Alkaline magmatic rocks are igneous rocks that contain more alkalis (Na2O + K2O) than feldspars alone can accommodate. They commonly contain feldspathoids like nepheline, sodalite, or leucite. Examples include phonolites, nepheline syenites, and basanites. Alkaline rocks can be classified based on their silica and alumina content relative to alkalis. The most silica-undersaturated alkaline rocks are carbonatites. Debate occurred over whether carbonatites had an igneous or replacement origin, but experimental evidence and observations of carbonate lavas support a magmatic origin.

1. Alkaline Magmatism
Thanks to John Winter….

2. Alkaline Magmatism
 The term alkalic is generally used in broad sense to describe
magmatic rocks that contain more of alkalis (Na2O+K2O) than is
considered normal for the group of rocks to which they belong.
In petrology, the term alkaline is used to describe those rocks
that contain essential amounts of foids (nepheline, sodalite, leucite)
&/or alkali pyroxenes, &/or alkali amphiboles, &/or melilite.
Phonolites, foid-syenites, tephrites, foid-diorites,
basanites, foid-gabbros, foid-basalts and nephelinites are
all examples of alkaline igneous rocks.

3. Alkaline Magmatism
Alkaline rocks generally have more alkalis than can be
accommodated by feldspars alone. The excess alkalis appear
in feldspathoids, sodic pyroxenes-amphiboles, or other
alkali-rich phases
In the most restricted sense, alkaline rocks are deficient in
SiO2
with respect to Na2
O, K2
O, and CaO to the extent that
they become “critically undersaturated” in SiO2
, and
Nepheline or Acmite appears in the norm.

4. Alkaline Magmatism
In alkali feldspar, the molecular ratio of (Na2O+K2O) : Al2O3 : SiO2 is
1:1:6.
If the ratio is 1:3:6, magma will form mica instead of feldspars.
An alkaline rock has alkalis in excess of this proportion. This
enrichment in alkalis may be in respect of silica or alumina or both.
Therefore, as per this definition, a rock cannot be qualified as
“alkaline” because of say, mere dominance of alkali feldspars in it. Thus
a hbl granite is not alkaline but riebeckite granite is, because it contains
a sodic mineral deficient in Al2O3.
Aegirine, aegirine augite, spodumene, arfvedsonite etc.

5. Classification of
Igneous Rocks
Figure 2.3. A classification and nomenclature
of volcanic rocks. After IUGS.

6. Classification of Igneous Rocks
Figure 2.4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Maitre
(2002) . Igneous Rocks: A Classification and Glossary of Terms. Cambridge University Press.

7. The alkaline rocks can be grouped as
1.Rocks having adequate or excess silica but deficient in alumina.
1. These excess alkalis enter into alkaline mafic minerals.
2. Alkali feldspar+Na pyx/Na amph.
3. Silica oversaturated rocks carry quartz.
4. These rocks having molecular proportion (Na2O+K2O)/Al2O3 > 1 are
designated as peralkaline rocks.
5. The ratio is called peralkalinity index or agpaicity index and are called as
agpaitic rocks
6. Typical rocks are aegirine-riebeckite granites.
7. Eg. Peralkaline silicic volcanics represented by pantellerites (Na rich
rhyolite) and comendite (K rich rhyolite) form the ring complex in
Malani Volcanic Province of Barmer, Rajasthan.

8. The alkaline rocks can be grouped as
2.Rocks in which alumina is adequate (to saturate feldspar
composition) or in excess but silica is deficient.
1. The rocks are then composed of feldspars and feldspathoids
along with mica hornblende, corundum etc.
2. Typical rocks are nepheline syenites, leucitophyre,
leucite monzonites.
3. Syenites or nepheline syenites in which alumina is in excess after
saturating aluminosilicate minerals and (Na2O+K2O)/Al2O3 < 1,
are called miaskites. Biotite is a characteristic mafic mineral in
these rocks.

9. The alkaline rocks can be grouped as
3.Rocks deficient in both silica and alumina relative to
feldspar composition.
1. The rocks contain besides alkali feldspars, both silica
undersaturated minerals, feldspathoids and also alkali rich
mafic minerals.
2. Aegirine &/or riebeckite bearing nepheline syenite
is a typical example.
3. These rocks are also known as plumaskitic rocks.

10. Table 19.1. Nomenclature of some alkaline igneous rocks (mostly volcanic/hypabyssal)
Basanite feldspathoid-bearing basalt. Usually contains nepheline, but may have leucite + olivine
Tephrite olivine-free basanite
Leucitite a volcanic rock that contains leucite + clinopyroxene ± olivine. It typically lacks feldspar
Nephelinite a volcanic rock that contains nepheline + clinopyroxene ± olivine. It typically lacks feldspar.
Urtite plutonic nepheline-pyroxene (aegirine-augite) rock with over 70% nepheline and no feldspar
Ijolite plutonic nepheline-pyroxene rock with 30-70% nepheline
Melilitite a predominantly melilite - clinopyroxene volcanic (if > 10% olivine they are called olivine melilitites)
Shoshonite K-rich basalt with K-feldspar ± leucite
Phonolite felsic alkaline volcanic with alkali feldspar + nepheline. See Fig. 14-2. (plutonic = nepheline syenite)
Comendite peralkaline rhyolite with molar (Na2
O+K2
O)/Al2
O3
slightly > 1. May contain Na-pyroxene or amphibole
Pantellerite peralkaline rhyolite with molar (Na2
O+K2
O)/Al2
O3
= 1.6 - 1.8. Contains Na-pyroxene or amphibole
Lamproite a group of peralkaline, volatile-rich, ultrapotassic, volcanic to hypabyssal rocks. The mineralogy is variable, but most contain phenocrysts of
olivine + phlogopite ± leucite ± K-richterite ± clinopyroxene ± sanidine. Table 19-6
Lamprophyre a diverse group of dark, porphyritic, mafic to ultramafic hypabyssal (or occasionally volcanic), commonly highly potassic (K>Al) rocks.
They are normally rich in alkalis, volatiles, Sr, Ba and Ti, with biotite-phlogopite and/or amphibole phenocrysts. They typically occur as shallow
dikes, sills, plugs, or stocks. Table 19-7
Kimberlite a complex group of hybrid volatile-rich (dominantly CO2
), potassic, ultramafic rocks with a fine-grained matrix and macrocrysts of olivine
and several of the following: ilmenite, garnet, diopside, phlogopite, enstatite, chromite. Xenocrysts and xenoliths are also common
Group I kimberlite is typically CO2
-rich and less potassic than Group 2 kimberlite
Group II kimberlite (orangeite) is typically H2
O-rich and has a mica-rich matrix (also with calcite, diopside, apatite)
Carbonatite an igneous rock composed principally of carbonate (most commonly calcite, ankerite, and/or dolomite), and often with any of clinopyroxene
alkalic amphibole, biotite, apatite, and magnetite. The Ca-Mg-rich carbonatites are technically not alkaline, but are commonly associated with, and
thus included with, the alkaline rocks. Table 19-3
For more details, see Sørensen (1974), Streckeisen (1978), and Woolley et al. (1996)

