Stratigraphy,structure and geochronology of ore leads in the Matsitama Schist belt of Northern Botswana

The Matsitama schist belt in northeastern Botswana comprises an area of metasediments, notably quartzites, limestones, shales and amphibolites that are bounded by granites and gneisses. The belt lies southwest of the Rhodesian cration and north of the Limpopo mobile belt.Stratigraphic, structural and lead isotopic evidence indicates that the Matsitama metasediments are equivalent to the Shashi metasediments in the Limpopo belt. There is strong evidence that the Matsitama and Shashi metasediments stratigraphically underlie volcanic rocks of the Tati belt which have been correlated with Archaean schist belts of about 2700 Ma of Rhodesia. Therefore, the Matsitama and Shashi rocks are at least as old as the schist belts of the Rhodesian craton and may represent a shallow-water facies that occurs only in the Limpopo area.There is no structural evidence that the Matsitama and Shashi metasediments were deposited unconformably on basement rocks, although the presence of gneiss, amphibolite and ironstone pebbles in a Matsitama conglomerate, as well as the presence of orthoquartzites, shows the existence of a basement source region. However, the surrounding granites intrude the Matsitama and Shashi metasediments and all underwent several deformation phases.The structural history of the Matsitama rocks can be described in terms of five phases of deformation. The main cleavage-producing deformation phase, F₂, folded the rocks into a major synform and intensely deformed them. Before this, however, the rocks had been folded and thrust so that part of the succession shows downward-facing F₂ structures and there are possibly repetitions of the stratigraphy due to imbrication. Structures of the F₃ and F₄ phases fold the main cleavage but locally are sufficiently intense to modify the shape of the finite strain ellipsoid. There is a major ductile shear zone of F₄ age, south of which F₄ folds are tight, while to the north, F₄ deformation is negligible. All of these structures can be correlated with deformation phases in the Tati schist belt to the east and in the northern part of the Limpopo mobile belt.Lead isotope evidence suggests that mineralization in the Matsitama metasediments occurred at least 2200 Ma ago, and that leads from Dihudi/Thakadu and Messina, in the centre of the Limpopo belt, underwent a two-stage history of events at 2600–2700 Ma and 2000–2100 Ma ago, agreeing with other geochronological evidence. The leads from Matsitama and Messina are isotopically distinct from leads from the Rhodesian schist belts, which show evidence of transfer to the crust some 3500 Ma ago. The absence of this 3500 Ma-old lead from the Matsitama and Messina environments may indicate different crustal conditions and possibly the absence of the Rhodesian-type early basement.     

