Geochronology and geochemistry of the Panjal Traps from the southern Pir Panjal Range, Kashmir, India

Abstract

The eruption of the Panjal Traps is associated with rifting along the Tethyan margin of Gondwana. A CA-ID-TIMS zircon U-Pb age of basalt collected from the southern Pir Panjal Range (SPPR) yielded a weighted-mean 206Pb/238U age of 288.2 ± 0.3 Ma. The rocks from the SPPR can be divided into three compositionally distinct groups (basalt: SiO2 < 52 wt%; Mg# = 56–36, basaltic andesite: SiO2 = 52–60 wt%; Mg# = 32–48, and silicic: SiO2 > 64.0 wt%; Mg# = 27–23) and are chemically correlated with rocks from the eastern Kashmir Valley rather than the northern Pir Panjal Range (NPPR). The Sr-Nd isotopes (87Sr/86Sri = 0.70860–0.71104; εNd(t) = −6.1 to −4.4) of the mafic rocks are similar and indicate that their magmas were derived from a subduction modified subcontinental lithospheric mantle (SCLM) source. The silicic rocks were derived by partial melting of the continental crust (87Sr/86Sri = 0.71022–0.71093; εNd(t) = −8.7 to −7.2) and their discovery in the SPPR indicates that silicic volcanism was far more extensive than previously thought. The 187Os/188Osi isotopic ratios of basalts from different regions of the Panjal Traps range from 0.1230 to 0.2832. Two samples from the NPPR have ratios of 0.1230 and 0.1256 that are slightly higher than primitive upper mantle at the time of eruption whereas the remaining samples show enriched values (0.1343–0.2832). The new data suggest that it is likely that the mantle source of the Panjal Traps transitioned from a SCLM source to a sublithospheric source over time as seafloor spreading developed.