Metamorphic <i>P</i>–<i>T</i>–<i>t</i>–<i>d</i> evolution of the Mesoproterozoic Pur‐Banera supracrustal belt, Aravalli Craton, northwestern India: Insights from phase equilibria modelling and zircon–monazite geochronology of metapelites

Abstract

Abstract The Pur‐Banera belt of metasedimentary rocks, in the Bhilwara region of the Aravalli Craton, northwestern India, constitutes a Mesoproterozoic rift‐related supracrustal sequence. The Pur‐Banera belt, while containing psammitic, calcareous, and ferruginous metasediments, is dominated by garnet–staurolite–kyanite‐bearing metapelites. Here we integrate data on outcrop‐ and microscopic‐scale deformation structures with pseudosection modelling and zircon–monazite geochronology to decipher the P–T–t–d evolution of these metapelites. The metapelites record three distinct deformation events. The first two (D 1 –D 2 ) produced a composite S 1 //S 2 foliation, and a younger event of dextral shearing (D 3 ) produced crenulations superimposed on the foliation. Textural and compositional data indicate two stages of growth of garnet, whose core compositions were significantly altered during peak metamorphism. The textures combined with pseudosection modelling and conventional thermobarometry reveal that the garnet cores continued to grow up to staurolite‐grade prograde metamorphism, followed by a second stage of garnet growth (syn‐D 2 ; 7.3–7.9 kbar and 665–690°C) during kyanite‐grade peak metamorphism. Fibrolite grew in the matrix during post‐peak readjustments due to uplift and decompression, whereas fluid‐induced retrograde metamorphism (480–540°C) resulted in the formation of euhedral staurolite prisms and narrow rims around garnet porphyroblasts. Detrital zircons of magmatic origin yield a weighted mean 207 Pb/ 206 Pb age of 1,827 ± 7 Ma (95% confidence) indicating a Palaeoproterozoic igneous source rock for the sediments, consistent with a Mesoproterozoic (1.6–1.3 Ga) age of deposition as inferred in previous work on the Pur‐Banera quartzofeldspathic gneisses. Monazite U–Th–(total) Pb ages indicate the timing of commencement of prograde metamorphism at c . 1.3 Ga, peak metamorphism at c . 1.2–1.1 Ga, and fluid‐induced retrograde metamorphism at c . 0.85–0.75 Ga. Our combined results indicate that the metapelites experienced two deformation events (D 1 and D 2 ) during the closure of the Pur‐Banera basin and the consequent burial of the sediments to lower crustal depths. These were followed by post‐collisional uplift and exhumation, and fluid‐induced retrograde metamorphism during D 3 shearing. Southwestward tilting of the rocks during the D 3 event, and subsequent erosion, have exposed rock units in the western, central, and southeastern parts of the belt which were buried deeper than those in the southwestern parts. This explains the observed northeastward increase in the metamorphic field gradient along the NE–SW strike of the belt. These new insights into the Meso‐ to Neoproterozoic tectonometamorphic evolution of the Pur‐Banera belt help relate it to the amalgamation and fragmentation of the supercontinent Rodinia.

Topics & Concepts