新疆哈尔里克山二叠纪碱性花岗岩-石英正长岩组合的成因及其构造意义

Petrogenesis and tectonic implications of the Permian alkaline granite and quartz-syenite assemblage in Harlik Mountains,XinJiang.

哈尔里克奥姆尔塔格碱性花岗岩(OMAG)侵入于小铺东二长花岗岩中。该岩体岩性较均一,呈灰紫色,主要矿物组合为石英、钾长石、钠长石、碱性角闪石和霓石;化学成分上富硅、碱,贫钙、镁,低铝;富Rb、Th、U和Zr,Ga/Al比值大(3.18×10~4～3.56×10~4),贫Ba、Sr,岩石中Co、Cr、V和Ni等过渡元素显著亏损,U、Hf、Zr和Nb等高场强元素的含量高;稀土总量较高,配分模式右倾,并具有显著的负铕异常;LA-ICP-MS锆石^(206)Pb/^(238)U加权平均年龄为287.6±2.1Ma;具有正的ε_(Nd)(t)值(+4.96),较低的(^(87)Sr/^(86)Sr),值(0.70159)。同样侵入于小铺东二长花岗岩中的恰干昆多沟含角闪石石英正长岩(QGQS)也呈灰紫色,其结构和构造与OMAG非常相似。两个岩体在主量元素和REE图解上均呈现有规律的分布和一致的演化趋势。从QGQS到OMAG,SiO_2含量增加(从64.71%～64.83%到71.23%～74.31%)、全碱含量降低(从11.72%～11.79%到8.49%～9.87%)和Al_2O_3含量减少(从16.09%～16.19%到11.44%～13.34%)、稀土总量减少(从592.3×10^(-6)～656.0×10^(-6)到273.9×10^(-6)～405.9×10^(-6))、轻/重稀土比值降低(从18.2～20.1到4.4～6.0)、稀土配分曲线从平滑到Eu显著亏损。基于OMAG的岩相学、矿物学、地球化学、锆石年代学和与QGQS的对比研究,笔者认为奥姆尔塔格碱性花岗岩可能是由石英正长岩母岩浆经过分离结晶作用演化的产物,且这种石英正长岩岩浆起源于亏损地幔;根据哈尔里克造山带地质演化历史的分析,这两个岩体均形成于紧随主碰撞后的挤压-伸展转折阶段。

The Omortage alkaline granite ( OMAG) was intruded in the Xiaopudong monzogranite of the Kelameili-Harlik orogenic belt. The rock of the OMAG is massive, grayish purple and mainly composed of quartz (21% similar to 35%), k-feldspar (61% similar to 77%), albite (4% +/-), alkaline amphibole (1% similar to 4%) and aegirine ( < 1%). Chemically, the OMAG is rich in silica and alkali, and poor in Ca, Mg and Al. It is also enriched in lib, Th, U, Zr, Hf and Nb with Ga/Al values between 3. 18 x 10(4) similar to 3. 56 x 10(4), but depleted in Ba, Sr, Co, Cr, V and Ni. The OMAG shows higher Sigma REE and right-inclined distribution patterns with strongly negative Eu anomalies (delta Eu =0.26 similar to 0. 33). Zircon LA-ICP-MS U-Pb isotope determination gave an age of 287. 6 +/- 2. 1Ma for the OMAG. The OMAG is higher in epsilon(Nd) (t) ( + 4. 96) and lower in (Sr-87/Sr-86)(i)(0. 70159). The Qiagankunduogou hornblende-bearing quartz-syenite ( QGQS), which was also intruded in the Xiaopudong monzogranite, exhibits very similar texture, structure and grayish-purple in comparison with the OMAG. Plots of these two plutons show regular distribution on the major elements and REEs diagrams, and indicating consistent evolutional trends. From QGQS to OMAG, SiO2 increases from 64. 71% similar to 64. 83% to 71. 23% similar to 74. 31%, whereas ALK ( Na2O + K2O) deceases from 11.72% similar to 11.79 % to 8. 49% similar to 9. 87 %, Al2O3 from 16. 09% similar to 16. 19% to 11.44% similar to 13. 34%, REE from 592.3 x 10(-6) similar to 656. 0 x 10(-6) to 273.9 x 10(-6) similar to 405.9 x 10(-6), and the ratio of LREE/HREE from 18.2 similar to 20. 1 to 4. 4 similar to 6. 0 with distribution patterns varying from being smooth to intensive Eu depletion. Based on comparative study between the OMAG and OGQS in petrography, mineralogy, geochemistry and zircon chronology, it is concluded that the Omortage alkaline granites ( OMAG) was derived from the quartz-syenite parental magma by fractional crystallization, the quartz-syenite magma came from the depleted-mantle-sourced. Geological evolution of the Harlik orogen indicates that these two plutons were formed during tectonic conversion of post-collision from compression to extension.