东天山四顶黑山层状岩体岩石地球化学特征

The Geochemistry Characteristic of Sidingheishan Mafic-Ultramafic Layered Intrusion, East Tianshan, Xinjiang

位于东天山黄山岩带最东端的四顶黑山镁铁质-超镁铁质层状杂岩体规模较大,呈北东向展布,可分为南、北2个岩体。主要岩石类型有单辉橄榄岩、橄榄辉石岩、橄榄苏长辉长岩、橄榄辉长岩、辉石角闪岩、辉长岩和角闪辉长岩,超镁铁岩相常与辉长岩相呈韵律互层。主量元素化学组成主要属拉斑玄武岩系列。岩石稀土元素配分曲线呈平坦型以及Eu的正异常,普遍富集大离子亲石元素(Rb、Ba、U),显示明显的Sr正异常以及Nb、Ta的亏损。通过橄榄石最高的Fo值(79.6)估算出母岩浆中MgO和FeO含量分别为6.23%和6.60%。岩浆在演化过程中发生了橄榄石、斜方辉石、单斜辉石、斜长石的分离结晶/堆晶作用以及较弱的上地壳同化混染作用。岩石高的(La/Yb)N值1.550和1.744值表明岩浆源区的深度可能大于90km;Th/U值特征以及Th/Yb-Nb/Yb和La/Ba-La/Nb图解表明,四顶黑山岩体的岩浆源区可能为受俯冲作用改造的岩石圈地幔。

The Sidingheishan layered intrusion is located in the eastern of the Huangshan mafic-ultramafic rocks belt, East Tianshan, large-scale, northeast trending, can be divided into north and south intrusion, composes of clinopyroxene peridotite, olivine pyroxenite, olivine norite-gabbro, olivine gabbro, pyroxene amphibolite, gabbro, hornblende gabbro, diorite, and the ultramafic rock phase and gabbro phase often showed a rhythm with interbedded. The petrochemical compositions belong to the tholeiitic series. The chondrite-normalized REE patterns are flat type and positive Eu anomalies, generally enriched in LILE （i. e. , Rb, Ba, U）, significant positive anomalies of Sr and Nb, Ta loss. Estimate the MgO and FeO content of the parental magma is 6.23% and 6.60% by the highest Fo values （79. 6） of olivine, respective- ly. The magma experiences olivine, orthopyroxene, clinopyroxene, plagioclase fractionation / cumulate and weakly upper crustal contamination in the parental magma. The high （La/Yb）N ratio （1. 550） and （Gd/Yb）N ratio （1. 744） indicates that the depth of the magma source may be greater than 90Km; The Th/U ratio characteristic and Th/Yb versus Nb/Yb and La/Ba versus La/Nb diagrams show that the magma source of the Sidingheishan intrusion may be the modified lithosphere mantle by subduction.