Petrography and geochemistry of granitoid and metamorphite pebbles from the early Archaean Moodies Group,Barberton Mountainland/South Africa

Pebbles of potassic granitoids and metamorphites constitute up to 5% of the basal conglomerate of the Moodies Group in a ratio of 2 : 1. The granitoid pebbles frequently show micrographic quartz–feldspar intergrowth, whereas the metamorphites—of a modal composition similar to that of the granitoids—are characterized by large quartz grains which could represent original quartz phenocrysts in felsic volcanic precursors.The granitoids show high K₂O, Sr, K₂O/Na₂O, and K/Rb, small enrichment of light REE, large negative Eu-anomalies, and slightly depleted and fractionated heavy REE. Compared to the granitoids the metamorphites show higher Fe₂O₃, TiO₂, and Cr concentrations, greater enrichment of light REE, and also large negative Eu-anomalies.There is little similarity between the Moodies pebbles and the majority of the rocks of the Ancient Gneiss Complex of Swaziland (AGC). There is only some similarity of the REE distribution patterns between the pebbles and the Mkhondo Metamorphic Suite, possibly an areally restricted phase of the AGC. The geochemical data, and especially the large negative Eu-anomalies suggest that the Moodies pebbles were derived from granites which represent residual magmas from which much plagioclase had been removed. The granites crystallized at depths of < 7 km from magmas with low H₂O-pressures in a rather thick sialic crust. It appears possible that the pre-Moodies granitoids originated through partial melting of low-Al₂O₃ siliceous gneisses of the AGC. A chronologic connection of the formation of the granitoids with the late Onverwacht Group volcanicity is possible.     

Stratigraphy and petrology of the Archaean Nsuze Group,northern Natal and southeastern Transvaal,South Africa

The Nsuze Group is the lower, dominantly volcanic, division of the Pongola Supergroup that accumulated on a sialic basement between 3.1 and 2.9 Ga. The Nsuze Group is subdivided into a lower sedimentary unit (800 m thick), a middle volcanic unit (± 7500 m thick) and an upper volcaniclastic-sedimentary unit (5–600 m thick). The predominant sediments in the lower unit are immature, medium- to very coarse-grained quartz wackes with thin intercalated lenses of quartz and arkosic arenites, and minor conglomeratic wackes. These sediments were deposited in a distal braided stream environment.There followed a major period of volcanism during which lavas showing a continuous spectrum of compositions from basalt to rhyolite were extruded subaerially. Flows of both different and similar compositions are complexly interfingered on both regional and local scales. As volcanism waned, pyroclastic and sedimentary rocks became dominant in the upper unit. The Nsuze Group is gently dipping and is metamorphosed to low greenschist grade.The Nsuze Group is significant in that it provides evidence for the existence of high-standing continental sialic crust in the southeastern part of the Kaapvaal province at ca. 3.0 Ga. Volcanism and sedimentation in the Pongola Supergroup are more typical of Proterozoic basins than of Archaean environments, despite their age. Komatiitic and high-Mg basalts were, however, being extruded in Zimbabwe contemporaneously with the Nsuze lavas.     

P–T and structural evolution during exhumation of high-T, medium-P basement rocks in the Barberton Mountain Land, South Africa

The P–T evolution of amphibolite facies gneisses and associated supracrustal rocks exposed along the northern margin of the Paleo to MesoArchean Barberton greenstone belt, South Africa, has been reconstructed via detailed structural analysis combined with calculated K(Mn)FMASH pseudosections of aluminous felsic schists. The granitoid‐greenstone contact is characterized by a contact‐parallel high‐strain zone that separates the generally low‐grade, greenschist facies greenstone belt from mid‐crustal basement gneisses. The supracrustal rocks in the hangingwall of this contact are metamorphosed to upper greenschist facies conditions. Supracrustal rocks and granitoid gneisses in the footwall of this contact are metamorphosed to sillimanite grade conditions (600–700 °C and 5 ± 1 kbar), corresponding to elevated geothermal gradients of ～30–40 °C km^?1. The most likely setting for these conditions was a mid‐ or lower crust that was invaded and advectively heated by syntectonic granitoids at c. 3230 Ma. Combined structural and petrological data indicate the burial of the rocks to mid‐crustal levels, followed by crustal exhumation related to the late‐ to post‐collisional extension of the granitoid‐greenstone terrane during one progressive deformation event. Exhumation and decompression commenced under amphibolite facies conditions, as indicated by the synkinematic growth of peak metamorphic minerals during extensional shearing. Derived P–T paths indicate near‐isothermal decompression to conditions of ～500–650 °C and 1–3 kbar, followed by near‐isobaric cooling to temperatures below ～500 °C. In metabasic rock types, this retrograde P–T evolution resulted in the formation of coronitic Ep‐Qtz and Act‐Qtz symplectites that are interpreted to have replaced peak metamorphic plagioclase and clinopyroxene. The last stages of exhumation are characterized by solid‐state doming of the footwall gneisses and strain localization in contact‐parallel greenschist‐facies mylonites that overprint the decompressed basement rocks.     

