Tracing the Transhimalayan magmatic belt and the Lhasa block southward using zircon U–Pb, Lu–Hf isotopic and geochemical data: Cretaceous – Cenozoic granitoids in the Tengchong block, Yunnan, China

Abstract

A number of Cretaceous – Cenozoic granitoid bodies in the Tengchong block were analyzed for their U–Pb zircon ages, Lu–Hf zircon isotopic compositions and whole-rock geochemistry. Zircons from six different granite bodies in the central Tengchong block have weighted mean 206Pb/238U ages ranging from 76 to 65Ma, and negative εHf(t) values ranging from −2.7 to −15.5 with TDMC ranging from 1312 to 1928Ma. One dextrally sheared and gneissic granodiorite sample of the Nabang shear zone in the western Tengchong block yielded a weighted mean 206Pb/238U age of 19.7±0.4Ma, and positive εHf(t) values ranging from +0.2 to +10.7 with TDMC ranging from 418 to 1092Ma. The late Cretaceous granites have metaluminous and peraluminous affinities with strong enrichment of U and Th, and significant negative anomalies of Eu, Ba, Sr, Nb, P, and Ti. The Miocene granodiorite displays strong negative anomalies of Nb, Ta, P, and Ti and a weak negative Eu anomaly. The data suggest that the late Cretaceous granites in the Tengchong block were derived from partial melting of ancient, most likely Proterozoic continental crustal material whereas the Miocene gneissic granodiorite was derived from partial melting of juvenile crustal material possibly within a supra-subduction setting and intruded synkinematically into the active Nabang shear zone.Lu–Hf-isotopic data of late Cretaceous – Paleocene granites are indistinguishable from those of granites of the Central Lhasa block. Thus, we tentatively correlate the late Cretaceous granites of the Tengchong block with those of the Central Lhasa block. The boundary between the South and Central Lhasa blocks in western Tengchong may be exposed in the NNE-striking, dextral Nabang shear zone along the Chinese-Myanmar border region, which hosts early Eocene granitoids with εHf(t) values >0. The Lu–Hf-data of the zircons from the gneissic Miocene granodiorite indicate a juvenile crustal substrate in the western Tengchong block as well. The synkinematic Miocene granodiorite provides an important age constraint for dextral shear at mid-crustal levels, which was active until at least c. 19–20Ma in the Nabang shear zone. This Miocene intrusion may be a first indication that active margin tectonics lasted 20–25Ma longer than in the Gangdese belt and possibly was associated with deformation partitioning in the hinterland produced by increasing obliquity of subduction during clockwise rotation of Sundaland during the India–Asia collision.