塔里木板块东北部古堡泉岩体岩石地球化学特征与成因

Lithogeochemistry and Petrogenesis of the Gubaoquan Intrusion in the Northeastern Part of Tarim Plate

古堡泉岩体位于甘肃北山裂谷带的南带,岩体呈不规则的岩株状,出露面积约1.7km2。主体岩性为辉长岩,含有较多的橄榄辉长岩、辉石橄长岩析离体,各岩相之间呈渐变过渡关系。大部分样品属于拉斑玄武岩系列,少数样品属碱性玄武岩系列。岩石中TiO2、K2O、P2O5、大离子亲石元素和稀土元素丰度很低(∑REE=5.29×10-6～17.30×10-6)。εNd(t)=+6.9‰～+8.1‰,εSr(t)=+2.3‰～+17.3‰,206Pb/204Pb=18.463～18.645,207Pb/204Pb=15.604～15.634,208Pb/204Pb=38.351～38.497。岩体源区可能为先期发生过部分熔融的亏损型地幔,岩浆在运移过程中遭受了显著陆壳物质的混染。原生岩浆为富铁贫镁的玄武质岩浆,在上侵和运移过程中主要发生了以单斜辉石和橄榄石为主的暗色矿物的分离结晶作用和不同程度斜长石的堆晶作用。岩体是地幔尖晶石橄榄岩在较低压力下发生较大程度部分熔融的产物。

Gubaoquan intrusion,located at south of Beishan Rift Zone,Gansu province,occurs as irregular stockwork,with and area of 1.7km2.The intrusion consists mainly of hornblende gabbros including a large amount of olivine gabbros and pyroxene troctolites,both of which show a relation of gradual transition.Most of samples belong to tholeiite series,with a few sample belonging to alkali series.All kinds of rocks are characterized with very low TiO2,Na2O,P2O5 content and LILE,REEs（∑REE=5.29×10-6-17.30×10-6）.εNd（t）=＋6.9~＋8.1,εSr（t）=＋2.3~＋17.3,206Pb/204Pb=18.463~18.645,207Pb/204Pb=15.604~15.634,208Pb/204Pb=38.351~38.497 respectively.The magma source is depleted mantle by previous melt extraction,and the magma experienced contamination to some extent.The primary magma was basaltic magma characterized by high Fe and poor Mg.Fractional crystallization of melanocrratic minerals such as clinopyroxene and olivine and various accumulation occurred during its emplacement and migration.The Gubaoquan intrusion was the product of highly partial melting of spinel peridotite at low pressure level in mantle.