Silver and mercury in single gold grains from the Witwatersrand and Barberton,South Africa

The contents of silver and mercury in 323 spots on gold grains from seven localities of the Witwatersrand palaeo-placer and Archaean vein deposits from Barberton were measured using an electron microprobe. The objective was to obtain information on the extent of gold alteration during fluvial transport and post-depositional geological processes. The results, however, show that Ag and Hg are distributed homogeneously in the gold grains studied. No indications were found that the gold was transported in solution nor that leaching took place in an oxidizing fluvial environment. This strongly suggests that the Ag and Hg contents in Witwatersrand gold grains represent geochemical ‘fingerprints’ inherited from their eroded primary sources. Combined analysis for Ag and Hg in Witwatersrand gold grains by electron microprobe can therefore be a valuable tool in establishing the types of primary sources for the gold.     

Origin of phlogopite and potassic richterite bearing peridotite xenoliths from South Africa

Peridotite xenoliths containing primary phlogopite with or without potassic richterites as major constituent (up to 12 vol. %) are rarely found in kimberlite from the Bultfontein Floors. Chemically, these rocks are similar in compositions with those of the granular type garnet peridotite xenoliths from South Africa and Lesotho, except for an abnormally high content of K₂O in the former. Phlogopite and potassic richterite are thought to have the following genesis: garnet peridotites at a depth from 170 to 100 km suffered local introduction of a potash-rich fluid, and garnet and enstatite reacted with this fluid to form phlogopite and diopside. Potassic richterite may have been produced by the reaction between diopside and fluid at the same time as crystallization of phlogopite at depths shallower than 120 km.     

Late Neoarchean synchronous TTG gneisses and potassic granitoids in southwestern Liaoning Province,North China Craton: Zircon U-Pb-Hf isotopes,geochemistry and tectonic implications

Abundant late Neoarchean granitoids occur in southwestern Liaoning Province, part of the Eastern Ancient Terrane of the North China Craton. These rocks include intermediate gneiss, TTG gneisses and potassic granitoids, and we report on the geochemistry and zircon SHRIMP ages as well as Hf-in-zircon isotopes of these granitoids in order to determine their petrogenesis. Field relationships suggest that most of these granitoids experienced widespread metamorphism and deformation, associated with anatexis at some localities. The intermediate gneisses, TTG gneisses and potassic granitoids were all emplaced at the end of the Neoarchean (2.50–2.53 Ga), and CL images document widespread recrystallization in the zircons. The intermediate and TTG gneisses yielded similar Hf isotopic systematics (ε_Hf(t) = −3.73 to +6.42) as the associated potassic granitoids (ε_Hf(t) = −2.44 to +7.80), and both rock types yielded mean Hf crustal model ages of 2.8–2.9 Ga. Combined with the geochemistry, we propose that the formation of the intermediate and TTG gneisses was related to partial melting of mafic rocks at different depth, whereas the potassic granitoids have variable petrogenesis. The nearly coeval TTG gneisses and potassic granitoids and their widespread metamorphism, deformation and zircon recrystallization suggest that a large-scale heat source must have been present at or near the base of the crust in southwestern Liaoning Province at the end of the Neoarchean. We propose that collision and post-collisional extension is the most likely tectonic environment for generation of the above granitoids, and the formation of widespread potassic granitoids played an important role in the maturation of continental crust in the North China Craton.     

