Geochronology, geochemistry and petrogenesis of the Yinjiagou complex in western Henan Province, China.

The Yinjiagou complex, located in Lingbao City, western Henan Province, is mainly composed of monzogranite porphyry, K-feldspar granite porphyry and quartz diorite porphyry, with minor granodiorite and diorite porphyry dykes, which is closely related to the Yinjiagou porphyry-skarn pyrite-polymetallic deposit. The monzogranite porphyry, K-feldspar granite porphyry and granodiorite porphyry yiled zircon SHRIMP U-Pb ages of 147.5±2.1Ma, 147.8±1.6Ma and 142.0±2.0Ma, respectively, indicating that the Yinjiagou complex formed during the transition from Jurassic to Cretaceous. Both the monzogranite porphyry and K-feldspar granite porphyry have high contents of SiO2(65.17%~73.94%), Al2O3 (13.53%~15.96%) and K2O (5.03%~9.89%, mostly 6.0%), but low contents of Na2O (0.24%~1.86%, mostly 1.0%), Fe2O3 (0.82%~3.71%), FeO (0.02%~1.62%), MgO (0.23%~1.47%, mostly 1.0%) and CaO (0.08%~1.98%, mostly 0.4%). They have high aluminum saturation index (1.18~2.04, all 1.1) and have been altered. The REE contents range from 71.38×10-6 to 276.4×10-6, with mediately negative Eu anomalies (δEu=0.57~0.88) and strong REE fractionation ((La/Yb)N=6.93~30.09). The Yinjiagou complex is characterized by low Sr (104×10-6~461×10-6, generally 300×10-6) and Yb contents (0.90×10-6~2.10×10-6, mostly 1.9×10-6). The complex is enriched in Rb, Ba, Th, U, K, La, Ce, Nd, Hf and Zr, but strongly depleted in Sr, P and Ti, with moderate depletion of Nb and Ta. The geochemical signatures above are similar to those of post-collisional granitoids, suggesting that the Yinjiagou complex can be classified into the group of post-collisional granites. The complex has initial 87Sr/86Sr ratios of 0.7069~0.7091 (mostly 0.7085), eNd(t) values of -14.99 to -10.57, tDM1(Nd) of 1.41~1.76Ga and tDM2(Nd) of 1.51~1.81Ga, respectively. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of K-feldspar range 17.376~17.628, 15.455~15.502 and 37.867~38.090, respectively. The Nd-Sr-Pb isotope systematics suggests that the Yinjiagou complex most likely originated from partial melting of a mixed source composed of the Kuanping, Erlangping and Taihua groups. The complex formed in a transitional tectonic setting from nearly EW-trending to nearly NNE-trending structures during the Late Jurassic-Early Cretaceous. The tectonic transition resulted in decompression in the East Qinling area, accommodating large-scale partial melting of the lower crust, including the formation of the Yinjiagou complex.