Spinifex Zones Research Articles

Major, trace and platinum-group element geochemistry of Mesoarchean komatiites from the Banasandra greenstone belt, Western Dharwar Craton (southern India): implications for multi-stage plume magmatism in generating compositionally diverse komatiites

ABSTRACT The early Archean Al-undepleted komatiites (AUK) from two locations (Kunikenahalli, Kodihalli) of the Banasandra greenstone belt in the Western Dharwar Craton are studied for understanding their petrogenesis. The field distribution, mineralogy and geochemistry indicate their cogenetic nature. The samples are mainly cumulates (MgO: 37–45 wt%), now completely serpentinized, while the spinifex zone is obscure. Using geochemical modelling, the MgO content of the primary undifferentiated magma is estimated to be nearly 25 wt%. The depleted LREE values from this study, and the positive εNd values from previous studies suggest that the komatiites are derived from a depleted mantle source. Trace-element and PGE geochemistry eliminate the possibility of crustal contamination and sulphide-saturation in these komatiites after emplacement. Modelling results estimated in this study suggest ~45% melting of a deeper mantle (~6 GPa) to generate the komatiites. Fractionation of olivine (Fo94) from the parental melt and their accumulation formed the mesocumulates in Kodihalli, and the more fractionated orthocumulates in Kunikenahalli. The compositional spectrum from AUK through Al-depleted (ADK) to Al-enriched (AEK) varieties in the Banasandra greenstone belt reported in this study and others, are proposed to be a product of melting of a single mantle plume at different depths.

Mineralogical and petrographic features of metakomatiites of the Kostomuksha greenstone structure (Karelia)

Object of study. The paper presents the mineralogical and petrographic study results of metamorphosed and metasomatized komatiites and komatiitic peridotites from the Ozerki soapstone deposit and Pentinsuo prospect, located in the Kostomuksha greenstone structure of the Karelian Craton, Fennoscandian Shield. Material and methods. Surface and drill core samples of various mineral and structural-textural varieties of altered komatiites were studied by optical microscopy, electron microscopy with an attachment for microanalysis, XRD phase, thermogravimetric and chemical analysis. Results. Soapstone formation in the investigated localities displays a multistage alteration and associates with the superimposed alteration of Mg-rich metakomatiite flows and olivine cumulates due to influx of carbon dioxide bearing fluids enriched in calcium and potassium. The chemical composition of initial komatiite (MgO content of the rock) and the degree of its fracture intensity are the main factors controlling soapstone formation. In differentiated lava flows soapstone is formed mainly in high-magnesium cumulate zones characterized by a high content of serpentine. In the flow top and spinifex zones an early amphibole-chlorite-magnetite mineral association is preserved in varying degrees. Talc and carbonate are formed by the decomposition of serpentine and amphibole. The chlorite content in soapstone is controlled by the Al2O3 concentration in the respective flow zones. Conclusion. The carbonate-chlorite-talc is the general natural type of soapstone associated with thin komatiite lava flows of distal volcanic facies. In more thick proximal lava flows the formation of soapstone of chloritecarbonate-talc and talc-carbonate composition is possible. The most prefered prospecting areas for soapstone are fields proximal to the eruptive vent, main lava conduits and subvolcanic analogues of komatiites.

Petrography and geochemistry of Mesoarchaean komatiites from the eastern Iron Ore belt, Singhbhum craton, India, and its similarity with ‘Barberton type komatiite’

The Mesoarchaean supracrustals of the Gorumahishani–Badampahar belt, eastern India record sedimentation–volcanism like most other contemporary greenstone belts over the world. The current study reports unambiguous komatiitic rocks from Tua-Dungri hill, Gorumahishani–Badampahar belt, Jharkhand and presents a petrological and geochemical inventory of these very interesting rocks. The Tua-Dungri komatiites are characterised by a well distinguishable cumulate, platy and random spinifex zone. These Tua-Dungri komatiites are rich in SiO2 (47–50wt%) like Barberton type komatiite or modern day boninite. Their Al depleted nature (Al2O3=1.36–2.95wt%) with very low Al2O3/TiO2 (3.4–6.5) and high CaO/Al2O3 (2–3), high LREE/HREE ratios show further resemblance with the Barberton komatiite. The Tua Dungri komatiite data along with published geochemical, sedimentological and stratigraphic data from the Iron Ore Group of rocks suggest mantle plume activity during the Mesoarchaean on the Singhbhum craton.

