The ~860-Ma, Cordilleran-Type Guandaoshan Dioritic Pluton in the Yangtze Block, SW China: Implications for the Origin of Neoproterozoic Magmatism

摘要

The tectonic setting of the widespread Neoproterozoic igneous rocks in South China has long been a matter of debate. Situated in the western margin of the Yangtze Block of South China, the Guandaoshan pluton is composed of diorite in the core and quartz diorite in the margin. Both types contain abundant mafic enclaves and mafic dikes. This pluton is dated at by the SHRIMP zircon U-Pb method. The rocks have SiO2 contents of 52.8–63.8 wt%, Al2O3 contents of 17.4–20.6 wt%, Na2O contents of 2.14–3.74 wt%, and K2O contents of 0.44–1.61 wt%. They are calc-alkaline with an Al saturation index, and , and they have Fe numbers <0.8 . Their total rare earth element (REE) contents range from 19 to 45 ppm, with concave chondrite-normalized patterns slightly depleted in middle REEs, indicating extensive amphibole fractionation. Their whole-rock Nd(t) values range from +3.9 to +5.1, and initial 87Sr/86Sr ratios range from 0.7028 to 0.7033. They have zircon Hf(t) values ranging from +11 to +17, with mean Hf model ages of around 860 Ma, clearly indicating a depleted-mantle source. In the primitive mantle–normalized trace element spider diagram, the rocks show pronounced enrichment of Rb, Ba, Th, and Pb and depletion of Nb, Ta, and Ti, a typical arc signature. The Guandaoshan pluton is an amphibole-rich, calc-alkaline, Cordilleran-type granitoid, a typical product of subduction-zone magmatism. The occurrence of this Neoproterozoic Cordilleran-type granitoid suggests that the western margin of the Yangtze Block was an Andean-type continental margin at that time, rather than mantle plume–related rifting setting.