Ultramafic Volcanic Rocks Research Articles

The Central Slave Basement Complex, Part II: age and tectonic significance of high-strain zones along the basement-cover contact

The basement-cover high-strain zone enveloping parts of the Sleepy Dragon Complex, northeast of Yellowknife, Slave Province, Canada, has been reinvestigated. Integrated stratigraphic, structural, and geochronological data show that the high-strain zone is of regional extent and is best interpreted as a décollement between crystalline, ca. 2.9-3.3 Ga rocks of the Central Slave Basement Complex and pre-2687 Ma cover rocks. Three temporally distinct mafic dyke swarms occur within the high-strain zone. The two oldest of these constrain the timing of the high-strain event to between 2734 ± 2 and 2687 ± 1 Ma. At the time of décollement development, the cover stratigraphy consisted of (i) the Central Slave Cover Group, a thin, pre-2734 Ma succession of mafic and ultramafic volcanic rocks, conglomerates, fuchsitic quartzites, minor rhyolites, and banded iron formation; and (ii) an overlying sequence of tholeiitic pillow basalts. The Central Slave Cover Group is considered to be autochthonous, whereas a variety of evidence suggests that the pillow basalts are parautochthonous to possibly allochthonous. The transport direction in the décollement was from northeast to southwest, and maximum displacement was probably on the order of 10 to several tens of kilometres. Presently, the décollement appears discontinuous due to younger intrusive and erosional events. Around most of the southern flanks of the Sleepy Dragon Complex, the crystalline core of the complex consists of post-décollement intrusive rocks and (or) is unconformably overlain by parts of the Yellowknife Supergroup that are younger than 2687 Ma. Lineation patterns in these younger rocks reflect regional deformation events that postdate and are unrelated to the décollement. The new data allow two tectonic models for development of the décollement: (i) a contractional thrusting model, involving collision of an eastern Slave Province arc terrane; or (ii) a syn-greenstone belt extensional model.

Structural-metamorphic evolution of the Palaeoproterozoic Bryah and Padbury Groups during the Capricorn orogeny, Western Australia

Volcano-sedimentary successions of the Bryah and Padbury Groups were deposited and deformed along the northern margin of Archaean Yilgarn Craton, Western Australia, during the ca 2.0-1.8 Ga Capricorn Orogeny. This orogeny is interpreted to record convergence and collision of the Yilgarn Craton with the Pilbara Craton to the north. The Bryah Group with its extensive mafic and ultramafic volcanic rocks represents a short-lived proto-oceanic backarc basin whereas the sedimentary Padbury Group records the filling in of a retro-arc foreland basin. The two groups were deformed as a single package and are now everywhere in faulted contact with older Palaeoproterozoic rocks or underlying Archaean granite-greenstone. Four events are recognized in a progressive compressional deformation regime. During D1 subhorizontal high strain shear zones and tectonic interleaving developed between the volcanosedimentary succession and reworked Archaean crustal fragments that were underthrust or underplated during initial basin closure. Movement directions were mainly north-to-south. D2 NS shortening resulted in upright EW folds dominated by two regional, basement-cored, anticlines, which are interpreted as fault bend folds above a crustal scale frontal thrust ramp. Uplifted deeper crustal rocks are now represented by zones of higher grade metamorphic assemblages in the core of these anticlines. D3 folds are upright, NS striking and restricted to a zone (depression) between the two basement-cored anticlines (or domes). D2 and D3 folds are largely mutually exclusive with little or no evidence for overprinting, and are regarded as contemporaneous. The D3 structures are interpreted to have formed above lateral ramps in the crustal scale thrust fault during continued NS shortening. D4 is a minor event of NNE-SSW compression and did not influence the regional geometry. This deformation history records the closure and deformation of marginal back-arc and foreland basins.