Chromite in komatiites: 3D morphologies with implications for crystallization mechanisms

High-resolution X-ray computed tomography has been carried out on a suite of komatiite samples representing a range of volcanic facies, chromite contents and degrees of alteration and metamorphism, to reveal the wide range of sizes, shapes and degrees of clustering that chromite grains display as a function of cooling history. Dendrites are spectacularly skeletal chromite grains formed during very rapid crystallization of supercooled melt in spinifex zones close to flow tops. At slower cooling rates in the interiors of thick flows, chromite forms predominantly euhedral grains. Large clusters (up to a dozen of grains) are characteristic of liquidus chromite, whereas fine dustings of mostly individual ~20-μm grains form by in situ crystallization from trapped intercumulus liquid. Chromite in coarse-grained olivine cumulates from komatiitic dunite bodies occurs in two forms: as clusters or chains of euhedral crystals, developing into “chicken-wire” texture where chromite is present in supra-cotectic proportions; and as strongly dendritic, semi-poikilitic grains. These dendritic grains are likely to have formed by rapid crescumulate growth from magma that was close to its liquidus temperature but supersaturated with chromite. In some cases, this process seems to have been favoured by nucleation of chromite on the margins of sulphide liquid blebs. This texture is a good evidence for the predominantly cumulus origin of oikocrysts and in situ origin of heteradcumulate textures. Our 3D textural analysis confirms that the morphology of chromite crystals is a distinctive indicator of crystallization environment even in highly altered rocks.

Hydrous komatiites from Commondale, South Africa: An experimental study

This study examines the emplacement conditions of komatiites in the 3.33 Ga Commondale Ultramafic Suite in South Africa. The komatiites of Commondale are unlike any other komatiites in both their physical structure and chemical nature. Komatiite unit chill margins preserve original komatiite liquid compositions with an Mg# of 0.91, MgO = 31.9 wt.%, Al 2O 3 wt.%/TiO 2 = 80 (wt.%), and SiO 2 content of 49.7 wt.%. A common feature throughout the komatiite sequence is the presence of orthopyroxene spinifex, where original orthopyroxene crystals are still preserved. The compositional information preserved in the most primitive of the natural pyroxenes present in these spinifex zones (Mg# = 0.92), provides insight into the original emplacement conditions of the komatiites. This study used anhydrous and hydrous equilibrium experiments, along with disequilibrium cooling-rate experiments, to quantify the crystallization conditions of the Commondale komatiites. The anhydrous, 1-atm liquidus was found at 1550 °C, with Fo97 olivine being the initial crystallizing phase, followed by spinel and then by protoenstatite, Mg# 0.95, at 1335 °C. The phase relations were also examined at 200 MPa under H 2O saturated conditions. The addition of ~ 4 wt.% H 2O lowers the appearance temperature of the initial pyroxene by 210 °C, thereby producing orthopyroxene with a Mg# closer to that of the most primitive preserved orthopyroxenes found in the komatiites. Additionally, dynamic cooling-rate experiments show that the natural pyroxenes preserve a chemical signature indicative of crystallization and cooling within an inflated flow complex. Estimates of the pre-eruptive H 2O content for the Commondale komatiites are between ~ 2 and 4.3 wt.% H 2O in the liquid. This range is similar to that estimated for 3.5 Ga komatiites of the Barberton Mountainland and may indicate formation of both suites in similar tectonic environments.

