Komatiitic Basalts Research Articles

Light-rare earth element-rich minerals associated with platinum-group element mineralization in the Archean Boston Creek Flow, Ontario

A variety of LREE-rich minerals are associated with late magmatic-stage platinum-group element (PGE) mineralization [Σ(PGE + Au) = 300 ppb) in unsheared clinopyroxenite and gabbro proximal to sheared amphibolite in the Boston Creek Flow (BCF) Al-depleted komatiitic basalt, Archean Abitibi greenstone belt, Ontario. The LREE-rich minerals are LREE-rich apatite (La2O3 + Ce2O3 ≥ 1.5 wt%), LREE-rich epidote (Ce, La: 12 wt% ΣREE), and bastnaesite [(Ce,La)(CO3)(F,OH)]. The LREE-rich apatite forms rare zones in altered apatite grains and discrete, multifaceted micrometric-sized grains. LREE-rich epidote forms large (up to 100 ,μm), compositionally zones grains in amphibolitized plagioclase. Bastnaesite forms areas marginal to and veinlets within the LREE-rich epidote and analyses are characterized by up to 0.4 wt% Cl. Compared to other unsheared rocks from the flow, the REE-rich mineral host rocks contain: intermediate REE contents (ΣREE = 38 to 71 ppm), Ba contents (up to 240 ppm), and U/Th values (0.3 to 7.2); variable Cl contents (21 to 60 ppm); and slightly elevated δ34S values (up to 3.3). In contrast, the sheared amphibolite is characterized by low contents of REE (ΣREE = 25 ppm), Cl (15 ppm), Ba (20 ppm), U (0.5 ppm), and Th (0.4 ppm), and a distinctive chondrite-normalized whole-rock REE pattern profile [(La/Sm)n = <2 and (Tb/Yb)n = < 1).

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.

Sulfide-poor platinum-group mineralization in komatiitic systems; Boston Creek flow, layered basaltic komatiite, Abitibi Belt, Ontario

Geochemical studies of platinum-group elements (PGE), Au, Ag, and other metals (S, Se, As, Sb, Cu, Ni, Cr, Co, V, Ti) and electron microprobe studies of them have provided new constraints on the character and origin of the recently discovered sulfide-poor PGE mineralization in the Boston Creek flow layered basaltic komatiite of the Abitibi belt. High PGE levels characterize a 2-m-thick interval overlapping the contact of the clinopyroxenite and gabbro layers (50 ppb Pt, 75 ppb Pd, 10 ppb Au), the basal part of the clinopyroxenite layer (20 ppb Pt, 33 ppb Pd, 49 ppb Au), and the basal part of the peridotite layer (3 ppb Ir, 15 ppb Pd). The PGE-rich clinopyroxenite and gabbro layers contain grains of Pd, Pt, and Rh bismuthotellurides, antimonides and arsenides, and gold and silver minerals, which occur in spatial association with sparsely disseminated interstitial chalcopyrite, skeletal magnetite-ilmenite, and secondary amphibole alteration of chalcopyrite and magnetite-ilmenite. These rocks are spatially associated with zones of sheared amphibolite that are relatively poor in PGE and rich in Ni, Co, S, and Se. Minor anorthosite patches, dikes, and discontinuous layers immediately up section of the mineralized gabbro are relatively poor in PGE, especially Ir (< or =0.02 ppb), and Ni, Cu, Co, and S. The PGE-enriched peridotite is slightly richer in Cr and Ni but poorer in Co, Ag, S, and Se compared to the overlying peridotite.The textural and geochemical characteristics indicate a sulfide control in the origin of the PGE mineralization, but they contradict an important role for settling of segregated sulfide, Cl-rich late-stage magmatic volatiles, or hydrothermal fluid activity during metamorphism. Alternatively, they suggest PGE mineralization by in situ segregation of sulfide accompanying the crystallization of Fe-Ti oxide from upwardly mobile, volatile-charged interstitial melt that evolved in the clinopyroxenite and basal gabbro and subsequently formed the anorthosite. Nickel, Co, Se, S, Ag, and Cu were selectively mobilized relative to the PGE in the mineralized rocks by hydrothermal fluids and reconcentrated in zones of shear deformation and brittle failure during greenschist facies contact metamorphism. Iridium and Pd probably concentrated in the basal peridotite by coprecipitation of an alloy-sulfide phase during the fractionation of olivine + or - chromite. Exploration criteria potentially useful in the search for economic concentrations of PGE in geologically similar environments of the Abitibi belt and elsewhere may include magnetite-ilmenite-rich clinopyroxenite and gabbro within thick layered mafic-ultramafic flows and especially intrusions; and Cu, V, Fe, and Ti fractionation trends and anomalies.