Neoarchean evolution of differing convergent margin assemblages in the Wabigoon Subprovince: geochemical and geochronological evidence from the Lake of the Woods greenstone belt, Superior Province, Northwestern Ontario

The Neoarchean Lake of the Woods greenstone belt is subdivided into three lithostratigraphic assemblages: a lowermost mafic volcanic assemblage: a compositionally diverse, predominantly volcanic, middle assemblage; and a predominantly sedimentary uppermost assemblage. The secular evolution and geodynamic settings of these assemblages are constrained with detailed geochronological and geochemical data. The Lower Keewatin is the oldest assemblage with a zircon UPb age of 2738 ± 2.0 Ma. It consists of Mg and Fe tholeiitic basalts (Mg number = 30–65) with flat to slightly enriched rare earth elements (REE) [( La/ Yb) CN = 0.9−1.6], elevated large ion lithophile elements (LILE), and undepleted high field strength elements (HFSE). The tholeiites do not appear to have exact modern analogs but do have some similarities to primitive arc tholeiites and back-arc basin basalts suggesting an Archean destructive margin setting. The Upper Keewatin assemblage disconformably overlies the Lower Keewatin assemblage. It consists of volcanic and sedimentary groups. Two geochemically distinct volcanic suites are recognized. The dominant suite consists of compositionally diverse calc-alkalins basalt to rhyolite. Felsic end-members have yielded zircon UPb ages ranging from 2712 to 2723 Ma. The suite exhibits highly fractionated REE [( La/ Yb) CN = 4−35], elevated LILE and Nb and Ta depletion anomalies which are very similar to modern island arc volcanic rocks developed in proximity to continental margins. A coeval suite of mafic to ultramafic volcanic rocks consists of Mg tholeiite, komatiitic basalt and ultramafic komatiite (Mg numbers = 45–80). The suite exhibits depleted LREE [( La/ Yb) CN = 0.6−1.0], slightly elevated LILE and undepleted to slightly enriched HFSE suggesting that this geochemically primitive suite was produced by partial melting of a refractory mantle source, perhaps as second-stage melts of the same mantle source from which the Lower Keewatin assemblage basalts were previously derived. The Electrum assemblage unconformably overlies the Upper Keewatin assemblage in the northern part of the belt. It consists of conglomerates, sandstones and minor volcanic rocks having detrital ages indicating deposition after 2699 Ma. The volcanic rocks consists of andesite to rhyolite with shoshonitic to calc-alkaline geochemical affinity as indicated by highly fractionated REE [( La/ Yb) CN = 12−112] highly enriched LILE and pronounced Nb, Ta and Ti depletion anomalies. Samples from the syenitic Falcon Island pluton appear to be coeval with Electrum assemblage volcanics having an U-Pb isotopic age of 2694 Ma and similar but even more alkaline chemistry consisting of highly fractionated LREE and LILE trace elements with very pronounced Nb, Ta and Ti depletion anomalies. These late tectonic suites are geochemically similar to modern shoshonites with a continental influence formed in the late stages of a mature island arc or a continental arc setting.

The 89 Ma Tortugal komatiitic suite, Costa Rica: Implications for a common geological origin of the Caribbean and Eastern Pacific region from a mantle plume

