New SHRIMP Age and Microstructures from a Deformed A-Type Granite, Kanigiri, Southern India: Constraining the Hiatus between Orogenic Closure and Postorogenic Rifting

Abstract

A new U-Pb zircon SHRIMP age of 1284 Ma from the Kanigiri granite, India, is reported to help constrain the middle to late Mesoproterozoic tectonic evolution of the Nellore schist belt (NSB). The Kanigiri granite has whole-rock chemical characteristics of A-type granites and is marked by light rare earth element enrichment, a strong negative Eu anomaly, and negative Ba, Sr, P, Ti, and Yb anomalies, indicating feldspar, apatite, and ilmenite/magnetite fractionation. Samples show Y/Nb versus Yb/Ta ratios in the range for granites associated with ocean island basalts. This two-mica granite is peraluminous and alkali-calcic to calc-alkalic, and it has high annite to phlogopite proportions (92%-98%). Strong alignment of flattened mafic microgranular enclaves in the granite, together with relatively high-to moderate-temperature crystal plastic deformation fabric in shear zones within the granite, suggest overprinting of subsolidus deformation over a relict magmatic fabric, a feature not very common in true anorogenic granites but reported in late-to postorogenic granites elsewhere. An intrusive relationship with the 1334 Ma Kanigiri ophiolitic mélange, within the NSB, indicates that there is a ≤50 m.yr. gap between the Mesoproterozoic subduction-accretion, represented by the ophiolite mélange and late-to postorogenic granite emplacement. Although the Kanigiri granite occurs in close proximity to mafic and felsic alkaline plutons belonging to the 1250-1400 Ma Prakasam alkaline province (PAkP) in the northern NSB and there is overlap in age, the Atype granitic magma source is apparently unrelated to PAkP alkaline magmatism. Our work further substantiates the observation that A-type granites originate in varied tectonic settings, not necessarily only in a rift-related (intraplate) setting.

Proterozoic felsic and mafic magmatism in India: Implications for crustal evolution through crust-mantle interactions

“…An intrusive relationship with the 1334 Ma Kanigiri ophiolitic mélange, within the NSB, indicates that a ca 50 Ma gap exists between Mesoproterozoic subduction-accretion. Although Kanigiri granite occurs close to alkaline plutons (1250-1400 Ma) in Prakasam alkaline province, A-type granite appears unrelated to intra-plate rift magmatism (Sain et al, 2017).…”

“…A-type two-mica Kanigiri granite (ca 1284 Ma, Sain et al, 2017) in the NSB contains enclaves, which display high-to moderate-T crystal plastic deformation fabric, suggesting an overprinting of subsolidus deformation over a relict magmatic fabric. An intrusive relationship with the 1334 Ma Kanigiri ophiolitic mélange, within the NSB, indicates that a ca 50 Ma gap exists between Mesoproterozoic subduction-accretion.…”

Tectonothermal evolution of a collisional orogen in the Khammam region, southeastern India: insights from structures, phase equilibria modeling and U–Th–(total) Pb monazite geochronology

“…The 1479-1377 Ma tectonic overprint in the quartzofeldspathic gneiss may be related to the terminal rifting of the Columbia supercontinent that led to the dispersal of continental blocks with simultaneous alkaline magmatism in southeastern India and East Antarctica (Fig. S9; Dobmeier and Raith 2003;Simmat and Raith 2008;Upadhyay 2008;Dharma Rao et al 2011a;Sarkar et al 2015;Subramanyam et al 2016;Sain et al 2017). However, due to the absence of alkaline magmatism within the Khammam region, the significance of the Late Mesoproterozoic (1438 ± 25 Ma) event is unclear.…”

The Indian Subcontinent: Its Tectonics

“…The NKSB is divided into two sedimentary stratigraphic units. The Vinjamuru Group is made up of rocks of amphibolite facies showing characters of a volcanosedimentary assemblage (Sain et al, 2017) with gabbro yielding WR Sm-Nd isotope age of 2654 ± 100 Ma and a 1911 ± 88 Ma isochron age (Vadlamani, 2010). The andesites and rhyolites of the Vinjamuru Group are ~1868 and 1771-1791 Ma (Vadlamani et al, 2012).…”