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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 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   

3.
The Precambrian of Madagascar is divided into two sectors by the north-west trending sinistral Ranotsara shear zone, which continues in the Mozambique belt, probably as the Surma shear zone, and in Southern India as the Achankovil shear zone. South of Ranotsara six north-south trending tectonic belts are recognized that consist largely of granulite and high amphibolite facies paragneisses, phlogopite diopsidites, concordant granites and granulites. North of Ranotsara the central-northern segment is traversed by a north-trending axial 100–150 km wide dextral shear zone of probable Pan-African age, which was metamorphosed under granulite and high amphibolite facies conditions and which has reworked older basement. This shear zone continues across southern India as the Palghat-Cauvery shear zone. Major stratiform basic -ultrabasic complexes occur in the axial zone and in the basement to the west. Well preserved low grade continental margin-type sediments (quartzites, mica schists and stromatolitic marbles) of Kibaran age are present in western Madagascar. Two partly greenschist grade sedimentary groups lie unconformably on high grade basement in north-east Madagascar. Isotopic age data suggest the presence in Madagascar of Archaean, Early and Mid-Proterozoic crustal material that was extensively reworked in Pan-African times.  相似文献   

4.
Ruby Mineralization in Southwest Madagascar   总被引：2，自引：0，他引：2  
Gem-variety of red corundum (i.e. ruby) is produced in the Ejeda-Fotadrevo area, in southwestern Madagascar. The primary ruby deposits are closely associated with basic/ultrabasic complexes within the high grade metamorphic terranes of the Precambrian Vohibory unit. Ruby is recovered from amphibolite and anorthosite veins within these complexes. Petrographic data and P-T estimates indicate that the ruby-bearing rocks crystallized under granulites facies conditions of 750–850°C and 9–11.5kbar, in accordance with the conditions recorded from the surrounding granulites. The Malagasy ruby deposits present numerous similarities with East African deposits, especially Tanzanian, indicating similar geological context of ruby mineralization and suggesting that ruby formation in both these areas resulted from a same mineralizing event when Madagascar was still adjacent to East Africa (Kenya, Tanzania) in the Gondwanaland assembly at the end of Proterozoic times.  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.
The southern East African Orogen is a collisional belt where the identification of major suture zones has proved elusive. In this study, we apply U–Pb isotopic techniques to date detrital zircons from a key part of the East African Orogen, analyse their possible source region and discuss how this information can help in unravelling the orogen.U–Pb sensitive high-mass resolution ion microprobe (SHRIMP) and Pb evaporation analyses of detrital zircons from metasedimentary rocks in eastern Madagascar reveal that: (1) the protoliths of many of these rocks were deposited between 800 and 550 Ma; and (2) these rocks are sourced from regions with rocks that date back to over 3400 Ma, with dominant age populations of 3200–3000, 2650, 2500 and 800–700 Ma.The Dharwar Craton of southern India is a potential source region for these sediments, as here rocks date back to over 3400 Ma and include abundant gneissic rocks with protoliths older than 3000 Ma, sedimentary rocks deposited at 3000–2600 Ma and granitoids that crystallised at 2513–2552 Ma. The 800–700 Ma zircons could potentially be sourced from elsewhere in India or from the Antananarivo Block of central Madagascar in the latter stages of closure of the Mozambique Ocean. The region of East Africa adjacent to Madagascar in Gondwana reconstructions (the Tanzania craton) is rejected as a potential source as there are no known rocks here older than 3000 Ma, and no detrital grains in our samples sourced from Mesoproterozoic and early Neoproterozoic rocks that are common throughout central east Africa. In contrast, coeval sediments 200 km west, in the Itremo sheet of central Madagascar, have detrital zircon age profiles consistent with a central East African source, suggesting that two late Neoproterozoic provenance fronts pass through east Madagascar at approximately the position of the Betsimisaraka suture. These observations support an interpretation that the Betsimisaraka suture separates rocks that were derived from different locations within, or at the margins of, the Mozambique Ocean basin and therefore, that the suture is the site of subduction of a strand of Mozambique Ocean crust.  相似文献   