11. Plutonic
1. Foid-bearing-alkali
feldspar syenite
2. Foid-bearing-syenite
3. Foid-bearing-monzonite
4. Foid-bearing-monzodiorite
5. Foid-bearing-gabbro
6. Foid-syenite
7. Foid-monzosyenite
8. Foid-monzodiorite
9. Foid-gabbro
10. Foidolite
Hypabyssal/volcanic
1. Foid-bearing-alkali
feldspar-trachyte
2. Foid-bearing-trachyte
3. Foid-bearing-latite
4. Hawaiite and mugearite
5. Alkali basalt
6. Phonolite
7. Tephritic phonolite
8. Phonolitic tephrite
9. Tephrite
10. Foidite

12. Commonest of the alkaline rocks are the foid-syenites and the associated foid-
monzo-syenites which are coarse to medium grained intrusive rocks with
hypidiomorphic granular texture that essentially consist of alkali feldspar, >10%
foids and one or more alkaline mafic mineral (aegirine augite, arfvedsonite,
riebeckite).
Laurdalite are nepheline syenites containing anorthoclase
Foyaite is a variety in which feldspathoids and potassium feldspars are present in
equal proportion.
Ditroites are sodalite-bearing nepheline syenites containing sanidine and albite.
Juvite is a variety of nepheline syenite in which feldspar is exclusively potassium
feldspar.
Mariupolites are albite-rich nepheline syenites.
Mafic and opaque minerals 0-30% --- Foid syenites, 30-60% --- Malignite and 60-
90% --- Shonkinite (HERE FOIDS ARE 10-60% AND PLAGIOCLASE IN TF IS 0-
10%)

13. Foid-monzosyenite is a plutonic equivalent of tephritic phonolites.
They contain more plagioclase than foid syenites and often have a higher
amount of mafic minerals.
They are rare rocks; and they can be separated into leuco-foid-
monzosyenites (M’=0-15, PF in TF=10-50, Foids=10-60%), foid-
monzosyenites (M’=15-45, PF in TF=10-50, Foids= 10-60%) and mela-
foid-monzosyenites (M’=45-90, PF in TF=10-50, Foids=10-60%).
Sommaite (leucite monzonite, plutonic) grades into foid-
monzosyenites.
Husebyites are foid-monzosyenites.
Miaskite is a transitional between foid-monzosyenite and foid-
monzo-diorite in which mafic mineral is biotite.

14. Phonolites are the extrusive equivalents of foid-syenites and can be
grouped as Na-rich nepheline phonolites and K-rich leucite phonolites.
Phonolites essentially contain sanidine or anorthoclase and mafic
minerals like sodic pyroxene, ferroaugite, titanaugite and Fe-rich
olivine.
Leucitophyre (leucite phonolite) contains large phenocrysts of leucite.
Phonolites typically show trachytic texture.
Tephritic phonolites are extrusive equivalents of foid monzosyenite.

15. Foid Monzodiorite contains PF in TF=50-90%,
Foids=10-60% and M’=20-60% with plagioclase An<50.
Where An>50, its called as Foid-Monzogabbro
(Streckeisen).
Essexite is a foid-monzodiorite with plagioclase
(An60-35) dominating over alkali feldspar.
Phonolitic tephrite is an extrusive equivalent of foid-
monzodiorite.

16. Foid gabbro are the rocks that contain Calcic plagioclase.
The most abundant of foid gabbro is the nepheline gabbro described
as Theralite.
Tephrites are volcanic equivalents of foid-gabbro.
They are essentially composed of calcic plagioclase, cpx and a foid,
minor amounts of alkali feldspars and thus they grade with increasing
feldspar contents into the phonolitic tephrites and tephritic phonolites.
Tephrites that contain olivine are known as basanites.
Basanites ususally contain nepheline but leucite, sodalite and analcite
may also be present.
Analcite basanites with disappearance of plagioclase grades into
analcitites.

17. Foidolites are the coarser grained, plutonic equivalents of the foidites with
feldspathoids >60%.
They are first divided into Na-rich and K-rich rocks and then they are subdivided
using their mafic mineral content (M’).
In Urtite, Na>K and M’ <30% (82-86% is nepheline, 12-16% Pyx)
Ijolite , Na>K and M’ 30-70% (49-55% is nepheline, 35-42% Pyx)
Melteigite, Na>K and M’ 70-90% (48% Pyx, 21% nepheline, 6% biotite, 5%
calcite).
Potassic foidolites are less abundant and are named as Italites, Fergusites and
Missourites respectively.
They contain >90% leucite, 65% Leucite-24% Cpx, and 50% Cpx-16% Leucite
respectively.
Jacupirangite is a foidolite with >90% mafic minerals and is ultrabasic.

18. Foidites are alkaline extrusive rocks that are essentially devoid
of feldspar and comprise nephelinites, melanephelinites, leucitites
and melilitites.
Nephelinites are constituted of nepheline and cpx
(titanaugite/diopside). With modal olivine >10%, the rock
becomes olivine nephelinites.
Rocks in which there are more mafic minerals than foids are
called melanephelinites.
The plutonic equivalents of the nephelinites are the urtites and
ijolites that contain <10% mafic minerals; whereas the plutonic
equivalents of melanephelinites are the melteigites and the
ijolites that contain >50% mafic minerals.

19. Leucitites are a group fine grained, porphyritic essentially
composed of leucite and cpx (titanaugite/diopside/aegirine
augite) and feldspar is generaly absent. They are K-rich
equivalents of nephelinites.
When significant amounts of olivine are present, the rock is
olivine leucitite.
The melilitites are a group of rare fine grained, porphyritic
rocks that are essentially composed of melilite and cpx along
with leucite and nepheline.
Melilitites that contain olivine are called as olivine
melilitites.

22. The most silica-undersaturated rocks found in alkaline provinces are
the carbonatites.
The term Karbonatite was introduced by Brogger in 1921 to denote
carbonate rocks from the Fen district of southern Norway, which he
believed were of igneous origin.
The idea of magmatic carbonates met with immediate opposition from
Bowen, who thought that the carbonates were of replacement origin.
Despite experimental evidence that such melts could exist at low
temperatures and pressures, a magmatic origin for carbonatites was not
universally accepted until carbonate lavas were witnessed erupting from
the Oldoinyo Lengai volcano in Tanzania.
CarbonatitesCarbonatites

23. Coarse Med.-Fine
Calcite-carbonatite sövite alvikite
Dolomite-carbonatite rauhaugite* beforsite
Ferrocarbonatite
Natrocarbonatite
* Rarely used, beforsite may be applied to any grain size.
Table 19-3. Carbonatite Nomenclature
Alternative
Name
• Carbonatites, by definition, contain >50 modal % carbonate minerals.
• Shows the terminology that is applied to the more common carbonatites.
• The first column gives the recommended modern names based on the most abundant
carbonate mineral, while the second column gives the names that were common in the
older literature.
• Sovite is still used for the more abundant coarse-grained calcite-carbonatites.
• The corresponding term for coarse dolomitic carbonatites, rauhaugite, is much less
commonly used.
• Although a few ferrocarbonates contain ankerite or siderite, they are typically fine-
grained mixtures of calcite and hematite (or hydrated iron oxides) Natrocarbonatite
(Na-K-Ca carbonatite) is very rare, and known for certain from only one volcanic
center.