The behaviour of platinum-group elements during partial melting, crystal fractionation, and sulphide segregation: An example from the Cape Smith Fold Belt, northern Quebec

The behaviour of the Pt-group elements (PGE) during both normal igneous processes and during the formation of PGE deposits is poorly understood. This is in part because of the limited data set available for nonmineralized rocks. Accordingly, PGE concentrations have been determined in komatiitic basalts associated with Ni-Cu sulphide deposits rich in PGE from the Proterozoic Cape Smith Fold Belt of northern Quebec. The lavas have been divided into olivine-phyric, pyroxene-phyric, and plagioclase-phyric on the basis of the phenocrysts present. Olivine-phyric lavas have Mg#s greater than 0.66 and are thought to be primary partial melts. The bulk partition coefficients of the PGE during partial melting calculated from the average of olivine-phyric basalts are 6, 2, 0.6, and 0.2 for Ir, Rh, Pt, and Pd, respectively, which indicates a decrease in compatibility of the PGE in this order. The pyroxene and plagioclase-phyric lavas are thought to have formed from the olivine-phyric lavas by crystal fractionation of olivine and chromite in the pyroxene-phyric lavas and olivine, chromite, and pyroxene in the plagioclase-phyric lavas. Ir shows a strong positive correlation with Mg#, Cr, and Ni, and this is attributed to Ir having partitioned into olivine or chromite. If olivine, chromite, and clinopyroxene were the only phases to have crystallized, then Pd should have behaved as an incompatible element; however, it does not correlate with the lithophile incompatible elements. It does, however, correlate with Rh and Pt. This is interpreted to suggest that Rh, Pt, and Pd were controlled by sulphide segregation. The removal of a small amount of sulphide along with the olivine and chromite during crystal fractionation could have resulted in the mildly compatible behavior exhibited by Rh, Pt, and Pd. The composition of the sulphides in the sulphide deposits may be modelled by assuming that the silicate magma from which they segregated was similar in composition to the spinifex-textured komatiites containing 15% MgO.

Felsic fire-fountaining beneath Archean seas: pyroclastic deposits of the 2730 Ma Hunter Mine Group, Quebec, Canada

The lapilli tuff breccias (LTB-1 and LTB-2) of the Archean Hunter Mine Group in the south-central part of the Abitibi greenstone belt are inferred to be the product of subaqueous lava fountaining. Intercalated sub-wave base iron-formations, interstratified turbiditic tuffs, the absence of wave-induced sedimentary structures, and the stratigraphic position of lapilli tuff breccias beneath basaltic komatiites, support this contention. A complete eruptive sequence shows a tripartite division into (a) massive breccia, (b) stratified lapilli tuff, and (c) turbiditic tuff-lapilli tuff division. The massive breccia division is characterized by clusters of isolated and compressed irregular-shaped clasts inferred to be deposited directly from the hot magmatic lava fountain. Abundant vesicular pyroclasts with a vesicle content of up to 60% exhibit locally coalescing vesicles indicating bubble nucleation prior to eruption. The prevalence of irregular to amoeboid clast shapes suggests transport from the vent in a steamy-rich, high-density current to the site under a self-generated steam cupola. Ubiquitous subequant lapilli-size pyroclasts of the stratified lapilli tuff division suggest that significant ingress of water into the fountain changed the prevalent fragmentation process from magmatic to hydrovolcanic. The turbiditic tuff-lapilli tuff division composed of pumice, lithic fragments and vitric ash is envisaged to have formed by gravitational collapse of a subaqueous turbulent eruptive plume. This type of eruptive mechanism constituted a minor but important process of volcanic construction on the ocean floor during the Archean, and possibly during incipient arc and backarc formation in modern day settings.

