Al-undepleted Komatiites Research Articles

Geochemistry, geochronology, and radiogenic isotopes of the Balmer and Confederation assemblages of the Laird Lake Area, Red Lake greenstone belt, Canada: implications for Archean tectonic evolution

The Laird Lake property, southwest Red Lake greenstone belt, straddles the contact between the Balmer (2.99–2.96 Ga) and the Confederation (2.74–2.73 Ga) assemblages. The property is 10 km along strike from the Madsen and Starrat–Olsen Au mines that are hosted near the contact. The Balmer assemblage consists of fine-grained, aphyric, locally pillowed mafic volcanic rocks, ultramafic intrusive and volcanic rocks with flow breccia textures hosting local spinifex-bearing clasts, and banded-iron formations. In contrast, the Confederation assemblage comprises porphyritic mafic volcanic rocks intercalated with intermediate to felsic volcanic rocks that include crystal lapilli tuffs, crystal tuffs, and tuffs. The Balmer assemblage is composed of tholeiitic mafic volcanic rocks with minor Al-undepleted komatiites, whereas the Confederation assemblage is calc–alkalic. Neodymium isotopes, in conjunction with trace element geochemistry, suggests that parts of the Balmer assemblage were weakly contaminated by an older intermediate basement. Both arc and back-arc volcanism occurs in the Confederation assemblage, with the arc rocks showing a stronger crustal component than the back-arc rocks. A maximum U–Pb age of 2741 ± 19 Ma for a crystal tuff and an age of 2737.68 ± 0.79 Ma for a diorite are consistent with a Confederation assemblage affinity for the intermediate calc–alkaline rocks south of the Au-bearing horizon. The Balmer assemblage represents an oceanic plateau formed by plume magmatism on the margins of the North Caribou Terrane, whereas the Confederation assemblage at Laird Lake formed in an oceanic arc setting where both arc and back-arc volcanism occurred simultaneously.

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.

The Paleoarchean Buffalo River komatiites: Progressive melting of a single large mantle plume beneath the growing Kaapvaal craton

Several Archean granitoid-greenstone terranes are exposed on the southeastern Kaapvaal craton in South Africa, but they received little scientific attention compared to the archetypal greenstone belt successions of the Barberton Mountain Land at the eastern craton margin. This study reports on a detailed field and geochemical survey of the Buffalo River Greenstone Belt at the southern Kaapvaal craton margin in KwaZulu-Natal, with focus on hitherto unstudied komatiites and basaltic rocks from this volcanic succession. Cross-cutting relationships and new U-Pb zircon age determinations for several granitoid units establish a minimum age of 3.26 Ga for komatiitic volcanism, possibly as old as ca. 3.5 Ga if a 3.47 Ga granodiorite sheet is interpreted as ‘intrusive’ into the greenstone succession.Geochemical data reveal three types of Paleoarchean komatiites at Buffalo River. Spinifex textured lava flows represent Al-depleted komatiites, with subchondritic Al2O3/TiO2 ratios and enrichment of LREE over HREE. The second type comprises Al-undepleted komatiites that have chondritic Al2O3/TiO2 and flat REE patterns. The third type identified comprises Al-enriched komatiites that display suprachondritic Al2O3/TiO2 ratios, with significant LREE depletion. The Al-depleted and Al-undepleted komatiites from Buffalo River are geochemically similar to komatiites from the 3.48 Ga Komati and 3.26 Ga Weltevreden formations of the Barberton Supergroup respectively, whereas the Al-enriched komatiites resemble the 3.33 Ga Commondale komatiites on the southeastern Kaapvaal craton.To explain the co-occurrence of three discrete komatiite types within a single volcanic succession at Buffalo River, we suggest that each major komatiite magmatic pulse originated from the same upwelling mantle source, from which melt was extracted at different pressure but similarly hot temperature conditions. 187Os/188Os data for the Al-depleted komatiites suggest an ultimate magma origin from a primitive mantle reservoir. The contrasting γOs values for Kaapvaal craton komatiites (zero to positive) and peridotitic mantle xenoliths (zero to negative) support a complementary nature of these lithologies as high-degree melts and depleted residues linked by vigorous mantle plume activity at around 3.5 Ga. Such a relationship can explain the contrasting Re/Os systematics of komatiites and lithospheric mantle peridotites, which creates the contrasting γOs over time.The highly siderophile element patterns of the Al-depleted komatiites from Buffalo River are similar to those of Barberton-type komatiites, for which an origin from the deepest upper mantle with high melt retention in an upwelling plume source was suggested. We confirm that this ca. 3.5 Ga mantle source had only 60–80 % of the platinum-group element budget of the modern ambient mantle, which points indirectly to a location at great depth in the aftermath of the meteoritic late accretion. Progressive melting of such an upwelling mantle source, to the point of majoritic garnet exhaustion, may explain the Al-undepleted and Al-enriched komatiites at Buffalo River. The presence of all three major komatiite types within a single volcanic succession may be linked to deep critical melting of a large mantle plume associated with growth of the Kaapvaal ‘continent’ at 3.5 Ga.

