Mantle-derived mafic-ultramafic xenoliths and the nature of Indian sub-continental lithosphere

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/m 2 ) and harzburgitic to lherzolitic rocks with median X mg 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 (X mg 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/m 2 ) and contains few subcalcic harzburgites, and has a median X mg olivine = 0.925. In contrast, the kimberlites of the Uravakonda and WK-7 clusters sampled quite fertile (median X mg olivine ~0.915) SCLM with an elevated geotherm (> 40 mW/m 2 ). 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 o 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/m 2 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.

Short-scale variability of the SCLM beneath the extra-Andeanback-arc (Paso de Indios, Argentina): Evidence from spinel-faciesmantle xenoliths

“…Petrological studies of mantle xenoliths provide valuable information on the structure and composition of the litho-spheric mantle [ [1,2], among others], sometimes providing the direct evidence of metasomatic processes, such as those produced by slab-derived components at convergent margins [e.g. [3,4]].…”

Small-scale convection beneath oceans and continents

“…This inference is mathematically correct, but may not represent actual thick chemical lithosphere. That is, a range of cratonal lithospheric thicknesses greater than platform lithosphere thickness exists [61,62]. Such lithosphere is deeper than the postulated lower equilibrium.…”

“…The depths to the bases of the chemical lithosphere and the thermal lithosphere vary within cratons [61,62]. The trend of scalloped relief on the base of the chemical lithosphere may sometimes align with the direction of plate motion and hence the strike of the axes of convection cells.…”

“…Depleted samples extend to ~250 km depth and ~1300°C. Karmalkar et al [62] compiled results for Wajrakarur district of India. Depleted samples extend to ~185 km and ~1200°C.…”

“…The curved geotherm between the arrays and the MORB adiabat is not resolved. The chemical base of the lithosphere in the Wajrakarur region of India [62] provides an upper limit for the rheological boundary layer. Its geotherm is similar to the cratonal one.…”