24. • For carbonate-bearing rocks with 10 to 50%
carbonates, the IUGS recommends the use of the
modifying terms "calcitic" or "dolomite" before
the igneous rock name based on the remaining
silicate assemblage (for example, "calcite
ijolite").
• "Silico-carbonatite" is a term that appears in the
literature for rocks with 10 to 50% carbonate.
• It is not among the terms recommended by the
IUGS.

25. In East African rift, they
clearly occur in volcanic
cinder cones and flows and as
shallow intrusive bodies.
They are high in Na and Ca
and contain Na-rich pyx and Na-
rich amphibole.
Pyrochlore (Nb And tantalum-
rich oxide mineral)
Carbonatites Carbonates Sulfides
Calcite Pyrrhotite
Dolomite Pyrite
Ankerite Galena
Siderite Sphalerite
Strontanite Oxides-Hydroxides
Bastnäsite (Ce,La)FCO3) Magnetite
* Nyerereite ((Na,K)2Ca(CO3)2) Pyrochlore
* Gregoryite ((Na,K)2CO3) Perovskite
Silicates Hematite
Pyroxene Ilmenite
Aegirine-augite Rutile
Diopside Baddeleyite
Augite Pyrolusite
Olivine Halides
Monticellite Fluorite
Alkali amphibole Phosphates
Allanite Apatite
Andradite Monazite
Phlogopite
Zircon
Source: Heinrich (1966), Hogarth (1989) * only in natrocarbonatite
Table 19-4. Some Minerals in Carbonatites.

26. African carbonatite occurrences and approximate
ages in Ma. OL = Oldoinyo Lengai natrocarbonatite
volcano. After Woolley (1989) The spatial and
temporal distribution of carbonatites.
• Of the approximately 350 known
carbonatites, over half occur in
Africa.
• Most carbonatites occur in stable
continental intraplate settings.
• Only two carbonatites are known
from ocean basins (also intraplate,
but oceanic): one is in the Cape
Verde Islands, and the other in the
Canary Islands.
• The proximity of these islands to the
carbonatite-rich African continent
suggests that these occurrences
might be related to African
continental (probably sub-crustal)
processes.

27. Carbonatites can occur as volcanics or intrusive bodies, they
commonly occur within or satellitic to alkaline intrusive centers.
Carbonatite complexes are generally <25km2
, and are
composite, with multiple intrusions of both silicate and
carbonatite magma.
Exposed intrusive carbonatites include small plugs, cone
sheets, and occasional ring-dikes along with planar dikes or dike
swarms.
The wall rocks may have a fractured appearance suggesting a
high volatile content of the carbonatite melts.
Carbonatites-Field characteristics

28. In a typical sequence, shallow early
ijolite and/or nepheline syenite plugs
are followed by carbonatites that cut
the earlier silicate complex.
Sovites (typically with over 90%
calcite) are the most common type of
carbonatite in these complexes, and
may represent the only carbonatite at a
locality.
Other common carbonatites contain
both calcite and dolomite; less
common are those in which dolomite
or ankerite are predominant.
Idealized cross section of a carbonatite-alkaline silicate complex with early ijolite cut by more evolved urtite. Carbonatite (most commonly calcitic)
intrudes the silicate plutons, and is itself cut by later dikes or cone sheets of carbonatite and ferrocarbonatite. The last events in many complexes are late
pods of Fe and REE-rich carbonatites. A fenite aureole surrounds the carbonatite phases and perhaps also the alkaline silicate magmas.

29. The later manifestations of
igneous activity in many complexes
is the emplacement of dikes or cone
sheets of iron-rich carbonatites,
collectively called
ferrocarbonatite.
The most common of these
contain fine-grained calcite and
hematite, but some are ankeritic, and
only a few contain siderite.
Finally, pipe-like bodies of Fe and
REE-rich carbonatite (some are
distinctly radioactive) may be
emplaced.

30. The last episodes are typically brecciated, and
exhibit replacement textures and fluorine
addition.
They appear to involve late-stage fluids that
may be hydrothermal in nature. All of the stages
are rarely developed in a single locality.
Estimated temperatures of emplacement,
calculated from mineral geothermometry, range
from 550°C to over 1000°C.
Carbonatites-Field characteristics

31. An almost universal characteristic of carbonatite complexes is the presence of a
distinctive metasomatic aureole in which the wall rocks (most commonly
quartzo-feldspathic gneiss) has been converted to aegirine-rich and alkali
amphibole-rich rocks, and in some cases to K-feldspar-rich rocks.
The metasomatic rocks are commonly called fenites, and the process
fenitization, after the Fen alkaline complex in S. Norway.
Fenitization begins along a network of fractures, and typically involves
addition of alkalis and progressive desilification in which the original quartz and
feldspars of the country rock are replaced by alkaline pyroxene and amphibole.
The prevalence of such metasomatized rocks around carbonatite complexes
indicates that large volumes of alkali-bearing solutions are given off during
cooling.
Carbonatites-Field characteristics

32. Calcite- Dolomite- Ferro- Natro-
% carbonatite carbonatite carbonatite carbonatite
SiO2 2.72 3.63 4.7 0.16
TIO2 0.15 0.33 0.42 0.02
Al2O3 1.06 0.99 1.46 0.01
Fe2O3 2.25 2.41 7.44 0.05
FeO 1.01 3.93 5.28 0.23
MnO 0.52 0.96 1.65 0.38
MgO 1.80 15.06 6.05 0.38
CaO 49.1 30.1 32.8 14.0
Na2O 0.29 0.29 0.39 32.2
K2O 0.26 0.28 0.39 8.38
P2O5 2.10 1.90 1.97 0.85
H2O+ 0.76 1.20 1.25 0.56
CO2 36.6 36.8 30.7 31.6
BaO 0.34 0.64 3.25 1.66
SrO 0.86 0.69 0.88 1.42
F 0.29 0.31 0.45 2.50
Cl 0.08 0.07 0.02 3.40
S 0.41 0.35 0.96
SO3 0.88 1.08 4.14 3.72
Table 19-5. Representative Carbonatite Compositions Calcite- Dolomite- Ferro- Natro-
% carbonatite carbonatite carbonatite carbonatite
ppm
Li 0.1 - 10 -
Be 2 < 5 12 -
Sc 7 14 10 -
V 80 89 191 116
Cr 13 55 62 0
Co 11 17 26 -
Ni 18 33 26 0
Cu 24 27 16 -
Zn 188 251 606 88
Ga < 5 5 12 <20
Rb 14 31 - 178
Y 119 61 204 7
Zr 189 165 127 0
Nb 1204 569* 1292 28
Mo - 12 71 125
Ag - 3 4 -
Cs 20 1 1 6
Hf - 3 - 0
Ta 5 21 1 0
W - 10 20 49
Au - - 12 -
Pb 56 89 217 -
Th 52 93 276 4
U 9 13 7 11
La 608 764 2666 545
Ce 1687 2183 5125 645
Pr 219 560 550 -
Nd 883 634 1618 102
Sm 130 45 128 8
Eu 39 12 34 2
Gd 105 - 130 -
Tb 9 5 16 -
Dy 34 - 52 2
Ho 6 - 6 -
Er 4 - 17 -
Tm 1 - 2 -
Yb 5 10 16 -
Lu 1 0 - 0
Wooley & Kempe (1989), natrocarb. from Keller & Spettel (1995).
* one excluded analysis contained 16,780 ppm Nb.
Table 19-5. Representative Carbonatite Compositions