Corona-Type Textures in Ultrabasic Complexes of the Serpentinite Belt, Kola Peninsula, Russia

For the first time, corona-type textures are described in ultrabasic rocks in three complexes of the Serpentinite Belt on the Kola Peninsula in the northeastern Fennoscandian Shield. Three variants of the corona texture formed at different stages during the crystallization of a komatiitic, Al-undepleted melt emplaced in a subvolcanic setting. The first type crystallized at an early stage (Mg# Ol = 87) in a fine-grained harzburgite of the Chapesvara-I sill, with the following order in the corona: Ol → Opx → Cpx → Pl → Amp (aluminous sodic-calcic). The second type displays the sequence Opx → Cpx → Amp → Pl → Qz, which is observed in the orthopyroxenite zone in the Lotmvara-I sill. The third type involves a symplectitic corona in a plagioclase-bearing orthopyroxenite in the Lyavaraka complex, in which the inferred order is: Cpx → Amp (aluminous hornblende) + symplectitic Qz, formed in direct contact with grains of Pl. The corona-type textures occur in fresh rocks and are not related to regional metamorphism. They likely formed as consequences of two important factors: (1) rapid cooling, leading to unsteady conditions of crystallization in a shallow setting; and (2) an intrinsic enrichment in H2O and other volatiles in the parental magma, giving rise to fluid-saturated environments at advanced stages of crystallization. This was followed by a deuteric deposition of Amp rims as a result of the accumulation of H2O and reaction of H2O-bearing fluid with early grains of pyroxene and late plagioclase. The likely existence of a close relationship is suggested by the drusites of the Belomorian complex, which are coeval. In addition, unusual occurrences of lamellar inclusions of phlogopite and Al2SiO5 are documented, hosted by interstitial grains of plagioclase in the orthopyroxenite zone of the Lotmvara-I sill. These are attributed to crystallization from late portions of remaining melt enriched in Al, K, Na, H2O, and Cl, which is indicated by the recorded occurrence of chlorapatite in this association. Thus, our findings indicate the presence and abundance of intrinsic volatiles, Cl, F, CO2, and especially magmatic H2O, which were important to lower the liquidus, decrease the density and viscosity of the highly magnesian melt of Al-undepleted komatiite, thus enabling its transport from the mantle to a shallow level in the crust.

Geochemistry of the Abiete-Toko komatiite-like ultrabasites: Petrogenesis, evolution and geodynamic implications

The Greenstone Belt of the Paleoproterozoic Nyong Unit, in the northwestern Congo Craton, consists of metasediments and basic-to ultrabasic magmatic rocks. Ultrabasic rocks are of two types in the Abiete-Toko portion of the Nyong Unit. Type 1 has a mesh texture and consists of olivine (∼40 vol %), serpentine (∼30 vol %), orthopyroxene (∼15 vol %), clinopyroxene (∼5 vol %), brucite (∼5 vol %) and magnetite (∼5 vol %), while type 2 typified by spinifex texture is composed of serpentine (∼30 vol %), clinopyroxene (∼25 vol %), magnetite (∼20 vol %), hornblende (∼10 vol %), plagioclase (∼10 vol %) and pyrite (∼10 vol %). These petrographic features indicate that Abiete-Toko ultrabasic rocks have experienced both alteration of the main petrogenic minerals (olivine and pyroxene) and greenschist facies metamorphism. The behaviour of incompatible elements (HFSE and LREE) combined with chemical index of alteration values (35.6 = CIA ≤75.9), suggest that the investigated rocks have retained much of their original chemistry. Olivine is highly forsteritic (Mg# between 89.73 and 90.65), and contains high FeO (9.14–9.89 wt %), low NiO (0.45–0.47 wt %) and CaO (˂ 0.1 wt %), thus suggesting a mantle source. Based on bulk-rock chemical composition, the ultrabasites of the Paleoproterozoic Nyong Unit are hyper-magnesian [(MgO)adj ˃ 39 wt %] and have the Al-undepleted komatiites signature (Munro type). The low (0.17–0.70) (CaO/Al2O3)adj, low (0.60–0.94) (Gd/Yb)N and high (18.18–62.5) (Al2O3/TiO2)adj ratios, associated with the high ƩHREE (0.38–2.03 ppm), Y (0.87–3.57 ppm) and Zr (2–55 ppm) contents, suggest a shallow upper mantle source. The observed chemical characters including, (MgO)adj, (FeO)adj, (Al2O3/TiO2)adj, (CaO/Al2O3)adj, and Na, Ni, Cr, Y, Zr, Hf and REE contents reveal that the parent magma of these rocks was generated through partial melting of a heterogeneous and depleted mantle, at temperatures close to 1550 °C. Multi-element discrimination diagrams are consistent with crystallisation of these rocks in the arc setting, thus suggesting a Paleo subduction zone in the north-western part of the Congo Craton. These results are in favour of plate tectonic dynamics already operating in a hotter and ancient Earth environment point of view.

Synsedimentary rifting and basaltic-komatiitic volcanism in the Pontiac subprovince, Superior craton (Canada): Implications for Neoarchean geodynamics

New field observations from the Pontiac subprovince, Superior Province, supported by whole-rock geochemistry provide strong evidence for extensional magmatism and volcanism during deposition of the ca. 2685 Ma Pontiac sediments. Contacts between basaltic and komatiitic volcanic rocks and sedimentary rocks are concordant and weakly deformed, inconsistent with an allochthonous emplacement model proposed for volcanic successions in the Pontiac subprovince. Structural observations and aeromagnetic imagery show that the volcanic rocks are present as thin (<500 m), laterally continuous (>30 km) conformable successions within the turbiditic succession and were emplaced prior to the earliest deformation of the Pontiac subprovince. Sediment-matrix breccias with fluidal and blocky igneous clasts identical to the compositions of the mafic–ultramafic rocks occur along intrusive contacts with Pontiac Group wacke. These textures are interpreted as peperite, representing the interaction of magma or lava with wet, unconsolidated sediment. Peperite was observed along the margins of tholeiitic-komatiitic rocks and calc-alkaline lamprophyre intrusions, indicating that these magmas were broadly synchronous with the deposition of the Pontiac sediments. The volcanic rocks are characterized as tholeiitic basalt and Al-depleted to Al-undepleted komatiite, have high Nb/Th, flat to positive light rare earth element (REE) slopes, and flat heavy REE slopes, consistent with partial melting of a mantle plume. The calc-alkaline lamprophyre intrusions have low Nb/Th and prominent negative light and heavy REE slopes, consistent with partial melting of a previously metasomatised mantle. The presence of autochthonous komatiitic and tholeiitic rocks is consistent with the Pontiac subprovince developing as an extensional basin within the Abitibi subprovince in response to an upwelling mantle plume ca. 2.7 Ga. This model is incompatible with uniformitarian evolutionary models for the Superior craton interpreting the sedimentary subprovinces as accretionary prisms that formed during subduction processes. Non-uniformitarian models invoking stagnant-lid tectonics predict the formation of extensional basins and ascent of depleted mantle-derived magmas associated with upwelling mantle plumes, but the ability to produce source-metasomatism magmas without subduction is highly contested. The evolution of the Pontiac basin may thus reflect a transitional period in the Neoarchean where stagnant-lid and proto-subduction styles of tectonism were operating intermittently.

