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1.
ABSTRACT The Pan-African orogeny left a strong imprint on the basement rocks of Madagascar, which were metamorphosed up to granulite facies conditions. The supracrustal Itremo Group of central Madagascar, comprising quartzites, schists and carbonates of lower metamorphic grade, has to date been described as a folded sedimentary sequence. Despite their fine-grained 'sugary' appearance, most quartzites are plastically deformed tectonites. Quartzite microstructures are mainly of the elongate mosaic type, indicating significant grain boundary migration, and are compatible with dynamic recrystallization under lower amphibolite facies conditions. Consistent asymmetric quartz c -axis fabrics indicate a dominant top-to-the-east shear sense. Hence, the Itremo quartzites bear evidence for a major eastward-directed tectonic event of Pan-African age, possibly resulting from an early Pan-African thrust motion. Younger deformational events, responsible for localized mylonites with top-to-the-WSW sense of motion and N–S-trending folds and shear zones, were superimposed on this first fabric.  相似文献   

2.
The Ranotsara shear zone in Madagascar has been considered in previous studies to be a >350-km-long, intracrustal strike-slip shear zone of Precambrian/Cambrian age. Because of its oblique strike to the east and west coast of Madagascar, the Ranotsara shear zone has been correlated with shear zones in southern India and eastern Africa in Gondwana reconstructions. Our assessment using remote sensing data and field-based investigations, however, reveals that what previously has been interpreted as the Ranotsara shear zone is in fact a composite structure with a ductile deflection zone confined to its central segment and prominent NW–SE trending brittle faulting along most of its length. We therefore prefer the more neutral term “Ranotsara Zone”. Lithologies, tectonic foliations, and axial trace trajectories of major folds can be followed from south to north across most of the Ranotsara Zone and show only a marked deflection along its central segment. The ductile deflection zone is interpreted as a result of E–W indentation of the Antananarivo Block into the less rigid, predominantly metasedimentary rocks of the Southwestern Madagascar Block during a late phase of the Neoproterozoic/Cambrian East African Orogeny (c. 550–520 Ma). The Ranotsara Zone shows significant NW–SE striking brittle faulting that reactivates part of the NW–SE striking ductile structures in the flexure zone, but also extends along strike toward the NW and toward the SE. Brittle reactivation of ductile structures along the central segment of the Ranotsara Zone, confirmed by apatite-fission track results, may have led to the formation of a shallow Neogene basin underlying the Ranotsara plain. The present-day drainage pattern suggests on-going normal fault activity along the central segment. The Ranotsara Zone is not a megascale intracrustal strike-slip shear zone that crosscuts the entire basement of southern Madagascar. It can therefore not be used as a piercing point in Gondwana reconstructions.  相似文献   

3.
The present paper correlates the southern Madgascar terrain, south of the Ranotsara shear with the granulite terrain of southern India, occurring south of the Palghat-Cauvery (P-C) shear zone. Both the terrains have witnessed high temperature to ultra high temperature granulite metamorphism at 550 Ma and are traversed by shear zones and deep crustal faults. The 550 Ma old granulite terrains of Madagascar and southern India have similar lithologies, in particular, sapphirine bearing pelitic assemblages. Graphite deposits and gem occurrences are common to both these terrains. The 550 Ma old southern granulite terrain of southern India comprises of different blocks, the Madurai and the Kerala Khondalite belt, but all the blocks have similar lithologies with pelite—calc silicate rocks inter-banded with two pyroxene granulite bodies. These lithologies occur amidst an essentially charnockitic terrain. The protolith ages of the southern granulite terrain, south of the P-C shear zone ranges between 2400–2100 Ma. The terrain as a whole has witnessed the 550 Ma old granulite event. The granulite metamorphism took place under temperatures of 800–1000°C and at pressures of 9.5 to 5 Kbar.The source of heat for the high temperature granulite event of the southern Madagascar terrain has been linked to advective heat transfer along mantle deep faults. The source for the high temperature granulite metamorphism for the southern granulite terrain may be attributed to high temperature carbonatite and alkaline intrusives in an extensional setting which followed an initial crustal thickening.Many workers have linked Madagascar to southern India by connecting the Ranotsara shear either to the P-C shear zone or to the Achankovil shear zone, further south. The important factor is the lithologies of the Madagascar terrain, south of Ranotsara shear zone and the 550 Ma. old southern Indian granulite terrain are similar in many aspects. It will be more appropriate to link the Ranotsara shear to the curvilinear lineament bounding the Anaimalai-Kodaikanal ranges and which merges with the southern margin of the P-C shear zone.However, north of the Ranotsara shear/fault, the northern Madagascar terrain comprises of a dominant Itremo sequence (< 1850 Ma) and 780 Ma old calc-alkaline intrusives. The latter have similarities with that of Aravallis and the Sirohi, Malani sequences occurring further north east. The Rajasthan terrain has witnessed igneous intrusive activity at 1000–800 Ma. If we can broaden the area of investigations and include the above areas, the Madagascar-India connection can be better understood.  相似文献   