Endogenous growth in channelized komatiite lava flows: evidence from spinifex-textured sills at Pyke Hill and Serpentine Mountain, Western Abitibi Greenstone Belt, Northeastern Ontario, Canada

Spinifex-textured sills (i.e., veins) characterized by komatiitic magmas that have intruded their own volcanic-piles have long been recognized. For instance, in the early 1970s, Pyke and coworkers, in their classic work at Pyke Hill in Munro Township, noted that not all spinifex-bearing ultramafic rocks formed as lava flows, rather some were clearly emplaced as small dikes and sills. Several hypotheses have been proposed to explain spinifex-textured sills: intrusion into a cold host, filter pressing, or drainage of residual liquid. However, these do not satisfactorily explain the phenomenon. Field and petrographic observations at Pyke Hill and Serpentine Mountain demonstrate that spinifex-bearing komatiite sills and dikes were emplaced during channel inflation processes when new magma was intruded into a cooler, semi-consolidated but permeable cumulate material. Komatiitic liquids were intruded into the olivine cumulate rocks near the boundary between the spinifex and the cumulate zones of well-organized to organized komatiite flows. Spinifex-textured sills are generally tabular in morphology, stacked one above another, with curviplanar contacts sub-parallel to stratigraphy. Some sills exhibit complex digitated apophyses. Thinner sills typically have a random olivine spinifex texture similar, though generally composed of coarser crystals, to that of komatiite lava flows. Thicker sills exhibit more complex organization of their constituent crystals characterized by zones of random olivine spinifex, overlying zones of organized coarse spinifex crystals similar to those found in lava flows. They have striking coarse dendritic spinifex zones composed of very large olivine crystals, up to several centimetres long and up to 1 cm wide that are not observed in lava flows. Typically, at the sill margins, the cumulate material of the host flow is composed of euhedral to subhedral olivine crystals that are larger than those distal to the contact. Many of these margin-crystals have either concentric overgrowth shells or dendritic olivine overgrowths that grew from the cumulate-sill contact toward the sill interior. The dendrites grew on pre-existing olivine cumulate at the contact in response to a sharp temperature gradient imposed by the intrusion of hot material, whereas the concentric overgrowths formed as new melt percolated into the unconsolidated groundmass of the host-flow cumulate material. Spinifex-textured sills and dikes occur in well-organized to organized flows that are interpreted to have formed by “breakouts” above and peripheral to lava pathways (channels/conduits) as a result of inflation that accompanied voluminous komatiitic eruptions responsible for the construction and channelization of komatiitic flow fields. The spinifex-textured dikes and sills represent komatiitic lava that was originally emplaced into the channel roof during periods of episodic inflation that resulted in lava breakouts and was subsequently trapped in the “roof rocks” during periods of channel deflation. Accordingly, the occurrence of spinifex-textured sills and dikes may indicate proximity to, and aid in the identification and delineation of lava channel-ways that could potentially host Ni–Cu–(PGE) mineralization within komatiitic lava flow-fields.

Contemporaneous ultramafic and felsic intrusive and extrusive magmatism in the Archaean Boorara Domain, Eastern Goldfields Superterrane, Western Australia, and its implications

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.

Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: inflation of submarine lavas and origin of spinifex zones

Komatiites of the 3.5-Ga Komati Formation are ultramafic lavas (>23% MgO) erupted in a submarine, lava plain environment. Newly discovered vesicular komatiites have vesicular upper crusts disrupted by synvolcanic structures that are similar to inflation-related structures of modern lava flows. Detailed outcrop maps reveal flows with upper vesicular zones, 2–15 m thick, which were (1) rotated by differential inflation, (2) intruded by dikes from the interior of the flow, (3) extended, forming a flooded graben, and/or (4) entirely engulfed. The largest inflated structure is a tumulus with 20 m of surface relief, which was covered by a compound flow unit of spinifex flow lobes. The lava that inflated and rotated the upper vesicular crust did not vesiculate, but crystallized as a thick spinifex zone with fist-size skeletal olivine. Instead of representing rapidly cooled lava, the spinifex zone cooled slowly beneath an insulating upper crust during inflation. Overpressure of the inflating lava may have inhibited vesiculation. This work describes the oldest vesicular komatiites known, illustrates the first field evidence for inflated structures in komatiite flows, proposes a new factor in the development of spinifex zones, and concludes that the inflation model is useful for understanding the evolution of komatiite submarine flow fields.

