Geochemistry of the Archean Bulawayan Group,Midlands greenstone belt,Rhodesia

The Bulawayan Group in the Midlands greenstone belt can be divided into three formations. The Mafic Formation is composed principally of pillowed, low-K tholeiites and minor bedded chert. The Maliyami Formation and conformably overlying Felsic Formation are composed of calc-alkaline tholeiites, andesites, and dacites with andesites dominating in the Felsic Formation. Minor rhyolite quartz porphyries and ultramafic bodies also occur in the section. The Bulawayan Group near Que Que is perhaps the least altered and metamorphosed Archean greenstone succession known. The absence of andesite and related rocks, the association of bedded chert, and the consistently low K₂O, Rb, and Sr contents of Mafic Formation tholeiites suggest that they represent Archean oceanic rise tholeiites. The compositions of tholeiites and andesites of the Maliyami Formation, however, suggest that they represent an emerging arc system. The Felsic Formation is interpreted as a more advanced stage in the evolution of this arc system.Trace-element model calculations favor an origin for Mafic Formation tholeiites involving about 30% partial melting of a lherzolite source. Similar calculations are consistent with an origin for Maliyami Formation tholeiites, Maliyami and Felsic Formation andesites, and Midlands rhyolites involving, respectively, 50, 20–30, and 10% equilibrium melting of eclogite or garnet amphibolite (of Mafic Formation tholeiite composition). The low K₂O, Rb, and Sr contents of Mafic Formation tholeiites suggest that they were derived from an upper mantle source as depleted in these elements as the oceanic upper mantle is today.A plate tectonic model is proposed for the Bulawayan Group in which the Mafic Formation is derived from a depleted lherzolite source beneath a spreading center in a marginalsea basin and the Maliyami and Felsic Formations and associated rhyolites are produced by partial melting of eclogite in a descending slab located west of the basin.     

Seismic evidence for preservation of the Archean Uchi granite–greenstone belt by crustal-scale extension

In the westernmost Superior Province of Canada, the east–west alignment of granite–greenstone belts and the adjacent, highly deformed gneiss belts led to the first proposals that plate tectonics existed before 2.5 Ga ago, with the belts thrust against one another by east–west-oriented subduction zones. Here, we present seismic reflection data, which demonstrate that in this region the present juxtaposition of the Uchi granite–greenstone belt and the North Caribou gneiss terrane occurred along a late southeast-dipping extensional shear zone that extends from the surface into the lower crust. The preservation of the Uchi belt and probably the English River metasedimentary belt is directly related to their dropping along extensional shear zones, which limited subsequent erosion. The relative lateral transport of these greenstone rocks implies that they were neither derived from the immediately underlying crust, nor preserved by vertical crustal movements as might occur in the absence of plate tectonics. Extension may have been associated with the emplacement of mantle-derived magmas at 2700 Ma, which has been linked to slab break-off or lithospheric delamination, making the extension approximately coeval with local gold mineralisation. Since crustal-scale faults can facilitate the circulation of gold-bearing fluids, we suggest that greenstone rocks preserved in the hanging walls of syn- to post-accretion extensional shear zones may preferentially host Archean lode-gold deposits. In the westernmost Superior Province, our seismic observations imply that some of the late structures in the well-developed belts defined by surface mapping arose through the collapse of a collage of laterally accreted terranes.     

The geochemistry of Archaean shales derived from a Mafic volcanic sequence, Belingwe greenstone belt, Zimbabwe: provenance, source area unroofing and submarine versus subaerial weathering

