Petrological characterization of the Cryogenian lower crust of southern India: Evidence from charnockites and metapelites from the northern part of Madurai Block

Abstract

<p>The granulite terrane of southern India exposes Precambrian granulites that are dismembered into several crustal blocks with intervening shear/suture zones. Madurai Block forms the largest crustal block, preserving rock record from the Neoarchean to Neoproterozoic, acting as a time capsule to understand the Precambrian crustal evolution in southern India. In this context, magmatic and metamorphic signals acquired from various rock suites in the terrane are crucial as they serve as proxies of changing tectonic style and thermal evolution of crust with time. There is protracted evidence of multiphase magmatism (~2.8, 1.0, 0.8, 0.5Ga) from the metaigneous rocks in Madurai Block. The Cryogenian (~0.8Ga) magmatism specifically is characterized by voluminous A-type granite and anorthosite magmatism reported throughout the block and beyond. Temporally spaced multiphase metamorphic events (~2.5, 1.0, 0.8, 0.6, 0.5Ga) are reported from Madurai Block, wherein, the Ediacaran-Cambrian (~0.5Ga) high-grade metamorphism dominates the metamorphic signals, with sporadic evidence of older metamorphic events. Further, the ~0.5Ga metamorphism has completely obliterated the signals of older metamorphic events in the supracrustal rocks, rendering characterization of older metamorphic events difficult. In this study, we provide the evidence of Cryogenian metamorphic event from a charnockite and metapelites from the Dindigul region in Madurai Block, and for the first time characterize the Cryogenian metamorphism from the lower crustal rocks of southern India.</p><p>The charnockites preserve peak assemblage of Grt+Opx+Qz+Kfs+Pl, wherein the garnet has been replaced by Opx+Pl corona. The associated metapelites preserves peak assemblage of Grt+Sil+Bt+Kfs+Pl+Qz±Ilm±Spl, which has been replaced by retrograde biotite. The peak P-T conditions for both the rock types are constrained at 850±30℃ at 8±0.3kbar, using exchange thermometry and phase-equilibria modeling. The P-T estimates for the corona formation yield similar temperatures (820±30℃) at relatively lower pressures (7.4±0.2kbar). Conventional thermometry utilizing the garnet and retrograde biotite yields P-T condition of 650±30℃ at 6.5±0.3kbar for the retrograde metamorphism.</p><p>The timing of different stages of metamorphism has been constrained using in-situ monazite geochronology from the metapelites. BSE imaging and X-ray elemental maps obtained from individual monazite grains display patchy zonation with three distinct compositional domains which corresponds to distinct age clusters. The Th-poor and HREE enriched dark cores of matrix monazite, yield a weighted mean age of 844±35Ma, dating the prograde evolution prior to or during the initial stages of garnet growth. On the other hand, relatively Th-enriched and HREE poor, cuspate mantles replacing the core, yield a weighted mean age of 815±30Ma, dating garnet growth due to biotite dehydration melting at the peak stage. In contrast, Th and HREE poor fine rims, cross cutting both core and mantle, yield a weighted mean age of 615±35Ma, dating the timing of Ediacaran-Cambrian overprinting. Obtained ages are suggestive of a Cryogenian granulite facies metamorphism in the Madurai Block. The spatially and temporally associated A-type granite and anorthosite magmatism were most likely the heat source. Similar ages obtained from metaigneous rocks from several places within the block shows that Cryogenian granulite facies metamorphism was widespread, although its signal has been substantially overprinted by a higher grade Ediacaran-Cambrian metamorphism.</p>