3. CONTINENTAL CRUST
Upper and lower continental crust
Upper continental crust- granites and granodiorites
Lower continental crust- granulites
Overall mineralogical abundance- quartz, felspar, with minor
proportion of amphibole, muscovite and biotite
Orthopyroxene present in lower continental granulitic crust.
Maximum thickness of continental crust at collisional settings.Ex.
Himalaya (quartz polymorphism)
4. Oceanic Crust – Basalt (Olivine, plagioclase pyroxene)
At subduction zone mineral present undergo subsequent changes.
Xenolith provide the best evidence of the upper mantle
Petrological study of xenoliths- peridotites-olivine, ortho-pyx, clino-pyx along with
aluminous phase that changes with depth.
Crust mantle boundary – 30 km plagioclase
30km – 60 km spinel
Below 60 km garnet
At dept of nearly 300 km----pyroxene forms solid solution with garnet
5.
6. SALIENT GEOCHEMICAL CHARACTERISTICS OF
MAGMA FORMED IN MOR SETTING
-Depleted
-LILE, HFSE, REE with respect to chondrite and primitive mantle
-Decrease in pressure (decompression melting)
- adiabatic decompression melting
- MOR magma generated by high degree of partial melting of
asthenospheric mantle under an anhydrous condition.
- geochemical comp of the mantle derived magma reflects the composition
of the source mantle where melting occurs.
7. In this case asthenospheric mantle beneath a diverging oceanic plate is
anhydrous and depleted in trace elements.
As a result magma errupted at oceanic spreading centre are characterised by
depleted concentration of trace elements w.r.t primitive mantle and chondrite.
Factor contributing to depleted nature in a MOR rift system include
a) Absence of volatiles and anhydrous conditions
b) previous melt extraction events during ancient tectonic cycles.
8. The overall geochemical characteristics of magma derived from the mantle through
partial melting provide a first order estimation of the geochemical composition of
the source mantle.
Mantle having greater abundance of trace and REE compare to primitive manlte or
chondrite is called an enriched mantle.
Mantle having trace and REE values lower then primitive mantle or chondrite is
called an depleted mantle.
MOR rift system source is asthenosphere, having a depleted geochemical comp.
MOR magma generated from partial melting of depleted asthenosphere
N-MORB and E-MORB
9. The process by which depleted asthenospheric mantle beneath MOR undergo
elemental enrichment includes
a) Plume and asthenosphere interaction
b) percolation of melt modified by paleo subduction processes.
10. GEOCHEMICAL CHARACTERISTICS OF MAGMA FROM
SUBDUCTION ZONE
Convergent plate margins have been classified as:
a) Ocean-Ocean
b) Ocean-Continent
c) Continent-Continent system
11. Magma produced in subduction zone typically reflects the processes that the
underlying mantle undergoes during subduction.
The subducting oceanic lithosphere and adjacent mantle wedge are the
principal contributer of subduction zone magmatism.
The subduction zone process occurring at different convergent margins setting
is the driving force for the magmatism generation of juvenile crustal material
which finally results into orogenic cycles and mountain building.
The orogenic cycles are divided as
a) Accretionary orogeny and
b) collisional orogeny
12. The accretionary orogeny is associated with ocean –ocean and ocean continent
convergent processes.
The collisional orogeny is associated between the collision between two continental
blocks with an intermediate ocean in between them, this oceanic plate between two
continental blocks undergoes one sided or two sided subduction beneath the
continent, the ocean basin is consumed followed by collision of continental plates.
13. During the initial stage of subduction the subducting oceanic lithosphere undergoes
dehydration and all the fluid and volatiles are released from it.
These slab dehydrated fluids and volatiles are transported to the adjacent mantle
wedge, the geothermal gradient is disturbed and partial melting starts to produce
magma.
Island arc tholeitic basalts are the first products of the subduction zone magmatism.
During dehydration of the subducting oceanic slab fluid mobile elements like LILE
(Rb, Sr, Ba etc) and LREE (La, Ce, Pr) are released with the fluid and transfer to the
mantle wedge thereby causing enrichment of the mantle wedge that was depleted
earlier. This process is called metasomatism of the mantle wedge.
14. Thus, magma generated by partial melting of the matasomatic mantle wedge are
enriched in LILE and LREE.
These magmas are relatively depleted in HFSE and HREE because owing to the
fluid immobile nature of these elements, they are retained in the subducting
oceanic slab and not released in the fluid.
These processes continued to the intermediate stage of the subduction where
tholeiitic magmas are changed to calc- alkaline magma and a basalt –andesite-
dacite-rhyolite (BADR) spectrum is generated, however trace element composition
remains the same.
Therefore the initial and intermediate stage of subduction are characterised by
oceanic slab dehydration and mantle wedge metasomatism and the magma
produced are characterisd by LILE and LREE enrichment with HFSE and HREE
depletion.