The evolution of the archaean crust of northeast Botswana

Some 1800 km² of Archaean terrain have been mapped (the Eastern Geotraverse of the Botswana Geological Survey, Geodynamics Project) including a part of the schist belt/granitic terrain of the Rhodesian Craton, and part of the Limpopo Mobile Belt in the south. The cratonic area includes the whole or part of four schist belts (greenstone belts) now referred to as ‘schist relics’ as they are shown to be mega-xenoliths surviving regional deformation and granitisation.The schist relics display a typical greenstone belt composition with basal ultra-mafic schist, extrusive meta-basalts, serpentinites, and meta-dacites, -andesites, -rhyolites and volcanoclastic rocks appearing up the succession. These are termed the Volcanic Group. They also contain marble, graphitic phyllite, meta-greywacke, banded ironstone and aluminous schist.The schist relics overlie thick sequences of granitised clastic sediments with intercalated volcanic and sedimentary layers which are regarded as an integral, lower part of the succession. The total sequence, including the Volcanic Group and underlying rocks is at least 30 km thick.The schist relics originally formed a more continuous and lithostratigraphically equivalent succession, which may have included some of the Rhodesian schist belts. Such a succession would be of the order of size of an island arc system in length and width.An early stage of regional folding (F1) is recognized, from preserved fabrics and structural analysis, which is tentatively proposed as nappe folding on a regional scale. This was accompanied by low-grade metamorphism. This first stage of deformation determined the regional pattern as it is still seen.Tonalite/monzonite plutons were emplaced within the Volcanic Group and probably protected the relics during regional granitisation.Regional granitisation, which isolated the schist relics, was accompanied by pyroxene hornfels metamorphism. This was followed by a regional, penetrative deformation (F2) with amphibolite facies (Barrovian type) metamorphism. This is the most prominent style of deformation which, although most intensely developed in the Limpopo Mobile Belt, is imposed throughout the whole area. The Limpopo Belt cuts across the area in the south as a large ductile shear zone and affects rocks largely similar to those of the craton, although anorthosites are not known outside it.The Limpopo Belt has its own style of deformation — interference folds, and several stages of folding related to transcurrent movement, and intense cataclasis.Metamorphism is similar throughout Limpopo Mobile Belt and Craton, each event being recognized in both domains and being of similar grades.In many respects the Botswana Eastern Geotraverse area resembles other Archaean terrains and analogies can be drawn. In attempting to find a geotectonic model into which it can be placed there is a considerable range of choices, and not enough constraints existing either within the imperfectly preserved geological record, nor within the expanding and elastic framework of Global tectonic theory.No direct evidence for subduction zones or plate boundaries has been found, nor is this surprising considering the subsequent history. On the other hand we see nothing in the rather detailed body of evidence that has been accumulated by mapping to preclude a place within Plate theory as outlined for the Phanerozoic. It allows a moderately uniformitarian interpretation rather than one supposing unique conditions preculiar to the Archaean, such as the concept of isolated volcanic depositaries, or of unique and peculiar crustal conditions.     

Seismic evidence for preservation of the Archean Uchi granite–greenstone belt by crustal-scale extension

In the westernmost Superior Province of Canada, the east–west alignment of granite–greenstone belts and the adjacent, highly deformed gneiss belts led to the first proposals that plate tectonics existed before 2.5 Ga ago, with the belts thrust against one another by east–west-oriented subduction zones. Here, we present seismic reflection data, which demonstrate that in this region the present juxtaposition of the Uchi granite–greenstone belt and the North Caribou gneiss terrane occurred along a late southeast-dipping extensional shear zone that extends from the surface into the lower crust. The preservation of the Uchi belt and probably the English River metasedimentary belt is directly related to their dropping along extensional shear zones, which limited subsequent erosion. The relative lateral transport of these greenstone rocks implies that they were neither derived from the immediately underlying crust, nor preserved by vertical crustal movements as might occur in the absence of plate tectonics. Extension may have been associated with the emplacement of mantle-derived magmas at 2700 Ma, which has been linked to slab break-off or lithospheric delamination, making the extension approximately coeval with local gold mineralisation. Since crustal-scale faults can facilitate the circulation of gold-bearing fluids, we suggest that greenstone rocks preserved in the hanging walls of syn- to post-accretion extensional shear zones may preferentially host Archean lode-gold deposits. In the westernmost Superior Province, our seismic observations imply that some of the late structures in the well-developed belts defined by surface mapping arose through the collapse of a collage of laterally accreted terranes.     

西藏阿里雅鲁藏布江缝合带韧性剪切带的磁组构特征

应用磁组构测量方法,厘定、划分了札达地区印度河—雅鲁藏布江缝合带内的韧性剪切带,两条强韧性带分别位于缝合带北缘和南缘,均具有南盘(下盘)俯冲、平面右旋扭动运动特征和压扁型应变特征。韧性剪切发生于65Ma以前,它们应是印—欧两大板块俯冲-碰撞剪切应变产物,是板块缝合带的重要组成部分。     

山东鲁西地区绿岩带型金矿床特征及成因浅析

山东鲁西地区绿岩带地层总体金背景值较高,是金矿形成的矿源层.无论是保存相对完好的绿岩带还是被TTG改造后的花岗-绿岩带,均有利于金矿形成.已知金矿床的形成与分布均与韧性剪切带有关,韧性剪切带是形成绿岩型金矿的主要控矿构造.强烈韧性变形和破碎并富含绿岩带残留体的TTG岩系与金成矿关系密切.中生代燕山期岩浆热液活动的叠加有利于进一步富集成矿.      

