmas. Crustal anatexis has locally resulted in the
formation of peralkaline rhyolites.
• The southwest Ethiopia volcanism ranges from essentially tholeiitic compositions
40-45 Ma to nepheline-normative basalts since ~19 Ma. MER basalts are
transistional-tholeiitic overall, with evolved products encompassing primarily
rhyolites and trachytes.

32. • The northern Kenya rift mafic lavas are transitional-tholeiitic basalts, the
corresponding felsic lavas are dominantly trachytic in composition. Central and
southern Kenya rift volcanics are dominantly trachytic to pantelleritic.
• The basalts are dominantly transitional-tholeiitic Oldoinyo Lengai (N Tanzania) is
the only active carbonatite volcano, typically involving extreme low viscosity
magmas.
• The origin of its unique natrocarbonatites remains a key topic for debate. Western
rift mafic lavas are silica-undersaturated including ultrapotassic, hypersodic and
carbonatitic compositions. Virunga volcanics show silica undersaturated,
ultraalkaline, alkalic-mafic compositions.

33. Trace element
• Available trace element data in the East African Rift lavas show general patterns
in the extent of melting and source mineralogy. Turkana basalts and Main
Ethiopian Rift lavas record the highest degrees of melting and, correspondingly,
the lowest proportion of amphibole in the source rock.
• Simil provinces of the Western Rift, as well as lavas from throughout Kenya and
the Turkana areas, derive by higher degrees of melting of amphibole lherzolite,
On the other hand the silica-undersaturated lavas of the Chyulu Hills province
are derived by the lowest degrees of melting of amphibole-bearing source
material (i.e., lowest K/Th).
• Nyiragongo lavas formed at greater depths by low degree partial melting of a
garnet, clinopyroxene, and phlogopite-bearing carbonated mantle, while the
Nyamuragira lavas are products of larger degree partial melting at comparatively
shallower mantle depths with a recycled crustal component.

34. • Geodynamic Implications
• EARS volcanism is supported by two distinct regions of upwelling at upper mantle
levels, both probably connected to deeper mantle reservoirs
1. A super plume with multiple plume stems appears consistent with South
African Superswell
2. The South African Superswell controls magmatism throughout the African Rift
Sea-floor spreading could be initiated (in Afar) above a mantle plume.

35. Dubbi volcano,
• located in the northeast part of the Afar triangle, erupted explosively in May
1861, with the activity switching to basaltic fire-fountaining focused along a 4-
km-long summit fissure that fed several lava flows that traveled as far as 22 km.
• The volume of lava flows alone, 3.5 km3 , makes this the largest reported
historical eruption in Africa. Mount Nyamuragira and Mount Nyiragongo (D. R.
Congo) are amongst the most active volcanoes on the continent.
• Nyiragongo is one of very few volcanoes regularly hosting a lava lake. Oldoinyo
Lengai (N. Tanzania) is the only active carbonatite volcano, typically involving the
ascent and eruption of extreme low viscosity magmas.
• Oldoinyo Lengai is the only know active carbonatite volcano in the world.. Rising
over 2200 meters from the valley floor, the bulk of this volcanic cone is
composed phonolitic tephra.
• However, its upper portion is dominated by natrocarbonatite lava flows. Historic
eruptions of natrocarbonatite have filled much of the summit crater.

36. Isotope
• Many researchers have suggested one or more mantle plume models that they
consider consistent with the geophysical features and with the geochemical and
geological evidences. High He isotopic values (9 and 19RA ) in Oligocene flood
basalts as well as in young mafic lavas from the Main Ethiopian Rift require a
contribution from an undegassed lower mantle source region, i.e., an upwelling
plume.
• In addition, interaction of mantle plume-derived magmas with various proportion
of upper mantle (DM) and lithospheric mantle and/or continental crust sources
were suggested for the genesis of EARS lavas on the basis of trace elements and
Sr-Nd-Pb-He isotopic compositions.
• Recently, Furman [2007] provided petrological, geochemical and geodynamic
overview of the plume-related magmatism of the EARS and concluded that three
distinct source components (or domains) control magmatism throughout the
broad area from the Afar triangle in northern Ethiopia to the Rungwe province in
southern Tanzania.

37. (1) the subcontinental lithospheric mantle, which is the ultimate source of
ultrapotassic and other silica-undersaturated lavas erupted in the Western and
Southern Kenya Rifts;
(2) a plume source with high-μ (HIMU) Sr-Nd-Pb-He isotopic affinities and is
present in all volcanic systems within and south of the Turkana Depression;and
(3) a plume source with nonradiogenic Sr-Nd-Pb isotopic values,
but radiogenic He isotopic signature, analogous to those observed in some ocean
islands, and is recorded in all Oligocene flood basalts and younger volcanic systems
throughout the Main Ethiopian Rift and northward to the Afar region.

38. • Furthermore, Furman [2007] integrated the geochemical observation and
geophysical features and proposed a modified oneplume model that allows for
multiple plume stems arising from a common large plume at depth.
• The plume stems are proposed to contain lenses of isotopically distinct materials
that record largescale heterogeneity within the South African Superplume.
• Much recently, the geochemical variations have been reinterpreted to reflect the
involvement of at least four mantle plume components as sources for the
northeastern Africa magmatism (Meshesah and Shinjo, 2008). The debate on the
number of mantle plumes existing under the East African rift system remains to
be resolved.

39. Conclusion
• Within Ethiopia, we distinguish three separate volcanic subprovinces: pre-rift
plateau, Afar rift and the Ethiopian rift. The northern Kenya rift comprises the
Turkana, Huri Hills and Marsabit regions, as well as the Quaternary central volcanic
complexes of Emuruangogolak, Namurunu, Silali, Paka and Korosi Volcanic
complexes in central and southern Kenya rift include the within-rift centres
Menengai, Eburru, Naivasha, Longonot, Suswa and Lenderut as well as off-rift
volcanic field at Chyulu Hills In the Western rift, eruptive activity is restricted to
four spatially distinct sub-provinces
• Along the rift axis. From north to south these are: the Toro-Ankole region in
western Uganda, the Virunga and Kivu sub-provinces along the border of the
Democratic Republic of the Congo with Uganda, Rwanda and Burundi and the
Rungwe volcanic field in southwestern Tanzania. The Rungwe Volcanic Province
has been a long-lived volcanic area in the past 9 Ma and is developed at a triple
junction of the EARS.

40. • The Oligocene flood basalt province, within Ethiopia and Eritrea, covers an area
of about 600,000 km2 , with an estimated total volume of about 350,000 km3 .
The thickness of this lava pile varies, reaching up to 2000 m in northwest Ethiopia
and thinning to ~500 m towards both north and south.
• Well-developed polygenetic calderas are situated in the center of the Main
Ethiopian Rift axis. Recent basalts are erupted from within fissures adjacent to
and distal from these central volcanoes. Quaternary volcanism covers about two-
thirds of the Afar Depression at the northern end of the Triangle.
• The axial part of Afar is characterized by discrete active magmatic segments (axial
ranges) that are considered nascent oceanic ridges in stretched continental
lithosphere

41. source
Advanced Workshop on Evaluating, Monitoring and Communicating Volcanic and
Seismic Hazards in East Africa
Yirgu Gezahegn 17 - 28 August 2009
Addis Ababa University Ethiopia
An outline of the East African Rift Volcanism

42. Answer c
Intrusive igneous rocks of Ethiopia (15 ppt)
Introduction
• For centuries people living near volcanoes have noticed that the red-hot molten
material that erupted onto Earth’s surface as lava cooled and solidified to give
solid rocks. Lava (from the Italian lavare: to wash) was originally applied to
streams of water, and in the eighteenth century in Neapolitan dialect to streams
of molten rock from the Vesuvius volcano. The term is now used for the molten
material that erupts from volcanoes as well as for the rock that forms on
solidification of this material.
• Rocks resulting from volcanic eruption represent only a small proportion of those
rocks formed by the cooling and crystallization of magma, most of which occurs
at depths beneath the Earth’s surface. All rocks represent the final products of a
multitude of physical and chemical processes (see Igneous and Metamorphic
Petrology;
• Processes of Magma Evolution, Magmatic Suites and Consequences for the
Composition of Continental Crust). Throughout their cooling history all magmatic
rocks try to achieve mechanical and chemical equilibrium, but rarely succeed. As
a result, they leave behind trails consisting of wide chemical and mineralogical
variations, disequilibrium mineral assemblages, disequilibrium textures, fluid
inclusions.