Komatiite hosted Hunters Road nickel deposit, central Zimbabwe: physical volcanology and sulfide genesis

The nickel sulfide bearing Main Flow at Hunters Road is a thick komatiite unit situated at the base of a well developed lava channel complex overlying a prominent banded iron formation that caps a thick sequence of felsic lavas and volcaniclastic rocks intruded by a probably comagmatic feeder sill. The 300–350 m thick inner flank comprises a 200–250 m thick central olivine meso to adcumulate, relatively narrow lower and upper ortho to mesocumulates and a 2–10 m thick olivine spinifex zone at the top. Approximately 700 m towards the 90 m thick outer flank, the spinifex zone is up to 30 m thick, the central meso to adcumulate lenses out between the upper and lower mesocumulates, and the lower orthocumulate is more rich in clinopyroxene. In places, the flanks are directly overlain by rubbly hyaloclastite. The less well preserved, 500 m thick central axis coincides with a floor rock embayment of demonstrably primary origin, which is 200 m deep and at least 800 m wide. Nickel sulfide mineralisation is disseminated, texturally cocumulus to olivine and confined to the meso to adcumulate, the highest nickel grades being located towards the top. Large floor rock xenoliths occur in the mineralised meso to adcumulate of the inner flank within reaction envelopes of barren, pyroxene bearing, olivine ortho to mesocumulate. Formation of the embayment and xenoliths (clear evidence of large scale thermomechanical floor rock erosion) and of the olivine sulfide meso to adcumulate (the sulfur probably derived by the assimilation of sulfidic wall rocks adjacent to the feeder sill) is attributed to prolonged focused flow of hot turbulent lava close to the vent. The Main Flow is interpreted as the product of a thick channelised sheet flow comprising: (i) a turbulent lava river, open along the central axis and partly tubed over along the inner flanks; and (ii) a tubed over levée facies along the outer flanks accommodating overflow from the central axis by inflationary growth under laminar flow conditions.

Crustal-contaminated komatiitic basalts in Southern China: products of a Proterozoic mantle plume beneath the Yangtze Block

Abundant mafic and ultramafic rocks including basalts, komatiitic basalts, and peridotites occur in the Proterozoic Sibao Group, northern Guangxi Province, China. Whereas the basalts are generally pillow lavas, the komatiitic basalts are typically spinifex-textured and, in a few cases, show pyroxene accumulation associated with NiCu(PGE) sulfide deposits. The peridotites occur in the lower portions of differentiated sills, which contain gabbro and diorite in their upper parts. The sills are believed to be co-magmatic with the komatiitic basalts. The spinifex rocks of the Jiepai and Hejia Flows have MgO ranging from 8.9 to 14.3 wt%. The Zhongkui Flow is highly fractionated to form a spinifex zone with lower MgO (5.3–5.9 wt%) and a cumulate zone with higher MgO (17.3–17.9 wt%). Overall the rocks have TiO 2=0.44–0.74 wt%. Relative to primitive mantle, they are enriched in Th and LREE, but exhibit negative Ti-, Nb-, and P-anomalies. These features are consistent with their formation from a crustally-contaminated komatiitic magma. During this process of crustal contamination, the magmas assimilated sulfur from sediments, which caused sulfide-segregation resulting in the formation of NiCu(PGE) sulfide deposits. The occurrence of the komatiitic basalts in the Sibao Group can be explained by the ascent of a mantle plume beneath a continental rift environment, and implies that the Yangtze Block may have had an Archean basement through which the Sibao komatiitic basalts erupted.

