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

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 melange, within the NSB, indicates that there is a 50 m.yr. gap between the Mesoproterozoic subduction-accretion, represented by the ophiolite melange 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 A-type 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.