Discovery and Geochemistry of Upper Permian Volcanic Rocks in Tongren Area, Qinghai Province and Their Tectonic Significance

Tongren area is located in the lower reaches of the Longwuhe River, east of Qinghai Province, which is between the West Qinling and South Qilian orogenic belts. Along the two sides of Longwuhe River broadly develops stratum in Permian and Triassic. Through the analysis of rock assemblages, this area is determined to be turbidite fan facies of hemipelagic from the end of Late Permian to the beginning of Early Triassic. Through the geological research, we found volcanic rocks in the Shiguan Formation of Upper Permian. The volcanic rock assemblage is mainly composed of basalts, basaltic andesites and andesites. According to the characteristics of the ordinary elements, the volcanic rocks can be divided into two parts. The lower part: SiO₂ content is 49.04%-52.43%, with the average of 51.28%; the TiO₂ content is 0.67%-1.11%, with the average of 0.9%; and the ALK value is 2.64%-3.6%.The rock type is mainly tholeiitic basalt series, with high titanium quantity. The upper part: SiO₂ content is 56.25%-60.36%, the average being 58.76%; The TiO₂ content is 0.52%-0.63%, the average being 0.58%; the ALK value is 5.26%-5.54%. The rock type is calc-alkaline series with low titanium quantity. REE distribution patterns of the volcanic rocks are different. The lower part has lower REE, with (La/Lu)_N average being 3.07, and its distribution patterns are consistent with those of transitional mid-oceanic ridge basalt. REE is rich in the upper part, especially the LREE. And its distribution patterns are similar to those of island arc andesite. The trace elements have the same characteristics, the distribution type of which is something like that of island arc basalt rock. Tectonic setting distinction reveals that the volcanic rocks of this area were formed in transitional mid-oceanic ridge basalt to island arc tectonic setting.