北秦岭双龙-夏馆地区镁铁质麻粒岩地球化学、变质演化及其对岛弧环境的限定

北秦岭双龙-夏馆地区出露有镁铁质麻粒岩,其岩石成因及构造意义不甚清楚,该文对其进行了矿物学、变质温压条件和岩石地球化学研究。结果表明,研究区镁铁质麻粒岩变质演化经历了3个变质阶段:峰期麻粒岩相变质阶段(M1)矿物组合为石榴子石(Grt)+单斜辉石(Cpx)+斜方辉石(Opx)+斜长石(Pl1)+石英(Qtz)+角闪石(Hb1)(高Ti)+钛铁矿(Ilm);退变角闪岩相阶段(M2)矿物组合为角闪石(Hb2)+斜长石(Pl2)+榍石(Sph2);晚期绿片岩相阶段(M3)以形成低温退变质矿物组合绿泥石(Chl3)+黑云母(Bi3)特征。峰期变质条件为T=807~890℃,P=7.8~8.1 kbar,其变质演化过程具有逆时针型P-T轨迹,为中低压麻粒岩。地球化学数据显示原岩具有拉斑质岩石的地球化学特征,SiO₂、Al₂O₃、P₂O₅含量相对较低,TFeO、CaO、MgO含量相对较高。镁铁质麻粒岩稀土元素配分模式为轻稀土元素弱富集、重稀土元素相对平坦的右倾型,Eu异常不明显(δEu=0.78~0.94)。岩石富集K、Th、Ba等大离子亲石元素,亏损Nb、Hf、P等高场强元素的特点,并且Ti具弱的负异常,原岩兼具E-MORB(富集型洋中脊玄武岩)和IAB(岛弧玄武岩)的特征,形成于与俯冲有关的岛弧环境。

Geochemistry and metamorphic evolution of mafic granulite in Shuanglong-Xiaguan area, North Qinling:Constraint on the formation in island-arc setting

Mafic granulites occur on Shuanglong-Xiaguan area of North Qinling,but their petrogenesis and structural significance are not clear,therefore the mineralogy,metamorphic P-T condition and rock geochemistry are studied in this paper.The results show that the metamorphic evolution of mafic granulite in this investigated area has experienced three metamorphic stages:peak granulite facies metamorphic stage(M1)mineral assemblage is Grt+Cpx+Opx+Pl1+Qtz+Hb1+Ilm,Retrograde amphibolite facies stage(M2)mineral assemblage is Hb2+Pl2+Sph2,and late greenschist facies stage(M3)is characterized by low temperature retrograde metamorphic mineral assemblage chl3+Bi3.The peak metamorphism condition is T=807-890℃,P=7.8-8.1 kbar,and its metamorphic evolution process has a anticlockwise P-T evolution track,indicates that it is middle-low pressure granulite.The geochemical data show that the protolith has the geochemical characteristics of tholeiitic rocks.The contents of SiO₂,Al₂O₃ and P₂O₅ are relatively low,while the contents of TFeO,Cao and MgO are relatively high.The REE distribution pattern of mafic granulite is right dipping type with weak LREE enrichment and relatively flat hree.Eu anomaly is not obvious(δEu=0.78-0.94).The rocks are enriched in large ion lithophile elements such as K,Th,Ba,and depleted in high field strength elements such as Nb and Hf and P,and Ti has weak negative anomaly.The protolith has the characteristics of E-MORB(enriched mid ocean ridge basalt)and IAB(island arc basalt),and formed in the island arc environment related to subduction.