Komatiitic Basalts Research Articles

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 > 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.

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.

Geochemistry and origin of late Archean volcanics from the ventersdorp supergroup, South Africa

The Ventersdorp Supergroup (2700–2750 Ma) is comprised chiefly of mafic to intermediate subaerially erupted volcanics with smaller amounts of clastic sediments. The lower unit, the Klipriviersberg Group, consists of basal basaltic komatiites overlain by basalts which show a progressive increase in Mg number and Ni content and a decrease in incompatible element contents with increasing stratigraphic height. Volcanics of the overlying Platberg and Pniel Groups consist of basaltic andesites and andesites, and some felsic volcanics. All Ventersdorp volcanics exhibit a subduction zone geochemical component and are similar in incompatible element distributions to volcanics from continental-margin arc systems. Both tholeiitic and calc-alkaline trends occur in the Ventersdorp succession, with the former dominating. Crustal contamination appears to have played a minor role in the evolution of Ventersdorp magmas. With the exception of basaltic komatiite, the Klipriviersberg lavas can be related by progressive melting of a garnet lherzolite source (containing a subduction zone component) followed by up to 30% of shallow fractional crystallization. The basaltic komatiite can be produced from the same source at higher temperatures and the Pniel lavas by shallow fractional crystallization of basaltic komatiite. The Platberg volcanics must come from a within-plate enriched mantle source, and mafic and felsic members of this group can be related by shallow fractional crystallization. Geochemical and geologic data are consistent with a model for Ventersdorp magmas involving a subduction zone in which Klipriviersberg and Pniel magmas are produced from the mantle wedge and Platberg magmas come from an enriched mantle lithosphere with a superimposed subduction zone component. Secular trends in Klipriviersberg lavas can be explained by progressive adiabatic melting of an ascending plume rooted in the mantle wedge.

Basic lavas of the Archean La Grande Greenstone belt: Products of polybaric fractionation and crustal contamination

Despite the fact that some greenstone belts preserve the record of contemporaneous komatiitic and tholeiitic volcanism, a genetic link between the two is not widely accepted. The significance of a compositional gap seperating these magma types and differences in their respective degree of light rare earth element (LREE) enrichment, cited as evidence against a derivative relationship, are complicated by the possibility of crustal assimilation by magmas of komatiitic affinity. In the Archean La Grande Greenstone belt of northern Quebec a succession of metamorphosed tholeiitic basalts and younger, high-Mg, LREE-enriched andesites are preserved. The tholeiites are differentiated basaltic rocks whose chemical compositions appear to have been controlled by low pressure, gabbroic fractional crystallization and are similar to Type 1 MORB. Parental magmas were probably high-Mg liquids of compositions similar to komatiitic basalts which also occur in the greenstone belt. These high-Mg liquids are believed to be themselves the product of high pressure, OLIV+OPX fractional crystallization of more magnesian primary liquids of komatiitic composition. The higher La/Sm ratios of komatiitic basalts and tholeiites relative to komatiites in this belt, can be explained by small degrees of crustal assimilation. In the central part of the belt, late-stage, mafic igneous rocks have chemical compositions similar to Archean examples of contaminated volcanic rocks (e.g., Kambalda, Australia). These late-stage lavas consist of basalts and andesites with high-Mg, Ni and Cr abundances, LREE-enriched profiles and low Ti abundances. They are believed to be the products of crustal assimilation and crystallization of OPX-PLAG-CPX from high-Mg liquids of komatiitic affinity. The volcanic stratigraphy records the progressive effects of crustal contamination through time. A light sialic crust may have initially acted as a density barrier, preventing the eruption of primary high-Mg liquids and forcing fractionation at depth which produced more buoyant compositions. With subsequent thinning of the crust, the density barrier presumably failed, and primary liquids migrated directly toward the surface. Reaction of these liquids with tonalitic crust produced contaminated differentiates.