43. Mode of Occurrence of Igneous Rocks
• Magmas erupted from volcanoes are either poured out as coherent fluidal lava
flows or blown out as fragments of various sizes. A body of magma may also be
emplaced and cool beneath the surface of the Earth. Igneous rocks result from
the final solidification of magma at the surface or at variable depths within the
Earth, as well as from the eventual consolidation of fragmented debris.
• Igneous rocks thus occur in two ways, either as “extrusive” (on the surface) rocks
or as “intrusive” (below the surface) bodies. Intrusive rocks are also called
“plutonic” (Pluto, the Greek god of infernal regions, therefore deep-seated) and
extrusive rocks “volcanic.”
• The terms intrusive and extrusive only refer to the place where the rock
solidified. Extrusive rocks cool rapidly because they have erupted at the Earth’s
surface, but intrusive rocks cool more slowly within an insulating blanket of
surrounding rocks into which they have been emplaced. The rapid cooling of
magma gives a fine-grained rock, which may even be glassy, whereas slower
cooling gives coarse-grained rock with large crystals.

44. There are two main types of igneous rocks:
1 Plutonic or intrusive igneous
rocks that are formed when magma or red-hot liquid rock cools down
and hardens before it reaches the surface of the Earth, e.g. granite
2 Volcanic or extrusive igneous
rocks that form when the liquid magma erupts during volcanic activity
and cools and hardens on the Earth’s surface, forming e.g. basalt or
andesite lava.

45. Plutonic Rocks
• Recall that an igneous rock is a rock formed by the solidification and
crystallization of a cooling magma. If a magma remains well below the surface
during cooling, it cools relatively slowly, insulated by overlying rock and soil.
• Under these conditions, the crystals have ample time to form and to grow very
large, and the rock eventually formed, has mineral grains large enough to be seen
individually with the naked eye. Such a rock that crystallizes below the earth’s
surface is referred to as Plutonic rock (The name is derived from Pluto, the Greek
god of the lower world.)
• Sometime plutonic rocks are referred to as Intrusive rocks. Granite is probably the
most widely known example of a plutonic rock. Compositionally, a typical granite
consists of quartz and feldspars, and it usually contains some ferromagnesian
minerals or other silicates

46. The proportions and compositions of these constituent minerals may vary, but all
granites show the coarse, interlocking crystals characteristic of a plutonic rock.
The three basic characteristics of plutonic rocks are:
1 They cut across older rocks
2 They are coarse grained, often with chilled margins
3 They are formed underground

47. THE FORMS OF IGNEOUS INTRUSIONS.
• Dykes (or Dikes):
These are tabular, or wall-like igneous intrusions that are usually steeply inclined
and cut across the bedding or foliation of the country rocks.
• Sills:
These are tabular igneous bodies that are flat-lying, and have been intruded
parallel to the planar structures in the surrounding rocks.
They are generally injected between bedded units, at relatively shallow depths
within the upper crust .
• Batholiths:
These are large, generally discordant bodies of plutonic rocks that have an outcrop
area that is greater than 100 km2 (Classical Greek, bathos, depth).
• Stocks:
These are small, generally discordant bodies of plutonic rock that have an outcrop
area that is less than 100 km2 .

48. • Lapoliths:
These are large, generally concordant bodies of plutonic rocks that have a plano-
convex or lenticular shape. They differ from sills in that they are depressed in the
center (Classical Greek, lopas, a basin).
• Cone Sheets:
These are conical dykes that converge towards a central point. In plan they usually
occur as concentric sets of dykes arranged about and dipping towards a center of
igneous activity.
• Ring Dykes:
These are dykes that are arcuate or circular in plan. Their dip is vertical or inclined
away from a local center of igneous activity.

49. Igneous Geochemistry
• The chemistry of igneous systems provides clues to a number of important
whole-earth processes, including the processes and timing of planetary
differentiation, the production and destruction of the lithosphere, and the
relationships between magmatic styles, composition and plate-tectonic
environment.
• Magmatism occurs in extrusive (volcanic) and intrusive (plutonic) forms.
Estimates of the volumes of magmatic rock produced each year in the four types
of plate-tectonic environment are listed in the table below, broken into
categories of extrusive (volcanic) and intrusive (plutonic)
• The magmas that ultimately produce the crust are commonly referred to as
mafic (rich in Mg and Fe, poor in Si) and silicic (the opposite). But, the bimodality
is far from perfect. A range of compositions and processes form more of a
continuum of igneous rock compositions.
• Some magmas represent melts of the mantle whereas others represent melts of
the crust, particularly on the continents.

50. Early Igneous geochemistry History
• By the early 1900s, a great deal was already known about the chemical
compositions of igneous rocks. However, an understanding of why certain
compositions occurred in certain places had to wait until the advent of plate
tectonics theory in the 1960s.
• The first large compendium of major-element analyses of igneous rocks from
around the world was published in 1917. Using 5159 samples, it demonstrated
that most igneous rocks are mixtures of just 10 major elements (O, Si, Al, Mg, Fe,
Ca, Ti, Na, K, P), plus minor.

51. Major Elements
• Chemical compositions of igneous rocks are usually reported as weight % (wt%)
of each element as an oxide. There are typically 9 major and 2 minor element
oxides listed in a rock analysis. This is true when Fe is reported as total FeO or
total Fe2O3 .
• Sometimes both oxidation states [Fe2+ (FeO) and Fe3+ (Fe2O3 )] are analyzed
and reported separately, as in the olivine thermometry homework problem we
had earlier this semester. Be sure to check this aspect when scrutinizing rock
analysis data.
• The early data base was strongly biased toward rocks from easily-accessed
locations, nearly all on land, so some details about global variations in
composition have changed
• The major elements are found in different proportions in the main minerals of
igneous rocks, and these minerals vary in proportion and composition with rock
type.

52. Trace element
• We will instead focus mainly on evidence provided by the trace elements, which
are covered in less detail in petrology courses. Unlike major element analyses,
trace element data are usually are reported as an element’s relative
concentration by mass (e.g., ppm, ppb). By definition, trace elements are present
at concentrations less than about 0.1 wt%.
• As we saw last week, trace elements provide key insights into the composition of
the mantle. We will look in the next few days at how they are used as tracers of
the composition of mantle and crustal source rocks that melt to produce
magmas. Trace elements also yield important information about the processes
and conditions of melting and crystallization.
• Trace elements usually do not form the major rock-forming phases. Instead, they
partition themselves among the different major phases as “contaminants”,
according to ionic radius , ionic charge , electronegativity and lattice energy of
substitution site.

53. "Goldschmidt's Rules"
• These outline the conditions for trace element partitioning between
igneous phases.
Ions will substitute readily for each other in a mineral lattice if...
1. Size: Their ionic radii differ by <15%.
2. Charge: They have the same charge or ±1 unit of charge
difference (substitution with greater charge differences may occur
but to a significantly lesser degree).
Of two ions with the charge and radius to occupy a lattice site...
3. The ion with the higher ionic potential (z/r) is favored because it
will make stronger bonds.
A fourth rule was added more recently by Ringwood:
4. The ion with the most similar electronegativity to that of the major
element being replaced will be favored because it destabilizes the
crystal lattice the least.

54. Trace element distribution
• We can determine whether a particular substitution is favored or not by using
the solid-melt distribution (partition) coefficient. Recall the equation for the
simple case of melt + one solid:
• Akd = [conc. of A]solid/[conc. of A]melt
• For a multi-phase system (one melt + more than one solid; e.g., several
minerals), we use the bulk distribution coefficient:
• Bulk Akd = AKd = DA = kA = [modal conc. of A]solids/[conc. of A]melt.