Precambrian ophiolite belts of southern Siberia, Russia, and their metallogeny

Abstract Greenstone and ophiolite belts of different ages in southern Siberia are compared. The Archaean Olondo greenstone belt (3.0–2.9 Ga) within the Aldan Shield is different from younger ophiolite belts. The Early Proterozoic Baikal-Muya ophiolite belt (∼ 1.9 Ga) containing both komatiites and a complete ophiolitic sequence of the tholeiite series, and the Late Proterozoic-Early Cambrian (0.6-0.52 Ga) Kurtushibin and Ilchir ophiolite belts in western and eastern Sayan are fairly comparable with young ophiolites and modern crust of oceanic type. In the Olondo greenstone belt, there are komatiite and tholeiite series that are represented by both volcanic rocks and differentiated sills, but no dyke complex is present. In the Baikal-Muya ophiolite belt there are metavolcanic rocks (including komatiites and komatiitic basalts), a dyke complex, upper cumulate gabbros and lower metaperidotites. Ophiolites of the Kurtushibin belt in the western Sayan and Ilchir belt in eastern Sayan are examples of complete ophiolitic sequences with long and complicated evolution histories including development of a marianite-boninite series of primitive island-arc type. Asbestos, chromite, FeTi oxide, polymetallic and gold deposits occurring Precambrian ophiolites are briefly described. When ophiolites appeared about 2.0–1.9 Ga ago, Cr, Ni, Cu and asbestos were concentrated within them. The composition of ophiolites and associated ores changed significantly with time. A change in upper mantle convection may be the key to the magmatic and metallogenic evolution.

The composition of nickel‐copper sulphide deposits and their host rocks from the Cape Smith Fold Belt, Northern Quebec

Ni‐Cu sulphide deposits containing 2–4% Ni, 1% Cu and 2–3 ppm Pt + Pd occur within sills and flows formed from basaltic komatiite in the Cape Smith Fold Belt of Northern Quebec. The sulphide compositions in these deposits range from metal‐rich (10–15% Ni, 3–4% Cu and 10–20 ppm Pt+Pd) to metal‐poor (5–7% Ni, 1–2% Cu and 3–7 ppm Pt + Pd). Assuming that all of these sulphides formed from silicate magma of the same composition the difference in metal content of these deposits may be due to a larger weight fraction of sulphide having segregated to form the metal‐poor sulphides (0.3 vs 0.1 wt%). All of the sulphides have higher Pd/ Ir, Rh/Ir and Pd/Pt ratios and lower Ni/Cu ratios than the chilled margins of the flows and sills or of the pillowed basalts that overlie the deposits and contain 20% MgO. It is difficult to model the composition of the sulphides if the metal contents of the 20% MgO liquid are used. However, the composition of the sulphides may be modelled using the metal contents of the spinifex‐tex...

