南非太古代巴伯顿绿岩带的构造—热力场演化

10年来对3.1～3.5Ga巴伯顿绿岩带在构造学、地层学、地质年代学中的重大进展促使我们研究其构造热力场的演化,组成绿岩带的岩石从时间序列上说至少横跨和经历400Ma的地区性构造-热力场历史,并且受到了多于1500Ma的区域性热力场干扰。最早的热事件(3490～3450Ma)作为类似于大洋中脊玄武岩的热力场变化过程记录下来,之后,两期以海弧-海沟作用为主的以200Ma为时间间隔的地质作用发生了,碰撞作用发生于3230～3080Ma之间,此后,陆台阶地扩张且受到热扰动,在3070Ma,在区域性的     

坦桑尼亚克拉通太古代绿岩带造山型金矿床地质特征及成因

为了更系统地了解坦桑尼亚克拉通太古代绿岩带内金矿成矿地质背景、矿床类型、成矿规律及矿床成因.笔者通过对绿岩带内典型金矿床的系统总结,认为金矿床可以赋存于绿岩带各种类型的岩石中,绿岩岩石及其周围花岗岩接触带附近是重点找矿方向;金矿床主要受构造剪切带控制,构造强烈变形的部位往往是最有利的成矿部位,北西-南东向和北东-南西向...     

Archaean lode gold mineralisation in banded iron formation at the Kalahari Goldridge deposit, Kraaipan Greenstone Belt, South Africa

The Kalahari Goldridge Mine is located within the Archaean Kraaipan Greenstone Belt, about 60 km southwest of Mafikeng in the North West Province, South Africa. The ore body thickness varies from 15 to 45 m along a strike length of about 1.5 km within approximately N–S striking banded iron formation (BIF). The stratabound ore body is hosted primarily by BIF, which consists of alternating chert and magnetite–chlorite–stilpnomelane–sulphide–carbonate bands of millimetre- to centimetre scale. A footwall of sericite–carbonate–chlorite schist underlain by mafic amphibolite occurs to the west and carbonaceous metapelites in the hanging wall to the east. Overlying the hanging wall, carbonaceous metapelites, units of coarse-grained metagreywackes fining upwards, become increasingly conglomeratic up the stratigraphy. Small-scale isoclinal folds, brecciation, extension fractures and boudinage of cherty BIF units reflect brittle-ductile deformation. Fold axial planes have foliation, with subvertical plunges parallel to prominent rodding and mineral lineation in the footwall rocks. Gold mineralisation is associated with two generations of quartz–carbonate veins, dipping approximately 20° to 40° W. The first generation consists of ladder-vein sets (group IIA) preferentially developed in centimetre-scale Fe-rich mesobands, whereas the second generation consists of large quartz–carbonate veins (group IIB), which locally crosscut the entire ore body and extend into the footwall and hanging wall. The ore body is controlled by mesoscale isoclinal folds approximately 67° E, orthogonal to the plane of mineralised, gently dipping veins, defining the principal stretching direction and development of fluid-focussing conduits. The intersections of the mineralised veins and foliation planes of the host rock plunges approximately 08° to the north. Pervasive hydrothermal alteration is characterised by chloritisation, carbonatisation, sulphidation and K-metasomatism. Gold is closely associated with sulphides, mainly pyrite and pyrrhotite, and to a lesser extent, with bismuth tellurides and carbonate minerals. Mass balance transfer calculations indicate that hydrothermal alteration of BIF involved enrichment of Au, Ag, Bi, Te, S and CO₂ (LOI), MgO, Ba, K and Rb, but significant depletion of SiO₂ and, to a lesser extent, Fe₂O₃. Extensive replacement of magnetite and chlorite in BIF and other pelitic sedimentary rocks by sulphide and carbonate minerals, both on mesoscopic and microscopic scales, is evidence of interaction of CO₂- and H₂S-bearing fluids with the Fe-rich host rocks. The fineness of gold grains ranges from 823 to 921, similar to that of other epigenetic Archaean BIF-hosted gold deposits, worldwide.     

华北克拉通绿岩带型金成矿系统初析

华北克拉通中绿岩带型金矿床是我国重要的矿床类型之一，也是我国黄金产量和储量的主要来源。从构造动力体制来看，华北克拉通在漫长的演化历史中曾经达过中晚元古代的离散型陆缘，早古生代到晚古生代的汇聚型陆缘和显生宙中生代克拉通活化多个构造阶段。金矿床主要形成于克拉通活化阶段，其总的构造背景可概括为由挤压，碰撞作用晚期或期后向伸展构造的转变时期。金成矿具有明显的区域地球化学背景。虽然华北克拉通绿岩带型金矿床形成时代不尽相同，但具有相似的地质地球化学特征，并与显生宙花岗岩类侵入体具晚于花岗岩，指示金矿化是构造－花岗岩浆演化晚期的产物。因此，这些金矿床基本上可归为一个成矿系统，即绿岩带中与花岗质岩浆作用于有关的热液成矿系统。     