7.
Sm-Nd model ages of orthopyroxene-bearing massif charnockites from the Cardamom Hills Massif and adjoining supracrustal rocks from the Kerala Khondalite Belt in southernmost India are used to infer some of the relationships within these rocks and between them and neighboring areas. Most of these rocks have model ages of 2.1–2.8 Ga with most charnockites in the range 2.2–2.6 Ga. Thus, 3.0–3.4 Ga Archean rocks to their north did not contribute material to either suite and the two suites may have been juxtaposed after formation of the supracrustal rocks. The similarity of Sm-Nd isotope systems in the two units studied here supports an argument that the massif charnockites were the primary sole source of the detritus incorporated into the supracrustal rocks. A cordierite gneiss, representative of a relatively minor lithology in the supracrustal belt, has a model age of 1.3 Ga. The protolith of this gneiss not only formed from much younger material than the rest of the belt but also formed significantly after the other metasedimentary rocks. The source material of the gneiss protolith may have been located in the Wanni and Vijayan Complexes of Sri Lanka. The overlap of the model ages of rocks in this area and those in the Highland Complex of Sri Lanka supports the notion that these two sets of rocks were joined to each other in Gondwana. They belong to a belt that ran from Antarctica through Sri Lanka and India into Madagascar. This belt was involved in Pan-African tectono-metamorphism, as reflected in the 550 Ma age of the last, granulite-forming, event throughout the belt.  相似文献   

8.
The southeast Reynolds Range, central Australia, is cut by steep northwest‐trending shear zones that are up to hundreds of metres wide and several kilometres long. Amphibolite‐facies shear zones cut metapelites, while greenschist‐facies shear zones cut metagranites. Rb–Sr and 40Ar–39Ar data suggest that both sets of shear zones formed in the 400–300 Ma Alice Springs Orogeny, with the sheared granites yielding well‐constrained 40Ar–39Ar ages of ca 334 Ma. These data imply that the shear zones represent a distinct tectonic episode in this terrain, and were not formed during cooling from the ca 1.6 Ga regional metamorphism. A general correlation between regional metamorphic grade and the grade of Alice Springs structures implies a similar distribution of heat sources for the two events. This may be most consistent with both phases of metamorphism being caused by the burial of anomalously radiogenic heat‐producing granites. The sheared rocks commonly have undergone metasomatism implying that the shear zones were conduits of fluid flow during Alice Springs times.  相似文献   

9.
The Aravalli mountain range (AMR) in the northwestern part of the Indian Peninsula consists of two main Proterozoic metasedimentary and metaigneous sequences, the Aravalli and Delhi Supergroups, respectively, which rest over the Archaean gneissic basement. A synthesis and reinterpretation of the available geological, geochronological and geophysical data, including results of own field work and geophysical interpretations pertaining to the AMR, indicate its origin as an inverted basin: rifting into granitoid basement began ca. 2.5; Ga ago with Aravalli passive rifting (ca. 2.5–2.0 Ga) and Delhi active rifting (ca. 1.9–1.6 Ga). Associated mafic igneous rocks show both continental and oceanic tholeiitic geochemistry and are comparable with Phanerozoic, rift-related magmatic products. Available data showed no conclusive evidence for oceanic lithoshere and island-arc/active margin magmatic activity in the AMR. Subsequent inversion and orogeny (Delhi orogeny, ca. 1.5-1.4 Ga) lead to complex deformation and metamorphism. Only in the western and central zones has the basement been involved in this mid-Proterozoic (Delhi) deformation, whereas it is unaffected in the eastern part, except for local shear zones mainly along the basement/cover interface. The grade of metamorphism increases from the greenschist facies in the east to the amphibolite facies in the west with local HP assemblages. These latter are explained by rapid burial and exhumation of thin and cool continental lithosphere. Subsequently, during a final, mild phase of inversion, the Vindhyan basins consisting mainly of sandstones, limestones and shales, flanking the AMR formed which are comparable to foreland basins. The tectonic evolution of the AMR is therefore interpreted as an example of a major inverted continental rift and of a Proterozoic intra-continental orogen.  相似文献   

