Geochemistry of the mafic xenoliths from the Kinnaur Kailash Granite, Baspa valley, Himachal Pradesh, India

Mafic enclaves occur in a variety of geologic and tectonic settings and play an important role in the crustal evolutionary processes.  The geochemistry of the mafic xenoliths from Baspa valley of Himachal Pradesh, India has been investigated to characterize their protoliths on the basis of immobile elements, especially trace elements including REE.  The mafic xenoliths occur within the Kinnaur Kailash Granite (KKG). They are elongated in shape with commonly 5-10 cm length, although a few of them attain surface dimensions of even 20-100 cm. These xenoliths are fine-grained and massive to foliated, sometimes with thin felsic stringers parallel to layering. Contact between host granite and mafic xenolith is sharp. They are comprised of clinopyroxene, hornblende, plagioclase, quartz, ilmenite, sphene, ±orthopyroxene, ±garnet.  Hornblende constitutes the dominant mineral of the mafic xenoliths, it occurs as a well-developed matrix mineral and as a cleavage-less massive retrograde mineral around clinopyroxene. Mineral chemistry of the hornblende shows that they are of magnesio-hornblende, ferro-hornblende to ferro-tschermakite variety. Orthopyroxene when present occurs as both discrete grains and microscopic needles exsolved parallel to prismatic cleavage of the clinopyroxene host. Garnetiferrous mafic xenoliths display coronae of garnet around plagioclase and clinopyroxene, and of sphene around ilmenite. The most common retrograde mineral is hornblende, retrogression is distinctly observed as corona textures such as, garnet around clinopyroxene and/or plagioclase, sphene around ilmenite, and hornblende around clinopyroxene.

Their geochemistry show that they have tholeiitic nature with basaltic composition. Compositionally, they range from ‘depleted’ to ‘enriched’ MORB as observed on the binary diagrams of Ti vs V and Zr vs Ti and on ternary diagrams of Zr-Ti-Y and Th-Zr-N.  Likewise, they match with various enriched or ‘transitional’ MORB types as evident from their Zr vs Nb binary plot. Their enriched character when compared with N-MORB, E-MORB and OIB rocks on chondrite and primordial mantle normalized plots reveals that it is intermediate to that of E-MORB and OIB. The geochemistry of the rocks suggest that the enriched components are probably derived by melting of a mantle source with E-MORB or OIB rather than due to the crustal contamination. The study carried out emphasize that the mafic xenoliths have developed in rift environment, and that they are not volcanic rocks of island arc related to subduction tectonics. It is visualized that the mafic xenoliths were formed as cumulate rocks from the tholeiitic magmas that were rising to lower crust levels in a rift environment, which at a later stage got entrapped as restitic material in the host Kinnaur Kailash Granite formed in a collision environment, and propose a change of regime from rift related to collision environment prior to Palaeozoic period.

Summarizing the overall implications of the studies carried out on the mafic xenoliths of Baspa valley of Himachal Pradesh, it can be suggested that, the region under study had experienced change of regime from rift environment to collision environment prior to Paleozoic period. The mafic xenolith were formed as cumulate rocks from the tholeiitic magmas that were rising to lower crust levels in a rift environment, which at latter stages occurred as restitic material within the host Kinnaur Kailash Granite (KKG) that were formed in a collision environment during Paleozoic times. However, the enigma of age of mafic xenolith from Baspa valley can be resolved only by dating them.  The geochemical comparison of mafic xenolith under study with metabasites from other parts of Himalaya further strengthen our view of mafic xenolith formation in a rift environment.

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