Evidence of a Continental Slab Break Off During Orogeny in Tonian Neoproterozoic India?

Neoproterozoic massif-type anorthosites and related magmatic suites from the Eastern Ghats Belt, India: implications for slab window magmatism at the terminal stage of collisional orogeny

Authors:

C.V. Dharma Rao, M. Santosh and Shuan-Hong Zhang

Abstract:

The Jugsaipatna massif-type anorthosite complex (JAC) in the Eastern Ghats Belt of India comprises anorthosite–leuconorite–norite in the central part and gabbros, gabbronorites and porphrytic granites in the periphery. In this study, we report laser ablation ICP-MS zircon U-Pb data and REE geochemistry from the anorthosites, gabbros and porphyritic granites from the JAC. The zircon data yield weighted mean 207Pb/206Pb ages of 918 ± 33 Ma (MSWD = 2.2) for the anorthosite and 931 ± 38 Ma (MSWD = 2.2) and 928 ± 35 Ma (MSWD = 0.99) for two leuconorites. The gabbros bordering the anorthosite body yield weighted mean 207Pb/206Pb ages of 984 ± 10 Ma (MSWD = 0.41) and 969 ± 12 Ma (MSWD = 1.5). Zircons from the associated porphyritic granites define two concordant age groups: an older group with 207Pb/206Pb weighted mean age of 996 ± 11Ma (MSWD = 2.1) and a younger group with an age of 964 ± 29Ma (MSWD = 2.1). Zircons from another granite sample yield an age of 957 ± 17 Ma (MSWD = 1.1), identical to the Neoproterozoic age data obtained from the zircons in the anorthosites and gabbros of the JAC. The zircons from anorthosites show moderate REE contents, prominent HREE enrichment and a conspicuous positive Eu anomaly. The zircons from the granites show high REE contents, prominent HREE enrichment and a conspicuous positive Eu anomaly, suggesting a common melt source. The ages reported in study correlate well with similar ages of 983 ± 2.5Ma for the anorthosites from Chilka Lake complex and the ca. 930 Ma for Bolangir anorthosite in the Eastern Ghats Belt. The early Neoproterozoic ages reported from the magmatic suite in this study remarkably coincide with the timing of ultrahigh-temperature metamorphism reported from various localities in the Eastern Ghats Belt in recent studies. The coeval nature of mantle-derived magmatism and ultrahigh-temperature metamorphism in a collisional orogen following a prolonged subduction-accretion history along the eastern periphery of the Indian lithosphere suggests asthenospheric upwelling, probably through a slab-window mechanism. We correlate the geodynamic setting to post-collisional slab-break off at the terminal stages of the orogeny.