A geochemical and Nd isotopic study of Barberton komatiites (South Africa): implication for the Archean mantle

The Komati Formation of the Barberton greenstone belt (BGB), South Africa, is composed of both Al-undepleted and -depleted komatiites. The Al-undepleted komatiites are characterised by Al₂O₃/TiO₂ and CaO/Al₂O₃ ratios of 15–18 and 1.1–1.5, respectively, and exhibit chondritic trace element contents and (Gd/Yb)_N ratios. In contrast, the Al-depleted komatiites show significantly lower Al₂O₃/TiO₂ ratios of 8–12, highly variable CaO/Al₂O₃ (0.19–2.81) ratios combined with (Gd/Yb)_N ratios varying from 1.08 to 1.56. A Sm–Nd whole rock isochron for komatiites of the Komati Formation gives an age of 3657±170 Ma. ¹⁴⁷Sm/¹⁴⁴Nd ratios (0.1704 and 0.1964) are all lower than the chondritic value of 0.1967. The komatiite _i,Nd(3.45) values cluster at +1.9±0.7.

Trace element distribution indicates that most of the primary geochemical and isotopic features of the komatiites were preserved in line with the conservation of the primary chemical composition of clinopyroxene. High field strength element and rare earth element abundances indicate that crustal contamination and post-crystallisation processes did not disturb the primary features of komatiites.

The Sm/Nd and Nb/U ratios of komatiites indicate that the Barberton greenstone belt mantle source has undergone melt extraction prior to komatiite formation. Variations of Al₂O₃/TiO₂, (Gd/Yb)_N, Zr/Sm and Sm/Nd ratios of komatiites indicate that a batch melting of slightly depleted mantle source during with garnet and/or clinopyroxene remained in the residue can produce the geochemical isotopic feature of the Barberton greenstone belt komatiites. Typical geochemical fingerprints of subduction-related processes (LILE enrichment, HFSE depletion compared to REE), as known from modern subduction zones, are not observed. Komatiites exhibit Ti/Zr, La/Nb, Nb/U, Sr/Nd and Ba/La ratios comparable to those of oceanic island basalt and mid-ocean ridge basalt. (La/Nb)_PMN, (Sm/Yb)_PMN, positive δNb values and flat or slightly enriched REE patterns suggest that BGB komatiites are part of an oceanic plateau rather than an oceanic island such as Iceland. Therefore, an oceanic plateau or mid-ocean ridge, in connection with an oceanic plateau, such as Ontong Java plateau or Caribbean–Colombian oceanic plateau, is a suitable tectonic setting for the formation of the BGB komatiites.     

Granitoid Series in Terms of Magnetic Susceptibility: A Case Study from the Barberton Region,South Africa

Magnetic susceptibilities were measured on a representative collection of Archaean granitoids of the Barberton region using a portable KT5 magnetic susceptibility meter. The studied granitoids comprise, (1) syn-tectonic tonalite-trondhjemite-granodiorite (TTG) granitoids (132 samples), (2) late-tectonic calc-alkaline granitoids (402 samples) and (3) post-tectonic low-Ca and high-Ca granitoids (12 samples). Most of the early-stage syn-tectonic granitoids (∼3450 Ma) have low magnetic susceptibilities, less than 3 × 10⁻³ SI units, and correspond to ilmenite-series granitoids. The late-stage Kaap Valley tonalite pluton (∼3230 Ma) contains sporadically distributed higher magnetic susceptibility values (greater than 3 × 10⁻³ SI units), which are less than one-third in magnetic susceptibility of typical magnetite-series TTG of the Japanese Island Arc and thus strictly belong to an intermediate series. The Barberton TTG suite is essentially derived from reduced amphibolitic lower crust that reflects the anoxic nature of the Earth surface during the Archaean Eon. The more oxidized nature of the Kaap Valley tonalite may be generated in an oxidized lower crust by fluids squeezed out of the subducting plate.Late-tectonic granodiorite - adamellite batholithic complexes (∼3105 Ma) belong mostly to the magnetite series, and seem to suggest that relatively oxidized continental crust, reflecting oxic atmosphere and subduction mechanism operating, had evolved it by this time. Post-tectonic granitic plutons formed largely between circa 2900 Ma and 2700 Ma can be subdivided into low-Ca ilmenite series and high-Ca magnetite series.     

