Shonkinites from Salem, southern India: Implications for Cryogenian alkaline magmatism in rift-related setting

Abstract

Alkaline and potassic igneous rocks, although minor components of the continental crust, provide important constraints on magma processes and tectonic settings. Here we report petrology, mineral chemistry, whole rock geochemistry, zircon U–Pb and Lu–Hf data on shonkinites from the Salem Block in the Southern Granulite Terrane of India. The shonkinite is composed mainly of porphyritic clinopyroxene, K-feldspar and olivine, and intrudes into the surrounding ultramafic rocks composed of wehrlite (clinopyroxene and olivine) and dunite (mostly olivine with minor spinel). The geochemical data on shonkinites from Salem show alkaline and ultrapotassic features with marked enrichment in incompatible elements, and high K2O and K2O/Na2O ratios suggesting magma derivation from metasomatized lithospheric mantle. Their HFSE depletion relative to LILE compositions, coupled with positive Nb anomalies suggests continental rift setting. The zircon grains in shonkinite show typical magmatic crystallization features and their LA-ICP-MS U–Pb data yield weighted mean age of 818±6.3Ma (MSWD=0.32). The zircon εHf values range between −11.1 and −12.7, consistent with magma derivation from an enriched mantle, with depleted mantle model ages (TDM) in the range of between 2406 and 2513Ma, indicating the involvement of Neoarchean–Paleoproterozoic subduction-related components. Together with similar-aged alkaline plutons with comparable geochemical features, we envisage a major alkaline magmatic event associated with rifting during Cryogenian in the northern part of the Southern Granulite Terrane.