新疆磁海铁矿区镁铁质岩及正长岩锆石U-Pb年代学、岩石地球化学特征:对成岩、成矿作用的制约

对新疆磁海铁矿区镁铁-超镁铁质岩与铁成矿关系、正长岩与镁铁质岩关系的解剖,是认识磁海矿区成岩、成矿过程及构造背景的关键。本文利用SIMS锆石U-Pb测年法,获得磁海辉绿岩、辉长辉绿岩、磁南辉长岩、磁海北角闪石英正长岩的206Pb/238U-207Pb/235U谐和年龄分别为275.1±2.2Ma、281.9±3.2Ma、273.0±1.9Ma和273.0±1.8Ma,这与北山乃至北疆地区主要含铜镍-钒钛磁铁矿的镁铁-超镁铁质岩年龄一致。岩石地球化学特征研究显示,从辉石岩到辉长岩,再到辉绿岩,经历了Ti逐渐富集、Mg#和m/f值先增加后降低的过程,角闪石英正长岩具有A型花岗岩特征,与辉长岩、辉绿岩在成因上存在互补关系。综合年代学和地球化学特征,磁南辉石岩、辉长岩、磁海辉绿岩、辉长辉绿岩以及磁海北边的角闪石英正长岩为同源岩浆演化的产物,岩浆演化过程中受地壳混染作用微弱,在岩浆演化的早期,磁铁矿的结晶分离主导着岩浆成分的改变,当岩浆演化到辉长岩阶段,岩浆开始以结晶分异作用为主;磁铁矿的分离结晶时间早于钛铁矿,岩浆型的金属硫化物为磁铁矿和钛铁矿结晶过渡阶段的产物。磁海镁铁-超镁铁质岩石在成岩及成矿作用上可能与在时间和空间上相邻近的塔里木早二叠世大火成岩省有密切关系。

Lithological, chronological and geochemical characteristics of Cihai iron deposit, Eastern Xinjiang: Constraints on genesis of mafic-ultramafic and syenite intrusions and mineralization

Interpretation of the relationship between mafic-ultramafic rocks and iron mineralization, as well as the relationship between syenite and mafic rocks in Cihai iron district is the key to understand the diagenesis, metallogenetic process and tectonic settings. SIMS Zircon U-Pb dating for diabase, gabbro-diabase in Cihai, gabbro in Cinan, and amphibole-quartz-syenite in Cibei yields results of 275.1±2.2Ma, 281.9±3.2Ma, 273.0±1.9Ma and 273.0±1.8Ma respectively, which is coincident with the age of mafic-ultramafic rocks containing nickel, copper and V-Ti-magnetite in Beishan and Xinjiang regions. Characteristics of rock geochemistry show the process that from peroxenite to gabbro then to diabase, the Ti enriches gradually and the value of Mg^# and m/f first increase and then decrease. The amphibole-quartz-syenite, with signature of A-type granite, has complementary relation with gabbro and diabase in rock genesis. The features of geochronology and geochemistry indicate that pyroxenite, gabbro in Cinan, diabase, gabbro-diabase in Cihai and amphibole-quartz-syenite in Cibei belong to the comagmatic evolutionary series. The magma evolution is slightly contaminated by crust. During the early stage of magma evolution, the fractional crystallization of magnetite dominates the change of magma component. While the magma developed to the gabbro stage, crystallization differentiation plays the key role. Fractional crystallization of magnetite is earlier than ilmenite. Magmatic sulphide is the transitional product of magnetite and ilmenite crystallization. The diagenesis and metallogeny of mafic-ultramafic rocks in Cihai iron district may be temporarily and spatially associated with the Early-Permian Tarim Large Igneous Province.