Archaean greenstone belts: A structaral test of tectonic hypothesis

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

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

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

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

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

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.     

The deformation patterns of two Archaean greenstone belts in Rhodesia and Botswana

Variations in intensity of deformation have been examined in the Fort Victoria greenstone belt in southern Rhodesia and in the Tati belt in northeast Botswana, both of which are on the diffuse northern border of the Limpopo mobile belt. Much of the deformation is related to widespread Limpopo events, and not to diapiric granite intrusions previously considered of major importance in the development of the Early Precambrian crust.Variations in intensity of deformation are due to the form of the major structures and to localised major ductile shear zones which cut both granites and greenstone belts. The pattern of deformation within the belts depends partly on how the granites behaved. Whether the schist belts deformed homogeneously or were part rotated and cut by shear zones depended on whether the granites deformed homogenously with the schist belt matrix or whether they were deformed by block sliding.     

Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization

An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (X_CO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al₂O₃–SiO₂–H₂O–CO₂, using a new, expanded, internally consistent dataset. Greenstone metamorphism cannot have involved much crustal over-thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low-P high-T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper-earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper-later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid-crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes. Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower X_CO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids.     

Apatite-bearing greenstone belts in Russia

The paper discusses the important and urgent problem of enlarging the mineral resources of Russia. Volcanosedimentary complexes of greenstone belts are suggested as a new alternative source of apatite ores. The distinguishing features and minerageny of greenstone complexes in the Karelian, Aldan, and Anabar Shields have been considered. Apatite occurrences have been described in brief. The paper suggests the main lines of study and geological exploration of apatite-bearing metavolcanic sedimentary complexes.     

Geochemistry of Archaean metavolcanic rocks from the Holenarsipur and Shigegudda volcano—sedimentary belts of Karnataka,South India

Major and trace element data for ultramafic and mafic metavolcanic rocks from the volcano-sedimentary belts at Holenarsipur and Shigegudda are presented. Although the Holenarsipur belt has been regarded as representing two stratigraphic groups separated widely in time — the Sargur (pre-3,4 Ga) and Dharwar (post-3,4 Ga) Groups — and Shigegudda is clearly younger than the 3.4 Ga gneisses, representative samples from all three suites are part of the same geochemical population. This comprises komatiitic to tholeiitic lavas which are genetically related by progressive fractional crystallization of olivine + pyroxene ± Cr-spinel. Some of the variability in the high-MgO rocks may reflect differential, partial melting of the mantle. There are two sub-populations, separable into light rare-earth-enriched and light rare-earth-depleted, which may reflect different depths of melting of compositionally homogeneous mantle. Many of the geochemical characteristics of the population bear a strong resemblance to modern basic volcanics formed in destructive plate margin environments.     

Trace-element geochemistry of archean greenstone belts

Progressive alteration, diagenesis, and low-grade metamorphism of Archean greenstone belts often leads to redistribution of alkali and related trace elements. Transition metals and rare earths are relatively resistant to these processes and hence are most useful in evaluating petrologic problems.Depleted Archean tholeiite (DAT) exhibits flat REE distributions and low LIL-element contents while enriched Archean tholeiite (EAT) exhibits slightly enriched REE patterns and moderate LIL-element contents. DAT is grossly similar to modern rise and are tholeiites and EAT to cale-alkaline and oceanic island tholeiites. Archean and esites fall into three categories: depleted Archean andesite (DAA) exhibits flat REE patterns, negative Eu anomalies and low LIL-element contents; low-alkali Archean andesite (LAA) shows minor light REE enrichment and low LIL-element contents; and high-alkali Archean andesite (HAA) shows light REE enrichment and high LIL-element contents. LAA is grossly similar to modern cale-alkaline andesites, but DAA and HAA do not have modern analogues. Archean depleted siliceous volcanics (DSV) exhibit depletion in heavy REE and Y compared to modern siliceous volcanics whereas undepleted varieties (USV) are similar to modern ones. Almost all Archean volcanic rocks, regardless of composition, are enriched in transition metals compared to modern varieties. Archean graywackes are similar in composition to Phanerozoic graywackes. Rock associations in Archean greenstones suggest the existence of two tectonic settings.Magma model studies indicate that partial melting has left the strongest imprint on trace-element distributions in greenstone volcanics. Three magma source rocks are necessary (listed in order of decreasing importance): ultramafic rock, eclogite, and siliceous granulite. Trace-element studies of Archean graywackes indicate a mixed volcanic—granitic provenance with minor ultramafic contributions.Alkali and related trace-element contents of Archean volcanics have been interpreted in terms of both undepleted and depleted upper mantle sources. Preferential enrichment of transition metals in Archean volcanics may have resulted from upward movement of immiscible liquid sulfide droplets with Archean magmas, depleting the source area in these elements. Initial Sr isotope distributions in Archean volcanics indicate the upper mantle during the Archean was heterogeneous in terms of its Rb/Sr ratio.     

