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

摘要

The Palaeoproterozoic Luoling granites occur along the southern margin of the North China Craton. They are rich in silica and total alkalis with SiO_2 contents ranging from 65.18 to 72.72 wt.%, K_2O from 4.68 to 6.62 wt.%, and Na_2O from 1.35 to 4.88 wt.%. They have high Fe*[FeO_t/(FeO_t + 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.%), TiO_2 (0.4-1.12 wt.%), and P_2O_5 (0.04-0.36 wt.%). Geochemically, they show typical characteristics of A-type granites, such as high contents of alkalis (i.e. high K_2O + Na_2O, with K_20/Na_20 > 1), Rb, Y, Nb, and REEs (except for Eu);;high FeO_t/MgO and Ga/Al ratios;;and low CaO, A1_2O_3, 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.