东天山雅满苏北岩体的年代学、地球化学及其构造意义

Geochronology,geochemistry and tectonic significances of the Yamansubei pluton in eastern Tianshan,Northwest China

雅满苏北岩体位于新疆东天山地区的觉罗塔格晚古生代构造带。岩性为钾长花岗岩,主要矿物成分为石英(25%～40%)、条纹长石(40%～70%)、斜长石(10%～25%,An18～30)和黑云母(2%～5%)。LA-ICP-MS锆石U-Pb定年结果表明,该岩体侵位于中三叠世(227.90±0.47Ma),为印支中期产物。该岩体的主量元素以高硅(SiO2为74.27%～75.99%)、富碱(ALK为7.64%～8.29%)、高钾(K2O为4.37%～4.79%)和准铝质(A/CNK为0.96～1.00)以及较低的Ti,Ca,Fe和Mg含量为特征,属于高钾钙碱性岩花岗岩。岩体的微量元素总体富集大离子亲石元素(LILE)K、Rb和Cs等和高场强元素(HFSE)Th、U、Zr和Hf,贫大离子亲石元素Ba、Sr和高场强元素Nb、Ta及Y。稀土元素的球粒陨石标准化配分曲线呈右倾型,轻重稀土分馏明显,轻稀土分异明显,而重稀土分异并不显著,Eu呈现弱负异常(δEu=0.48～0.86)。来自岩体的锆石原位Hf同位素测定结果表明,εHf(t)值变化于8.60～11.31,加权平均值为9.96±0.53(n=8),二阶段Hf模式年龄(tDM2)变化于538～711Ma。通过该岩体岩相学、地球化学及Hf同位素特征的综合研究以及该区区域构造演化表明,雅满苏北岩体是幔源岩浆底侵带来的大量热能使得新元古代新生的下地壳物质发生部分熔融的产物。结合前人的研究,笔者认为,东天山地区的印支期岩浆活动的动力来源可能与中-晚石炭世天山陆壳整体化进入板内阶段后又受到特提斯构造体制显著影响有关。

The Yamansubei granite pluton （YB） is situated in the Late paleozioic Qoltag orogenic belt,eastern Tianshan,Northwest China. Petrographically,the pluton is a K-feldspar granite,which is composed of quartz （25% ～ 40%）,perthite （40% ～ 70%）, plagioclase （10% ～ 25%,An = 18 ～ 30） and biotite （2% ～ 5%）. Zircon LA-ICP-MS U-Pb dating shows that the YB pluton intruded at 227. 9 ± 0. 47Ma of Middle Triassic. Geochemically,the pluton is characterized by high silicon （SiO2 = 74. 27% ～ 75. 99%）, alkali （ALK = 7. 64% ～ 8. 29%）,potassium （K2O = 4. 37% ～ 4. 79%） and metaluminous,whereas low titanium,calcium,iron and magnesium,which attributes to high K calc-alkaline granite. The pluton is also rich in LILE （K,Rb,Cs etc.）,HFSE （Th,U,Zr, Hf etc.） and depleted in Ba,Sr,Nb,Ta and Y. Chondrite-normalised REE distribution pattern for the YB pluton displays right skewed shapes with slightly negative Eu anomalies （δEu = 0. 48 ～ 0. 86） ,indicating fractionation of LREE,and fractionation between LREE and HREE. Zircon in-situ Hf isotopic composition analysis yields the εHf（t） and t DM2 values of the YB pluton varying ＋8. 60 ～ ＋11. 31 and 538 ～ 711Ma,respectively. Integrated studies in petrography,geochemistry and Hf isotope and regional tectonic evolution indicate that the YB pluton resulted from mantle magma underpalting induced partial melting of the Neoproterozoic new-born lower crust. After comprehensive study on previous contributions,this paper suggests that the geodynamic source for Indosinian magmatism in eastern Tianshan could attribute to the influence of the Paleo-Tethys ocean tectonic regime.