55. • Kd and DA
values tell us about the tendency of an element to be proportioned between
coexisting melt and solids.
DA > 1 The element is compatible (a.k.a. “captured”).
DA = 1 The element is neutral (a.k.a. “camouflaged”).
DA < 1 The element is incompatible (a.k.a. “released”)

56. • To summarize, the energy of substitution is minimized (made most favorable) for
substitution by the “right” ion; that is, the one with the best combination of;
a size,
b charge,
c electronegativity.
• As a function of T and P substitution into a phase becomes less favorable (i.e.,
kd goes down) the more any of these values vary from the ideal.
The incompatible elements have kd < 1 in all the common mantle minerals (and D
< 1 in mantle rocks) because their substitution energies are high in all of these
minerals

57. some Intrusive Rocks
The main granitoid intrusives in Tigrai are grouped into the Forstaga diorite and the
Mereb granite (Alene, 1998).
Forstaga Diorite
It occurs as large elongated intrusions near the eastern escarpment (in the Dahar
River near Berahale (Arkin et al. 1971 as cited in Alene, 1998) and in the Gabal
River (Garland, 1972 as cited in Alene, 1998).
This diorite intruded both the Tsaliet metavolcanics and rocks of the Tambien
Group (Alene, 1998). In the Mai Kenetal-Negash area, the quartz diorite intrusion is
associated with and migmatized by, a granitic stock (Alene, 1998).

58. • The Mereb granites
• occur in many parts of Tigrai mainly in the upper Mereb river (north-west of
Adwa), in the Shire area, and in the upper Tekeze river (Alene, 1998). They occur
also in the Mai Kenetal-Negash area as rounded or slightly elongated stock
(Alene, 1998). They have intruded the Tsaliet metavolcanic and the Tambien
Group (Alene, 1998). The Mereb granites are generally coarse grained, pink to
grey, alkali porphyritic granites with microcline phenocrysts up to 5 cm
long(Alene, 1998).
Source
Igneous geochemistry(GG325 L36, F2013) .lecture 6
MATERIALS OF THE EARTH ( C.M. NYAMAI Lecture 5)

59. 2)Discusses the geodynamic evolution intrusive and volcanic rocks of Ethiopia (30 ppt)
Introduction
• The geodynamic evolution of the "Afro-Arabian Rift System" rook place over a
Jong period of time (at least 35 m.y.) through a set of extension movement pulses
(stages), chemically developed during a process -which produced crustal chinning
up to continental separation and associated with magmatism changing from
transitional (fissural volcanism) to alkaline (central volcanism) compositions.
• Trace element geochemistry raid composition of primary compositions were
performed on selected basaltic samples retrospective of the main volcanic stages
(including Ashangi, Aiba, Alaji and Termaber formations) of the Central eastern
Ethiopian plateau representing the earlier evidence of the geodynamic evolution
from plateau to rift.
• The behaviour of hygromagmatophile elements (H. E.) supports the view that the
different basalts' of each stage are refaced sub spatially to different degrees of
partial melting which affected mantle sources with H. E. Both for major and trace
elements, the transitional basalts of the plateau (.high crustal thickness) and the
Afar rift show similar compositions, stream than the chemistry of basaltic melts
is better related to the intensity of extension movements rather than to difference
tectonic settings.

60. • The Cainozoic Ethiopian volcanism began 60-45 m.y. B.P. and obviously
represents one of the starting-points for the geodynamic interpretation of the Afro-
Arabian rift system (MOHR 1971).
• The first model suggested by GASS (1970) for the "Afro-Arabian Dome"
proposed three main tectonic-magmatic regimes characterized by:
(1) under saturated, mildly alkaline flood basalts (Ethiopian and Yemen "Traps") in
regions of the normal crustal thickness (plateaus);
(2) transitional basalts and associated peralkaline acid volcanics in the rift areas of
crustal attenuation; and
(3) oversaturated rholeijcic basalts in secrors of "oceanic" crust (Red Sea, Gulf of
Aden and north-central Afar Depression}. The transition from alkali basalts to
tholeiites was essentially related to the decreasing depth (pressure) of magma
generation, or segregation from an uprising anomalous upper mantle during a
process of progressive continental separation.

61. • However, the existence of large volumes of basaltic melts, similar in composition
to those of the Afar, from the beginning of Ethiopian volcanism in the north-
western and south-eastern plateaus (ZANETIIN et al. 1974a, b and 1976;
BROTZU et al. 1974; SCHUBERT 1975; RASCHKA &
• MULLER 1975; BARBIERI et al. 1976) rules out the relationship between
tectonic setting and related basic melts as proposed by GASS (1970). This fact
induced this Author (1975) to propose a new and more realistic model which takes
into account also the relevant role of (1) the degree of partial.

62. • Geodynamic Evolution from Plateau to Rift
1)melting; (2) the composition of the mantle; (3) the mantle water content, and (4)
the processes of crystal fractionation, zone refining and gas fluxing in the magma
generation from the anomalous upper mantle and the subsequent modifications
during magma ascent to the surface.
• It is therefore apparent that some tectonic-magmatic regimes are the result of an
evolutionary process that develops over a long period of time. This evidently
implies that a process producing crustal thinning up to continental separation can
be completely described only if considered from its beginning.
• On these grounds, the trace element geochemistry of the basalts emitted during the
various volcanic stages of the Ethiopian plateau (ZANETTIN et al. 1978) may
supply a record of the evolution of partial melting processes in time (e. g.
MYSEN & HOLLOWAY 1976), and some constraints regarding the interpretation
of the geodynamic evolution of the "Afro-Arabian System".

63. Geological outlines
•The volcanism of the north-western Ethiopian plateau occurred
in two main stages. separated by a long period of volcanic
quiescence during which uplifting and deep erosion, producing
the "Ashangi pene plain", took place (ZANETTIN et al. 1978).
•The first stage is essentially represented by basaltic activity
(Ashangi formation) characterized by alkaline to tholeiitic
basalts, the latter clearly related to fissural volcanism.
•The second stage (Oligocene to Miocene, 34-13 m.y.) instead is
represented by: (1) basaltic activity (Aiba formation, 34-28
m.y.); (2) bimodal volcanism (32-26 m.y.) with basalts and
abundant 'rhyolitic' ignimbrites (Oligocene and Miocene Alaji
formations), and (3) basaltic volcanism (25-13 m.y.) associated
with subordinate intermediate terms up to ophiolites (Termaber
Guassa and Termaber Meghezez formations).

64. • The Cainozoic volcanics of Ethiopia and Yemen were erupted during
development of the Arabo-Ethiopian swell and its traversing rift valleys. They
form what is perhaps the world's largest alkaline volcanic province, though calc-
alkaline lavas (e. g., tholeiitic basalts) are not unknown.
• The province is notably situated on a triple junction of the world ridge-rift
system, where the Gulf of Aden, Red Sea, and African rift structures converge at
Afar . Oceanographic data strongly imply a Cainozoic drift of Arabia
northeastward from Africa [Laughton, 1966;
• Tramontini and Davies, 19691, with the floors of the Gulf of Aden and Red Sea
being composed of young oceanic crust generated by typical tholeiitic
magmatism [Schilling, 196 91 , contrasting with the thick sialic crust of the
continental plates [Niazi, 19681. However, the occurrence of Afar and its
bordering sialic horsts within the triple junction is an unexplained paradox in the
simple theory of plate separation of Arabia from Africa.

