关键词: 石英闪长岩体, 早古生代, 岩石成因, 北祁连东段

Abstract: This paper presents comprehensive zircon U-Pb ages and whole-rock geochemistry for the Laohushan quartz diorite in the
eastern segment of the North Qilian Orogen, with aims to elucidate their petrogenesis and constraints on the early Paleozoic tectonic
evolution of North Qilian Orogen. Zircon LA-ICP-MS U-Pb dating of the Laohushan quartz diorite yielded an age of 440 Ma. Rocks
from the Laohushan pluton show sub-alkaline and metaluminous signatures, relatively low CaO, MgO and Fe2O3T contents, agpaitic
index (AKI) values of less than 0.85, and aluminum saturation index (A/NKC) values of mostly less than 1.0. The Laohushan pluton can
thus be grouped into metaluminous calc-alkaline I-type granitoids. All of the rocks are also enriched in LREEs and LILEs (e.g., Rb, Cs,
Th and U) and depleted in HFSEs (e.g., Nb and Ta), similar to the geochemical characteristics of typical arc magma rocks. They have
homogeneous Sr and Nd isotopic compositions with (87Sr/86Sr)i values of 0.7058~0.7071 and εNd(t) values of -1.06~-0.31. Integrated
petrological, elemental, and isotopic compositions indicate that the Laohushan quartz diorites were generated by mixing of
mantle-derived material with crustal-derived magmas that were formed by dehydration melting of basaltic rocks from the lower crust. By
comprehensively synthesizing geochemical compositions and geological setting for magma rocks within the study region, we propose
that the northward subduction of back-arc basin induced by the subduction of the North Qilian oceanic plate and the subsequent
collision between the Qilian-Qaidam block and magma arc were major dynamic mechanisms responsible for the emplacement of the
Minleyaogou and Laohushan granitoids and the adakitic rocks within the Maozangsi-Laohushan granitoid belt.

Key words: quartz diorite pluton, Early Paleozoic, petrogenesis, eastern segment of North Qilian Orogen