Shales of the ∼2.7 Ga Zeederbergs Formation, Belingwe greenstone belt, Zimbabwe, form thin (0.2-2 m) horizons intercalated with submarine lava plain basalts. Shales of the overlying Cheshire Formation, a foreland basin sedimentary sequence, form 1-100 m thick units intercalated with shallow-water carbonates and deep-water, resedimented basalt pebble conglomerates. Zeederbergs shale is characterized by high contents of MgO and transition metals and low concentrations of K₂O and LILE as compared to average Phanerozoic shale, indicative of an ultramafic to mafic source terrain. Cheshire shales have similar major and trace element contents, but MgO and transition metals are less enriched and the LILE are less depleted. Zeederbergs shales have smoothly fractionated REE patterns (La_N/Yb_N = 2.84-4.45) and no significant Eu anomaly (Eu/Eu* = 0.93-0.96). REE patterns are identical to those of the surrounding basaltic rocks, indicating local derivation from submarine reworking. Cheshire shales have rather flat REE patterns (La_N/Yb_N = 0.69-2.19) and a small, negative Eu anomaly (average Eu/Eu* = 0.85), indicative of a mafic provenance with minor contributions of felsic detritus. A systematic change in REE patterns and concentrations of transition metals and HFSE upwards in the sedimentary succession indicates erosion of progressively more LREE-depleted basalts and ultramafic volcanic rocks, followed by unroofing of granitoid crust. Weathering indices confirm the submarine nature of Zeederbergs shale, whereas Cheshire shale was derived from a source terrain subjected to intense chemical weathering.     

The eastern boundary of the Baikal collisional belt: Geological,geochronological, and Nd isotopic evidence

New geological. geochronological, and Nd isotopic data are reported for the rocks occurring at the interfluve of the Barguzin, Nomama, and Katera rivers, where the main structural elements of the Early Paleozoic collisional system have been established. The strike-slip and thrust Tompuda-Nomama and Barguzin boundary sutures separate the Svetlaya and the Katera zones of the Baikal-Muya Belt from the Barguzin terrigenous-carbonate terrane. The age estimates of syntectonic (prebatholithic) gneissic granite and gabbrodiorite intrusive bodies (469 ± 4 and 468 ± 8 Ma, respectively) coincide with the age of collisional events in the Ol’khon, Southwest Baikal, and Sayan regions (480–470 Ma). A linear zone with zonal metamorphism and granite-gneiss domes dated at 470 Ma is revealed in the allochthonous fold-nappe packet of the Upper Riphean Barguzin Formation. This zone of Caledonian remobilization marks the collisional front between the Riphean structural units of the Barguzin Terrane consolidated 0.60–0.55 Ga ago and the Baikal-Muya Belt. New data allow us to recognize this zone as the northeastern flank of the Baikal Collisional Belt. The Nd isotopic data for the reference igneous complexes of the collisional zone indicate that the Late Riphean juvenile crust was involved in the Ordovician remobilization in the zone of conjugation of the consolidated Baikalian structural elements at the northeastern flank of the Baikal Belt and likely was a basement of the entire Barguzin Terrane or, at least, its frontal portion. The lateral displacements of the terranes to the northeast during the Early Ordovician collision were constrained by the rigid structural framework of the Baikalides in the Muya segment of the Baikal-Muya Belt, where the Riphean blocks were involved in strike-slip faulting and the Vendian-Cambrian superimposed basin underwent deformation. Finally, it may be concluded that the Early Ordovician was an epoch of collision, complex in kinematics, between heterogeneous blocks of the continental crust: the Baikalides of the Baikal-Muya Belt and polycyclic Barguzin-Vitim Superterrane.     

PGE mineralization in Archean volcanic systems: geochemical evidence from thick, differentiated mafic-ultramafic flows, Abitibi greenstone belt, Ontario, and implications for exploration

