Zircon U–Pb geochronology and Hf isotope of granitoids in East Kunlun: Implications for the Neoproterozoic magmatism of Qaidam Block, Northern Tibetan Plateau

摘要

The Neoproterozoic granitoids in East Kunlun Orogen (EKO) record critical information on the composition and evolution of the early crustal materials, as well as the Precambrian tectonic evolutionary history of the Qaidam Block, northern Tibetan Plateau. Zircon U–Pb geochronology and Hf isotopic compositions of three newly discriminated gneissic granitoids in the Hongshuihe, Dangennaomuru and south Bokalike areas are employed in this study to elucidate the Neoproterozoic tectonics of EKO. Petrological studies reveal that these granitoids are S–type granite with abundant Al–rich minerals like muscovite and garnet in their mineral assemblages. Zircon U–Pb geochronological results show that the Hongshuihe and Dangennaomuru plutons yield weighted mean206Pb/238U ages of 914?±?5?Ma (MSWD?=?0.25) and 935?±?6?Ma (MSWD?=?0.56), respectively, indicating their magmatic crystallization ages. And the south Bokalike pluton yields upper and lower intercept ages of 1002?±?31?Ma and 388?±?39?Ma (MSWD?=?0.63), disclosing its magmatic crystallization age and the time of subsequent Paleozoic metamorphic event, respectively. TheεHf(t) values of zircons in this study are dominated by negative with minor positive values ranging from ?7.18 to +2.76. The two–stage Hf model ages of the zircons having negativeεHf(t) values are ca. 1.7–2.0?Ga, whereas those with positiveεHf(t) values are ca. 1.5?Ga. Both theεHf(t) values and the two–stage Hf model ages indicate that these granitoids were probably derived from the reworking of ancient continental crustal materials with minor mantle contribution. Combined with available data of the granitoids in EKO, central and north Qaidam Block, we propose that the Neoproterozoic granitoids in Qaidam Block have similar crystallization ages of ca. 0.9–1.0?Ga, as well as same or similar geochemical and zircon Hf isotopic compositions revealing the Qaidam Block was involved into the amalgamation of the supercontinent Rodinia.