A 40Ar39Ar geochronological study of komatiites and komatiitic basalts from the Lower Onverwacht Volcanics: Barberton Mountain Land, South Africa

40Ar39Ar step-heating analyses have been carried out on domatiites and komatiitic basalts from the Komati Formation, the uppermost formation of the Tjakastad Subgroup in the Barberton Greenstone Belt. The results for the komatiites fall into two groups, which have integrated ages either greater than 3.3 Ga or less than 3.0 Ga. The most precise plateau ages in the first group correspond to an age of ∼3.49 Ga, which we take to be the age of metamorphism since the whole rocks analysed are composed of metamorphic minerals. Other samples in the first group appear to preserve a record of a later event at ∼3.43 Ga. Evidence of this younger age is also seen in the only komatiitic basalt displaying a stable age spectrum. The second group of komatiites (integrated ages less than 3.0 Ga) on a 40Ar39Ar correlation diagram suggest that a 3.3 Ga event may have introduced paleoatmospheric argon with a 40Ar39Ar ratio of ∼190 into some of the komatiites. Comparison of the results of detailed electron microprobe analyses with the Ca/K spectra obtained in the step-runs, suggests that tremolite is the mineral tenaciously recording the great ages of the komatiites, while hornblende is the dominant argon retainer in the komatiitic basalts. The argon ages are completely consistent with recently published U-Pb ages of zircons from the Barberton Greenstone Belt and suggest that the original metamorphism of the komatiites occurred shortly after their eruption.

Terrane accretion in the internal zone of the Ungava orogen, northern Quebec. Part 1: Tectonostratigraphic assemblages and their tectonic implications

The tectonostratigraphic record of the Ungava orogen contains evidence for the interaction of divergent, transform, and convergent plate boundaries over a &gt; 0.2 Ga period in the Early Proterozoic. Three principal tectonic domains are recognized: (1) autochthonous basement plutonic and supracrustal rocks of Archean age; (2) autochthonous and allochthonous sedimentary and volcanic units associated with a ca. 1.99–1.92 Ga rift-to-drift margin; and (3) "suspect' ' crustal components of a ca. 2.00 Ga ophiolite and ca. 1.90–1.83 Ga island-arc terrane. Domain 1 is the stratigraphic or structural basement to domains 2 and 3. Field relationships indicate that domain 2 units were accumulated on or near domain 1, whereas no tectonostratigraphic linkages are known between domain 3 and the others. Accumulation of continent-derived sedimentary rocks, continental tholeiites, komatiitic basalts, and mafic lavas equivalent to modern normal mid-oceanic ridge basalts (n-MORB's) in domain 2 is interpreted as recording continental rifting leading to the possible formation of oceanic crust. Pillowed basalts, sheeted dykes, and cumulate rocks suggest that the crustal portion of an ophiolite is preserved within domain 3. The ophiolite is interpreted as a "suspect" fragment of oceanic crust, which may represent an earlier rifted portion of domain 2, possibly juxtaposed by transform plate boundary motions. Within domain 3, calc-alkaline volcanic and plutonic units and sequences of volcanogenic and siliciclastic sedimentary rocks are thought to indicate the development of a magmatic arc and fore-arc basin and thus the establishment of a convergent plate boundary. The most internal lithotectonic packages in domain 3 are predominantly crystalline thrust sheets containing (i) an older intrusive suite, interpreted as the plutonic foundation of the magmatic arc, and (ii) younger plutons, interpreted as the result of a reversal in subduction polarity. The reversal may have followed attempted subduction of domains 1 and 2 beneath the upper-plate arc assemblage of domain 3.

Sargur-Dharwar Relationship Around the Komatiite-Rich Jayachamarajapura Greenstone Belt in Karnataka