The petrogenesis of Gorgona komatiites, picrites and basalts: new field, petrographic and geochemical constraints

Gorgona Island, Colombia is remarkable not only because it contains the only Phanerozoic komatiites, but also because it has mafic to ultramafic lavas with a wide range of compositions, from moderately enriched to extremely depleted (relative to Bulk Earth). The komatiite flows are, in many respects similar to Archaean komatiites; they formed from MgO-rich (18%) liquids and have upper spinifex zones and lower cumulate zones. The cumulate zones of Archaean komatiites contain many solid grains, in contrast more than 90% of the olivine in the Gorgona cumulates is highly skeletal. This combined with the fact that the Gorgona cumulate zones are thinner than those in Archaean komatiites, suggests that the komatiite magma became strongly superheated en route to the surface. The komatiites have trace element contents intermediate between those of the basalts and the ultramafic tuffs. Some basalts have isotope compositions indicative of long-term enrichment in incompatible elements, whereas other basalts and ultramafic volcanics have isotopic signatures that imply corresponding depletion. It is apparent that the plume source region of the Gorgona magmas was markedly heterogeneous, with at least two source components contributing to the observed variation in composition. This heterogeneity may have resulted from the incorporation of different components into the plume source, or it may be the result of complex melting and melt extraction processes during the ascent of a heterogeneous plume. Despite earlier suggestions that there may have been a significant age gap between depleted komatiite and basalt flows and the enriched basalts, new 40Ar- 39Ar dating of basalts and gabbros are more consistent with all being generated at 87 Ma during formation of the Caribbean/Colombian plateau, possibly at the Galapagos hotspot.

The origin of fluids and the effects of metamorphism on the primary chemical compositions of Barberton komatiites: New evidence from geochemical (REE) and isotopic (Nd, O, H, [formula omitted]) data

Numerous greenstone relics, all containing the two lowermost formations of the Onverwacht Group, occur in the Archean trondhjemitic/tonalitic gneiss terrains south of the Barberton Greenstone Belt. In this study, we report detailed petrological, geochemical and isotopic (Nd, O, H, 40Ar 39Ar ) data obtained on komatiites from the Schapenburg Greenstone Remnant (SGR), the largest and best-preserved greenstone relic. The main goals are 1. (1) to date the metamorphism affecting the SGR using the 40Ar 39Ar dating method on amphiboles, 2. (2) to evaluate the effect of metamorphism on the preservation of primary isotopic and chemical signatures, 3. (3) to estimate the temperature and water/rock ratios that prevailed during metamorphic recrystallization in order to constrain the composition and origin of the reacting fluid phase. 40Ar 39Ar ages of 2.9 Ga obtained on two amphibole separates from the Schapenburg metavolcanics reveal the existence of a metamorphic event younger than the emplacement age (3.5 Ga). This metamorphic event belongs to a series of discrete periods of thermal activity from 3.4 to 2 Ga, each of which coincides with a major episode of magmatic activity. The ultrabasic lava flows acquired their δ 18O values (from +3.2 to +5%.) at high temperature (≈450°C) under high water/rock ratios. The reacting water had initial isotopic values typical of metamorphic fluids ( δ 18 O = +5 to +7%.; δD = −65 to −50%.). REE patterns were not disturbed by metamorphic recrystallization. Despite the long time interval between emplacement and metamorphism (≈600 Ma), ϵ Nd ( T) values are uniform throughout the whole magmatic suite, indicating that the Sm-Nd system was closed on the sample scale during metamorphism. The mantle source of these greenstones was depleted in LREE as evidence by ϵ Nd ( T) ≈ +2.5. Chemical fluxes during metamorphism were calculated for elements unfractionated by olivine removal (e.g. Na, Ca, Ti, Al, and Sr), by normalizing to Nd. They suggest a significant mobility of most major elements in the cumulate zones of the lava flows. By contrast, spinifex zones appear to have preserved most of their primary chemical signatures during metamorphic recrystallization. Their CaO Al 2O 3 and Al 2O 3 TiO 2 ratios can be used with confidence to determine the PT conditions of melting in the mantle source. A general model of water-rock interactions applied to the sedimentary and magmatic rocks of the Onverwacht Group is also presented. The model involves conditions of metamorphism deduced from this study and available data from the literature. In addition to metavolcanic rocks, most of the oxygen isotope compositions of carbonates and cherts (except the highest values) can be explained by reequilibration with metamorphic fluids under greenschist-amphibolite facies conditions.