10.
郯庐剪切带的性质和意义   总被引：11，自引：0，他引：11  
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。  相似文献   

11.
The U–Pb age determinations of zircon and rutile from the Aar massif reveal a complex evolution of the Central Alpine basement. The oldest components are found in zircons of metasediments, which bear cores of Archean age; the U–Pb age of discordant prismatic zircons of the same rocks ranges between 580 and 680 Ma, an age that is typical for Pan-African metamorphism. The zircons are interpreted as Pan-African detritus with Archean inheritance. The provenance region of the Pan-African zircons is assumed to be a terrane of Gondwana-affinity, i.e. the W. African craton or the Pentevrian microplate. The Caledonian metamorphism left a pervasive structural imprint in amphibolite facies on the rocks of the Aar massif; it is dated at 456±2 and 445 Ma by zircons of a layered migmatitic gneiss and a migmatitic leucosome, respectively, both occurring in the northernmost zones of the massif. Hercynian metamorphism never exceeded greenschist-facies conditions and is recorded by zircon in a garnet-amphibolite and by rutile in a meta-psammite that yield an age of 330 Ma. Both zircon and rutile are considered to be products of retrograde mineral reactions and therefore do not date the peak conditions of Hercynian metamorphism. The Gastern granite at the western end of the Aar massif is a contaminated granite that intruded at 303±4 Ma, contemporaneously with the wide-spread late Hercynian post-collisional I-type magmatism. The study demonstrates the potential of isotope dilution U–Pb dating of single grains and microfractions in deciphering complex evolutionary histories of polymetamorphic terrains.  相似文献   

12.
Trace element and U–Pb isotopic analyses of inherited zircon cores from a sample of Gil Márquez granodiorite (South Portuguese Zone, SPZ) and Almonaster nebulite (Ossa-Morena Zone, OMZ, in the Aracena Metamorphic Belt) have been obtained using laser ablation-inductively coupled plasma-mass spectrometry. These data reveal differences in the age of deep continental crust in these two zones. Inherited zircon cores from the Ossa-Morena Zone range at 600±100 Ma, 1.7–2 Ga and 2.65–2.95 Ga, while those from the South Portuguese Zone range at 400–500 and 700–800 Ma. These data support the “exotic” origin of the South Portuguese Zone basement relative to the rest of Iberian Massif. The young ages of inherited zircon cores and Nd model ages of magmatic rocks of the South Portuguese Zone are comparable to results from granulite facies xenoliths and granitic rocks from the Meguma Terrane and Avalonia and support a correlation between the basement of the southernmost part of the Iberian Massif and the northern Appalachians.  相似文献   

13.
An assessment of the southern Betsimisaraka Suture (B.S.) of southeastern Madagascar using remote sensing and field investigation reveals a complex deformation history. Image processing of Landsat ETM+data and JERS-I Synthetic Aperture Radar (SAR) imagery was integrated with field observations of structural geology and field petrography. The southern B.S. divides the Precambrian basement rocks of Madagascar in two parts. The western part includes Proterozoic rocks whereas the eastern part is an Archean block, named the Masora block. The southern part of the B.S. includes high-grade metamorphic rocks, recording strong deformation and has mineral deposits including chromite, nickel, and emerald, characteristic of oceanic material that is compatible with a suture zone.Large-scale structural features indicate ductile deformation including three generations of folding (F1, F2, and F3) associated with dextral shearing. The first folding event (F1) shows a succession of folds with NE striking axial planes. The second folding event (F2) mainly has north–south striking axial planes and the last event (F3) is represented by mega folds that have ENE–WSW axial plane directions and have NNW and SSE contractional strain patterns. Closure of the Mozambique Ocean between two components of Gondwana sandwiched rocks of the B.S. and formed upright folds and shortening zones which produced N–S trending lineaments. Later dextral movements followed the contraction and formed NW–SE trending lineaments and N–S trending normal faults associated with dextral strike slip faults and fractures.  相似文献   

