Rift‐related origin of the P aleoproterozoic K uncha F ormation, and cooling history of the K uncha nappe and T aplejung granites, eastern N epal L esser H imalaya: a multichronological approach

Abstract In order to decipher the origin and tectonothermal history of the K uncha nappe, we undertook a geological investigation in the T aplejung window in eastern N epal, and carried out multichronological analyses of zircon, apatite, and mica of the K uncha F ormation and T aplejung granites. Three granite bodies that intrude into the K uncha F ormation show fission‐track ( FT ) ages of 6.2 to 4.8  M a for zircon and 2.9 to 2.1  M a for apatite, although the granites yielded zircon U – P b ages of 1852 ± 24  M a and 1877 ± 21  M a, and muscovite 40 A r– 39 A r ages of ca 1650  M a. The U – P b intercept age of detrital zircons from the K uncha schist is 1888 ± 16  M a, and this age represents a probable depositional age of the lower part of the K uncha F ormation. The FT ages of zircon and apatite from the same schist are found to be 5.4 and 2.5  M a, respectively. No zircon younger than 1.6  G a was identified from any of these samples. It means that the K uncha nappe has never undergone thermal events after 1.6  G a until the H imalayan orogeny in M iocene. The K uncha F ormation and overlying K ali G andaki S upergroup as well as the T aplejung granites can be correlated with the C oronation S upergroup and the H epburn intrusives in the W opmay orogen, northwest C anada. They are interpreted to be deposits in the basins of continental rift system and subsequent passive‐margin settings. All the FT age data of zircon and apatite provide evidence that both the K uncha nappe and overlying C rystalline nappe seem to have cooled laterally from the nappe front to the north. The FT ages from the front to central part of the nappe in T aplejung suggest that isotherm of 240° C , closure temperature of zircon FT , retreated toward north at about 10 mm/y during the middle to late M iocene and P liocene.