Petrogenesis of the ultramafic pluton in the Jianglang dome, western margin of the Yangtze block: Zircon U-Pb dating, geochemistry and Sr-Nd isotopes

The Jianglang dome is located in the western margin of the Yangtze block.A set of ultramafic pluton were newly discovered in the south of the Jianglang dome and they intruded into the Permian and Silurian. The rocks are mainly composed of serpentine (ca. 60%), olivine (ca. 30%) with minor magnetite (ca. 5%) and hornblende (ca. 5%). In order to discuss the petrogenesis, we performed LA-ICP-MS zircon U-Pb dating, geochemical and Sr-Nd isotopic studies. Dating results show that the ultramafic pluton have numerous 2427~430 Ma captured zircons, and ²⁰⁶Pb/²³⁸U weighted mean age of the youngest magmatic zircons is 222.3±4.4 Ma (MSWD=1.9, n=6). Geochemical analyses results show that: (1) the rock samples exhibit low SiO₂contents (46.76%~39.07%) with high Mg# values (82.3~74.0) and Cr, Co, Ni abundances; (2) low ΣREE (average 31.8 μg/g) and (La/Yb)_N values (5.26~1.38) with flat REE pattern and negative Ce anomalies (Ce/Ce^*=0.80~0.67); (3) enrichment in large ion lithophile elements (e.g., Rb, Ba and U) and depletion in high field strength elements (e.g., Zr and Hf); (4) low (Th/Yb)_PM values (29.8~1.56), (Th/Ta)_PM values (0.22~0.03), (La/Nb)_PM values (1.91~0.39) and La/Sm values (5.88~1.11). Sr-Nd isotopic analyses indicate that the samples have low (⁸⁷Sr/⁸⁶Sr)_ivalues (0.706872~0.702598) and high ε_Nd(t) values (8.02~5.64). Their compositions are close to the depleted mantle and island arc basalts with contamination of crustal materials lower than 5%. Combining with the previous studies, we propose that the crystallization age of ultramafic pluton is ca. 222.3 Ma which probably formed under island arc setting of the Paleo-Tethys closure stage. The primitive magma was possibly derived from highly partial melting of mantle source region. The magma might experience crystallization differentiation of chromite and olivine. In addition, the age spectrum of captured zircons reflects that Archean-Palaeoproterozoic metamorphosed basements most likely exist in the Jianglang dome, where also have geological age records of the Rodinia supercontinent convergence-disintegration processes and Pan-African events.