33. Ancient intrusive carbonatites are composed predominantly of
calcite, whereas modern volcanic ones contain abundant sodium
carbonate.
Rainwater rapidly dissolves sodium carbonate, so it is not
surprising that natrocarbonatite lavas are unlikely to be preserved in
the geological record.
One explanation is that carbonatite magmas do indeed
have high concentrations of sodium carbonate, but during
solidification and cooling, hydrothermal solutions remove
the sodium from the carbonatite (leaving it composed
essentially of calcite) and transporting the sodium into the
country rocks to form fenites.
Carbonatites

34. Some carbonatites are of economic value as ores of niobium and rare earths.
Initially carbonatites were mined only for iron and limestone, which is used for
cement and as a flux in smelting iron.
Some carbonatites contain zones of high concentration of pyrochlore (Nb and
Tantalun rich mineral). REE and Th can also be present in economic
concentrations, occurring mainly in perovskite (CaTiO3).
Carbonatites, as a group, have exceptionally high concentrations of Ti, Nb,
Zr, REE, P, F, Ba, Sr and Th.
These elements are abundant in alkaline magmas in any case, but it appears that
during the process of generating carbonatites they are further concentrated,
perhaps through strong liquid-liquid partitioning into an immiscible carbonate
liquid.
Once fractional crystallization of this liquid takes place, Sr and Nb are depleted
by entering early-crystallizing carbonate and pyrochlore, respectively, and the
residual liquid becomes enriched in other elements.
Carbonatites

35. Figure 19.16. Schematic cross
section of an asthenospheric
mantle plume beneath a
continental rift environment, and
the genesis of nephelinite-
carbonatites and kimberlite-
carbonatites. Numbers
correspond to Figure 19-13. After
Wyllie (1989, Origin of
carbonatites: Evidence from
phase equilibrium studies. In K.
Bell (ed.), Carbonatites: Genesis
and Evolution. Unwin Hyman,
London. pp. 500-545) and Wyllie
et al., (1990, Lithos, 26, 3-19).
Winter (2001) An Introduction to
Igneous and Metamorphic
Petrology. Prentice Hall.
CarbonatitesCarbonatites

36. They are described from several mafic alkaline and
nepheline syenite complexes of different ages, like
Mer Mundwara ring intrusion of Sirohi - Rajasthan,
Amba Dongar-Siriwasan alkaline complex of Gujarat
of possible post-Deccan trap age and Elchuru, Borra,
Kunavaram alkaline complexes, Andhra Pradesh and
Niwania pluton, Rajasthan of Precambrian age.
Carbonatites-Indian occurrences

37. Carbonatites-Indian occurrences

38. Niggli (1923) introduced the term lamproite to describe a group of
lamprophyre like subvolcanic, and extrusive igneous rocks, that are enriched
in both K and Mg.
Lamproites, group II kimberlites, and minette lamprophyres are about the
only rock types that are both ultrapotassic (molar K/Na>3) and perpotassic
(molar K/Al>1.0). Lamproites are also peralkaline ([K + Na]/Al commonly >
1.0), and have high Mg# (usually>70) as well as high concentrations of the
compatible trace elements Ni and Cr.
At the same time they are highly enriched in incompatible elements like K,
Ti, Rb, Zr, Sr, Ba, and F.
They are depleted in Ca, Na, and Al, which indicate that the mantle source
was depleted in these elements by earlier episodes of partial melting.
The high concentration of K2O is usually reflected in an abundance of
minerals such as leucite, phlogopite and K-richterite.
LamproitesLamproites

39. The mineralogy of lamproites reflects their peralkaline-perpotassic
nature.
Lamproites are characterized by widely varying amounts (0 to 90%) of
the following primary phases: phenocryst and groundmass Ti-rich
phlogopite, Ti- and K-rich richteritic amphibole, olivine, diopside, leucite,
and sanidine.
The hydrous nature of many phases indicates high H2O content.
Lamproites notably lack primary plagioclase, melilite, monticellite,
kalsilite, nepheline, and sodalite.
Diamond-bearing olivine lamproites have recently been discovered in
NW Australia; Prairie Creek, Arkansas; and Majhgawan, India.
LamproitesLamproites

40.  The old parochial type-locality lamproite terminology has been
replaced by Scott-Smith and Skinner (1984a, b) and Mitchell (1985)
with a more descriptive classification reflecting the rock's
constituents.
 Only the term "madupite" has been retained as a modifier
("madupidic") to signify the presence of poikilitic groundmass
phlogopite.
Old Nomenclature
wyomingite diopside-leucite-phlogopite lamproite
orendite diopside-sanidine-phlogopite lamproite
madupite diopside madupidic lamproite
cedricite diopside-leucite lamproite
mamilite leucite-richterite lamproite
wolgidite diopside-leucite-richterite madupidic lamproite
fitzroyite leucite-phlogopite lamproite
verite hyalo-olivine-diopside-phlogopite lamproite
jumillite olivine diopside-richterite madupidic lamproite
fortunite hyalo-enstatite-phlogopite lamproite
cancalite enstatite-sanidine-phlogopite lamproite
From Mitchell and Bergman (1991).
Table 19-6. Lamproite Nomenclature
Recommended by IUGS

41. Although compositionally diverse, lamproites are rare, having been
described from only 30-40 localities.
They are predominantly extrusive (both flows and pyroclastics).
Occasional intrusive forms are generally hypabyssal (shallow) dikes,
sills, and vent pipes.
Lamproites are produced in a short magmatic episode (<3 to 10 Ma),
and show few effects of differentiation.
They occur strictly in continental-intraplate areas with thick crust (>40
to 55 km) and thick lithosphere (>150 to 200 km).
Lamproites do not occur within ancient cratons, but concentrate at
cratonal margins in areas that have experienced one or more stages of
compressive orogeny, aborted rifting, and/or post-collisional collapse.
LamproitesLamproites

