敦煌地块南缘小多坝沟超基性岩体岩石地球化学特征及构造地质意义

ZIRCON U-Pb DATING, GEOCHEMISTRY AND GEOLOGICAL SIGNIFICANCE OF THE DUOBAGOU GRANITE PLUTON FROM THE SOUTHERN MARGIN OF THE DUNHUANG BLOCK

小多坝沟超基性岩体位于敦煌地块南缘的大红山—三危山断裂带上,由两处岩体组成,呈脉状、扁豆状产出,侵位于太古宙—中元古代敦煌岩群。岩体形成于早古生代,Sr-Nd同位素的Nd模式年龄Nd模式年龄TDM2=1511 Ma,属中元古代早期阶段。岩体主要由绿黑色、黑色辉石橄榄岩组成,岩体边缘局部有强蛇纹石化辉橄岩存在。岩石具高MgO、m/f,低Al2O3、TiO2、Na2O+K2O的特征,为镁质超基性岩。岩石可能是高程度玄武质熔体抽取之后的难熔残余,是幔源岩浆受到了早期俯冲洋壳交代作用,或上升侵位过程中受到了地壳的混染,高程度玄武质熔体抽取之后的难熔残余的产物。表明小多坝沟超基性岩体的形成受早古生代阿尔金洋闭合作用影响,属造山带变质橄榄岩。

Xiaoduobagou ultrabasic rock body is located in the Dahongshan-Sanweishan fault zone on the south edge of Dunhuang block,which is composed of two rock bodies,which are vein and lenticular in shape and intruded into the Archean-Mesoproterozoic Dunhuang Rock Group.The rock body was formed in the early Paleozoic.The Nd model age of Sr-Nd isotope is TDM2=1511 Ma,which belongs to the early stage of Mesoproterozoic.The rock body is mainly composed of greenish black and black pyroxene peridotite,and there are strong serpentinite pyroxenite in the margin of the rock body.The rocks are characterized by high MgO and m/f,low Al2O3,TiO2 and Na2O+K2O.The rock may be the refractory residue after the extraction of high-level basaltic melt.It is the product of the refractory residue after the extraction of high-level basaltic melt after the mantle derived magma was metasomatized by the early subducted oceanic crust,or mixed by the crust during the process of ascending emplacement.It shows that the formation of xiaoduobagou ultrabasic rock body is affected by the early Paleozoic closing of the Altun Ocean,which belongs to the orogenic belt metamorphic peridotite.