Geochemistry of 1.78 Ga A-type granites along the southern margin of the North China Craton: implications for Xiong'er magmatism during the break-up of the supercontinent Columbia

Abstract

The Palaeoproterozoic Luoling granites occur along the southern margin of the North China Craton. They are rich in silica and total alkalis with SiO2 contents ranging from 65.18 to 72.72 wt.%, K2O from 4.68 to 6.62 wt.%, and Na2O from 1.35 to 4.88 wt.%. They have high Fe*[FeOt/(FeOt + MgO)] ranging from 0.84 to 0.95 wt.% and low MnO (0.03–0.09 wt.%), MgO (0.27–1.55 wt.%), CaO (0.36–2.04 wt.%), TiO2 (0.4–1.12 wt.%), and P2O5 (0.04–0.36 wt.%). Geochemically, they show typical characteristics of A-type granites, such as high contents of alkalis (i.e. high K2O + Na2O, with K2O/Na2O > 1), Rb, Y, Nb, and REEs (except for Eu); high FeOt/MgO and Ga/Al ratios; and low CaO, Al2O3, and Sr contents. New secondary ion mass spectroscopy (SIMS) zircon U–Pb ages reveal that the Luoling granites were emplaced at 1786 ± 7 Ma and thus were approximately coeval with Xiong'er volcanic rocks in the area. Their negative bulk-rock initial Nd and zircon initial Hf isotopic ratios suggest that they have affinities to EM-I-type mantle and both are the products of Xiong'er magmatism during the Palaeoproterozoic. We regard them as produced under a continental rift setting during the Palaeoproterozoic, genetically related to the break-up of the Columbia supercontinent.