42. That virtually all lamproites occur in areas that overlie
extinct subduction zones must have genetic significance, and
the hydrous, incompatible element-enriched fluids released
above these subduction zones are likely to play an important
role in developing the unique chemical composition and
mineralogy of these rocks.
The lamproites tend to occur in association with kimberlites; and they are often
interpreted as having evolved from kimberlitic parental magmas that differentiated,
and were possibly contaminated by crustal materials, on their passage to the surface.
According to Scott (1979), the lamproites of central West Greenland evolved
during the relatively slow upward movement of batches of kimberlitic magma.
However, this theory has been modified by later workers.
LamproitesLamproites

43. Mitchell and Bergman (1991) suggested the following model for the generation of
lamproites:
1.A depleted harzburgite is created, either by partial melting within a rising
asthenospheric plume or by long term depletion of the sub-continental lithospheric
mantle (SCLM).
2.Later enrichment adds incompatible elements to the harzburgite. This may occur in
the form of subduction zone fluids rising from the dehydrating slab into the over lying
SCLM, or via melt infiltration, underplating, stalled and crystallizing hydrous melts in
rift zones, escaping juvenile fluids, or a combination of these factors. Enriched
aqueous fluids will produce phlogopite, and perhaps K-richterite, which act as
incompatible element repositories. Other than the introduction of K, the enrichment
affects trace elements far more than major elements. Enrichment processes may occur
in several stages, and affects only portions of the lithosphere, resulting in a
heterogeneous sub-continental mantle with variably fertile pockets.

44. 3. The enriched heterogeneous SCLM source is partially
melted. This may be triggered by a new plume that supplies
thermal energy and/or a sudden volatile influx, or it may
result from collapse of an orogen and decompression melting
of the rising asthenospheric blob. Given the complex and
speculative nature of the source, it is impossible to constrain
the degree of partial melting from the geochemistry of the
lamproites. 1-10% partial melting probably occurs under
H2O- and F-rich conditions at a single eutectic point,
resulting in a primitive phlogopite-lamproite magma with 52-
55wt. % SiO2 and a limited compositional range.

47. The term lamprophyre was introduced by Gumbel (1874) to describe a
group of dark colored dyke rocks from Germany.
It is derived from the classical Greek word Lampros meaning “bright”
or “glistening” as the rocks of the type area contained prominent flakes of
lustrous biotite.
Lamprophyres are defined by the IUGS Subcommission (LeMaitre,
1989) as follows:
“a distinctive group of rocks which are strongly porphyritic in mafica distinctive group of rocks which are strongly porphyritic in mafic
minerals, typically biotite, amphiboles and pyroxenes, with anyminerals, typically biotite, amphiboles and pyroxenes, with any
feldspar being confined to the groundmass. They commonly occur asfeldspar being confined to the groundmass. They commonly occur as
dykes or small intrusions and often show signs of hydrothermaldykes or small intrusions and often show signs of hydrothermal
alteration.”alteration.”
LamprophyresLamprophyres

48. Traditionally they are distinguished based on following characteristics
1.They normally occur as dykes and are not simply textural varieties of
common plutonic or volcanic rocks.
2.They are porphyritic, with M’ (modal % mafics) typically 35-90, but rarely
>90.
3.Feldspars &/or feldspathoids, when present, are restricted to the
groundmass.
4.They usually contain essential biotite/phlogopite &/or amphibole and
sometimes clinopyroxene, olivine.
5.Hydrothermal alteration of olivine, pyroxene, biotite and plagioclase (when
present) is common.
6.Calcite, zeolite and other hydrothermal minerals may appear as primary
phases.
7.They tend to have contents of K2O, Na2O, H2O, CO2, S, P2O5 and Ba that are

49. biotite, hornblende, Na- Ti- amphib., melilite, biotite,
feldspar foid diopsidic augite, diopsidic augite, Ti-augite, ± Ti-augite
(± olivine) (± olivine) olivine, biotite ± olivine ± calcite
or > pl -- minette vogesite
pl > or -- kersantite spessartite
or > pl feld > foid sannaite
pl > or feld > foid camptonite
-- glass or foid monchiquite polzenite
-- -- alnöite
Lamprophyre branch: Alkaline Melilitic
After Le Maitre (1989), Table B.3, p. 11.
Calc-alkaline
constituents
Table 19-7. Lamprophyre Nomenclature
Light-colored Predominant mafic minerals
 Earlier, lamprophyres were classified into 3 types by Streckeisen
(1980)
1. Calc-alkaline lamprophyres (lamprophyres sensu stricto) (SiO2=50-
54%)
2. Alkaline lamprophyres (SiO2=38-44%)
3. Melilitic lamprophyres (SiO2=<30%)
• Rock (1987,1991) preferred to call melilitic lamprophyres “ultramafic
lamprophyres instead, arguing that melilite-free and melilite-rich varieties
commonly co-exist.

50. biotite, hornblende, Na- Ti- amphib., melilite, biotite,
feldspar foid diopsidic augite, diopsidic augite, Ti-augite, ± Ti-augite
(± olivine) (± olivine) olivine, biotite ± olivine ± calcite
or > pl -- minette vogesite
pl > or -- kersantite spessartite
or > pl feld > foid sannaite
pl > or feld > foid camptonite
-- glass or foid monchiquite polzenite
-- -- alnöite
Lamprophyre branch: Alkaline Melilitic
After Le Maitre (1989), Table B.3, p. 11.
Calc-alkaline
constituents
Table 19-7. Lamprophyre Nomenclature
Light-colored Predominant mafic minerals
• The latest attempts by the IUGS Subcommission to classify the melilitic rocks
considered them as varieties of a separate melilitic rock group, and not
lamprophyres at all.
• Calc-alkaline lamprophyres consist of minette, vogesite, kersantite and
spessartites. They occur in subduction zone environments, generally in
association with calc-alkaline granitoid suites or with the more alkaline
shoshonites.

51. biotite, hornblende, Na- Ti- amphib., melilite, biotite,
feldspar foid diopsidic augite, diopsidic augite, Ti-augite, ± Ti-augite
(± olivine) (± olivine) olivine, biotite ± olivine ± calcite
or > pl -- minette vogesite
pl > or -- kersantite spessartite
or > pl feld > foid sannaite
pl > or feld > foid camptonite
-- glass or foid monchiquite polzenite
-- -- alnöite
Lamprophyre branch: Alkaline Melilitic
After Le Maitre (1989), Table B.3, p. 11.
Calc-alkaline
constituents
Table 19-7. Lamprophyre Nomenclature
Light-colored Predominant mafic minerals
• The four common lamprophyres are most readily
classified using modal data; and the minerals that are
characteristically present in these rocks include orthoclase,
oligoclase/andesine, biotite-phlogopite, diopsidic-augite,
hornblende/kaersutite, olivine, apatite, Fe-Ti oxides and
calcite.

