西昆仑塔什库尔干苦子干碱性杂岩体的成因及其构造意义

苦子干碱性杂岩体是塔什库尔干新生代碱性岩带的主要组成之一，主要由霓辉正长岩、石英霓辉正长岩和碱性花岗岩组成。霓辉正长岩、石英霓辉正长岩和碱性花岗岩的SiO2含量差别较大，分别为50．26％～54．11％、59．74％～60．43oA、69．59％～72．13％；霓辉正长岩与石英霓辉正长岩的里特曼指数相近，分别为7．83～10．97、8．51～9．05，明显不同于碱性花岗岩（2．49～3．71）。这3种碱性岩石都表现出富集轻稀土，以及K、Rb、Sr、Ba等大离子亲石元素（LILE），亏损Nb、Ta、Ti等高场强元素（HFSE）的特征。其中异常高的Sr、Ba和∑REE含量暗示可能还有富集地幔物质的加入。利用原位锆石LA—ICP—MS同位素测试方法，获得石英霓辉正长岩和碱性花岗岩的锆石U—Pb年龄分别为10．9±0．1Ma和11．9土0．4Ma，代表了岩石的结晶年龄；石英霓辉正长岩锆石εHf（t）值为-9．78～3．24，碱性花岗岩中锆石εHf（t）值为-13．54～5．78，均显示岩浆源区具有壳幔混合的特点。依据这些资料，并结合区域地质研究的综合分析，笔者初步提出西昆仑塔什库尔干苦子干碱性杂岩体的形成是由于青藏高原在～25Ma期间主体拆沉作用发生后，造成软流圈物质向北逃逸，并受到北面塔里木克拉通岩石圈山根阻挡而上涌，进而发生壳幔混合作用的产物。因此，该杂岩体是青藏高原大规模拆沉后浆活动的远程效应，且标志着研究区在～11Ma期间已处于伸展构造背景。

Petrogenesis and Tectonic Implications of the Cenozoic Alkaline Complex in Kuzigan,Taxkorgan, West Kunlun

Kuzigan alkaline complex is one of the major parts of the Taxkorgan alkaline complex- terrane. It is mainly composed of aegirine-augite syenite, quartz aegirine-augite syenite, and alkali granite. There is great difference among the three rocks in the content of SiO2, 50.26%-54. 11%, 59.74%-60.43% and 69. 59 %-72.13 % for aegirine-augite syenite, quartz aegirine- augite syenite and alkali granite, respectively. Aegirine-augite syenite and quartz aegirine-augite syenite has similar Rittman index, namely 7.83-10.97 and 8.51-9.05, distinctly differing from 2.49-3.71 of alkali granite. These three rocks are strongly enriched in light rare earth elements （LREE） as well as some large ion lithophile element （LILE）, such as Rb, Ba, St, and depleted in High Field Strength Element （HFSE） such as Nb, Ta and Ti. Abnormal high Sr, Ba and REE content may imply the mixing of enriched mantle materials. LA-ICP-MS in-situ zircon U-Pb dating obtain the ages of 10.9±0.1 Ma and 11.9!0. 4Ma for the quartz aegirine-augite syenite and alkali granite respectively, representing the crystallization ages of the two rocks. The εHf （t） values of the quartz aegirine-augite syenite and alkali granite are of -9.78-3.24 and -13.54- 5.78, showing the crust-mantle mixing in magma source area. According to the comprehensive a- nalysis and combined with regional geological data, we preliminarily hypothesized that the Kuzigan alkaline complex is a product of the crust-mantle mixing, which is caused by the as the- nosphere flowing northward due to the delamination of the Tibetan plateau at -25Ma, and the asthenosphere upwelling as a result of the obstruction of the root of the Tarim craton lithosphere. Therefore, the formation of the complex is a fernwirkung of the magmation after Tibetan plateau large-scale delamination, and it signifies that during - 11 Ma the study area was in extensional tectonic setting.