Partial melting and recrystallization of Archeaan komatiites by residual heat from rapidly accumulated flows

Spinifex-textured komatiites at Honeymoon Well, Western Australia, show evidence of partial melting and recrystallization of original igneous textures. Their textures and mineral compositions differ markedly from those typical of komatiites. Spinifex olivine plates are bent and broken, while interstitial space between spinifex and cumulus olivine is occupied by polygonal aggregates of clinopyroxene, orthopyroxene, minor olivine and plagioclase. Similar granular pyroxene-plagioclase aggregates occur as diffuse veins cutting spinifex zones and cumulate zones of the flows and, in places, form the matrix to a breccia containing corroded fragments of spinifex rock. Thermometry based on the two pyroxene assemblages yields temperatures of 1055° to 1141° C, just below the low-pressure komatiite solidus. Mineral compositions are different from those of typical komatiites: clinopyroxenes are Al-poor and Cr-rich, olivines are unusually iron-rich and depleted in Cr and Ca, and the low-Ca pyroxene is bronzite rather than the more typical pigeonite. We interpret these observations as the results of thermal metamorphism, partial remelting and subsequent slow crystallization of originally normal spinifex-textured komatiite flows. The rocks in question occupy a 40–70 m interval sandwiched between two olivine-rich units: an underlying 90 m-thick olivine adcumulate layer, forming part of the cumulate zone of a basal 160 m-thick flow, and an overlying 1 km-thick extrusive body composed mostly of olivine mesocumulate and adcumulate and capped in turn by spinifex-textured flows. Thermal modelling shows that a sinusoidal temperature profile of cool flow tops and hot flow centres would exist within this sequence shortly after eruption. Conductive thermal relaxation of this profile could reheat spinifex zones to the extent of inducing partial melting and textural reconstitution. Such reheating is largely dependent on the time interval between the emplacement of successive flows. Calculations suggest that at Honeymoon Well the emplacement interval must have been of the order of 10 years or less. Textural reconstitution may have contributed to the development of the thick orthocumulate sequences characteristic of komatiites in the Agnew-Wiluna belt.

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.

Quantitative REE SIMS analyses of komatiite pyroxenes, Munro Township, Ontario, Canada

In situ REE analyses of fresh clinopyroxene from samples of a thick, layered komatiite flow (Fred's Flow), Munro Township, Ontario, are presented and compared with whole-rock analyses. Analyses were obtained by secondary ion mass spectrometry (SIMS) with very good precision and accuracy. Matrix problems were avoided by using clinopyroxene calibration curves. The partition coefficients of rare-earth element (REE) between melt and clinopyroxene have been calculated and found to approximate those of basalt systems. The abundance levels of rocks and contained clinopyroxenes support the hypothesis that spinifex zones and cumulate zones formed at the same time, and that the melt may have assimilated significant material from its base.

Pyroxene accumulation in spinifex-textured rocks

SummaryNatural pyroxene spinifex-textured rocks, free of olivine, may have MgO contents as high as 15%. If these rocks are allowed to crystallize in the laboratory under equilibrium conditions, olivine is the liquidus phase at 1350°C and pyroxene does not appear until the temperature has fallen to 1180°C. A similar problem is encountered in high MgO komatiites. If spinifex-textured and aphyric komatiites from Munro Township are plotted on an MgO-CaO-Al2O3 diagram, olivine control is evident until the MgO content of the melt falls to 15%. At this point the liquid trend moves away from the CaO-MgO boundary, indicating the crystallization of pyroxene. If a high MgO komatiite is crystallized in the laboratory under equilibrium conditions, pyroxene does not appear until the MgO content of the liquid reaches 9%. It is suggested that experimental and natural systems behave differently because pyroxene has crystallized metastably in the natural system under conditions of extreme supercooling. The early metastable crystallization of pyroxene can only affect the liquid trend if the spinifex zone contains a cumulate component.