4.
Proterozoic terrains in South India and Madagascar provide important clues in understanding the Gondwanaland tectonics, especially the assembly of this mega-continent during the Pan-African period. The Archaean terrains in both Madagascar and India are characterized by N-S trending greenstone belts occurring within gneissose granitic rocks in the northern part. Extensive development of K-rich granitic rocks of ca. 2.5 Ga is also characteristic in both areas. Such a broad age zonation of younger Dharwar (ca 2.6–3.0 Ga) in the north and the older Sargur (ca 3.0–3.4 Ga) in the south as in South India remains to be identified in future studies from Madagascar. The occurrence of greenschist facies rocks in the northeastern part and higher grade rocks in most of other parts in the north-central terrain of Madagascar is comparable with the general tendency of increasing metamorphic grade from northwestern to southern areas ranging from greenschist to granulite facies in South India. The Proterozoic crystalline rocks in both continents show pronounced lithological similarity with the wide occurrence of graphite-bearing khondalite in association with charnockitic rocks. While the Archaean-Proterozoic boundary is well defined in southern India by the Palghat-Cauvery or the KKPT shear zones as recently identified, this boundary is ill-defined in Madagascar due to extensive Pan-African overprinting, as well as the development of the Proterozoic cover sequence, the Itremo Group. There is also a possible general correlation between the Mesoproterozoic cover sequences in Madagascar and India, such as between the Itremo Group of west-central Madagascar and the Kaladgi and Cuddapah sequences of South India. The Pan-African granulite facies metamorphism of ca. 0.5 Ga extensively developed in both India and Madagascar is generally comparable in intensity and extent. P-T conditions and P-T-t paths also appear comparable, with the general range of ca. 700–1000°C and 6–9 kb, and near-isothermal decompressional paths. A-type granite plutons and alkaline rocks including anorthosites and mafic plutonic rocks of ca. 500–800 Ma develop in both terrains, provide strong basis for the correlation of both terrains, and define a Pan-African igneous province within East Gondwanaland. Major shear zones in both continents are expected to play a critical role in the correlation, albeit are still poorly constrained. Detailed elucidation of the tectonic history of the shear zones, and the timing of various events along the shear zones would provide important constraints on the correlation of the two continental fragments.  相似文献   

5.
ABSTRACT The nature of the Indian crust underthrusting the Himalaya may be studied in xenoliths within Ordovician granites in the external part of the Himalaya. These peraluminous S-type granites have travelled for c . 200 km in the Main Central (or related) thrust. The granites and xenoliths sample Indian basement now buried beneath the High Himalayan thrust pile. In low-strain granites the xenoliths reveal polyphase tectonite fabrics older than the fabrics in the country rocks. Most xenoliths show greenschist/lower amphibolite facies assemblages; none is typical granulite facies of the Indian Shield. Therefore, the portion of the Indian crust underthrusting the Himalaya may be early/middle Proterozoic reworked Indian Shield, as in peninsular India. Alternatively reworking may be assigned to the Pan-African (late Proterozoic) orogeny. This prospect is raised by recent work in East Antarctica but evidence in the Himalaya is rather ambiguous. If confirmed, a Pan-African event calls for reassessment of the geological history of the Himalayan region, particularly with respect to the placing of India in Gondwanaland.  相似文献   

