青海热水地区晚三叠世火山岩地球化学特征及构造环境
Geochemistry and Tectonic Setting of Late Triassic Volcanic Rocks in Reshui Area, Qinghai
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摘要: 青海热水地区位于秦祁昆三大造山带的结合部位，该区发育晚三叠世鄂拉山组火山岩。岩石类型以英安岩及英安质火山碎屑岩为主，其次为中性的安山质火山岩和酸性的流纹质火山岩，为一套高K、低Ti、准铝质的钙碱性系列火山岩。SiO2含量为59.62%～76.46%，总碱含量ALK为6.59%～8.04%，ANK在1.14～1.80，里特曼系数σ在1.72～2.41，岩石矿物结晶分离程度高，但固结程度一般。稀土总量REE在126.76×10-6～240.07×10-6，LREE/HREE均值为10.19，属轻稀土富集型，且轻稀土富集程度高于重稀土富集程度，δEu值多在0.37～0.56亏损较为明显，稀土元素标准化配分曲线均呈右倾较缓的平滑曲线。大离子半径亲石元素Rb、Ba、U、K、Th较为富集，高场强元素Nb、Ta、Ti、P等较为亏损，原始地幔标准化配分曲线呈明显左高右低的“W”型，具有明显的弧火山岩特点。Mg#大部分在16~36，206Pb/204Pb=18.181～18.218，207Pb/204Pb=15.570～15.588，208Pb/204Pb=38.368～38.529，显示岩浆来源于下地壳。该区火山岩与A型俯冲构造活动有关，利用Hf/3-Th-Nb/16等构造环境判别图解分析，其形成于陆缘弧环境。Abstract: 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.
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Key words:
- geochemistry /
- tectonic setting /
- Elashan formation /
- reshui area /
- epicontinental arc
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