新疆北山地区坡北镁铁-超镁铁岩体单斜辉石的矿物学特征及其地质意义

单斜辉石是坡北镁铁-超镁铁杂岩体的主要造岩矿物之一,在各岩相中普遍存在。本文对岩体各岩相中单斜辉石的矿物化学组成进行了电子探针和LA-ICPMS分析,以探明岩体性质、形成的构造背景及演化过程。分析表明,单斜辉石主要氧化物含量为:SiO21.05%~54.00%、MgO 13.95%~17.64%、CaO 19.87%~23.56%、FeO3.44%~6.58%、TiO20.13%~1.36%、Al2O32.17%~4.21%、Na2O 0.17%~0.50%、Cr2O30.01%~1.27%。岩石判别图解表明寄主岩浆为亚碱性拉斑玄武岩,形成于岛弧环境。单斜辉石的结晶温度范围介于1 141~1 221℃。∑REE和(Ce/Yb)N值表明从(异剥)橄榄岩和橄榄单辉辉石岩到橄榄辉长岩和角闪辉长岩,岩浆结晶作用增强。与岩浆正常结晶演化相比,(异剥)橄榄岩和橄榄单辉辉石岩中单斜辉石的SiO2、Na2O含量有所增大,角闪辉长岩和橄榄辉长岩中单斜辉石的CaO含量略有富集,结合前人研究成果和围岩性质,初步认为成岩过程中,可能发生过地壳同化混染作用,其中大理岩为角闪辉长岩和橄榄辉长岩提供了部分Ca元素,而黑云母石英片岩则为(异剥)橄榄岩和橄榄单辉辉石岩贡献了Si、Na等。(异剥)橄榄岩和橄榄单辉辉石岩具较低的FeO含量,此外黑云母石英片岩混入其中,这可能是引起硫化物熔离的重要因素。

Mineralogy of clinopyroxene from Pobei mafic-ultramafic complex in Beishan area, Xinjiang, and its geological significance

As the major rock-forming mineral in Pobei mafic-ultramafic complex,clinopyroxene is distributed in almost every rock phase.In order to find out the nature,evolution and tectonic setting of magma,the authors analyzed chemical compositions of clinopyroxene by means of EPMA and LA-ICPMS.The results demonstrate that cliopyroxene is characterized by SiO2(51.05%～54.00%),MgO(13.95%～17.64%),CaO(19.87%～23.56%),FeO(3.44%～6.58%),TiO2(0.13%～1.36%),Al2O3(2.17%～4.21%),Na2O(0.17%～0.50%),and Cr2O3(0.01%～1.27%).The clinopyroxene discrimination diagrams show that the parent magma was sub-alkaline tholeiite and probably formed in an island arc setting.Clinopyroxene thermobarometer calculation shows that the crystallization temperature was from 1 141℃ to 1 221℃.ΣREE and(Ce/Yb)N values suggests that magma crystallization was intensified from wehrlite and olivine pyroxenite to olivine gabbro and hornblende gabbro.Combined with other data available,the authors believe that the magma was assimilated when it experienced fractional crystallization.Wehrlite and olivine pyroxenite have more SiO2 and Na2O,which might have come from biotite quartz schist,whereas olivine gabbro and hornblende gabbro have more CaO,which probably came from marble.Wehrlite and olivine pyroxenite have lower FeO and were assimilated by biotite quartz schist,which might have led to sulfide segregation.