Metamorphism of the Central Bundelkhand Greenstone Complex, Indian Shield: Mineral Compositions, Paragenesises, and P–T Path

Abstract

The paper presents data acquired on epigenetic processes in rocks of the Meso- to Neoarchean greenstone complex of the Bundelkhand Craton of the Indian Shield. The study was focused on felsic and mafic metavolcanic and mafic–ultramafic rocks of the Central Bundelkhand Greenstone Complex (CBGC). The rocks contain metamorphic mineral assemblages produced during an earlier high-temperature [Grt–Prg–Pl; Cpx–Prg–Pl; Cpx–Grt–Pl; Cpx–Prg (± Ksp, Qz, Mag)] and later retrograde [Grt–Pl–Hbl; Grt–Chl–(Hbl–Act); Grt–(Zo–Czo)–(Hbl–Act); Grt–Chl–Pl (± Ksp, Qz, Mag, Ms)] episodes of metamorphism. The retrograde metamorphism was associated with the origin of zoned Ep and Qz–Ep (±Grt, Act) veins, patches, stripes, and small bodies of epidosites and actinolites. The mineral assemblages of the final episodes of the rock transformations contain prehnite and pumpellyite (Prh–Pmp–Alb; Prh–Pmp–Grs; Pmp–Grs–Alb; Prh–Cc–Act; and Prh–Grs–Cc). The composition of the garnet and amphibole was determined to broadly vary (Alm–Sps to Grs and Prg–Ts to Act and Cum, Gru, respectively). The garnet is contrastingly zoned (comprises three zones), with the boundaries of the zones marking drastic changes in the crystallization parameters (episodes of transformations). The identified sequence of the epigenetic events was as follows: amphibolite-facies metamorphism under elevated pressures (580–680°C at 7.2–10 kbar), with a clockwise P-T-t path and retrogression to the greenschist facies → Ca(Mn) metasomatism → metamorphism to the prehnite–pumpellyite facies. The metamorphic evolution of the CBGC rocks in the Bundelkhand Craton is in good agreement with the geodynamic model of cratonic crustal growth, with Archean subduction–accretion processes and Paleoproterozoic intraplate rifting.

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