Newly discovered MORB-Type HP garnet amphibolites from the Indus-Yarlung Tsangpo suture zone: Implications for the Cenozoic India–Asia collision

High pressure (HP) and ultrahigh pressure (UHP) oceanic rocks remain very scarce along the Indus-Yarlung Tsangpo suture zone in the Tibetan Plateau. Such rock types, however, could provide new and important insight into the understanding of the India-Asia collision tectonics and regional metamorphic processes. This contribution focuses on newly discovered HP garnet amphibolites that occur in the North Ayilari range along the Indus suture zone in the Western Himalaya. Petrographic and mineralogical studies as well as pressure–temperature (P–T) calculations show that the garnet amphibolites underwent at least three metamorphic stages. A prograde amphibolite-facies stage at 0.9–1.2 GPa and 605–680°C (M1) was followed by HP granulite-facies metamorphism at 680–750°C and 1.1–1.5 GPa (M2) and cooling to amphibolite-facies metamorphism at 628–650°C, 0.6–0.8 GPa (M3). Zircon and apatite U–Pb dating shows that the garnet amphibolites experienced granulite-facies conditions at 33–32 Ma and a cooling age of 12.5 ± 2.6 Ma at temperatures of 375–550 °C. The garnet amphibolites have light rare earth element (LREE)-depleted distribution patterns, low (87Sr/86Sr)i = 0.70727–0.70833, but high εNd(t) (+8.9 – +9.8) in whole-rock, and high εHf(t) (+13.9 – +16.0) values of zircon, which are similar to those of normal mid-ocean ridge basalt. This confirms that the protolith represents an oceanic slice that dominantly sourced from depleted mantle material. Geochemical, geochronological and geothermobarometrical data demonstrate that the garnet amphibolites represented a part of the Neo-Tethyan ocean, that experienced a profound amphibolite facies metamorphism before reaching HP granulite-facies conditions at ca. 32 Ma as a consequence of crustal thickening during the India–Asian collision. Thus, the studied HP oceanic-type slice represents an important puzzle piece that, together with other continental portions that experienced different P-T conditions such as HP and UHP eclogite and different metapelites and quartzo-feldspathic gneisses, helps to decipher the tectonic evolution of the Western Himalaya orogen, from oceanic subduction to continental collision and subsequent exhumation.