Thenmudiyanur Mafic–Ultramafic Complex from the north eastern margin of Southern Granulite Terrane, India: A possible Neoproterozoic Alaskan-type Complex.

Alaskan-type of plutonic complexes represent arc magma chambers and they occur throughout the earth's history from Archean to Phanerozoic periods around the globe. The ultramafic rocks from these complexes commonly contain spinels of varied chemistry, especially rich in Cr, Fe, and Ti coupled with low-Al contents when compared to the spinels from oceanic or continental rift domains. An attempt is made here to study Thenmudiyanur Mafic–Ultramafic Complex (TMUC) from the Belur Shear Zone (BLSZ) in the northeastern margin of Southern Granulite Terrane, located about 20 km SW of Tiruvannamalai Town. The important lithologies of TMUC include dunites, peridotites and pyroxenites surrounded by metagabbros and amphibolites. The dunites and peridotites are highly serpentinized with well-preserved cumulate olivines along with magnesite, chrome spinel and clinopyroxenes. The mineral chemistry of olivines represents higher forsterite (Fo88–90) and Mg# 87–90 [100×Mg/(Mg+Fe2+)] contents. The gradational contact within the rock types, compositional variations and cumulate nature of mineralogical characteristics suggest fractional crystallization of magma from a single parental source. The chrome spinels from these lithologies represent higher Cr# [100×Cr/(Cr+Al)] that ranges from 65 to 86 along with lower MgO (1.06–4.57 wt%) and MnO (<1 wt%) contents. The presence of chrome spinels together with olivine in dunites as well as peridotites and clinopyroxene mineral chemistry from pyroxenites reveal arc signatures for the evolution of TMUC, which seems to be similar to that of the Alaskan-type. Considering the tectonic setting and the age relationships of adjacent lithologies, it is opined that the TMUC might have evolved during the Neoproterozoic period. [ABSTRACT FROM AUTHOR]