Chromitites of komatiitic affinity from the Archaean Nuggihalli greenstone belt in South India

Summary Mineral chemistry and petrological data of chromites from chromitite bands in the N–S trending schist belt of Nuggihalli (southern Karnataka, India), belonging to the Dharwar craton of South India, are presented in this paper. Crystal chemical data indicate a komatiitic affinity of the chromitite. P–T calculations of the chromite-hosting peridotites yielded a pressure range of 13 to 28 kbar and temperatures ranging from 775 to 1080 °C; the oxygen fugacity (log fO₂) varies from +0.5 to +1.6 above the QFM buffer. The P, T and fO₂ data indicate that Nuggihalli chromitites crystallized in an environment akin to the upper mantle. The studied samples also show partial resetting; the lower temperatures ranging from 515 to 680 °C are ascribed to subsequent metamorphism of the area.     

U-Pb SHRIMP ages of detrital zircons from Hiriyur Formation in Chitradurga greenstone belt and its implication to the Neoarchean evolution of Dharwar craton,south India

We report newly obtained U-Pb SHRIMP ages of detrital zircons from metagreywackes in the Hiriyur Formation (Chitradurga Group, Dharwar Supergroup) from the central eastern part of the Chitradurga greenstone belt. U-Pb analyses yield three major Neoarchean age populations ranging from 2.70–2.54 Ga with some minor age population of Mesoarchean. The maximum age of deposition is constrained by the youngest detrital zircon population at 2546 Ma. This is the first report of the occurrence of supracrustal rocks less than 2.58 Ga in the central part of Chitradurga greenstone belt. Close evaluation of detrital ages with the published ages of surrounding igneous rocks suggest that the youngest detrital zircons might be derived from rocks of the Eastern Dharwar craton and the inferred docking of the western and eastern Dharwar cratons happened prior to the deposition of the Hiriyur Formation. The Chitradurga shear zone, dividing the Dharwar craton into western and eastern blocks, probably developed after the deposition. Furthermore, the lower intercept is interpreted as evidence for the Pan-African overprints in the study area.     

The lead isotope geochemistry and geochronology of late-kinematic intrusives from the Abitibi greenstone belt,and the implications for late Archaean crustal evolution

Pb isotope abundances are reported for six late-kinematic granitoid intrusives from the Quebec sector of the Abitibi greenstone belt. Leaching experiments on K-feldspar separates reveal the presence of radiogenic Pb, attributed to in situ decay of U and Th. Pb-Pb mineral isochrons were constructed with the K-feldspar data plus results obtained on the total-rock, sphene, apatite and other mineral phases; five localities show no evidence of post-emplacement disturbance and yield ages ranging from 2616 ± 19 to 2718 ± 12 Ma. These ages, which are corroborated by U-Pb dating of small populations of sphene, imply that the orogenic events in the Abitibi belt were terminated 2700–2710 Ma ago, and followed by a period of granitization which lasted for 80 to 100 Ma.The initial Pb isotope composition of the magmas shows that their source regions were isotopically heterogeneous; the time integrated ${}^{238}\text{U}{}^{204}\text{Pb}$ values for the source regions vary from 7.62 to 7.92 and the K-feldspar data indicate that similar heterogeneities were present at the scale of a single intrusion. The range of isotopic composition spans the compositional domain of the mantle, defined by sulfides associated with komatiites and some galenas, and that of the continental crust, defined by sulfides associated with Abitibi iron-formations. Consequently, the granitoid magmas are interpreted as partial melts of a continental crust comprising juvenile, mantle-derived rocks and non-negligible amounts of earlier formed sialic material. The Pb isotope data for the latter are consistent with the presence in the area of 3.0 to 3.4 Ga old sialic crust. The episode of crustal anatexis occurred as a consequence to the orogenic events which resulted in burial of altered supracrustal rocks rich in water and heat-producing elements.     

The Kostomuksha greenstone belt,NW Baltic Shield: remnant of a late Archaean oceanic plateau?

