Triassic granitoids of the Qinling orogen, central China: Genetic relationship of enclaves and rapakivi-textured rocks

Abstract

Granitoids varying in composition from biotite–amphibole granodiorite to monzogranite with rapakivi textures have recently been recognized in the Qinling orogen (central China). The granitoids are magnesian, calc-alkalic and metaluminous (A/CNK = 0.71–0.95). They contain many mafic to intermediate microgranular enclaves. The enclaves are quenched mafic melts in granitic host magma as evidenced by fine-grained sinuous margins against or transitional contacts with the host granitoids as well as disequilibrium textures and mineral assemblages. U–Pb zircon dating of the granitoids yields: 209 ± 2 Ma and 212 ± 2 Ma and the enclaves 210 ± 2 Ma and 211 ± 2 Ma, respectively. The host granitoids and enclaves show obvious silica gap but similar REE and trace element patterns. The whole-rock 87Sr/ 86Sr( t), ε Nd( t) and zircon ε Hf( t) values for the granitoids vary from 0.70516 to 0.70634, − 2.3 to − 4.4 and 0.7 to − 7.6, and for the enclaves from 0.70467 to 0.70600, − 1.1 to − 4.0, and 1.2 to − 8.8, respectively, showing overlapped ε Nd( t) and zircon ε Hf( t) values. All these suggest that the two magmatic systems represented by the host rock and enclave are probably derived from distinct sources with sufficient interaction, or common origin but underwent different degrees of crustal contamination. Mineralogical and geochemical data point to enclave dissolution in silicic magma as a major process accounting for the origin of the rapakivi texture. The possible bimodal magmatism suggested by the coeval granitoids and lamprophyre dykes, combined with the structural pattern, geochemical features and regional tectonics, suggest a post-collision setting for the Qinling rapakivi-textured granitoid plutons.