西南天山东德沟镁铁-超镁铁岩中尖晶石的矿物学特征及其地质意义

Mineralogy of spinel from mafic-ultramafic rocks in Dongdegou,southwestern Tianshan and its geological significance

中国西南天山东德沟镁铁-超镁铁岩出露于天山伊犁-中天山板块和塔里木板块之间的碰撞造山带中,岩石类型主要为（方辉）橄榄岩,且普遍经历了含水流体（熔体）的渗透交代作用,含有形态和成分变化复杂的尖晶石,表现为从红褐色到黄褐色、再到深绿色和浅绿色,尖晶石的Cr2O3和FeO的含量依次降低,Al2O3和MgO的含量逐渐增高。研究认为红褐色和黄褐色的铬尖晶是原生矿物,而绿色的镁铝尖晶石为次生矿物。原生铬尖晶石的Cr#为0.41~0.87,Mg#为0.21~0.61,且成分变化大,显示其可能具多期熔融的复杂成因;次生绿色的尖晶石形态不一、成分相对有较大变化,表明其至少形成于两个期次的变质作用或流体交代过程。原生铬尖晶石具有SSZ环境橄榄岩的特征,指示其寄主岩石可能是俯冲上板片地幔楔岛弧环境的橄榄岩残片。

The mafic-ultramafic rocks of Dongdougou are located in the orogenic belt between the Tarim plate and the Yili-central Tianshan plate,southwestern Tianshan.Harzburgite is the main type of peridotite,generally metasomatized by hydrous melt or fluid.It includes different types of spinels,being red-brown,brown,dark green,and light green in color.Cr2O3 and FeO decrease gradually whereas Al2O3 and MgO increase with the metasomatism.Studies indicate that red-brown and brown picotiles constitute the primary type,while green magnesium spinels possess the second place.The compositions of the primary picotiles are variable with Cr# and Mg# in the range of 0.41~0.87 and 0.21~0.61 respectively,which may indicate polyphase origin of melting.Secondary spinels have different shapes,and their compositions vary tremendously,exhibiting two stages of metasomatism or metamorphism.The primary picotiles have characteristics of peridotite in a subducting upper plate mantle wedge zone,which implies the host rocks might have been the fragments of peridotite from a SSZ volcanic arc environment.