Geochemistry and zircon geochronology of the Neoarchean volcano-sedimentary sequence along the northern margin of the Nilgiri Block, southern India

• Volcano-sedimentary sequences from the north-eastern periphery of the Nilgiri Block.
• Zircon 207Pb/206Pb ages indicate prominent Neoarchean arc magmatism.
• The dominantly positive εHf (t) values suggest depleted mantle source.
• Evidence of Early Paleoproterozoic suturing of southern granulite terrane to the Dharwar Craton.

The Nilgiri Block is one of the major Archean crustal blocks that define the tectonic framework of southern India. Here we report geologic, petrologic, geochemical, and zircon U–Pb, –REE, and –Lu–Hf data of a highly metamorphosed and disrupted sequence of amphibolite, meta-gabbro, websterite, volcanic tuff, meta-sediment, and banded iron formation (BIF) from the northern fringe of the Nilgiri Block. Geochemically, the amphibolite shows altered ocean floor basalt signature, whereas the meta-gabbro and the websterite samples form part of a volcanic arc. The metamorphosed volcanic tuff shows subalkaline rhyolitic signature. U–Pb isotope analysis of zircon grains from the volcanic tuff and meta-gabbro shows 207Pb/206Pb ages of 2490 ± 12 Ma and 2448 ± 16 Ma, respectively. Zircons from the meta-sediments show an age range of 2563 ± 33 Ma to 2447 ± 34 Ma. The dominantly positive εHf (t) values of the zircons in the analyzed rock suite suggest that the magmas from which the zircons crystallized evolved from a Neoarchean depleted mantle source. The Hf model ages (TDM) of volcanic tuff, meta-sediment and meta-gabbro samples are ranging between 2908–2706 Ma, 2849–2682 Ma, and 2743–2607 Ma, respectively. The ca. 2500 Ma ages for the arc-related magmatic rock suite identified along the northern periphery of Nilgiri Block suggest prominent Neoarchean arc magmatism and early Paleoproterozoic convergent margin processes contributing to the early Precambrian crustal growth in Peninsular India.

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