西藏雅鲁藏布江缝合带普兰地幔橄榄岩铬尖晶石成因研究

普兰蛇绿岩位于雅鲁藏布江缝合带西段,其中地幔橄榄岩由方辉橄榄岩、含单斜辉石方辉橄榄岩以及少量二辉橄榄岩及纯橄岩组成。尖晶石是地幔橄榄岩中常见的副矿物,可以作为重要的岩石学成因指示剂。在野外地质调查基础上,通过岩相观察、电子探针、尖晶石成分面分析、电子背反射衍射分析,可将普兰地幔橄榄岩铬尖晶石分为三类:第一类铬尖晶石呈自形,粒径较小(<100μm),或包裹于斜方辉石中,或杂乱分布于橄榄石和辉石之间,具有高Cr^#(>0.6)、低Mg^#(0.43~0.57)的特征,为部分熔融+玻安质熔体交代成因;第二类铬尖晶石呈半自形-他形,粒径较大(>100μm),常含有橄榄石、辉石包裹体,具有中Cr^#(0.17~0.42)、高Mg^#(0.63~0.77)的特点,主要受部分熔融作用影响;第三类铬尖晶石呈他形蠕虫状与辉石交生在一起构成后成合晶结构,粒径变化较大,具有低Cr#(0.17~0.28)、高Mg^#(0.67~0.77)的特点。EBSD分析结果显示尖晶石、辉石的结晶学优选方位(CPO)较为相似,表明为同一矿物分解而来,单斜辉石与大陆岩石圈地幔捕掳体中石榴子石的稀土元素对比表明构成后成合晶结构的辉石和铬尖晶石为具有大陆岩石圈地幔属性的高压石榴子石退变分解而成。综合分析表明:普兰蛇绿岩的地幔橄榄岩体在从石榴子石相深度上升过程中发生了石榴子石退变、岩石部分熔融及熔体渗透作用,岩体经历了威尔逊旋回初期的大陆裂谷阶段,主体经历了中-低程度的部分熔融,类似大洋中脊环境,局部受到了富硅、富镁玻安质熔体的影响。     

滇西富碱斑岩及其中包体岩石的地幔流体交代作用特征

滇西富碱斑岩体中,有多处产出深浅来源不同的包体岩石。针对寄主岩石和包体岩石的岩相学和岩石化学成分的对比研究发现:寄主岩石较包体岩石更富S iO2,A l2O3,N a2O和K2O,而包体岩石较寄主岩石更富C aO,M gO,F e2O3和F eO,岩石蚀变以角闪石化、钠黝帘石化和硅化为主,结合具超基性岩性的包体岩石发育碎裂结构,碎裂粒间出现与交代成因角闪石共生的微晶硅质、碳质、碳酸盐和金云母等表现地幔流体显交代作用的特征矿物组合,由此判定交代主岩和包体岩石的流体富含碱质和硅质,是与富碱岩浆同源的来自富集地幔源区的地幔流体,其交代作用的功能和能量明显优于一般意义上的地壳流体作用,而此交代作用促进了成矿元素和挥发份的聚集,且富碱斑岩与地幔流体的交代作用之间存在有利的时差关系。     

东天山维权银铜多金属矿控矿因素及成因分析

维权银铜多金属矿床位于新疆东天山地区,是具有典型特征的大型银铜多金属矿,大地构造划属觉罗塔格沟弧带中的雅满苏岛弧带。该矿床的地质特征、控矿因素、矿床成因长期存在争议。本次研究建立在多年野外地质勘查的基础上,对矿床的地质特征、控矿因素、矿床成因等进行了系统研究,认为银铜、铅锌矿体均分布于火山碎屑岩中,受地层控制的铅锌矿体被断裂构造控制的银铜矿体所切穿;铅锌矿成矿严格受地层层位控制,主要的蚀变类型为矽卡岩化,为接触交代成因类型,成矿时代为早石炭世晚期;银铜矿体形成于晚石炭世中晚期,矿体就位空间为断裂构造,矿体形态明显受断裂形态和性质所控制,有着明显的矿化边界,即断层面,认为其属岩浆期后远程中低温热液成因,成矿晚于铅锌成矿。     

大陆科学钻探工程CCSD主孔中发现的大量流体交代脉体

在对位于苏北榴辉岩分布区中国大陆科学钻探工程CCSD5000多m主孔岩心进行岩矿鉴定的过程中, 普查发现存在各种各样的脉体, 可大致分为四大系列(1) 与超高压/高压变质阶段有关的脉体系列; (2) 与减压部分熔融阶段有关的脉体系列; (3) 与退变质(主要绿帘角闪岩相和绿片岩相) 有关的脉体系列; (4) 折返晚期超低压-低温热液脉体.尤其特别的是于3578.5~4006m深度发现了大量特殊的富含稀土矿物的脉体.CCSD主孔岩心的脉体分布广泛、类型繁多, 有的与矿化作用密切有关, 在成因上可能属于大规模地质流体交代的结果.      

