Mineralogy and petrology of the mafic Panjal Traps, Kashmir, India: constraints on magmatic conditions and post-emplacement metamorphism

The Early Permian (c. 290 Ma) Panjal Traps is the most extensive sequence of flood basalts within the Tethyan domains of the Himalaya. The Panjal Traps erupted during a period of tensional plate stress related to the rifting of Cimmerian terranes from the Tethyan margin of Gondwana. The majority of the mafic Panjal Traps were affected by postemplacement low-temperature deuteric alteration and/or regional deformation. Consequently, constraining the magmatic conditions of the rocks is difficult. The least altered Panjal Traps are located within the Guryal Ravine section of the western Zanskar Range and the southern Pir Panjal Range. The rock and mineral textures are preserved and they have a primary mineralogy of clinopyroxene (Wo_30.3-42.5En_29.4-49.7Fs_13.8-34.2) and plagioclase (An_61.0-43.5) phenocrysts within an aphanitic matrix. Secondary minerals include chlorite, epidote, actinolite, quartz, albite, orthoclase, rutile, and titanite and indicate that some of the rocks underwent greenschist to sub-greenschist facies metamorphism. Clinopyroxene-liquid geothermobarometers were used to assess the equilibrium crystallization temperature and pressure of the least altered Panjal Traps. The clinopyroxene-liquid saturation conditions yielded temperatures of 1104-1184 °C and pressures of 1.8-7.0 kbar which are within the uncertainty of the jadeite-diopside-hedenbergite exchange thermometer (1064-1167 °C) and the Al exchange barometer (1.1-6.8 kbar). The equilibrium temperatures are not anomalously high and similar to lavas that erupt at a passive rift setting rather than from a mantle plume. The whole rock V/Sc and V/Ga ratios suggest that the oxygen fugacity of the lavas was likely at or below the fayalite-magnetite-quartz buffer (ΔFMQ = 0 to –1). Rhyolite-MELTS modeling (ΔFMQ = –1; P = 2 kbar) indicates the water content within the basalts was variable and probably did not exceed 2.25 wt% prior to eruption. The post-emplacement metamorphic conditions are constrained using Perple_X modeling and demonstrate that the rocks from both Guryal Ravine and southern Pir Panjal Range (P = < 3.0 kbar and T = 390 – 415 °C) underwent greenschist facies metamorphism at similar temperature. However, the pressure is not well constrained. We attribute the metamorphism to regional deformation associated with terrane accretion to the Indian plate during the Mesozoic and Cenozoic that occurred after the deposition of the Late Permian to Early Triassic Pangea megasequence but before the Oligocene.