西昆仑库地蛇绿岩地质、地球化学及其成因研究

西昆仑库地蛇绿混杂岩由方辉橄榄岩和纯橄榄岩等地幔变质橄榄岩、豆荚状铬铁矿、堆晶橄榄岩、堆晶辉石岩和辉长岩、辉绿岩墙、块状和枕状玄武岩等组成。强亏损方辉橄榄岩为主的地幔岩组合,二辉石的低Al含量和铬尖晶石的高Cr^#,以及岩石的富Mg、Ni和贫Al、Ca特征一致表明地幔橄榄岩类是经较高程度部分熔融后的地幔残余,与消减带之上蛇绿岩中的同类岩石相近。岩石富Rb、Ba、U、Th、LREE,说明地幔残余岩石受到了来自消减带的洋壳重熔组分的混染。堆晶岩以辉石岩和辉长岩为主,可能属PPG系列,指示岩浆是在消减带环境和含水条件下熔融的。辉长岩为低Ti蛇绿岩型,代表洋内弧后盆地早期环境或弧前环境。辉绿岩和玄武岩为洋中脊拉斑玄武岩和岛弧拉斑玄武岩的过渡类型;玄武岩和辉绿岩相比富Ba、Th、LREE,贫Ta,指示玄武岩较辉绿岩更多地受到来自消减带洋壳重熔组分的影响。库地蛇绿岩形成时的古构造环境是消减带之上的弧间或弧后盆地。     

Gravity variations along the Southeast Bohol Ophiolite Complex (SEBOC), Central Philippines: Implications on Ophiolite Emplacement

Abstract The basement complex of Bohol Island consists of the Southeast Bohol Ophiolite Complex (SEBOC), Cansiwang Melange and Alicia Schist. The SEBOC is a complete, but dismembered ophiolite with outcrops generally trending northeast– southwest and dipping north-west. The harzburgite units of the SEBOC are almost always observed to be thrusted onto the Cansiwang Melange, which in turn is thrusted onto the Alicia Schist. Bouguer gravity values on Bohol range from about +60 mGal in the west to +120 mGal in the east, in the region to the north-east of the SEBOC outcrops. Based on the present distribution of the SEBOC units and their thrust fault relationship with the Cansiwang Melange and Alicia Schist, it is proposed that the SEBOC was emplaced by onramping towards the south-eastward direction. However, the orientation of the Bouguer highs suggests that the thrusting direction of the ophiolite units is towards the south-west and not towards the south-east.     

Pb, Sr AND Nd-ISOTOPIC COMPOSITIONS OF PALEO AND NEO-TETHYAN OCEANIC CRUSTS IN THE EASTERN TETHYAN DOMAIN: IMPLICATION FOR THE INDIAN OCEAN-TYPE ISOTOPIC SIGNATURE

Pb, Sr AND Nd-ISOTOPIC COMPOSITIONS OF PALEO AND NEO-TETHYAN OCEANIC CRUSTS IN THE EASTERN TETHYAN DOMAIN: IMPLICATION FOR THE INDIAN OCEAN-TYPE ISOTOPIC SIGNATURE     

TWO SUITES OF OPHIOLITE RECOGNIZED IN THE MAQEN AREA, NORTHEASTERN TIBETAN PLATEAU

TWO SUITES OF OPHIOLITE RECOGNIZED IN THE MAQEN AREA, NORTHEASTERN TIBETAN PLATEAU1　JiangCF ,YangJS ,FengBG ,etal.OpeningandClosingTectonicsofKunlunMountains.Beijing :GeologicalPublishingHouse (inChinese) ,1992 . 2　XuZQ ,YangJS ,LiHB ,etal.TheA’nyemaqenSutureBeltandtheDynamicsinSubductionandCollision .TheStudyoftheOphioliteandGeodynamics[M ].(inChinese) .1996 .185～ 189. 3　YangJS ,XuZQ .TheA’nyemaqenOphioliteBeltEastKunlunMts.,NWChi…     

Oceanic ridge off-axis deep structure in the Mansah region (Sumail massif,Oman ophiolite)

Large scale structural mapping of the Oman ophiolite indicates that the Mansah area (Sumail massif) was a ridge off-axis region at the time the ophiolite was detached. This paper presents a detailed structural mapping of the region. We show that, as opposed to other off-axis areas, it contains plunging lineations, correlated with a thick Moho transition zone and chromite pods, indicative of a mantle diapir. However this diapir has a discontinuous structure, it is bounded by shear zones and types of diabases that are not found elsewhere in Oman; it also has a broken crust, strongly affected by hydrothermal alteration. This suggests that Mansah fossilized an off-axis diapir intruding a cooling lithosphere. It may be a good candidate to be the root of an off-axis seamount such as those found in the East Pacific, and may bring new insights into this particular volcanism which we are only beginning to explore.     

