甘-新北山地区二叠纪镁铁-超镁铁质岩体造岩矿物化学特征及其岩石学指示

二叠纪镁铁-超镁铁质岩体在北山造山带广泛分布, 其中多发育铜镍硫化物矿化, 并集中在新疆北山坡北和甘肃北山柳园地区。本文对柳园地区二叠纪骆驼山(283Ma)和西南山(277Ma)矿化岩体中早期结晶的造岩矿物开展矿物化学分析, 并对甘-新北山地区矿化岩体的主要造岩矿物进行系统对比研究。骆驼山和西南山岩体主要由单辉橄榄岩、二辉橄榄岩、橄榄辉石岩、橄榄辉长岩和辉长岩组成, 主要造岩矿物为橄榄石、单斜辉石、斜方辉石、斜长石和角闪石, 及少量铬尖晶石和金云母。铬尖晶石Cr^#值范围为52.2~70.4, TiO₂含量为1.1%~4.0%, Al₂O₃含量为8.1%~18.9%。橄榄石Fo值范围为78~86, Ni含量为856×10^-6~2121×10^-6, Ca含量为71.3×10^-6~720×10^-6。斜方辉石主要为顽火辉石和少量古铜辉石; 单斜辉石为透辉石和普通辉石, Mg^#范围为86~88, Al^Ⅳ/Al^Ⅵ比值范围为1.42~3.49。矿物成分计算结果表明岩体的母岩浆为高Mg拉斑玄武质岩浆, 结晶温度范围为1132~1323℃, 结晶压力为0.23~0.25GPa, 氧逸度范围为FMQ +1.0~+1.7, 具有富集LREE以及Nb-Ta-Zr-Hf负异常的特征。富含含水矿物及母岩浆含水量反演结果显示骆驼山和西南山岩体单斜辉石结晶时其岩浆含水量分别为3.4%和3.6%, 表明岩体原始岩浆形成于富水源区, 可能与俯冲交代作用有关。与坡北地区岩体对比, 骆驼山和西南山岩体母岩浆结晶温度略低, 较高的单斜辉石Al^Ⅳ/Al^Ⅵ值以及压力计算表明岩体形成于相对低压环境, 岩浆氧逸度范围略高于坡北地区岩体, 可能与含水的镁铁质岩浆分异演化作用相关。结合区域地质资料认为柳园地区二叠纪岩体形成于碰撞后伸展构造环境, 母岩浆所具有的岛弧岩浆特征来源于地幔源区曾经历的俯冲板片来源熔/流体的改造作用。

Mineral chemistry and petrological implications of rock-forming minerals from Permian mafic-ultramafic intrusions in the Gansu-Xinjiang Beishan area

Permian mafic-ultramafic intrusions are widely distributed in the Gansu-Xinjiang Beishan area. They are mainly located in the Pobei and Liuyuan areas and some host Cu-Ni sulfide mineralization. Here we report the chemical compositions of rock-forming minerals from Luotuoshan (283Ma) and Xinanshan (277Ma) intrusions in the Liuyuan area. The two intrusions are mainly composed of wehrlite, lherzolite, olivine pyroxenite, olivine gabbro and gabbro. The major rock-forming minerals include olivine, clinopyroxene, orthopyroxene, plagioclase and amphibole together with minor Cr-spinel and phlogopite. Cr-spinels have Cr# values of 52.2 to 70.4, TiO2 contents of 1.1%~4.0% and Al2O3 contents of 8.1%~18.9%. Olivines have Fo values of 78~86, Ni contents of 856×10-6~2121×10-6 and Ca of 71.3×10-6~720×10-6. Orthopyroxenes are mostly enstatite with minor bronzite in composition, while clinopyroxenes are mainly diopside and augite, with Mg# of 86~88 and AlⅣ/AlⅥ ratios of 1.42~3.49. Calculations using mineral chemistry indicate that the parental magmas of the two intrusions were high-Mg basaltic magmas and had liquidus temperatures of 1132~1323℃, crystallization pressures of 0.23~0.25GPa, and oxygen fugacities of FMQ +1.0~+1.7. The parental magmas were characterized by LREE enrichment and negative Nb-Ta and Zr-Hf anomalies and contained 3.4%~3.6% H2O when earlier clinopyroxene crystallized. These, together with whole rock geochemistry and presences of hydrous minerals, indicate that the magmas were derived from a mantle source that had been metasomatized by subduction-related materials, and had slightly lower crystallization temperatures and higher oxygen fugacities than those formed in the Pobei area. The intrusions in the Liuyuan area were formed in a relatively lower pressure environment. Considering the regional geological data, it is concluded that Permian intrusions in the Liuyuan area were formed in a post-subduction extensional environment and that the arc-like features of their parental magmas were inherited from previous metasomatism of their mantle sources by slab-derived melts/fluids.