东天山哈尔里克山区二叠纪高钾钙碱性花岗岩成因及地质意义
Petrogenesis and geological implications of the Permian high-K calc-alkaline granites in Harlik Mountains of eastern Tianshan, NW China
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摘要: 本文从岩石学、地球化学和同位素等方面讨论东天山克拉麦里-哈尔里克岛弧东段八大石和小铺东两个岩体的特征和成因。八大石岩体和小铺东岩体主要为二长花岗岩,SiO2含量分别为61.92%~74.40%和69.17%~74.92%,K2O+Na2O的含量分别为6.50%~8.32%和7.74%~8.14%,绝大部分岩石具有高钾钙碱性花岗岩特征;∑REE分别为105×10-6~210×10-6和100×10-6~172×10-6,(La/Yb)CN分别为4.1~8.9和9.1~15.3,配分模式右倾,δEu分别为0.40~0.93和0.59~0.80,为中-低负异常;两个岩体均富Rb、Ba等大离子亲石元素和Th、U、Hf、Zr等高场强元素,而贫Ta、Nb、Sr、Ti等。通过LA-ICP-MS分析得到八大石岩体中锆石206Pb/238U加权平均年龄为298±2Ma,表明岩体形成于早二叠纪,计算得到八大石和小铺东岩体的模式年龄tDM分别为944Ma和648Ma;八大石和小铺东岩体的εNd(t)u分别为+3.06和+4.47,(87Sr/86Sr) i分别为0.70475和0.70384,表现出高εNd(t)u 低(87Sr/86Sr) i。综上所述,认为哈尔里克高钾钙碱性花岗岩可能为碰撞后挤压-伸展转折阶段的产物,主要由来自新生地壳的中钾钙碱性岩浆经过结晶分异作用而成。Abstract: Based upon petrologic, geochemical and isotopic investigations, this paper expounds genesis of the Badashi and Xiaopudong plutons in the eastern part of the Kerameili-Harlik volcanic-arc belt of eastern Tianshan. The Badashi and Xiaopudong plutons are mainly composed of monzogranites with SiO2 of 61.92%~74.40% and 69.17%~74.92%,K2O+Na2O of 6.50%~8.32% and 7.74%~8.14%, respectively. SiO2 and K2O values of most samples are plotted in the area of high-K calc-alkaline granites. These two plutons are characterized by REE of 105×10-6~210×10-6 and 100×10-6~172×10-6, (La/Yb)CN of 4.1~9.0 and 9.1~15.3, respectively, but consistently show right-inclined distribution patterns. They show weakly negative Eu anomalies with δEu being 0.40~0.93 and 0.59~0.80, respectively. These two plutons are enriched in Rb, Ba (LILE) and Th, U, Hf, Zr (HFSE), and depleted in Ta, Nb, Sr, Ti, etc. Zircon LA-ICP-MS U-Pb isotope determination gave 298±2Ma for the Badashi pluton, indicating their Early-Permian emplacement, model ages (tDM) are calculated to be 944Ma and 648Ma for the Badashi and Xiaopudong plutons, respectively. They are higher in εNd(t)u (+3.06 and +4.47) and lower in (87Sr/86Sr) i(0.70475 and 0.70384).It is suggested that these high-K calc-alkaline granites in the Harlik region were emplaced during post-collisional conversion from compression to extension, and were derived as a result of fractional crystallization of medium-K calc-alkaline magmas from juvenile crustal sources.
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Key words:
- High-K calc-alkaline granite /
- Calc-alkaline /
- Post-collision /
- Underplating /
- East Tianshan /
- Xinjiang
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