Mantle-derived mafic-ultramafic xenoliths and the nature of Indian sub-continental lithosphere |
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| Authors: | N R Karmalkar R A Duraiswami N V Chalapathi Rao and D K Paul |
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| Institution: | (1) Department of Geology, University of Pune, Pune, 411 007, India;(2) Department of Geology, Banaras Hindu University, Varanasi, 221 005, India;(3) Present address: Institute of Mineralogy and Mineral Resources, Clausthal Technical University, 38678 Clausthal-Zellerfeld, Germany;(4) Department of Geology, Presidency College, Kolkata, 700 073, India |
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| Abstract: | Mantle derived xenoliths in India are known to occur in the Proterozoic ultrapotassic rocks like kimberlites from Dharwar
and Bastar craton and Mesozoic alkali igneous rocks like lamrophyres, nephelinites and basanites. The xenoliths in kimberlites
are represented by garnet harzburgites, lherzolites, wehrlite, olivine clinopyroxenites and kyaniteeclogite varieties. The
PT conditions estimated for xenoliths from the Dharwar craton suggest that the lithosphere was at least 185 km thick during
the Mid-Proterozoic period. The ultrabasic and eclogite xenoliths have been derived from depths of 100–180 km and 75–150 km
respectively. The Kalyandurg and Brahmanpalle clusters have sampled the typical Archaean subcontinental lithospheric mantle
(SCLM) with a low geotherm (35 mW/m2) and harzburgitic to lherzolitic rocks with median Xmg
olivine > 0.93. The base of the depleted lithosphere at 185–195 km depth is marked by a 10–15 km layer of strongly metasomatised
peridotites (Xmg
olivine > ∼0.88). The Anampalle and Wajrakarur clusters 60 km to the NW show a distinctly different SCLM; it has a higher geotherm
(37.5 to 40 mW/m2) and contains few subcalcic harzburgites, and has a median Xmg
olivine = 0.925. In contrast, the kimberlites of the Uravakonda and WK-7 clusters sampled quite fertile (median Xmg
olivine ∼0.915) SCLM with an elevated geotherm (> 40 mW/m2).
The lamrophyres, basanites and melanephelinites associated with the Deccan Volcanic Province entrain both ultramafic and mafic
xenoliths. The ultramafic group is represented by (i) spinel lherzolites, harzburgites, and (ii) pyroxenites. Single pyroxene
granulite and two pyroxene granulites constitutes the mafic group. Temperature estimates for the West Coast xenoliths indicate
equilibration temperatures of 500–900°C while the pressure estimates vary between 6–11 kbar corresponding to depths of 20–35
km. This elevated geotherm implies that the region is characterized by abnormally high heat flow, which is also supported
by the presence of linear array of hot springs along the West Coast. Spinel peridotite xenoliths entrained in the basanites
and melanephelinites from the Kutch show low equilibrium temperatures (884–972°C). The estimated pressures obtained on the
basis of the absence of both plagioclase and garnet in the xenoliths and by referring the temperatures to the West Coast geotherm
is ∼ 15 kbar (40–45 km depth). The minimum heat flow of 60 to 70 mW/m2 has been computed for the Kutch xenolith (Bhujia hill), which is closely comparable to the oceanic geotherm. Xenolith studies
from the West Coast and Kutch indicate that the SCLM beneath is strongly metasomatised although the style of metasomatism
is different from that below the Dharwar Craton. |
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| Keywords: | Mantle xenoliths Kimberlites Alkali magmatism Subcontinental lithospheric mantle Deccan volcanic province Dharwar craton |
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