The evolution of Archaean greenstone terrains,Eastern Goldfields Province,Western Australia

Available petrological, structural and geochronological data suggest that metamorphism and deformation of greenstone sequences and the evolution of intrusive granitoids in the Eastern Goldfields Province, Yilgarn Block, were related to a widespread and integrated tectonic event in the time interval 2700-2600 m.y.Polyphase deformation of the greenstone sequences involved the superimposition of a series of upright folds and related subvertical foliations on earlier macroscopic recumbent folds. Metamorphism was imposed rapidly on these previously deformed but relatively unaltered greenstone sequences, synchronously with a third phase of deformation. Static-style metamorphic recrystallization at very low to medium grades occurred over most of the province, but contemporaneous high grade recrystallization of dynamic style was restricted to elongate narrow zones which were also the sites of synkinematic granitoid diapirism. These zones commonly mark the present margins of greenstone belts.The extensive areas between greenstone belts are dominated by outcrops of post-kinematic granitoids whose abundance may be overestimated because of the limited exposure. Their emplacement caused only minor contact metamorphic overprinting on the pre-existing metamorphic patterns. Also present are banded gneisses interpreted as modified basement to the greenstone sequences. These gneisses are enclosed in post-kinematic granitoid batholiths or occur as remnants in synkinematic diapirs within the dynamic domains. All major granitoid groups, including gneisses, are geochemically similar and show parallel but limited variations. Both field and chemical evidence points to the gneisses being parental to intrusive granitoids derived by both anatectic and solid-state processes.The data provide important constraints on any model for greenstone belt evolution. Our preferred model involves a widespread disturbance resembling the kind currently referred to as a “mantle plume”, which initially led to extrusion of mafic and ultramafic magmas via tensional fractures in a sialic crust, then subsequently caused their deformation and metamorphism and generated the intrusive granitoids by widespread reactivation of the basement. The dynamic metamorphic domains may reflect pre-greenstone crustal lineaments that controlled the initial vulcanism. The evolution of Archean greenstone terrains proposed here appears distinct from that of subsequent Proterozoic and Phanerozoic tectonic belts.     

坦桑尼亚西北部苏库马绿岩带含金石英脉成矿特征

坦桑尼亚克拉通西北部的苏库马绿岩带是坦桑尼亚环维多利亚湖绿岩带重要的金矿聚集区。区内金矿化类型以含金石英脉为主,矿体的产出主要受剪切构造带控制。通过研究区域构造特征和岩石地球化学特征,认为苏库马绿岩带中玄武岩为弧后环境形成,流纹岩和花岗岩形成于陆弧环境,N--S向挤压作用和E--W向伸展作用是区域内主要的构造展布特征。建立了含金石英脉的成矿模式,表明具有金矿化作用的石英脉集中分布在EW、SEE、NE向剪切构造破碎带中,含金石英脉富矿体的形成经历了多期构造变形和成矿流体的叠加、富集作用。     

Archaean greenstone belts: A structaral test of tectonic hypothesis

Tectonic hypotheses for Archaean greenstone belts are tested against structural data from the Agnew belt, Western Australia. This belt shows the following critical features:

1. (1) A sialic infrastructure, formed by semi-concordant tonalitic intrusions, was present before tectonism began.

2. (2) An early deformation formed recumbent folds and a flat-lying schistosity; a second deformation formed major upright folds and steep ductile shear zones that outline the present tectonic belt. Neither deformation caused major disruptions in the stratigraphy. Both were accompanied by metamorph ism under upper greenschist to amphibolite facies conditions and low pressure.

3. (3) The belt is bounded on either side by tonalitic gneiss of unknown age, emplaced along steep shear zones.

Comparison with Phanerozoic orogenic belts representing a subduction complex, a collisional suture zone, and a collapsed marginal basin, indicates that the belt was not formed in any of these plate-tectonic environments.The second deformation in the belt resulted from regional crustal distortion, accomplished by right-lateral ductile wrenching along major N- to NNW-trending shear zones. Associated en-echelon buckle folding formed large granite-cored anticlines and tight synclines. The detailed structural pattern is not consistent with a diapiric origin for these folds. The ductile wrench faults may have been related to mantle flow patterns in a manner analogous to modern transform faults.     

