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

Abstract

This paper presents three different age domains, obtained by electron microprobe monazite dating, for granulitic gneisses collected from the Shillong-Meghalaya Gneissic Complex in Sonapahar, NE India, which contain radioactive materials, e.g. thorium (3.32–7.20 wt %), uranium (0.133–1.172 wt %) and lead (0.101–0.513 wt %). The microprobe analyses of monazite grains in the rock samples show that the monazites have three different ages ranging from Mesoproterozoic to Neoproterozoic. The oldest age (1571 ± 22 Ma) represents a peak metamorphic event, the youngest dominant age indicates the Pan-African tectonic event (478 ± 7 Ma) and the intermediate age marks the Grenvillian orogeny (1034 ± 91 Ma) or may be a mixing artefact; these ages are located at the cores, rims and intermediate parts of the monazite grains, respectively. The equilibrium mineral phases calculated for the granulitic gneisses from Sonapahar lie in a P–T range from 5.9 kbar/754 °C to 8.3 kbar/829 °C in the NCKFMASH system. Plotting the P–T conditions of the granulitic gneisses reveals a clockwise P–T path. Two major metamorphic events are observed in Sonapahar. The M1 metamorphic stage is represented by peak mineral assemblages of prograde garnet-forming reactions (8.2 kbar/∼713 °C) during Mesoproterozoic time (1571 ± 22 Ma). The M2 metamorphic stage featured decompression (3.9 kbar/∼701 °C) in which garnet–sillimanite broke down to form cordierite along an isothermal decompression path during the Pan-African tectonic event (478 ± 7 Ma).

Geochemistry of meta‐mafic and meta‐tonalite‐trondhjemite intrusives from Jaintia and Karbi Anglong hills of Shillong Plateau, North East India: Implications on the evolution of the Proterozoic Shillong Basin

“…Nonetheless, the Na and K‐rich transitional trend (Figure 8c) and its intermediate characteristics between Archaean TTG/Adakite and BADR (Figure 11a–d) for the Umnem—Laru MTT, suggest that it may have originated under low to medium pressure between 10–15 kbar (Figure 11c,d) similar to the medium‐pressure TTG group with some adakite affiliations (Figure 10f). These observations support crustal thickening (Dwivedi et al, 2020; Dwivedi & Theunuo, 2011) followed by collapse and exhumation possibly related to the Pan African Orogeny. Furthermore, post‐Archaean TTG‐like suites, Adakites, and other tonalitic rocks as is the case with MTT, are usually diagnostic of arc tectonic setting or, continental active margin (e.g., Condie, 2005; Martin et al, 2005; Moyen & Martin, 2012; Winter, 2014).…”

Mineralogical Constraints on the Pressure–Temperature Evolution of Granulites in the Bunger Hills, East Antarctica

“…Along with field relations and geochronology, mineral evidence (rock microstructures, mineral parageneses, mineral chemistry, zoning in minerals, etc.) gives an answer to a question concerning P-T path continuity or discontinuity which is very important for deciphering metamorphic history [1][2][3][4][5][6][7][8][9] In this article, we discuss the essential features of the metamorphic history of granulites in the Bunger Hills of East Antarctica deduced from petrography, mineral chemistry, and thermobarometry as well as phase equilibrium modeling. Many researchers consider the Bunger Hills to be a former westernmost part of the Albany-Fraser Orogen [10][11][12] ( Sheraton et al, 1993;Clark et al, 2000;Morrissey et al, 2017), which was positioned along the southeastern margin of the Archaean Yilgarn Craton in Western Australia (Figure 1) and was formed in the late Mesoproterozoic during Rodinia assembly [13,14] (Smits et al, 2014; Aitken et al, 2015).…”

Field and Petrographic Studies of the Guwahati Metapelites, Assam, NE India: Evidence for Post- Metamorphic K-Metasomatism

“…The U-Th-Pb total monazite, xenotime and zircon geochronology provides a powerful technique to elucidate the timing of thermal events for establishing their relationship with regional and global tectonic processes. For this reason, they have been subjected to extensive petrological, chronological, geochemical and tectonic scrutiny by several workers [1][2][3][4][5][6][7][8][9][10]. However, the petrological, thermochronological history of the Guwahati metapelites [3] is somewhat cloudy and it is obscure whether these high-grade metapelites are a continuation of those situated towards the south in Meghalaya [4,5,7,8].…”