Ultramafic Volcanic Rocks Research Articles

Ti- and Ba-rich phlogopitic micas in alkaline basic and upper mantle igneous rocks; stoichiometry, stability, and Fe valence estimation reassessed and rationalised

Ti- and Ba-rich tri-octahedral micas occur in fractionated basic igneous rocks, metasomatized mantle peridotites, metamorphosed pelites/carbonates, and hydrothermally altered mineral deposits. Electron microprobe analyses (EMP), with all iron reported as FeO, were widely used in the 1970/80s to interpret Ti and Ba substitution mechanisms, based on 22 O2– unit cell calculations, implying that cation vacancies occur in octahedral and/or intersheet sites. In 1996 EMP with chemical and physical analyses for ferric and total Fe, H2O, (OH), and element-specific Fe X-ray Absorption Spectroscopy (both K and L-edges) established valence states for Fe and Ti and cation site occupancies, that ∼50 % O replaces (OH) molecules, and that 24 anion cell formulae show the absence of cation vacancies. Cell formula calculation protocol for phlogopitic micas is refined here and results tested against the stoichiometric formula for vacancy-free phlogopite, XIIK2VIMg6IV[Si6Al2]O20(OH)4. Hypothetical sheet silicate compositions, calculated with fixed contents of vacancies linked to particular mixed-valence element substitutions, confirm that reliable unit cell formulae for natural mica solids require that each stoichiometric vacancy must be accounted for. If reliable estimates for ‘excess O’ (denoted WO2−) are assigned to EMP analyses, the proportion of the oxy-mica component in a mica solid solution can be defined. This approach is tested using published analyses for Ti- and Ba-rich biotites from fractionated basic and ultramafic volcanic igneous rocks (oxymica range 2.5–45 %; TiO2 up to 14 %; BaO up to 23 %), upper mantle peridotites (equivalent values 7–18 %; 6 %; 0.7 %), and metasomatised upper mantle (2–37 %; 9 %; 23 %). Enrichments of Ti and Ba in micas are clearly linked to the extra oxygen charge required to neutralise the more highly charged Ba2+ and Ti4+ replacing K+ and Mg2+.Substitution mechanisms involving Ba, Ti, and Fe3+ in ideal phlogopite involve coupled inter-site / inter-valence interactions so both site-ordering and valence-balance between the different cation sites must be accounted for. The cation exchange 2XIIK+ + 4VI(Mg2+ + Fe2+) + 4IVSi4+ ↔ XIIBa2+ + 3VITi4+ + 4(Altotal + Fe3+ + Crtotal)3+ is used here so that valence and site order can be considered together. Compositional variations show well-defined trends towards high-oxymica content and vacancy-free Ti and Ba mica end-members, rather than to oxy-free end-members with essential vacancies. Average compositions for different source rock micas are used to assess compositional trends; molecular WO2− values are refined by relating initial estimates directly to the wt.% TiO2 contents; and refined WO2− and stoichiometrically calculated Fe3+/Fetotal ratios ranging ∼0.1–0.5) are correlated with possible primary magmatic values.

The discovery and petrogenetic significance of komatiites

This review paper reports on the discovery of komatiites and the impact this has had on the geosciences internationally. Komatiites were first discovered by Richard and Morris Viljoen in the Barberton greenstone belt of South Africa and proposed as a new class of ultramafic volcanic rock with >18 wt% MgO. Komatiites are in places associated with spinifex textures (elongated bladed and plumose olivine crystals). The discovery and petrological study of komatiites and understanding of the field controls have given rise to intense debate about their mantle origin, water content and geodynamic setting of eruption. The observation that komatiites are found predominantly in the Archean and to some extent in the Palaeoproterozoic, has led to the proposition that the early Earth was much hotter (by c. 400 °C) than the present day. However, some workers have challenged this model involving secular cooling of the Earth since Archean times. These workers argue that komatiites formed in a cooler Archean subduction zone, rather than in a mantle plume. Komatiites of the mid-to late-Archean have also proven to be important economically (e.g. Yilgarn Craton, Australia; Superior Craton, Canada) as they host important nickel-sulphide deposits. Today, novel petrological techniques (e.g. secondary ion microprobe analyses of olivine melt inclusions) are being applied to komatiites to address important outstanding questions in the geosciences, including the question of whether or not plate tectonics was operating in the early Archean.