Two types of Archaean supracrustal belts in the Bundelkhand craton,India: Geology,geochemistry, age and implication for craton crustal evolution

Groundwater sampling was carried out in fast growing Vijayawada urban agglomeration in Andhra Pradesh state with a view to generate base line data and to assess groundwater quality, its variations vis-à-vis urbanization and hydrochemical characteristics. The groundwaters are found to be fresh to brackish, hard to very hard, often enriched with nitrate, phosphate,and faecal colliform indicating anthropogenic influence on groundwater on account of urbanization. Manganese and iron of geogenic origin are also found to be in high concentrations in some localities. The quality deterioration is more in shallow aquifers, as also in core urban and red soil covered areas. Spatial variations in groundwater quality are discernible, with high concentrations of SO₄^-4–,NO₃^- in core urban area suggesting the impact of urbanization. A wide range of chemical constituents indicate water rock interactions as influenced by anthropogenic activities controlling the urban aquifers. The order of abundance of cations is Na>Mg>Ca>K, while the order of abundance of anions is Cl>HCO₃>SO₄>NO₃. The baseline study suggests the need to protect the groundwater resource through sound environmental protection measures for the welfare of the inhabitants in the city around which the new capital is proposed to be constructed for newly formed Andhra Pradesh state in India.     

Rock-water interaction and evolution of sulphide ores in the late Archaean Chitradurga greenstone belt of southern India

Sulphide ore mineralization developed in proximity to iron formations in a cyclic volcano-sedimentary sequence that forms the offshore part of the late Archaean intracratonic basin of the Chitradurga greenstone belt, south India. The chemistry of volcanic suite attests to an extensive rock-water interaction resulting in exchange of Si and Na for divalent cations, including Cu, Pb and Zn. The exchange took place during hydrothermal alteration of basalt prior to its involvement in regional deformation and metamorphism. A hydrothermal convection system extracted Cu, Zn and Pb from sea-floor basalts during a static phase of evolution of the Chitradurga basin and developed the stratabound ore deposit within the pyroclastics and the stringer ores below.     

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» , , , .

     

津巴布韦绿岩带中构造型铁矿成矿特征

津巴布韦铁矿层分布普遍,是组成太古界津巴布韦克拉通绿岩带地层常见岩性单元,其岩石特征类似于"硫化物相"含铁建造层,实际上是一种伴有硅化和硫化物矿化的剪切带.过去这种构造成因的铁矿层被认为是一种原始沉积岩层,但从野外露头上可明显辨别出呈交织状分布的片理化带、褶皱变形带、石香肠构造和糜棱岩带;区域上铁矿层与层理或片理相交切,岩层呈交织状分布,同一岩层单元会重复出现.在花岗岩底辟上升作用导致岩石发生变形之前,一系列逆推断层使得原始火山沉积岩单元层产生水平伸展、叠瓦状构造推覆以及岩层重复出现.因此绿岩带地层中这种包含有构造型铁矿层的"馅饼状"岩层模式,愈来愈成为地质工作者研究的焦点.     

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.     