6.
We report here the occurrence of pink sapphires in association with a variety of gemstones from the Trivandrum Granulite Block south of the Achankovil Shear Zone in southern India. The mineralization is associated with pegmatites or veins emplaced within granulite facies aluminous supracrustals. The sapphires show near-pure A1,0, composition (98.43-99.48 wt.%) with traces of Cr, O, (0.02-0.12 wt.%) and FeO (0.01-0.12 wt.%). The available radiometric age of 513-2 Ma for gem quality zircon associated with pink sapphire in the Melankode locality confirms that the mineralization is of late Pan-African age. Pink sapphires have been widely reported from a number of localities in southern Madagascar including Betroka, Illakaka, Antranondambo and Ambossary. Sapphires of various hues also occur in the Ratnapura gem district in the southwestern part of Sri Lanka. The pink sapphire occurrences in southern Madagascar and southern Kerala provide strong evidence for India-Madagascar juxtaposition in the Gondwana assembly with the Ranotsara Shear Zone in southern Madagascar extending into the Achankovil Shear Zone in southern India.  相似文献   

7.
黄雄南  张家声  彭澎  李天斌 《岩石学报》2013,29(7):2353-2370
贺兰山北段结晶基底中保留有不同程度的韧性变形剪切带.通过详细的野外考察和室内显微构造研究,明确贺兰山北段的古元古代基底经历了4期韧性剪切变形:(1)早期顺层剪切带表现出中下部地壳层次的变形样式,运动学特征一致反映了近南北向的伸展;(2)麻粒岩相变质的糜棱片麻岩剪切带为南北向挤压的产物,导致经历高温高压变质的孔兹岩系从下地壳向中部地壳抬升;(3)高级糜棱岩(低角闪岩相-高绿片岩相)剪切带涉及的2次伸展运动(北西-南东向伸展和北东-南西向伸展)使得基底进一步向中部地壳抬升,可能发生在形成孔兹岩系的同一造山运动的晚期伸展垮塌过程中;(4)北东-近东西向左行逆冲绿片岩相糜棱岩剪切带则将结晶基底抬升到中上部地壳层次,其运动学特征与高级糜棱岩剪切带明显不同,可能是另一造山运动的产物.贺兰山北段与大青山-乌拉山地区有相似的韧性剪切带和构造变形,表明华北克拉通西部北缘存在一致的近东西走向的古元古代碰撞造山运动以及随后另一造山运动的改造.  相似文献   

8.
The Heimefrontfjella mountains, Western Dronning Maud Land (East Antarctica), are dominantly composed of Grenville-aged (≈ 1.1 Ga) rocks, which were reworked during the Pan -African orogeny at ≈500 Ma. Three discontinuity-bounded Grenville-aged terranes have been recognized namely (from north to south) the Kottas, Sivorg and Vardeklettane terranes. The terranes contain their own characteristic lithological assemblages, although each is made up of an early supracrustal sequence of metavolcanic and/or metasedimentary gneisses, intruded by various (predominantly granitoid) suites. No older basement upon which the protoliths of these older gneisses were deposited has been recognized. In each terrane the older layered gneisses were intruded by various plutonic suites ranging in age from ≈ 1150 to ≈1000 Ma. The Vardeklettane terrane is characterized by abundant charnockites and two-pyroxene granulite facies parageneses in metabasites, whereas the Sivorg and Kottas terranes were metamorphosed to amphibolite facies grade. P-T estimates show that peak metamorphic conditions changed from ≈600°C at 8 kbar in the south, to ≈700 °C at 4 kbar in the northern Sivorg terrane. Regional greenschist retrogression of high-grade assemblages may be of Pan-African age. The Heimefrontfjella terranes were juxtaposed and pervasively deformed during a complex and protracted period of E-W collision orogenesis in a transpressive regime at ≈ 1.1 Ga. This is manifest as early, gently dipping thrust-related shear fabrics (D1), succeeded by the initiation of an important (D2) steep dextral shear zone (Heimefront shear zone, HSZ), during which the early fabrics and structures were steepened and rotated in an anticlockwise sense. The HSZ is a curvilinear structure which changes from a dextral oblique strike-slip lateral ramp in the north to a steep dip-slip frontal ramp in the south, where it forms the boundary between the Sivorg and Vardeklettane terranes. The Pan-African event is manifested as discrete, low- to medium-temperature ductile to brittle shears (D3) and numerous K/Ar cooling ages.  相似文献   

9.
The western Musgrave Ranges are broadly divided into three groups of metamorphic rocks. A central granulite‐facies core is bounded on the north by rocks of amphibolite grade and on the south by rocks transitional between the granulite and amphibolite facies. Faults trending east‐west separate the three groups of rocks.