52. biotite, hornblende, Na- Ti- amphib., melilite, biotite,
feldspar foid diopsidic augite, diopsidic augite, Ti-augite, ± Ti-augite
(± olivine) (± olivine) olivine, biotite ± olivine ± calcite
or > pl -- minette vogesite
pl > or -- kersantite spessartite
or > pl feld > foid sannaite
pl > or feld > foid camptonite
-- glass or foid monchiquite polzenite
-- -- alnöite
Lamprophyre branch: Alkaline Melilitic
After Le Maitre (1989), Table B.3, p. 11.
Calc-alkaline
constituents
Table 19-7. Lamprophyre Nomenclature
Light-colored Predominant mafic minerals
• The alkaline and the melilitic lamprophyres contain alkaline rocks and are usually
associated with alkaline complexes and the rocks of the carbonatite-nepheline-
ijolite association.
• According to Streckeisen, the common alkaline lamprophyres are camptonites,
sannaites and monchiquites; and they are chemically akin to the alkali basalts,
basanites and nephelinites.
• They typically occur in intraplate and rift environments.

53. biotite, hornblende, Na- Ti- amphib., melilite, biotite,
feldspar foid diopsidic augite, diopsidic augite, Ti-augite, ± Ti-augite
(± olivine) (± olivine) olivine, biotite ± olivine ± calcite
or > pl -- minette vogesite
pl > or -- kersantite spessartite
or > pl feld > foid sannaite
pl > or feld > foid camptonite
-- glass or foid monchiquite polzenite
-- -- alnöite
Lamprophyre branch: Alkaline Melilitic
After Le Maitre (1989), Table B.3, p. 11.
Calc-alkaline
constituents
Table 19-7. Lamprophyre Nomenclature
Light-colored Predominant mafic minerals
• Camptonite is the most common alkaline lamprophyre. It contains phenocrysts of
kaersutitic (high TiO2) or barkevitic (high Fe) amphibole, titanaugite, and possibly
olivine, and biotite in a fine grained groundmass of amphibole, titanaugite,
plagioclase and Fe-Ti oxides with minor amounts of feldspathoid and apatite.
• Chemically they are equivalent to alkali basalts.
• Sannaites are similar to camptonites, but plagioclase is subordinate to K-feldspar
in the groundmass.

54. biotite, hornblende, Na- Ti- amphib., melilite, biotite,
feldspar foid diopsidic augite, diopsidic augite, Ti-augite, ± Ti-augite
(± olivine) (± olivine) olivine, biotite ± olivine ± calcite
or > pl -- minette vogesite
pl > or -- kersantite spessartite
or > pl feld > foid sannaite
pl > or feld > foid camptonite
-- glass or foid monchiquite polzenite
-- -- alnöite
Lamprophyre branch: Alkaline Melilitic
After Le Maitre (1989), Table B.3, p. 11.
Calc-alkaline
constituents
Table 19-7. Lamprophyre Nomenclature
Light-colored Predominant mafic minerals
• At still lower silica, melilite becomes an important mineral taking place of
pyroxene. This gives rise to Alnoite which is composed essentially of phlogopite,
melilite, olivine, titanaugite, monticellite, Fe-Ti oxides and calcite with accessory
nepheline, apatite and perovskite.
• Alnoites are remarkable in that they are rich in both Mg and K, a characteristic
shared with kimberlites.

55. By increase in olivine and decrease in augite,
some alnoites grade into kimberlites (mica-
peridotites).
Others, by increase in the content of
carbonates, pass naturally into carbonatites that
still carry olivine, melilite, apatite as accessories.
Polzenite is a melilitic lamprophyre that
usually contains between 10-30% feldspathoids
and it normally contains the same minerals as
occur in alnoite.
LamprophyresLamprophyres

56. The high volatile content (particularly H2O) and the resulting abundant
mica-amphibole phenocrysts, are the predominant uniting characteristic of the
group.
The implication is that lamprophyres develop as a consequence of volatile
retention via crystallization at high pressure, or by prolonged normal
differentiation processes (Mitchell, 1994a).
If so, many lamprophyres may be nothing more than the hydrous
crystallization products of common magma types that occur under unusually
H2O-rich conditions.
They are still worth considering, however, because they record part of the
long-term evolution of some magma chambers, and the mechanisms by which
such hydrous variants are accomplished are both interesting and probably
diverse (depending on the initial magma type and the conditions under which
it crystallizes).
LamprophyresLamprophyres

57. In india lamprophyres are mostly studied from the Gondwana basins
and have been reported as “mica traps” from Indian coal fields.
From Raniganj coal field, minette and kersantite are the most
frequently reported types.
Leucitites, microsyenites and quartz-carbonate rocks occur in
association.
Monchiquite is reported from Mount Girnar.
Calc-alkali lamprophyres are reported from Mundwara Complex of
Rajasthan.
Amba Dongar carbonatite complex of Gujarat is intruded by
camptonite and kersantite.
Lamprophyres-Indian occurrencesLamprophyres-Indian occurrences

58. Lamprophyres-Indian occurrencesLamprophyres-Indian occurrences

59. Lamprophyres-Indian occurrencesLamprophyres-Indian occurrences

60. Lamprophyres-Indian occurrencesLamprophyres-Indian occurrences

61. Kimberlites are fascinating for a number of reasons. They are K-rich,
typically ultramafic hybrid rocks that occur in ancient cratons.
They are volatile rich, tend to rise from mantle depths rapidly and are
emplaced violently, contain diamonds and coesite, which indicate a fairly
deep mantle origin.
They also contain xenoliths from deep mantle.
The term Kimberlite was introduced by Lewis (1887) to describe the
diamond bearing, porphyritic mica-peridotites of the Kimberly area of S.
Africa.
The petrography of kimberlites is both unusual and complex because
1. They are hybrid rocks that contain minerals, rock fragments that formed in diverse physical
and chemical environments
2. They vary greatly in modal composition. Olivine, serpentine group minerals, phlogopite,
pyropic garnet, diopside, bronzite, calcite, monticellite (Ca-Mg Olivine). Chromite, perovskite.
KimberlitesKimberlites

62. Mitchell (1970) defined kimberlite as
“A porphyritic, alkali peridotite, containing rounded and corroded phenocrysts of
olivine (serpentinized, carbonatized or fresh) phlogopite (fresh or chloritized)
magnesian ilmenite, pyrope and chrome-rich pyrope set in a fine-grained groundmass
composed of second generation olivine and phlogopite together with calcite &/or
dolomite, serpentine &/or chlorite, magnetite, perovskite and apatite. Diamond and
garnet peridotite xenoliths may or may not occur.”
More recently Clemente et al. (1984), gave kimberlite (sensu stricto)
definition as
“A volatile rich, potassic, ultrabasic igneous rock which occurs as small volcanic
pipes, dykes and sills. It has a distinctively inequigranular texture resulting from the
presence of macrocrysts set in a finer-grained matrix. This matrix contains, as prominent
primary phenocrystal &/or groundmass constituents, olivine and several of the following
minerals : phlogopite, carbonate (calcite, serpentine, cpx, diopside), monticellite, apatite,
spinels, perovskite and ilmenite.”
KimberlitesKimberlites

