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

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 (Sr-87/Sr-86 = 0.707490-0.707840; epsilon(Nd)(t) = +4.0-4.3; zircon epsilon(Hf)(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.