High Ti- bearing Gabbros from Chalk Hills of Salem, Southern India: A Co-genetic Origin during Neoproterozoic Alaskan-type Evolution

Abstract

The Chalk hills of Salem complex also called Salem mafic and ultramafic complex (SMUC), occurs in northern part of Cauvery suture zone (CSZ), southern granulite terrane (SGT), India with the lithological sequence of ultramafic cumulates, mafic intrusions of gabbros and amphibolites with quartzo-feldspathic veins and ultrapotassic dykes. Recently the complex has been described as Neoproterozoic Alaskan-type. The Ti- rich gabbros are in association with serpentinized dunites and pyroxenites in the form of several intrusions at various locations of the complex. The petrography of these gabbros shows sub-alkaline to alkaline in nature with mineralogy of two pyroxenes and plagioclases of calcicsodic assemblages along with magnetite-titanomagnetite-ilmenite (Ti-spinels). The whole rock geochemistry indicates that they are tholeiitic in nature with enrichment of LILE (Rb, Ba, Th, Sr) and depletion of HFSE (Nb, Ta, Ti) with reference to N-MORB. The mineral chemistry of clinopyroxenes from these gabbros show slightly higher Mg# (0.68–0.80), TiO2 (0.40–0.73 wt%) with lower Al2O3 (4.4–5.5 wt%), Na2O (0.53–0.94 wt%) and Cr2O3 (0.1–0.17 wt%) contents. The orthopyroxenes also show low Al2O3 (1.4–2.3 wt%), Cr2O3 (up to 0.13 wt%), and relatively higher MgO (17.11–21.79 wt%). The Ti- bearing oxides (ilmenites) of these gabbros are characterized by higher contents of TiO2 (38–51 wt%) and FeO (46–56 wt%), similar to that of Ti-Fe rich Alaskan- type of mafic rocks. On various tectonic discrimination diagrams of whole rock and clinipyroxene mineral chemistry of these gabbros reveal that they are derived from subduction related components of arc magmas of titanium rich, evolved through the process of high degree of crystal fractionation. From the two-pyroxene thermometry, the temperature of formation of these gabbros are determined as 893–1014°C and the clinopyroxene barometry results variable pressures of 8–13 kbars. The tectonic setting with the available age relationships from the complex suggest these intrusions are syntectonic and co-genetic of late emplacements along with the ultramafics during Neoproterozoic subduction events. Such occurrences of high Ti- bearing gabbros are common in many Alaskan- type of complexes like Neoproterozoic Arabian Nubian Shield and other parts of the world.