Abstract New geological and preliminary geochemical data presented here from J ayachamarajapura (JC Pura) reinforce the idea of orogenic separation of two sequences across a deformed angular un confonnity . The JC Pura belt forms a major structural dome linking a trail of enclaves to the north and south. Its main lithology is altered komatiites preserving excellent polyhedral joints, pillows, ocelli and spinifex texture. These peridotitic komatiites are closely associated with the basaltic komatiites now represented by dark shiny amphibolites.Cherty quartzites and BIF form minor units. This association is typical of the older greenstones (Sargur Group) of Kamataka. These lithologies are overlain unconformably by the basal quartz-pebble conglomerate and associated amygdular metabasalt - crossbedded quartz arenite alternations constituting the Bababudan Group in the Kibbanahalli arm of Chitradurga schist belt. Kibbanahalli volcanics are mainly tholeiites as distinct from the dominant komatiites of the JC Pura belt. Such chemical distinction, but of a different kind, is reported recently from the Kalasapura area of Bababudan belt. JC Pura area therefore offers yet another spectacle of Sargur-Dharwar orogenic discrimination in Kamataka.

Spinifex basalts with komatiite-tholeiite trend from the Nansen-Gakkel Ridge (Arctic Ocean)

During cruise ARK IV/3 with RV Polarstern (1987) volcanic rocks were recovered from the Nansen-Gakkel Ridge (NGR), a slow spreading (half rate approximately 0.5 cm) ridge with an axial depth of more than 5000 m. The NGR is one of the slowest and deepest mid-ocean ridges so far known and calculations based on the distance of sampling location from the axial valley yielded ages of approximately 600 ka for the rocks investigated here. According to petrographic and geochemical results i.e. spinifex textures, mg > 70 and MgO > 9 wt.%, the volcanics are termed komatiitic basalts. Dark spherical droplets of basanitic composition within the komatiitic basalts are believed to be relicts of an incomplete magma-mixing whose basanitic end-member could well account for the enriched character of the NGR basalts in terms of rare earth elements, Ti and incompatible trace elements. Based on Nd-isotope as well as high Sm/Nd ratios, mantle metasomatism (i.e. veined-mantle model) could be responsible for the enrichment of incompatible trace elements in the source region of komatiitic basalts of the NGR.

Isovolumetric Silicification of Early Archean Komatiites: Geochemical Mass Balances and Constraints on Origin

Stratiform quartz-phyllosilicate-carbonate alteration zones are a common feature of Archean greenstone terranes, and determining their origin is an important aspect of understanding the nature of shallow crustal alteration during the early history of the earth. Komatiites and komatiitic basalts of the uppermost Onverwacht Group (ca. 3.4-3.5 Ga), Barberton greenstone belt, South Africa, were regionally metasomatized prior to major tectonic deformation to the assemblage: quartz + dioctahedral mica + chlorite dolomite. Subsequent metamorphism of these rocks has been restricted to greenschist facies, and they thus record processes that may be obscured in analogous sequences from more highly metamorphosed terranes. The isovolumetric nature of much of the alteration and the relative immobility of elements such as Al, Zr, and Y permit a quantitative assessment of elemental gains and losses during metasomatism. For each (one ton) of MgO were removed and added. There were also significant enrichments in K, Ba, and Rb, and the nearly complete removal of Fe, Ca, and Sr. Thermodynamic considerations support the hypothesis that the komatiites were altered by cooling, mildly-acidic fluids. Constraints imposed by the aqueous solubility of mobile components require in excess of the equivalent of of rock. Low-temperature, thermally-driven subseafloor convection of pore fluid provides a credible mechanism for producing this alteration. Local igneous sources of heat are not required, but convection could have been enhanced by the higher regional heat fluxes postulated for the Archean. There is no evidence to support the hypothesis that silicification was the result of subaerial exposure and weathering. The ultimate sources and sinks of the mobile components involved in metasomatism are not yet known but may have included rocks of the underlying Onverwacht Group and/or superjacent Early Archean sea water.

