The Early Permian mafic-ultramafic complexes in the Beishan Terrane, NW China: Alaskan-type intrusives or rift cumulates?

摘要

The petrogenesis and geodynamic setting of the Early Permian mafic-ultramafic complexes in the Beishan Terrane, NW China have important bearing on the prospects of Ni-Cu-PGE sulfide and Ti-Fe oxide deposits, as well as in understanding the history of evolution of the southern Central Asian Orogenic Belt (CAOB). Here we present results from a detailed study on a representative suit of a mafic-ultramafic rocks from the Bijiashan complex in the Beishan Terrane. The complex is composed of dunite, troctolite, olivine gabbro and gabbro without clinopyroxenite or hornblendite. In addition to olivine and clinopyroxene, orthopyroxene and ptagioclase are also present in all these rock types, in the absence of chromite or primary hornblende. The minerals display marked compositional variations with the Fo content in olivines ranging from 66 to 83 and the clinopyroxenes ranging from diopside to augite. The whole-rock geochemistry shows good correlations among the major elements, and the trace elements are characterized by flat REE patterns with Eu positive anomalies, and HFSE depletion relative to L1LE. These features are comparable to other mafic-ultramafic complexes in the Beishan and Eastern Tianshan terranes, but obviously deviate from the typical features of Alaskan-type intrusives. The present study suggests that the Beishan complexes were formed in a post-orogenic extensional environment with potential for Ni-Cu mineralization, rather than in arc-related setting with Alaskan-type PGE mineralization. Our study confirms that the Beishan area does not conform to an arc-related setting, but was located within a rift setting in Permian probably related to Permian mantle plume event, suggesting that the subduction of the Paleo-Asian ocean had culminated by this time, and the southern CAOB witnessed a post-orogenic extensional regime in late Paleozoic.