In situ carbon storage potential in a buried volcano

Abstract In situ mineral carbonation in porous and permeable mafic and ultramafic volcanic rocks is proposed to be a promising process that can contribute toward safe and permanent CO2 sequestration. Here, we investigated a partially buried Late Cretaceous composite volcano located offshore the central West Iberian margin as a proxy for potential in situ mineral carbonation in volcanic edifices on continental margins worldwide. Based on seismic data, geochemistry, and petrophysical properties, deterministic scenarios for permanent carbon storage were estimated. Overall analysis of the nature of the volcano and its internal architecture revealed that this single edifice has the potential to store 1.2–8.6 Gt CO2 in newly formed carbonate mineral assemblages. Results suggest that ancient, buried volcanoes on continental margins constitute auspicious sites for safe carbon storage, with a total storage capacity of hundreds of gigatonnes of CO2.

Heavy Rare Earth Elements and the Sources of Continental Flood Basalts

AbstractHeavy rare earth elements (HREEs) in mafic and ultramafic volcanic rocks are useful recorders of mantle source processes because their ratios are not easily modified by differentiation. Here we utilize REEBOX PRO, a simulator of adiabatic decompression melting of the mantle, to study the behavior of HREEs in the formation of continental flood basalt (CFB) parental magmas in the mantle. We simulate partial melting of depleted peridotite, pyrolitic peridotite, pyroxenite, and peridotite-pyroxenite mixtures at mantle potential temperatures of 1350–1650°C and lithospheric thicknesses of 50–150 km, and compare the results to natural data. Many large igneous provinces are typified by low-Ti and high-Ti CFBs with contrasting HREE patterns. Our results show that low-Ti CFBs originate mainly from peridotitic sources. Flat mid-ocean ridge basalt-like HREE patterns typical of low-Ti CFBs can be generated beneath thick lithosphere (~100 km), given that mantle potential temperatures are high (>1500°C) and garnet is completely consumed from the source. We thus challenge the common interpretation that flat HREE patterns always indicate shallow sources for CFB parental magmas. High-Ti CFBs require pyroxenite-bearing sources (≥10%). Contrary to a common view, their steep oceanic island basalt-like HREE patterns can be generated beneath quite a thin lithosphere (~50 km), which is due to increased garnet stability in pyroxenite sources. When applied to CFBs of the Karoo large igneous province, the results are compatible with a model where a mantle plume penetrates a progressively thinning Gondwana lithosphere.

Stratigraphy of the Agnew-Wiluna Greenstone Belt: review, synopsis and implications for the late Mesoarchean to Neoarchean geological evolution of the Yilgarn Craton

Our ability to understand the geological evolution of Archean cratons is fundamentally dependent on stratigraphic reconstructions across greenstone belts. Here, we integrate published stratigraphic, geochemical and geochronological data to produce a revised stratigraphic model for the Agnew-Wiluna Greenstone Belt of the Yilgarn Craton. The Agnew-Wiluna Greenstone Belt records ca 130 m.y. of magmatism that occurred between ca 2820 and 2690 Ma and can be divided into three cycles. Cycle 1 is marked by ca 2820–2740 Ma mafic–ultramafic volcanic rock successions. Cycle 2 is associated with ca 2740–2720 Ma felsic volcanic rocks, associated fragmental deposits and contemporaneous mafic–ultramafic intrusions. Cycle 3 consists of ca 2720–2690 Ma mafic–ultramafic volcanic rock successions and co-magmatic intrusions. Exhumation and reworking of these continental rift basin successions after ca 2690 Ma resulted in two younger, additional, depositional cycles in the Agnew-Wiluna Greenstone Belt. Cycle 4 is associated with intermediate to felsic volcanoclastic rocks and epiclastic rocks erupted/deposited between ca 2690 and 2665 Ma. Cycle 5 is associated with the deposition of polymict conglomerates and sandstones in localised depocentres between ca 2665 and 2655 Ma. The stratigraphic scheme identified at the scale of the Agnew-Wiluna Greenstone Belt is the local expression of craton-wide tectono-thermal events. Our revised stratigraphic model for the Agnew-Wiluna Greenstone Belt and proposed correlations at the craton scale support a para-autochthonous model whereby crust from a proto-Yilgarn continental mass was extended/thinned, favouring the activation of translithospheric pathways associated with the ca 2720–2690 Ma Kalgoorlie Large Igneous Province. Inversion of this intra-cratonic rift basin after ca 2680 Ma led to the protracted activation of shear zones and multiple shifts in sedimentary depocentres. The widespread occurrence of previously unidentified fragments of ca 2820–2720 Ma substrate within the Kalgoorlie-Kurnalpi Rift has significant implications for craton-scale stratigraphic reconstructions and challenges existing terrane and domain boundaries. KEY POINTS We propose a revised, coherent and testable stratigraphic model for the Agnew-Wiluna Greenstone Belt of the Kalgoorlie Terrane. We compare our revised stratigraphic model for the Agnew-Wiluna Greenstone Belt with that of the Murchison and Southern Cross domains of the Youanmi Terrane in order to build a new space–time–integrated stratigraphic cross-section across the western half of the Yilgarn Craton. We demonstrate for the first time the existence of a magmatic continuum (i.e. no significant magmatic hiatus) between ca 2820 and 2690 Ma, providing the missing link between the geological record of the Youanmi Terrane and that of the Eastern Goldfields Superterrane.