撒哈拉地台霍嘎地区某金矿地质特征

太古宙绿岩带是世界主要金矿赋存区,在古老的非洲地台分布很多绿岩带,其上叠加剪切带即伴随有含金建造的出现。本文根据安徽地矿"走出去"实施的某绿岩区金矿勘查资料、西霍嘎地块地质背景、区域上金矿赋存规律及邻区正在开采的金矿考察,初步分析了该区金矿地质特征、控矿要素、矿床成因及找矿标志,供后续勘查投资和周边中方相同地质背景矿权区找金参考。     

Crustal lineaments and shear zones in Africa: Their relationship to plate movements

Many of the major lineaments in southern Africa are major ductile shear zones with large displacement, occurring within, though often bounding orogenic belts. An example is the boundary to the Limpopo belt in Botswana and Zimbabwe. However, some of these shear zones only record slight displacement when considered on a crustal scale; they are merely planes recording differential movement on much larger, flat to gently dipping, shear zones where the boundary to the orogenic belt is a low-angle thrust zone. These different types of shear zones are clearly shown in the Pan-African belt of Zambia where large ENE-trending lineaments have been recorded. Recent work has shown the northern group of shears to be large lateral ramps; for example, the rocks of the copper belt are part of an ENE-verging thrust package, the southern boundary of which is a major, oblique to lateral ramp. In southern Zambia shears are more analogous to major transform faults; they form as tear faults separating zones of different thrust vergence. A possible plate tectonic model is given for this part of Africa, showing the different relative plate movement vectors estimated from the geometry of the Pan-African shear zones.     

大别变质地体的构造样式及变形序列

大别山区晚太古代大别群变质岩系,经历了复杂的变形过程。在大别变质地块内部识别出两套大型剪切带,它们在该区构造格架形成过程中起着重要的作用。本文还分析了变质的上壳岩、片麻岩和剪切带内的变形特点。实际资料说明,剪切带内的褶皱是由于剪切的不稳定性及平行面理局部缩短形成的,这些大型地壳剪切作用多伴随有挤压流动或伸展流动。根据褶皱叠加关系以及剪切带、新成体交切关系建立了大别群总体构造序列。该区提供了一个研究中下地壳作用的窗口。     

Deformation partitioning during transpression in response to Early Devonian oblique convergence,northern Appalachian orogen,USA

Transpressive deformation was distributed heterogeneously within the Central Maine belt shear zone system, which formed in response to Early Devonian oblique convergence during the Acadian orogeny in the northern Appalachians. ‘Straight’ belts are characterized by tight folds, S>L fabrics and sub-parallel form lines, and asymmetric structures that together indicate dextral–SE-side-up kinematics. In contrast, intervening zones between ‘straight’ belts are characterized by open folds and L≫S fabrics. Within both types of zone, metasedimentary rocks have fabrics defined by the same minerals at the same metamorphic grade, including a penetrative, moderately to steeply NE-plunging mineral lineation. Thus, we interpret accumulation of plastic deformation and regional metamorphic (re-) crystallization to have been synchronous across the Central Maine belt shear zone system. Discordance between inclusion trails in regionally developed porphyroblasts of garnet and staurolite and matrix fabrics in ‘straight’ belt rocks records shortening by tightening of folds and greater reorientation of matrix fabrics with respect to porphyroblasts. Kinematic partitioning of flow was responsible for the contrasting states of finite deformation recorded in the Central Maine belt shear zone system. Perturbations in the flow were caused by serially developed thrust-ramp anticlines in the stratigraphic succession immediately above the Avalon-like basement, at which décollement of the shear zone system was initially rooted. General shear deformation at the ramps involved strain softening with an enhanced component of noncoaxial flow. In contrast, deformation during extrusion in the intervening zones involved strain hardening with a greater component of coaxial flow. Part of the thickening stratigraphic succession exceeded T_solidus, reflected by the occurrence of migmatites and granites. The latter were partly sourced from the underlying Avalon-like basement that was involved in the deformation and melting.     

