西秦岭甘青交界一带晚三叠世火山岩岩石成因及构造指示意义

西秦岭甘肃、青海交界一带中生代火山岩较为发育，火山岩组成以安山质、流纹质为主，均为高钾钙碱性火山岩，主、微量元素含量变化较大。SiO2的质量分数为5667%～7817%，平均6978%， Al2O3含量较高(1277%～1729%，平均1464%)。TiO2含量少(002％～075%，平均022%)，Na2O/K2O比值较小(003％～268%，平均065%)。轻稀土元素中等富集，重稀土元素相对亏损，稀土元素配分曲线明显右倾，具Eu负异常。安山质火山岩与流纹质火山岩稀土元素球粒陨石配分曲线中重稀土型式明显不同，暗示二者来自不同的源区。微量元素中P、Nb、Ti、Ta等高场强元素(HFS)相对亏损，Th、 Ba、K、Rb等大离子亲石元素相对富集。研究区安山岩LA ICP MS锆石U Pb同位素定年结果为(236±12)Ma，代表了火山岩的喷发年龄。它们形成于隆务峡蛇绿岩洋壳向南俯冲的活动大陆边缘弧环境，为俯冲洋壳在地幔深部发生高程度部分熔融作用的产物，并在上升过程中受到陆壳物质作用。在岩浆演化过程受部分熔融和分异结晶作用的控制，但安山质火山岩主要受控于分异结晶作用，而流纹质火山岩受部分熔融作用较大。

Petrogenesis and Tectonic Implications of Late Triassic Volcanic Rocks at the Gansu Qinghai Junction in the West Qinling Mountains

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.