Early Neoarchean oceanic crust in the North China Craton: Evidence from geology, geochemistry and geochronology of greenstone belts in western Shandong

Western Shandong is the only area in the North China Craton where abundant supracrustal rocks of the early and late Neoarchean are developed. This study presents an integrated geological investigations, whole-rock geochemistry and SHRIMP UPb zircon dating investigation of the supracrustal rocks at Yanlingguang, one of the most typical areas of Neoarchean supracrustal rocks in western Shandong. The original Yanlingguan ‘formation’ is now subdivided into three belts: the Northeastern, Central and Southwestern belts, which are all in tectonic contact. The Northeastern and Southwestern belts are mainly composed of meta-komatiitic and basaltic rocks. Four granitic dykes intruding these belts have magmatic zircon ages of 2731–2530 Ma, supporting the conclusion that the supracrustal rocks were formed in the early Neoarchean. Meta-basalt with pillow structures has been identified in several locations, with the largest being ~35 m in thickness. The meta-komatiitic rocks are Al-undepleted komatiites and have ∑REE and (La/Yb)n of 2.30–26.21 ppm and 0.29–2.35, respectively, whereas the meta-basaltic rocks have ∑REE and (La/Yb)n of 16.45–53.26 ppm and 0.67–1.78, respectively, and εNd(t) values of 3.36–4.76, being similar to N-MORB. Eleven samples of meta-sedimentary rocks (including meta-sandstones-siltstones and meta-conglomerates) from the Central Belt have zircon UPb ages extending from 2760 to 2520 Ma, providing evidence for the first time that they were deposited during the late Neoarchean, rather than the early Neoarchean as previously thought. The meta-sandstones-siltstones show large variations in major element contents, but have low ∑REE (49.17–102.30 ppm) and (La/Yb)n (1.53–7.00) ratios, with negligible to weakly negative or positive Eu anomalies (Eu/Eu* = 0.60–1.17), and record εNd(t) values of 0.96–3.21. Combined with regional geological investigations, we conclude that the early Neoarchean supracrustal rocks were formed in an oceanic environment. It is also suggested that the sandstones-siltstones of the late Neoarchean were derived from the early Neoarchean basement, and that the conglomerates were derived from two separate source regions; one of early, and the other of late Neoarchean age. The identification of basal conglomerates is consistent with the arc model established in early studies. We suggest that the criteria employed in this study can be used for determining whether or not an Archean greenstone belt is of oceanic affinity.

Double-Front Crystallization in the Chapesvara Ultramafic Subvolcanic Complex, Serpentinite Belt, Kola Peninsula, Russia

Dunite–harzburgite–olivine-bearing orthopyroxenite successions in the subvolcanic Chapesvara-I and Chapesvara-II intrusions in the Serpentinite Belt, western Kola Peninsula, are notably magnesian. The mean Mg# value (whole-rock) is 86.6, and the olivine is Fo84−89. The upper contact facies (UCF) displays a lower Mg# (81.6). It consists of grains of Fo92 and abundant chromian spinel, implying rapid crystallization of an almost unfractionated melt. On average, the whole-rock Al2O3/TiO2 value is 22.45, close to 22.9 (UCF) and to the primitive mantle, ~22. The rise of primitive ultramafic magma presumably occurred in a special tectonic setting at the boundary of the Paleoproterozoic Lapland Granulite Terrane and the Belomorian Composite Terrane of Archean age. The Chapesvara suite resembles examples of the Al-undepleted komatiites in the Barberton Belt, South Africa, with magmas of up to 30–35% MgO. The UCF rock yields an anomalously low molar MgO/SiO2 value, close to that of dunitic rocks located at the center of the Chapesvara-II body. This rock is the most primitive, as indicated by the maximum Fo content of olivine, the lowest value of (Gd/Yb)N, 0.52, and the lowest abundances of middle to heavy rare-earth elements (REE) in the chondrite-normalized spectrum. The crystallization of the Chapesvara-II sill-like intrusion likely proceeded in two stages, which are evident from the olivine compositions varying from the maximum Fo92 (UCF) to Fo≤89.5 (the central dunite zone). At Stage 1, the UCF rock (Fo92) crystallized first, close to the upper contact. The area of crystallization then shifted to a central portion of the Chapesvara-II body, in which the dunitic zone (Fo89.5) formed in situ (Stage 2). The compositional variations in chromian spinel are consistent with this suggestion. Two crystallization trends were recognized. The type-1 trend displays a relative maximum or minimum close to the center, and then diverges into two linear subtrends directed upward and downward. This pattern is manifested in the variations of Mg# in olivine and chromian spinel, the whole-rock contents of Al and Ca, and in levels of incompatible elements: Ti, V, Zr, Y, and Hf. The type-2 trend decreases or increases uniformly from top to bottom. Variations in amount of Ni in olivine, the Fe3+# index in chromian spinel, and in values of Mg# (rocks), follow a type-2 trend. Variations in total amounts of REE, Nb, and Th, which gradually increase downward, are also related to a type-2 trend. Thus, a contrasting development and possible interference of the two types of evolutionary trends were observed in the crystallization history of the Chapesvara-II sill-like body. A double-front crystallization, hitherto unreported, involved two fronts moving upward and downward, respectively. The upward subtrend appeared to be of subordinate importance, whereas the extent of fractional crystallization of the downward front was much greater. Crystallization proceeded from the top to the bottom, presumably because of the preferential loss of heat at the roof. Variations in the Fe3+# index indicate that the level of fO2 also increased downward with progressive crystallization. Convection cells were presumably the key mechanism of accumulation of the crystallizing olivine grains to form the central dunite zone close to the center of the sill-like intrusion. The observed characteristics of the Chapesvara complex indicate the existence of a primitive-mantle source and imply a highly magnesian composition of intruding magma not only for Chapesvara, but also for the Pados-Tundra layered complex and associated suites of the Serpentinite Belt in the Kola Peninsula.