Petrogenesis and redox state of late Mesozoic granites in the Pingmiao deposit: Implications for the W–Cu–Mo mineralization in the Dahutang district

The Pingmiao W (Cu) deposit is located in the central part of the newly-discovered giant Dahutang ore-concentrated district, South China. Field relationships and mineral assemblages confirm that the W (Cu) mineralization in the Pingmiao deposit is genetically related to multiphase late Mesozoic granites, which include the porphyritic two-mica granite, biotite granite porphyry, fine-grained muscovite granite, and Li-mica albite granite. Zircon U–Pb dating shows that these granites emplaced at ca. 146–143 Ma, which are coeval with the W–Cu–Mo-bearing granitoids from other ore deposits in the Dahutang district. The strongly peraluminous (A/CNK > 1.1) feature, negative correlations between Rb and Th and between Rb and P2O5, low Zr + Nb + Ce + Y values (mostly lower than 200), and relatively low Zr saturation temperatures (lower than 800 °C) suggest that these granites are typical S-type granite. These granites show similar Nd isotopes (−8.91 to −4.61), CaO/Na2O (0.2–0.57), and Al2O3/TiO2 (mostly ranging of 50–90) ratios with those W–Cu–Mo-bearing granitoids from other ore deposits in the Dahutang district, suggesting that they derived from partial melting of metapelites in the Shuangqiaoshan Group (or the Proterozoic metamorphic basement geochemically similar to the Shuangqiaoshan Group), with involving of minor mafic–ultramafic volcanic rocks. Geochemical fractionation trends recorded by whole-rocks and micas permit to distinguish the evolutionary trend from the porphyritic two-mica granite and biotite granite porphyry to the fine-grained muscovite granite and Li-mica albite granite. The apparent rare earth element tetrad effect, high Li and F contents, low K/Rb, Zr/Hf, and Nb/Ta ratios, as well as the zoned features of micas suggest that the melt–fluid interaction occurring during the formation of Li-mica albite granite. The Pingmiao granites have reduced redox state, revealed by their low whole-rock Fe2O3/FeO ratios (mostly <0.5), biotite Fe3+/Fe2+ ratios (ranging of 0.02–0.07), zircon Ce4+/Ce3+ ratios (median value of 18.70), and oxygen fugacity (near or below the FMQ buffer). The data in this study indicate that the late Mesozoic granitoids in the Dahutang district are favorable for W, Cu (<1 Mt), and Mo (<0.3 Mt) mineralization. Notably, the more evolved Li-mica albite granite has potential for W, Sn, Nb and Ta mineralization.

Volcanoes in the estuaries: Insights into Earth’s oldest (3.22 Ga) terrestrial microbial habitats, Moodies Group, Barberton Greenstone Belt

The Moodies Group of the Paleoarchean Barberton Greenstone Belt (BGB; ca. 3.22 Ga) provides an exceptional window into Archaean sedimentary and magmatic processes interacting with microbial ecology. Its magmatic component, the Moodies Igneous Complex (MIC), has to date been largely overlooked. The MIC consists of several mafic-to-intermediate sills, peperitic dike stockworks, extensive basaltic lava flows, and various volcaniclastic deposits and tuffs, all of Moodies depositional age. Excellent outcrop exposures illustrate diverse and dynamic interactions between volcanic activities and shallow-water sedimentary processes; they constrain the conditions in which microbial communities thrived in estuarine and tidal environments: (1) When the Moodies Basaltic Lava flooded the basin, some lava fragments and pillows were reworked as boulder-sized conglomerate clasts and in gravelly sandstones while still plastic and presumably hot. (2) Aerodynamically shaped ejecta of mafic lava and common tephra shards in cross-bedded sandstones demonstrate explosive lava-water interactions. (3) Dacitic (lapilli) tuffs up to several m thick were eroded by estuarine channels and filled back-beach lagoons, modifying and smothering concurrent aqueous transport of epiclastic sediment. (4) The paleo-groundwater table and the MIC interacted vigorously by in situ fluidization of poorly to unconsolidated sediment. This led to entrainment of grains and sandstone xenoliths in mafic (sub-)volcanics, widespread peperite formation, and formation of lava-filled fractures and abundant quartz veins in surficially consolidated sandstones.The proximity of numerous shallow intrusives, lavas, and volcanic tephra to the tidal-zone habitat of abundant benthic microbial mats provided microorganisms with readily accessible thermal and chemical energy. Heavy mineral lags in the estuaries, combined with potentially aggressive weathering of felsic to ultramafic volcanic rock, released ample nutrients as colloidally-bound transition metals and phosphorus. Drying-and-wetting cycles on tidal flats and coastal plains, the high porosity and permeability of the well-sorted quartzose sand in shallow water, the variably energetic current regime, and the anoxic, reducing-to-oxidizing chemistry of these habitats also benefitted microbial communities greatly.