Iron‐sulfur mineralogy of Mars: Magmatic evolution and chemical weathering products

Models for the evolution of sulfide minerals on Mars and reaction pathways to their oxidative weathering products in Martian regolith have been proposed based on petrogenetic associations between komatiitic rock types, Viking geochemical data, SNC meteorites, and terrestrial Fe‐Ni sulfide deposits. To test the weathering model, komatiitic pyrrhotites and olivines were exposed to sulfuric acid solutions, with and without dissolved ferric iron added to simulate deep‐weathering processes, and the reaction products were identified by Mossbauer spectroscopy. Secondary FeS2 (pyrite or marcasite), FeOOH (goethite), and possibly jarosite were formed from pyrrhotite, while olivine was oxidized to nanophase goethite. These results suggest that on Mars, acidic groundwater induced pyrrhotite → FeS2 → FeOOH (+ jarosite) oxidative weathering reactions, particularly in the presence of dissolved Fe3+. Such gossaniferous materials occurring in Martian regolith were derived mainly from Fe‐Ni sulfides associated with komatiitic basalts and not from sulfides occurring in calc‐alkaline porphyry copper deposits, granitic hydrothermal veins, sediment‐hosted PbS‐ZnS ores, etc., which presumably did not evolve on Mars due to the virtual lack of plate tectonic activity there.

Magmatic and geotectonic evolution of a Proterozoic oceanic basin system: the Cape Smith Thrust-Fold Belt (New-Quebec)

The Cape Smith Thrust-Fold Belt (1999-1840 Ma, Parrish, 1989) is divided into two tectonostratigraphic domains separated by the major east-west Bergeron fault. In the southern domain, the Povungnituk Group lies on the Archean craton of the Superior Province. It contains a sedimentary sequence (Lamarche sub-group) grading to a continental LREE-enriched tholeiitic basaltic sequence (Beauparlant sub-group, MgO < 10%, TiO2 = 1.2–3.6%) that is locally overlain by phyllites and alkaline volcanic rocks. The Chukotat Group to the north includes several sequences evolving from olivine-phyric komatiitic basalts (MgO = 19−11 %, TiO < 0.9%) to pyroxene-phyric tholeiitic basalts (MgO = 12.5 − 7%, TiO2 = 0.8–1.1%) with moderately enriched to depleted LREE profiles. Finally, in its northernmost part, the Chukotat Group contains several sequences of LREE-depleted tholeiitic plagioclase-phyric basalts (MgO < 8%, TiO2 = 1.3–2.8%). The northern domain presents: (1) a vast dismembered ophiolitic complex (Watts Group) with plutonic peridotitic to gabbroic rocks, sheeted dykes and associated LREE-depleted tholeiitic basalts (MgO = 9.8-5.7%, TiO2 = 0.3–2.2%); and (2) a volcano-sedimentary sequence of shales, siltstones and greywackes intercalated by basaltic to rhyolitic calc-alkaline lavas and volcaniclastites (Parent Group). With respect to their structural, petrological and geochronological characteristics, the two tectono-stratigraphic domains are interpreted as having resulted from a rifting process with (1) the formation of an early oceanic crust (the Purtuniq ophiolite) in the northern domain, and (2) the subsequent opening of the more southern Povungnituk-Chukotat basin. A comparison with present-day rifting processes suggests that the younger Povungnituk-Chukotat basin evolved by an asymmetric rifting model implying: (1) the creation of a fault-bound basins into which shallow water sediments (Lamarche sub-group) accumulated; (2) the formation of an ensialic proto-rift into which LREE-enriched tholeiitic basalts of the Beauparlant sub-group were emplaced, with local emission of alkaline volcanic rocks on volcanic islands; (3) the progressive opening of an oceanic rift and subsequent eruption of weakly LREE-enriched then depleted olivine- and pyroxene-phyric basalts of the Chukotat Group; and (4) the formation of an oceanic crust characterized by the LREE-depleted plagioclase-phyric basalts of the Chukotat Group. Finally, in response to a north-south compression, the northern Chukotat basalts were probably subducted beneath the northern domain, thus initiating the development of a magmatic arc system (Parent Group).