The Geochemistry of Ultramafic to Mafic Volcanics from the Belingwe Greenstone Belt, Zimbabwe: Magmatism in an Archean Continental Large Igneous Province

The evolution of the late Archean Belingwe greenstone belt,Zimbabwe, is discussed in relation to the geochemistry of theultramafic to mafic volcanic rocks. Four volcanic types (komatiite,komatiitic basalt, D-basalt and E-basalt) are distinguishedin the 2·7 Ga Ngezi volcanic sequence using a combinationof petrography and geochemistry. The komatiites and D-basaltsare rocks in which isotopic systems and trace elements are depleted.Chemical variations in komatiites and D-basalts can be explainedby fractional crystallization from the parental komatiite. Incontrast, komatiitic basalts and E-basalts are siliceous anddisplay enriched isotopic and trace element compositions. Theirchemical trends are best explained by assimilation with fractionalcrystallization (AFC) from the primary komatiite. AFC calculationsindicate that the komatiitic basalts and E-basalts are derivedfrom komatiites contaminated with 20% and 30% crustal material,respectively. The volcanic stratigraphy of the Ngezi sequence,which is based on field relationships and the trace elementcompositions of relict clinopyroxenes, shows that the leastcontaminated komatiite lies between highly contaminated komatiiticbasalt flows, and has limited exposure near the base of thesuccession. Above these flows, D- and E-basalts alternate. Thekomatiite appears to have erupted on the surface only in theearly stages, when plume activity was high. As activity decreasedwith time, komatiite magmas may have stagnated to form magmachambers within the continental crust. Subsequent komatiiticmagmas underwent fractional crystallization and were contaminatedwith crust to form D-basalts or E-basalts. KEY WORDS: komatiite; crustal assimilation; Belingwe greenstone belt; continental flood basalt; plume magmatism     

Metamorphic evolution of the late Archaean Cadillac tectonic zone, McWatters, Abitibi belt, Quebec

Abstract Archaean greenstone belts are often cut by major shear zones, for example the Cadillac tectonic zone (CTZ) of the southern Abitibi region in north-western Quebec. At McWatters, the CTZ contains slices of metavolcanic units bounded by corridors of highly strained and altered rocks. Mineral assemblages of the metabasites record the metamorphic evolution of the CTZ.
The McWatters metabasalts and metagabbros have similar chemistry but different mineral assemblages consisting of variable amounts of actinolite, hornblende, chlorite, albite, epidote, quartz, carbonates, titanite, biotite, rutile, magnetite, ilmenite and sulphides. The different mineral assemblages, which coexist in a single tectonic slice, can be divided into three types, characterized by (A) presence of hornblende and actinolite, (B) presence of actinolite and epidote, and (C) absence of amphibole and epidote. Partial replacements indicate that these mineral assemblages are not in equilibrium. The hornblende of the least altered and deformed samples of the type A assemblage is a relict of a prograde metamorphic event, contemporaneous with the development of the main schistosity. The prograde conditions are estimated at P = 5 kbar, T = 475° C with low P_f . The more altered and deformed samples of the type C assemblage record a later retrograde metamorphic event. Conditions of the later event are estimated at P = 4 kbar, T = 400° C with higher P_f . Widespread calcite precipitation occurred during a later episode. The diversity of the mineral assemblages results from permeability variations along the high-strain zones of the CTZ.     

Occurrence of komatiites in the Sandstone greenstone belt,north-central Yilgarn Craton *

Recent geological mapping and exploration drilling has identified widespread but poorly exposed komatiites in the southern part of the Sandstone greenstone belt, which represent the most significant occurrence of komatiites so far recognised in the north-central Yilgarn Craton. Despite serpentinisation and talc – carbonate alteration, relict olivine-cumulate and less common olivine-spinifex textures are preserved. Whole-rock geochemistry indicates the presence of aluminium-depleted and aluminium-undepleted komatiites, both of which are also found in the Forrestania greenstone belt of the south-central Yilgarn Craton.     

Field Relationships, Geochemistry, Zircon Ages and Evolution of a Late Archaean to Palaeoproterozoic Lower Crustal Section in the Hengshan Terrain of Northern China

