Geochemistry and Tectonic Setting of Late Triassic Volcanic Rocks in Reshui Area, Qinghai

Reshui area is located in the binding site of Qinling, Qilianshan and Kunlunshan orogenic belt, and the late Triassic volcanic rock developed in the area is part of Elashan formation. As a set of volcano rocks with high K content, low Ti content and metaluminous calc-alkaline characteristics, the late Triassic rock are mainly consisted of dacite and dacitic pyroclastic rock, followed by neutral andesitic volcano rock and acidic rhyolitic volcano rock. The content of SiO₂ is between 59.62% and 76.46%, the content of ALK ranges from 6.59% to 8.04%, the content of ANK is from 1.14 to 1.80, and the value of Rittmann coefficient varies between 1.72 and 2.41, the degree of rock mineral fractionation crystallization is high, but the degree of consolidation is common. The total of REE is between 126.76×10^-6 and 240.07×10^-6, the average value of LREE/HREE is 10.19, indicating that the rock belongs to the LREE enrichment type. Besides, the enrichment degree of LREE is higher than HREE, the δEu value mostly falls between 0.37 and 0.56 and the loss is more obvious. The REE patterns are right dipping slow smooth curves. LILE such as Rb, Ba, U, K, and Th are enriched, and HFSE such as Nb, Ta, Ti, P are depleted. Primitive mantle trace element spider diagram show a significant W type high on the left and low on the right, with obvious characteristics of arc volcanic rocks. Mg# is almost within a range of 16 to 36, the value of ²⁰⁶Pb/²⁰⁴Pb is from 18.181 to 18.218, while the value of ²⁰⁷Pb/²⁰⁴Pb is between 15.570 and 15.588, then the value of ²⁰⁸Pb/²⁰⁴Pb is among 38.368 to 38.529. All these values indicate that the magma derived from the lower crust. The volcanic rocks are related to Type A subduction, and according to Hf/3-Th-Nb/16 and other tectonic environment discrimination diagrams, the rocks are found formed in an epicontinental arc environment.