Petrology of blueschist from the Western Himalaya (Ladakh, NW India): Exploring the complex behavior of a lawsonite-bearing system in a paleo-accretionary setting.

Although the Himalaya is the archetype of collisional orogens, formed as a consequence of the closure of the Neo-Tethyan ocean separating India from Asia, high-pressure metamorphic rocks are rare. Beside few eclogites, corresponding to the metamorphosed continental Indian crust dragged below Asia or underthrusted beneath southern Tibet, blueschists occur seldom along the Yarlung-Tsangpo Suture zone, i.e. the suture marking the India-Asia collision. These blueschists, mostly interpreted as related to paleo-accretionary prisms formed in response to the subduction of the Neo-Tethyan ocean below the Asian plate, are crucial for constraining the evolution of the India-Asia convergence zone during the closure of the Neo-Tethyan Ocean. In the Western Himalaya, the best occurrence of blueschist is that of the Sapi-Shergol Ophiolitic Mélange in Ladakh. This unit is dominated by volcanoclastic sequences rich in mafic material with subordinate interbedding of metasediments, characterized by very fresh lawsonite blueschist-facies assemblages. In this paper, the lawsonite blueschist-facies metasediments have been petrologically investigated with the aims of (i) constraining the P-T evolution of the Sapi-Shergol Ophiolitic Mélange, (ii) evaluating the influence of Fe2O3 and of H2O on the stability of the high-pressure mineral assemblages, (iii) understanding the processes controlling lawsonite formation and preservation, and (iv) interpreting the P-T evolution of the Sapi-Shergol blueschists in the framework of India-Asia collision. Our results indicate that (i) the Sapi-Shergol blueschists experienced a cold subduction history along a low thermal gradient, up to peak conditions of ca. 470 °C, 19 kbar; furthermore, in order to preserve lawsonite in the studied lithologies, exhumation must have been coupled with significant cooling, i.e. the resulting P-T path is characterized by a clockwise hairpin loop along low thermal gradients (< 8-9 °C/km); (ii) the presence of ferric iron in the investigated system has a non-negligible (lowering) effect on pressure estimates, whereas temperatures estimates are not influenced by the oxidation state of the system; (iii) the observed sequence of mineral growth (i.e. simultaneous growth of lawsonite and garnet) suggests that (a) the system was initially H2O-undersaturated and lawsonite growth was triggered by a protracted H2O influx at high pressure (equilibrium approach), or (b) the system was H2O-saturated since the beginning, but lawsonite growth was delayed due to the predominance of kinetic factors over equilibrium dynamics (nonequilibrium approach); (iv) the inferred P-T evolution is consistent with a cold subduction zone system in an intra-oceanic subduction setting. Moreover, the estimated peak P-T conditions roughly coincide with the maximum P-T estimates predicted by thermo-mechanical models for the metasediments exhumed in accretionary wedges, and with the maximum P-T conditions recorded by natural occurrences of blueschist accretionary complexes worldwide.