65. • Some workers have considered Afar to be merely subsided sialic crust once
contiguous with the bordering Ethiopian and Somalian plateaus [e. g. , Gortani,
19501. More recently, the concept that the Afar depression represents exposed
oceanic crust has become widely accepted because it fits with the oceanographic
data [Bullard, 19691 ).
• Other workers point to the problem of the existence of the Danakil and Aisha
horsts within the triple junction, and the implications from structural geology
that fragmented blocks of sialic crust form an appreciable proportion of the Afar
floor under the cover of young lavas and sediments.
• [Mohr, 1967; Crass and Gibson, 19691As a contribution toward resolving this
controversial problem, an attempt is made here to examine and contrast the
volcanic chemistry of the plateau and rift subprovinces of the Ethiopian region.
• functions can then be discussed in terms of presumed differences in crustal and
subcrustal nature of the two subprovinces, though a major handicap remains our
lack of understanding of the petrogenesis of silicic magma in a rift environment
[Sigurdsson, 1967; McBirney, 19691 . Molly [ 19341 attempted to find a
petrographic basis' for distinction between the older (plateau) and Any chemical
discount

66. AGE AND DISTRIBUTION OF THE ETHIOPIAN VOLCANIC
The sequence of major vulcanism can be summarized as follows
• Aden Series
• C) Fissure basalts and caldera silicics in rift. Rare basaltic cinder-cone activity on
plateau (Upper r Pleistocene- Holocene).
• B) Basalts, tephrites, phonolites on the plateaus, especially in the Tana rift. strong
silicic volcanism in main Ethiopian rift and Afar (Middle Pleistocene).
• A) Extensive pantelleritic ignimbrites in southcentral Ethiopia, and possibly
contemporaneous silicics in Afar. Peralkaline intermediate lavas of re s tricted
extent on the plateaus. Rather minor basalt and nephelinitic activity, particularly
on the Somalian plateau (Plioceneearly Pleistocene). Minor basaltic.

67. Afar Series
• Flood basalts and mugearitic derivatives, with minor end-phase silicic volcanism,
in central and eastern Afar.
Shield Group
• Central-type flood alkali olivine basalts and hawaiites on plateaus. Occasional
end-phase comenditic lavas and stocks. Abundant minor intrusive activity, with
dike straps Series
• Fissure flood basalts forming a very extensive cover on the Arabo-Ethiopian
swell. bas alts predominate. unconformities are too localized to be used for
subdividing this thick series on a regional basis. Silicic lavas very rare. under the
margins of the present rift floor, but its extent under Afar is debatable (Eocene-
Oligocene). Alkali olivine Some intraformational The Trap Series plungeswarms of
both basaltic and silicic composition (Lower MiocenePlio).

68. • The rift grabens were not in existence until the Plio-Pleistocene. Before then the
rifts were sites of troughs formed by crustal downwarping [Baker and Mohr,
19701).
• its tectonic development commenced earlier, in the Mesozoic, with major
boundary faulting accompanying the Miocene phase of swell uplift.,
• The Tana rift, containing Lake Tana, is wholly within the Ethiopian plateau its
volcanic are included with plateau volcanics in this study, though some specific
features are discussed.
• local variations on the general succession given above influence interpretation of
plateau-rift volcanic chemistry. more important variations are brought out in the
discussion, but their full significance cannot be realized until many more data are
available. Afar is distinct from the true rift valleys in that It must be emphasized.

69. • The placement of individual analyses in either the plateau or the rift category has
been based on the best available structural maps. main Ethiopian rift, the major
graben faulting provides a clear boundary between young plateau and rift
volcanism, though for pre- graben volcanics the distinction is more obscure.
• For Afar, the inner structural margin of Mohr [ 19671 has been chosen as the
boundary, and thus lavas within the down warped region between the inner and
the outer structural margins are included with the plateau. included with rift
volcanics, excepting the southern portion of the sialic Aisha horst, where it is
contiguous with the Somalian plateaued of the Afar horsts can be reconsidered
when more analyses are available,
• though existing data reveal a character that is intermediate between rift and
plateau and that even in some instances tends to plateau-type chemistry.
Volcanics of the Tana rift, within the Ethiopian plateau, are considered to belong
to the plateau if only because of the small size and sialic foundation of this
interesting rift [Mohr and Rogers, 1966; Mohr, 1970k11 . The boundaries
between plateau and rift in Yemen have proved For the Lavas of the Danakil and
Aisha horsts are currently This treat.

70. • more difficult to delineate, and the volcanics of the Aden coast have been
considered as rift volcanics despite the overlap of at least one center onto Trap
Series lavas [Gass and Mallick, 19681)
• The data suggest that, both in Ethiopia and in Yemen, the rift volcanics compare
with the plateau volcanic as follows:
1. The rift basalts may have higher Si, Fe, Ca, and Na/K and lower Al, K, and Mg/Fe.
2. The rift intermediates may have higher Fe, Mg, Ca, Na/K, Mg/Fe, 32 Ti, and P
and lower Al, Na, K, and Fe /Fe . 32
3. The rift silicics may have higher Na and Na/K and lower Fe /Fe . Some of these
distinctions are comon to more than one class: for example, Na/K is higher in the
rift volcanics of all three classes, though for reasons discussed below it is unlikely
that there is only one cause.

71. Basalts.
• The rift basalts show a distinctly tholeiitic tendency compared with the plateau
basalts. basalts are quartz normative and the plateau basalts nepheline
normative. While there are localized occurrences of true tholeiites on both the
Ethiopian and the Somalian plateaus, it is in northern Afar that olivine-rich
basalts with tholeiitic tendencies are particularly abundant [Tazieff and Varet,
1969; Barberi et al., 19701).
• from en-echelon volcanic alignments paralleling the Red Sea and are considered
by Tazieff and Varet [lo691 to be of oceanic type in a region lacking sialic crust.
southern Afar and in the main Ethiopian rift, where they are also more alkaline
This is emphasized in the norms.
• where the rift These young basalts of Afar have been extruded Quaternary
basalts are much less voluminous in central and An important distinction
between plateau and rift volcanism in Ethiopia is the restriction of feldspathoid-
bearing lavas to the plateau environment [Le Bas and Mohr, 19681. between rift
and plateaus in the even more alkaline Kenyan volcanic province.

72. • [Wright, 19651. marked than that in Ethiopia, in conformity with the pattern of
increasing alkalinity southward along the African rift system [Harris, 1967; Mohr,
1963, 19681. and thus to a steeper thermal gradient under the Red Sea
(characterized by oceanic-type tholeiites [SchillinG 196 91) than under eastern
Africa.
• It will be interesting to test this plausible hypothesis, which relates the degree of
swell uplift and crustal distension to heat flow, with seismic refraction studies
now in progress. The alkaline character of the Yemeni plateau basalts is less
Harris relates this pattern to increasing depth of melting southward and thus to a
steeper thermal gradient under the Red Sea (characterized by oceanic-type
tholeiites [SchillinG 196 91) than under eastern Africa.
• It will be interesting to test this plausible hypothesis, which relates the degree of
swell uplift and crustal distension to heat flow, with seismic refraction studies
now in progress.

73. • The tholeiitic character of the Ethiopian rift basalts compared with the plateau
basalts invites comparison with other flood- basalt associations on the world
ridge-rift system. from their study of a compendium of Pacific and Atlantic ridge
lavas, that the more undersaturated lavas emerge on the flanks of the ridges, and
the more saturated lavas in or close to the median rift.
• Detailed study of a section of the mid-Atlantic rift shows a relatively rapid
transition from tholeiites on the rift floor to alkali basalts on the hills overlooking
the rift [Aumento, 1967, 19681 . The chemical pattern of the Ethiopian basalts
therefore appears to be analogous to oceanic ridge-rift basalts in general, despite
the presence of sialic crust forming the Ethio-Yemen plateaus.
• McBirney and Gass [l967] have suggested, Gass - [1970] considers that the
relation of alkali basalts to tholeiites Gass - [1970] considers that the relation of
alkali basalts to tholeiites in the Ethio-Yemen regions is largely a temporal one:
"alkali basalts were erupted during the early period when vertical [swell] uplift
was dominant. . . whereas the 'oceanic' crust in the Red Sea, Gulf of Aden and
Afar Depression, formed during the lateral movement of sialic blocks in the late
Tertiary, is of tholeiitic character.