The Nsuze Group,Pongola Sequence,South Africa: Geochemical evidence for Archaean volcanism in a continental setting

The Archaean Nsuze Group in southeast South Africa represents an important volcano-sedimentary succession that is markedly different compositionally and lithologically from older and contemporaneous sequences elsewhere in southern Africa.The Nsuze volcanic rocks cropping out in the vicinity of the Pongola River in northern Natal display a complete spectrum of chemical compositions from basalt to rhyolite, with lavas of intermediate compositions predominant but ultramafic lavas absent. Flows of different compositions are complexly interdigitated. The uppermost rocks of the Nsuze Group reflect a gradual decrease in volcanic activity accompanied by an increase in sedimentation.The Nsuze lavas are tholeiitic with total Fe (as Fe₂ O₃) contents approaching 17% in the basaltic andesites. All lithologies are characterized by moderate to strong light rare earth element (REE) enrichment. Heavy REE slopes range from moderate in the basalts and basaltic andesites to flat in the rhyolites, with the exception of two basaltic andesites and a dacite which have flat heavy REE slopes. Basalts have small or no negative Eu anomalies but increasingly larger negative anomalies are a feature of the intermediate and acid lavas. Variations in Cr content appear to be related to stratigraphic position. Basalts and basaltic andesites in the lower part of the volcanic sequence have higher Cr contents (by a factor of six to eight times) than lavas with similar MgO abundances in the upper part of the sequence.Provisional modelling of the available chemical data favours low pressure, crystal fractionation from and evolved basaltic parent, that could be derived by initial non-modal melting of a garnet-lherzolite source. Variable degrees of crustal contamination of the evolved magmas are considered probable.     

Effects of alteration on element distributions in archean tholeiites from the Barberton greenstone belt,South Africa

Major and trace element and modal analyses are presented for unaltered, epidotized, and carbonated tholeiite flows from the Barberton greenstone belt. Au, As, Sb, Sr, Fe⁺³, Ca, Br, Ga, and U are enriched and H₂O, Na, Mg, Fe⁺², K, Rb, Ba, Si, Ti, P, Ni, Cs, Zn, Nb, Cu, Zr, and Co are depleted during epidotization. CO₂, H₂O, Fe⁺², Ti, Zn, Y, Nb, Ga, Ta, and light REE are enriched and Na, Sr, Cr, Ba, Fe⁺³, Ca, Cs, Sb, Au, Mn, and U are depleted during carbonization-chloritization. The elements least affected by epidotization are Hf, Ta, Sc, Cr, Th, and REE; those least affected by carbonization-chloritization are Hf, Ni, Co, Zr, Th, and heavy REE. Both alteration processes can significantly change major element concentrations (and ratios) and hence caution should be used in distinguishing tholeiites from komatiites based on major elements alone. The amount of variation of many of the least mobile trace elements in the altered flows is approximately the same as allowed by magma model calculations. Hence, up to about 10% carbonization and 60% epidotization of tholeiite do not appreciably affect the interpretation of trace-element models for magma generation.     

Deformation of the Assegaai supracrustals and adjoining granitoids,Transvaal, South Africa

The structures in Archaean quartzites of the Assegaai greenstone belt in the Transvaal of South Africa can be interpreted in terms of three major deformation events. Earliest D₁, involved thrusting along brittle ramp and flat crush zones. Rising temperatures then led to the mobilization of SiO₂ and the development of syn-D₁ stylolites and omnidirectional quartz veins in the quartzites. Increasingly ductile behaviour results in some of the early D₁ crush zones being overprinted by late D₁ mylonitic shear zones while eastward-verging recumbent folds formed about N-S axes in the layered quartzites.The Assegaai supracrustals were then separated from a contiguous basement of Ancient Gneiss Complex (AGC) by the intrusion of a granitoid sheet about 3 km thick along their sheared subhorizontal contact either late in D₁ or in the interval between D₁ and D₂. The supracrustals were then refolded about steep NNE-trending axial surfaces by concentric F₂ folds on a variety of scales. Such folds were the second major structures to affect the supracrustals but the first to deform the granitoid sheet.The older of two suites of quartzo-feldspathic veins in the AGC were probably generated penecomtemporaneously with the syn-D₁ quartz veins in ther Assegaai quartzites. This interpretation can be tested by removing the intense D₂ constriction (and 30% volume increase) recorded by a later network of syn-granitoid quartzofeldspathic veins in the AGC from the D₁ + D₂ flattening recorded by the older veins. This procedure can be carried out by geometric vector subtraction on a Hsü diagram. The result reveals that the D₁ strains in the Assegaai supracrustals and the then contiguous AGC were identical penetrative subhorizontal flows prior to the intrusion of the intervening granitoid. The last significant strain in the region is represented by a brittle NNE-trending strike-slip fault and, locally in the quartzites, disharmonic conjugate folds.     