14.
There are several geological, geochemical and geophysical evidences, which corroborate reconstruction of Gondwanaland and juxtaposition of India and Antarctica. Petrology of the Precambrian mafic dykes of East Antarctica and Central-East India also support juxtaposition of India and Antarctica. Mafic dykes of different generations are emplaced in the Archaean granite gneisses of these regions. These dykes appear to be an important tool to support juxtaposition of India and Antarctica. Geological and petrological data of the Central-East India Precambrian mafic dykes suggest four episodes of mafic magmatism in the region - three tholeiitic and one noritic (?). Similarly, East Antarctica also comprises four dyke suites, emplaced during three distinct periods. These suites are 2.4 Ga meta-tholeiites, 2.4 Ga high-Mg tholeiites, 1.8 Ga dolerites and 1.2–1.4 Ga dolerites. Geochemical compositions of these mafic dykes are compared and they show good relationships with each other. Similarities in petrological and geochemical characteristics of Precambrian mafic dykes of East Antarctica and Central-East India strongly support juxtaposition of these two continents.  相似文献   

15.
We discuss the question whether the late Mesoproterozoic and early Neoproterozoic rocks of eastern, central and southern Africa, Madagascar, southern India, Sri Lanka and South America have played any role in the formation and dispersal of the supercontinent Rodinia, believed to have existed between about 1000 and 750 Ma ago. First, there is little evidence for the production of significant volumes of ˜1.4–1.0 Ga (Kibaran or Grenvillian age) continental crust in the Mozambique belt (MB) of East Africa, except, perhaps, in parts of northern Mozambique. This is also valid for most terranes related to West Gondwana, which are made up of basement rocks older than Mesoproterozoic, reworked in the Brasiliano/Pan-African orogenic cycle. This crust cannot be conclusively related to either magmatic accretion processes on the active margin of Rodinia or continental collision leading to amalgamation of the supercontinent. So far, no 1.4–1.0 Ga rocks have been identified in Madagascar. Secondly, there is no conclusive evidence for a ˜1.0 Ga high-grade metamorphic event in the MB, although such metamorphism has been recorded in the presumed continuation of the MB in East Antarctica. In South America, even the Sunsas mobile belt, which is correlated with the Grenville belt of North America, does not include high-grade metamorphic rocks. All terranes with Mesoproterozoic ages seem to have evolved within extensional, aulacogen-type structures, and their compressional deformation, where observed, is normally much younger and is related to amalgamation of Gondwana. This is also valid for the Trans-Saharan and West Congo belts of West Africa.Third, there is also no evidence for post-1000 Ma sedimentary sequences that were deposited on the passive margin(s) of Rodinia. In contrast, the MB of East Africa and Madagascar is characterized by extensive structural reworking and metamorphic overprinting of Archaean rocks, particularly in Tanzania and Madagascar, and these rocks either constitute marginal parts of cratonic domains or represent crustal blocks (terranes or microcontinents?) of unknown derivation. This is also the case for most terranes included in the Borborema/Trans-Saharan belt of northeastern Brazil and west-central Africa, as well as those of the Central Goíás Massif in central Brazil and the Mantiqueira province of eastern and southeastern Brazil.Furthermore, there is evidence for extensive granitoid magmatism in the period ˜840 to <600 Ma whose predominant calc-alkaline chemistry suggests subduction-related active margin processes during the assembly of the supercontinent Gondwana. The location of the main Neoproterozoic magmatic arcs suggests that a large oceanic domain separated the core of Rodinia, namely Laurentia plus Amazonia, Baltica and West Africa, from several continental masses and fragments now in the southern hemisphere, such as the São Francisco/Congo, Kalahari and Rio de La Plata cratons, as well as the Borborema/Trans-Saharan, Central Goiás Massif and Paraná blocks. Moreover, many extensional tectonic events detected in the southern hemisphere continental masses, but also many radiometric ages of granitois that are already associated with the process of amalgamation of Gondwana, are comprised within the 800–1000 age interval. This seems incompatible with current views on the time of disintegration of Rodinia, assumed to have occurred at around 750 Ma.  相似文献   