Evolution of the Archean crust in Finland

An Archean age for Finnish rocks in the range 2500–3000 Ma has been determined north of the NW-striking Ladoga—Raahe shear belt. The Archean may be divided into two main units: the granitoid association and the greenstone-belt association. The complex is characterized by stockwork tectonics. The granitoid association forms the basement infrastructure and the greenston-belt association forms the suprastructure which is present in synforms between granitoid diapirs. The infrastructure has been subjected to ultrametamorphism, and the second and third generation palingenetic magmas so formed have intruded the suprastructure. The granitoid association contains widespread migmatized relicts of the greenstone-belt association, indicating that the latter originally covered much larger areas, but the granitoids are also thought to be partly transformed primitive ensialic crust on which rocks of the greenstone-belt association were deposited. The Archean rocks have been deformed in at least four subsequent phases, of which part developed in Proterozoic time. The youngest deformation is the overthrust of the granulite belt of Lapland towards SSW. NWSE striking transcurrent faults played a major role in Proterozoic time and affected cratonized Archean crust. On the whole the greenstone belts in eastern and northern Finland form a NNW-trending zone 750 km long. On a geochemical basis the volcanic rocks of the greenstone belts can be divided into two groups: tholeiites with a low potassium content and extremely low aluminium content and a calc-alkalic group with some alkalic affinities.     

Tectonic significance of deformation patterns in granitoid rocks of the Menderes nappes, Anatolide belt, southwest Turkey

Deformation fabrics in Proterozoic/Cambrian granitic rocks of the Çine nappe, and mid-Triassic granites of the Bozdag nappe constrain aspects of the tectonometamorphic evolution of the Menderes nappes of southwest Turkey. Based on intrusive contacts and structural criteria, the Proterozoic/Cambrian granitic rocks of the Çine nappe are subdivided into older orthogneisses and younger metagranites. The deformation history of the granitic rocks documents two major deformation events. An early, pre-Alpine deformation event (D_PA) during amphibolite-facies metamorphism affected only the orthogneisses and produced predominantly top-to-NE shear-sense indicators associated with a NE-trending stretching lineation. The younger metagranites are deformed both by isolated shear zones, and by a major shear zone along the southern boundary of the Çine submassif. We refer to this Alpine deformation event as D_A3. D_A3 shear zones are associated with a N-trending stretching lineation, which formed during greenschist-facies metamorphism. Kinematic indicators associated with this stretching lineation reveal a top-to-south sense of shear. The greenschist-facies shear zones cut the amphibolite-facies structures in the orthogneisses. ²⁰⁷Pb/²⁰⁶Pb dating of magmatic zircons from a metagranite, which crosscuts orthogneiss containing amphibolite-facies top-to-NE shear-sense indicators, shows that D_PA occurred before 547.2ǃ.0 Ma. Such an age is corroborated by the observation that mid-Triassic granites of the Çine and Bozdag nappes lack D_PA structures. The younger, top-to-south fabrics formed most likely as a result of top-to-south Alpine nappe stacking during the collision of the Sakarya continent with Anatolia in the Eocene.     

双山子太古代绿岩带金矿的基本特征

文章简要介绍了双山子太古代绿岩带的基本特征,指出双山子绿岩带同加拿大阿比提比绿岩带有许多相似之处;论述了绿岩带中三期不同类型金矿的各自主要成矿地质特征,指出早期含金石英脉是太古代的产物,为冀东地区典型的太古代绿岩带金矿;毒砂型金矿可能是太古代最晚期的产物;晚期石英脉型金矿形成于燕山期.在该区东部南北长约30km的矿带中...     

