新疆青河县阿拉图拜一带晚石炭世侵入岩地球化学特征及就位机制浅析
Geochemical Characteristics and Emplacement Mechanism of the Late Carboniferous Intrusive Rocks in Alatubai of Qinghe County, Xinjiang
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摘要: 新疆青河县阿拉图拜一带晚石炭世二长花岗岩呈北西—南东向长条状展布于玛因鄂博断裂南西侧, 整体分布严格受断裂构造控制, 具明显的糜棱岩化变形特征。岩石中K2O+Na2O为7.55%~9.09%, K2O/Na2O为1~1.88, 里特曼组合指数σ为2.35~2.86, 属高钾钙碱性岩;CIPW标准矿物Q含量23.14%~31.41%, C含量0.54%~1.69%, 属SiO2过饱和的准铝质-过铝质岩石, "I"型花岗岩类。稀土元素∑REE为227.04×10-6～326.42×10-6, L/H大多小于10, δEu为0.42～0.58, 轻稀土富集程度中等, 具有中等程度的Eu负异常, 微量元素K、Rb、Th、Ce等呈正异常, Sr、Ba、Nb、P、Ti等为负异常, 经历过一定程度的斜长石、磷灰石、钛铁矿等矿物分离结晶作用;全岩δ18O值4.4‰～4.6‰, 岩石在亚固态条件下与沿裂隙渗入的低18O的大气降水进行过氧同位素交换, 87Sr/86Sr为0.704 34, 具幔源岩浆的特征;全岩Rb-Sr等时线年龄为(300±4)Ma。晚石炭世中晚期, 哈萨克斯坦-准噶尔板块与西伯利亚板块碰撞对接后的伸展阶段, 深部幔源玄武质岩浆以底僻方式沿断裂构造上升并部分熔融壳源物质, 最终以岩墙扩张的方式定位, 后期具有中等程度的剥蚀。Abstract: The late Carboniferous adamellite spread to the southwest of Mayin'ebo fracture on a NW-SE long strip in Alatubai area of Xinjiang Qinghe County. They have obvious mylonitized deformation characteristics and are strictly controlled by faults. The grade of the rock K2O + Na2O is 7.55%-9.09%,the ratio of K2O/Na2O is 1-1.88, the Rittman combination index σ is 2.35-2.86. The data suggest that the late Carboniferous adamellite is high-K calc-alkaline rocks; the CIPW standard mineral Q content of 23.14%-31.41% and C content of 0.54%-1.69% indicate that the intrusive body is supersaturated SiO2 metaluminous-peraluminous rock, known as type I granite. The ∑REE value of late Carboniferous adamellite is (227.04-326.42)×10-6, with most L/H values less than 10. The enrichment of light REE is middle, with medium δEu anomaly of 0.42-0.58. The trace elements took on positive anomaly of K, Rb, Th, Ce and negative anomaly of Sr, Ba, Nb, P, Ti, and experienced some degree of plagioclase, apatite, ilmenite and other minerals fractional crystallization. The whole-rock δ18O value is 4.4‰-4.6‰, a strong oxygen isotopic exchange occurred between the rock and low 18O meteoric water along fissures infiltration under subsolidus conditions. Together with 87Sr/86Sr ratio of 0.70434, it is speculated that the adamellite would be variants of mantle-derived magma. The Rb-Sr isochron age of whole rock minerals is (300±4) Ma. In the middle-late period of Neo-Carboniferons epoch, after the extension stage colliding and connecting between Kazakstan-Junggar and Siberia Plate, the deep mantle-derived basaltic magma raised along faults with partial melting of crustal material, and eventually emplaced by dike spreading with moderate erosion in the latter stage.
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