Spinifex Textures Research Articles

Petrology and stratigraphy of some texturally well preserved thin komatiites from Kambalda, Western Australia

AbstractArchaean komatiite volcanics at Kambalda, Western Australia have been metamorphosed to upper greenschist–lower amphibolite grade and have experienced intense though heterogeneously developed polyphase deformation. Despite this, preservation of igneous textural features is often good, particularly in areas which underwent only ‘static style’ metamorphism. Thin lavas from the Tripod Hill Member of the Kambalda Komatiite Formation over the western margin of the Hunt nickel shoot display textural elements and facies variations which are virtually identical to those found in fresher thin komatiite sequences in other Archaean greenstone belts. Four principal flow profile (facies) types are defined, comprising nine subtypes. These represent stages in a facies continuum, ranging from ‘mature’ profiles which comprise thick spinifex textured tops and close packed cumulate bases through to massive, jointed ‘immature’ profiles devoid of mesoscopic spinifex texture. The causes of textural diversity within and between profiles are many and complex. However, facies variations can be attributed mainly to the effects of lava velocity at the time of major heat loss, combined with relative lateral position within any flow. The most mature textural (and geochemical) profiles developed in parts of lavas which had become ponded prior to major heat loss, whereas the least evolved profiles developed along the lateral margins (levees) of moving lavas. The study area komatiites occur as alternating stacks of flows of similar type. This stratigraphy records temporal and spatial shifts in the locus of lava ponding over the western margin of the Hunt nickel shoot. Such shifts may have been caused by irregularities in the underlying volcanic topography and/or by synvolcanic faulting and subsidence.

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.

Processus de fractionnement dans les coulées komatiitiques archéennes: cas des laves à spinifex de la ceinture de roches vertes de Tipasjärvi (Finlande orientale)

Komatiitic flows (ultramafic lavas with MgO > 18%) from the small Tipasjärvi Archean greenstone belt (eastern Finland) are of two types: (1) thin lava flows with microspinifex textures showing no chemical variation along a vertical cross section; (2) a thicker flow with important chemical variation in which macroscopic spinifex textures are found within small veins in the upper part of this flow. Two different trends of fractional crystallization can be demonstrated by major- and some trace-element variations in the upper and the lower parts of the flow: olivine crystallizes and settles to the base of the flow; in the upper part, the major phase that crystallizes seems to be clinopyroxene.Such double-trend fractionation has also been shown in some other komatiitic flows of similar ages from Australia and Canada. However, some komatiitic flows indicate a simpler fractionation process involving olivine only.Rare-earth-element (REE) distribution patterns in the thick Finnish flow show two quite different patterns. The upper part of the flow has strongly LREE-depleted patterns, whilst those in the lower part of the flow are flat. It is suggested that the flow is composite and that two source regions are involved.

Volcanic Geochemistry of the Northern Segments of the Circum-Superior Belt of the Canadian Shield

Summary The northern segment of the early Proterozoic Circum-Superior Belt extending from the Ungava region of Quebec (Cape Smith Belt) through the islands of eastern Hudson Bay to northeastern Manitoba (Fox River and Thompson Nickel Belts) is distinguished by an extraordinary magmatic assemblage composed of two principal suites; komatiitic and tholeiitic. The older tholeiitic suite comprising basaltic flows and dolerite sills associated with a stable-shelf sedimentary sequence lies next to the Superior Craton in the Ungava-Hudson Bay region and is adjoined on its outboard side by the komatiitic suite of predominantly pillowed lavas. Their relationships are obscured by faulting but the presence of sills of the komatiitic suite at a number of levels in, and folded with, the tholeiite-sedimentary assemblage is evidence of its younger age. In northeastern Manitoba a discrete tholeiitic suite is absent and komatiitic flows and sills, in part differentiated to tholeiitic compositions, are associated directly with siliceous sediments of apparent continental provenance. The komatiitic suite differs from its Archaean counterparts in that magma compositions rarely exceed about 16–18% MgO, yet spinifex texture (albeit clinopyroxene-olivine) like that of Archaean komatiites is one of its distinctive features. It is typically developed in the upper parts of certain layered (differentiated) flows that are otherwise little distinguished from the majority of layered flows common to the komatiitic sequence. The tholeiitic suite is little differentiated but the komatiitic suite ranges from olivine- to clinopyroxene- and in places to plagioclase-phyric lavas through both long- and superimposed short-range stratigraphic intervals. Differentiation in the komatiitic suite is effectively represented by declining MgO/CaO ratios through the olivine fractionation stage (to about 1) then by increasing contents of oxides of the less refractory minerals, notably A1 2 O 3 , through clinopyroxene and clinopyroxene-plagioclase fractionation. The short-range variation is attributable to fractionation in the layered flows which are interpreted as principal surface distributors of lava and the long-range variations to major high-level sills as represented by those exposed in the Fox River Belt.

