新疆东天山黑白山镁铁超镁铁岩地球化学、U-Pb年代学、Hf同位素特征及地质意义

Geochemistry,U-PB chronology,Hf isotope Characteristics and Geological Significance of the Heibaishan Mafic Ultramafic Rocks of East Tianshan,Xinjiang

黑白山镁铁超镁铁岩位于新疆中天山地块与觉罗塔格构造带的分界断裂——阿其克库都克大断裂南侧约4 km处。该岩体主要由阳起石化橄榄辉石岩、辉橄岩、辉石岩、蚀变细粒辉长岩组成,对岩体的主微量及稀土元素地球化学特征研究表明,岩体属铁质镁铁超镁铁岩,具低钾拉斑玄武质岩的分异特征;岩石具相对较低SiO_(2)(43.90%~52.80%)、相对富Na_(2)O(0.38%~2.97%),贫K_(2)O(0.11%~0.47%)及Na_(2)O>K_(2)O特征;MgO值变化大且较高(4.25%~32.49%),较高Mg#(63.94~82.98),相对富集轻稀土((La/Yb)N为2.03~5.76),富集大离子亲石元素Rb,Ba,U,La,亏损高场强元素Nb,Ta,Ce,Ti,强烈富集Pb,说明岩石具幔源特征和岛弧岩浆特征。LA-ICP-MS锆石UPb年龄(392.7±4.5)Ma表明,该岩体属早泥盆世岩浆活动的产物。Hf同位素等地球化学特征和岩相学显示,岩浆起源于含有早期俯冲洋壳的亏损软流圈地幔,在岩浆演化过程中受到地壳及围岩混染,但混染作用较弱。黑白山岩体的发现为北天山洋在早古生代沿阿其克库都克断裂向南俯冲提供了新的证据。

The Heibaishan mafic ultramafic rock is located about 4 km south of the Akkekuduk fault,the boundary fault between the Middle Tianshan block and the Jelotag tectonic belt in Xinjiang province.The rock mass is mainly composed of anolitic pyroxenite,pyroxenite,pyroxenite and altered fine gabbro.The study on the geochemical characteristics of the main,trace and rare earth elements of the rock mass shows that the rock mass belongs to ferromagnesian ultramafic and has the differentiation trend of low potassium tholeiite.Rocks with relatively low SiO_(2)(43.90%~52.80%),relatively rich Na_(2)O(0.38%~2.97%),poor K_(2)O(0.11%~0.47%)and Na_(2)O>K_(2)O,the characteristics of the MgO style change of value and higher(4.25%~32.49%),high Mg#(63.94~82.98),the relative enrichment of light rare earth(La/Yb N 2.03~5.76),the enrichment of large ion lithophile Rb,Ba,U,La,loss of high field strength elements Nb,Ta,Ce,Ti,strong enrichment of Pb,It shows that the rocks have mantle-derived characteristics and island arc magmatic characteristics.The zircon LA-ICP-MS U-PB age(392.7±4.5Ma)indicates that the rock mass is a product of early Devonian magmatic activity.The geochemical characteristics of hafnium isotopes and petrology show that the magma originated from the asthenosphere with the loss of the subducted oceanic crust at the early stage,and was mixed by the crust and surrounding rocks during the magma evolution,but the mixing was weak.The discovery of the Heibaishan rock mass provides a new evidence for the southern subduction of the Northern Tianshan Ocean along the Akhikkuduk fault in the early Paleozoic.