130-4 Contrasting Thermal Regimes in Granulites of the Assam–Meghalaya Gneissic Complex, Northeast India: Insights into Melt Retention and Migration

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism, Part II

Presenting Author:

Suvankar Samantaray

Authors:

Samantaray, Suvankar¹, Ghosh, Anuj², Gupta, Saibal³, Mohanty, William Kumar⁴

(1) Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, Kharagpur, India, India, (2) Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, KHARAGPUR, INDIA, India, (3) Department of Geology and Geophysics, Indian Institute of Technogy, Kharagpur, Kharagpur, West bengal, India, (4) Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, Kharagpur, West Bengal, India,

Abstract:

Granulite facies metamorphism reflects deep crustal processes involving partial melting, melt migration, and melt extraction. The interplay of these processes critically governs the thermal and rheological evolution of orogenic roots. This study examines two spatially co-located but petrologically distinct metapelitic granulites from the Assam-Meghalaya Gneissic Complex, namely the Nongstoin metapelite (SH 34D) and the Shangbangla metapelite (SH 2), which record significantly different peak pressure-temperature (P–T) conditions: 930 ± 50 °C at 7.5 ± 1 kbar for SH 34D and 780 ± 30 °C at 5.5 ± 1 kbar for SH 2. Despite these contrasts, zircon U-Pb geochronology indicates a synchronous metamorphic event. The Nongstoin sample exhibits a peak assemblage of Grt + Opx + Kfs + Pl + Qz + Ilm+ melt, formed well beyond the biotite stability field under (ultra) high temperature ((U)HT) conditions. Limited retrogression and the presence of low modal proportion of hydrous phases suggest efficient melt extraction and minimal melt retention. We interpret these findings as the result of high melt productivity but low melt connectivity and permeability, allowing significant melt escape. This open-system behavior likely produced a dry residuum with elevated thermal conditions. Conversely, the Shangbangla metapelite (SH 2), with a peak assemblage of Grt + Crd + Kfs + Qz + Pl + Sill + melt, equilibrated closer to the solidus and is located within a high-strain zone. This facilitated enhanced melt retention and channelized migration, as evidenced by abundant granitic melt layers, both parallel and cross-cutting the foliation. The presence of retrograde melt-derived hydrous minerals (Bt ± Ms) further supports late-stage melt crystallization and fluid influx. We propose that melts extracted from deeper levels (e.g., the SH 34D source region) migrated into mid-crustal levels such as the SH 2 domain, where they acted as thermal buffers, absorbing heat and moderating local temperatures. These contrasting thermal regimes underscore the critical influence of strain localization, melt connectivity, and structural permeability on melt retention versus extraction during granulite facies metamorphism. While SH 34D evolved into a dry, high-temperature residuum, SH 2 developed into a melt-rich, thermally buffered domain. This highlights the importance of rheological and structural heterogeneities in shaping crustal thermal architecture during orogenesis.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4782

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