PGE-rich disseminated zones with discrete platinum-group minerals (Pd, Pt and Rh mineral phases) have been discovered in three thick (80–130 m), differentiated (peridotite-gabbro) mafic-ultramafic flows of the Archean Abitibi greenstone belt, Ontario. Three mineralization zones (whole-rock ∑PGE + Au = up to 1000 ppb) occur along four stratigraphic cross sections through a 2 km strike-length of the Boston Creek Flow ferropicritic basalt. Their occurrence most strikingly correlates with lenticular-podiform concentrations of disseminated chalcopyrite (1 %) and clinopyroxene + interstitial magnetite-ilmenite intergrowths (15–20% oxide), high concentrations of related metals (3000 ppm Cu, 3000 ppm S, 1200 ppb Ag, and 1000 ppm V), strong PGE depletion in adjacent rocks and along strike, and lithological and textural complexity in the margins of the central gabbro-diorite layer. The mineralization zone (whole-rock Ir + Pt + Pd + Au = 110 ppb) within Theo's Flow tholeiitic basalt is somewhat similar in occurrence, style, and composition to those within the Boston Creek Flow. In contrast, the mineralization zone (whole-rock Ir + Pt + Pd + Au = 340 ppb) in Fred's Flow komatiitic basalt most strikingly correlates with vesicle-filling intergrowths of pyrrhotite + pentlandite ± chalcopyrite (2 modal %) and high whole-rock concentrations of Ni (2500 ppm), Cu (700 ppm), and S (1.1%) in the upper chilled margin of the flow.Although apparently uneconomic, these flow-hosted PGE mineralization zones are of interest in exploration, because they are more similar in stratigraphie setting, style, and composition to PGE-rich disseminated Fe-Cu sulfide mineralization zones within thick differentiated intrusions than to mineralization zones in other Archean volcanic rocks. The characteristics of the mineralization zones and their host rocks, especially high degrees of PGE enrichment, vertical and horizontal patterns of PGE depletion, and accumulation of clinopyroxene + magnetite-ilmenite intergrowths, indicate a critical genetic role for variations in the regime of melt flowage. The mineralization zones in the Boston Creek and Theo's Flows are interpreted to have formed by simultaneous in situ formation of PGE-rich Fe-Cu sulfide and Fe-Ti oxide from flowing silicate liquid in the margins of internal lava channels. The mineralization zone in Fred's Flow is interpreted to have formed by ponding and coalescence of PGE-enriched sulfurous vapor bubbles in the upper chilled margin during olivine accumulation on the base of a dynamic lava channel. The relative abundance of PGE mineralization zones and high degree of PGE enrichment in the Boston Creek Flow suggest that the most favorable exploration targets are rocks crystallized from late-stage, highly fractionated derivative liquids in large differentiated terropicritic units.     

Stable isotope and Rare Earth Element evidence for recent ironstone pods within the Archean Barberton greenstone belt, South Africa

There is considerable debate about the mode and age of formation of large (up to ∼200 m long) hematite and goethite ironstone bodies within the 3.2 to 3.5 Ga Barberton greenstone belt. We examined oxygen and hydrogen isotopes and Rare Earth Element (REE) concentrations of goethite and hematite components of the ironstones to determine whether these deposits reflect formation from sea-floor vents in the Archean ocean or from recent surface and shallow subsurface spring systems. Goethite δ¹⁸O values range from −0.7 to +1.0‰ and δD from −125 to −146‰, which is consistent with formation from modern meteoric waters at 20 to 25 °C. Hematite δ¹⁸O values range from −0.7 to −2.0‰, which is consistent with formation at low to moderate temperatures (40-55 °C) from modern meteoric water. REE in the goethite and hematite are derived from the weathering of local sideritic ironstones, silicified ultramafic rocks, sideritic black cherts, and local felsic volcanic rocks, falling along a mixing line between the Eu/Eu* and shale-normalized HREE_Avg/LREE_Avg values for the associated silicified ultramafic rocks and felsic volcanic rocks. Contrasting positive Ce/Ce* of 1.3 to 3.5 in hematite and negative Ce/Ce* of 0.2 to 0.9 in goethite provides evidence of oxidative scavenging of Ce on hematite surfaces during mineral precipitation. These isotopic and REE data, taken together, suggest that hematite and goethite ironstone pods formed from relatively recent meteoric waters in shallow springs and/or subsurface warm springs.     

