Fluid evolution during burial and exhumation of the Tso Morari UHP complex, NW India: Constraints from mineralogy, geochemistry, and thermodynamic modeling

Abstract

The Tso Morari terrane within the Himalayan orogenic belt underwent ultrahigh-pressure (UHP) coesite-eclogite metamorphism due to northward subduction of the Indian continent under the Eurasian continent during the early Eocene. The Tso Morari UHP terrane has been intensely studied petrologically, mineralogically, and geochemically over the past several decades. However, the fluid history (e.g., phases and pressure–temperature conditions, fluid compositions and sources, and processes of fluid–rock interactions) and thermal structure during exhumation remain unresolved. To address these issues, we sampled a traverse from the center of an eclogite boudin out into the host orthogneiss. Three major fluid evolution stages (FESs) were identified and characterized using petrography, mineral and bulk-rock chemistry, and thermodynamic modeling. FES 1 constrained mineral dehydration and hydration reactions during prograde metamorphism before reaching peak pressure at 29.0 ± 0.8 kbar and 591 ± 9 °C by modeling garnet growth in the eclogites. FES 2 constrained mineral reactions in the eclogite matrix due to destabilization of internal hydrous minerals. This FES caused the formation of epidote at 22.8 ± 0.6 kbar, amphibole core domains (glaucophane) at 19.0 ± 0.4 kbar, amphibole rim domains (barroisite) at 14.5 ± 1.0 kbar, and symplectite at 9.0 ± 1.0 kbar, during isothermal decompression (600–650 °C). FES 3 caused amphibolization of eclogite at the boudin rim at 625 ± 50 °C and 9.0–14.0 kbar. Metasomatism resulted in increased K2O, CO2, and bulk-rock Fe3+/ΣFe in the amphibolized eclogites. Large ion lithophile elements (LILE) (e.g., K, Rb, Cs, Sr, Ba) and trace element ratios of Ba/Rb and Cs/Rb are also elevated relative to the eclogite core. The fluid most likely originated from dehydrating host orthogneiss and/or metasediments. Thermodynamic modeling also predicts that the Tso Morari complex was exhumed through a low-temperature (< 650 ± 50 °C) regime in the subduction channel