Controls on the formation of Cu-rich magmas: Insights from the Late Triassic post-collisional Saishitang complex in the eastern Kunlun Orogen, western China

Abstract

Petrogenesis of high Sr/Y andesitic to dacitic Cu-rich magmas remains highly controversial and has been attributed to be related to either particular sources or differentiation processes/conditions. This study provides a comprehensive petrological and geochemical dataset including Cu, Au and Pt systematics in the Saishitang complex in the eastern Kunlun Orogen to investigate the influence of the magmatic source and differentiation processes on Cu enrichment in a post-collisional, slab-derived magmatic system. The Saishitang complex contains a ~233Ma plutonic main body and tens of porphyritic dykes intruded at ~218Ma, and hosts skarn copper mineralization formed at 218–224Ma. The main intrusion consists of quartz diorite, granodiorite and minor amounts of monzogranite, whereas the porphyritic dykes vary from diorite to quartz diorite, granodiorite and granite. The porphyritic diorites, with high MgO, Ni and Cr contents and moderately high Sr/Y and La/Yb ratios, were most likely derived from partial melting of the subducted slab and overlying sediments and then interacted with mantle peridotite. Mineral compositions and geochemical data, as well as mass-balance, Rayleigh fractionation and isotopic mixing modeling show that the parent magmas subsequently experienced simultaneous assimilation and fractional crystallization (AFC) at both the lower and the mid-to-upper crustal levels, and were finally emplaced to different upper crustal levels. Systematic Cu and Au variations during the Saishitang magmatic evolution suggest that the lower crustal AFC process improved the Cu budget by approximately one order of magnitude and therefore was the most critical factor in controlling the formation of Cu-rich magmas, although other processes such as slab melting and sulfide saturation were also important.