15. During the maturation stage of subduction, the subducted oceanic slab is
metamorphosed to amphibolite-eclogite facies and start melting.
The melting of the oceanic slab cause breakdown of amphibole and garnet present and
the HFSE and HREE compatible with amphibole and garnet are released into the melt.
These slab derived melt interact with the mantle wedge, hybridize it and leads to
generation of magma enriched in HFSE like Nb enriched basalts and adakites.
Therefore the mature stage of subduction are characterised by oceanic slab melting and
mantle hybridization and the magma produced are enriched in HFSE along with LILE and
LREE.
16. Characteristics
On primitive mantle normalised diagram subduction zone magmas exhibit
positive anamolies of LILE and LREE and negative anamolies of HFSE and HREE.
Trace and REE characteristics of magma generated in ocean –ocean and ocean –
continent subduction setting shows overall similar features like REE enrichment
or HFSE depletion.
However continental Arc magmas suffer assimilation of continental crust from the
overriding plate and show the greater abundance of Rb and Th.
17. GEOCHEMICAL CHARACTERISTICS OF MAGMAS GENERATED IN
INTRAPLATE TECTONIC SETTING
Magmatism in intra plate conditions are associated with mantle plume activities
either in an intracontinental or oceanic setting.
Mantle plume rises either from the core mantle boundary at 2900km or from
660km depth marking the boundary between upper mantle and lower mantle or
from superficial depth.
The mantle plume rise and impinges beneath the oceanic and continental
lithosphere.
The reducing pressure in an upwelling mantle plume causes melting of the plume
head.
Due to plume effect oceanic and continental lithosphere is fractured and faulted
to provide pathway for melt propagation and magmatism.
18.
19. Melts derived by pathway of plume head interact with the lithospheric mantle and
finally travel through oceanic continental crust before eruption.
Magma erupted in oceanic and continental settings are referred to as oceanic island
basalts (OIB) and continental flood basalts (CFB).
The geochemical characteristics of these magmas are controlled by:
a) Composition of the mantle plume
b) Composition of the lithospheric mantle
c) assimilation and contamination from oceanic and continental crust.
A mantle plume owing to its origin 660-2900 km depth carries ancient recycled
subducted slabs
20. These 660 km and 2900 km regions serve as graveyards of the ancient subducted
slabs that got detached and recycled into the mantle after the termination of the
subduction.
These recycled subducted slabs are enriched in HFSE elements and therefore the
mantle plume carrying these components are also enriched in HFSE compared to
primitive mantle, chondrite, NMORB and arc basalts.
The mantle plume with HFSE enriched component interacts with the lithospheric
mantle.
The lithospheric mantle is enriched in LILE and LREE, that were fluxed from paleo-
subduction events.
Thus the plume incorporate LILE and LREE with interaction with lithospheric mantle
21. The magma generated through the partial melting of the plume head and lithosphere
mantle and their mutual interaction travels through oceanic or continental crust before
eruption.
Assimilation of oceanic crust causes enrichment in Ba and Sr while contamination and
assimilation of continental crust results into enrichment of Rb, Th and U
Therefore magma generated in oceanic and intraplate tectonic setting are
geochemically characterised by enrichment of LILE, HFSE and LREE, show positive
anamolies of LILE (Nb, Ta, La, Ce etc) on primitive mantle and chondrite normalised
diagrams.
OIB are influenced by oceanic crust while CFB are influenced by continental crust.
22. REE COMPOSITION AND MANTLE MELTING CONDITIONS
Relative abundance of LREE MREE and HREE are primarily controlled by the
degree and depth of partial melting of the mantle.
The aluminous phase present at different depths of the mantle controls REE
distribution.
Partial melting at depth greater than 60 km---- garnet peridotie----HREE (Yb, Lu)
more compatible and hence trapped in garnet------while MREE and LREE are conc.
In melt.
Melt will have high LREE/HREE and MREE/HREE ratios and mantle residue will
have higher abundnce of HREE
23. Partial melting at depth less than 60 km----- garnet is not a stable phase hence HREE
abundance will be more in melt phase
Depth less than 60 km is represented by plagioclase and spinel peridotite----melt
will have low LREE/HREE and MREE/HREE ratio because
a) MREE are very much compatible with spinel and absence of garnet will
automatically elevate HREE causing low MREE/HREE
b) LREE being compatible to plagioclase will lower LREE content in the melt and
absence of garnet will increase HREE and hence low LREE/HREE ration in melt.
24. In terms of degree of partial melting of the mantle, it has been observed that lower
degree of partial melting causes higher LREE abundance in the melt and high degree
of partial melting HREE concentration increases.
Theses observation can help to draw inferences
Case I
Basalt is having La/Yb >2 and Gd/Yb > 2. interpret the mantle melting condition.
Case II
Basalt having La/Yb < 2 , Sm/Yb < 2, Gd/Yb <2. interpret the mantle melting condition
Case III
Basalt having La/Sm >2