Diversity of 1.8 Ga potassic granitoids along the edge of the Archaean craton in northern Scandinavia: a result of melt formation at various depths and from various sources

The edge of the Archaean craton in northern Scandinavia had been intensively reworked during the Svecofennian orogeny 1.93-1.86 Ga ago and was subsequently intruded by potassic granitoids of 1.79–1.80 Ga age. Despite similar or even identical ages and overlapping areas of occurrence, these rocks belong to two different groups, the Edefors and Lina granitoids, which have contrasting geochemistries and Sm---Nd isotopic characteristics. The Edefors granitoids range from syenites to granites, and are alkali-rich and distinctly metaluminous. They crystallized from dry magmas. This is indicated by the scarcity of pegmatites and aplites. The contacts to older rocks are often distinct, but gradual transitions to Lina-type granitoids are common. The Edefors granitoids have high contents of Zr but not of elements such as Y, REE, Ta and Nb, and have low Mg/Mg+Fe ratios. They also frequently have positive Eu anomalies, even in the quartz rich varieties. Initial ε_Nd values range from −2.1 to +1.4, indicating that the Edefors granitoids were formed by the mixing of mantle-derived magmas and continental crustal materials. The amount of crustal component was probably less than 35% in most cases. The Lina granitoids are accompanied by abundant pegmatites and aplites. Ghost structures and remnants of country rock are common. True granites predominate, but also quartz monzonites occur. The content of HFS elements is low and the Mg/Mg+Fe ratios are higher than in the Edefors granitoids. Initial ε_Nd values range from −9.3 to −3.7, reflecting a significant portion of Archaean Nd in the source materials. The Lina granitoids are largely the result of remobilisation of continental crust with a small input of juvenile material. However, the dominant source for these crustally derived granitoids are c. 1.9 Ga old granitoids. These carry a large proportion of Archaean Nd. The most probable environment of the formation of potassic migmatite granitoids, such as the Lina type, is a collision zone between two masses of felsic crust (e.g. arc-continent or continent-continent), but the details of such a collision in the Baltic Shield remain to evaluated. The formation of the Edefors granitoids could have been associated with an extensional zone developed due to delamination caused by separation of the down-dip oceanic lithosphere from the continental lithosphere.     

南天山花岗岩类的年代学,岩石学,地球化学及其构造环境

在南天山褶皱带,分布有大量的花岗岩类岩石。按照由老到新的侵位顺序,岩石类型依次为：闪长岩、似斑状花岗岩、二云母花岗岩和碱长花岗岩。精确的锆石Ｕ－Ｐｂ法研究证明,它们形成于石炭纪末期至早二叠世末期。闪长岩、似斑状花岗岩的岩相学和岩石化学具有Ｉ型花岗岩的特征,而二云母花岗岩充分表现了Ｓ型花岗岩的岩相学和岩石化学特征。与洋中脊花岗岩相比,它们不同程度地富集Ｋ２Ｏ、Ｒｂ、Ｔｈ而贫于Ｔａ、Ｎｂ、Ｚｒ,显示了碰撞造山阶段花岗岩的地球化学特征。碱长花岗岩主要由钾长石、钠长石、石英和富羟铁云母组分的黑云母组成。它们普遍具有高的ＳｉＯ２、Ｎａ２Ｏ＋Ｋ２Ｏ、Ｎｂ、Ｙ、ＲＥＥ（Ｅｕ除外）丰度和Ｆｅ／Ｍｇ比值,以及低的ＣａＯ、ＴｉＯ２、Ｓｒ、Ｂａ、Ｅｕ丰度。从而,系统地显示了Ａ２型花岗岩的普遍特征。证明它们形成于后造山垮塌阶段。各种岩石的εＳｒ（ｔ）界于＋３７．７５和＋２１００．７之间,而εＮｄ（ｔ）界于－４．４２和－６．５１之间,证明它们全部源自于陆壳物质的深熔作用。北天山古洋盆于晚石炭世的闭合导致了南天山碰撞造山作用。而在早二叠世末期,更大范围的大陆伸展作用与南天山的后造山垮塌作用同步。     