The Kostomuksha greenstone belt consists of two lithotectonic terranes, one mafic igneous and the other sedimentary, separated by a major shear zone. The former contains submarine 2.8 Gyr old komatiite-basalt lavas and volcaniclastic lithologies with trace element and isotopic compositions resembling those of recent oceanic flood basalts [?Nd(T) =+ 2,8, μ.₁= 8.73 (Nb/Th)N= 1.5–2.1 (Nb/La)N= 1.0–1.5]. We suggest that the mafic terrane is a remnant of the upper crustal part of an Archaean oceanic plateau derived from partial melting of a mantle plume head. When the plateau reached the continental margin, it collided with the sedimentary terrane but was too buoyant to subduct. As a result, the volcanic section of the plateau was imbricated and obducted thus becoming a new piece of continental crust. The deeper zones were subducted and disappeared from the geological record.     

Archaean crustal thickness of greenstone granite belts of South India

Archaean crustal thickness for the Dharwar craton is estimated using potash index and Rb?Sr crustal thickness grid. The volcanics of the Dharwar greenstone belts appear to have evolved in a less than 20 km thick crust. Whereas the tonalite-trondhjemite pebbles of the Dharwar conglomerates (3250±150 m.y.) were derived from gneisses that evolved in a crust less than 20 km thick, the bulk of the peninsular gneisses and associated granitoids were emplaced in a crust 25 to 35 km thick. The 2000 m.y. old Closepet granite suite was emplaced in a crust thicker than 30 km. It is deduced that the continental crust in the region thickened from 15 to 35 km during a span of about 1000 m.y. between 3250±150 to 2000 m.y. ago. Calculations show that Archaean gecthermal gradients in Dharwar craton were three to four times steeper when compared to the present 10.5°C/km. The thin crust and the steep geothermal gradients are reflected by the emplacement of high magnesia basalts, layered igneous complexes and the strong iron enrichment trend shown by Dharwar metavolcanics.     

Archaean greenstone belt from the Central African Republic (Equatorial Africa)

The Bandas belt, one of two prominent Archaean greenstone belts in the Central African Republic (Equatorial Africa), is ca. 250 km long. At the southernmost part of the belt, a metasedimentary—metavolcanic rock suite is preserved only in brachysynclines. The suite can be divided into two lithostratigraphic units. The lower unit is composed predominantly of volcanic rocks, while the upper one contains mainly metasedimentary rocks. The volcanic rocks, which are part of a sequence ca. 3600 m thick, can be sub-divided according to stratigraphic position, lithology and geochemistry into three groups. The lowermost group includes low-K tholeiitic basalts depleted in light REE. The second group consists of tholeiitic basalts with light REE-enriched patterns and the third, uppermost, group includes andesites, which are similar in several respects to Recent calc-alkaline andesites.The tholeiitic basalts of the first two groups are probably related to different upper mantle sources. The andesites of the third group were produced either by fractional crystallization from a basaltic magma enriched in light REE or equilibrium melting of eclogite or garnet amphibolite.     

Synplutonic mafic dykes from late Archaean granitoids in the Eastern Dharwar Craton,southern India

We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56–2.52 Ga calcalkaline to potassic plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic mafic dykes and (4) Mafic injections enter into largely crystallized (>80% crystals) granitic host results in continuous dykes with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic event and spatially associated with reworking and cratonization of Archaean crust in the EDC.     