Research Article| May 01, 1997 The 89 Ma Tortugal komatiitic suite, Costa Rica: Implications for a common geological origin of the Caribbean and Eastern Pacific region from a mantle plume Guillermo E. Alvarado; Guillermo E. Alvarado 1Instituto Costarricense de Electricidad, Departamento Ingeniería Geológica, Apdo. 10032-1000, Costa Rica Search for other works by this author on: GSW Google Scholar Percy Denyer; Percy Denyer 2Escuela Centroamericana de Geología, Universidad de Costa Rica, Apdo. 35-2060, San José, Costa Rica Search for other works by this author on: GSW Google Scholar Christopher W. Sinton Christopher W. Sinton 3Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882 Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (5): 439–442. https://doi.org/10.1130/0091-7613(1997)025<0439:TMTKSC>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Guillermo E. Alvarado, Percy Denyer, Christopher W. Sinton; The 89 Ma Tortugal komatiitic suite, Costa Rica: Implications for a common geological origin of the Caribbean and Eastern Pacific region from a mantle plume. Geology 1997;; 25 (5): 439–442. doi: https://doi.org/10.1130/0091-7613(1997)025<0439:TMTKSC>2.3.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 SocietyGeology Search Advanced Search Abstract Komatiites are reported for the first time in the northern part of the Gulf of Nicoya, Costa Rica. These rocks, dated at 89.7 ± 1.4 Ma (Turonian) by 40Ar/39Ar methods, occur as a large, elongated (14 km long, 1.5 km wide) N60°W striking body in the ophiolitic Nicoya Complex. These lavas have high MgO (26%–29%), Ni, and Cr, have high CaO/Al2O3 (0.98–1.08) and moderate Al2O3/TiO2 (5.55–8.44) ratios, and are depleted in Al2O3 (4%–5.5%), K2O (0.02%–0.37%), and TiO2 (0.59%–0.9%). Although these lavas are cumulates, their geochemical composition indicates an origin from a primary komatiitic magma, with a melting temperature of 1700 °C at a depth of 150 km. Similarities in the petrology and age (88–90 Ma) of Gorgona, Curaìao, and Nicoya-Tortugal mafic and ultramafic volcanic rocks suggest that these rocks had a common origin. These occurrences suggest a single hotspot center over a large area of the Caribbean and Eastern Pacific Mesozoic region due to a major thermal anomaly in the mantle, such as a hot, rising, convective plume. 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.

Geochemistry and provenance of metasedimentary rocks from the Archean Golden Pond sequence (Casa Berardi mining district, Abitibi subprovince)

The Archean Golden Pond sequence is made up of deformed and metamorphosed conglomerates, greywackes, and mafic volcanic rocks, and is overlain by ferrugineous metasedimentary rocks of the North iron formation. The clastic rocks were derived mainly from a volcanic source that had undergone weak chemical weathering. Their source area was dominated by the presence of 60–80% high-Al2O3 felsic volcanics having strongly fractionated [La/Sm]N (= 3.7 ± 0.3) and very low Ta/Th ratios (= 0.09 ± 0.02), with lesser proportions of basaltic (10–30%) and ultramafic volcanic rocks (1–10%). The ferrugineous metasedimentary rocks can be modelled by mixing 20–40% siliciclastic material, of the composition of the average Golden Pond greywacke, with an Fe- and Si-rich precipitate (molecular Fe/Si = 0.6 ± 0.2). The high-Al2O3 felsic source rocks were most likely produced by subduction processes within an oceanic arc environment, but the mafic and ultramafic volcanic rocks were derived by different processes from an asthenospheric mantle source, possibly in an oceanic rift environment. Therefore, it is suggested that the ultramafic, mafic, and felsic volcanic rocks were brought to the same erosional level by dissection of the arc system and rapid exhumation of the felsic arc lithologies and the deeper ocean floor. Intrabasinal hydrothermal activity associated with contemporaneous mafic volcanism and (or) graben development may have also been responsible for the local production of the Fe-rich precipitates of the North iron formation.

Noncarbonate, skarnlike Au-Bi-Te mineralization, Lucky Draw, New South Wales, Australia

