华北板块北缘中段新太古代的陆-陆碰撞事件: 来自合教S型花岗岩的证据

通过对内蒙古达茂旗合教地区具有明显构造意义的新太古代S型花岗岩的主量元素、微量元素及SHRIMP锆石U-Pb年代学的研究, 为揭示华北板块北缘新太古代末的构造环境提供了进一步的证据。合教花岗岩的主量元素特征表明它是高分异的强过铝 S 型花岗岩; 其里特曼指数 σ为0. 54 ～0. 57, 属于低钾钙碱性岩石。稀土元素总量为 243. 8 ～427. 1 μg/ g。球粒陨石标准化分配模式显示 LREE 相对富集, HREE 相对亏损; ( La/ Yb)_N为 14. 88～18. 18, Eu 具有较大的负异常( Eu*/ Eu = 0. 31 ～0. 65); 模式图总体呈一右倾的海鸥型。微量元素显示Nb, Ta, Zr, Hf, Ti 等高场强元素相对亏损。合教花岗岩的微量元素组成表明其来源于正常厚度的大陆地壳, 形成于低压高温的后碰撞环境, 相当于澳大利亚拉克伦造山带的S型花岗岩。合教花岗岩的SHRIMP锆石U-Pb年龄为2556±8Ma, 属于新太古代晚期。合教强过铝 S型花岗岩反映在新太古代晚期华北板块北缘中段可能发生了一次陆-陆碰撞事件, 也表明固阳地区同时期的赞岐岩可能形成于后碰撞环境而非板片消减带。

Late Archean Continent-Continent Collision Event of Middle Segment of North Margin of North China Plate:Evidence from S-Type Granite of Hejiao Area

The study of Neoarchean S-type granite, with obvious tectonic significance, which lies in Hejiao area, Darhan Muminggan Lianheqi, Inner Mongolia, in major elements, trace elements and SHRIMP zircon U-Pb geochronology provided further evidence for understanding the nature of tectonic setting of the northern margin of North China plate at the end of Neoarchean. The major elements features of Hejiao granite indicate that it belongs to cal-alkaline series with strongly peraluminous characteristics and Rittman index σis between 0. 54 and 0. 57. The granite has high total rare earth elements (REE) values(243.8- 427. 1 μg/g), and enriches light REE relative to the heavy ones, and has negative anomalies characteristics in Eu ( ( La/ Yb)_N = 14. 88-18. 18, Eu^* / Eu = 0. 31- 0. 65) . In addition, it depletes high field strength elements( HFSE) , such as Nb, Ta, Zr, Hf and Ti. The trace elements characteristics of Hejiao granite reflect that it is formed under post-collisional tectonic setting and it is from normal thick continental crust in low pressure-high temperature conditions, which are similar to the formation conditions of Lachlan fold belt in Australia. The SHRIMP U-Pb zircon age of Hejiao granite is 2556 ±8 Ma. Strongly peraluminous S-type granite of Hej iao indicates that, in the end of Neoarchean, a continent-continent collision event probably occurred at the middle segment of north margin of North China plate, and that the sanukite with the same age in Guyang area, was possibly formed at post-collisional tectonic setting rather than subduction zone.