Emplacement,crystallization and alteration of spinifex-textured komatiitic basalt flows in the Archaean Nondweni greenstone belt,southern Kaapvaal Craton,South Africa

A well exposed succession of spinifex-textured komatiite flows is reported from the Archaean Nondweni greenstone belt located near the southern margin of the Kaapvaal Craton. The flows are relatively thin (1–5 m) compared to similar occurrences in other greenstone belts. They are characterised by well developed cone structures of highly elongate amphibole crystals (after augite) which fan downwards from the tops of the flows. Extreme development of coned spinifex has not been reported from other greenstone belts and points to specific thermal conditions prevailing in the Nondweni environment. The zones of bladed spinifex are contained between layers of random spinifex and overlie a lower cumulus layer originally of augite, orthopyroxene and minor olivine. The observed major and trace element distributions through a 1.7 m thick spinifex-textured flow are consistent with a model involving concentration of phenocryst phases resulting in significant fractionation upwards in the flow. Approximately 40% of the spinifex-textured phenocrysts grew in situ after the lithological units were established. Collapse and displacement of the coned crystal networks, originally attached to the top of the flow, are shown to have influenced the distribution of liquid within the flow and accentuated the fractionation. Associated with the spinifex-textured units are massive aphyric and brecciated flows which show distinct chemical cycles through the succession. The brecciated zones have compositions with <18% MgO and are characterised by ovoid bodies that are not pillows and may represent magmatic reworking and movement of a partly congealed flow. Post-solidus alteration is considered to have caused early hydration of the original mineralogy and also introduced SiO₂ and Na₂O into the upper part of the flow by way of microfractures. The observed alteration is different to that of Mid-Ocean Ridge basalts, and a subaerial/shallow water environment is suggested.     

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.     

Constraints upon models of greenstone belt evolution by gravity modelling,Birch-Uchi greenstone belt,northern Ontario

Eight two-dimensional gravity models, incorporating most of the major hypotheses proposed for the development of Precambrian greenstone belts, are computed. Maximum use is made of geological and geophysical constraints, in an attempt to explain the disparity between the 11 000 m measured stratigraphic thickness and the maximum 5000 m interpreted vertical extent of a portion of the Birch-Uchi metavolcanic-metasedimentary belt.The two-dimensional modelling concentrates on an east-trending profile across a dominantly volcanic portion of the belt comprising three superposed basalt to rhyolite cycles. It is supplemented by a computed second vertical derivative map of the gravity field.Metamorphic and geophysical considerations show simple, isoclinal, parallel folding of the mapped stratigraphic thickness to be unacceptable. Proposed mechanisms which could explain the shallow interpreted vertical extent are a density gradient, a granite root, more complex folding of the belt and restriction of the metavolcanic basin. Taken individually, however, none of these mechanisms is entirely adequate.The model which best fits the gravity and geological data suggests that the shallow vertical extent is the result of both magmatic stoping by a subjacent granitic magma chamber during the caldera stage of cycle III volcanism and partial melting of the basaltic rocks during earlier cycles of volcanism.     

晚太古代Sanukite（赞岐岩）与地球早期演化

Shirey and Hanson(1984)将某些太古代的高镁闪长岩套称为sanukite(赞岐岩),类似于日本中新世(11～15Ma)Setouchi火山岩带的高镁安山岩。Sanukitoids由闪长岩-二长闪长岩-花岗闪长岩组成,不同于TTC岩套(奥长花岗岩-英云闪长岩-花岗闪长岩)。Sanukitoids具有下列地球化学特征:富Mg,Mg~#>0.60,Ni和Cr>100μg/g,Sr和Ba>500μg/g,LREE富集(大于球粒陨石100倍),无Eu异常。高镁安山岩在太古代很少见,而其相应的侵入岩高镁闪长岩或sanukitoids,虽然数量也很少,但却是各地晚太古代地体中随处可见的。Sanukitoids的原始岩浆是交代的地幔楔部分熔融形成的,随后可能经历了广泛的分离结晶作用。TTC和sanukitoids岩套可以相伴产出,二者均与板片熔融有关,TTG与其直接有关,sanukitoids可能与其间接有关。全球Sanukitoids主要集中在晚太古代时期,可能暗示板块的消减作用在～3.0Ga以后才起了重要的作用。     

Exceptionally Cr-rich basalts in the komatiitic volcanic association of the Archaean Kuhmo greenstone belt, eastern Finland