Gold-bismuth-tellurium mineralization at the Lucky Draw mine is associated with pods, lenses, and veins of Fe-Mg-Al silicates, adjacent to the late Carboniferous Burraga Granodiorite. Alteration and mineralization is enclosed by micaecous quartzite and quartz-mica schist of the Triangle Group, immediately below ultramafic shoshonitic volcanic rocks of the Rockley Volcanics. Geologic relationships imply that mineralization and alteration are probably synchronous with later stages of contact metamorphism and, hence, postdate upper greenschist facies regional metamorphism. Barren stage I assemblages consisting of combinations of gedrite, biotite, staurolite, cordierite, quartz, albite, ilmenite, and hercynite are overprinted by stage II assemblages of chlorite, almandine, and biotite. Stage III alteration, consisting of green-brown biotite, is developed where the granodiorite intrudes stage II alteration. Stage I assemblages are poorly constrained to T nearly equal 600 degrees C at 2 to 3 kbars, whereas garnet-chlorite thermometry and phase equilibria indicate T nearly equal 550 degrees C for stage II assemblages. Native Bi, bismuthinite, native Au, maldonite, and various Bi tellurides were deposited interstitial to chlorite grains and along chlorite cleavage planes, during stage II alteration. Ore minerals are characterized by low melting points (< or =370 degrees C) but are in apparent equilibrium with stage II assemblages; this can be reconciled if the ore minerals collected as melts at higher temperatures. Laminae of heavy minerals, which are present in stage I and II alteration assemblages, are identical to those marking bedding and crossbedding in unaltered metasedimentary rocks enclosing the mineralization. Whole-rock immobile element concentrations- of the alteration assemblages are very similar to unaltered metasedimentary rocks of the local Triangle Group. However, stage I assemblages and mineral compositions have some similarities with local occurrences of cordierite-orthoamphibole rocks derived from altered mafic volcanics. Therefore, silicate assemblages at Lucky Draw may be the product of contact metamorphism and metasomatic alteration of quartz-rich metasedimentary and interbedded, mafic volcanic rocks.

Sulfur and carbon isotopes in Archean banded iron formations: Implications for sulfur sources

Sulfur isotope ratios of pyrite from two Late Archean banded iron formations were used to constrain possible sources of sulfur. The iron formations studied were the Temagami iron formation which hosts the Sherman Mine near Temagami and the Boston iron formation including the Adams Mine south of Kirkland Lake, both in Ontario. In both cases Algomantype oxide-facies magnetite-chert ores predominate. The sulfur studies were supplemented by limited carbon isotopic data for the Sherman Mine. Iron formation in the Sherman Mine occurs on both limbs of a regional synformal structure. Each iron formation unit caps a thick calc-alkaline andesite-dominated volcanic sequence with one band hosted in sedimentary rocks of turbidite origin. The Adams Mine includes four major iron formation bands interbedded with mafic and ultramafic volcanic rocks and greywacke of turbidite origin. The sulfur isotope statistics for averages of pyrites from these three units are: Sherman Mine volcanic-hosted suite, δ 34 S= −1.7 ± 4.6‰. ( n=14); Sherman Mine sediment-hosted suite, δ 34 S=+1.4±1.6‰ ( n=43); and Adams Mine, δ 34 S=−1.8±2.9‰ ( n=17). These averages differ by <2‰ from juvenile sulfur, and together with their limited dispersions, suggest a predominantly unfractionated source of sulfur. It is argued that this sulfur has been derived from the associated volcanic piles. Most of the pyrite occurs as disseminated grains in oxide-facies iron formation and interbedded clastic sediments. A distinctive nodular pyrite in the Sherman Mine sediment-hosted suite occurs in cherty graphitic sulfide-facies iron formation. While the sulfur isotopic properties of this subset ( δ 34 S=0.0±0.8‰ ) are not suggestive of biogenic sulfur, the geological setting and carbon isotopic compositions of associated graphite ( δ 13 C=−34‰ ) imply that anaerobic bacterial processes were active in this reducing environment. The apparent lack of biogenic sulfide is rationalized by models suggesting that Archean marine basins differed from Phanerozoic analogues in various parameters which influence sulfur isotopic characteristics. It is argued that biogenic sulfide is probably a significant component of the sulfur in Sherman Mine nodular pyrite.