Ultramafic rocks of the Widgiemooltha-Norseman area, Western Australia: Petrological diversity, geochemistry and mineralisation

Mineral exploration of the Widgiemooltha-Norseman region of Western Australia has located massive and disseminated nickel sulphide mineralisation. This paper discusses the geological setting of the nickel sulphide mineralisation with reference to the stratigraphy, structure and metamorphism of the ultramafic sequence and spatially associated rocks. The amphibolite facies metamorphosed ultramafic rocks of the Widgiemooltha area are compared with greenschist metamorphosed ultramafic rocks at Eundynie, which exhibit excellently preserved pseudomorphs of primary igneous textures. Some 2000 ultramafic rock samples were analysed for 13 element/oxides and divided into four major mineralogically and texturally distinct groups. The data were analysed statistically by determination of means and standard deviations and multiple regression analyses. The data suggest differentiation of a magma at depth followed by a vast outpouring of lava to produce picritic-peridotitic rocks. The flows crystallised with upper spinifex zones and lower euhedral-olivine zones in varying proportions dependent on magma composition. The development of spinifex texture is compositionally controlled. This texture is not developed in rocks with MgO contents greater than 26% (± 2%). The comparison of the ultramafics of the Widgiemooltha and Eundynie areas indicates that talc-carbonate formation and serpentinization have modified primary igneous textures and geochemistries, resulting in the production of a diverse group of rock types. Such post-magmatic alteration processes in association with structural elements are considered important in the upgrading of nickel sulphide mineralisation.

Archean Ultramafic Flows in Munro Township, Ontario

Research Article| March 01, 1973 Archean Ultramafic Flows in Munro Township, Ontario D. R. PYKE; D. R. PYKE 1Ontario Division of Mines, Geological Branch, Whitney Block, Queen's Park, Toronto, Ontario, Canada Search for other works by this author on: GSW Google Scholar A. J. NALDRETT; A. J. NALDRETT 2Department of Geology, University of Toronto, Toronto, Ontario, Canada Search for other works by this author on: GSW Google Scholar O. R. ECKSTRAND O. R. ECKSTRAND 3Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada Search for other works by this author on: GSW Google Scholar GSA Bulletin (1973) 84 (3): 955–978. https://doi.org/10.1130/0016-7606(1973)84<955:AUFIMT>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D. R. PYKE, A. J. NALDRETT, O. R. ECKSTRAND; Archean Ultramafic Flows in Munro Township, Ontario. GSA Bulletin 1973;; 84 (3): 955–978. doi: https://doi.org/10.1130/0016-7606(1973)84<955:AUFIMT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A unique exposure within the Abitibi orogenic belt of northeastern Ontario provides an exceptionally clear view of early Precambrian (Archean) ultramafic volcanic rocks. Approximately 60 ultramafic flow units are exposed over a stratigraphic thickness of 125 m. Individual flow units range in thickness from 0.5 to 15 m, average about 3 m, and display remarkable strike lengths and internal structure, which suggest extreme fluidity.The upper margin of the flow units (0.1 to 1.0 m) is thicker than the lower margin (about 1 cm), and the upper margin is highly fractured, thereby providing an easily recognizable top determinator. Many flow units contain a zone of spinifex, ranging in width from zero to 5 m, which underlies and is gradational with the flow top. Spinifex is a textural term to describe an array of criss-crossing sheafs or booklets characterized by numerous closely spaced and parallel blade- or plate-like crystals of olivine. The texture is generally accepted as having formed by rapid cooling of a crystal-free ultramafic liquid in situ. The variation in size of bladed olivine crystals within a spinifex zone is always from coarse (up to 1 m long) at the bottom to fine at the top, thereby providing a primary top determination. In flow units not containing a spinifex zone the chilled and fractured flow top grades downward into a zone of moderately foliated peridotite which generally constitutes the remaining part of a flow unit.Chemically, the flow units resemble similar ultramafic rocks described from northeastern Ontario, western Australia, and the Barberton area of South Africa; one unique feature is the exceptionally high alumina content of the clinopyroxene This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.