Constraints from monazite and xenotime growth modelling in the MnCKFMASH‐PYCe system on the P–T path of a metapelite from Shillong‐Meghalaya Plateau: implications for the Indian shield assembly

Abstract

The Palaeo‐Mesoproterozoic metapelite granulites from northern Garo Hills, western Shillong‐Meghalaya Gneissic Complex (SMGC), northeast India, consist of resorbed garnet, cordierite and K‐feldspar porphyroblasts in a matrix comprising shape‐preferred aggregates of biotite±sillimanite+quartz that define the penetrative gneissic fabric. An earlier assemblage including biotite and sillimanite occurs as inclusions within the garnet and cordierite porphyroblasts. Staurolite within cordierite in samples without matrix sillimanite is interpreted to have formed by a reaction between the sillimanite inclusion and the host cordierite during retrogression. Accessory monazite occurs as inclusions within garnet as well as in the matrix, whereas accessory xenotime occurs only in the matrix. The monazite inclusions in garnet contain higher Ca, and lower Y and Th/U than the matrix monazite outside resorbed garnet rims. On the other hand, matrix monazite away from garnet contains low Ca and Y, and shows very high Th/U ratios. The low Th/U ratios (<10) of the Y‐poor garnet‐hosted monazite indicate subsolidus formation during an early stage of prograde metamorphism. A calculated P–T pseudosection in the MnCKFMASH‐PYCe system indicates that the garnet‐hosted monazite formed at <3 kbar/600 °C (Stage A). These P–T estimates extend backward the previously inferred prograde P–T path from peak anatectic conditions of 7–8 kbar/850 °C based on major mineral equilibria. Furthermore, the calculated P–T pseudosections indicate that cordierite–staurolite equilibrated at ~5.5 kbar/630 °C during retrograde metamorphism. Thus, the P–T path was counterclockwise. The Y‐rich matrix monazite outside garnet rims formed between ~3.2 kbar/650 °C and ~5 kbar/775 °C (Stage B) during prograde metamorphism. If the effect of bulk composition change due to open system behaviour during anatexis is considered, the P–T conditions may be lower for Stage A (<2 kbar/525 °C) and Stage B (~3 kbar/600 °C to ~3.5 kbar/660 °C). Prograde garnet growth occurred over the entire temperature range (550–850 °C), and Stage‐B monazite was perhaps initially entrapped in garnet. During post‐peak cooling, the Stage‐B monazite grains were released in the matrix by garnet dissolution. Furthermore, new matrix monazite (low Y and very high Th/U ≤80, ~8 kbar/850–800 °C, Stage C), some monazite outside garnet rims (high Y and intermediate Th/U ≤30, ~8 kbar/800–785 °C, Stage D), and matrix xenotime (<785 °C) formed through post‐peak crystallization of melt. Regardless of textural setting, all monazite populations show identical chemical ages (1630–1578 Ma, ±43 Ma). The lithological association (metapelite and mafic granulites), and metamorphic age and P–T path of the northern Garo Hills metapelites and those from the southern domain of the Central Indian Tectonic Zone (CITZ) are similar. The SMGC was initially aligned with the southern parts of CITZ and Chotanagpur Gneissic Complex of central/eastern India in an ENE direction, but was displaced ~350 km northward by sinistral mo...

Characterization and metamorphic evolution of Mesoproterozoic granulites from Sonapahar (Meghalaya), NE India, using EPMA monazite dating

“…The Proterozoic metasedimentary rocks and the basement gneisses are composed of amphibolite-to granulite-facies rocks derived from Neoarchaean-Palaeoproterozoic gneissic rocks (Chatterjee, 2017 and references therein). The Sonapahar area is composed of granulite-facies metapelites and quartzofeldspathic gneisses, including sillimanite-bearing gneisses, basic granulites, amphibolites and granite gneisses (Lal et al 1978;Nandy, 2001;Dwivedi & Theunuo, 2013, 2017. However, the Sonapahar area has a significant occurrence of metapelitic granulites and has experienced multiple phases of deformation (Chatterjee et al 2007;Chatterjee, 2017).…”

“…The Sonapahar area is composed of granulite-facies metapelites and quartzofeldspathic gneisses, including sillimanite-bearing gneisses, basic granulites, amphibolites and granite gneisses (Lal et al 1978;Nandy, 2001;Dwivedi & Theunuo, 2013, 2017. However, the Sonapahar area has a significant occurrence of metapelitic granulites and has experienced multiple phases of deformation (Chatterjee et al 2007;Chatterjee, 2017). There are three stages of metamorphism occurring in the metapelites, and they were metamorphosed along a counter-clockwise P-T path (Chatterjee et al 2007).…”

“…Ghosh & Saha (1954) and Lal et al (1978) reported sapphirine-bearing granulites from Sonapahar. The exposure of the basement rocks in the SMGC is limited; thus, granulite-facies rocks are reported only from some parts of the plateau, namely, the Sonapahar area (Lal et al 1978;Chatterjee et al 2007;Dwivedi, 2011;Dwivedi & Theunuo, 2013, 2017, Goalpara Hills (Chatterjee et al 2007(Chatterjee et al , 2011 and Patharkhang (Dwivedi & Theunuo, 2011).…”

“…The evolutionary records of granulites are preserved in the form of different mineral assemblages and associations in different rock types, fabrics and textural relationships. The EPMA monazite dating of the metapelitic granulites of the SMGC reveals predominantly Mesoproterozoic ages of 1621-1596 Ma for a post-S 1 /pre-S 2 stage of metamorphism, with poorly constrained 1141-946 Ma ages and more significant ages of 649-524 Ma, which are considered syn-S 2 and post-S 2 stages of metamorphism, respectively (Chatterjee et al 2007(Chatterjee et al , 2011Chatterjee, 2017). U-Pb zircon dating of granite gneiss basement rocks from the central SMGC yielded Neoarchaean to Neoproterozoic ages, which range from ∼2600 to ∼1100 Ma (Bidyananda & Deomurari, 2007;Kumar et al 2017).…”

Two Tertiary metamorphic events recognized in high‐pressure metapelites of the Nevado‐Filábride Complex (Betic Cordillera, S Spain)

“…In garnet-bearing metapelites, monazite ages have been successfully related to the growth of garnet (e.g. Chatterjee, 2017;Massonne, 2016a;Regis, Warren, Mottram, & Roberts, 2016).…”