Abstract
Abstract In comparison to the eastern Dharwar Craton, the mantle-derived xenocrysts/xenoliths are extremely rare or even unreported from the western Dharwar Craton, southern India. A Neoproterozoic (ca. 800-900 Ma) lamprophyre cropping out in the Mysuru area of southern India contains chrome-diopside xenocrysts (Cr2O3 content varying from 0.2 – 1.23 wt%) which provide important evidence about the pressure-temperature conditions and lithospheric thickness beneath the western Dharwar Craton. Studied chrome-diopsides show compositional zoning which is lacking in the liquidus phases (amphiboles and feldspars) of the lamprophyre which additionally favors a non-cognate origin of the former. Based on the compositional zoning, all the chrome-diopside xenocrysts can be divided into three groups: (i) Group I- which are euhedral and show reverse zoning with increasing Cr-content from core to rim; (ii) Group II- which are characterized by fractures and resorption textures, show complex reverse zoning and display up to three distinct compositional layers, and (iii) Group III- which evidence the reaction of chrome-diopsides with lamprophyric melt and are marked by alteration phases, such as actinolite and chlorite, together with relicts of some unaltered xenocrysts. High Cr2O3, moderate MgO and low Al2O3 content of all the three varieties of chrome-diopside suggest them to represent disaggregated xenocrysts of mantle-derived garnet peridotite. Temperature-pressure estimates for chrome-diopside xenocrysts ranges from 895 - 1026 °C (± 30 °C) and 32 – 38 kbar respectively and correspond to depth range of 106 – 127 km. The study reveals that lithospheric thickness during the Neoproterozoic beneath the western Dharwar craton was at least ~115 km and is similar in composition to that of the cratonic lithosphere found in the other cratonic domains.
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