The detailed structural relationships between the granulites and the lower grade rocks are described and discussed. The granulites are structurally relatively simple and are characterised by the presence of a strong southwesterly‐plunging, mineral‐streaking lineation. In marked contrast, the transitional rocks are more complexly folded on a macroscopic scale and they also have a well‐developed mineral lineation plunging to the southeast. These two lineation orientations are considered to be directions of maximum elongation. The amphibolite‐facies rocks are also complexly folded and at least two lineations related to different phases of deformation have been recognized.

A suite of foliated and lineated dolerite dykes which occurs throughout the area inherited their fabric during a period of intense deformation and recrystallization, which resulted in the development of numerous mylonite zones.

It is suggested that the granulite‐facies rocks may represent a suite of cover rocks which have been thrust in a northerly direction over a pre‐existing amphibolite‐facies basement.  相似文献   

10.
喜马拉雅地体的泛非-早古生代造山事件年龄记录   总被引：35，自引：24，他引：35  
喜马拉雅地体是55±10Ma以来印度陆块与欧亚大陆碰撞而形成的增生地体,位于其中的高喜马拉雅与特提斯-喜马拉雅构造单元的变质基底主要由角闪岩相的富铝变质沉积岩和花岗质片麻岩组成。对两类岩石中锆石的SHRIMPU-Pb测年结果表明,除了记录了20Ma以来的构造事件年龄外,主要保存了529-457Ma的变形和变质事件记录,另外还保存了更早期(>835Ma)的年龄信息。根据20Ma以来崛起的喜马拉雅挤出岩片中包含早期强烈褶皱和向南的斜向逆冲构造以及伴随的角闪岩相变质作用记录,结合岩石测年所获得的大量泛非-早古生代年龄和奥陶纪底砾岩的发现,说明曾位于南半球印度陆块北部的变质基底岩石经历过泛非-早古生代造山事件,同位素年代学数据表明:(1)原始喜马拉雅山是泛非-早古生代造山事件的产物;(2)印度陆块早-中元古代变质基底的再活化在原始喜马拉雅山形成中起重要的作用;(3)现在的喜马拉雅山是在泛非-早古生代造山事件基础上再造山的结果。  相似文献   

11.
苏鲁变质带北部的岩石构造单元及结晶块体推覆构造   总被引：20，自引：0，他引：20       下载免费PDF全文
虽然苏鲁变质带北部的超高压变质岩石的类型及其特征与大别山超高压带相似,但是要划出与大别山相对应的岩石构造单元是困难的。详细的区域地质、岩石学、同位素年代学及地球化学的研究已将超高压带的西界大致圈定在牟平断裂至米山断裂的范围内。特别需要提出的是,在荣成超高压变质岩石分布区的南北两侧,出露有3 种不同成因的麻粒岩,即榴辉岩化的麻粒岩;由超高压变质岩经高压升温变质作用生成的麻粒岩;以及未经过榴辉岩相变质的麻粒岩。它们有规律地成带分布。这些麻粒岩带以及在荣成地区出露的未经过麻粒岩相叠加变质的超高压变质带,各自都有着完全不同的变质历史,并且都以深大韧性剪切带为其边界。由此笔者将苏鲁变质带北部由南向北划分为海阳所变质地块(榴辉岩化扬子陆块基底变质单元)、荣成变质地块(超高压变质单元)、威海变质地块(麻粒岩相叠加变质的超高压构造岩片)、昆嵛山边界杂岩带。这些来源于地壳深层的结晶块体是超高压带形成和演化的产物,在后来的进一步碰撞挤压中,这些来源于深层而就位于中上地壳水平的结晶块体,有可能发生了与薄皮构造机制(thin-skin thrust)类似的构造过程。它们有如一系列的推覆体挤压叠置,使华北和华南陆块最后挤压在一起。  相似文献   