Summary Exceptionally Cr-rich metabasalts related to a komatiite-dominant volcanic sequence occur in the Archaean Kuhmo greenstone belt, eastern Finland. These basalts contain more chromium (1300–4500 ppm) than any other terrestrial basalts analysed so far, and also have high nickel concentrations (200–1700 ppm). In stratigraphy, these Cr-rich basalts occur above the komatiite and komatiitic basalt units forming the uppermost units of the komatiite sequence. Extremely reducing conditions during magma generation or fractional crystallization are suggested as a reason for these exceptional compositions. Low oxygen fugacity has prevented crystallization of chromite and decreased the olivine/liquid partition coefficient for nickel in olivine fractionation, thus causing the enrichment of Cr and Ni in the residual melt.

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Zusammenfassung Au?ergew?hnlich Cr-reiche Basalte in der komatiitischen vulkanischen Assoziation des Archaischen Kuhmo-Grünsteingürtels in Ostfinnland Im archaischen Kuhmo-Grünsteingürtel in Ostfinnland treten au?ergew?hnlich Cr-reiche Metabasalte auf, die mit einer komatiitdominanten vulkanischen Sequenz verbunden sind. Diese Basalte enthalten mehr Chrom (1300–4500 ppm) als alle anderen terrestrischen Basalte, die bislang analysiert worden sind, und sie weisen zudem auch hohe Nickelkonzentrationen auf (200–1700 ppm). In der Stratigraphie treten die Cr-reichen Basalte über dem Komatiit und den komatiitischen Basalteinheiten auf und bilden die obersten Einheiten der Komatiitsequenz. Extrem reduzierende Bedingungen w?hrend der Magmabildung oder fraktionierte Kristallisation werden als eine Ursache für diese au?ergew?hnlichen Zusammensetzungen vorgeschlagen. Die niedrige Sauerstoff-Fugazit?t hat die Kristallisation des Chromits gehemmt, den Verteilungskoeffizienten Olivin/Schmelze für Nickel bei der Olivinfraktionierung herabgesetzt und dadurch die Anreicherung von Cr und Ni in der Restschmelze verursacht.

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Received November 24, 1997; revised version accepted July 22, 1999     

The Archaean gold mineralization in Rämepuro, llomantsi greenstone belt, Eastern Finland

The Rämepuro gold mineralization is situated in the Hattu sub-belt of the Archaean Ilomantsi greenstone belt, Eastern Finland. It consists of gold-bearing quartz veins located mainly within the shear zone contact between felsic volcanics and sedimentary rocks. The quartz veins cutting all the types of Archaean rocks are composed mainly of quartz and tourmaline and minor amounts of pyrite, pyrrothite, chalcopyrite, sphalerite, siderite, hedleyite, native bismuth and gold. The highest amounts of gold are found in the quartz-tourmaline veins, with abundant chalcopyrite and sphalerite. Gold has its best correlation with Bi, Te, S, Zn and Cu. The minor mineralogical alterations (sericirization, saussuritizarion, chloritization) are connected with variations in some major elements. The quartz veins were emplaced under greenschist facies conditions after the peak in the lower amphibolite facies metamorphism.     

Middle to late Archaean geology of the eastern Baltic shield,with a note on its similarity and contrast with the Archaean of southern India

The middle to late Archaean rocks of Kola and Karelia in the eastern Baltic shield consist of the Infracomplex overlain by the Saamian complex, and the Lopian greenstone belts. The Infracomplex which forms the basement is a polymigmatite, parts of which are at least 3100 Ma old. The Saamian in the central Belomorian region comprises granite gneiss, amphibolite, garnet-kyanite gneiss and high alumina gneisses which belong to the Keret, Hetolombina and Chupa suites. The Lopian greenstone belts ranging in age from 3000 to 2700 Ma are composed of peridotitic, pyroxenitic and basaltic komatiites, tholeiitic basalts, andesites, dacites and rhyolites, together with tuffs, graywackes and iron formations. Whereas there is a dominance of volcanic over sedimentary rocks in the greenstone belts of the Baltic shield, a significant proportion of detrital and chemogenic sedimentary rocks characterizes the Dharwar succession of approximately the same time span in the southern Indian shield. Association of mature and immature detrital sedimentary rocks with bimodal volcanic assemblages points to a back-arc setting for the Dharwar belts. This contrasts with the association of immature sediments with calc-alkaline volcanic rocks in the greenstone belts of the eastern Baltic shield, suggesting an island arc environment there.     