74. • However, this does not present a valid picture of the tectonic evolution of the
Ethiopian region. has been operating during periods of varying intensity all
through the Cainozoic, and likewise, distension crustal movements are probably
longstanding despite the late replacement of rift troughs by graben [Mohr -'
19671.
• Furthermore, the Quaternary basalts of the plateaus remain strongly alkaline. It
might be considered more accurate to state that tholeiitic Swell uplift basalts are
associated with lines of crustal thinning, regardless of age, but this in turn fails to
explain the occurrence of Miocene tholeiites on the Ethiopian and Somalian
plateaus. plateau tholeiites and alkali basalts is intimate and complex and that
'oceanic' and 'continental' tholeiites are distinct entities [Schilling, 19691.
• It, therefore, seems that an association Several earlier workers [e. g., Hieke-
Merlin, 1950, 1953; Colucci, 19501 have used Niggled values to investigate the
chemistry of the Ethiopian volcanic. ing inter- oxide relationships independently
of variations in silica content [Leake, 19701. McBirney and Gass [1967] found QZ
values to be the most sensitive single parameter in the pattern of volcanic
variations across midnight values are currently finding favor as a means of
determining oceanic ridge

75. Intermediate volcanics.
• The intermediate volcanics show greater distinction between rift and plateau
than do either basalts or silicics. For the Ethiopian region (excluding Yemen), K20,
Ti0 MgO, and especially CaO provide possible single discriminants, and
associations of these oxides and also of volatiles would undoubtedly improve
discrimination.
• intermediates have lower Mg/Fe than do the Ethiopian intermediates, and like
the Yemen basalts, include members with very high Niggli C and low ALK values.

76. • The rift intermediates have a calc-alkaline tendency that is quite distinct from the
peralkaline nature of the plateau intermediates. mediates magnify a feature of
the plateau basalts in showing strong potash enrichment relative to soda. In this
regard, some field relationships are noteworthy. with more voluminous units of
alkaline rhyolite, the plateau intermediates form plugs, stocks, and thick flows
that often lack associated silicics or basalts.
• The rift intermediates can therefore be considered to lie on a general line of
crystal fractionation (or extensive partial melting at depth), whereas the plateau
intermediates are considered to be derived from an independent magmatic
source [see also Williams, 19701 . The plateau inter Whereas the rift
intermediates are usually intimately associated Like the basalts,
• the intermediate volcanics of the plateau are less saturated in terms of normative
quartz and Niggli QZ values, are more oxidized, and are much more salic
compared with the rift volcanics. excessively salic and alkaline (especially
potassic) character of the plateau intermediates makes it difficult to explain their
origin in terms of normal fractionation from or partial melting of alkali olivine
basalt such as forms the bulk of the Trap Series and Shield Group.

77. • both Sutherland [ 19651 and Nixon and Clark E19671 note the inadequacy of
normal fractionation processes to generate these rocks; they propose a crustal
origin involving fenitisation and mobilization The question occurs as to whether
the magmas supplying the alkali olivine basalts of the plateau ever gave rise to
fractionated intermediate derivatives. appear peculiarly rare in Ethiopia and
Yemen.
• the Ethiopian phonolites are intimately related to contemporaneous basalts
(basanites), whereas the trachytes are associated with andesites. Molly's
conclusions are accepted, however, the overwhelming proportion of Ethiopian
basalts show no associated phonolites. On the basis of known field evidence,
such possible derivatives Molly E19341 considers that Even if Similar objections
to.
• partial fusion origin for the plateau intermediates derive from the infrequency of
associated silicic lavas, unless the degree of basaltic fusion was peculiarly
consistent.

78. Silicic volcanic
• Possible distinctions have been tentatively suggested by Mohr [19701?], who
has emphasized the influence of local anomalies on any general interpretation.
Thus, although the rift tends to be characterized by pantelleritic and the plateau
by comenditic rhyolites, occurrences of pantellerite are known from the
southern Ethiopian plateau and of comendite from central Afar.
• on the basis that the rift silicics have higher Na/K than do the plateau silicics
and, like the intermediates, may be less oxidized. Degrees of over saturation and
femininity are almost identical Nevertheless, that Niggli plots reveal separate
tendencies between plateau and rift volcanics.
• value corner of the Chabbi obsidians [Macdonald and Gibson, 19691 having
higher QZ values than do the Afar silicic, perhaps owing to their situation well
inside the continental African shield

79. • McBirney [ 19691 states that extensive rhyolitic ignimbrites are characteristic of
continental epeirogenic tectonism, whereas in oceanic regions only small
quantities of pantellerite-comendite occur. sialic crust, the Ethiopian swell is
considered to be a continental tectonic phenomenon rather than one analogous
to a mid-oceanic ridge, then the enormous extent of silicic ignimbrites confirms
McBirney's observation, though the Ethiopian ignimbrites are pantelleritic
[Mohr, 19681 . If, on the basis of its McBirne.
• further notes that "the extreme composition and great volume of rhyolitic
ignimbrites places a severe constraint [on their possible genesis] " and concludes
that large-scale crustal fusion is a more plausible explanation than is crystal
fractionation.
• Barberi et al. [ 19701 postulate assimilation of sialic crust to explain the large
quantities of 'variegated' silicic volcanics in northern and central Afar, excluding
the Retinal and Alanita silicics, whose low Sr /Sr values suggest an
uncontaminated mantle origin. This presumption of sialic crust under Afar,
supported by geological and other evidence [Jones, 1968; Mohr, 1962, 19671,
introduces well-known problems for the original geometric fit of Arabia against
Africa.

80. • Sigurdsson [1967] has discussed the origin of the silicic lavas of Iceland in view of
the absence of a pre-volcanic sialic crust there and is forced to conclude that
differentiation from basaltic magma is the only obvious source.
• Gibson [ 19691 has suggested the possibility that partial fusion of solid basalt,
rather than differentiation from basaltic magma, has been the source for the
Icelandic silicic lavas. inadequate to provide the voluminous quantities of silicic
magma erupted in Ethiopia during the Neogene (see also Sigurdsson [l967], p.
39).
• case of partial fusion, the required thermal conditions at the base of the crust
are too discrete to be at all probable. such that 5'7'0 of basalt is partially fused to
yield silicic magma, 1 million km of basalt are required to provide the 50, 000
km3 of Ethiopian Neogene silicic.
• The Upper Pliocene ignimbrites make up the bulk of this volume, and as their 2
occurrences in south-central Ethiopia covers a maximum of 150,000 km , there is
a required partial fusion of a 6-km-thick plate of underlying basalt. In view of the
temperature gradient that would exist through a plate of this thickness, such a
model is implausible.

81. Geochemistry
• The major element behaviour of the central-eastern Ethiopian plateau volcanics
has already been discussed extensively by ZANETTIN et al. (19746); the relative
chemical data (337 analyses) can be found in the catalogue of ZANETTIN et al.
(1976).
• The present trace element investigation was performed on 41 selected
"basaltic" samples representative of the main volcanic stages (Ashangi,
Aiba, Alaji and Termaber formations). They are fresh, normally aphyric
to sub aphyric and, only rarely, weakly porphyritic lavas, which
therefore can be assumed as representing effective 'liquids'.. The
relative major elements are listed in the above-mentioned catalogue,
while in this paper they are shown in diagrammatic form (Fig. 4) and as
average values (a) with relative standard deviations (s) for the different
formations Short petrographical notes for the analyses samples are to be
found in the Appendix.

82. Nomenclature and major elements
• The basaltic rocks were divided into three groups, mainly on mineralogical,
petrographic ,and, lastly, chemical criteria. The tholeiitic basalts (Ashangi: S. 56
to 63; Aiba: 91; Miocene Alaji: 194) are characterized by orthopyroxene and/or
Ca-poor clinopyroxene 1, also as a reaction relation between Mg-rich olivine and
liquid, scarcity or absence of olivine in the groundmass, and late crystallization of
opaques.
• S. 45 to 52; Termaber Guassa: S. 174, 258; Termaber Meghezez: 278, 280, 282,
285) are typically composed of stable olivine, early crystallization of opaques and
Ca-rich and/or Ti-rich clinopyroxene, relative abundance of olivine and
sometimes interstitial alkali feldspar in the groundmass. The transitional basalts
(Ashangi: S. 54; Aiba: S. 74, 78, 80, 83, 85, 86, 88, 93
• to 97; Alaji: S. 169, 170, 172, 173, 189, 191 to 193; Termaber Guassa: S. 257),
instead, are intermediate between the preceding ones: unstable olivine,
moderately Ca-rich clinopyroxene, late crystallization of opaques, etc.
).