Petrology and metamorphic evolution of ultramafic rocks and dolerite dykes of the Betic Ophiolitic Association (Mulhacén Complex, SE Spain): evidence of eo-Alpine subduction following an ocean-floor metasomatic process

The Betic Ophiolitic Association, cropping out within the Mulhacén Complex (Betic Cordilleras), is made up of numerous metre- to kilometre-sized lenses of mafic and/or ultramafic and meta-sedimentary rocks. Pre-Alpine oceanic metasomatism and metamorphism caused the first stage of serpentinization in the ultramafic sequence of this association, which is characterized by local clinopyroxene (Cpx) breakdown and Ca-depletion, and complementary rodingitization of the basic dykes intruded in them. Subsequent eo-Alpine orogenic metamorphism developed eclogite facies assemblages in ultramafic and basic lithotypes, which were partly retrograded in Ab-Ep-amphibolite facies conditions during a meso-Alpine event. The heterogeneous development of the oceanic metasomatism in the ultramafic rock-types led to the patchy development of highly serpentinized Ca-depleted domains, without gradual transition to the host, and less serpentinized, Cpx-bearing ultramafites, mainly lherzolitic in composition. The high-pressure eo-Alpine recrystallization of these ultramafites in subduction conditions originated secondary harzburgites in the Ca-depleted domains, consisting of a spinifex-like textured olivine+orthopyroxene paragenesis, and a diopside+Ti-clinohumite paragenesis in the enclosing lherzolitic rocks. During the meso-Alpine event, secondary harzburgites were partly transformed into talc+antigorite serpentinites, whereas the diopside and clinohumite-bearing residual meta-lherzolites were mainly transformed into Cpx-bearing serpentinites. Relics of mantle-derived colourless olivine may be present in the more or less serpentinized secondary harzburgites. These relics are overgrown by the eo-Alpine brown pseudo-spinifex olivine, which contains submicroscopic inclusions of chromite, ilmenite and occasional halite and sylvite, inherited from its parental oceanic serpentine. The same type of mantle-derived olivine relics is also preserved within the Cpx-bearing serpentinites, although it has been partly replaced by the eo-Alpine Ti-clinohumite. The dolerite dykes included in the ultramafites were partly rodingitized in an oceanic environment. They were then transformed during the eo-Alpine event into meta-rodingites in their border zones and into eclogites towards the innermost, less-rodingitized portions. Estimated P–T conditions for the high-pressure assemblages in ultramafic and basic lithotypes range from 650 to 750°C and 16–25 kb.     

西北准噶尔地区碱性花岗岩体中角闪石的地质地球化学意义

角闪石的种类、化学组成、微量元素特征显示所研究的碱性花岗岩分为两类：达拉布特才恰库尔特型。前者结晶温度为６６５－７５０℃,定位深度为４．９５－８．２５ｋｍ,形成于相对封闭的环境中岩浆演化晚期水－岩作用不强烈;角闪石主要为钙质角闪石,富Ｆｅ、Ｃｏ、Ｃｒ、Ｒｂ、Ｓｃ、Ｃｓ、ｈｆ、贫Ｔａ、Ｚｎ;其球粒陨石标准化稀土模式曲线呈左高右低的台阶式“Ｖ”型。后者结晶温度相对较高、定位较浅,形成了开放环境中,岩浆     

The trace element geochemistry of some high-grade Bergen-Jotun kindred gneisses from the Jotun Nappe around Gjendebu, central southern Norway

