Omphacite breakdown, symplectite formation and carbonate metasomatism in a retrograded continental eclogite: Implications for the exhumation of the Tso Morari Crystalline Complex (Trans-Himalaya, NW India)

Abstract

The India-Eurasia collision zone in eastern Ladakh, India, is marked by the Tso Morari Crystalline Complex (TMCC), which underwent continental subduction, eclogite grade metamorphism, and rapid exhumation during the Himalayan orogeny. The TMCC is characterized by eclogitic enclaves, metamorphic evolution of which has been extensively studied in the past. Detailed petrography, mineral chemistry, Raman spectroscopy, and P-T-X pseudosection modelling of two such eclogite samples from the core and western margin of TMCC were carried out to address the differences and discrepancies from previous studies. Our study suggests peak metamorphism at ∼2.75 ± 0.1 GPa and ∼550 ± 50 °C, followed by a decompression stage at ∼1.50 ± 0.1 GPa and ∼580 ± 50 °C and subsequent high-temperature overprint at ∼0.85 ± 0.1 GPa and ∼680 ± 50 °C. Fine-grained grain boundary omphacite + garnet + amphibole symplectite colonies indicate exhumation along with fluid infiltration. The transition of omphacite into a more calcic variety and its subsequent replacement with amphibole are also observed. Presence of diopside + plagioclase symplectite indicates post-peak decompression and fluid infiltration. Carbonates in the matrix as veins and within fractures indicate late-stage fluid infiltration and metasomatism. Our study shows that lawsonite and/or Na-amphibole have not played any significant role in the metamorphic evolution of TMCC. In contrast to most of the previous studies carried out in TMCC, we opine that omphacite breakdown and metasomatism through externally derived carbonaceous fluids played a vital role in the exhumation and retrogression. We further opine that our mineral chemistry data and metamorphic modelling resemble a continental eclogite and mark differences from oceanic eclogites.