Crustal outgassing and LILE enrichment in major lithosphere structures,Archean Abitibi greenstone belt: evidence on the source reservoir from strontium and carbon isotope tracers

Major structural discontinuities in the Abitibi greenstone belt acted as conduits for outgassing of the Archean crust, as reflected in fixation of a select group of lithophile elements including Si, C, K, Rb, Ba, Li, Cs, B and Pb, in metasomatized faults. For two of the largest structures, the Destor-Porcupine (DP) and Kirkland Lake — Cadillac (KC) fault zones 6×10¹⁵ g Si, 3×10¹⁵ g CO₂ and 10¹⁵ g K were introduced into the faults during expulsion of an estimated 6×10¹⁸ g aqueous fluids. Strontium isotope ratios of tourmaline, piemontite, actinolite and scheelite mineral separates, characterized by Rb/Sr0.02, are concordant with respect to ⁸⁷Sr/⁸⁶Sr initial ratios over local sectors of the faults. The Sr isotope data record geographic variations which, from east to west on the KC fault is 0.7031–0.7041 (Val d'Or), 0.7008–0.7022 (Bourlemaque), 0.7017–0.7019 (Bousquet), 0.7029–0.7031 (Noranda), and 0.7013 to 0.7015 (Kirkland Lake). At Timmins, on the PD fault, ⁸⁷Sr/⁸⁶Sr initial ratios cluster at 0.7010 to 0.7020. Metasomatised fault zones are systematically more radiogenic than contiguous host lithologies, and imply a source reservoir (0.7010 to 0.7041) generally more radiogenic than the upper mantle at 2690 Ma (0.700±0.001), or contemporaneous volcanic rocks of mafic to ultramafic composition (0.700 to 0.7012). Whereas certain minerals are concordant and retentive, Rb-Sr isochrons based on suites of rocks at progressive intensities of metasomatism, have been systematically reset over an elpased time of 200 Ma after termination of outgassing, due to disturbance accompanying incremental displacements on structures.Carbon isotope compositions of ferroan dolomites in faults are tightly clustered along local fault sectors, but also display a marked provinciality: from east to west ¹³C=–6.0 to –8.5 (Malartic), –8.0 to –9.0 (Cadillac), –2.0 to –4.5 (Kirkland Lake), and –0.5 to –3.5 (Timmins). The observed provinciality of both ¹³C values and ⁸⁷Sr/⁸⁶Sr initial ratios is interpreted to reflect compositional heterogeneities in a radiogenic sialic crust and the green-stone belt supracrustal sequence, both of which supplied volatiles, magmas and lithophile elements to the fault structures during late stage transpressive tectonics.     

Elemental geochemistry and Nd isotopic characteristics of the metasedimentary rocks from the metamorphic belt in central Jiangxi： Provenance and tectonically environmental constraints

The metamorphic belt in central Jiangxi, located in the compound terrain within the Cathaysia, Yangtze Block and Caledonian fold zone of South China, is composed dominantly of meta-argillo-arenaceous rocks, with minor amphibolite. These rocks underwent amphibolite-facies metamorphism. The meta-argillo-arenaceous rocks show large variations in major element composition, but have similar REE patterns and trace element composition, incompatible element and LIE enrichments [ high Th/Sc (0.57-3.59) , La/Sc ( 1.46 - 12.4), La/Yb (5.84 - 19.0) ] and variable Th/U ratios, with ∑REE = 129-296μg/g, δEu =0.51 -0.86, and (La/Yb)N = 3.95 -12.9. The Nd isotopic model ages tDM of these rocks vary from 1597 to 2124 Ma. Their 143 Nd/144 Nd values are low [εNd (0) = - 11.4 to -- 15.8]. Some conclusions have been drawn as follows: (1) The metamorphic rocks in central Jiangxi Province are likely formed in a tectonic environment at the passive continental margin of the Cathaysia massif. (2) The metamorphosed argillo-arenaceous rocks are composed dominantly of upper crustal-source rocks (Al- and Krich granitic or/and sedimentary rocks of Early Proterozoic), which experienced good sorting, slow deposition and more intense chemical weathering. (3) According to the whole-rock Sm-Nd isochron ages (1113±49 to 1199 ± 26 Ma) of plagioclase-amphibole (schist) and Nd isotopic model age tDM ( 1597 - 2124Ma) of meta-argillo-arenaceous rocks, the metamorphic belt in central Jiangxi Province was formed during the Middle Proterozoic ( 1100 - 1600 Ma).     