Transition metals in komatiitic olivine: Proxies for mantle composition, redox conditions, and sulfide mineralization potential

AbstractWe present the results of a comprehensive study on the concentrations of first-row transition elements (FRTE: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), as well as Ga and Ge, in liquidus olivine from 2.7–3.5 Ga old Al-undepleted and Al-depleted komatiites from the Kaapvaal and Zimbabwe Cratons in southern Africa, the Yilgarn Craton in Australia, and the Superior Craton in Canada. The sample set includes komatiites that remained sulfur-undersaturated upon emplacement, as well as komatiites that reached sulfide saturation owing to assimilation of crustal sulfur.All olivine grains display concentrations of Mn, Zn, Ge, Co, Fe, Mg, and Ni similar to the Bulk Silicate Earth (BSE) values, with significant negative anomalies in Sc, Ti, V, Cr, Ga, and Cu. Olivine from the studied Al-depleted komatiites displays on average higher 100×Ga/Sc ratios (&gt;5) than olivine from Al-undepleted komatiites (≤5). Because garnet preferentially incorporates Sc over Ga, the data suggest that elevated Ga/Sc ratios in komatiitic olivine are indicative of garnet retention in the source region of komatiites, highlighting the potential of olivine trace element chemistry as a proxy for the depth of komatiite melting and separation of the magma from the melting residue. Copper concentrations in the studied olivine grains are controlled by sulfur saturation of the host komatiite during olivine crystallization. Olivine from sulfur-undersaturated komatiite systems displays Cu concentrations mostly between 1 and 10 ppm, whereas olivine from sulfide-bearing komatiites has Cu contents of &lt;0.5 ppm. Because komatiites contain some of the world's highest metal tenor magmatic Ni-Cu sulfide deposits, the Cu variability in olivine as a function of the sulfide-saturation state highlights a potential application of olivine chemistry in the exploration for sulfide ore deposits.Olivine from the Paleo-Archean (3.5–3.3 Ga) komatiites displays overall higher V/Sc ratios (V/Sc = 2.1 ± 0.96; 2 S.D.) than olivine from their Neo-Archean (2.7 Ga) counterparts (V/Sc = 1.0 ± 0.81, 2 S.D.). Vanadium and Sc behave similarly during partial melting of the mantle and are similarly compatible in majorite garnet. However, V is redox-sensitive and its compatibility in olivine increases as the system becomes less oxidized, whereas Sc is redox-insensitive. We argue that olivine from the studied Paleo-Archean komatiites crystallized from more reduced magmas than their Neo-Archean counterparts. Elevated Fe/Mn ratios in olivine from Paleo-Archean komatiites mimic the V/Sc signatures and are interpreted to reflect that Fe2+ is more compatible in olivine than Fe3+. These results imply that V/Sc and Fe/Mn in komatiitic olivine may potentially provide insight into the evolution of the oxidation state of the Archean mantle. Additional studies that integrate the chemistry of komatiitic olivine with those of relict interstitial glass and melt/fluid inclusions are encouraged to fully understand and quantify the potential of FRTE in olivine as a proxy for the oxidation state of the mantle sources of komatiite magmas.

Genetic relationship among komatiites and associated basalts in the Badampahar greenstone belt (3.25–3.10 Ga), Singhbhum Craton, Eastern India

The ultramafic–mafic volcanic rocks of Archean greenstone belts are important archives for lithospheric and asthenospheric processes of the early Earth. Despite decades of research on this context, many issues still remain unsolved. For example, the process of komatiite magma genesis and the genetic relationship among komatiites, komatiitic basalts and tholeiitic basalts in Archean greenstone belts are not clearly understood. The metavolcanic rocks of the Badampahar greenstone belt (BGB), Singhbhum Craton are studied by major–trace element geochemistry to address the said problems and better understand the evolution of melts in Archean lithosphere. Our research suggests that the protoliths of the metavolcanic rocks were komatiites (both Al –depleted and –undepleted), komatiitic basalts and tholeiitic basalts. The Al–heavy rare earth element (HREE) depleted komatiites were formed by moderate degree mantle melting at a higher depth and the Al–HREE undepleted komatiites are products of moderate to high degree mantle melting at a shallower depth. The melting–assimilation–fractional crystallization modelling result shows that komatiitic basalts were generated from Al–undepleted komatiites, and tholeiitic basalts were generated from evolved komatiitic basalts by assimilation and fractional crystallization processes. The older age limit of the BGB is determined to be 3.25 Ga. and the basement of sedimentation and volcanism was composed of plutonic felsic rocks.