Existence of a marginal basin within the Circum-Superior Belt: geochemical evidence from the Churchill-Superior boundary in Manitoba, Canada

Mafic volcanic rocks incorporated within the Churchill-Superior Boundary Zone at Ospwagan Lake, Moak Lake, Assean Lake and the Fox River Belt constitute part of the Circum-Superior Belt. Major and trace element data have been obtained from mafic and ultramafic volcanic rocks from four localities. The data provide a means of comparing and contrasting the geochemistry of the rocks from the portion of the Circum-Superior Belt surrounding the Archean Superior Province in Manitoba with other units from elsewhere in the belt, and with rocks from the adjacent Proterozoic Churchill Province. Major element geochemical data are consistent with the rocks having originally formed in a oceanic environment. Trace element and rare earth element data suggest that the environment was a marginal basin. Al 2O 3/TiO 2 ratios indicate that the magmas were derived from a mantle source by small degrees of partial melting. High field strength element patterns show that the mantle source region giving rise to the rocks was relatively uniform in composition. Trace element, mid-ocean ridge basalt (MORB) and chondrite-normalized patterns indicate similarities in alkali element enrichment for the rocks at Asean and Ospwagan Lakes. Virtually all the rocks have Ti, Zr, Nb, Y and P levels < MORB, which is consistent with the mantle source region having undergone a small degree of partial melting. Zr/P ratios for the rocks from the Fox River Belt are consistently lower than those from the other localities, and the Assean Lake rocks show slight enrichment in light rare earth elements, which may suggest local compositional variability in the mantle source regions supplying the magmas to the Fox River Belt and the Ospwagan, Assean and Moak Lake areas.

High-Mg extrusive rocks from the Romeral Zone ophiolites in the southewestern Colombian Andes

In the southwestern Colombian Andes, high-Mg extrusive rocks occur within the Mesozoic (?Late Cretaceous) ophiolitic fragments from the Romeral fault zone, a major suture between the continental domain of the Central Cordillera, and the oceanic domain of the Western Cordillera. The high-Mg rocks include picrites and picritic basalts, and are associated in places with peridotites showing transitional to extrusive textures. Plagioclase-free picrites characterized by quench textures have been found in two localities, and occur as pillow-lava (Rio Boloblanco) or as clasts of monogenic and polygenic volcanic breccias (El Encenillo). They show similar olivine-phyric micro-spinifex textures with groundmass consisting of hopper and plate olivine (or Ca-poor pyroxene at El Encenillo), skeletal clinopyroxene and glass. Hyaloporphyritic texture occurs in the Rio Boloblanco picrites at pillow rims. Chemically, these picrites are characterized by MgO contents of >20%, and are distinguished by the content of TiO 2 and incompatible elements in a high-Ti type (Rio Boloblanco) and a very high-Ti type (El Encenillo), the latter particularly remarkable for the high contents of Ti, Zr and Nb which give evidence of an alkaline tendency. Phase chemistry is consistently different between the two picrite types, but comparably high Al contents (up to 14%) of skeletal pyroxenes suggest that the magmas were erupted and cooled in similar conditions. Olivine-phyric picritic rocks characterized by microlitic non-quenched groundmasses occur as clasts of volcanic breccias at El Encenillo (picritic basalts), and are present with comparatively high frequency within major (Los Azules) and minor (Upper Chapungo Valley) ophiolitic fragments from the Romeral Zone, either as lavas and volcanigenic breccias. These rocks are mostly richer in olivine than the quench-textured picrites, contain modal plagioclase, and relatively low-Al pyroxenes. As regards chemistry, a high-Ti and very low-Al group (Los Azules, Upper Chapungo Valley) and a very high-Ti group (El Encenillo) can also be distinguished within these picrites. A comparison with the ultramafic volcanic rocks from Gorgona Island, in the nearby Pacific domain off Colombia, among which the unique Phanerozoic komatiites were discovered, indicates close similarities between the Rio Boloblanco picrites and some Gorgona Island komatiites (olivine-phyric komatiites with micro-spinifex groundmass) regarding their petrochemical features. It is suggested that they record a unique event related to the same tectonic environment, and consequently that they belong to the same structural unit, developed in the Late Cretaceous and extending from continental Colombia to the Pacific domain. This is in contrast with previous subdivisions of the ophiolitic complexes from southwestern Colombia into distinct age groups progressively younger from the Central Cordillera toward the Pacific Ocean.