Equilibration and reaction in Archaean quartz-sapphirine granulite xenoliths from the Lace kimberlite pipe, South Africa

Ultrahigh-temperature quartz-sapphirine granulite xenoliths in the post-Karoo Lace kimberlite, South Africa, comprise mainly quartz, sapphirine, garnet and sillimanite, with rarer orthopyroxene, antiperthite, corundum and zinc-bearing spinel; constant accessories are rutile, graphite and sulphides. Comparison with assemblages in the experimentally determined FMAS and KFMASH grids indicates initial equilibration at >1040 °C and 9–11  kbar. Corona assemblages involving garnet, sillimanite and minor cordierite developed on a near-isobaric cooling P–T  path as both temperature and, to a lesser extent, pressures decreased. Garnet-orthopyroxene Fe-Mg exchange thermometers record temperatures of only 830–916 °C. These estimates do not indicate the peak metamorphic conditions but instead reflect the importance of post-peak Fe-Mg exchange during cooling. Correction of mineral Fe-Mg compositions for this exhange using a convergence approach of Fitzsimons & Harley (1994 ) leads to retrieved P–T  estimates from garnet-orthopyroxene thermobarometry ( c . 1000 °C and 10.5±0.7  kbar) that are consistent with the petrogenetic grid constraints. U-Pb dating of a single zircon grain gives an age of 2590±83  Ma, interpreted as the age of the metamorphic event. Protolith major and trace element chemistries of the xenoliths differ from sapphirine-quartzites typical of the Napier Complex (Antarctica) but are comparable to less siliceous, high Cr and Ni, sapphirine granulites reported from several ultrahigh temperature granulite terranes.     

Trace element geochemistry and origin of early Precambrian acid igneous series,Barberton Mountain Land,Transvaal

Sixteen new major and trace element analyses of granitic and acid volcanic rocks from the Barberton Mountain Land, eastern Transvaal, are reported. The data indicate very low abundances of LIL elements in albite porphyries incorporated in the lower ultramafic to mafic succession of the volcanic Onverwacht Group, and in the ‘ancient tonalites’ plutonic suite which intruded the volcanics at about 3.2–3.4 aeons. Some of the albite porphyries display highly fractionated REE patterns, which indicate equilibration with garnet and therefore high-pressure origin. The REE patterns of the ‘ancient tonalites’ are characterized by pronounced positive Eu anomalies, and their derivation by partial melting of eclogite within the depth range of 30–50 km is suggested. Some of the albite porphyries may represent shallow-level hypabyssal equivalents of the ‘ancient tonalites’. The Dalmein and Bosmanskop plutons are regarded as late members of the ‘ancient tonalites’ suite, derived by decreasing partial melting of basic source rocks. A contrasting model of origin is suggested for the ca. 3.0 aeons old Nelspruit migmatite and Hood granite, which are thought to have been derived by anatexis of the ‘ancient tonalites’ and incorporated ultramafic to mafic xenoliths of the Onverwacht Group. The chemistry of greywackes of the Fig Tree Group, which overlie the Onverwacht Group, is consistent with the erosion of a terrain consisting of Onverwacht Group and ‘ancient tonalites’—including late K-rich members of the latter suite. The data lend support to models involving a secular transformation from an ensimatic to an ensialic tectonic environment, but important geochemical differences are indicated between the Archaean rocks and acid igneous suites of island arcs and oceanic domains.     