Petrogenesis of ultramafic rocks of komatiitic composition from the Central Zone of the Limpopo Belt, South Africa: Evidence from O and H isotopes

Komatiites are MgO-rich ultramafic volcanic rocks, mostly interpreted to be essentially anhydrous at the time of extrusion, and are often characterized by spinifex texture. They mainly occur in Archean or Proterozoic greenstone belts within cratons. However, in the Central Zone of the Limpopo Belt in South Africa, ultramafic rocks of komatiitic composition occur within a high-grade metamorphic terrane. Their chemical composition overlaps with that of Al-undepleted (Munro-type) komatiites from elsewhere, but unlike komatiites, they are mainly composed of pargasitic amphibole, along with olivine, orthopyroxene, Cr-rich spinel and magnetite. The studied rocks have a narrow range of whole rock δ18O values (+5.4 to +6.3‰), unlike δ18O values from komatiites, which are generally altered and have δ18O values from ∼+2 to +9‰. However, they are within the range estimated for unaltered komatiites (+5.4 to +6.0‰). Amphibole separates from the studied rocks have δ18O values between +5.5 and + 6.1‰, similar to that of the bulk rock. The δD values of amphiboles range between −78 and −83‰, similar to primary amphiboles from other komatiites and many other mantle-derived mafic igneous rocks. The δ18O values, along with textural and mineralogical evidence, are consistent with the ultramafic rocks of komatiitic composition from the Limpopo Belt being intrusive and unaffected by low-temperature alteration. The O and H isotope composition of the amphiboles are consistent with them being primary magmatic in origin. It cannot be distinguished with certainty whether the water was introduced to the magma through partial melting of hydrated mantle or during ascent through hydrous mantle subsequent to their formation. The minerals observed at present most likely crystallized from komatiite magma that intruded into the lower crust and subsequently re-equilibrated at the ambient P-T conditions.

Geochemistry and Sm-Nd isotopic characteristics of the Paleoarchean Komatiites from Singhbhum Craton, Eastern India and their implications

Komatiites near Kapili village, located in Palaeo-Mesoachean Gorumahishani-Badampahar greenstone belt of Singhbhum Craton, Eastern India preserve excellent igneous textures and exhibit co-association of Al-depleted (ADK) and Al-undepleted komatiites (AUK). The Kapili komatiites exhibit repeated spinifex textured flows (cooling units) overlying massive cumulate zone. The basal cumulate part of Kapili komatiite is chemically similar to ADK which is characterized by subchondritic Al2O3/TiO2 (9.38–10.5) and HREE depleted nature (average (Gd/Yb)PM∼1.35) elucidating presence of majorite in residuum whereas compound upper spinifex lava typically display superchondritic Al2O3/TiO2 (∼22–28.5) and HREE enriched nature (average (Gd/Yb)PM∼0.81) demonstrating majorite free residuum.Calculated primary magma compositions of Kapili ADK and AUK reveal that generation of primary liquid of Kapili ADK or basal cumulate took place at greater depth (>300km) or at higher pressure (∼9GPa) at around 30% melting of source in contrast with primary liquid of AUK (spinifex lava), which was generated at shallower depth (∼150km) or at lower pressure (∼6GPa) beyond majorite stability and at higher degree of melting (∼30–40%). ‘Hump-shaped’ pattern of primitive mantle (PM) normalized REE spectra of Kapili ADKs and their primary liquid is caused by depletion of both LREE and HREE with respect to MREEs. Depletion in LREE and HREE is explained by depletion of source prior melting and sequestration of HREE in residual majorite respectively. However, Kapili AUK or spinifex lava samples show flat, near primitive mantle (REE/PM∼1) pattern exclusive of any significant depletion and implying higher degree of melting (between 30 and 40%) of a PM like source.Sm-Nd isotopic studies of Kapili ADK and AUK reveal that they both were derived from a mantle source with long term depletion (εNd∼+2 to +4). Except differences in LREE/HREE ratio featured due to differences in melting conditions, similarity in overall immobile HREE and high field strength element (HFSE) concentrations of both the types and their corresponding primary magmas suggest a single plume source rather than multiple sources, whose composition was closer to primitive mantle and eventually segregated chemically different komatiitic melts (Al-depleted and undepleted) at different depths. Whole rock Th/Yb, Th/Nb, Zr/Nb, Nb/Y, (La/Nb)PM ratios of both Kapili AUK and AUKs are comparable to oceanic island basalt (OIB) and mid oceanic ridge basalt (MORB) similar to typical greenstone belt volcanics from oceanic provenance. In addition, presence of copious pillow basalt and oceanic chert associated with komatiites, absence of negative δNb values recommend emplacement of komatiitic lava in oceanic rather than continental or arc setting.

Hydrogen-rich hydrothermal environments in the Hadean ocean inferred from serpentinization of komatiites at 300 °C and 500 bar

Serpentinization of Al-depleted and Al-undepleted komatiites (and olivine for comparison) was experimentally characterized under high-temperature and high-pressure conditions of 300 °C and 500 bar to evaluate the H2 generation potential in komatiite-hosted hydrothermal systems in the early Earth. From the results, the steady-state H2 concentrations of fluids were estimated to be approximately 20 and 0.05 mmol/kg during the serpentinization reactions for Al-depleted and Al-undepleted komatiites, respectively (60 mmol/kg in the case of olivine). The H2 concentration of hydrothermal fluid generated from the serpentinization of Al-depleted komatiite is lower than that from olivine but is comparable to that of typical modern peridotite-hosted hydrothermal systems (~16 mmol/kg). The relatively low H2 concentration from Al-undepleted komatiite is similar to the levels in modern basalt-hosted hydrothermal fluids. Considering that the generation of Al-depleted komatiite melt requires a hotter mantle upwelling (plume) than the generation of Al-undepleted komatiite melt and that the temperature of the mantle has gradually decreased throughout Earth’s history, Al-depleted komatiite may have constituted ultramafic volcanism in Hadean oceanic islands/plateaus. Furthermore, it seems unlikely that seafloor exposure of mantle peridotites occurred frequently in the Hadean because the oceanic crust of that time was presumably much thicker than the modern equivalent. Therefore, the serpentinization of Al-depleted komatiites may have been the main process that provided abundant H2-rich seafloor hydrothermal environments in the Hadean ocean, which potentially acted as a nursery for the prebiotic chemical evolution and the emergence and early evolution of life on Earth.

