Geochronology and petrogenesis of the post-orogenic Cu-Ni sulfide-bearing mafic-ultramafic complexes in Jilin Province, NE China

摘要

Northeastern (NE) China is a well-documented example of a collisional zone characterized by widespread post-orogenic granites and mafic-ultramafic complexes. Based on a study of the Hongqiling and Piaohechuan Cu-Ni sulfide-bearing mafic-ultramafic complexes in central Jilin province, we present geological, petrological, geochemical and geochronological data which indicates their post-orogenic origin. The Hongqiling complex comprises pyroxenite, olivine websterite, Iherzolite, gabbro and leucogabbro. Zircon U-Pb SHRIMP analyses on a leucogabbro of the Hongqiling complex yield a weighted mean ~(206)Pb-~(238)U age of 216 +- 5 Ma. The Piaohechuan complex is composed of gabbro, pyroxenite and dolerite, exposed as dikes. A plagioclase-bearing pyroxenite has a U-Pb zircon weighted mean ~(206)pb-~(238)U age of 217 +- 3 Ma, identical to that of the Hongqiling complex. These ages are coeval with the emplacement of A-type granites in the area, but slightly younger than the regional metamorphism (～ 240 Ma) and syn-orogenic granitic magmatism (246 +- 4 Ma). This suggests that these mafic-ultramafic complexes are post-orogenic in origin. The age data also indicated a short period of lithospheric stabilization of about 30 Ma after cessation of erogenic activity. Geochemical investigation indicates that the primary mafic magma was a lithospheric mantle-derived basalt resulting from the upwelling of asthenosphere due to lithospheric delamination during post-orogenic processes. The magmatic source was contaminated by a small amount of crustal material, and subsequent crystal fractionation resulted in the Cu-Ni mineralization. The widespread occurrence of mafic-ultramafic complexes in the Xing'an-Mongolian Orogenic Belt of NE China and in the Altay-Tianshan-Junggar Orogenic Belt of Northern Xinjiang indicates that mafic intrusions are an important magmatic suite that evolved during post-orogenic processes. Portions of this mafic magma could have underplated the lower crust, and served as the heat source for associated late-stage granitic magmas.