Miocene orbicular diorite in east-central Himalaya: Anatexis, melt mixing, and fractional crystallization of the Greater Himalayan Sequence

Abstract

The Greater Himalayan Sequence and leucogranite forming the core of the Himalayan orogen provide an excellent natural laboratory in which to study processes related to crustal melting, granitoid formation, and the tectonic evolution of mountain belts. In contrast to most previous studies, which considered the Himalaya-aged granitoids as leucogranites, here we report a Miocene orbicular diorite from the Greater Himalayan Sequence in the east-central Himalaya. The diorite consists of ellipsoidal orbicules in a diorite matrix. The orbicules have a diorite core with or without garnet-sillimanite-biotite schist enclaves, an inner shell of tangentially oriented biotite laths, and an outer shell of radial or plumose plagioclase crystals. The diorite is aluminous and calcic, shows a fractionated rare earth element pattern with a strongly positive Eu anomaly, and has elevated Sr concentration. The schist enclaves underwent high-temperature metamorphism and partial melting under conditions of 9.2–12 kbar and 765–900 °C, followed by a retrograde pressure-temperature path of decompression and cooling. The inherited magmatic cores, metamorphic mantles, and magmatic rims of zircon from the diorite yield a protolith age of ca. 494 Ma, metamorphic ages ranging from ca. 26 Ma to 22 Ma, and melt crystallization ages of ca. 18–14 Ma. The inherited magmatic cores of zircon show variable but mostly negative e Hf ( t ) values, whereas the metamorphic mantles and the magmatic rims of zircon yield variable and lower e Hf ( t ) values. Our study shows that the orbicular diorite is a plagioclase + biotite + cordierite cumulate rock that formed by the fractional crystallization of peraluminous melt, which was generated by mixing of melts derived from the partial melting of both early Paleozoic granitoids and old pelitic rocks. The orbicular diorite and the trapped metapelitic schist have a prolonged high-temperature metamorphic, anatectic, and crystallization history that was initiated at ca. 26 Ma and lasted until ca. 14 Ma. The formation of orbicular structures was probably related to decompression during the ascent of anatectic melt, and subsequent rapid cooling. Melt mixing was also a key factor that aided in the formation of the orbicular structure of the diorite.