Petrogenesis of Late Archaean Flood-Type Basic Lavas from the Klipriviersberg Group, Ventersdorp Supergroup, South Africa

The Klipriviersberg Group is a small continental flood-typetholeiitic suite forming the basal unit of the Ventersdorp Supergroup,an undeformed late-Archaean supracrustal sequence covering 200000km² in the SW part of the Kaapvaal Craton. From the base up,the Klipriviersberg Group consists of the Westonaria, Alberton,Orkney, Jeannette, Loraine, and Edenville formations, with amaximum combined thickness of 1–8 km. Samples were obtainedfrom several borehole cores in the Klerksdorp goldfield closeto the type area of the Klipriviersberg Group. This sample suiteincludes a detailed collection from a 1–36-km core intersection.These samples reveal that the Westonaria Formation is absentfrom the succession and that the Jeannette Formation is representedby a thin sequence of breccias. The remaining units are builtof lavas ranging from siliceous picrite basalts to tholeiiticandesites in composition. All lavas exhibit the effects of greenschistfacies metamorphism but igneous textures are well preservedand reveal that most lavas are aphyric to sparsely porphyritic.A well-developed geochemical stratigraphy exists in the lavasequence and is characterized by an upward trend from evolvedto primitive lavas. The Alberton (mg-number = 0.53–0.43;Zr= 108–137 ppm) and Orkney (mg-number = 0.51–O.42;Zr = 90–110ppm) lavas have a narrow compositional rangeand are more differentiated than the compositionally more variableLoraine/Edenville lavas (mg-number = 0.75 0.47; Zr = 34–97ppm). Unusual inter-element correlations in the Alberton andOrkney lavas suggest that magma mixing processes were importantin their petrogenesis. Compositional variation in the Loraine/Edenvillelavas is consistent with simple fractional crystallization dominatedby orthopyroxene. Collectively, all these lavas are light rareearth element (LREE)-enriched and have negative Nb-Ta, P, andTi anomalies on mid-ocean ridge basalt (MORB)- or primitivemantle-normalized abundance diagrams. In this they resemblemafic lavas from modern subduction-zone environments and manyArchaean and early Proterozoic siliceous high-magnesium basalts.The Klipriviersberg lavas also have initial Nd-isotopic compositionsthat cluster about the chondritic uniform reservoir (CHUR) valueregardless of degree of differentiation. It is unlikely thatcrustal contamination has influenced the composition of theKlipriviersberg lavas to any significant extent, and their geochemicalfeatures have probably been inherited from lithospheric mantlesources.     

The origin of large varioles in flow-banded pillow lava from the Hooggenoeg Complex,Barberton Greenstone Belt,South Africa

Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex of the Barberton Greenstone Belt exhibit both flow banding and large leucocratic varioles. The flow banding is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt (Robins et al. in Bull Volcanol 72:579–592, 2010a). Varioles occur exclusively in the dark chlorite-, MgO- and FeO-rich metabasalt. Varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows. In the central parts of some pillows, they impinge to form homogeneous pale patches, bands or almost homogenous cores. Individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite. Many varioles exhibit concentric zones and finer-grained rims. Some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer-grained rim. The matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain-like, skeletal Ca-rich pyroxenes that are partly overgrown by the margins of varioles. Varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile TiO₂, Cr, Zr and REE that are similar to the host metabasalts. The shape, distribution, texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass. Nucleation of plagioclase was strongly inhibited and took place on vesicles, on the bases of drainage cavities and along early fractures. Eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows.     

Carbonaceous cherts of the Barberton Greenstone Belt, South Africa: Isotopic, chemical and structural characteristics of individual microstructures

