Implications for Rodinia reconstructions for the initiation of Neoproterozoic subduction at ~860Ma on the western margin of the Yangtze Block: evidence from the Guandaoshan Pluton
Neoproterozoic igneous rocks are widespread along the western margin of the Yangtze Block, but their petrogenesis and tectonic setting is debated. The Guandaoshan pluton is located at the southwestern margin of the Yangtze Block and is mainly composed of diorite and subordinate gabbro, with quartz diorite in its margin. Hornblende is an ubiquitous mineral in different phases of the pluton. SHRIMP zircon U-Pb dating of quartz diorite, gabbroic diorite, and gabbro from the pluton yielded 206Pb/238U ages of 857 ± 7 Ma, 856 ± 6 Ma, and 856 ± 8 Ma, respectively. Guandaoshan pluton samples show a large range of SiO2 (47.02-67.66%), MgO (1.12-7.5%), Fe2O3T (2.8-12.22%) and CaO (2.95-11.88%), low rare earth element (REE) contents from 22 to 49 ppm, and enrichment of Sr, Ba and Rb and depletion of Nb, Zr and Ti with characteristics of island arc magma. They also exhibit low initial 87Sr/86Sr ratios from 0.7030-0.7033, and positive εNd(t) values from + 4.8 to + 5.2. These features suggest that the parental magma of the Guandaoshan pluton originated at a convergent plate boundary from a depleted mantle source modified by slab fluids, and underwent the fractional crystallization of amphibole and magnetite, without significant crustal assimilation, during the formation from gabbro-diorite to quartz diorite. Neoproterozoic magmas with age of 860-740 Ma are abundant on the western Yangtze Block, and there is a gap of magmatism in early Neoproterozoic (from about 1000 Ma to 870 Ma). Therefore, it can be deduced that the ~ 860 Ma Guandaoshan pluton and the contemporary magmatism represent initial subduction at the western margin of the Yangtze Block. Based on the Neoproterozoic paleomagnetic data, detrital zircon ages, magmas with low δ18O values in South China and our new data, we prefer that the South China Block was located at the margin of Rodinia in the Neoproterozoic, and not at the center of the supercontinent.