新疆中天山白石泉含铜镍矿镁铁-超镁铁岩体地球化学特征与岩石成因

Geochemistry and petrogenesis of the Baishiquan Cu-Ni sulfide-bearing mafic-ultramafic intrusion in the Central Tianshan,Xinjiang,NW China.

白石泉含Cu-Ni硫化物镁铁-超镁铁侵入体位于中天山地块,其主要岩石类型有辉石橄榄岩、橄榄辉石岩、橄长岩、辉长岩及角闪辉长岩等,具有明显的堆晶结构。对岩体的主要、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,具有拉斑玄武质岩浆的分异趋势;并相对富集大离子亲石元素(Rb、Ba、Sr等)、亏损高场强元素(Nb、Ta、Zr、Hf),以及具有LREE富集((La/Yb)_n=1.27～9.95)的右倾型稀土元素分布模式。Pb同位特征表明母岩浆与软流圈地幔和EMⅡ地幔物质有关。综合地球化学特征表明,母岩浆为来源于受早期俯冲物质混染的软流圈地幔的高镁拉斑玄武岩浆。岩浆在上升过程中,没有受到上地壳及围岩的混染。原生岩浆形成后,经过了橄榄石、辉石、斜长石及铬铁矿的分离结晶作用和硫化物的熔离作用。目前出露的岩体为富含橄榄石及硫化物的"晶粥"在高位岩浆房中演化的产物。

Located in the Central Tianshan block, northern Xinjiang, NW China, the Baishiquan Cu-Ni sulfide-bearing mafic ultramafic intrusion is composed of harzburgite, olivine pyroxenite, troctolite, gabbro and hornblende gabbro. Many of them have a cumulate texture. The major element, trace element and REE geochemistry of the intrusion indicates that: the intrusion is mafic-ultramafic and shows a trend of tholeiitic magmatic differentiation; it is relatively enriched in large ion lithophile elements (Rb, Ba, Sr etc.) and depleted in high field strength elements (Nb, Ta, Zr, Hf) and exhibits LREE-enriched ((La/Yb)(n) = 1.27 to 9.95)), right-inclined REE distribution patterns. According to these geochemical characteristics, combined with their Ph isotope characteristics, it may be inferred that the primary magma may have been derived from the asthenospheric mantle and EM 11 mantle materials. The integrated geochemical characteristics suggest that the parent magma originated from the high-Ma tholeiitic magma in the asthenospheric mantle that had undergone contamination with early-subducted oceanic crustal materials. During the rise of the magma, the upper continental crust and country rocks had no contribution to it. After its formation, the primary magma underwent fractional crystallization of olivine, pyroxenes, plagioclase and spinel (or chromite) and liquation of sulfides. The intrusion exposed now is the product of the evolution of olivine- and sulfide-laden crystal mush in the high-level magma chamber and its in-situ differentiation.