扬子地块北缘～635 Ma周庵超镁铁质岩体是一个新发现的含铜镍硫化物矿化的隐伏岩体,主要由二辉橄榄岩组成.文章通过研究周庵岩体中橄榄石、铬铁矿和辉石的矿物成分变化探讨了岩浆演化过程和含矿岩体成因.根据岩石的矿物组合和蚀变程度,岩体从上到下分为3个部分:上部绿泥石-蛇纹石化二辉橄榄岩相带、中部二辉橄榄岩相带和下部绿泥石-角闪石化二辉橄榄岩相带.根据岩体中部带橄榄石和铬铁矿的成分,计算得到母岩浆的Mg#值为0.63,MgO/FeO摩尔比值为1.72,w(Al2O3)为10.2％～11.7％,w(Ni)为476×10-6,说明其为高镁玄武质岩浆；岩体中部带原生铬铁矿和粒间相铬铁矿核部的Cr2O3和Al2O3呈正相关关系,说明铬铁矿与粒间硅酸盐熔体发生了平衡交换,铬铁矿的高w(TiO2)和Cr#值与拉张环境中层状岩体的铬铁矿特征一致；根据辉石温压计得到岩体中部单斜辉石和斜方辉石的共结温度为1017～1077℃,压力为(3.6～4.5)×108 Pa,暗示形成岩体的浅部岩浆房深度约为12 km.岩体上部和中部带的橄榄石Fo值大部分集中在80 mol％～85 mol％,w(Ni)介于2 255×10-6～4 455×10-6,说明这些橄榄石是从没有经过强烈分离结晶和硫化物熔离的岩浆中结晶出来的.岩体下部带橄榄石的Fo值(67 mol％～68 mol％)和w(Ni)(1500×10-6～2 000×10-6)都低于岩体上部和中部带的橄榄石相应值,说明岩体下部带的橄榄石可能形成于演化程度较高、并经历了硫化物熔离的岩浆.因此,笔者认为周庵岩体是由相对原始的和演化了的高镁玄武质岩浆两期侵位形成的.%The ～ 635 Ma Zhouan ultramafic intrusion on the northern margin of the Yangtze Block hosts Ni-Cu-(PGE) sulfide mineralization and is mainly composed of lherzolite. The compositions of olivine, chromite and pyroxene from a drill core cutting through the intrusion were documented to discuss the fractionation of magmas rnand the origin of the intrusion and related Ni-Cu-(PGE) sulfide mineralization. According to mineral assemblage and degrees of alteration, the intrusion can be divided into three zones: the upper zone composed of chloritized-serpentinized lherzolite, the middle zone composed mainly of fresh lherzolite and the lower zone composed of chloritized-amphibolized lherzolite. Parental magma is estimated to have Mg# of 0.63 with MgO/FeO molar ratio of 1.72 and contain 10.2%～11.7% AI2O3 and 476 × 10 ～"6 Ni on the basis of the compositions of olivine and chromite of the middle zone, indicating that the parental magma is of high-Mg basaltic nature. Positive correlation of Cr2Q3 and AI2Q3 for primary chromite of the middle zone indicates that there was subsolidus equilibration between chromite and interstitial liquid. High TiO2 and Cr# values of chromite are consistent with the chromite of layered intrusions formed in the extensional setting. Eutectic temperatures of clinopyroxene and orthopyrox-ene in the upper and middle zone were 1 017 ～ 1 077℃ on the basis of pyroxene thermometry and pressures were (3.6～4.5) × 108 Pa on the basis of clinopyroxene geobarometry, indicating that the shallow magma chamber from which the Zhouan intrusion formed was about ~ 12 km in depth. The forsterite (Fo) and Ni contents of olivine of the upper and middle zones are 80 mol% ~85 mol% and 2 255 X 10-6～4 455 × 10-6, respectively, indicating that olivine crystallized from magmas that might have not experienced intensive fractionation and sulfide segregation. Olivine of the lower zone has Fo values ranging from 67 mol% to 68 mol% and w(Ni) from 1 500 × 10-6 to 2 000 × 10-6, much lower than Fo values of the olivine from the upper and middle zones, suggesting the formation of the olivine from evolved magmas that may have undergone sulfide segregation. It is therefore proposed that the Zhouan intrusion was formed by both evolved and less evolved magmas that emplaced the shallow magma chamber in multiple replenishments.
展开▼