Source processes for Archean Au–Ag vein deposits: evidence from lithophile-element systematics of the Hollinger–McIntyre and Buffalo Ankerite deposits, Timmins

Potassic alteration domains of Archean greenstone belt lode gold deposits, typified by the Hollinger–McIntyre and Buffalo Ankerite mines, Timmins, are characterized by systematic partitioning between different groups of incompatible elements such that Al, Ga, Th, U, Ti, and V are decoupled from K, Rb, Ba, Li, Cs, and Tl. The former group of elements has concentrations and interelement ratios in alteration domains that reflect host-rock control, implying approximately isochemical behaviour. In contrast, the lithophile elements K, Rb, and Ba are generally coenriched and are linearly correlated over three orders of magnitude in abundance, where K/Rb = 230–380 (r = 0.92–0.99) and K/Ba = 34–97 (r = 0.82–0.99), values close to average crustal ratios of 285 and 36, respectively. K/Rb and K/Ba ratios trend towards higher values with respect to increasing concentrations of Rb and Ba, possibly because of mixing between host rock and hydrothermal reservoirs of the lithophile elements. Cs and Tl contents average 1.4 and 1.1 ppm, respectively, such that K/Cs and K/Tl, as well as K/Rb and K/Ba, trend to higher than bulk crustal ratios, signifying a mafic to ultramafic source variably depleted in Cs, Tl, Rb, and Ba relative to K. Lithium is generally not coenriched with K but rather correlates with Mg content, reflecting the primary magmatic substitution of Li1+ for Mg2+ in the parent magmas. Al and Ga are highly correlated: Al/Ga = 2070–3790 and r = 0.85–0.98. Accordingly, Al and Ga were not fractionated during mineralization. These interelement trends, collectively, are present in deposits variously hosted by ultramafic, mafic, or felsic volcanic rocks and by sediments or granitoids.Magmatic processes involving crystal fractionation of biotite, K-feldspar, and plagioclase generate trends to systematically diminished K/Rb (≥50), K/Li, K/Cs, and K/Tl but enhanced K/Ba (≤8 × 103) and Rb/Sr ratios in most late-stage differentiates. Al/Ga ratios diminish systematically during late-stage magmatic evolution, and Th–U and Ti–V display mutual partitioning governed by crystallization of trace phases. Such "late-stage" trends are the rule in "magmatophile" deposits, including the Archean Cadillac molybdenite deposit, Phanerozoic Cu- and Mo-"porphyry" deposits, Sn–W greisens, and most pegmatites. Accordingly, magmatic processes of this type can be ruled out as the dominant source of volatiles for gold-forming systems. There is no complementarity between large-ion lithophile-element (LILE) (K, Rb, Ba, Cs, U, Th) depletion patterns in granulites and K, Rb, and Ba enrichment in Au deposits. The enhanced K/Rb, K/Cs, and Rb/Cs ratios but diminished K/Ba ratios, characteristic of many granulites, are not reflected in the gold deposits, where K/Rb and K/Ba ratios approximate average crustal values and Th and U behave isochemically. In many granulites, especially Archean examples, LILE depletion is a primary feature; accordingly, the granulitization model for Au–Ag vein deposits can also be ruled out.The compliance of K/Rb and K/Ba ratios in potassic alteration domains of Au deposits with values characteristic of main-trend igneous rocks or "average" crust implies that K, Rb, and Ba were partitioned into the hydrothermal ore-forming fluids in approximately the same ratios as in the source rocks, consistent with known fluid–mineral distribution coefficients. Dehydration reactions in dominantly mafic to ultramafic source rocks under amphibolite-facies conditions or equilibration of ore-forming fluids with source rocks under conditions of low water/rock ratio, rather than either purely magmatic or granulitization processes, may satisfy the requirement for proportional K, Rb, and Ba coenrichment, Li retention, and the absence of Al–Ga fractionation.

