Neoproterozoic arc magmatism in the southern Madurai Block, India: Subduction, relamination, continental outbuilding, and the growth of Gondwana

Abstract

The Madurai Block in southern India is a composite collage of at least three sub-blocks, with Neoarchean–Paleoproterozoic segments in the north and central domains, and a Neoproterozoic segment in the south. Here we investigate a suite of rocks with magmatic protoliths that constitute the basement in the southern margin of the Madurai Block including alkali granites, charnockites, enderbites and gabbros. The alkali granites are dominantly composed of perthitic K-feldspar, minor plagioclase and quartz, with hornblende as the main mafic mineral suggesting a calc-alkaline nature. The enderbites and charnockites have a broadly similar mineralogical constitution except for the variation in the modal content of plagioclase, K-feldspar and quartz, as well as the additional presence of clinopyroxene in some of the enderbites. The high modal content of hornblende in the gabbros suggests crystallization from hydrous basaltic melts. The geochemical features of this suite are identical to those of arc magmatic rocks, with distinct Nb, Ta, and Ti depletion suggesting magmatism in a subduction-related environment. We envisage that the underplating of basaltic magmas within a convergent margin setting provided the heat input for lower crustal melting generating the charnockitic suite of rocks. The intrusion of the underplated mafic melts as gabbroic dykes and sills into the crystallizing felsic magmas resulted in magma mixing and mingling generating the widespread enclaves of gabbroic rocks. The alkali granites were derived from the differentiation of lower crustal melts. Zircon U–Pb data from the alkali granites yield weighted mean 206Pb/238U ages of 786±10 to 772±11Ma for the oldest and the most dominant group of magmatic grains, with a 662±20Ma subordinate group. The oldest group of magmatic zircons in the charnockite samples shows ages of 938±27Ma, 896±12Ma, and 786±9Ma, suggesting multiple magmatic pulses during early and mid-Neoproterozoic. A subordinate population of magmatic zircons with ages of 661±9Ma and 632±7Ma is also present. In the enderbites, the magmatic zircon population yields weighted mean ages of 926±22Ma, 923±36Ma, 889±13Ma, 803±10Ma, 787±23Ma, 786±10Ma, 748±27Ma, 742±11Ma, 717±8Ma and 692±10Ma suggesting continuous and multiple pulses of magmas emplaced throughout early to mid-Neoproterozoic. Magmatic zircons from the gabbros show weighted mean 206Pb/238U ages of 903±13Ma, 777±10Ma, 729±10Ma and 639±27Ma. Metamorphic zircons from all the rock types show latest Neoproterozoic-Cambrian ages in the range of 567±19Ma to 510±8Ma suggesting prolonged heating. Zircon Lu–Hf data show that the alkali granite-charnockite-enderbite suite has depleted mantle ages (TDM) in the range of 1164–2172Ma and crustal residence ages (TDMC) of 1227–3023Ma. These spots show both negative εHf(t) and positive εHf(t) values (−22.1 to 10.6), suggesting magma derivation from mixed juvenile mid- to late-Mesoproterozoic components and reworked Mesoarchean to mid-Mesoproterozoic components. Zircon grains from the gabbroic rocks show depleted mantle ages and (TDM) in the range of 1112–2046Ma, crustal residence ages (TDMC) of 1306–2816Ma, and both negative and positive εHf(t) values (−17.8 to 7.9), suggesting that the magmas were dominantly derived from juvenile mid-Mesoproterozoic to Neoproterozoic components as well as reworked Mesoarchean to mid-Mesoproterozoic sources.Our data clearly reveal multiple arc magmatism along the southern Madurai Block during distinct pulses throughout early to late Neoproterozoic, suggesting an active convergent margin along this zone at this time. Crustal thickening occurred through relamination by mafic magmas associated with slab melting. Continental outbuilding and southward growth of the Madurai Block were associated with the lateral accretion of the vast sedimentary belt of Trivandrum Block, culminating in collisional metamorphism during latest Neoproterozoic–Cambrian associated with Gondwana assembly.