Liquid immiscibility in the Archean Greenstone Belt of Piumhi (Minais Gerais, Brazil)

Liquid immiscibility has been used for explaining several modern situations either in effusive or in intrusive rocks. Liquid immiscibility is also proposed for explaining the variolitic structures in the Archean rocks of the Abitibi Greenstone belt, Ontario. The volcanic sequence of the lower portion of the Greenstone belt of Piumhi (Minas Gerais, Brazil) shows the occurrence of variolitic structure, that may have been formed by liquid immiscibility. This possibility is supported by the following points: (a) the contact between varioles and matrix is very sharp; (b) spinifex texture occurs both in the varioles and in the matrix and the single crystal may cross the varioles; (c) the varioles and the matrix show contrasting compositions: the latter has the composition of a basaltic komatiite and the former varies form that of an alkali-basalt to andesite; and (d) a test by a thermodynamic model from literature shows that, although with some doubt, the free energy of the possible parent liquid is higher than that of the two unmixed liquids.

Spinifex and swirling olivines in a komatiite lava lake, Munro Township, Canada

Spinifex veins, and veins filled with swirling, tabular olivine grains, intrude the upper part of a 120 m thick lava lake in Munro Township, Ontario, Canada. These veins are 20–200 cm wide, tens of metres long and are oriented roughly parallel to the upper surface of the lava lake. The spinifex texture differs from that found in simple layered komatiite flows: rather than increasing downwards, the size of skeletal olivine grains is greatest in the centres of the veins, or varies irregularly. Upper contacts meander and veinlets penetrate overlying rock; lower contacts are transitional. The other type of vein contains 50–70% of ∼5 mm long hopper olivine grains whose orientation varies widely to produce a swirling, contorted fabric. Both types of vein are believed to have formed when lava from the centre of the lake flowed into fractures in the solidified crust. The lava lake has a dunitic lower portion and an olivine porphyry upper portion. The composition of chill samples, the compositions of olivine phenocrysts, and variations in major element abundances throughout the unit are used to show that the lava originally was komatiite with about 22% MgO. The lava lake is overlain by mafic scorias and breccias with moderate vesicularity and welding structures suggesting eruption in shallow water.

Archaean flow-top alteration zones formed initially in a low-temperature sulphate-rich environment

The 3,500–3,400 Myr Onverwacht Group in the southern part of the Barberton Greenstone Belt, South Africa, includes up to 12 km of komatiitic and tholeiitic lavas containing minor sedimentary and felsic volcanic units1. Komatiitic lavas showing spinifex textures, chrome spinels and high Ni and Cr contents are common throughout the sequence2. The rocks have been subject to polyphase Archaean deformation and low-grade metasomatism2–4. At several positions within the Onverwacht Group (Fig. 1), the tops of komatiitic lavas underlying chert beds are marked regionally by 1–50-m thick zones of silicification, carbonatization and K2O enrichment (Fig. 2). Because these zones have been variously interpreted to represent the felsic tops of mafic-to-felsic volcanic cycles, weathering horizons and shear zones, their origin is germane to any interpretation of the stratigraphy, structure and evolution of the Barberton belt. Here we present evidence that they formed inlarge part by low-temperature, near-surface alteration of komatiitic lavas during intervals of volcanic quiescence. They do not mark fault zones and cannot be used to subdivide the greenstone belt sequence into a series of fault-bounded structural slices.

Emplacement and cooling of komatiite lavas

We propose that komatiite lavas were emplaced as turbulent flows, accompanied by vigorous forced convection with cooling rates often in excess of hundreds of °C h−1. They melted and assimilated up to 10% of the ground over which they flowed, forming deep channels. Nickel sulphide mineralization may have resulted from incorporation of sulphur-rich sediment. After emplacement, high cooling rates persisted, resulting in spinifex textures due to compositional convection.

