Abstract:

Volcanic rocks are developed in the West Qingling Mountains, near the border between Gansu and Qinghai Provinces. The rocks are dominantly andesitic and rhyolitic, and are all high-K calc-alkaline volcanic rocks.These volcanic rocks differ greatly in their major and trace element contents. The SiO₂ contents range from 56.67% to 78.17% (average 69.78%). The Al₂O₃ are high (12.77% to 17.29%,average 14.64%) and the TiO₂ contents are low (0.02% to 0.75%, average 0.22). The Na₂O/K₂O ratios are low (0.03% to 2.68%,average 0.65%). Light rare earth elements (LREEs) are enriched whereas heavy rare earth elements (LREEs) are depleted. REE patterns are obviously right inclining with distinct negative Eu anomalies. The chondrite-normalized HREE patterns of the andesitic and rhyolitic volcanic rocks are markedly different, which implies that the both volcanic rock types were derived from different magma sources. The volcanic rocks are depleted in high field strength elements(HFSEs) such as P,Nb,Ti, and Ta, but are enriched in large ion lithophile elements (LILEs) such as Th,Ba,K, and Rb. LA-ICP-MS zircon U-Pb dating of the andesitic volcanic rock yielded (236±1.2) Ma,which represents the volcanis mage.These rocks were likely oceanic subduction products with high-degree partial melting in the deep mantle, and the magma is also influenced by fractionation and crustal assimilation during its ascent. The volcanism may have formed in an active continental margin arc setting, related to the south-dipping subduction of the Longwu Gorge Ocean(remnants of which preserved now as ophiolite). Magmatic evolution of these volcanic rocks was likely controlled by partial melting and fractional crystallization: the andesitic magma was mainly controlled by fractional crystallization, whereas the rhyolitic magma was largely dominated by partial melting.

Key words: West Qinling Mountain, calc-alkaline, volcanic rock, subduction of oceanic crust