Abstract The high-grade meta-plutonic rocks of this study lie entirely within the Jotun Nappe of the southern Norwegian Caledonides. They are divisible on the basis of metamorphic grade and petrographic character into three units, the Storadalen Complex (SCX), the Svartdalen Gneiss (SG), and the Mjølkedøla Purple Gabbro (MPG). The SCX is a differentiated series of ultrabasic to intermediate rocks now showing only tectonite fabrics. It has been metamorphosed to spinel-Iherzolite granulite facies grade. The broadly monzonitic SG is weakly tectonized and internally differentiated. Its metamorphic grade does not exceed plagioclase-lherzolite granulite facies grade. The mis-named MPG is also broadly of monzonitic composition but it retains a coarse ophitic texture, and is of amphibolite facies grade. A gradational boundary exists between the MPG and SG, but the contact between these two units and the SCX is the steeply dipping Tyin-Gjende Fault. The three units represent a comagmatic body of mid-Proterozoic age, metamorphosed during a Sveconorwegian event and finally dismembered and upthrust during the Caledonian Orogeny. The new trace element analyses reported here show that the three rock units have remarkably similar trace element abundances and trends. K-Rb covariation shows increasing K/Rb ratios with increasing K. These patterns were produced by magmatic fractionation processes acting at deep crustal levels, possibly in the presence of a non-aqueous fluid phase. With the exception of K and Sr, close similarities exist between the rocks of this study and present-day calc-alkaline basalts and andesites from island arcs. The high K content is regarded as a primary magmatic feature, but the available data are insufficient to indicate its origin. The Sr contents are abnormally high and are ascribed to metasomatism which occurred during either high-grade metamorphism or post-climactic cooling. There are no systematic geochemical variations with metamorphic grade or degree of deformation.     

Element mobility associated with syn-metamorphic shear zones near Scotchport, NW Mayo, Ireland

Abstract Three major blastomylonitic synmetamorphic (epidote amphibolite to mid amphibolite facies) shear zones are seen on the NW coast of the Mullet Peninsula in NW Mayo. These shear zones occur at the contacts of major structural units and in an imbricated slice where rocks of the Erris Complex are deformed and chemically modified. Chemical changes associated with individual shear zones have been deduced by comparing the compositions of various gneisses both within and adjacent to the shear zones. Compositional changes are different in the constituent rock-types within each unit and many elements normally considered immobile have been selectively mobilized within the shear zones. Little evidence of wholesale metasomatic introduction of components into these shear zones was found to accompany the selective mobilization.     

粤北竹筒尖矿床碱交代花岗岩地球化学特征及地质意义

竹筒尖矿床位于粤北下庄矿田西北部,区内碱交代作用较发育,碱交代花岗岩主要呈面状展布,与铀矿化关系密切。为研究区内碱交代过程元素变化特征及其与铀成矿的关系,文章系统采集了5件碱交代花岗岩样品,分析了其主量及微量元素组成。结果表明,与新鲜花岗岩相比,碱交代花岗岩中SiO_2、Na_2O含量明显降低,而K_2O含量显著偏高,表明区内碱交代以钾质交代为主;碱交代花岗岩中相对富集Rb、Th及U元素,亏损Ba、Sr、Ti等元素,w(LREE)/w(HREE)值相对降低,δEu值与新鲜花岗岩一致。上述特征表明,区内碱交代花岗岩与新鲜花岗岩可能具有相同的形成环境及物质来源,同时暗示流体中的U可能来自花岗岩;碱交代作用的发育有利于富铀花岗岩中铀的活化转移,为成矿流体中铀的富集创造了有利条件。     

Les xénolites ultramafiques du volcanisme alcalin quaternaire d'Oranie (Tell,Algérie occidentale), témoins d'une lithosphère cisaillée et enrichieUltramafic xenoliths from Quaternary alkali volcanism from Oranie (Tell,western Algeria): witnesses of a sheared and enriched lithosphere

Numerous ultramafic xenoliths occur within the A??n–Temouchent volcanic complex (Northwestern Oranie, Algeria). Most of them are type I mantle tectonites (lherzolites and harzburgites) and composite xenoliths (harzburgite/clinopyroxenite) are rare. Only a few samples of spinel lherzolites display relatively fertile compositions when the major part of type I xenoliths have refractory major element compositions but enriched LREE contents showing that they have been affected by mantle metasomatism. The composite xenoliths are witnesses of reactions of alkaline magmas with the upper mantle. An asthenospheric rising, in relation with the large strike slip fault affecting the North African plate margin at Trias time is proposed as a possible geodynamical setting. To cite this article: M. Zerka et al., C. R. Geoscience 334 (2002) 387–394.