紧闭褶皱翼部的剪切变形作用——五台山绿岩带金矿化的一种构造控矿机制

五台山绿岩带中 ,与变质火山 沉积岩系直接相关的金矿化分布广泛 ,具有多种成矿和控矿机制。但就两种主要类型金矿———剪切带型金矿和铁建造型金矿的时空分布、地质特征和赋存的构造部位来看 ,它们都明显地与剪切变形 (韧性的或脆性的 )所产生的构造 热液活化作用有关 ,具显著的构造控矿意义。区域构造分析表明 ,五台山绿岩带的构造格架是一个多级和多期褶皱与断裂的组合 ,总的构造样式是一个紧闭程度向中心增强的复式倒转向形。它们是在北西 南东向挤压作用下递进变形的结果。在这一区域应力场作用下 ,处于不同构造部位的岩石发生变形分解作用 ,从而在紧闭褶皱的翼部产生一系列剪切变形带 ,构成了五台山绿岩带构造控矿机制。本文将以两类绿岩金矿的典型矿床为例 ,对这一控矿构造机制进行全面分析。     

阿尔泰额尔齐斯构造带岩石应变特征及其构造意义

阿尔泰额尔齐斯构造带东段岩石变形强烈,最大应变椭球轴比(Rxz)可达10～30,剪应变(γ)达4～6。该带可分为2个层次4个次级构造带。第一层次为韧性—韧脆性中—中-浅层次变形带,包括萨尔布拉克—科克萨依脆性挤压劈理化带、达拉维孜—阿热勒托别剪切流变构造带和库尔提—玛因鄂博韧性剪压变形带,第二层次是齐伯度—富蕴深层次超韧性变晶糜棱岩带。第一层次变形带中沿达拉维孜—阿热勒托别一线为最强变形带,向两侧依次减弱。中心带以剪切—拉伸变形为主,两侧以压扁变形为主兼有剪切,均为左行剪切。结合区域地质分析,可认为这3个变形带共同组成一个完整的板块(微陆块)聚合变形带,显示喀拉通克岛弧以北东偏东方向俯冲于可可托海—青河陆缘岩浆弧之下,并发生弧—陆碰撞。齐伯度—富蕴变晶糜棱岩带岩石变形温度压力条件高,以右行剪切为特点,具有完全不同的地质演化历史,反映它可能是在古生代期间被强烈改造的具有前震旦纪结晶基底的微大陆的残块(或断片)。     

郯庐剪切带的性质和意义

沿郯庐断裂带从大别山东麓经山东中部至辽北吉南的新宾—桦甸地区,暴露的早前寒武纪结晶岩石中存在着一系列北北东走向的大型韧性剪切带,其最大宽度达20km,走向断续延伸近2000km,它们分别在大别、鲁西和新宾地区形成了巨大的弧形牵引构造。剪切带中不同尺度组构要素的几何学,指示其中曾经发生了一致的大幅度左行位移;变形岩石的显微构造和矿物组合特征,说明这一韧性剪切带早期形成于低角闪岩相条件下,并且在抬升和冷却过程中经历了绿片岩相条件下的递进变形。山东中部晚元古代以后的沉积不整合于韧性剪切带及其变形岩石之上,中生代未期脆性的郯庐断裂系统追踪并改造了基底岩石中的韧性剪切带。     

Tectonics of proterozoic Cuddapah Basin,southern India: A conceptual model

A tectonic map of Cuddapah Basin (CB) and its adjacent regions has been compiled using LANDSAT TM data and the available literature on the prominent tectonic features which include Archaean greenstone belts, disposition of Proterozoic mobile belts along with the associated mid-crustal shear zones, and the mafic dyke swarms. The field characteristics of the two major fault zones: Gudur-Cuddapah and the Veldurti-Kalva-Gani fault system that cut across the CB have been re-examined and are re-interpreted as typical of transfer faults, which are generated exclusively by extensional tectonics. The earlier concept that these fault systems form a conjugate set of “Anderson type” causing the ‘Cuddapah salient’, needs to be reviewed.     

后坪-大洛韧性剪切带初步研究

位于政和-大埔断裂带中北段的后坪-大洛韧性剪切带，规模较大，由一系列北北东向平行状、网结状的糜棱岩带组成。是由加里东期和印支期四套运动学性质各不相同的韧性剪切系统构成，是不同期次、不同形变叠加变形的产物，是造山带中浅层位复杂构造运动的产物。