Age, petrogenesis and significance of 1 Ga granitoids and related rocks from the Sendra area, Aravalli Craton, NW India

Abstract

We present new geochronological, petrological, geochemical and isotopic data for granitic and related rocks from the Aravalli Craton, Rajasthan, northwestern India. In the Sendra area, five variably deformed granitoid plutons, ranging in composition from tonalite to granite, cut across carbonate-rich metasedimentary rocks of the Delhi Supergroup. The largest of these bodies, the Chang pluton (∼15 km 2) is dominated by monzogranitic gneisses and aplitic dykes, composed of subequal proportions of quartz, plagioclase (An 7–20) and microcline (Or 92–98), with lesser biotite (Fe ∗=0.8–0.9) and accessory muscovite (Fe ∗=0.7–0.8). U–Pb zircon data (TIMS method) for a biotite granite gneiss yield a weighted mean 207Pb/ 206Pb age of 967.8±1.2 Ma, which we interpret as representing the time of magmatic crystallization. Rb–Sr whole-rock isotopic data for the Chang pluton, including new analyses as well as previously published ones, yield a regression of 906±67 Ma (MSWD=82), which is barely within error of the U–Pb age. There is evidence for open-system behaviour in the Rb–Sr system, particularly for whole-rock samples with low Sr concentrations, and consequently high Rb/Sr. Sm–Nd isotopic data fail to yield meaningful age information. Initial isotopic ratios (at 968 Ma) for Chang pluton granitoids ( I Sr=0.7110±14; ε Nd=−3.28±0.47) are compatible with source materials similar to Archaean amphibolitic rocks of the Banded Gneiss Complex. Spatially associated with the Chang pluton is a massive metagabbro, composed of plagioclase (An 45–68) and magnesio-hornblende (Fe ∗=0.3–0.4), with secondary Cl-rich scapolite and ferrian zoisite. The scapolite and zoisite likely crystallized from metamorphic fluids that interacted with nearby calc-silicate schists and gneisses of the Delhi Supergroup. Aside from slight enrichments in Rb, U, Th and Ba, this metagabbro retains a primitive chemical signature similar to N-MORB (LREE depletion, low K), and initial isotopic ratios ( I Sr=0.7058; ε Nd=+2.9) that approach model depleted mantle at 968 Ma. The metagabbro is chemically and isotopically similar to mafic metavolcanic and related rocks that have been considered to represent ocean-floor and island-arc basaltic magmas. All available data are compatible with the idea that these rocks represent the products of convergent margin processes during the Early Neoproterozoic, a conclusion that could have important implications for the construction of the Rodinia supercontinent.