雅鲁藏布江缝合带西段东波地幔橄榄岩体钻孔岩心研究

The drilling core(DSD-1) study of the Dongbo peridotite massif,Yarlung Zangbo Suture Zone,western Tibet

东波地幔橄榄岩是雅鲁藏布江缝合带内代表性超镁铁岩体,位于缝合带西段,面积超过400km2。为查明岩体成因,在岩体中实施了一口千米深的科学钻探(DSD-1)。除上部有约23m厚的第四系堆积物外,钻孔均钻进在地幔橄榄岩中,孔深1002.06m,岩心采取率96.62%。岩心编录结合显微镜下鉴定将岩心划分出44个岩性单元层,并进一步归并为上、下两套岩性:上部(23.1~340.17m)为含单辉方辉橄榄岩(斜方辉石含量为15~20%,单斜辉石含量不足5%);下部(340.17~1002.06m)为方辉橄榄岩(斜方辉石含量为10%~15%,几乎不含单斜辉石),其中发育薄层状纯橄岩和辉石岩以及辉绿岩脉。矿物学及全岩地球化学研究揭示:①东波地幔橄榄岩以方辉橄榄岩为主,其次为含单辉方辉橄榄岩,它们均具有亏损的全岩地球化学及矿物成分组成,指示它们为经历过中高程度部分熔融后的地幔残余岩石;经历过富水流体(熔体)交代作用,表现为角闪石呈柱状交代斜方辉石;②纯橄岩和辉石岩以透镜状或薄层状脉体发育于方辉橄榄岩中,具有岩浆成因的矿物组成,见交代矿物角闪石,指示它们可能为交代成因;③辉绿岩脉兼具N-MORB和弧玄武岩的化学属性,以低SiO2、高MgO和高Al2O3含量为特征,具有与Western Lau Basin玄武岩一致的REE配分型式,形成于较为成熟的弧后盆地环境。东波地幔橄榄岩(及其中的纯橄岩、辉石岩和辉绿岩脉)在成因上与俯冲带关系密切,都受到了与俯冲带相关地质作用的影响。

The Dongbo peridotite massif in the western part of the Yarlung Zangbo Suture Zone in western Tibet represents a mantle thrust sheet of more than 400km2.In order to find out the origin of the massif,a borehole(DSD-1)has been drilled in the massif.The upper-most part of the drilling core is composed of Quaternary sediments with a thickness of 23m.The rest part of the core is dominated by peridotite which could be divided into two main rock units(further divided into 44sub-rock units).The upper unit of the core(from 23.1mto 340.7m)consists of strongly serpentinized clinopyroxene(Cpx)-bearing harzburgites with orthopyroxene(Opx)contents ranging from 15%to 20%and Cpx contents less than 5%;while the lower unit(from 310.17mto 1002.06m)mainly consists of less serpentinized harzburgites(with Opx ranging from 10%to 15%,and no or little Cpx).Thin layers of dunite and pyroxenite,as well as diabase dikes,occur in the harzburgites.Detailed studies on the mineralogy and whole-rock geochemistry of these rocks have revealed that:①the Dongbo peridotite massif is dominated by harzburgites,accompanied by subordinate Cpx-bearing harzburgites;they have depleted whole-rock major-element and mineral compositions,suggesting that they are residues of mantle rocks after middle-high degrees of basaltic extraction;the occurrence of amphibole replacing Opx indicates that these rocks have experienced melt(fluid)-rock interactions;②the dunites and pyroxenites occurring as thin layers are composed of minerals of magmatic origin,and amphibole of metasomatic origin is seen locally,which indicates the matasomatic origin for the dunites and pyroxenites;③the diabases exhibit both N-MORB-like and arc-like geochemical affinities;they have low SiO2contents,and high contents of MgO and Al2O3,and their REE patterns are similar to those of the basalts in Western Lau Basin;these features suggest that they are formed in a mature back-arc basin.The formation of the Dongbo peridotites(including the dunite,pyroxene and diabase veins)are closely related with the subduction process.