Late Cambrian tonalite-trondhjemite association in the eastern segment of North Qilian suture zone: petrogenesis and geodynamic implications

Abstract

ABSTRACT The late Cambrian tonalite-trondhjemite association in the eastern segment of North Qilian suture zone (E. NQSZ) is important for understanding of the sub-arc magmatic process and tectonic evolution. Here we report data for feldspar-quartz porphyry and fine-grained trondhjemite with abundant tonalite enclave/interlayer from the Leigongshan (LGS) pluton, and medium-grained tonalite from the Beilinggou (BLG) pluton. Zircon U‒Pb ages suggest emplacement of the plutons at ca. 506‒481 Ma, associated with early subduction of the North Qilian Ocean. The LGS tonalite enclave/interlayers and trondhjemite hosts share many characteristics in common, including the same mineralogy and texture, similar mineral chemistry, whole-rock trace element ratio and Sr‒Nd isotopic compositions. The tonalite enclave/interlayers probably represent quenched blobs/layers of an evolved, tonalitic magma branch resulting from periodic injection of magma into the magma chamber. All the LGS-BLG tonalite-trondhjemites display low abundances of incompatible trace elements (e.g., K, Rb, Th, Zr and REE) and depleted Sr‒Nd‒Hf isotopes (87Sr/86Sr = 0.707490‒0.707840; εNd(t) = +4.0‒4.3; zircon εHf(t) = +4.0‒13.1), indicating little/no contribution of the continental crust. Geochronological and geochemical consistency between the coeval arc sodium-rich volcanic rocks and studied tonalite-trondhjemites further suggests crystal fractionation of a common basaltic parental magma. The parental magma was generated in continental-arc rifting environments due to slab rollback in the early stage of subduction.