Ultramafic flows of the Crixas greenstone belt, Goias — Brazil

Abstract The Crixas greenstone belt, together with the Guarinos and Pilar belts, has a basal, intermediate and an upper unit consisting of extrusive and intrusive ultramafic, mafic and sedimentary rocks, respectively. The Crixas belt, the main object of this study, has a basal unit formed of peridotitic and pyroxenitic lava flows intercalated with iron formation, chert, graphite schist, mafic and acid tuffs, and intrusive bodies of pyroxenite and gabbro. The ultramafic unit becomes thicker towards the south end of this belt and it is there that the flows are best exposed, particularly along the Alagadinho creek, where detailed mapping was carried out. Three types of ultramafic flows were recognized: flows with olivine spinifex texture in the central zone, flows with polyhedral jointing at the base, clinopyroxene spinifex texture towards the top and flows without spinifex, but showing polyhedral jointing throughout. The flows are composed of olivine, clinopyroxene, glass and opaques. Greenschistfacies metamorphism, together with deformation, have obliterated part of the primary volcanic textures and structures in these rocks, and partially changed olivine to serpentine and chlorite, clinopyroxene to tremolite, and glass to a very fine chlorite. Preliminary chemical work suggests that the ultramafic rocks are komatiites, characterised by MgO > 20%, CaO/Al 2 O 3 > 1 and low TiO 2 , Na 2 O and K 2 O; these compositions coupled with the presence of spinifex texture indicate an origin from an ultramafic liquid. The flows from Crixas represent the first recognition of spinifex-textured komatiites in the central-western part of Brazil.

REE geochemistry of komatiites and associated rocks from Piumhi, southeastern Brazil

Komatiitic rocks with spinifex textures have recently been found in two areas in Brazil: (1) in the vicinity of Piumhi, State of Minas Gerais and (2) in the Goias Central Massif. This adds to the notion of a world-wide occurrence of komatiites in most, if not all, Precambrian shields. This report documents major element compositions and rare earth abundances in komatiites and associated rocks from the Piumhi Massif, of probable Archean age. The komatiites are found to have flat heavy REE and somewhat enriched light REE distribution patterns. Basaltic komatiites have near-chondritic Al 2O 3/TiO 2 ratios. However, their low CaO/Al 2O 3 ratios (∼ 0.6) suggest that these rocks might have lost CaO by alteration, or might have undergone clinopyroxene fractionation. Modal calculations indicate that the LREE enrichment more likely reflects mantle source characteristics, and is also consistent with the LREE-enriched pattern in peridotic komatiite. Komatiites and associated rock types including tholeiites, minor andesites, sedimentary sequences and granitic intrusions in the Piumhi Massif are entirely comparable with those of typical Archean greenstone—granite terrains. However, the preliminary results of SmNd isotopic analyses suggest that they were emplaced in the Proterozoic, ∼ 1.84 Ga ago (Appendix).

The petrogenesis of the Lac Guyer komatiites and basalts and the nature of the komatiite-komatiitic basalt compositional gap

A sequence of ultramafic rocks in the Lac Guyer Archean greenstone belt exhibit brecciated flow tops, pillow structures, and spinifex textures testifying to their volcanic origin. Massive, spinifex-textured and differentiated flows in the sequence have the chemical characteristics of peridotitic komatiite, with MgO ranging from 19–25 wt.%. Associated pillowed flows have compositions that straddle the conventional boundary between komatiite and komatiitic basalt with MgO contents ranging from 16 to 19 wt.% MgO and are best termed pyroxenitic komatiites. Unlike other komatiitic occurrences, the peridotitic and pyroxenitic komatiites at Lac Guyer constitute a continuous chemical spectrum with no evidence of population minimum near 18 wt.% MgO. The contrasting behaviour of highly compatible elements, such as Ni and Cr, versus incompatible elements, such as Zr, indicate that this compositional spectrum was produced by a variation in the extent of partial melting (10–40%) of a garnet lherzolite source in the Archean mantle. The pyroxenitic komatiites represent liquids produced during lower (10–20%) degrees of melting during which garnet remained in the mantle residue. However, a change in slope in the distribution of Zr vs. Y between the pyroxenitic and the peridotitic komatiites indicates that garnet was completely consumed at the more extensive degrees of melting which produced the peridotitic komatiites. The Lac Guyer volcanic rocks display a population minimum at 15 wt.% MgO separating komatiitic magmas whose compositions are controlled by partial melting from basalts whose composition is controlled by crystal fractionation. The population minimum near 18 wt.% MgO which is taken as the boundary between komatiite and komatiitic basalt may have a similar origin.

