华北板块北缘乌梁斯太A型花岗岩体锆石SHRIMP U-Pb定年及构造意义
Zicron SHRIMP U-Pb dating of Wuliangsitai A-type granite on the northern margin of the North China Plate and tectonic significance
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摘要: 内蒙古乌拉特中旗乌梁斯太岩体位于华北板块北缘中段，岩性以黑云二长花岗岩为主。它具有较高的SiO2 (74.69%~76.16%)、Na2O+K2O (8.63%~9.62%)、Rb (73×10-6~200×10-6)含量和10000×Ga/Al (3.21~3.32)、Rb/Sr (3.27~8.68)比值，较低的CaO、Ba和Sr含量，碱性指数AI=mol(K2O + Na2O)/ Al2O3) ≥0.90，属于碱性A型花岗岩。稀土球粒陨石标准化图解上，轻稀土相对富集，Eu负异常较为明显，δEu=0.03~0.06；在原始地幔标准化蛛网图上，亏损Ba、Sr、P、Nb、Ta和Ti，富集Rb、K；这些特征类似中国东北二叠纪A型花岗岩。在判别图解上，所有样品均显示了A2型(后碰撞)花岗岩特征。其形成可能与后碰撞板块断离作用有关。乌梁斯太岩体的形成过程可能为，俯冲板片断离导致软流圈物质上涌，发生底侵作用，底侵的玄武质岩浆的加热作用以及地壳伸展的减压效应导致中下地壳的脱水部分熔融，随后经历了分离结晶和晚期的交代作用。乌梁斯太岩体中的锆石SHRIMP U-Pb年龄为277±3Ma，表明其形成时代并非原来认为的三叠纪，而是早二叠世晚期。Abstract: Wuliangsitai Pluton from Urad Zhongqi, central Inner Mongolia, is located on the middle segment of the northern margin of the North China Plate. The Pluton mainly consists of biotite monzogranites. All samples show chemical signatures of alkaline A-type granite and are characterized by high contents of SiO2 (74.69%~76.16%), Na2O + K2O (8.63%~9.62%) and Rb (173×-6~200×10-6), high ratios of 10000×Ga/Al (3.21~3.32), Rb/Sr (3.27~8.68) and alkaline index (A.I.=mol(K2O + Na2O)/ Al2O3)≥0.90), low contents of CaO, Ba and Sr. They invariably exhibit light rare earth elements (LREE) enrichment with significant negative Eu anomalies (δEu=0.03~0.06) in the chondrite-normalized REE patterns, depletion of Ba, Sr, P, Nb, Ta, Ti and enrichment of Rb, K in the primitive mantle normalized spidergram, which are similar to the Permian A-type granitoids in northeastern China. In the discrimination diagrams, all the samples show geochemical characteristics of A2-type (post-collisional) granitoid, which means that their formation may be associated with post-collisional slab break-off. Wuliangsitai Pluton may be produced by partial melting of middle or lower crust by heating via mantle-derived basaltic magma and decompression due to crustal extension, followed by fractional crystallisation and late-stage replacement. SHRIMP U-Pb zircon age of 277±3Ma from Wuliangsitai Pluton implies it was emplaced during the late stage of Early Permian, not the Triassic as previously thought.
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Key words:
- The northern margin of the North China Plate /
- A-type granite /
- Post-collision /
- Permian
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