Geochemistry and petrogenesis of 270 Ma Ni-Cu-(PGE) sulfide-bearing mafic intrusions in the Huangshan district, Eastern Xinjiang, Northwest China: Implications for the tectonic evolution of the Central Asian orogenic belt

Abstract

The Central Asian orogenic belt (CAOB) is composed of remnants of microcontinents, island arcs and ophiolites. The presence of voluminous Phanerozoic A-type granites suggest that continental growth occurred through addition of juvenile crust, and the presence of mafic-ultramafic intrusions provides direct evidence for a mantle contribution. The origin of the mafic-ultramafic intrusions in the Huangshan area, Eastern Xinjiang, has been debated because of the lack of precise age data or trace element and isotopic analyses. We have studied the geology, petrology, geochemistry and geochronology of two of the intrusions: the Huangshanxi and Huangshandong intrusions. Both are typical layered mafic-ultramafic intrusions composed of diorite, troctolite,norite, gabbronorite, gabbro wehrlite, websterite and lherzolite with systematic geochemical variations that are consistent with the following crystallization sequence: Ol-(Chr), Ol-Pyx-(Chr), Cpx-Pl and Hbl-(Bt). Igneous zircons separated from a diorite in the Huangshanxi intrusion yield a 269±2 Ma SHRIMP U-Pb age. The rocks are enriched in Rb-Ba-U-K-Sr relative to Th and LREE, depleted in Nb-Ta relative to Th and exhibit positive εNd values (+6.7 to +9.3). Ni-Cu-(PGE) sulfide mineralization occurs in the lower parts of the intrusions, primarily strata-bound within wehrlite or lherzolite and locally stratiform along the margins of the intrusions. The parental magmas appear to have been derived from a mantle source that was previously contaminated by subduction of oceanic crust and to have been variably contaminated by upper continental crustal components. The intrusions appear to have formed by plume-related activity within a continental setting, likely related to post-orogenic extension following Carboniferous amalgamation of arc-related terranes in the CAOB. This plume activity was most likely also responsible for providing heat sources to generate large amounts of A-type granites across the CAOB. © 2004 Elsevier B.V. All right reserved.