The Hengshan complex forms part of the central zone of the North China Craton and consists predominantly of ductilely-deformed late Archaean to Palaeoproterozoic high-grade, partly migmatitic, granitoid orthogneisses, intruded by mafic dykes of gabbroic composition. Many highly strained rocks were previously misinterpreted as supracrustal sequences and represent mylonitized granitoids and sheared dykes. Our single zircon dating documents magmatic granitoid emplacement ages between 2.52 Ga and 2.48 Ga, with rare occurrences of 2.7 Ga gneisses, possibly reflecting an older basement. A few granitic gneisses have emplacement ages between 2.35 and 2.1 Ga and show the same structural features as the older rocks, indicating that the main deformation occurred after -2.1 Ga. Intrusion of gabbroic dykes occurred at -1920 Ma, and all Hengshan rocks underwent granulite-facies metamorphism at 1.88-1.85 Ga, followed by retrogression, sheafing and uplift. We interpret the Hengshan and adjacent Fuping granitoid gneisses as the lower, plutonic, part of a late Archaean to early Palaeoproterozoic Japan-type magmatic arc, with the upper, volcanic part represented by the nearby Wutai complex. Components of this arc may have evolved at a continental margin as indicated by the 2.7 Ga zircons. Major deformation and HP metamorphism occurred in the late Palaeoproterozoic during the Luliang orogeny when the Eastern and Western blocks of the North China Craton collided to form the Trans-North China orogen. Shear zones in the Hengshan are interpreted as major lower crustal discontinuities post-dating the peak of HP metamorphism, and we suggest that they formed during orogenic collapse and uplift of the Hengshan complex in the late Palaeoproterozoic （〈1.85 Ga）.     

太古宙克拉通型下地壳剖面：华北怀安－丰镇－尚义的麻粒岩－角闪岩系

华北克拉通北缘晋冀内蒙交界地区有变质程度连续变化、从高压基性麻粒岩、麻粒岩到角闪岩相的变质岩系出露。它们的古变质压力由＞１．４ＧＰａ（５０ｋｍ）变化到０．５ＧＰａ（１５ｋｍ）；岩石类型从变辉长岩、中酸性正片麻岩到表壳岩变化；变质矿物组合由不含水的耐火组合变为富含云母和角闪石的组合；地球化学性质表现出从贫Ｓｉ和Ａｌ、轻度亏损生热元素到具有正常的化学成分的连续变化；包裹体流体性质在轻度亏损生热元素的麻粒岩中均为ＣＯ２流体，而在麻粒岩相的表壳岩和角闪岩系中Ｈ２Ｏ流体逐渐增加。上述变质岩系剖面的组成和特点符合大陆下地壳的定义，可能代表了包括最下部地壳在内的华北地块太古宙克拉通型大陆下地壳。据此本文建立了我国第一条克拉通型大陆下地壳剖面，并讨论其地质意义以及相关的问题     

Patterns and Dynamics of Rifting on Passive Continental Margin from Shelf to Slope of the Northern South China Sea:Evidence from 3D Analogue Modeling

Affected by thermal perturbation due to mantle uprising,the rheological structure of the lithosphere could be modified,which could lead to different rifting patterns from shelf to slope in a passive continental margin.From the observed deformation style on the northern South China Sea and analogue modeling experiments,we find that the rift zone located on the shelf is characterized by half grabens or simple grabens controlled mainly by long faults with large vertical offset,supposed to be formed with normal lithasphere extension.On the slope,where the lithosphere is very hot due to mantle upwelling and heating,composite grabens composed of symmetric grabens developed.The boundary and inner faults are all short with small vertical offset.Between the zones with very hot and normal lithosphere,composite half grnbens composed of half grabens or asymmetric grabens formed,whose boundary faults are long with large vertical offset,while the inner faults are relatively short.Along with the thickness decrease of the brittle upper crust due to high temperature,the deformation becomes more sensitive to the shape of a pre-existing weakness zone and shows orientation variation along strike.When there was a bend in the pre-existing weakness zone,and the basal plate was pulled by a clockwise rotating stress,the strongest deformation always occurs along the middle segment and at the transition area from the middle to the eastern segments,which contributes to a hotter lithosphere in the middle segment,where the Baiyun (白云) sag formed.     

恒山中段新太古代高压麻粒岩区的构造特征

恒山中段高压麻粒岩区是华北克拉通新太古代高压麻粒岩最好的露头区之一，北部为强烈重熔的灰色片麻岩区，构造样式以穹窿、近水平韧性剪切带为主，中部以近东西向的右旋走滑剪切带与南部变形变质程度相对较低的岩石单元相接，后者主要的岩石类型为均匀的英云闪长质片麻岩和变质辉绿岩墙。高压麻粒岩以透镜体形式出现在北部。本文通过对恒山高压麻粒岩区构造特征和构造样式的分析，认为高压麻粒岩区主要经历了4期构造变形：①高压麻粒岩透镜体里的逆冲型韧性剪切变形和走滑剪切变形；②向南或南西的伸展变形；③右行走滑剪切；④向东或南东伸展变形。对恒山地区的高压麻粒岩的出露起决定作用的是两次伸展构造运动。     

早前寒武纪大陆块壳构造理论——华北克拉通早前寒武纪基底构造格架、地壳演化及其与区域地球化学不均一性的关系研究

早前寒武纪大陆块壳构造理论——华北克拉通早前寒武纪基底构造格架、地壳演化及其与区域地球化学不均一性的关系研究张福勤＊＊（中国科学院地球化学研究所，贵阳５５０００２）关键词大陆块壳并合带地壳演化进程地球化学不均一性早前寒武纪地质学研究是地球科学最基础的...