Sm-Nd and Rb-Sr ages and P-T history of the Archean Sittampundi and Bhavani layered meta-anorthosite complexes in Cauvery shear zone, South India: Evidence for neoproterozoic reworking of archean crust

Sittampundi and Bhavani Archean layered meta-anorthosite complexes occur as tectonic lenses within the Cauvery shear zone (CSZ), a crustal scale shear dividing the Precambrian granulite crust of south India into late Archean (> 2.5 Ga) and Proterozoic (c. 0.55 Ga) blocks. They and their host supracrustal-gneiss rocks record at least two stages of tectonometamorphic history. The first is seen as regional scale refolded isoclinal folds and granulite metamorphism (D-1-M(1)) while the second stage is associated with dominantly E-W dextral transcurrent shearing and metamorphic recrystallisation (D-2-M(CSZ)). Whole rock Sm-Nd isochrons for several comagmatic rocks of the layered complexes yield concordant ages: Sittampundi - 2935+/-60 Ma, epsilon(Nd) +/- 1.85+/-0.16 and Bhavani - 2899+/-28 Ma, epsilon(Nd) + 2.18+/-0.14 (2 sigma errors). Our Sm-Nd results suggest that: (1) the magmatic protoliths of the Sittampundi and Bhavani layered complexes were extracted from similar uniform and LREE depleted mantle sources; (2) M(1) metamorphism occurred soon after emplacement at c.3.0 Ga ago. P-T estimates on garnet granulites from the Sittampundi complex characterise the M(CSZ) as a high-P event with metamorphic peak conditions of c. 11.8 kbar and 830 degrees C (minimum). The M(CSZ) is associated with significant isothermal decompression of the order of 4.5-3.5 kbar followed by static high-temperature rehydration and retrogression around 600 degrees C. The timing of M(CSZ) is inferred to be Neoproterozoic at c. 730 Ma based on a whole rock-garnet-plagioclase-hornblende Sm-Nd isochron age for a garnet granulite from the Sittampundi complex and its agreement with the 800-600 Ma published age data on post-kinematic plutonic rocks within the CSZ. These results demonstrate that the Cauvery shear zone is a zone of Neoproterozoic reworking of Archean crust broadly similar to the interface between the Napier and Rayner complexes of the East Antarctic shield in a model Proterozoic Gondwana supercontinent.(1

U-Pb zircon geochronology of granites and charnockite from southern India: Implications for magmatic pulses associated with plate tectonic cycles within a Precambrian suture zone

The Palghat-Cauvery Suture Zone (PCSZ) in southern India marks the trace along which continental blocks were sutured during the Precambrian. We report here U-Pb geochronological data based on laser ablation ICP mass spectrometry for zircons in two granite plutons and an associated charnockite from within the PCSZ. The weighted mean of206Pb/238U ages for a rapakivi granite from the southern part of the PCSZ yields 819±26Ma (error: 2σ; N=13). On the other hand, a metamorphosed A-type granite and a charnockite from the central part of the PCSZ yield a markedly older upper intercept age of 2468±36Ma (error: 2σ) and a lower intercept age of 588±36Ma (error: 2σ). Our results provide important constraints on the Precambrian crustal evolution in the PCSZ and demonstrate: (1) Neoarchaean to early Palaeoproterozoic magmatism, generating A-type granites which we correlate with a major global crustal formation event; (2) mid Neoproterozoic magmatism, and generation of the rapakivi granite, probably associated with the subduction of the Mozambique Ocean floor; and (3) latest Neoproterozoic high-grade metamorphism (generating the charnockite) associated with the collisional amalgamation of the Gondwana supercontinent. Together with the previous geochronological data from the PCSZ and adjacent crustal blocks, our results provide information on magmatic pulses associated with the Archaean cratonization event, as well as the Neoproterozoic Wilson Cycle of the Mozambique Ocean culminating in continent-continent collision

Subduction-accretion-collision history along the Gondwana suture in southern India: A laser ablation ICP-MS study of zircon chronology

We report the petrological characteristics and preliminary zircon geochronology based on laser ablation ICP mass spectrometry of the various units in an accretionary belt within the Palghat-Cauvery Shear/Suture Zone in southern India, a trace of the Cambrian Gondwana suture. Zircons extracted from a plagiogranite in association with an ophiolite suite within this suture possess internal structure that suggests magmatic crystallization, and yield mid Neoproterozoic206Pb/238U age of 817±16Ma (error: 1σ) constraining the approximate timing of birth of the Mozambique Ocean floor. Compiled age data on zircons separated from a quartzite and metamorphosed banded iron formation within the accretionary belt yields a younger intercept age of 759±41Ma (error: 1σ) which we relate to a mid Neoproteozoic magmatic arc. Detrital zircons extracted from the quartzite yield207Pb/206Pb age peaks of about 1.9-2.6Ga suggesting that they were sourced from multiple protolithis of Neoarchean and Paleoproterozoic. Metamorphic overgrowths on some zircon grains record ca. 500-550Ma ages which are in good harmony with the known ages for the timing of high-grade metamorphism in this zone during the final stage of continent collision associated with the birth of the Gondwana supercontinent in the latest Neoproterozoic-Cambrian. The preliminary geochronological results documented in our study correlate with the subduction-accretion-collision history associated with the closure of the Mozambique Ocean and the final amalgamation of the Gondwana supercontinent

Prograde and retrograde högbomites in sapphirine + quartz bearing Mg-Al rock from the Palghat-Cauvery Suture Zone, southern India.

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Laser ablation ICP mass spectrometry for zircon U-Pb geochronology of metamorphosed granite from the Salem Block: Implication for Neoarchean crustal evolution in southern India

We report here petrological characteristics of metamorphosed granite and metagabbro from the Kanja Malai hills, the southern margin of the Salem Block in southern India. Field observations suggest that both these rock types experienced high-grade metamorphism, and exhibit strong foliation. Many of cores of zircons extracted from the granite show an internal structure suggesting magmatic crystallization. In contrast, rims of the zircons commonly display metamorphic overgrowth. We also present geochronological data utilizing laser ablation ICP mass spectrometry technique on the zircons that show the weighted means of207Pb/206Pb age of 2647 ± 11 Ma (error: 1σ) for the timing of magmatism, and 2442 ± 20 Ma (error: 1σ) for the subsequent high-grade metamorphism. Together with a previous geochronological study on charnockites from the Salem Block, our results also provide an important constraint on the Neoarchean magmatism (2.65-2.53 Ga) in southern India, correlating the magmatism with a major global crust formation event during the Neoarchean