Petrology and shock metamorphism of Pampa del Infierno chondrite

Pampa del Infierno, an L6 chondrite, displays strong evidence of impact metamorphism. Rare chondrules and two types of dark-colored clasts occur in a light-colored matrix. Granular clasts are similar in mineralogy and chemistry to the host meteorite, but display shock metamorphic features, produced mainly by deformation, such as mosaicism, undulatory extinction, and fracturing. Partial melting in the granular clasts is manifested by the presence of selvages of mafic glass with troilite-iron eutectic intergrowths around remnants of low-Ca pyroxene and plagioclase glass with skeletal poikilitic inclusions of olivine. Clasts with spinifex texture are believed to have crystallized from a supercooled, impact-generated, ultramafic melt of the host chondrite or a chondritic source of similar composition. The light-colored matrix mainly displays evidence of shock metamorphism under subsolidus conditions as manifested by kinking and deformation twinning in pyroxene; high-pressure phase transitions of olivine and low-Ca pyroxene to ringwoodite and majorite, respectively; and lineation that still preserves the deformation features in the different mineral phases. Pertinent shock-wave data used to interpret the metamorphic history of the Pampa del Infierno chondrite suggest metamorphism by impact at a minimum peak pressure greater than 300 kbar.

Komatiites: An early precambrian phenomenon

Using the presently accepted definition, komatiites are ultramafic volcanic rocks characterised by bladed and/or skeletal olivine texture (spinifex texture). Where textural evidence is absent,the geochemical definition is equivocal and as a result there is considerable ambiguity, particularly among the basaltic types. If the added control of Al 2O 3/TiO 2 ⩽ 25 is added to the definition, this eliminates rocks of the boninite group and also the low-Ti basalts found in some ophiolite complexes. The bulk of komatiites occur in the Archaean and the most obvious explanation is that the eruption of high (up to 30%) MgO melts was controlled by an enhanced geothermal gradient. There is some evidence to suggest that the high CaO/Al 2O 3 ratio characteristic of some komatiites (e.g. Barberton) is found only in those greenstone belts older than 3.0 b.y.

Archaean and Lower Proterozoic units in Central Brazil

The following main units have been recognized as Archaean basement rocks in central Brazil: a) High-grade granite-gneiss terrain composed predominantly by granodioritic and tonalitic gneisses; these rocks are cut by amphibolite dykes and present several nucleii locally surrounded by an array of Archaean greenstone belts, b) Greenstone belts presenting basal komatiitic ultramafic-mafic lavas with abundant spinifex textures, followed by basalts and chemical sediments such as cherts, limestones and graphitic pelites, that are often interbedded with acid and intermediate volcanics; metamorphism is within greenschist facies. Granulite mobile belt, including the granulitized mafic-ultramafic complexes of Barro Alto, Niquelândia and Brava. The Lower Proterozoic is represented by: a) metasedimentary sequences overlying the granite-greenstone terrains, with several tin bearing granite intrusions. b) Gabbro-anorthosite layered plutons and their associated mafic volcanics which are followed by acid and intermediate volcanics and ocean floor sediments. c) Mafic-ultramafic layered intrusions in the granulite terrain.

Geochemical characteristics of the Crixás greenstone belt, Goiás, Brazil

Geochemical data (major elements and Ni, Cr, Co, Cu, Pb. Zn, V, and Zr) are presented for the Crixas Greenstone Belt (Goias, Brazil). The chemical composition of the ultramafic flows, displaying spinifex texture, as well as that of the pillow-lava structured metabasalts of Crixas, indicate a komatiitic nature. The granitic archean rocks show a potassium enrichment (progressive decrease of the Na/K ratio) during their evolutionary history. Iron formation (Lower Metasedimentary Unit) and metachert intercalated with tale and chlorite schists (Upper Ultramafie Unit) exhibit gold and arsenium anomalies. In both sequences the highest gold concentrations are located in the hinge line region of folds.

Archaean ultramafic rocks with relict spinifex textures in the Umanak area, central West Greenland

Conspicuous olivine and serpentine textures were found in a number of ultramafic bodies in the Umanak area while it was being mapped on a scale of 1:20000. The rocks in the area mapped by the writer are mainly a suite of quartzo-feldspathic gneisses (Pulvertaft et al., this report) in which there are distinct amphibolite-dominated supracrustal horizons which enable the major structures of the area to be recognised. The structure which dominates the area is a large recumbent/rec1ined fold plunging ESE at a low angle and closing to the NNE. Isoclinal folds belonging to an earlier phase, and late open folding have also been recognised in the area.