Shear sense inversion in the Hilti mantle section (Oman ophiolite) and active mantle uprise

Structural analyses in the well-exposed Hilti mantle section in the Oman ophiolite suggest a model of forceful horizontal flow in the uppermost mantle at the edge of a diapir below a oceanic spreading center. Detailed structural mapping, focussed on high-T deformation (i.e., asthenospheric flow), revealed a gently undulated flat structure with a uniform east-west flow direction. When it is related to the N–S to NNW–SSE trending, vertical sheeted dike complex located to the east, this mantle flow is parallel to the spreading direction. Because the Moho is so flat lying, a large dunite occurrence at the south-western region is possibly ascribed to the Moho Transition Zone. Kinematic analysis shows that the shear direction generally changes from top-to-the west in the upper level, to top-to-the east in the lower level with respect to the Moho. This shear sense inversion is explained by a model of forceful flow due to an active mantle uprise and it is not compatible with a passive mantle uprise. In the plan section, the boundary of the shear sense inversion is subparallel to the flow direction and subperpendicular to the spreading axis. In cross section, the boundary appears to occur at various depths in the range of 200 m to 500 m. It shows that the active mantle uprise in the diapir center resulted in a channelled horizontal flow.     

西藏东巧蛇绿岩中辉长岩锆石SHRIMP定年及其地质意义

东巧蛇绿岩位于分割羌塘地块和拉萨地块的班公湖—怒江缝合带中段,是该带最著名的产铬铁矿床的蛇绿岩残片之一。该蛇绿岩遭受构造肢解,恢复后的蛇绿岩剖面各岩石单元齐全。本文应用SHRIMPⅡ锆石UPb法,对东巧蛇绿岩中堆晶辉长岩进行了测年,获得辉长岩年龄为187.8±3.7 Ma。该年龄代表了东巧洋盆的形成时代,为确定研究区特提斯洋盆的古构造格局及其发育演化过程等提供了重要的年代学制约。     

昆仑造山带中段蛇绿混杂岩的地质地球化学特征

昆仑造山带中段蛇绿混杂岩由镁质超基性岩、块状辉石质玄武岩、枕状玄武岩、堆晶岩、斜长岩脉、灰岩岩块及其间的薄层状粉砂质泥岩、泥质粉砂岩组成 ,并有硅泥组合。不同岩性之间、不同块体之间由剪切带分开 ,沉积岩显示出浅深水岩块的混杂。其岩石化学主要特征是MgO含量特别高 ,达 33.2 2 %～ 37.2 8% ,SiO2 、CaO、Al2 O3 、TiO2 、K2 O含量低 ,FeO /(FeO 十MgO)、Ca/Al2 O3 比值非常低。属镁质超基性岩。超基性岩的常量元素、微量元素 ,以及稀土元素特征表明 ,其形成的环境为多硅铝质陆壳成分的大陆边缘盆地 (具有洋脊及岛弧的过渡性质 )。     

新疆北天山巴音沟南侧发育早泥盆纪蛇绿岩及其构造意义

北天山晚古生代蛇绿岩带的巴音沟蛇绿岩是该带的重要组成部分和典型代表。传统意义的巴音沟蛇绿岩带在区内分南北两支,南支为贝勒克蛇绿岩带,北支为巴音沟蛇绿岩带,分别代表早泥盆世末和早石炭世末的两条不同古洋壳残片。     

蛇绿岩及蛇绿岩中浅色岩的SHRIMP U-Pb测年

文中简要评述了蛇绿岩的层状辉长岩,斜长岩和斜长花岗岩,以橄榄岩为主岩的花岗岩和蛇绿岩中的埃达克岩的锆石SHRIMP U-Pb年龄的地质意义。层状辉长岩(或堆晶层状辉长岩)通常起源于洋脊下的岩浆房,因而它的形成年龄代表洋壳形成的时代。斜长岩与层状辉长岩的时代相近或略晚。斜长花岗岩年龄的解释极其依赖锆石组成和地球化学证据。橄榄岩为主岩的花岗岩,可能记录蛇绿岩的侵位时代。蛇绿岩中的埃达克岩是消减洋壳在深部的部分熔融的产物。文中发表了新疆扎河坝蛇绿岩SHRIMP定年的中间成果,并简略地介绍了滇川西部金沙江和内蒙古图林凯等地的研究实例。根据层状辉长岩的测定结果,扎河坝蛇绿岩形成于(489±4)Ma,金沙江蛇绿岩形成于(328±8)Ma。内蒙古图林凯蛇绿岩中埃达克岩形成于(467±13)Ma～(429±7)Ma。块状辉长岩、斜长花岗岩和橄榄岩为主岩的花岗质岩石记录了蛇绿岩的复杂演化。新疆扎河坝蛇绿岩中的块状辉长岩中存在多组锆石年龄值。较老的一组为468～511 Ma,与层状辉长岩和斜长岩相似,记录了蛇绿岩或洋壳的形成时代,但是,岩石中的大部分锆石年龄为396～419 Ma,加权平均年龄为(406±4)Ma,可能反映了一次部分熔融事件。滇川西部金沙江蛇绿岩中的斜长花岗岩的形成年龄为约300～285Ma,晚于层状辉长岩和?