郯庐断裂带晚中生代富碱侵入岩Nd、Sr、Pb同位素特征及源区性质探讨

郯庐断裂带是中国东部一条巨大的深大断裂带,它纵切华北克拉通、扬子克拉通、兴蒙-吉黑造山带和大别-苏鲁造山带四个大地构造单元,特别是在该断裂带出露许多晚中生代富碱侵入岩体,构成一条长达近2000km的富碱侵入岩带.本文是在前人研究基础上首次从大尺度对整个郯庐断裂带富碱侵入岩的Nd、Sr、Pb同位素做了比较系统深入的研究,发现四个大地构造单元的富碱侵入岩具有明显不同的Nd、Sr、Pb同位素特征,暗示四大构造单元富碱侵入岩的源区性质差别较大:兴蒙-吉黑造山带晚中生代富碱侵入岩源于和亏损地幔有成因联系的年青地壳(εNd(t)为ft.,平均值 2.25);华北克拉通、扬子克拉通和大别-苏鲁造山带三大构造单元晚中生代富碱侵入岩都主要源于EMI型富集地幔(εNd(t)均为负值).但是这三大构造单元岩石圈地幔富集程度差别较大.大别-苏鲁造山带岩石圈地幔相对富集程度最高(εNd(t)平均值为-17.06);华北克拉通次之(εNd(t)平均值为-10.03);扬子克拉通岩石圈地幔富集程度最低(εNd(t)平均值为-6.41).本文认为大别.苏鲁造山带岩石圈地幔相对最富集的原因可能是在早中生代扬子克拉通陆壳向华北克拉通深俯冲之后的折返过程中,因压力释放有利于熔/流体的形成、析出和向上交代并改造华北岩石圈地幔使之成为富集地幔.     

Tourmaline mineralization in the Barberton greenstone belt,South Africa: early Archean metasomatism by evaporite-derived boron

Tourmaline-rich rocks are common in the lowgrade, interior portions of the Barberton greenstone belt of South Africa, where shallow-marine sediments and underlying altered basaltic and komatiitic lavas contain up to 50% tourmaline. The presence of tourmaline-bearing rip-up clasts, intraformational tourmalinite pebbles, and tourmaline-coated grains indicates that boron mineralization was a low-temperature, surficial process. The association of these lithologies with stromatolites, evaporites, and shallow-water sedimentary structures and the virtual absence of tourmaline in correlative deep-water facies rocks in the greenstone bels strengthens this model.Five tourmaline-bearing lithologic groups (basalts, komatiites, evaporite-bearing sediments, stromatolitic sediments, and quartz veins) are distinguished based on field, petrographic, and geochemical criteria. Individual tourmaline crystals within these lithologies show internal chemical and textural variations that reflect continued growth through intervals of change in bulk-rock and fluid composition accompanying one or more metasomatic events. Large single-crystal variations exist in Fe/Mg, Al/Fe, and alkali-site vacancies. A wide range in tourmaline composition exists in rocks altered from similar protoliths, but tourmalines in sediments and lavas have similar compositional variations. Boron-isotope analysis of the tourmalines suggest that the boron enrichment in these rocks has a major marine evaporitic component. Sediments with gypsum pseudomorphs and lavas altered at low temperatures by shallow-level brines have the highest ¹¹B values (+2.2 to-1.9); lower ¹¹B values of late quartz veins (-3.7 to-5.7) reflect intermediate temperature, hydrothermal remobilization of evaporitic boron. The ¹¹B values of tourmaline-rich stromatolitic sediments (-9.8 and-10.5) are consistent with two-stage boron enrichment, in which earlier marine evaporitic boron was hydrothermally remobilized and vented in shallow-marine or subaerial sites, mineralizing algal stromatolites. The stromatolite-forming algae preferentially may have lived near the sites of hydrothermal discharge in Archean times.     