Carbonaceous matter occurring in chert deposits of the 3.4-3.2 Ga old Barberton Greenstone Belt (BGB), South Africa, has experienced low grade regional metamorphism and variable degrees of local hydrothermal alteration. Here a detailed study is presented of in situ analysis of carbonaceous particles by LRS (laser Raman spectroscopy) and SIMS (secondary ion mass spectrometry), reporting degree of structural disorder, carbon isotope ratio and nitrogen-to-carbon ratio. This combination of in situ analytical tools is used to interpret the δ¹³C values of only the best preserved carbonaceous remains, enabling the rejection of non-indigenous (unmetamorphosed) material as well as the exclusion of strongly hydrothermally altered carbonaceous particles. Raman spectroscopy confirmed that all carbonaceous cherts studied here have experienced a regional sub- to lower-greenschist facies metamorphic event. Although this identifies these organics as indigenous to the cherts, it is inferred from petrographic observations that hydrothermal alteration has caused small scale migration and re-deposition of organics. This suggest that morphological interpretation of these carbonaceous particles, and in general of putative microfossils or microlaminae in hydrothermally altered early Archean cherts, should be made with caution. A chert in the Hooggenoeg Formation, which is older than and has been hydrothermally altered by a volcanic event 3445 Ma ago, contains strongly altered carbonaceous particles with a uniform N/C-ratio of 0.001 and a range of δ¹³C that is shifted from its original value. Cherts of the Kromberg Formation post-date this volcanic event, and contain carbonaceous particles with a N/C-ratio between 0.002 and 0.006. Both the Buck Reef Chert and the Footbridge Chert of the Kromberg Formation have retained fairly well-preserved δ¹³C values, with ranges from −34‰ to −24‰ and −40‰ to −32 ‰, respectively. Abiologic reactions associated with hydrothermal serpentinization of ultramafic crust (such as Fischer-Tropsch synthesis) were an unlikely source for carbonaceous material in these cherts. The carbonaceous matter in these cherts has all the characteristics of metamorphosed biologic material.     

The New Consort Gold Mine,Barberton greenstone belt,South Africa: orogenic gold mineralization in a condensed metamorphic profile

The New Consort Gold Mine in the Palaeo- to Mesoarchaean Barberton greenstone belt, South Africa is one of the oldest recognized orogenic gold deposits on Earth. The gold mineralization is hosted by discrete mylonitic units that occur at, or close to, the contact between the mafic and ultramafic volcanic rocks of the c. 3,280 Ma Onverwacht Group and the mainly metasedimentary rocks of the overlying c. 3,260–3,230 Ma Fig Tree Group. This contact, locally referred to as the Consort Bar, formed during ductile D₁ imbrication of the metavolcanosedimentary sequence and predates the main stage of the gold mineralization. The imbricate stack is situated in the immediate hanging wall of the basal granitoid–greenstone contact along the northern margin of the greenstone belt. It is characterized by a condensed metamorphic profile in which the metamorphic grade increases from upper greenschist facies conditions (510–530°C, 4 kbar) in rocks of the Fig Tree Group to upper amphibolite facies grades (600–700°C, 6–8 kbar) in the basal Onverwacht Group. Detailed structural and petrological investigations indicate that the Consort Bar represents a major structural break, which is largely responsible for the telescoping of metamorphic isograds within the structural sequence. Two stages of mineralization can be distinguished. Loellingite, pyrrhotite, and a calc–silicate alteration assemblage characterize an early high-T mineralization event, which is restricted to upper amphibolite facies rocks of the Onverwacht Group. This early mineralization may correlate with the local D₁ deformation. The second and main stage of gold mineralization was associated with renewed ductile shearing during D₂. The D₂ deformation resulted in the reactivation of earlier structures, and the formation of a NNW trending, steeply dipping shear zone system, the Shires Shear Zone, which separates two regional SE plunging D₁ synclines. The mineralized shear zones are intruded by abundant syn-kinematic pegmatite dykes that have previously been dated at c. 3040 Ma. Petrological and geothermobarometric data on ore and alteration assemblages indicate that the main stage of gold mineralization, which affected a crustal profile of ca. 1.5 km, was characterized by increasing temperatures (c. 520 to 600°C) with increasing structural depth. Sulfide assemblages in the ore bodies change progressively with metamorphic grade, ranging from arsenopyrite + pyrite + pyrrhotite in the structurally highest to arsenopyrite + pyrrhotite + chalcopyrite + loellingite in the structurally deepest part of the mine. The main stage of gold mineralization was broadly syn-peak metamorphic with respect to the Fig Tree Group, but postdates the peak of metamorphism in upper amphibolite facies rocks of the structurally underlying Onverwacht Group. This indicates that the mineralization coincided with the juxtaposition of the two units. As the footwall rocks were already on their retrograde path, metamorphic devolatilisation reactions within the greenstone sequence can be ruled out as the source of the mineralizing fluids.