83. • In the present case this boundary can be tentatively used for distinguishing
transitional basalts with alkaline affinity (Alaji and part of the Aiba basalts) or
tholeiitic affinity (part of the Aiba basalts), see also BROTZU et al. 1974, and
BARBIERI et al. 1976 for the transitional steroid basalts of the south-eastern
Ethiopian plateau.
• Alaji, with a few samples from Termaber and Ashangi, plot between the alkaline
and tholeiitic suites from Hawaii (MACDONALD & KATSURA 1964).
Maximum iron enrichment is found in tholeiitic and part of alkaline Ashangi,
while the minimum is shown mainly by Termaber which therefore, contrary to the
alkaline Ashangi, suggests a high fO2 typical of an alkaline evolutionary
environment.
• The transitional basalts also show an intermediate character according to the classification of CHAYES (1966), Coo Mns
(1963), PoLDERVAART (1964) and de LA ROCHE (1976). However, from the normative (CIPW) viewpoint, only part of
the plateau transitional basalts (see later discus sion) plot in the olivine tholeiite field of YODER & TILLEY (1962), which
BASS (1972) assumes as typical for the transitional basalts.

84. • Trace elements
All the trace elements were determined by instrumental neutron
activation analysis (INAA).
• The trace element contents for the analyzed the variation patterns with
respect to the average values for the different formations are also
explained. As is well known, the concentration of hygromatophile
(LILE, REE) elements really does provide significant information on
the nature of basic melts, since it increases with the degree of alkalinity
(BASS 1971; SCHEI DEGGER 1973; TREUIL & VARET 1973 and
references therein).
• case it is relevant to observe that La, Rb, Sr, Ta, Th, U and, to a lesser
extent, Zr contents increase, for similar S.I. values, from tholeiitic to
transitional to alkaline basalts and this strongly supports the basalt
classification employed. This behaviour is also shown by K20 and P205
, while Hf, Eu and Tb seem to reveal an inverse tendency,

85. Since Ni and Cr are good tracers of olivine and clinopyroxene
respectively, in the absence of a spine! (MYSEN & Kushiro 1976;
MYSEN 1977a; ALLEGRE & MINSTER 1978), a lack which is
probable in the present case because of its absence in the porphyritic and
cumulative facies, the Ni and Cr contents in the lavas can supply a
quantitative evaluation of crystal fractionation of such minerals.
• The chemical compositions of 86 °/o Fo olivine, 80 0/o An plagioclase
(DEER et al. 1963), and different clinopyroxenes 2 depending on the
basalt type, were employed in the calculations of primary basaltic melts,
whose average values for the different formations are plotted in Fig. 4.
The high average values of S.I. (43), Mg/Mg+Fe" (0.66) and MgO
(10.7) are consistent with those proposed for possible primary basaltic
compositions (KUN0 1969; RINGWOOD 1975 and references therein).
• The compositional differences between all the primary liquids are lesser
than those among the corresponding lavas, except for SiO2, and
emphasize some distinctive general chemical features.

86. Major elements
• The alkali-silica shows that, among the axial ranges, most of the Asal, Manda Hararo, Alayta
and Erta Ale volcanics plot in the transitional field both with tholeiitic and alkaline affinity,
while the Boina and Manda Inakir volcanics plot mainly in the alkaline field,
• that in the same field where the alkaline central volcanics (Termaber) of the Ethiopian plateau
plot. Only a few samples of Asal and Erta Ale (see also BARBERI & V ARET 1977,) belong to
the tholeiitic.
• The Afar steroid basalts (see also BARBERI & VARET 1977, ) show the same distribution as
those of the axial range with a majority of transitional basalts.
• The major element variation diagrams (Figs. 9a, b), are very interesting in that they reveal how
the Afar basalts, for similar S.I. values, are generally poorer in SiO2 and partly in K2O,richer in
FeO and only with slightly higher contents of MgO and CaO with respect to the transitional
basalts of the plateau,

87. • As regards K
2O we observe that the lowest concentrations (0.3-0..4 0/o) are shown by
relatively few samples from Manda Hararo, Asal, Erta Ale and tholeiitic Ashangi.
Moreover, the Ena Ale range is really characterized by two distinct groups of
basalts with different K2O contents, between which the Manda Inakir, Alayta and
the transitional basalts of the plateau (Aiba and Alaji) plot.
• Most of the Boina volcanics, instead, have K2O concentrations which are
intermediate between the transitional and alkaline plateau basalts, while the
stratoid series covers the range between low-K Erta Ale and Boina basalts

88. Comparison between the Central-eastern Ethiopian plateau and the Afar basic
volcanism
• acorrect petrologic and geodynamic interpretation of the
expounded data relative to the Ethiopian plateau volcanics is
impossible without taking into account the nature of the basic
volcanism which occurred in the Afar rift, when the volcanic
activity of the plateau was substantially completed
(ZANETTIN et al. 1978 and references therein).
•on these grounds, the major and trace element comparison with
the basalts of the axial ranges and stratoid series of Afar is very
interesting, since they were outpoured in a very different
structural setting (significantly attenuated crustal thickness)
with respect to that of the Ethiopian plateau (high crustal
thickness).

89. • Source
•Geodynamic Evolution from Plateau to Rift: Major
and Trace Element Geochemistry of the Central
Eastern Ethiopian Plateau Volcanics
• By
• E. M. Piccirillo, E. Justin-Visentin, B. Zanettin (Padova)
•J. L. Joron and M. Treuil (Paris)With 17 figures and 2 tables in the text
• ETHIOPIAN RIFT AND PLATEAUS: SOME VOLCANIC PETROCHEMICAL DIFFERENCES
• P. A. Mohr

90. 3) Discusses in detail the petrological and geochemical (major element, trace
element and isotopic) characteristics of (50 ppt).
1. Mid-Ocean Ridges
2. Intra-continental Rifts
3. Island Arcs
4. Active Continental Margins
5. Back-Arc Basins 7. kimberlites, carbonatites, anorthosites

91. ANSWER FOR QUESTION 1
• The most volcanically active regions of our planet are concentrated along the
axes of the globe, encircling midocean ridges. These undersea mountain ranges,
and most of the oceanic crust, result from the complex interplay between
magmatic (i.e., eruptions of lavas on the surface and intrusion of magma at
depth) and tectonic (i.e., faulting, thrusting, and rifting of the solid portions of
the outer layer of the earth) processes. Magmatic and tectonic processes are
directly related to the driving forces that cause plate tectonics and seafloor
spreading.
• Exploration of midocean ridges by submersible, remotely operated vehicles
(ROV), deep-sea cameras, and other remote sensing devices has provided clear
evidence of the effects of recent magmatic activity (e.g., young lavas, hot springs,
hydrothermal vents and plumes) along these divergent plate boundaries.
Eruptions are rarely observed because of their great depths and remote
locations.

92. • However, over 60% of Earth’s magma Sux (approximately 21 km3 year~1 )
currently occurs along divergent plate margins. Geophysical imaging, detailed
mapping, and sampling of midocean ridges and fracture zones between ridge
segments followed by laboratory petrologic and geochemical analyses
• recovered rocks provide us with a great deal of information about the
composition and evolution of the oceanic crust and the processes that generate
midocean ridge basalts (MORB) Midocean ridges are not continuous but rather
broken up into various scale segments reciting breaks in the volcanic plumbing
systems that feed the axial zone of magmatism.
• Recent hypotheses suggest that the shallowest and widest portions of ridge
segments correspond to robust areas of magmatism, whereas deep, narrow
zones are relatively magma-starved. The unusually elevated segments of some
ridges (e.g., south of Iceland, the central portion of the Galapagos Rift, Mid-
Atlantic Ridge near the Azores) are directly related to the influence of nearby
mantle plumes or hot spots that result in voluminous magmatic.