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.     

Zircon ages and Nd isotopic and chemical compositions of orthogneisses from the Black Forest, Germany: evidence for a Cambrian magmatic arc

 Single zircon U–Pb dating combined with ²⁰⁷Pb/²⁰⁶Pb ages obtained by the evaporation method constrains the emplacement of tonalitic, trondhjemitic, and granodioritic orthogneisses of the Moldanubian zone in the Black Forest between 500 and 510 Ma. Two detrital zircon populations of 1.9 and 1.6 Ga indicate Early-Middle Proterozoic material in the former setting of the basement. The initial e_Nd values range from –0.1 to –3.4 and mean crustal residence ages of 1.0–1.4 Ga are consistent with involvement of Early-Middle Proterozoic crust, and a subordinate juvenile component probably originating from subduction-related melting of the mantle. The orthogneisses have fractionated REE patterns and slightly higher K₂O/Na₂O ratios than typical low-K tonalite–trondhjemite–granite suites. The chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust. The emplacement of the orthogneisses was superceded by a high-temperature metamorphic event at ∼480 Ma which we interpret as a result of lithospheric thinning in a marginal basin behind a Cambrian magmatic arc. Received: 29 March 1999 / Accepted: 25 August 1999     

Archaean stromatolites from the Ngesi Group,Belingwe greenstone belt,Zimbabwe; preservation and stable isotopes — preliminary results

The petrology and stable isotope chemistry of cyanobacterial stromatolites of Archaean age (2.7 Ga) from the Cheshire and Manjeri Formations of the Belingwe greenstone belt in Zimbabwe have been examined. Palaeomagnetic data suggest that the stromatolites formed in tropical to subtropical latitudes. The Cheshire Formation shows little evidence of either anion or cation exchange during metamorphism, and the stable carbon and oxygen isotope ratios suggest a formation at temperatures perhaps considerably below 80°C. The Manjeri Formation, only slightly older, but overlain by a thick volcanic sequence, shows a low grade of metamorphism, and isotope ratios that are consistent with a metamorphic temperature of around 200°C.     

Gold mineralization in the Mazowe area, Harare-Bindura-Shamva greenstone belt, Zimbabwe: II. Genetic relationships deduced from mineralogical, fluid inclusion and stable isotope studies, and the Sm-Nd isotopic composition of scheelites

In the Mazowe area some 40 km NW of Harare in Zimbabwe, gold mineralization is hosted in a variety of lithologies of the Archean Harare-Bindura-Shamva greenstone belt, in structures related to the late Archean regional D2/3 event. Conspicuous mineralzogical differences exist between the mines; the mainly granodiorite-hosted workings at Mazowe mine are on pyrite-rich reefs, mines of the Bernheim group have metabasalt host rocks and are characterized by arsenopyrite-rich ores, and Stori's Golden Shaft and Alice mine, both in metabasalts, work sulfide-poor quartz veins. In contrast to the mineralogical diversity, near-identical fluid inventories were found at the different mines. Both H₂O-CO₂-CH₄ fluids of low salinity, and highly saline fluids are present and are regarded to indicate fluid mixing during the formation of the deposits. Notably, these fluid compositions in the Mazowe gold field markedly contrast to ore fluids “typical” of Archean mesothermal gold deposits on other cratons. Stable isotope compositions of quartz from the various deposits (δ¹⁸O=10.8 to 13.2‰ SMOW), calcite (δ¹⁸O=9.5 to 11.9‰ SMOW and δ¹³C=−3.2 to −8.0‰ PDB), inclusion water (δD=−28 to −40‰ SMOW) and sulfides (δ³⁴S=1.3 to 3.2‰ CDT) are uniform within the range typical for Archean lode gold deposits worldwide. The fluid and stable isotope compositions support the statement that the mineralization in the Mazowe gold field formed from relatively reduced fluids with a “metamorphic” signature during a single event of gold mineralization. Microthermometric data further indicate that the deposits formed in the PT range of 1.65–2.3 kbar and 250–380 °C. Ages obtained by using the Sm/Nd and Rb/Sr isotope systems on scheelites are 2604 ± 84 Ma for the mineralization at Stori's Golden Shaft mine, and 2.40 ± 0.20 Ga for Mazowe mine. The Archean age at Stori's is regarded as close to the true age of gold mineralization in the area, whereas the Proterozoic age at Mazowe mine probably reflects later resetting. Received: 30 September 1998 / Accepted: 17 August 1999     