The geology and gravity field in the central core of the Vredefort structure

The centre of the Vredefort structure in South Africa comprises an uplifted granitic basement with an associated 300 g.u. gravity high ringed by upturned sedimentary strata. Detailed geological mapping of the basement core has delineated a three-fold concentric zoning of Outer Granite Gneisses (OGG), Steynskraal Formation (SF) and Inlandsee Leucogranofels (ILG). An increase in metamorphic grade occurs toward the centre of the structure from greenschist relics on the basement perimeter to upper amphibolite and granulite facies in the SF and granulite facies in the ILG. A marked lithological change occurs within the SF in which perimeter greenschists and inner amphibolites give way to paragneisses. In the ILG, paragneiss and mafic granulite relics predominate. These divisions are consistent with the “standing-on-edge” model for the basement 1. Three regional metamorphic episodes and two thermal events have been recognised. M 1 and M 3 both attained granulite facies at low-medium load pressures in the ILG. M 1 granulite-facies assemblages extended into the OGG, but the M 3 facies was restricted to the ILG. M 2 was a K-metasomatism event limited to the OGG. An extensive thermal episode, M 4 a , is provisionally correlated with the collar metamorphism. Pyroxene hornfels facies ( T = ± 800 ° C) was attained in the centre as opposed to conditions of P = 4–5 kbar and T = 650° C for alkali granite stocks in the sedimentary collar 2. This event was thus pre- or syn-uplift, but not post-uplift. A high-level, high-temperature ( P < 3 kbar and T = 800°–900° C) event, M 4 b , occurred at a late stage in the overall sequence. A suite of amphibole-orthopyroxene rocks of basaltic komatiite type (CaO/Al 2O 3 ⩾ 1) are chemically related to younger high-Mg tholeiites. These tholeiites are correlated with similar rocks in the sedimentary collar and were intruded syn- to post- M 4 a in several episodes of decreasing basicity. “Shock” metamorphic features, granitic breccias and pseudotachylite are linked to the late-stage history of the structure and a case is presented for a dynamic metamorphic “front” advancing outward and upward from the centre in response to unroofing of the basement. The central gravity high 3 is delineated by 1239 new gravity stations at 1 km and 0.5 km intervals. A low-frequency oval-shaped anomaly with a long axis trending NW-SE has eccentrically positioned on it a plateau extending southwards as a spur. A dyke is responsible for the spur, but the source for the plateau is, from a recent borehole, thought to be largely due to sheared altered harzburgite. Modelling of the low-frequency gravity field component was based on the mapped divisions of the OGG, SF and ILG, and a density contrast of 200 kg/m 3 was determined between the OGG and the SF and ILG lithologies. A purely crustal solution for the excess mass distribution in the basement is conclusively demonstrated.

A Sm [sbnd]Nd and Pb isotope study of Archaean greenstone belts in the southern Kaapvaal Craton, South Africa

A Sm Nd and Pb study on a wide variety of lithologies in Archaean greenstone belt fragments in the southern Kaapvaal Craton reveals a complex petrogenetic history. The fragments are important because they represent a 350 km transect through the craton south of Barberton to its southern margin. The Commondale greenstone belt yields a precise Sm Nd age of 3334 ± 18Ma on an exceptionally well preserved peridotite suite of komatiitic affinity. The wide range of Sm/Nd from 0.6 to 1.0 is attributed to the unusual occurrence of orthopyroxene in the spinifex-bearing rocks. A considerably younger age of about 3.2 Ga is suggested for the Nondweni greenstone belt close to the southern margin of the craton on the basis of separate Sm Nd isochrons on individual lithologies ranging from komatiite, through komatiitic basalt and basalt to felsic volcanic rocks. The most precise Sm Nd age for the greenstone belt of 3208 ± 87Ma is based on a combined mineral-whole-rock isochron for komatiitic basalts, but the possibility cannot be excluded that this represents a later metamorphic age. On the basis of the present study the greenstone belts appear to have been emplaced at progressively younger ages towards the southern margin of the craton. Of greater significance is that irrespective of age, the individual lithologies of the Nondweni Sequence have isotopically distinct initial ratios. Combining isotopic data from these lithologies invariably yields apparent older ages. The isotopic characteristics are consistent with a model of progressive contamination by older sialic crust thought to be the basement to the Nondweni Sequence.

