INVESTIGATING THE METAMORPHIC EVOLUTION, WATER-ROCK INTERACTION, AND GEOCHEMISTRY OF TSO MORARI UHP TERRANE, NW HIMALAYA

Collision of the Indian and Eurasian plates caused the uplift of the Tibetan Plateau, triggered volcanic eruptions and earthquakes, and led to global climate change. Accordingly, the Himalayan orogenic belt marks one of the largest sites of elemental cycling, crust-slab-mantle interactions, and areas of ultrahigh pressure (UHP) terranes on Earth. This study aims to test the hypothesis that some microcontinents were subducted to mantle depths surrounded by oceanic crust prior to the large scale Indo-Asian continental collision by looking for clues in the mineralogy and geochemistry of exhumed UHP rocks from the Tso Morari terrane, NW India.

The subduction and exhumation of small masses of continental crust can occur during oceanic subduction prior to final continental collision (Kylander-Clark et al. 2012). Fluids released by devolatilization of altered oceanic crust and sediments will interact with the subducted microcontinent in the subduction zone (Menold et al. 2016). Recrystallization at UHP depths and during exhumation will occur in the presence of these fluids, imparting a distinct geochemical signature. We analyzed a traverse across an eclogite and granitic gneiss from the Tso Morari to search for such a signal. The whole-rock geochemical data show that LILE (e.g. K, Rb, Cs, Sr, Ba) are enriched along the traverse and the Ba/Rb and Cs/Rb ratios are relatively lower at the eclogite-gneiss contact. These findings are consistent with fluid mediated exchange. EPMA analyses reveal garnet zonation in the eclogite: the X_Fe+Mn ((Fe+Mn)/(Fe+Mn+Mg)) drops from 83-90 to ~ 60 while the X_Mg (Mg/(Mg+Fe²⁺)) increases from ~10 to ~30 from the core to rim. Fe isotope analyses along the traverse also indicate interaction with a fluid at depth (Macris et al. 2016).

In order to estimate the P-T history of the rocks, we constructed metamorphic pseudosections using the whole rock data. Based on mineral assemblage, garnet growth zones and mineral inclusions, and pseudosection calculations, we determined that the eclogite experienced peak metamorphism at ~25 kbar and 575-625 °C. Future work will investigate these samples for evidence of fluid interaction at high P and T.

Kylander-Clark et al., Earth Planet Sci. Lett. 321, 115-120 (2012)

Macris et al., GSA Abstract with Programs 48, 7 (2016)

Menold et al., Earth Planet Sci. Lett. 446, 56-67 (2016)