16.
姜继圣  刘志宏 《岩石学报》1997,13(3):346-355
根据１５万区域地质调查，将区内的早前寒武纪基底划分为变质上壳岩、中粗粒黑云长英片麻岩、变黑云母钾长花岗岩和变质基性岩四个岩石单元。通过对上述岩石单元岩石类型、地球化学特征、变质变形作用及同位素年代学研究，对其形成时序进行了讨论，由此确定了本区早前寒武纪基底的地质演化轮廓，即在中晚太古时期，本区经历了由玄武安山岩和英安岩双峰式火山建造为主体的上壳岩系的形成阶段，并于２．６Ｇａ遭受角闪岩相变质；随即伴有大规模的ＴＴＧ深成岩浆活动，晚太古末经历绿帘角闪岩相的区域变质作用；至早元古初期，深熔成因的钾质花岗岩侵位，区内已存的早期变质岩石受到该期钾质岩浆的交代改造，并在其成岩之后遭受绿片岩相的区域变质。  相似文献   

17.
The northeastern part of Madagascar is characterized by Archaean to early Proterozoic rocks composed principally of Archaean granite and greenstone/amphibolite as well as reworked migmatite with subordinate Proterozoic paragneisses. The southern part is mostly occupied by Proterozoic rocks, composed mostly of Meso to Neo-Proterozoic and less metamorphic metasediments (Itremo Group) in the northwest, para- and ortho-gneisses in most other areas, with minor granitic gneisses with some Archaean components in the southeast. The north-northwest trending Central Granite-Gneiss-Migmatite Belt (CGGMB) is situated at the western margin of the Archaean-early Proterozoic terrain. The CGGMB is composed of granite, gneiss and migmatite with distinct lithologies and structures. They are: i) many types of granites including alkaline to mildly alkaline granites, and calc-alkaline granites; ii) batholitic granites, migmatitic granites and granite dyke swarm, iii) eclogite, and iv) the Ankazobe-Antananarivo-Fianarantsoa Virgation.

The CGGMB was formed by the collision of the palaeo-Dharwar Craton to the east and the East African Orogen to the west at ca. 820-720 Ma and suffered indentation by a part of the western part of the East African Orogen at ca. 530 Ma that produced the Ankazobe-Antananarivo-Fianarantsoa Virgation at the centre of the CGGMB. Thus, the CGGMB is proposed to be the continuation of the eastern suture between the palaeo Dharwar Craton and the East African Orogen, and carries the main feature of the Pan-African collisional event in Madagascar.  相似文献   


18.
The polyphase evolution of the Seridó Belt (NE-Brazil) includes D1 crust formation at 2.3–2.1 Ga, D2 thrust tectonics at 1.9 Ga and crustal reworking by D3 strike-slip shear zones at 600 Ma. Microstructural investigations within mylonites associated with D2 and D3 events were used to constrain the tectono-thermal evolution of the belt. D2 shear zones commenced at deeper crustal levels and high amphibolite facies conditions (600–650 °C) through grain boundary migration, subgrain rotation and operation of quartz c-prism slip. Continued shearing and exhumation of the terrain forced the re-equilibration of high-T fabrics and the switching of slip systems from c-prism to positive and negative a-rhombs. During D3, enhancement of ductility by dissipation of heat that came from syn-D3 granites developed wide belts of amphibolite facies mylonites. Continued shearing, uplift and cooling of the region induced D3 shear zones to act in ductile-brittle regimes, marked by fracturing and development of thinner belts of greenschist facies mylonites. During this event, switching from a-prism to a-basal slip indicates a thermal path from 600 to 350 °C. Therefore, microstructures and quartz c-axis fabrics in polydeformed rocks from the Seridó Belt preserve the record of two major events, which includes contrasting deformation mechanisms and thermal paths.  相似文献   