Evaluation of metallogenic potential of the Nuggihalli greenstone belt,South India

Chromite from the north–south-trending schist belt of Nuggihalli (southern Karnataka), belonging to the greenstone belt of South India (3.0–3.5 Ga) and crystallising at 1178 °C under oxygen fugacity of ?6.67 shows komatiitic affinity. The chemical data of the Cr-spinels when plotted in the form of the discriminant function of Johan (1979) show a remarkable follow-up of the curve A, which delineates the boundary between the ‘barren’ and ‘fertile’ (with respect to the plausible presence of sulphide phases) Cr-spinel bodies. Only a few samples plot marginally within the barren region (Fig. 1), suggesting that the chromite bands should hardly hold the possibility of becoming host of sulphide mineralisation of commercial relevance.     

A new discovery of possible primitive life-forms in conglomerates of the Archaean Pietersburg greenstone belt,South Africa

Spherical aggregates of carbonaceous matter measuring 0.2 to 1.0 mm in diameter were recently discovered in conglomerates of the Achaean Pietersburg greenstone belt in the Northern Transvaal, South Africa. Identical carbonaceous material, the so-called flyspeck carbon, occurs abundantly in the approximately 2'600 m. y. old sediments of the Witwatersrand Basin and has been considered to represent vegetative diaspores of primitive columnar plants. If this interpretation is correct, the occurrence of fly-speck carbon outside the Witwatersrand Basin indicates that differentiated life-forms also existed in other suitable depository environments and probably appeared earlier than previously thought.

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Zusammenfassung Im Nordtransvaal, Südafrika, wurde kürzlich kohlige Substanz in der Form rundlicher Aggregate entdeckt, die Durchmesser von 0,2 bis 1,0 mm besitzen und in Konglomeraten vorkommen, welche zum archaischen Pietersburg Greenstone Belt gehören. Ganz ähnlich ausgebildete kohlige Substanz, das sogenannte fly-speck carbon tritt in den rund 2600 Mio. Jahre alten goldführenden Konglomerathorizonten des Witwatersrand-Beckens verbreitet auf und wird dort als fossile Reste vegetativer Sporen von primitiven Pflanzen gedeutet. Trifft diese Interpretation der rundlichen Kohleaggregate zu, kann aus dem Auftreten von fly-speck carbon im Pietersburg Belt geschlossen werden, daß auch außerhalb des Witwatersrand-Beckens in geeigneten Ablagerungsräumen differenzierte Lebensformen existierten, möglicherweise schon vor der Ablagerung der Wirwatersrand-Sedimente, in denen solche Lebensformen bisher erstmals beschrieben wurden.

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Résumé Des agrégats spheriques de matière carbonée mesurant de 0.2 à 1.0 mm de diamètre ont été récemment découverts dans des conglomérats du Pietersburg Greenstone Belt d'âge archéen dans le Transvaal septentrional en Afrique du Sud. Du matériel carboné identique, connu comme »carbone en tâche de mouche« (fly-speck carbon), est abondant dans les sédiments datés de 2'600 m. a. du Bassin du Witwatersrand, et a été interprété comme les restes de spores végétatives de plantes columnaires primitives. Si cette interprétation est correcte, il s'en suit que la présence de »carbone en tâche de mouche« en dehors du Bassin du Witwatersrand indiquerait que des formes végétales différentiées existaient aussi dans d'autres environments de dépôts favorables et qu'elles ont apparu probablement plus tôt qu'on ne l'avait supposé jusqu'à présent.