Unscrambling the stratigraphy of an Archean greenstone belt; a U---Pb geochronological study of the Favourable Lake belt, northwestern Ontario, Canada

New zircon and baddeleyite U---Pb ages show that the major units of the Archean Favourable Lake greenstone belt in the northern Superior Province were formed by episodic magmatism spanning more than 250 Ma and demonstrate that the original stratigraphy is disrupted by thrusts that juxtaposed older supracrustal rocks on top of younger ones. The oldest rocks of the area are a 2950 Ma gneissic tonalite and a 3000-2960 Ma granodiorite clast from a ca. 2725 Ma conglomerate. Five distinct volcanic (and sedimentary) groups formed during presumably short-lived episodes at 2925 Ma (I), 2870 Ma (II), 2858 Ma (III), 2734 Ma (IV), and 2725 Ma (V). The youngest group contains the thickest sedimentary unit, a turbiditic and alluvial-fluvial sequence. Compression caused thrusting that placed Groups I and V on top of IV, III on V, and II on III. The thrust complex was subsequently isoclinally folded. Compression was accompanied by major plutonism that emplaced the bulk of the bounding batholiths between 2732 and 2711 Ma ago. The late tectonic, Mo-mineralized Setting Net Lake Stock in the centre of the belt has an age of about 2708 Ma and was overprinted by younger hydrothermal events that produced monazite (2706 Ma), titanite (ca. 2695-2690 Ma) and rutile (the youngest rutile at 2657 Ma). Similar late hydrothermal pulses are recorded by secondary titanite elsewhere in the belt and within the batholiths. The protracted magmatic evolution of the belt is typical of that observed in a number of greenstone belts of the northern Superior Province but is uncommon farther to the south. In contrast, the structural complications and put-of-sequence stratigraphy appear to be a quite common tectonic characteristic of greenstone belts of the whole Superior Province.     

Geochemistry of Archean shales from the Pilbara Supergroup,Western Australia

Archean clastic sedimentary rocks are well exposed in the Pilbara Block of Western Australia. Shales from turbidites in the Gorge Creek Group (ca. 3.4 Ae) and shales from the Whim Creek Group (ca. 2.7 Ae) have been examined. The Gorge Creek Group samples, characterized by muscovite-quartzchlorite mineralogy, are enriched in incompatible elements (K, Th, U, LREE) by factors of about two, when compared to younger Archean shales from the Yilgarn Block. Alkali and alkaline earth elements are depleted in a systematic fashion, according to size, when compared with an estimate of Archean upper crust abundances. This depletion is less notable in the Whim Creek Group. Such a pattern indicates the source of these rocks underwent a rather severe episode of weathering. The Gorge Creek Group also has fairly high B content (85 ± 29 ppm) which may indicate normal marine conditions during deposition.Rare earth element (REE) patterns for the Pilbara samples are characterized by light REE enrichment ($\text{La}{}_{\mathrm{N}}\text{Yb}{}_{\mathrm{N}}\text{≥ 7.5}$) and no or very slight Eu depletion ($\text{Eu}\text{Eu}{}^1\text{= 0.82 – 0.99}$). A source comprised of about 80% felsic igneous rocks without large negative Eu-anomalies (felsic volcanics, tonalites, trondhjemites) and 20% mafic-ultramafic volcanics is indicated by the trace element data. Very high abundances of Cr and Ni cannot be explained by any reasonable provenance model and a secondary enrichment process is called for.