Mode of emplacement of Archean komatiitic tuffs and flows in the Selkirk Bay area, Melville Peninsula, Nunavut, Canada

The 2.97Ga Prince Albert greenstone belt (PAGB) of Melville Peninsula (Nunavut, Canada) contains multiple komatiitic flows and laterally extensive units of komatiitic tuff in the Selkirk Bay area. These units are serpentinized, chloritized, amphibolitized, and less commonly, carbonate altered. The tuffs retain element abundances typical of Al-undepleted komatiites, identical to the associated komatiitic flows, indicating that the tuffs and flows are comagmatic. The flows exhibit excellent textural preservation with well-developed upper spinifex and lower cumulate zones typical of differentiated komatiite flows. However, the tuffs do not preserve primary microscale textures and are fine-grained and largely massive to planar-laminated. This, coupled with their extensive nature (up to 10km laterally; up to 10m thick vertically), lack of gradational contacts with komatiitic flows, and their komatiitic mineralogical and chemical composition support a pyroclastic origin, deposition via eruption-fed fallout and mass flow, and subaqueous deposition during intervals characterized by lower rates of effusive volcanism. The low volatile content of the komatiitic magma, indicated by lack of vesicles in komatiitic flows and the lack of scoriaceous lapilli in the tuff, suggests an origin through phreatomagmatic pyroclastic eruptions. The Selkirk Bay komatiitic lavas and tuffs have liquid compositions with up to 30wt% MgO, consistent with their derivation from a high magnesium komatiitic parental magma. There is evidence for up to 15% crustal contamination, which is also supported by slight enrichments in the LREE and Th relative to Nb. Major and minor elemental trends are indicative of olivine being the dominant liquidus mineral during early crystallization.Komatiitic flows of the PAGB were erupted onto massive to pillowed basalt where they formed thick, channelized, massive, undifferentiated flows flanked by spinifex-textured sheet/lobate flows. Synvolcanic structural basins and down-faulting are considered responsible for flow channelization and also controlled the deposition of local, framework-supported, ultramafic mass flow deposits and localized deposits of felsic volcaniclastic rocks. The komatiitic tuffs occur at two stratigraphic intervals indicating that they are a product of at least two pyroclastic eruptions. The uppermost strata document a change to a more mafic-dominated lava sequence interlayered with both differentiated and undifferentiated komatiitic flows, which are overlain by thick deposits of felsic volcaniclastic rock.

Geochemistry of two stratigraphically-related ultramafic (komatiite) layers from the Neoarchean Sigegudda greenstone terrane, Western Dharwar Craton, India: Evidence for compositional diversity in Archean mantle plumes

Two compositionally different ultramafic units are present in the Neoarchean Sigegudda greenstone terrane, Western Dharwar Craton, India. These ultramafic units occur in the same volcano-sedimentary sequence and are separated by a fault-bounded volcanic arc association. Because of deformation and amphibolite facies metamorphism, the primary igneous textures have been extensively modified in both ultramafic units. Given their spatial and temporal association with sedimentary and volcanic rocks and high MgO contents (12–29wt.%), these ultramafic rocks are interpreted as metamorphosed komatiites and komatiitic basalts. Field relationships and rock types indicate that the lower section of the Sigegudda volcano-sedimentary sequence was deposited in a peri-cratonic, continental rift setting. The lower ultramafic unit is characterized by 12.4–29.6wt.% MgO, Mg#=76–91, and 526–1150ppm Ni. The upper ultramafic unit is overall more magnesian and compositionally restricted: MgO=21.6–25.9wt.%, Mg#=86–89, and Ni=610–1000ppm. The lower unit features a combination of relatively elevated TiO2 (0.40–0.90wt.%), Al2O3/TiO2≤the chondritic ratio of 21, and (Gd/Yb)N ratios >1; these are Ti-enriched komatiites reported for the first time from greenstone belts in the Dharwar Craton. The upper unit is compositionally comparable to Al-undepleted komatiites and characterized by Al2O3/TiO2≥21, LREE-depletion coupled with near-flat chondrite-normalized HREE patterns. Geochemical data indicate that the Sigegudda komatiites and komatiitic basalts were variably contaminated by either continental crust or sub-continental lithospheric mantle, or both. The geochemical differences between the two units are explained by variable depths and degrees of partial melting. The Ti-enriched lower unit appears to have been formed through lower degrees of partial melting at depths ≥90km, whereas the Al-undepleted upper unit was generated by higher degrees of partial melting at depths ≤90km. Field relationships and geochemical characteristics of the Sigegudda komatiites can be explained by a geodynamic model in which the stratigraphically lower Ti-enriched komatiites were erupted from a mantle plume onto a rifting continental margin, whereas the stratigraphically upper Al-undepleted komatiites originated from a younger mantle plume and erupted onto a volcanic arc sequence that accreted to the rifted continental margin.

ON THE POSSIBLE OCCURRENCE OF KOMATIITES IN THE ARCHAEAN HIGH-GRADE POLYMETAMORPHIC CENTRAL ZONE OF THE LIMPOPO BELT, SOUTH AFRICA

