Zircon U–Pb age, Lu–Hf isotope, mineral chemistry and geochemistry of Sundamalai peralkaline pluton from the Salem Block, southern India: Implications for Cryogenian adakite-like magmatism in an aborted-rift

Abstract

The Sundamalai peralkaline pluton is one among the Cryogenian alkaline plutons occurring in the Dharmapuri Rift Zone (DRZ) of the Salem Block in the Southern Granulite Terrane (SGT) of India. Here we present zircon U–Pb age and Lu–Hf isotopic composition, mineral chemistry and geochemistry of the pluton to explore the petrogenesis and geodynamic implications. Systematic modal variation of orthoclase, Na-plagioclase, Ca-amphibole (ferro-edenite and hastingsite) and quartz developed quartz–monzonite and granite litho units in the Sundamalai pluton. Thermometry based on amphibole–plagioclase pair suggests that the pluton was emplaced and solidified at around 4.6kbar pressure with crystallization of the major phases between 748 and 661°C. Estimated saturation temperature of zircon (712–698°C) is also well within this range. However, apatite saturation occurred at higher temperatures between 835 and 870°C, in contrast with monazite saturation (718–613°C) that continued up to the late stage of crystallization. Estimated oxygen fugacity values (logfO2: −14 to −17) indicate high oxidation state for the magma that stabilized titanite and magnetite. The magmatic zircons from Sundamalai pluton yielded a weighted mean 206Pb/238U age of 832.6±3.2Ma.Geochemically, the Sundamalai rocks are high-K to shoshonitic, persodic (Na2O/K2O ratio>1), silica-saturated (SiO2:65–72wt.%), and peralkaline in composition (aluminum saturation index, ASI<1; Alkalinity index, AI<0). The initial magma was mildly metaluminous which evolved to strongly peralkaline as result of fractional crystallization (plagioclase effect) controlled differentiation between quartz–monzonite and granite. Both rock types have high content of Na2O (5.1–6.3wt.%), Ba (350–2589ppm) and Sr (264–1036ppm); low content of Y (8.7–17ppm) and Yb (0.96–1.69ppm); elevated ratios of La/Yb (11–46) and Sr/Y (46–69) and are depleted in Ti, with a positive Sr anomaly suggesting an adakite-like composition and garnet controlled melting of a plagioclase-poor source. The low content of MgO (<1wt.%), Cr (7–29ppm), Ni (6–19ppm), Sc(2–3ppm), positive Sr anomaly and predominantly negative zircon εHf(t) values (−10.8 to −9.3 with an average of −10.2) and initial 176Hf/177Hf ratios (0.281947–0.282022) confirm a Paleoproterozoic crustal source. Based on the field and geochemical evidences, we propose that a previously metasomatized mafic lower-crustal source enriched in alkalis has undergone CO2-present partial melting as a result of asthenospheric upwelling beneath an aborted rifting along the DRZ generating the magma that crystallized the Sundamalai rocks. Age of this pluton is comparable with that of the other Cryogenian felsic alkaline plutons from Salem Block suggesting extensive rift-related magmatism at this time in the SGT.