12.
何政伟 《矿物岩石》1995,15(3):24-31
在内蒙古大青山地区存在再造太古宙基底，它经历了太古宙麻粒岩相变质作用和早元古宙角闪岩相－麻粒岩相的再造作用，再造太古宙基底岩石即为再造岩系，它是一套混合岩化变晶糜棱岩。再造岩系中混合岩化作用与再造作用同时，亦受韧性剪切变形变质作用控制。  相似文献   

13.
Ｐｅｔｒｏｇｅｎｅｓｉｓ　ｏｆ　Ｅｃｌｏｇｉｔｅｓ　ｉｎ　ｔｈｅ　Ｌｉｇｈｔ　ｏｆ　ＰｕｎｃｔｕａｔｅｄＭｅｔａｍｏｒｐｈｉｃ　Ｅｖｏｌｕｔｉｏｎ　ｉｎ　Ｄａｂｉｅ　Ｔｅｒｒａｎｅ，Ｃｈｉｎａ￥ＹｏｕＺｈｅｎｄｏｎｇ；ＨａｎＹｕｊｉｎｇ；ＺｈｏｎｇＺ...  相似文献   

14.
Three fold generations have been recognized in Svecofennian rocks (±1,800 Ma) from West Uusimaa, SW Finland. The first one (F1) might be related to thrusting and imbrication tectonics at plate collision contacts. The main generation (F2) is due to a N-S horizontal crustal shortening, which created at first E-W trending upright folds in the whole region and later tightened these F2 folds in the western part of the belt, whereas conjugate shear zones and tectonic lenses of competent rock bodies developed in the eastern part. Simultaneously the metamorphic conditions rose from amphibolite- to granulite-facies in this eastern part, which is known as the West Uusimaa Complex. The amphibolite- to granulite-facies transition zone along the western boundary of the granulite-facies complex is studied in detail. A number of prograde mineral reactions are telescoped in this transition zone: the breakdown of biotite and amphibole to ortho- ±clino-pyroxene in metaigneous rocks, the appearance of garnet in cordierite-bearing metapelites and the appearance of scapolite in calcareous rocks. Distinct mineralogical changes also occur in this zone which cross cuts all major structures and rock units and are only affected by late-F3 folding (open, disharmonic folds with approximately N-S trending axial planes) and young shear zones, associated with pseudotachylite generation. The absence of any evidence of block faulting and tilting of the crust that could be associated with the granulite complex suggests that the whole region represents one crustal level. A fluid-inclusion study indicates similar pressures for the amphibolite facies and the granulite facies domains. Application of various independent geothermobarometric methods suggest a low pressure (3–5 K bar) and a temperature increase from 550–650° C to 700–825° C, associated with a decreasing water activity (0.12O<0.4) and a general increasing CO2 activity. Fluid inclusions strongly suggest an isobaric amphibolite/granulite transition. There-fore the granulite-facies complex is designated a thermal dome. Whole rock chemical data show that granulite-facies metamorphism is isochemical. Constraints for the Svecokarelian crustal evolution are discussed.  相似文献   

15.
The granulites of the Juiz de Fora complex occur within thick basement thrust slices associated with the Pan-African shortening process in the central segment of the Ribeira belt. Five lithological units of the Intermediate tectonic domain of the belt can be identified on the basis of detailed geological mapping: a) orthogranulites, b) orthogneisses; c) kinzigite; d) intrusive garnet charnockite and e) amphibolite facies metasediments of probable Meso to Neoproterozoic age, correlated to the cover of the belt. Petrological data indicate high temperatures and intermediate to low lithostatic pressure conditions for the Paleoproterozoic granulite facies metamorphism. Textures and CO2-rich fluid inclusions are probably related to an IBC path. Geochemical data do not show relevant compositional change as a result of the granulite metamorphism. Two calc-alkaline suites and tholeiitic to alkaline basic rocks can be related to compressional and extensional settings, respectively. The overall composition of the granulites, the lack of substantial LILE depletion as well as the composition of the fluid inclusion points to granulitization process driven by CO2-rich fluids. Orthogranulites gave rise to banded gneisses as a result of the Pan-African retrograde metamorphism and intense deformation. The U and Th depletion detected in few rocks is possibly related with the hydrated conditions of the retrograde reactions.  相似文献   

