Geochemistry and Significance of Paleoproterozoic Granitoids from Huangqikou, Central Helanshan Area

Widespread rocks in the khondalite belt record magmatism, metamorphism and tectonic evolution of the North China craton (NCC). Whereas the further research on Huangqikou granitic plutons exposed in the central Helanshan area is needed to offer information about the genetic mechanisms and the dynamic backgroud. Based on the field geological and petrographic studies, major, trace elements and Sr-Nd isotopic compositions were carried out on the Huangqikou granitic plutons, so as to discuss their petrogenesis, source characteristics and geological significance. The early intruded Huangqikou granites consist of alkali granite and monzogranite, while the later intrusions are mainly tonalite. The Huangqikou granites display typical geochemical characteristics of S-type granite, such as high contents of K₂O (2.97%-6.71%), A/CNK>1.1. However, the later intrusions generally show lower SiO₂ and higher Al₂O₃ relative to those early ones. All the studied granitic rocks show enriched LREE, and negative Eu anomalies. The samples generally show similar variation trend in the trace-element patterns, such as systematic enrichment of LILE (K, Rb, Th) and depletion of HFSE, e.g. Nb, Ta, P, Ti. All these geochemical features, combined with the majority of positive ε_Nd(t)(+1.81-+4.90) and Paleoproterozoic Nd model ages (T_DM1=2.10-2.37 Ga, T_DM2=2.10-2.35 Ga), suggest that the Huangqikou granites were mainly derived from the partial melting of the sedimentary rocks in the Zhaochigou Formation, but probably with minor mafic magma contributed from the mantle. Combined with previous regional metamorphic and magmatic studies, it is suggested that the early and later intruded granitic rocks in the Huangqikou region belong to orogenic granitoids, and were probably formed during the collision between the Ordos and Yinshan blocks and post-collisional extension setting, respectively.