19.
Mafic rocks of Western Dharwar Craton (WDC) belong to two greenstone cycles of Sargur Group (3.1–3.3 Ga) and Dharwar Supergroup (2.6–2.8 Ga), belonging to different depositional environments. Proterozoic mafic dyke swarms (2.4, 2.0–2.2 and 1.6 Ga) constitute the third important cycle. Mafic rocks of Sargur Group mainly constitute a komatiitic-tholeiite suite, closely associated with layered basic-ultrabasic complexes. They form linear ultramaficmafic belts, and scattered enclaves associated with orthoquartzite-carbonate-pelite-BIF suite. Since the country rocks of Peninsular Gneiss intrude these rocks and dismember them, stratigraphy of Sargur Group is largely conceptual and its tectonic environment speculative. It is believed that the Sargur tholeiites are not fractionated from komatiites, but might have been generated and evolved from a similar mantle source at shallower depths. The layered basic-ultrabasic complexes are believed to be products of fractionation from tholeiitic parent magma. The Dharwar mafic rocks are essentially a bimodal basalt-rhyolite association that is dominated by Fe-rich and normal tholeiites. Calc-alkaline basalts and andesites are nearly absent, but reference to their presence in literature pertains mainly to carbonated, spilitized and altered tholeiitic suites. Geochemical discrimination diagrams of Dharwar lavas favour island arc settings that include fore-, intra- and back-arcs. The Dharwar mafic rocks are possibly derived by partial melting of a lherzolite mantle source and involved in fractionation of olivine and pyroxene followed by plagioclase. Distinctive differences in the petrography and geochemistry of mafic rocks across regional unconformities between Sargur Group and Dharwar Supergroup provide clinching evidences in favour of distinguishing two greenstone cycles in the craton. This has also negated the earlier preliminary attempts to lump together all mafic volcanics into a single contemporaneous suite, leading to erroneous interpretations. After giving allowances for differences in depositional and tectonic settings, the chemical distinction between Sargur and Dharwar mafic suites throws light on secular variations and crustal evolution. Proterozoic mafic dyke swarms of three major periods (2.4, 2.0–2.2 and 1.6 Ga) occur around Tiptur and Hunsur. The dykes also conform to the regional metamorphic gradient, with greenschist facies in the north and granulite facies in the south, resulting from the tilt of the craton towards north, exposing progressively deeper crustal levels towards the south. The low-grade terrain in the north does not have recognizable swarms, but the Tiptur swarm consists essentially of amphibolites and Hunsur swarm mainly of basic granulites, all of them preserving cross-cutting relations with host rocks, chilled margins and relict igneous textures. There are also younger dolerite dykes scattered throughout the craton that are unaffected by this metamorphic zonation. Large-scale geochemical, geochronological and palaeomagnetic data acquisition through state-of-the-art instrumentation is urgently needed in the Dharwar craton to catch up with contemporary advancements in the classical greenstone terrains of the world.  相似文献   

20.
A biostratigraphic model of the temporal distribution of distinctive Proterozoic microfossil assemblages is suggested, based on studies of upper Precambrian chert-embedded and compression-preserved organic-walled microfossils from the reference sections of Eurasia, North America and Australia. Microfossils from 2.0 to 0.542 Ga can be divided into seven successive informal global units which can be compared to standard units of the International and Russian time scales. Each unit is characterized by a particular association of taxa, typified by the fossil assemblage that gives it its name. These form broad biostratigraphic units comparable to assemblage zones of Phanerozoic successions; in general (but with minor differences) they correspond to chronostratigraphic units accepted by the Internal Commission on Stratigraphy. The units are: (1) Labradorian, the upper part of the Paleoproterozoic (Orosirian and Statherian), 2.0–1.65 Ga; (2) Anabarian, lower Mesoproterozoic (Calymmian–Ectasian)/Lower Riphean–lower Middle Riphean, 1.65–1.2 Ga; (3) Turukhanian, upper Mesoproterozoic (Stenian)/upper Middle Riphean, 1.2–1.03 Ga; (4) Uchuromayan, lower Neoproterozoic (late Stenian–Tonian)/lower Upper Riphean, 1.03–0.85 Ga; (5) Yuzhnouralian, upper Neoproterozoic (Cryogenian)/upper Upper Riphean, 0.85–0.63 Ga; (6) Amadeusian, lower Ediacaran/lower Vendian, 0.63–0.55 Ga; (7) Belomorian, upper Ediacaran/upper Vendian, 0.55–0.542 Ga.  相似文献   

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