Petrogenesis of Paleoproterozoic alkali-feldspar granites associated with alkaline rocks from the Trans-North China Orogen

Abstract

Granitoids are the dominant component of the continental crust and can be used as probes for crustal formation and evolution. The origin of granites, especially ambiguous granites with alkaline affinities emplaced in orogenic settings, however, remains controversial. Here, we report the petrology, geochronology and geochemistry of Paleoproterozoic alkali-feldspar granites associated with carbonated alkaline rocks from the Trans-North China Orogen in the North China Craton. Corundum-bearing Al-rich gneisses are associated with the granites and occasionally contain granite pockets and veinlets. The rocks have high amounts of K2O (10–14 wt%) and low CaO (0.3–2.3 wt%) contents and show metaluminous to weakly peraluminous features. Moreover, they have low levels of total REE (18–100 ppm) and show strong LREE enrichment [(La/Yb)CN = 44–128] and positive Eu anomalies (δEu = 1.6–10.2). The zircons record a U–Pb age of 1809 ± 3 Ma and εHf(t) values ranging from −3.7 to −7.1, which are consistent with those in the gneisses (1827 ± 14 Ma, εHf(t) = −3.3 to −8.4). Inherited zircons occur in both rocks and show similar ages of ∼2.0 and ∼2.1 Ga. Considerably high δ18OSMOW ratios (9.9–10.9 ‰) in the zircons and low εNd(t) values (−4.0 to −6.2) of the granites were identified. The granite was generated by partial melting of pelitic rocks instead of differentiation of mantle-derived magmas, whilst the gneiss represents the residuum. The carbonated alkaline magmas arising from the mantle provided sufficient heat for anatexis of the crust. Granitoid rocks showing alkaline affinity in orogenic belts may not be the fractionation products of mantle-derived melts but may have formed from reworked sedimentary materials.