63. Olivine is usually the most abundant mineral, but it may be partly, or
completely replaced by secondary minerals (serpentine) and some have
3 generaions of olivine
1. Large rounded olivine megacrysts with Fo84-86
2. Medium-sized megacrysts with Fo90
3. Small groundmass olivines with intermediate types.
The abundance of phlogopite and carbonate minerals is also highly
variable, kimberlites can be divided into 3 groups
1. Kimberlites (sensu stricto)
2. Micaceous kimberlites; and
3. Calcareous kimberlites; depending on the proportions of olivine, phlogopite
and carbonate minerals that are present.
KimberlitesKimberlites

64. Kimberlites are currently divided into 2 groups.
Group 1 kimberlites are the archetypal ultramafic kimberlites, first
described from Kimberly, S. Africa, but known to occur on all continents.
Group 2 kimberlites are micaceous kimberlites (orangeites), the occurrence
of which is presently limited to S. Africa, where they are older (100-200 Ma)
than the group 1 kimberlites in the same area (100 Ma).
All known (200+) orangeite bodies lie in the Archean Kaapvaal craton in South Africa,
where they occur as early Cretaceous (125–110 Ma) and early Jurassic (165–145 Ma)
swarms of dikes and diatremes. Worldwide, there are >5000 known bodies of archetypal
kimberlites having an estimated volume >5000 km3
; most of these are in Precambrian
cratons. They typically occur in clusters <40 km across of a few dozen intrusions.
In southern Africa, kimberlite emplacement has occurred episodically several times
since 1600 Ma.
KimberlitesKimberlites

65. They are volatile rich (CO2) potassic ultramafic rocks.
In addition to their xenolith content, they commonly exhibit a distinctively
inequigranular texture caused by the presence of rounded, anhedral and
fragmented macrocrysts (0.5-10 mm diameter crystals) and, in some cases,
megacrysts (1-20 cm diameter crystals) set in a fine-grained matrix.
Some of these are xenocrysts.
Olivine is generally predominant, but may be accompanied by ilmenite, pyrope,
diopside, phlogopite, enstatite and chromite.
The matrix typically contains a second generation of fine euhedral to subhedral
olivine plus monticellite, phlogopite, perovskite, spinel and apatite.
Many contain a late, poikilitic Ba-rich phlogopite.
Many kimberlites exhibit 1-10 mm sized rounded globular to amoeboid-shaped
segregation masses of calcite+serpentine.
Group 1 KimberlitesGroup 1 Kimberlites

66. They are texturally similar to group 1 but are ultrapotassic, peralkaline,
and H2O-rich.
Phlogopite is the dominant macrocryst and groundmass phase.
Olivine is also common although essentially a xenocryst.
Other characteristic primary phase include diopside, spinel, perovskite,
apatite, REE-rich phosphates, rutile and ilmenite.
Mineralogically they are similar to lamproites.
They characteristically have higher K2O/TiO2 than group 1 kimberlites.
Diamonds occur principally in kimberlites, orangeites and some
lamproites.
A rich economic deposit may have a diamond concentration of 1 to 1.4
Group 2 Kimberlites or OrangeitesGroup 2 Kimberlites or Orangeites

67. KimberlitesKimberlites
Lamproite*
SiO2 33.0 27.8-37.5 35.0 27.6-41.9 45.5
TiO2 1.3 0.4-2.8 1.1 0.4-2.5 2.3
Al2O3 2.0 1.0-5.1 2.9 0.9-6.0 8.9
FeO* 7.6 5.9-12.2 7.1 4.6-9.3 6.0
MnO 0.14 0.1-0.17 0.19 0.1-0.6
MgO 34.0 17.0-38.6 27. 10.4-39.8 11.2
CaO 6.7 2.1-21.3 7.5 2.9-24.5 11.8
Na2O 0.12 0.03-0.48 0.17 0.01-0.7 0.8
K2O 0.8 0.4-2.1 3.0 0.5-6.7 7.8
P2O5 1.3 0.5-1.9 1.0 0.1-3.3 2.1
LOI 10.9 7.4-13.9 11.7 5.2-21.5 3.5
Sc 14 20 19
V 100 95 66
Cr 893 1722 430
Ni 965 1227 152
Co 65 77 41
Cu 93 28
Zn 69 65
Ba 885 3164 9831
Sr 847 1263 3860
Zr 263 268 1302
Hf 5 7 42
Nb 171 120 99
Ta 12 9 6
Th 20 28 37
U 4 5 9
La 150 186 297
Yb 1 1 1
Data from Mitchell (1995), Mitchell and Bergman (1991)
* Leucite Hills madupidic lamproite
Table 19-8. Average Analyses and Compositional Ranges
of Kimberlites, Orangeites, and Lamproites.
Kimberlite Orangeite

68. Kimberlitic magmas have to move rapidly through the lithosphere in order to
1. To transport the relatively high-density xenoliths of mantle origin that they
usually contain
2. To prevent the resorption, or inversion, of the diamonds they contain.
Experimental studies of the primary phases that they contain and also
xenoliths demonstrate that kimberlitic magmas have equilibrated at mantle
materials at depths of atleast 200km.
It is thus postulated that the magma has to travel rapidly through the
lithosphere at 25-70km/hr through a deep-seated fracture.
If this occurs, a low viscosity fluid phase may separate from the magma and
wedge open the fracture and first batch of melt moving at speeds in excess of
25km/hr.
Later a vent system develops, and the later batches move at 70km/hr.
EmplacementEmplacement

69. Kimberlites fieldKimberlites field
relationshipsrelationships
Kimberlites and orangeites can occur as
hypabyssal dikes or sills, diatremes, crater-fill, or
pyroclastics, depending largely on the depth of
erosion and exposure.
The dikes are generally 1 to 3 m thick and
commonly occur in swarms where they tend to
bifurcate into anastomosing stringers.
Most dikes tend to pinch out toward the surface
and thicken with depth.
Sills are less common, and may be up to several
hundred meters thick. Some dikes expand locally
near the top into lenticular enlargements called
"blows" which may be up to 10 to 20 times the dike
width and 100 m long. Blows may feed into the
root zones of diatremes.

70. Diatremes are 1 to 2 km deep carrot-
shaped bodies with circular-to-elliptical
cross sections, vertical axes, and steeply
dipping sides (80 to 85°).
They taper downward and terminate in
the "root zone," an irregularly-shaped
multiphase intrusion zone, transitional into
the hypabyssal kimberlites-orangeites.
The nature of the volcanic processes that
produce diatremes is still the subject of
much debate.
The diatreme represents the expansion of
the volatiles in the magma as it approaches
the surface and the confining pressure is
lowered.

71. In the model of Clement (1979)
multiple batches of magma exsolve
CO2 because of pressure reduction,
shattering the wall rocks to form sub-
surface breccias.
An upwardly progressing sequence
of stalled "buds" form in this fashion
until they reach approximately 300-
400m depth when hydrovolcanic
interaction with groundwater
produces gas violently enough that it
breaks through to the surface.