16.
A generalised crustal structure of Fiordland is proposed.Detailed mapping in part of Western Fiordland has led to the recognition of a basement granulite facies lower crustal material, probably Precambrian in age) separated by a regional thrust zone from a cover sequence (amphibolite facies gneisses, of Lower Paleozoic age). With the recognition of the basement—cover relationship and the aid of aeromagnetic anomalies Fiordland has been divided into four, generally north-northeast trending, regions. The Western Fiordland region is composed chiefly of basement rocks. The Central Fiordland and Southwestern Fiordland regions are made up predominantly of amphibolite and greenschist-facies metasediments and gneissic granodiorites of the cover sequence, which in Central Fiordland have a regional dip to the east, off the basement. The Eastern Fiordland region is characterised by a series of basic, intermediate and acid intrusive rocks. The more prominent magnetic anomalies in Eastern Fiordland, Southwestern Fiordland, and a large anomaly off the coast of Western Fiordland, are all considered to be caused by intrusive bodies. The presence of a positive gravity anomaly over Western Fiordland, coupled with a gravity low offshore, is consistent with the lower crust being uplifted and exposed in this area. Continuing shallow and intermediate-depth seismic activity beneath Fiordland, as well as the large size of the gravity anomaly, suggest that tectonic forces are currently acting to maintain Western Fiordland at its unusually high level.Fiordland thus displays a cross-section of continental crust: Precambrian(?) metaigneous granulites in the lower crust; Lower Paleozoic metasedimentary amphibolitefacies gneisses and melted equivalents in the middle crust; Mesozoic intrusives, and overlying Cretaceous and Tertiary sediments in the upper crust.  相似文献   

17.
New data on the metamorphic petrology and zircon geochronology of high‐grade rocks in the central Mozambique Belt (MB) of Tanzania show that this part of the orogen consists of Archean and Palaeoproterozoic material that was structurally reworked during the Pan‐African event. The metamorphic rocks are characterized by a clockwise P–T path, followed by strong decompression, and the time of peak granulite facies metamorphism is similar to other granulite terranes in Tanzania. The predominant rock types are mafic to intermediate granulites, migmatites, granitoid orthogneisses and kyanite/sillimanite‐bearing metapelites. The meta‐granitoid rocks are of calc‐alkaline composition, range in age from late Archean to Neoproterozoic, and their protoliths were probably derived from magmatic arcs during collisional processes. Mafic to intermediate granulites consist of the mineral assemblage garnet–clinopyroxene–plagioclase–quartz–biotite–amphibole ± K‐feldspar ± orthopyroxene ± oxides. Metapelites are composed of garnet‐biotite‐plagioclase ± K‐feldspar ± kyanite/sillimanite ± oxides. Estimated values for peak granulite facies metamorphism are 12–13 kbar and 750–800 °C. Pressures of 5–8 kbar and temperatures of 550–700 °C characterize subsequent retrogression to amphibolite facies conditions. Evidence for a clockwise P–T path is provided by late growth of sillimanite after kyanite in metapelites. Zircon ages indicate that most of the central part of the MB in Tanzania consists of reworked ancient crust as shown by Archean (c. 2970–2500 Ma) and Palaeoproterozoic (c. 2124–1837 Ma) protolith ages. Metamorphic zircon from metapelites and granitoid orthogneisses yielded ages of c. 640 Ma which are considered to date peak regional granulite facies metamorphism during the Pan‐African orogenic event. However, the available zircon ages for the entire MB in East Africa and Madagascar also document that peak metamorphic conditions were reached at different times in different places. Large parts of the MB in central Tanzania consist of Archean and Palaeoproterozoic material that was reworked during the Pan‐African event and that may have been part of the Tanzania Craton and Usagaran domain farther to the west.  相似文献   

18.
大青山地区韧性剪切带变形变质作用演化   总被引：3，自引：0，他引：3  
大青山早前寒武纪高级变质岩系中发育三种不同类型的韧性剪切带。第一种为高温（相当于麻粒岩相级）逆冲型，第二种为中温（相当于角闪岩相级）伸展型，第三种为低温（相当于绿片岩相级）近走滑型。不同类型韧性剪切带的变形变质作用演化反映了地壳深部高级变质杂岩在隆升过程中的主要再造特征。  相似文献   