93. • Major differences in the morphology, structure, and scales of magmatism along
midocean ridges vary with the rate of spreading. Slowly diverging plate
boundaries, which have low volcanic output, are dominated by faulting and
tectonism whereas fast-spreading boundaries are controlled more by volcanism.
• The region along the plate boundary within which volcanic eruptions and high-
temperature hydrothermal activity are concentrated is called the neovolcanic
zone. The width of the neovolcanic zone, its structure, and the style of volcanism
within it, vary considerably with spreading rate.
• In all cases, the neovolcanic zone on midocean ridges is marked by a roughly
linear depression or trough (axial summit collapse trough, ASCT), similar to rift
zones in some subaerial volcanoes, but quite different from the circular craters
and calderas associated with typical central-vent volcanoes. Not all midocean
ridge volcanism occurs along the neovolcanic zone.

94. • Recent evidence also suggests that significant amounts of volcanism may occur
up to 4 km from the axis off-axis mounds and ridges, or associated with faulting
and the formation of abyssal hills.
• Lava morphology on slow-spreading ridges is dominantly bulbous, pillow lava,
which tends to construct hummocks ((50 m high, (500 m diameter), hummocky
ridges (1}2 km long), or small circular seamounts (10s}100s of meters high and
100s}1000s of meters in diameter) that commonly coalesce to form axial volcanic
ridges (AVR) along the valley Soor of the axial rift zone.
• On fast-spreading ridges, lavas are dominantly oblong, lobate Sows and Suid
sheet Sows that vary from remarkably Sat and thin ((4 cm) to ropy and jumbled
varieties Although the data are somewhat limited, calculated volumes of
individual Sow units that have been documented on midocean ridges show an
inverse exponential relationship to spreading rate, contrary to what might be
expected.

95. • The largest eruptive units are mounds and cones in the axis of the northern Mid-
Atlantic Ridge whereas the smallest units are thin sheet/lobate Sows on the.
• Morphologic, petrologic, and structural studies of many ridge segments suggest
they evolve through cycles of accretion related to magmatic output followed by
magmatic periods dominated by faulting and extension.

96. Magma Generation
• Primary MORB magmas are generated by partial melting of the upper mantle;
believed to be composed of a rock type termed peridotite which is primarily
composed of the minerals olivine, pyroxenes (enstatite and diopside), and minor
spinel or garnet. Beneath ridges, mantle moves upward, in part, due to
convection in the mantle but possibly more in response to the removal of the
lithospheric lid above it, which is spreading laterally.
• Melting is affected by the decompression of hot, buoyant peridotite that crosses
the melting point (solidus curve) for mantle material as it rises to shallow depths
((100km), beneath the ridges.
• Melting continues as the mantle rises as long as the temperature of the
peridotite remains above the solidus temperature at a given depth. As the
seaSoor spreads, basaltic melts formed in a broad region (10s to 100s of
kilometers) beneath the ridge accumulate and focus so that they feed a relatively
narrow region (a few kilometers) along the axis of the ridge.

97. • During ascent from the mantle and cooling in the crust, primary mantle melts are
subjected to a variety of physical and chemical processes such a fractional
crystallization, magma mixing, crustal assimilation, and thermogravitational
diffusion that modify and differentiate the original melt composition.
Consequently, primary melts are unlikely to erupt on the sea floor without
undergoing some modification.
• Picritic lavas and magnesian glasses thought to represent likely primary basalts
have been recovered from a few ocean Soor localities; commonly in transform
faults (Table 1). MgO contents in these basalts range from &10 wt% to over 15
wt% and the lavas typically contain significant amounts of olivine crystals.
• Based on comparisons with high-pressure melting experiments of likely mantle
peridotites, the observed range of compositions may reSect variations in source
composition and mineralogy (in part controlled by pressure), depth and
percentage melting (largely due to temperature differences), and/or types of
melting (e.g., batch vs. fractional.

98. Ocean Floor Volcanism and Construction of the Crust
• Oceanic crust formed at spreading ridges is relatively homogeneous in thickness
and composition compared to continental crust. On average, oceanic crust is 6}7
km thick and basaltic in composition as compared to the continental crust which
averages 35}40 km thick and has a roughly andesitic composition.
• The entire thickness of the oceanic crust has not been sampled in situ and
therefore the bulk composition has been estimated based on investigations of
ophiolites (fragments of oceanic and back-arc crust that have been thrust up on
to the continents), comparisons of the seismic structure of the oceanic crust with
laboratory determinations of seismic velocities in known rock types, and samples
recovered from the ocean Soor by, dredging, drilling, submersibles, and remotely
operated vehicles.
• Rapid cooling of MORB magmas when they come into contact with cold sea
water results in the formation of glassy to Rnely crystalline pillows, lobate Sows,
or sheet Sows. These lava Sows typically have a &0.5}1 cm-thick outer rind of
glass and a Rne-grained, crystalline interior containing only a few percent of
millimeter-sized crystals of olivine, plagioclase, and clinopyroxene in a
microscopic matrix of the same minerals.

99. • MORB lavas erupt, Sow, and accumulate to form the uppermost volcanic layer
(Seismic Layer 2A) of ocean crust (Figure 1). Magmas that do not reach the
seafloor cool more slowly with increasing depth forming intrusive dikes at
shallow levels (0.5}3 km) in the crust (layer 2B) and thick bodies of coarsely
crystalline gabbros and cumulate ult Aramaic rocks at the lowest levels (3}7 km)
of the crust (layer 3)
• . Although most magma delivered to a MOR is focused within the neovolcanic
zone, determined by the axial summit collapse trough or axial valley, off-axis
volcanism and near-axis seamount formation add significant volumes of material
to the oceanic crust formed along ridge crests. In some portions of the fast-
spreading East Pair Rise, off-axis eruptions are related to syntectonic volcanism
and the formation of abyssal hills.
• Near-axis seamount formation is common along both the East Pair Rise and the
medium-spreading-rate Juan de Fuca ridges Oceanic transform faults are
supposed to be plate boundaries where crust is neither created nor destroyed,
but recent mapping and sampling indicate that magmatism occurs in some
transform domains. Volcanism occurs in these locales either at short,
intratransform spreading centers or at localized eruptive centers within shear
zones or relay zones between the small spreading center.

100. Mid Ocean Ridge Basalt Composition
• Ocean floor lavas erupted along midocean ridges are low-potassium tholeiites that can
range in composition from picrites with high MgO contents to ferro basalts and FeTi
basalts containing lower MgO and high concentrations of FeO and TiO2, and even to
rare, silica-enriched lavas known as celandines, ferroandesites and rhyodacites .
• In most areas, the range of lava compositions, from MgO-rich basalt to FeTi basalt and
ultimately to rhyodacite, is generally ascribed to the effects of shallow-level (low-
pressure) fractional crystallization in a sub-axial magma chamber or lens (Figure 1). A
pronounced iron-enrichment trend with decreasing magnesium contents (related to
decreasing temperature) in suites of genetically related lavas is, in part, what classes of
MORB as tholeiitic or part of the tholeiitic magmatic suite .
• Although MORB are petrologically similar to tholeiitic basalts erupted on oceanic
islands (OIB), MORB are readily distinguished from OIB based on their comparatively
low concentrations of large ion lithophile elements (including K, Rb, Ba, Cs), light rare
earth elements (LREE), volatile elements and other trace elements such as Th, U, Nb,
Ta, and Pb that are considered highly incompatible during melting of mantle mineral
assemblages.