Geochemistry of some ultramafic komatiite lava flows from the Barberton Mountain Land, South Africa

Thin ultramafic flows from the Komati Formation in the Barberton Greenstone Belt show striking textural variations similar to those found in some ultramafic flows from Munro Township, Ontario and Western Australia. The spinifex textured A2 zone has been divided into an A2r zone consisting of short randomly oriented olivine blades with vesicles in the interblade areas and an A2p zone made up of booklets of olivine blades forming a ‘thermotactic’ spinifex texture. The composition of the A2r zone is considered to be representative of the initial ultramafic liquid, while the variation in the composition of the A2p zone reflects the evolving residual liquid composition as the flow crystallized. Detailed modelling of major and trace element data obtained from a 3.1 m thick flow demonstrates that olivine fractionation and accumulation after extrusion probably generated the observed compositional range (25.7–31.7 wt.% MgO). The Ni content of olivine cores analysed from the B zone of this flow are consistent with these crystals having existed in equilibrium with a liquid composition represented by the random spinifex textured A2r zone. A DCr (Cr % in olivine/Cr % in ‘liquid’) of 0.5 found for these olivine crystals suggests that the high Cr content of olivines from Archaean ultramafic lavas is due to the very high Cr content of the liquids from which they crystallized. The textural and chemical data are combined to develop a cooling and crystallization model for this flow.

Tertiary or Mesozoic komatiites from Gorgona Island, Colombia: Field relations and geochemistry

An exceptional occurrence of ultramafic lavas within the volcanic member of the Mesozoic (or younger) Gorgona Igneous Complex represents the first known komatiites of post-Precambrian age. Gorgona komatiites are virtually unaltered and display typical spinifex textures, with 7–10 cm long plates of olivine (Fo 88 to 91) surrounded by acicular aluminous augite, subordinate plagioclase (An 56 to 78), basaltic glass, and two spinel phases. The MgO contents of the komatiites range from 15 to 22 wt.%. Sr and Nd isotopic compositions are indicative of depletion of incompatible elements in the mantle source region, as is the case for “normal” mid-ocean ridge basalts. The komatiites are low in total REE abundances and extremely depleted in LREE. They represent primary melts generated by high degree of partial melting of the mantle. Eruption temperatures are estimated at 1,450° to 1,500° C.

Lavas ultrabásicas da unidade basal do greenstone belt de Crixás (GO): uma nova classe de rochas ultrabásicas no Estado de Goiás

The basal unit of the Crixas Greenstone is constituted of peridotitic and pyroxenitic flows intercalated with iron formation, chert, maficand acid tuffs and intrusive bodies of piroxenite and babbro. At the south end of the Crixas belt, the ultrabasic unite becomes thicker and the flows are best exposed, particulary along the Alagadinho Creek where detailed mapping was carried out. Three types of ultrabasic flows were recognized: flows with spinifex texture (olivine) in the medium zone; flows with polyedral jointing and spinifex texture (clinopyroxene) in the upper zone; flows with polyhedral jointing. The ultrabasic flows are composed of olivine, clinopyroxene, glass and opaques. The metamorphism of greenschist facies, together with deformation, have destroyed most of the primary volcanic textures and structures and changed partially olivine to and chlorite, clinopyroxene to tremolite-actinolite and glass to very fine chlorite. A preliminary chemical work sugests that these ultrabasic rocks belong to the komatiitic suite, characterized by the ratio CaO/Al 2 O 3 ≥ 1 and low TiO 2 , Na 2 O and K 2 O. The identification in the greenstone of Crixas, of komatiitic peridotite, which represents a new of ultrabasic rock in the Central Western Part of Brazil, implies a reevaluation of the serpentine belt and ·alpine type ultramafic rocks of that region.

PETROLOGIA DAS LAVAS ULTRABÁSICAS E BÁSICAS DA SEQUÊNCIA VULCÂNO-SEDIMENTAR MORRO DO FERRO. FORTALEZA DE MINAS (MG)

In this work the petrology of the Archean Morro do Ferro volcano-sedimentary sequence is being defined. The sequence is located in southwest part of the Sao Francisco Craton in the vicinity of the Fortaleza de Minas Village. The basal part of the Greenstone belt of Morro do Ferro is constituted of ultramafic flows with spinifex texture intercalated with aluminous tuffs, and iron formation. The medium part is formed of massive basaltic flows, now represented by tremolite-chlorite-epidote schist associated with reworked volcanic rocks, iron formation and talc schist lenses. The petrological studies indicated a fractionation trend wich starts with rocks of komatiitic afiliation (peridotite, pyroxenite, basalt) to rocks of tholeiitic composition (basalt). The fractionation is caracterized by a decrease in MgO content, a constant CaO/Al 2 O 3 ration, that reflects the olivine crystalization in the initial stages, followed by clinopiroxene and plagioclase.