The early Archaean Nulliak (supracrustal) assemblage, northern Labrador

The Nulliak (supracrustal) assemblage, the remains of ca. 3800 Ma succession of volcanic and sedimentary rocks, was broken up by intrusion of the protoliths of the early Archaean Uivak orthogneisses and then deformed, metamorphosed, and variably metasomatised several times under upper amphibolite to granulite facies conditions in the Archaean. Amphibolites of "komatiitic basalt" and tholeiitic chemical affinity are the most important Nulliak assemblage lithologies. High Al2O3 metagabbroic rocks and anorthosites also occur. Interlayered with the amphibolites are marbles, calc-silicate rocks, and banded iron formation, interpreted as chemical sediments that were probably laid down in a shallow-water environment. Also found are felsic rocks probably derived by reworking of penecontemporaneous felsic volcanic rocks, and garnet- and sillimanite-bearing paragneisses derived from pelites. All these lithologies are randomly interlayered on a scale down to 1 m or less. The occurrence of 3850 – 3900 Ma cores for zircons in the surrounding polyphase Uivak gneisses suggests there may be an ancient sialic component in them, which could possibly represent basement upon which at least part of the Nulliak assemblage formed.

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.

Diapiric trondhjemites of the western Dharwar craton, southern India

Diapiric, comparatively massive trondhjemites were emplaced in the western part of the Dharwar craton of southern India about 3000 Ma ago. This age coincides with the age of (i) closure of Rb–Sr systems that now form the youngest isochrons in the predominantly gneissic terrane, and (ii) metasomatic enrichment of the gneisses in U. Younger events, principally about 2500 Ma ago, are recorded by sparse granites and by deformation of supracrustal sequences without major metamorphism. Thus, the trondhjemites appear to have formed during the last extensive thermochemical event in the craton. The trondhjemites are not associated with any mafic rocks and show very little fractionation within and among the various bodies.Presumably they formed as a single product of partial melting. Likely source materials for the magmas are rocks of basaltic composition (probably amphibolites) in the lower crust or along the crust–mantle boundary. Very low Zr and high Cr contents in the trondhjemites may indicate a slightly ultramafic (possibly basaltic komatiite) source. A lack of fractionation of Zr and Y and low light/heavy rare-earth element ratios in the trondhjemites may indicate an absence of equilibration of the magmas with major amounts of garnet. Lack of significant garnet equilibration could have resulted from production of hydrous magmas, either by partial melting of amphibolite or by introduction of water from an external source during the melting process.

Caractères géochimiques des formations birrimiennesdu supergroupe de Mako (Sabodala et ses environs)

The older suite being schistosed and metamorphosed as greenschist facies, is constitutedby basaltic pillow -lavas, alloclastic breccias and metasediments (metatuffites, metapelites, quartzites and limestones) associated with basaltic sills, metagabbros and metadolerites. Petrographically and geochemically the basaltic flow can be subdivided into a komatiitic basalt (type A) in the West and a tholeiitic basalt (type B) in the Eastof Sabodala locality. The younger being unmetamorphosed is constituted by small massives of gabbros, granodiorites, rhyolites, rhyodacites and andesites dykes found as intrusives and discordants in the older suite. This suite has a calco-alkaline affinity. This magmatic duality has been found in the Birrimian of West Africa, but the particularity in Senegal is the komatiitic affinity of volcanism. So the geological setting proposed by Tarney el al. (1976) for the archean greenstone belts is more consistent for the Birrimian of Senegal than the cratonic model proposed by Kröner (1984) which is suitable for Burkina Faso and Mauritania.