Recent developments in understanding the Archaean granitic basement of the Barberton Mountain Land and adjacent witwatersrand basin hinterland

Two komatiite emplacement styles are recognized in the Boorara Domain of the Kalgoorlie Terrane, Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia. (1) Large volume, dominantly cumulate-textured komatiite bodies, interpreted as high-level intrusions based on the presence of symmetrical spinifex-cumulate textural zonation, together with apophysy injection and xenolith incorporation along the top and basal margins of the komatiite. (2) Extrusive komatiite lavas exhibit asymmetrical textural zonation, with the top chilled flow margin preserving a fine-grained, fractured crust that coarsens downward into a supercooled former olivine spinifex zone and then into pseudomorphed olivine (now serpentine) orthocumulate texture above a thin basal chilled margin.The intrusive komatiite unit is observed to be intercalated with quartz- and feldspar-phyric dacite. The two lithologies exhibit primary irregular contacts against each other; komatiite apophyses have been injected into the dacite and magma mingling has occurred. These features are indicative of liquid–liquid interaction between the two units and komatiite magmatism is interpreted to have been contemporaneous with dacitic magmatism. The dacite was emplaced first and the komatiite intruded the dacite before it solidified.The reconstructed stratigraphy of the Boorara Domain comprises a basal unit of quartzofeldspathic turbidite beds, conformably overlain by high Mg tholeiite basalt, the intercalated dacite and komatiite units overlie the lower tholeiitic basalt with an unobserved contact, an upper komatiitic basalt unit overlies the komatiite and at the stratigraphic top of the sequence are felsic volcaniclastic turbidite units with felsic porphyry intrusions. These greenstone lithologies were emplaced within a deep submarine basin as indicated by carbonaceous metapelite units intercalated throughout the sequence.The tectonic-scale processes operating in, and around, the Boorara Domain depocentre include derivation of the komatiite unit from a likely mantle plume source. The contemporaneous dacite magmatism is interpreted to be the result of melting of pre-existing Archaean crust during plume ascension, although the exact composition of the primitive crust is yet to be determined.

Trace-element geochemistry of Archaean and Proterozoic rocks from eastern Karelia, U.S.S.R.

Several komatiite and komatiitic basalt occurrences have been found in the greenstone successions of the eastern part of the Baltic shield (Soviet Karelia), with both Archaean and Proterozoic ages being represented. Some flows exhibit internal layering with well-developed spinifex textures similar to those described from the classic komatiite provinces of Canada and Australia. Marked differences have been found in the trace-element geochemistry of these rocks. Some Archaean volcanics exhibit flat REE distributions with approximately chondritic ratios of Ca, Ti, Al, Zr, V and Y whereas others show depletion in LREE and Zr. Proterozoic komatiitic basalts differ from the Archaean ones by their relative LREE. Ba and Sr enrichment. These geochemical features are ascribed to an established heterogeneity in the Archaean mantle source and its further development by the Proterozoic. Major- and trace-element variations suggest relative immobility, during metamorphism, of all the analyzed elements except volatiles and alkalis. These trends being adequately explained by olivine with minor chromite fractionation.

Petrography and geochemistry of an unusual Fe-rich basaltic komatiite from Boston Township, northeastern Ontario

The Boston Creek Flow is a thick, Archean layered basaltic komatiite lava flow located 16 km south of Kirkland Lake, Ontario, within the Abitibi greenstone belt. It is superficially similar to the layered flows of Munro Township in that it consists of a basal pyroxenite overlain sequentially by peridotite, pyroxenite, gabbro, and pyroxene spinifex-textured rock and exhibits the typical cumulate komatiite crystallization sequence: olivine–clinopyroxene–plagioclase. The spinifex-textured unit is, however, unusually thick (33 m), and in comparison with other Archean lavas of similar magnesium content, the flow is Fe and Ti enriched and Al and Y depleted. CaO/Al2O3 and Al2O3/TiO2 ratios of 2–3 and 5, respectively, also distinguish it from other Archean magnesium-rich volcanic rocks.The preservation of typical tholeiitic chemistry in lavas underlying the Boston Creek Flow precludes metamorphism and metasomatism as explanations of the flow's enigmatic geochemistry. This is supported further by relict igneous textures exhibited by Ti-bearing oxides in the pyroxenite, gabbro, and spinifex-textured units of the flow. The presence of trellis-textured ilmenite lamellae in titanomagnetite, the textural evidence for symplectic intergrowth of titanomagnetite and clinopyroxene during crystallization, and the failure of assimilation models to explain the coupling of Fe and Ti enrichment with Al depletion suggest rather that the unusual geochemistry of the flow is a primary igneous feature. The Boston Creek Flow is more Al depleted and Fe and Ti enriched than Barberton and Minnesota Al-depleted komatiites (ADK) of the same magnesium number. It is therefore an Fe-rich, layered ADK.