Stratigraphy and geochemistry in the proterozoic mafic volcanic rocks of the Nagu-Korpo area, SW Finland

The volcanic rocks of the Nagu-Korpo area in SW Finland form a thin (< 1000 m) formation that is folded into narrow synforms separated by migmatite-filled antiforms. In Nagu the volcanic formation comprises three units: a lower unit of subvolcanic banded gabbro sills, a middle unit of amphibolitic volcanic rocks and an upper unit consisting of a thin ultramafic volcanic rock layer. The volcanic formation is interlayered within a thicker sequence of metasedimentary gneisses. Geochemically the metavolcanic rocks are tholeiitic and plots based on Ti, Y, Zr and other trace elements show that they resemble recent within-plate lavas. MORB- and mantle-normalised trace element plots show patterns strongly differing from those of recent arc lavas, indicating a tectonic environment of (plate boundary related?) initial rifting. The upper ultramafic lavas are enriched in LREE relative to the lavas of the much thicker amphibolitic middle unit below. A model for the initial formation of the volcanic rocks of the Svecofennian fold belt must acknowledge both the episode of initial rifting and the later evolution of an island arc-like environment.

On the origin of Archean vein-type gold deposits with reference to the Larder Lake ‘break’ of Ontario and Quebec

The vein-type gold deposits of middle to late Archean age constitute one of the major sources of gold production in the world. These deposits are highly diverse in character, but are invariably found in proximity to mafic and ultramafic volcanic rocks, and closely associated with tectonically disturbed zones, or ‘breaks’, in the ancient crust. Using the Larder Lake ‘break’ of northern Ontario and Quebec as a model, a general theory of genesis for such deposits is proposed.

A preliminary study of komatiites in Anshan-Benxi-Fushun region, Northeast China

Discovered for the first time in the Lower Anshan Group with a metamorphic grade from amphibolite to granulite facies, komatiites occur as bedded ultramafic volcanic rocks on the ocean-floor. The komatiites in the Anshan-Benxi-Fushun region occurring in the metamorphic rocks are some monomineralic rocks such as hornblendite, clinopyroxene and hypersthenite. Local komatiites are found retaining some typical pre-existing structures and textures of volcanic rocks, such as blastovariolitic and blastodoleritic textures. The chemical composition of komatiites in this region is much similar to that of other well-known model komatiites in the world, and so are their geochemical characteristics.The occurrence of komatiites in the Anshan-Benxi-Fushun region provides strong evidence showing that this region is an Archean greenstone belt. Studies on komatiites in this region will shed much light on the stratigraphic division of the Anshan Group and the paleo-structure of the greenstone belt as well as on the rules governing metallogenesis in the Anshan-Benxi-Fushun region.

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.