72. At this point either rapid
degassing and vapor exsolution in
response to progressive pressure
release resulting from unroofing, or
increase groundwater flow into the
crater and pipe, result in a
downward migrating zone of violent
brecciation and mixing to form the
diatreme (Mitchell, 1986).
Diatreme facies kimberlites-
orangeites, at least near the surface,
are more fragmented than their
hypabyssal equivalents, and take on
a volcaniclastic appearance.

73. Breccias containing abundant
country rock inclusions and
subordinate earlier hypabyssal
kimberlite-orangeite solid
fragments, from a few centimeters
to microscopic size, are the most
common rock type.
Megacrysts and macrocrysts are
also common.
The fragmental nature grades
downward into non-brecciated
kimberlite.

74. The rocks of kimberlite kindred are a paradox, as they generally have
major element compositions similar to picrites and yet are also enriched
in the incompatible elements.
Wagner (1914) and others have proposed that kimberlitic magmas are
generated in a source region that is relatively deep within the mantle and
the magmas are the products of low degree of partial melting.
This is known as incipient melting hypothesis.
This hypothesis would only work if the source materials had a special
composition, i.e. they were phlogopite bearing garnet lherzolites.
But one cannot explain that how the incompatible elements were
highly concentrated in source rocks.
Petrogenesis of KimberlitesPetrogenesis of Kimberlites

75. Another hypothesis is known as residual liquid hypothesis (1920, 1966,
1967).
It is based on the concept that there is possibly a genetic connection between
the extrusion of floods of tholeiitic basalt and the later emplacement of
kimberlites.
According to Verschure (1966), at the termination of a period of active
mantle convection and the extrusion of flood basalts, pockets of tholeiitic
basalt (picrites) magma remained in the upper mantle.
This magma cools under high pressure conditions and pyrope and omphacite
precipitate.
Fractional crystallization results in settling of garnets and this yields a residual
liquid enriched in alkalis.
Under favorable conditions, a kimberlitic residual magma is eventually
“explosively ejected” from the deep-seated source region.
Petrogenesis of KimberlitesPetrogenesis of Kimberlites

76. In order to explain why kimberlitic rocks contain high incompatible
element abundances and also why they normally contain megacrysts and
xenoliths that equilibrated at high pressures, Harris and Middlemost (1970)
proposed that kimberlitic magmas are generated in a 2-stage process.
In the first stage a tenuous magma, enriched in volatile components (H2O and
CO2) and possibly generated by volatiles degassing from the deep mantle, rises
by means of zone melting from a depth of approx. 600km.
At higher levels in the upper mantle (260km), the relatively hot, incompatible
element enriched tenuous magma induces partial melting to occur in the garnet
lherzolite mantle rock.
The new magma is picritic in major element composition, but significantly
enriched in the incompatible elements.
Petrogenesis of KimberlitesPetrogenesis of Kimberlites

77. Under ideal conditions, such a kimberlitic magma rises rapidly
(40km/hr) towards the surface from a depth of atleast 200km.
At 200km, the kimberlite material is essentially a
magma, but as it rises to higher levels it becomes a
mechanical mixture of liquid magma, phenocrysts,
xenocrysts, xenoliths, together with a large volume of a
separate low viscosity-fluid phase.
As this quasi-magma is propelled upwards through a variety of
physical and chemical environments, changes occur as the many
phases of which it is composed attempt to adjust to the changing
physical environment; and the phases also react with one another and
the surrounding wall-rocks.
Petrogenesis of KimberlitesPetrogenesis of Kimberlites

78. The first batch of quasi-magma that bursts explosively through to the
surface is likely to produce a maar, that is surrounded by crater-ring of
kimberlitic pyroclastic materials.
With the arrival of more batches of quasi-magma, the materials in the
surface vents and contiguous feeder-dykes are entrained and mixed; and
the solids are abraided in a vigorously-active fluidized system.
Eventually the fluidized system collapses, and the different materials
in the essentially degassed quasi-magma coalesce, and the typical rocks
of the kimberlite kindred form as the result of this process, assisted by
the crystallization and growth of a variety of low temperature and low-
pressure secondary minerals.
Petrogenesis of KimberlitesPetrogenesis of Kimberlites

79. Hypothetical cross section of an Archean craton with an extinct ancient mobile belt (once associated with subduction) and a young rift.
The low cratonal geotherm causes the graphite-diamond transition to rise in the central portion. Lithospheric diamonds therefore occur
only in the peridotites and eclogites of the deep cratonal root, where they are then incorporated by rising magmas (mostly kimberlitic-
“K”). Lithospheric orangeites (“O”) and some lamproites (“L”) may also scavenge diamonds. Melilitites (“M”) are generated by more
extensive partial melting of the asthenosphere. Depending on the depth of segregation they may contain diamonds. Nephelinites (“N”)
and associated carbonatites develop from extensive partial melting at shallow depths in rift areas. After Mitchell (1995) Kimberlites,
Orangeites, and Related Rocks. Plenum. New York. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice
Hall.
KimberlitesKimberlites

80.  This is a schematic cross section of an Archean craton with a Proterozoic mobile belt and a
modern rift.
 Due to the lower geothermal gradient in the poorly radioactive cratonal areas, the diamond-
graphite transition is elevated into the deep cratonal lithospheric mantle root.
 Diamonds thus occur only in the lherzolites, and depleted harzburgites-dunites of those roots,
and not beneath the rifts or mobile belts.
 Only melts generated in or beneath these roots can entrain and disaggregate diamond-bearing
KimberlitesKimberlites

81.  Kimberlites (K) may pass through various rock types, picking up harzburgite, eclogite, or
Iherzolite xenoliths.
 Lithospheric orangeites (O) may also traverse diamond-bearing levels.
 Lamproites (L) occur more commonly in the old mobile belts, but some may be generated in
diamond-bearing material at the cratonal edge.
 Diamond inclusions are commonly eclogitic in diamondiferous lamproites, and the source may
be the remnant subducted slab in which the oceanic crust is now converted to eclogite.
KimberlitesKimberlites

82. • Melilitites (“M”) are generated by more extensive partial melting of the asthenosphere. Depending
on the depth of segregation they may contain diamonds.
• Nephelinites (“N”) and associated carbonatites develop from extensive partial melting at shallow
depths in rift areas.
• This figure suggests that any melt that traverses the deep diamond-bearing horizons of the cratonal
roots may incorporate diamonds.
• Most melts rise sufficiently slowly and are oxidized enough to destroy any diamonds that they may
KimberlitesKimberlites

83. In India, the kimberlites of Wajrakarur, Anantpur district,
Andhra Pradesh occur in a cluster of pipes.
They are considered to be intermediate between type
kimberlite and olivine lamproite.
The Majgawan pipe of Andhra Pradesh is a typical
example of micaceous kimberlite.
The Chelima dyke of Kurnool district, Andhra Pradesh,
once considered to be kimberlite-carbonatite complex has
subsequently been described as olivine lamproite.
Kimberlites-Indian OccurrencesKimberlites-Indian Occurrences