The Paleoproterozoic diorite dykes in the southern margin of the North China Craton: Insight into rift-related magmatism

Abstract

The Xiong’er rift zone along the southern margin of the North China Craton (NCC) is characterized by exposure of a suite of volcanic rocks and associated mafic – intermediate dykes and sedimentary rocks. In this study, we report for the first time the newly identified late Paleoproterozoic diorite dykes from the Waifangshan Mountain in the Xiong’er rift zone and present results from the LA-ICP-MS zircon U–Pb dating, Sr–Nd–Hf isotope systematics, and whole-rock major and trace element geochemistry. The Wafang diorite dykes intruding into the upper part of the Xiong’er Group yielded emplacement mean ages of 207Pb/206Pb from 1746±22Ma to 1762±11Ma. These diorite dykes have extreme low MgO, Cr and Ni contents, and enriched in LREEs and LILEs but depleted in HFSEs (Nb, Ta, and Ti). They are characterized by having negative zircon ɛHf(t) values of −11.4 to −3.1, a whole-rock initial 87Sr/86Sr value of 0.70196 and ɛNd(t) value of −8.7, and Pb isotopes (206Pb/204Pb=16.072–16.295, 207Pb/204Pb=15.275–15.293, 208Pb/204Pb=36.538–37.255). These geochemical features, together with trace element modeling, suggest that the late Paleoproterozoic Wafang diorite dykes could be sourced from crustal melting with minor contribution of the mixing of mantle materials. These new data combined with previous studies on the late Paleoproterozoic the Xiong’er volcanism should have lasted for nearly 43Ma ranging from 1789Ma to 1746Ma. Correspondingly, the initiation timing of the Xiong’er rift should be younger than 1831Ma, but earlier than 1625. The Wafang diorite dykes formed in a rift environment, possibly related to breakup of the supercontinent Columbia.