101. • In other words, the most incompatible elements will be the most highly concentrated
in partial melts from primitive mantle peridotite. normal MORB (N-type or N-MORB)
exhibit characteristic smooth concave-down patterns respecting the fact that they were
derived from the incompatible element-depleted mantle.
• Isotopic investigations have conclusively shown that values of the radiogenic isotopes
of Sr, Nd, Hf, and Pb in N-MORB are consistent with their depleted characteristics and
indicate incompatible element depletion via one or more episodes of partial melting of
upper mantle sources beginning more than 1 billion years ago. Compared to ocean
island basalts and lavas erupted in arc or continental settings, MORB comprises a
relatively homogeneous and easily distinguishable rock association.
• Even so, MORB varies from very depleted varieties (D-MORB) to those containing
moderately elevated incompatible element abundances and more radiogenic isotopes.
These less-depleted MORB are called E-types (E-MORB) or P-types, indicative of an
‘enriched’ or ‘plum component typically associated with intraplate ‘hot spots’.
Transitional varieties are classified as T-MORB. Enriched MORB are volumetrically
minor on most normal ridge segments, but can comprise a significant proportion of the
crust around regions influenced by plume magmatism such as the Galapagos Islands,
the Azores, Tristan, Bouvet, and Iceland

102. Mineralogy of MORB
• The minerals that crystallize from MORB magmas are not only dependent on the
composition of the melt, but also the temperature and pressure during
crystallization. Because the majority of MORB magmas have relatively similar
major element compositions and probably begin to crystallize within the
uppermost mantle and oceanic crust (pressures less than 0.3 GPa), they have
similar mineralogy.
• Textures (including grain size) vary depending on nucleation and crystallization
rates. Hence lavas, which are quenched when erupted into seawater, have few
phenocrysts in a glassy to cryptocrystalline matrix. Conversely, magmas that cool
slowly in sub axial reservoirs or magma chambers form gabbros that are totally
crystalline (holocrystalline) and composed of well-formed minerals that can be up
to a few centimeters long Many of the gabbros recovered from the ocean Soor do
not represent melt compositions but rather respect the accumulation of crystals
and percolation of melt that occurs during convection.
• These cumulate gabbros are composed of minerals that have settled (or Soated)
out of cooling MORB magmas and their textures often respect compaction,
magmatic sedum.

103. • MOR lavas may contain millimeter-sized phenocrysts of the silicate minerals plagioclase
(solid solution that ranges from CaAl2Si2O8 to NaAlSi3O8) and olivine (Mg2 SiO4 to
Fe2SiO4) and less commonly, clinopyroxene (Ca[Mg, Fe]Si2O6). Spinel, a Cr-Al rich
oxide, is a common accessory phase in more magnesian lavas where it is often enclosed
in larger olivine crystals.
• Olivine is abundant in the most MgO-rich lavas, becomes less abundant in more evolved
lavas and is ultimately replaced by pigeonite (a low-Ca pyroxene) in FeO-rich basalts and
andesite. Clinopyroxene is only common as a phenocryst phase in relatively evolved
lavas. Titanomagnetite, ilmenite and rare apatite are present as microphenocrysts,
although not abundantly, in basaltic andesites and andesites.
• Intrusive rocks, which cool slowly within the oceanic crust, have similar mineralogy but
are holocrystalline and typically much coarser grained. Dikes form Rune- to medium-
grained diabase containing olivine, plagioclase and clinopyroxene as the major phases,
with minor amounts of ilmenite and magnetite. Gabbros vary from medium-grained to
very coarse-grained with crystals up to a few centimeters in length.
• Because of their cumulate nature and extended cooling histories, gabbros often exhibit
layering of crystals and have the widest mineralogic variation. Similar to MORB, the
leastevolved varieties (troctolites) consist almost entirely of plagioclase and olivine

104. • Some gabbros can be nearly monomineralic such as anorthosites (plagioclase-
rich) or contain monomineralic layers (such as olivine that forms layers or lenses
of a rock called dunites).
• The most commonly recovered varieties are composed of plagioclase, augite (a
clinopyroxene) and hypersthene (orthopyroxene) with minor amounts of olivine,
ilmenite, and magnetite and, in some cases, hornblende (a hydrous Fe-Mg
silicate that forms during the latest stages of crystallization). Highly evolved
liquids cool to form ferro gabbros and even rarer silica-rich plutonic known as
trondhjemites or plagiarists
• MOR basalts, diabases and gabbros are commonly metamorphosed to
greenschists and amphibolites. Plutonic rocks and portions of the upper mantle
rich in olivine and pyroxene are transformed into serpentinites.
• Oceanic metamorphic rocks are commonly recovered from transform faults,
fracture zones and slowly spreading segments of the MOR where tectonism and
faulting facilitate deep penetration of seawater into the crust and upper mantle.

105. • In environments where magma supply is low or mixing is inhibited, such as
proximal to transform faults, propagating rift tips and overlapping spreading
centers, compositionally diverse and highly differentiated lavas are common.
• In these environments, extensive fractional crystallization is a consequence of
relatively cooler thermal regimes and the magmatic processes associated with
rift propagation. Local variability in MORB can be divided into two categories: (1)
those due to processes that affect an individual parental magma (e.g., fractional
crystallization, assimilation) and (2) those created via partial melting and
transport in a single melting regime (e.g., melting in a rising diapir).
• In contrast, global variations reflect regional variations in mantle source
chemistry and temperature, as well as the averaging of melts derived from
diverse melting regimes (e.g. accumulative polybaric fractional melting). At any
given segment of MOR, variations may be due to various combinations of these
processes.

106. Chemical Variability
• Although MORB forms a relatively homogeneous population of rock types when
compared to lavas erupted at other tectonic localities, there are subtle, yet
significant, chemical differences in their chemistry due to variability in source
composition ) liquid composition, temperature, and pressure. In some MORB
suites, linear elemental trends may be due to the mixing of primitive magmas
with more evolved magmas that have evolved along an LLD.
• Suites of MORB glasses often describe distinctive LLDs that match those
determined by experimental crystallization of MORB at low to moderate
pressures. Much of the major element data from fast-spreading ridges like the
East Pacific Rise are best explained by low-pressure (&0.1 GPa) fractional
crystallization whereas at slow-spreading ridges like the Mid-Atlantic Ridge data
require higher pressure crystallization (&0.5}1.0 GPa).
• This is consistent with other evidence suggesting that magmas at fast-spreading
ridges evolve in a shallow magma lens or chambers and that magmas at slow-
spreading ridges evolve at significantly greater depths; possibly in the mantle
lithosphere or at the crust}mantle boundary. Estimated depths of crystallization
correlate with increased depths of magma lens or fault rupture depth related to
decreasing spreading rate.

107. • Cogenetic lavas (those from the same or similar primary melts) generated by
fractional crystallization exhibit up to 10-fold enrichments of incompatible trace
elements (e.g., Zr, Nb, Y, Ba, Rb, REE) that covary with indices of fractionation
such as decreasing MgO (Figure 5) and increasing K2O concentrations and
relatively constant incompatible trace element ratios irrespective of rock type.
• In general, the rare earth elements show systematic increases in abundance
through the fractionation sequence from MORB to andesite with a slight
increase in light rare earth elements relative to the heavy-rare earth elements.
The overall enrichments in the trivalent rare earth elements is a consequence of
their incompatibility in the crystals separating from the cooling magma.
• Increasing negative Eu anomalies develop in more fractionated lavas due to the
continued removal of plagioclase during crystallization because Eu partially
substitutes for Ca in plagioclase which is removed during fractional
crystallization

108. Global Variability
• MORB chemistry of individual ridge segments (local scale) is, in general,
controlled by the relative balance between tectonic and magmatic activity, which
in turn may determine whether a steady-state magma chamber exists, and for
how long. Ultimately, the tectonic magmatic evolution is controlled by temporal
variations in the input of melt from the mantle.
• Global correlation of abyssal peridotite and MORB geochemical data suggest that
the extent of mantle melting beneath normal ridge segments in creases with
increasing spreading rate and that both ridge morphology and lava composition
are related to spreading rate.
• The depths at which primary MORB melts form and equilibrate with
surrounding mantle remain controversial, as does the mechanism(s) of Sow of
magma and solid mantle beneath divergent plate boundaries. The debate is
critical for understanding the dynamics of plate spreading and is focused on
whether Sow is ‘passive’ plate-driven Sow or ‘active’ buoyantly driven solid
convection.
• At present, geological and geophysical observations support passive Sow which
causes melts from a broad region of upwelling and melting to converge in a
narrow zone at the ridge crest.