Neoarchean–Paleoproterozoic multiple tectonothermal events in the western Alxa block, North China Craton and their geological implication: Evidence from zircon U–Pb ages and Hf isotopic composition

Abstract

The Alxa block was traditionally considered to be part of the North China Craton, but its metamorphic basement has been poorly studied. Here we present a systematic zircon U–Pb and Hf isotopic investigation on four orthogneiss samples in the Beidashan area of the western Alxa block. The petrographic and geochemical data show that these rocks are granodioritic and trondhjemitic gneisses with TTG (tonalite–trondhjemite–granodiorite) characteristics. Zircons from the TTG gneisses display typical core–rim or core–mantle–rim structures. U–Pb datings and Hf isotopic analyses reveal two distinct age populations: the Latest Neoarchean (∼2.5Ga) and the Late Palaeoproterozoic (∼1.85Ga). The magmatic zircon cores and metamorphic mantles (rims) of the TTG gneisses were dated at similar ages around 2.5Ga, supporting the existence of Archean rocks in the western Alxa block. The short time interval between the Latest Neoarchean magmatism and the subsequent metamorphism suggests that they were related to the same Latest Neoarchean tectonothermal event. The ∼2.5Ga zircons have ɛHf(t) mainly between 0.8 and 5.0, TDM (Hf) model ages mainly between 2.6 and 2.8Ga (with a peak at ∼2.7Ga) and TDMC (Hf) model ages mainly between 2.7 and 3.0Ga (with a peak at ∼2.8Ga). The age of ∼1.85Ga obtained from two trondhjemitic gneisses is interpreted as the age of the Late Paleoproterzoic high-grade metamorphism. Our combined datasets show that the TTG gneisses in the Baidashan area of the western Alxa block experienced a main 2.7–2.8Ga crust growth, a ∼2.5Ga magmatic–metamorphic event and a ∼1.85Ga high-grade metamorphic event. The sequence of events is very similar to that of the other North China Craton. A Combination of the data of Paleoproterozoic metamorphic rocks in the Alxa block suggests that the Alxa block is the western extension of the Khondalite Belt rather than the Yinshan block.