Geochemical Characteristics and Tectonic Significance of Basic Rocks in the Yangla Ore District,Northwestern Yunnan Province

In order to study the tectonic settings of the Yangla ore district in the Jinshajiang zone,western Yunnan province,this work made a systematic geochemical analysis of major elements and trace elements for basic rocks from this ore district,and compared them with the geochemical properties of basalt lava coexisting with ophiolite in the route from the Baimang snow Mountains to Gongka.The result shows that the geochemical characteristics of basalt from Beiwu Formation in Lower Carboniferous of the Yangla ore district are similar to those from ophiolite units of the Baimang snow Mountains and Gongka,both being analogous to mid-ridge basalt,characterized by low K and Ti,and not obvious in fractionation between LREE and HREE.The trace elements indicate that they might originate from an enriched source,as the result of spreading of the ancient Jinshajiang ocean floor and presented as a part of the suture zone.The diabase dyke in the Yangla orefield,which intruded into the Devonian stratum,shows its epigenetic features.The normalized REE patterns of diabase are apparently inclined to right.The trace elements show enrichment in highly incompatible elements and depleted in Nb and Ta.These characteristics are similar to those of volcanic rocks in the epicontinental arc and different from basalt in ocean islands or intra-oceanic arcs.We infer that the diabase dyke were likely produced the Triassic magmatic activity of the epicontinental arc.This study does not support the previous conclusion that the Yangla district is situated in a tectonic setting of intra-ocean arcs.