19.
Oxide–sulphide–Fe–Mg–silicate and titanite–ilmenite textures as well as their mineral compositions have been studied in felsic and intermediate orthogneisses across an amphibolite (north) to granulite facies (south) traverse of lower Archean crust, Tamil Nadu, south India. Titanite is limited to the amphibolite facies terrane where it rims ilmenite or occurs as independent grains. Pyrite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade. Pyrrhotite is confined to the high‐grade granulites. Ilmenite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade and occurring primarily as hemo‐ilmenite in the high‐grade granulite facies rocks. Magnetite is widespread throughout the traverse and is commonly associated with ilmenite. It decreases in abundance with increasing metamorphic grade. In the granulite facies zone, reaction rims of magnetite + quartz occur along Fe–Mg silicate grain boundaries. Magnetite also commonly rims or is associated with pyrite. Both types of reaction rims represent an oxidation effect resulting from the partial subsolidus reduction of the hematite component in ilmenite to magnetite. This is confirmed by the presence of composite three oxide grains consisting of hematite, magnetite and ilmenite. Magnetite and magnetite–pyrite micro‐veins along silicate grain boundaries formed over a wide range of post‐peak metamorphic temperatures and pressures ranging from high‐grade SO2 to low‐grade H2S‐dominated conditions. Oxygen fugacities estimated from the orthopyroxene–magnetite–quartz, orthopyroxene–hematite–quartz, and magnetite–hematite buffers average 2.5 log units above QFM. It is proposed that the trends in mineral assemblages, textures and composition are the result of an external, infiltrating concentrated brine containing an oxidizing component such as CaSO4 during high‐grade metamorphism later acted upon by prograde and retrograde mineral reactions that do not involve an externally derived fluid phase.  相似文献   

20.
New petrological and geochronological data are presented on high‐grade ortho‐ and paragneisses from northwestern Ghana, forming part of the Paleoproterozoic (2.25–2.00 Ga) West African Craton. The study area is located in the interference zone between N–S and NE–SW‐trending craton‐scale shear zones, formed during the Eburnean orogeny (2.15–2.00 Ga). High‐grade metamorphic domains are separated from low‐grade greenstone belts by high‐strain zones, including early thrusts, extensional detachments and late‐stage strike‐slip shear zones. Paragneisses sporadically preserve high‐pressure, low‐temperature (HP–LT) relicts, formed at the transition between the blueschist facies and the epidote–amphibolite sub‐facies (10.0–14.0 kbar, 520–600 °C), and represent a low (~15 °C km?1) apparent geothermal gradient. Migmatites record metamorphic conditions at the amphibolite–granulite facies transition. They reveal a clockwise pressure–temperature–time (P–T–t) path characterized by melting at pressures over 10.0 kbar, followed by decompression and heating to peak temperatures of 750 °C at 5.0–8.0 kbar, which fit a 30 °C km?1 apparent geotherm. A regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise P–T–t path, characterized by peak metamorphic conditions of 7.0–10.0 kbar at 550–680 °C, which match a 20–25 °C km?1 apparent geotherm. These P–T conditions were reached after prograde burial and heating for some rock units, and after decompression and heating for others. The timing of anatexis and of the amphibolite facies metamorphic overprint is constrained by in‐situ U–Pb dating of monazite crystallization at 2138 ± 7 and 2130 ± 7 Ma respectively. The new data set challenges the interpretation that metamorphic breaks in the West African Craton are due to diachronous Birimian ‘basins’ overlying a gneissic basement. It suggests that the lower crust was exhumed along reverse, normal and transcurrent shear zones and juxtaposed against shallow crustal slices during the Eburnean orogeny. The craton in NW Ghana is made of distinct fragments with contrasting tectono‐metamorphic histories. The range of metamorphic conditions and the sharp lateral metamorphic gradients are inconsistent with ‘hot orogeny’ models proposed for many Precambrian provinces. These findings shed new light on the geodynamic setting of craton assembly and stabilization in the Paleoproterozoic. It is suggested that the metamorphic record of the West African Craton is characteristic of Paleoproterozoic plate tectonics and illustrates a transition between Archean and Phanerozoic orogens.  相似文献   

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