Research Article| June 01, 2013 ON THE POSSIBLE OCCURRENCE OF KOMATIITES IN THE ARCHAEAN HIGH-GRADE POLYMETAMORPHIC CENTRAL ZONE OF THE LIMPOPO BELT, SOUTH AFRICA H. MOURI; H. MOURI Department of Geology, University of Johannesburg, South Africa, e-mail: hmouri@uj.ac.za Search for other works by this author on: GSW Google Scholar W.D. MAIER; W.D. MAIER Department of Geology, University of Oulu, Finland, e-mail: wolfgang.maier@oulu.fi Search for other works by this author on: GSW Google Scholar G. BRANDL G. BRANDL Council for Geoscience, Limpopo Unit, P.O. Box 620, Polokwana, 0700, South Africa, e-mail: gbrandl@geoscience.org.za Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2013) 116 (1): 55–66. https://doi.org/10.2113/gssajg.116.1.55 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation H. MOURI, W.D. MAIER, G. BRANDL; ON THE POSSIBLE OCCURRENCE OF KOMATIITES IN THE ARCHAEAN HIGH-GRADE POLYMETAMORPHIC CENTRAL ZONE OF THE LIMPOPO BELT, SOUTH AFRICA. South African Journal of Geology 2013;; 116 (1): 55–66. doi: https://doi.org/10.2113/gssajg.116.1.55 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 SocietySouth African Journal of Geology Search Advanced Search Abstract Komatiites occur in many Archaean and Proterozoic greenstone belts metamorphosed to greenschist or upper amphibolite facies conditions. However, komatiites have so far not been reported from high-grade metamorphic environments (upper granulite facies conditions). Here we report the occurrence of Mg-, Cr-and Ni-rich ultramafic rocks (26 to 31% MgO, 2800 to 3800 ppm Cr, 1400 to 1800 ppm Ni) with unfractionated PGE patterns (8.15 to 12.36 ppb Pt, 6.17 to 13.49 ppb Pd, Pd/Ir ~2 to 6) in the high-grade polymetamorphic Central Zone of the Limpopo Belt, South Africa. The composition of the rocks overlaps with that of Al-undepleted (Munro-type) komatiites from elsewhere, except that the Central Zone samples are markedly enriched in highly incompatible trace elements and show negative Nb anomalies. Based on these data, we interpret the rocks to represent contaminated komatiites of Archaean age – the first such manifestation in the Limpopo Belt and in a high to ultra-high grade (≥900°C, 10 kbar) metamorphic environment. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Archean komatiite-hosted, PGE-bearing Ni–Cu sulfide deposit at Vaara, eastern Finland: evidence for assimilation of external sulfur and post-depositional desulfurization

Archean komatiites host important resources of Ni, Cu, Co, and PGE, particularly in Western Australia and Canada. In Finland, several small, low-grade sulfide deposits have been found in komatiites, including the ca. 2.8 Ga Vaara deposit in the Archean Suomussalmi greenstone belt. It occurs in the central part of the serpentinized olivine cumulate zone of a komatiitic extrusive body and is composed of disseminated interstitial sulfides consisting of pyrite, pentlandite, millerite, violarite, and chalcopyrite accompanied by abundant magnetite. Although currently subeconomic, the mineralization is interesting due to the very high chalcophile element contents of the sulfide fraction (38 wt% Ni, 3.4 wt% Cu, 0.7 wt% Co, 22.4 ppm Pd, and 9.5 ppm Pt). The sulfides occur in relatively Cr-poor olivine cumulates suggesting involvement of a chromite-undersaturated magma. The parental magma was an Al-undepleted komatiite with an estimated MgO content of at least 24 wt%. In contrast to the common komatiite types in the eastern Finland greenstone belts, the Vaara rocks are moderately enriched in LREE relative to MREE, suggesting that crustal contamination played an important role in the genesis of the Vaara deposit. Multiple sulfur isotope data reveal considerable mass-independent sulfur isotope fractionation both in country rock sedimentary sulfides (Δ33S ranges from −0.50 to +2.37 ‰) and in the Vaara mineralization (Δ33S ranges from +0.53 to +0.66 ‰), which provides strong evidence for incorporation of crustal sulfur. Extensive replacement of interstitial sulfides by magnetite and the presence of millerite- and violarite-bearing, pyrrhotite-free sulfide assemblages indicate significant post-magmatic, low-temperature hydrothermal oxidation of the primary magmatic pyrrhotite-pentlandite-chalcopyrite assemblages and associated sulfur loss that led to a significant upgrading of the original metal tenors of the Vaara deposit.

Geochemistry of komatiites from the Tipasjärvi, Kuhmo, Suomussalmi, Ilomantsi and Tulppio greenstone belts, Finland: Implications for tectonic setting and Ni sulphide prospectivity

Komatiites and komatiitic basalts within the Neo-Mesoarchaean, ca. 2.8Ga greenstone belts in the western part of the Karelian craton in Finland formed in diverse palaeoenvironments. The lavas in the Tipasjärvi and Kuhmo greenstone belts formed in an oceanic plateau environment, whereas the komatiites of the Suomussalmi belt formed in a continental rift setting, consistent with Nd and Pb isotope data of Suomussalmi felsic volcanics indicating the presence of older (>3Ga) crust in the belt. The komatiites and komatiitic basalts of the Ilomantsi greenstone belt show trace element systematics similar to those of modern continental arc volcanics, implying interaction of the lavas with felsic arc volcanics. PGE contents are low (2–4ppb Pt and Pd each) relative to most other Al-undepleted komatiites from elsewhere, interpreted to result from equilibration with sulphides during emplacement. Xenocrystic zircon grains, up to 3.2Ga in age, in some felsic volcanics and granitoids that are coeval with the komatiites indicate the presence of older continental basement in the Ilomantsi area. The Ilomantsi komatiites are interpreted to have formed at an active continental margin where basement rocks were extensively intruded and assimilated by felsic plutonic massifs. The komatiites of the Tulppio belt in northern Lapland resemble those of the Tipasjärvi, Kuhmo and Suomussalmi belts, but due to limited data, their emplacement environment remains uncertain.The Ni sulphide prospectivity of the Finnish komatiite belts appears to be relatively low, despite the identification of geochemical signatures of PGE depletion and crustal contamination in the Ilomantsi belt and, to a lesser degree, Suomussalmi and Tulppio belts, the presence of locally sulphide-bearing felsic volcanics and sedimentary rocks in the footwall of some of the komatiites, and the discovery of several small magmatic sulphide deposits in the Kuhmo, Suomussalmi, Ilomantsi and Tulppio belts. The main reasons are that the Finnish komatiites are relatively differentiated compared to those in mineralised belts elsewhere, and the Finnish belts contain a relatively large proportion of compound sheet flows relative to adcumulates, suggesting that dynamic lava channel environments where significant amounts of sulphide-bearing substrate could be assimilated and deposited are uncommon. Furthermore, the samples analysed to date lack the signature of scattered, but widespread PGE enrichment that is characteristic of mineralized komatiite belts and that has been interpreted to reflect wide-scale saturation of the magmas in Ni-sulphide melt.