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, , , 0,2 1 , , , , .. «fly-speck carbon», , 2600 , , . , «fly-speck carbon» , , , .

     

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.     

Geochemistry and genesis of metamorphosed volcanic rocks in the Kholodnikan greenstone belt,southern Aldan Shield

The Kholodnikan Complex consists of two units: lower volcanic and upper volcanic-sedimentary. The distributions of major and trace elements suggest that the protoliths of the lower unit were volcanics of the komatiite-tholeiite series (komatiite-basalt association) and those of the upper unit were volcanics of the calc-alkaline series (andesite-dacite-rhyolite association). The model assumed for the genesis of these associations involves two stages: (1) decompression-induced partial melting of the material of an ascending mantle plume with the derivation of melts of the komatiite-basalt association and (2) derivation of volcanic rocks of the andesite-dacite-rhyolite association via the partial melting of various rocks in the basement of the Aldan Shield under the effect of the heat of the ascending mantle plume. The magmatic protoliths of the Kholodnikan Complex were formed in the Paleoproterozoic at 2.41 Ga.     

Structural relations of charnockites of the Archaean Dharwar craton, southern India

Abstract Two varieties of charnockites are recognized in the Dharwar craton of southern India. The style and sequence of structures in one charnockite variety, and related intermediate to basic granulites, are similar to those in the supracrustal rocks of the Dharwar Supergroup and the adjacent Peninsular Gneiss. This style has isoclinal folds with long limbs and sharp hinges with an axial planar fabric in some instances. Additional evidence of flattening is provided by pinch-and-swell and boudinage structures, with basic granulites forming boudins in the more ductile charnockites/enderbites in the limbs of isoclinal folds. These folds are involved in near-coaxial upright folding resulting in the bending of the axial planes of the isoclinal folds and the associated boudins. All these structures are overprinted by non-coaxial upright folds with axial planes striking nearly N–S. The map pattern of charnockites suggests that this sequence of structures is present not only on a mesoscopic scale, but also on a macroscopic scale. Charnockites of this variety provide, in some instances, evidence of having been migmatized to give rise to hornblende–biotite gneiss and biotite gneiss, which form a part of the Peninsular Gneiss terrane.
The second variety comprises charnockite sensu stricto with an entirely different structural style. This type occurs in the tensional domains of the hinge zones of the later buckle folds, in the necks of foliation boudinage, in shear zones and in release joints parallel to the axial planes of the later folds in the Peninsular Gneiss. Because the non-coaxial later folds are associated with a strain pattern different from, and later than, that of the isoclinal folds of the first generation, it follows that charnockites of the Dharwar craton have evolved in at least two distinct phases, separate both in time and in process.     

Fluid evolution in the Pote Shear Zone Harare-Shamva-Bindura greenstone belt (northeast Zimbabwe)

A fluid inclusion study was completed on syn-deformational quartz veins of the Pote River Shear Zone, which is situated on the border between the Harare-Bindura greenstone belt and the granitoids of the Chinamora Batholith. The fluid inclusions were studied by means of microthermometry and Laser-Raman microspectrometry. The fluid inclusions consist of three major compositional types: (1) H₂OCO₂±N₂±halite inclusions in clusters and trails; (2) H₂OCO₂ inclusions (H₂O = 30–60 vol. %) in trails; and (3) H₂O-halite inclusions in trails. These fluid generations are explained by trapping at different P-T conditions of two different fluids: a high salinity aqueous fluid and a low salinity H₂OCO₂ fluid with XH₂O around 0.8. High salinity aqueous fluid inclusions are characteristic for the granite-greenstone contact and are absent within the Harare-Shamva-Bindura greenstone belt. The high salinity aqueous fluid has, therefore, been interpreted as magmatic in origin. The low salinity H₂OCO₂ fluid is most likely metamorphic in origin.