新疆东天山黄山东岩体橄榄石成因意义探讨

Correlations between Fo number and Ni content of olivine of the Huangshandong intrusion,eastern Tianshan,Xinjiang,and the genetic significances

黄山东岩体位于新疆东天山造山带中段,由四次岩浆侵入形成:第一次侵入形成了岩体上部的橄榄辉长岩、角闪辉长岩和闪长岩,构成岩体的主体;第二次侵入形成辉长苏长岩,呈岩墙状分布于岩体西端和西北部;第三次侵入岩石为斜长二辉橄榄岩,位于岩体下部,为主要的赋矿岩石;第四次侵入岩石为底部角闪辉长岩。橄榄石为第三次侵入的斜长二辉橄榄岩和第一次侵入的橄榄辉长岩主要造岩矿物之一,橄榄石的镁橄榄石牌号(Fo)值介于68.5～82.5之间。其中含硫化物斜长二辉橄榄岩中的橄榄石具有较高的Fo值(79.7～82.5);斜长二辉橄榄岩中橄榄石的Fo值为78.3～79.9;而基性程度较低的橄榄辉长岩中橄榄石具有较低的Fo值(68.5～72.2)。利用橄榄石矿物成分计算得出黄山东岩体母岩浆Mg#(Mg2+/(Mg2++Fe2+))为0.59,为原生玄武质岩浆经历结晶分异作用形成。模拟计算结果显示黄山东岩体不含矿岩石中橄榄石是母岩浆经过2%的橄榄石结晶分异且硫达到饱和后,在硫化物熔离的同时岩浆发生橄榄石结晶而形成,并且橄榄石︰硫化物≈50︰1,部分橄榄石成分投点在橄榄石结晶和硫化物熔离的模拟曲线右下侧,指示它们可能受到晶间硅酸盐熔浆作用的影响。含硫化物斜长二辉橄榄岩中Fo值与Ni含量呈负相关关系,说明橄榄石与硫化物熔体之间发生了Fe-Ni交换反应。

The Huangshandong mafic-ultramafic intrusion is located in middle of eastern Tianshan orogenic belt, Xinjiang. Four stages of magma intrusion have been identified. The first magmatic emplacement is comprised of olivine gabbro, hornblende gabbro and diorite, formed the upper part and the main portion of the intrusion. The resultant of the second stage emplacement formed gabbronorite dykes in the western and northwestern of the intrusion. Plagioclase lherzolite, lies in the lower part and regarded as the main host rock for the Cu-Ni ores, represents the products of the third magma intrusion. The last stage emplacement formed bottom hornblende gabbro. The olivine occurs mainly in the first stage olivine gabbro, and in the third stage plagioclase lherzolite. The forsterite number（Fo） of olivine ranges from 68.5 to 82. 5. The olivine from sulfide-bearing plagioclase lherzolite has highest Fo （ 79. 7 - 82. 5 ） ; the Fo of olivine in the plagioclase lherzolite range from 78.3 to 79. 9; the olivine from fractionated olivine gabbro has lower Fo（68.5 - 72. 2）. The Mg^#（ atomic Mg/（ Mg ＋ Fe） ）of parental magma estimated by using the compositions of most Fo-rieh olivine crystals is 0. 59 and indicates the parental magma was formed by fractional crystallization of primary basaltic magma. The model calculation results display the data of olivine from the ore-free rocks are explicable if the magma became saturated with sulfide after 2% olivine crystallization, and then simultaneously separated olivine and sulfide in a ratio of 50 ： 1. Scatter to the lower fight side of olivine crystallization and sulfide segregation line is explicable as result of the olivine interacting with trapped intercumulus liquid. The negative Fo-Ni relationship of olivine from the sulfide-bearing plagioclase lberzolite is apparently related to local equilibration of the olivine with trapped sulfide liquids.