Geochemistry of a komatiitic, boninitic, and tholeiitic basalt association in the Mesoarchean Koolyanobbing greenstone belt, Southern Cross Domain, Yilgarn craton: Implications for mantle sources and geodynamic setting of banded iron formation

Komatiite–basalt magmatism and Algoma-type banded iron formation (BIF) of Archean greenstone terranes have common time series, but there are few constraints on the geodynamic setting in which BIF were precipitated. The ∼3.0Ga Koolyanobbing greenstone belt (KGB), Southern Cross Domain, Yilgarn craton, exhibits an greenschist to lower amphibolite facies metamorphosed lithostratigraphic series, including four predominantly mafic to ultramafic volcanic sequences (S1–S4) with prominent BIF horizons (and local basal pyrite-dominated massive sulfide lenses) in between. Petrography, XRD, high-precision major, trace high field strength, and REE element chemistry of representative ultramafic and mafic rocks from S1, S2, and S3 are consistent with a komatiitic, boninitic, and tholeiitic basalt association. Komatiites are Al-depleted and Al-undepleted (Al2O3/TiO2 12–28 and (Gd/Yb)N, ≥1) endorsing a zoned plume and/or melting over a range of depths. The boninite-series rocks (Al2O3/TiO2 28–45 and (Gd/Yb)N, <1) preserve characteristic U-shaped REE patterns and positive Zr–Hf/MREE anomalies. Komatiites include uncontaminated members and counterparts contaminated by metasomatized continental mantle lithosphere, suggesting eruption proximal to a rifted arc setting, whereas boninite-series were erupted at an intraoceanic arc, collectively consistent with plume eruption into a convergent margin. Al-depleted and Al-undepleted komatiites plot close together to the mantle array on the Th/Yb versus Nb/Yb discrimination plot of Pearce (2008), implying a depleted, mildly heterogeneous, source for the mantle plume from which komatiites erupted. Boninites lie parallel to and above the mantle array, in the field of intraoceanic arcs, in keeping with their inferred oceanic convergent margin setting. Ratios of Nb/Yb extend to lower and higher values than komatiites implying a more heterogeneous upper mantle source of the subarc wedge.The KGB exhibits the first reported boninite-suite rocks in the Southern Cross Domain, Yilgarn craton. Conclusively, the komatiitic, boninitic, and tholeiitic basalt association suggests that BIF and basal massive pyrite lenses of the Koolyanobbing greenstone belt were precipitated from low-temperature submarine discharge of a seawater-dominated hydrothermal system driven by magmatic systems of a mantle plume erupting in a forearc convergent margin suprasubduction ophiolite.

The geochemistry of Neoarchean (ca. 2700 Ma) tholeiitic basalts, transitional to alkaline basalts, and gabbros, Wawa Subprovince, Canada: Implications for petrogenetic and geodynamic processes

Neoarchean (ca. 2700 Ma) Wawa greenstone belts, Superior Province, contain two lithologically and geochemically distinct volcanic associations: (1) a plume-derived oceanic plateau association; and (2) a subduction-derived oceanic island arc association. The oceanic plateau association is composed of three groups: (1) voluminous tholeiitic basalts with near-flat REE patterns; (2) volumetrically minor moderately LREE-depleted tholeiitic basalts and Al-undepleted komatiites; and (3) volumetrically minor moderately to strongly LREE-enriched transitional to alkaline basalts and Al-depleted komatiites. This geochemical diversity is comparable to those of Phanerozoic oceanic plateaus and ocean islands. The oceanic island arc association is characterized mainly by variably LREE-enriched and Nb-depleted (relative to Th and La) tholeiitic to calc-alkaline basalts, andesites, dacites and rhyolites, with minor adakites, magnesian andesites, and Nb-enriched basalts. All these lavas are intruded by subduction-derived late- to post-tectonic gabbros, tonalites, trondhjemites, granodiorites, and monzonites. Within the Wawa subprovince the Schreiber–Hemlo greenstone belt is dominated by plume-derived oceanic plateau volcanic rocks (60–80%), whereas the Winston Lake and Manitouwadge greenstone belts are composed dominantly of subduction-derived (70–90%) volcanic rocks. New major and trace element, and Sm–Nd isotope data are presented for volcanic rocks from the Schreiber–Hemlo, Winston Lake, and Manitouwadge greenstone belts, to address their petrogenetic origin. In addition, new major and trace element data are reported for subduction-derived gabbros from the Schreiber–Hemlo greenstone belt. The geochemical diversity of the oceanic plateau association is consistent with a compositionally heterogeneous mantle plume source. Depleted initial Nd isotopic compositions ( ɛ Nd = +1.7 to +2.2) indicate that transitional to alkaline basalts were derived from a long-term LREE-depleted mantle source. It is shown that the origin of transitional to alkaline lavas can be explained by mixing of LREE-depleted and LREE-enriched melts in an upwelling mantle plume. Depleted to enriched LREE patterns in the plume-derived association are consistent with variable degrees of melting that took place over a range of temperatures and pressures as the plume ascended towards the surface. On the basis of field relationships and geochemical data, it is suggested that the geodynamic evolution of the Schreiber–Hemlo greenstone belt began with the initiation of a subduction zone at the margin of an oceanic plateau at about 2725 Ma, forming an intra-oceanic island arc. In response to plate reorganization and subducted slab rollback, the island arc underwent extension leading to the opening of a backarc basin. Hydrothermal alteration of backarc oceanic crust resulted in the formation of volcanogenic massive sulphide (VMS) deposits in the Winston Lake and Manitouwadge greenstone belts.