Heat flow and crustal thermal structure in the Late Archaean Closepet Granite batholith,south India

The Late Archaean Closepet Granite batholith in south India is exposed at different crustal levels grading from greenschist facies in the north through amphibolite and granulite facies in the south along a ∼400 km long segment in the Dharwar craton. Two areas, Pavagada and Magadi, located in the Main Mass of the batholith, best represent the granitoid of the greenschist and amphibolite facies crustal levels respectively. Heat flow estimates of 38 mW m⁻² from Pavagada and 25 mW m⁻² from Magadi have been obtained through measurements in deep (430 and 445 m) and carefully sited boreholes. Measurements made in four boreholes of opportunity in Pavagada area yield a mean heat flow of 39 ± 4 (s.d.) mW m⁻², which is in good agreement with the estimate from deep borehole. The study, therefore, demonstrates a clear-cut heat flow variation concomitant with the crustal levels exposed in the two areas. The mean heat production estimates for the greenschist facies and amphibolite facies layers constituting the Main Mass of the batholith are 2.9 and 1.8 μW m⁻³, respectively. The enhanced heat flow in the Pavagada area is consistent with the occurrence of a radioelement-enriched 2-km-thick greenschist facies layer granitoid overlying the granitoid of the amphibolite facies layer which is twice as thick as represented in the Magadi area. The crustal heat production models indicate similar mantle heat flow estimates in the range 12–14 mW m⁻², consistent with the other parts of the greenstone-granite-gneiss terrain of the Dharwar craton.     

The petrogenesis and setting of Archaean metavolcanics from Karnataka State,South India

Major and trace element data for ultrabasic to silicic metavolcanics from five Archaean volcanic-sedimentary belts in South India are summarized. Their total alteration to greenschist and amphibolite facies assemblages poses problems for petrogenetic interpretation of these data. However, in the absence of unmetamorphosed lavas, some interpretation is essential.The data suggest that volcanism in each belt was petrogenetically similar. The presence of both light rare-earth element-enriched and -depleted lavas in individual volcanic sequences, together with variations in $\text{Zr}\text{Y}$ ratios is interpreted as a consequence of fractionation within ultramatic mantle diapirs, induced by upward vapour migration. Other chemical variations probably resulted from fractional crystallization of mafic phases from parental high MgO, high Cr magmas, the influence of plagioclase fractionation only being noticeable in late andesites and dacites. The consistent depletion of all basaltic samples in the high field strength elements Mb and Ta relative to the low field strength element Th and to La further emphasises the importance of a vapour phase in their petrogenesis. In this regard they resemble modern orogenic basalts. The favoured tectonic setting is that of ensialic, multiple back-arc basins above a thinner lithosphere than in modern back-arc environments, which rode over a shallow-angled subduction zone.     

Rare-earth element distributions in volcanic rocks from Archean greenstone belts

Rare-earth element distributions in Archean volcanic rocks from the South Pass (Wyoming), Yellowknife (NW Canada) and Abitibi (Quebec) greenstone belts and from the Upper Fig Tree Group of the Barberton (S. Africa) greenstone belt reveal two distinct types of Archean volcanism. One type, herein referred to as the arc-type, is characterized by flat (or slightly enriched) REE distributions in tho leiites and enrichment in total and light REE and a variable negative Eu anomaly in more siliceous volcanic members. The second type, herein referred to as the Abitibi-type, is characterized by rather flat REE patterns and negative Eu anomalies in all volcanic rock types.REE distributions in the arc-type volcanic successions can be produced by either progressive shallow fractional crystallization of tholeiitic magma or by decreasing amounts of equilibrium melting of a plagioclase-bearing mantle source. REE distributions in the Abitibi volcanic rocks are most readily explained in terms of progressively decreasing amounts of fractional melting of a source area in which REE are contained chiefly in minor minerals (with low melting temperatures) that are depleted in Eu. The melting models seem to necessitate the existence of one or more pre-greenstone magmatic episodes as well as a continuously replenished mantle source. Replenishment of source material could be accomplished in either of the melting models in subduction zones but the analogy to Phanerozoic plate tectonics should be used with caution. Melting models also imply either (or both) a decreasing geothermal gradient with time or systematic changes in mantle source-area composition.