| 新疆西准噶尔接特布调A型花岗岩年代学、地球化学及岩石成因 |
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| 引用本文：杨高学,李永军,张兵,汪雅兵,刘振伟,严镜,田陟贤.2013.新疆西准噶尔接特布调A型花岗岩年代学、地球化学及岩石成因[J].地球学报,34(3):295-306. |
| DOI：10.3975/cagsb.2013.03.05 |
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| 基金项目:中央高校基本科研业务费专项资金(编号: CHD2010ZY005)；新疆维吾尔自治区地质矿产勘查开发局国土资源大调查项目(编号: XJZBKD2008-04; XJQDZ2009-03) |
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| 中文摘要:西准噶尔乃至整个北疆地区广泛发育晚古生代后碰撞花岗岩类。接特布调岩体作为一个典型的代表, 岩石类型主要有中粗粒二长花岗岩和正长花岗岩, 是认识西准噶尔花岗岩岩石成因及构造-岩浆演化的关键。本文对接特布调岩体进行高精度锆石LA-ICP-MS U-Pb测年, 获得二长花岗岩和正长花岗岩的加权平均206Pb/238U年龄分别为(287±9) Ma(n=10, MSWD=0.92)和(278±3) Ma(n=14, MSWD=0.43), 确定其形成于早二叠世, 属于300 Ma前后准噶尔周边地区后碰撞岩浆活动的产物。岩石地球化学研究表明, 前人认为的接特布调I型花岗岩应归属于A型花岗岩。正长花岗岩具有高硅(SiO2: 76.11%~76.82%), 富碱(Na2O+K2O: 8.47%~8.49%), 低钛(TiO2: 0.04%~0.05%), 贫钙(CaO: 0.36%~0.42%)的特征。二长花岗岩与其类似, 高硅(SiO2: 68.35%~71.80%), 富碱(Na2O+K2O: 6.80%~7.86%), 低钛和钙(TiO2: 0.29%~0.82%, CaO: 1.76%~2.87%), 均属于准铝质或弱过铝质(ACNK: 0.98~1.09)高钾钙碱性系列。正长花岗岩相对于二长花岗岩具有相对较低的稀土元素总量(ΣREE)(分别为23.8×10-6~49.3×10-6, 95.23×10-6~222.2×10-6), 并具有明显的负Eu异常(Eu/Eu*分别为0.01~0.02, 0.57~0.72), 另外, 正长花岗岩相对二长花岗岩明显地富集大离子亲石元素(Rb、Th、K)及高场强元素(Zr、Hf、Nb), 而强烈亏损Ba、Sr、Eu、Ti等, 具有较高的10000Ga/Al比值(>2.44)。依据微量元素比值及相关判别图, 可将接特布调花岗岩体进一步细分为A1型和A2型。接特布调岩体就位于后碰撞环境, 来源于由年轻的地幔来源物质组成的下地壳。在后碰撞岩浆活动的初期, 年轻的下地壳部分熔融形成具有岛弧印迹的A2型二长花岗岩岩浆, 随着岩石圈进一步伸展, 可能在局部出现类似裂谷的环境, 即形成显示裂谷特征的A1型正长花岗岩岩浆。 |
| 中文关键词:A型花岗岩 锆石LA-ICP-MS U-Pb定年 岩石地球化学 岩石成因 后碰撞 西准噶尔 |
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| Geochronology, Geochemistry and Petrogenesis of the Jietebutiao A-type Granites in West Junggar, Xinjiang |
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| Abstract:Late Paleozoic post-collisional granitoids are widespread in West Junggar and even in the whole northern Xinjiang. As a representative of these intrusions, the Jietebutiao granite occurs in the southwestern margin of West Junggar, mainly comprises middle-coarse-grained monzogranite and syenogranite, and provides important clues for petrogenesis of granites and evolution of tectonic-magmatism in West Junggar. This paper reports the results of high-precision zircon LA-ICP-MS U-Pb dating of the Jietebutiao granite, which yields weighted mean 206Pb/238U ages of (287±9) Ma (n=10, MSWD=0.92)and (278±3) Ma (n=14, MSWD=0.43)for monzogranite and syenogranite, respectively, corresponding to the Early Permian and implying that the granite is the product of post-collisional magmatic activity around the Junggar area approximately at 300 Ma. Petrogeochemical analyses suggest that the Jietebutiao pluton which has long been thought to be of I-type granite actually belongs to A-type. The syenogranite is characterized by high silica (SiO2: 76.11%~76.82%), alkali (Na2O+K2O: 8.47%~8.49%), and low titanium and calcium (TiO2: 0.04%~0.05%, CaO: 0.36%~0.42%). Moreover, the monzogranite is similar to the syenogranite in such aspects as high silica (SiO2: 68.35%~71.80%) and alkali (Na2O+K2O: 6.80%~7.86%) as well as low titanium and calcium (TiO2: 0.29%~0.82%, CaO: 1.76%~2.87%); nevertheless, they both belong to metaluminous or peraluminous (ACNK: 0.98~1.09) high-K calc-alkaline series. Compared with the monzogranite, the syenogranite has relatively low REE content (ΣREE: 23.8×10-6~49.3×10-6, 95.23×10-6~222.2×10-6) with significant negative Eu anomalies (Eu/Eu*: 0.01~0.02, 0.57~0.72). They are also enriched with large ion lithophile elements such as Rb, Th and K and high strength field elements such as Zr, Hf and Nb and strongly depleted in Ba, Sr, Eu and Ti, with high 10000Ga/Al ratios (>2.44). These characteristics are more clearly in syenogranite than in monzogranite. Based on trace element ratios and related discrimination diagrams, the Jietebutiao pluton can be further subdivided into A1 and A2 type granites, which are usually believed to have been formed in an post-collisional tectonic setting and derived from the lower crust composed of juvenile mantle-origin substance. In the early post-collisional magmatism, A2-type monzogranite magma characterized by island arc features was derived from partial melting of the lower crust and, with the further extension of the lithosphere, probably formed a rift-like environment in local area, producing the A1-type syenogranite magma. |
| keywords:A-type granites zircon LA-ICP-MS U-Pb dating petrogeochemistry petrogenesis post-collision West Junggar |
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