Zircon and monazite geochronology of the granulites and associated gneisses from the Rengali Province, India: Growth of the southern margin of the Singhbhum Craton

Geological evidences show that cratons grew in size by multiple orogenic cycles which can best be studied in their marginal orogenic belts. Rengali Province, occurring at the southern margin of the Singhbhum Craton, India is one such belt, characterized by low- to high-grade metamorphic signatures. This is sharply different from the virtually unmetamorphosed cratonic rocks in north and ultrahigh temperature metamorphosed rocks of the Eastern Ghats Province in south. High-grade gneisses and granitoids (including charnockite) constitute the central highland of this province. Several large-scale faults and shear zones juxtapose rocks of contrasting metamorphic grades. Our geochronological investigation from samples of high-grade orthogneiss and paragneiss along with the intrusive granitoids from central and eastern part of the Rengali Province using zircon (SHRIMP) and monazite chemical techniques reveal a complex evolutionary history. Zircon grains of the metapelitic granulites from the eastern segment yield detrital ages in the time span of ca. 3528-3087 Ma, without perceptible evidence of subsequent metamorphism. In contrast, monazite grains from the same samples record strong metamorphic signature at ca. 2.8-2.7 Ga along with inherited populations of ca. 3.0-2.9 Ga. Charnockite from the eastern segment has protolith age of 3058×15 Ma while that from the central segment records high-grade metamorphism at 2818×15 Ma. Mafic granulite from the central segment preserves the oldest core age of 2844×7 Ma. Synkinematic leucogranite in the central and undeformed granitoid in the eastern segment were emplaced at 2807×13 Ma and 2809×13 Ma respectively. Most of the samples show zircon overgrowth at ca. 2500 Ma, which possibly caused by a separate tectonothermal overprinting. From the available geochronological data, we postulate the existence of marine basin which opened at ca. 3050 Ma, received sediments from the adjacent cratonic hinterland, switched to an active continental margin setting at ca. 2820 Ma before being cratonized at ca. 2800 Ma during the Rengali orogeny (ca. 2820-2800 Ma). Sedimentation in this basin appeared to be short-lived (~ 130 Ma). The geochronological history of the Rengali Province thus bears the signatures of crustal growth of Singhbhum Craton during the Neoarchean time which is similar to that of the North China Craton. Distinct monazite growth at ca. 500 Ma in leucogranite further implies rejuvenation of the continental margin during the Pan African orogeny. Interestingly, the studied craton-margin rocks were not affected by the tectonothermal pulses of the Meso-Neoproterozoic hot orogeny (1000-900 Ma) of the presently adjacent Eastern Ghats Province.