东准噶尔乌伦古富碱花岗岩类的成岩作用和源岩

This project is of great importance for elucidating petrogenesis of peralkaline granites in general, a topic of heated discussion in the world, and Carboniferous continental-crust growth of the Central Asian Orogenic Belt in particular. We will systematically analyze element concentrations of phenocrysts from the core through rim and groundmass minerals from granitoids and gabbro-diorites employing EMP and LA-ICP-MS. Based on the partition of elements such as Ba, Sr, Eu and LREE, HREE, HFSE between albite and clinopyroxene phenocrysts, which were trapped in the micro to fine-grained quartz alkali feldspar syenite from the magma chamber, and constituent minerals of alkali-rich granites, we will test the possibility of the phenocrysts as cumulate or crystal-rich mush zones and the alkali-rich granites as conjugate residual melt in the magma chamber. Based on the element partition between coexisting rocks and, more importantly, between constituent minerals (including the perthite phenocrysts and groundmass minerals) of the rocks or micro-domains of the rocks, the project will explore the filter pressing of residual melt from the cumulate or mush zones, immiscibility of the carbonated silicate melt and its incursion and trapping of the cumulate. Trace carbonates from the silicate rock are dissolved with anhydrous phosphoric acid to obtain CO2, whose 13C/12C and 18O/16O are analyzed with continuous-flow, gas chromatography interface to MAT-253 in order to determine whether the CO2 was exsolved from the silicate magma. The compositional variations will be found out from the core through rim of plagioclase, alkali feldspar and clinopyroxene phenocrysts to the groundmass feldspar and ferromagnesian minerals of porphyritic monzogranite, magmatic microgranular enclaves and intermediate-basic intrusions. From the gradual or abrupt variations we can judge whether the intermediate-basic magmas were parental to the alkali-rich granitoids with extensive fractional crystallization or the two rock types were mixing endmembers to produce the porphyritic monzogranite. In the latter case of mixing relationship, values of εNd (t), ISr and zircon εHf (t) of the alkali-rich granitoids are compared with those of potential source rocks in the area: the Late Paleozoic volcanic rocks; the ophiolites from the Zhaheba and Aermantai; gabbro and gabbro-diorites and volcanic rocks of the Carboniferous Batamayineishan Formation. Based on the above comparisons, together with the positive or negative δ18O shifts of crystalline (non-metamictized) zircons from the alkali-rich granitoids relative to the mantle zircon δ18O, the source rock of these alkali-rich granitoids, Ulungur of the East Junggar, can be identified among the candidates: the island arc volcanic rock; the upper oceanic crust; the lower oceanic crust; the intermediate-basic underplates.

本项目对于阐明碱性花岗岩的成因和中亚造山带石炭纪的大陆地壳生长具有重要意义。根据元素在堆晶与共轭岩石组成矿物之间的配分，揭示钠长石和单斜辉石的堆晶作用。根据元素在共生岩石之间尤其是它们的组成矿物（包括条纹长石斑晶、基质或基质微区域组成矿物）之间的配分，探讨残余熔体的压滤排出、碳酸盐化硅酸盐熔体的不混溶分离及其对堆晶的浸润裹挟。以氦气流质谱仪进样分析微量碳酸盐的C-O同位素，确定CO2是否从硅酸盐岩浆出溶。查明似斑状二长花岗岩、暗色微粒包体及中基性侵入岩的长石和单斜辉石斑晶从核心到边缘再到基质长石和暗色矿物的成分变异，确定中基性侵入岩与富碱花岗岩是混合还是母岩浆-残余岩浆关系。将富碱花岗岩与相关岩石的εNd (t)、ISr和锆石εHf (t)对比，结合前者的结晶质锆石的δ18O相对地幔锆石δ18O的正、负漂移，鉴别东准噶尔乌伦古富碱花岗岩类的源岩是岛弧火山岩、上/下洋壳还是底侵的中基性岩石。

项目主要研究内容如下：. 以乌伦古和东准噶尔地区的13个花岗岩体和辉长闪长岩体为研究对象。观察研究诸岩体各岩石类型之间、寄主岩与暗色微粒包体及岩墙的野外关系、岩相学；各岩体的SIMS锆石U-Pb定年。根据斑晶和基质矿物及岩石的主微量元素含量比值，确定石英正长斑岩是否富碱花岗岩的母岩浆，建立似斑状二长花岗岩与中基性侵入岩的成因联系。根据Nd-Sr-Hf-O同位素，鉴别乌伦古富碱花岗岩类的源岩组分。.重要结果及其科学意义如下：. 1. 本项目对研究区13个岩体做了高精度的SIMS锆石U-Pb定年，确定东准噶尔花岗岩类结晶于早石炭世末‒晚石炭世(335–301Ma)。从东准噶尔西北部(330.6–323.1Ma)和乌伦古(335‒315Ma)到卡拉麦里(315‒301Ma)，花岗岩类的年龄渐趋年轻。. 2. 确定乌伦古富碱花岗岩（包括轴部和北翼诸岩体）的母岩浆不是中基性岩浆，而是石英正长岩质的。液体不混溶是乌伦古富碱花岗岩形成的重要机制。液体不混溶形成碱性花岗岩是一项原创性成果。. 3. 具有海岭特征的早古生代洋壳仍然存在于东准噶尔的下地壳，构成了乌伦古富碱花岗岩和卡拉麦里花岗岩的主要源岩。提出幔源岩浆的底侵/内侵引起洋壳的熔融，幔源岩浆与洋壳岩浆混合，这些作用在非俯冲带环境下共同作用产生了东准噶尔花岗岩类。这显然不同于洋壳俯冲消减导致岛弧火成岩形成和大陆地壳生长的经典模式，具有明显的创新性。. 4. 确定东准噶尔西北部的I型花岗岩的源岩是底侵镁铁质岩石变质而成的石榴子石角闪岩。东准噶尔的俯冲在331‒323 Ma之前已经终止。. 5. 索尔库都克Cu-Mo矿床的成矿和矽卡岩化发生于320 Ma左右。远源流体沿着断裂和裂隙上升，淋滤围岩中的Ca、Fe及Cu、Mo成矿物质，单向渗滤交代火山岩-火山碎屑岩。. 6. 首次在国内建立了硅酸盐矿物和岩石中固定氮的同位素分析方法，提供了一个新的源区/岩示踪剂。