Southwest Boundary of Yangtze and Cathaysia Blocks: Constraints from the Luojiashan Gabbro in Yingyangguan Region, Northeastern Guangxi

The Luojiashan gabbro is a newly discovered mafic pluton emplaced in the Neoproterozoic Yingyangguan Formation in Yingyangguan region of northeastern Guangxi, South China. Comprehensive whole-rock geochemical and zircon geochronological and Hf isotopic analyses are performed on the gabbro and comparisons are made with the coeval mafic-ultramafic sills and dikes located in Longsheng region of northern Guangxi in order to understand the magmatic origin, evolution, and tectonic setting of the Luojiashan gabbro and to address the location of the suture zone of the Southwestern Jiangnan Orogen (SJO). LA-ICP-MS zircon U-Pb ages obtained from the Luojiashan gabbro show that it was emplaced at similar to 770 Ma. The gabbro contains abundant inherited zircons aged at 0.9-1.3 Ga, consistent with age spectrum of the Cathaysia Block. Chondrite-normalized REE pattern, primitive mantle-normalized trace element spider diagram, incompatible element ratios of Nb/Ta, Zr/Hf, La/Nb, Ba/Th, Th/La, and Ba/La, and Th/Yb-Ta/Yb discrimination diagram of the gabbro are indicative of OIB-like geochemical characteristics and of derivation from partially melted garnet peridotite of the asthenospheric mantle. Tectonic discrimination based on the trace and rare earth elements also indicate that the Luojiashan gabbro was emplaced in a within-plate extensional rift setting, probably as a result of Rodinia supercontinent dismantling, lithospheric thinning, and underplating and upwelling of the asthenospheric mantle. Based on zircon age, Hf isotopic data and comparison between northern and northeastern Guangxi, it is suggested that the Yingyangguan region was tectonically situated in a different tectonic locale from the Longsheng region of northern Guangxi at about 770 Ma during the post-orogenic mafic-ultramafic magmatic event, with the former within the Cathaysia Block and the latter along the southeast margin of Yangtze Block. The suture between the Yangtze and Cathaysia blocks must be located between Yingyangguan of northeastern Guangxi and Longsheng of northern Guangxi.