Komatiitic and Iron-rich Tholeiitic Lavas of Munro Township, Northeast Ontario

Munro Township, in the Archean Abitibi greenstone belt of northeast Ontario, contains volcanic and hypabyssal rocks of two magma series: (1) an Fe-rich tholeiitic series of basaltic to dacitic lava flows (3–10 m thick), layered peridotite-pyroxenite-gabbro flows (120 m thick), and layered sills (700 m thick); (2) an ultramafic-mafic komatiitic series, comprising discrete lava flows of peridotitic to andesitic composition (1–17 m thick), layered peridotite-gabbro flows (120 m thick), and layered sills (500 m thick). The komatiitie lavas form a succession about 1000 m thick that is both underlain and overlain by thicker successions of tholeiitic volcanic rocks. Three types of komatiite are recognized: peridotitic, pyroxenitic, and basaltic komatiites. The most ultramafic are peridotitic cumulates rich in forsteritic olivine (Fo89–94), at the bases of flows and sills. Many less mafic peridotitic komatiites (MgO: 20–30 per cent), which typically form the upper parts of flows and the marginal parts of small intrusions, exhibit spinifex textures indicative of their formation from ultrabasic liquids. Pyroxenitic komatiites (MgO: 12–20 per cent) also may contain olivine, but are dominated by clinopyroxene, usually in spinifex textures. Basaltic komatiites (MgO <12 per cent) are composed mainly of clino-pyroxene and plagioclase or devitrified glass, rarely in spinifex texture and more commonly in equigranular textures. Peridotitic komatiite with roughly 30 per cent MgO appears to represent a parental liquid from which the more ultramafic komatiites formed by accumulation of olivine, and the less mafic types were derived by fractionation of olivine, joined and finally succeeded in later stages by clinopyroxene and plagioclase. Komatiites of Munro Township share many of the characteristics of the komatiites from the Barberton Mountain Land, South Africa (Voljoen & Viljoen, 1969a and b), but lack the high CaO/Al2O3 ratios that distinguish the Barberton rocks. The Munro komatiites are identical in this respect to ultramafic volcanic rocks in Australia, Canada, Rhodesia, and India. It is proposed that the definition of the term komatiite be broadened so that it includes all members of this ultramafic-mafic rock series, not only those from Barberton Mountain Land. The proposed criteria are: (1) highly ultramafic compositions in noncumulate lavas; (2) unusual volcanic structures such as spinifex texture and polyhedral jointing; (3) low Fe/Mg ratios at given Al2O3 values or high CaO/Al2O3 ratios; low TiO2 at given SiO2; and high MgO, NiO, and Cr2O3.

The System Diopside-Nepheline-Akermanite-Leucite and Its Bearing on the Genesis of Alkali-Rich Mafic and Ultramafic Volcanic Rocks

The join Ne-Ak-Lc within the system, diopside (Di)- nepheline (Ne)- akermanite (Ak)- leucite (Lc), has been studied at 1 atm. The results show that it is pseudoternary with a four-phase point at $$Ne_{34.5}Ak_{20.5}Lc_{45}$$ and 1,170° ± 5° C where nepheline solid solution ($$Ne_{ss}$$) + melilite (Mel) + leucite solid solution ($$Lc_{ss}$$) coexist with liquid. Forsterite solid solution ($$Fo_{ss}$$), as a subliquidus phase, has a reaction relationship with liquid. A pseudobinary join $$(Di_{38}Ak_{3}Ne_{59})_{100-x} - Lc_{x}$$, investigated to determine the temperature of six phase assemblages within the Di-Ne-Ak-Lc system, cuts the primary phase volumes of $$Fo_{ss}$$ and $$Lc_{ss}$$ and has a pseudoeutectic at 39 wt% of Lc and 1,130° ± 5° C where $$Fo_{ss} + Lc_{ss}$$ + liquid coexist. An assemblage ($$Di_{ss} + Fo_{ss} + Mel + Ne_{ss} + Lc_{ss}$$ + liquid) occurs at 1,105° ± 10° C. Forsterite solid solution begins to react with liquid at this point and is eliminated completely at 1,075° ± 10° C. Study of the join Ne-Ak-Lc, the pseudobinary join $$(Di_{38}Ak_{3}Ne_{59})_{100-x} - Lc_{x}$$, and results on the remaining joins of the system Di-Ne-Ak-Lc suggest that olivine-melilite-nepheline leucitite magma can be generated at 1 atm pressure from the following magma types: (1) olivine-melilite nephelinite, (2) olivine-melilite leucitite, (3) olivine-nepheline leucitite, and (4) leucite-bearing katungite. A melilite nepheline leucitite rock can be produced from an olivine nepheline-melilite leucitite magma because of a reaction relationship of $$Fo_{ss}$$ and liquid.

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.