Rift-related origin of the Paleoproterozoic Kuncha Formation, and cooling history of the Kuncha nappe and Taplejung granites, eastern Nepal Lesser Himalaya: a multichronological approach

摘要

In order to decipher the origin and tectonothermal history of the Kuncha nappe, we undertook a geological investigation in the Taplejung window in eastern Nepal, and carried out multichronological analyses of zircon, apatite, and mica of the Kuncha Formation and Taplejung granites. Three granite bodies that intrude into the Kuncha Formation show fission-track (FT) ages of 6.2 to 4.8 Ma for zircon and 2.9 to 2.1 Ma for apatite, although the granites yielded zircon U–Pb ages of 1852 ± 24 Ma and 1877 ± 21 Ma, and muscovite ~(40)Ar–~(39)Ar ages of ca 1650 Ma. The U–Pb intercept age of detrital zircons from the Kuncha schist is 1888 ± 16 Ma, and this age represents a probable depositional age of the lower part of the Kuncha Formation. The FT ages of zircon and apatite from the same schist are found to be 5.4 and 2.5 Ma, respectively. No zircon younger than 1.6 Ga was identified from any of these samples. It means that the Kuncha nappe has never undergone thermal events after 1.6 Ga until the Himalayan orogeny in Miocene. The Kuncha Formation and overlying Kali Gandaki Supergroup as well as the Taplejung granites can be correlated with the Coronation Supergroup and the Hepburn intrusives in the Wopmay orogen, northwest Canada. 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 Kuncha nappe and overlying Crystalline 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 Taplejung suggest that isotherm of 240°C, closure temperature of zircon FT, retreated toward north at about 10 mm/y during the middle to late Miocene and Pliocene.