Linking the Wrangellia flood basalts to the Gal\xe1pagos hotspot

The Triassic volcanic rocks of Wrangellia erupted at an equatorial to tropical latitude that was within 3000 km of western North America. The mafic and ultramafic volcanic rocks are compositionally and isotopically similar to those of oceanic plateaux that were generated from a Pacific mantle plume-type source. The thermal conditions, estimated from the primitive rocks, indicate that it was a high temperature regime (TP > 1550 °C) consistent with elevated temperatures expected for a mantle plume. The only active hotspot currently located near the equator of the eastern Pacific Ocean that was active during the Mesozoic and produced ultramafic volcanic rocks is the Galápagos hotspot. The calculated mantle potential temperatures, trace elemental ratios, and Sr–Nd–Pb isotopes of the Wrangellia volcanic rocks are within the range of those from the Caribbean Plateau and Galápagos Islands, and collectively have similar internal variability as the Hawaii-Emperor island chain. The paleogeographic constraints, thermal estimates, and geochemistry suggests that it is possible that the Galápagos hotspot generated the volcanic rocks of Wrangellia and the Caribbean plateau or, more broadly, that the eastern Pacific (Panthalassa) Ocean was a unique region where anomalously high thermal conditions either periodically or continually existed from ~ 230 Ma to the present day.

Geologic-tectonic setting of the gold-endowed Kilo Terrane in the eastern Central Kibalian Superterrane, northeastern Democratic Republic of Congo

Solid geological mapping, granite geochemistry and robust U-Pb SHRIMP geochronology confirm our subdivision of the eastern Central Kibalian Superterrane (ECKS) into two terranes: the ca 2570–2552 Ma Western Ituri Terrane and the ca 2680–2570 Ma Kilo Terrane. The ECKS is part of the Central, Northern and Southern Kibalian Superterranes, first-order interpretative crustal blocks, which constitutes the Kibalian Greenstone Region in the Bomu-Kibalian craton. The Kilo Terrane comprises: 1) the lower Kibalian lithostratigraphic units of deep-sea facies mafic–ultramafic volcanic rocks, interlayers of pelitic-sedimentary rocks and mafic granite and diorite-tonalite complexes in the Galaya-Kilo Sub-Terrane; and 2) the upper Kibalian lithostratigraphic units of intermediate-felsic volcanic-sedimentary, mafic–ultramafic intrusive rocks and widespread quartz-dioritic, tonalitic to granodioritic granites in the Balazana-Bunia Sub-Terrane.Mafic granite and diorite-to-tonalite magmatism in the Kilo Terrane is demonstrated by: 1) the ca 2680–2640 Ma syn-volcanic mafic-dioritic granite magmatism, including the ca 2650–2640 Ma albitic granites extensive in the Nizi-Tsi Belt; 2) the ca 2640–2600 Ma syn-volcanic hornblende tonalite-to-granodiorite magmatism in the Kilo and Moto-Zani Terranes; and 3) a ca 2600–2570 Ma syenite, quartz-monzonite and felsic granite intrusions along extensional zones and reactivated major terrane boundaries. The coincidence between the ca 2603 Ma tectonic uplifting and ca 2605–2590 Ma hydrothermal activity in the Kilo Terrane is associated with the gold mineralization.The 2711–2790 Ma xenocryst ages point to potential existence of the > 2790 Ma basement rocks extending from underneath the Western Ituri Terrane into underneath the Bunia West Domain. It’s absence in the gold-hosting domains of the Kilo Terrane does not exclude the existence of > 2.8 Ga basement, but instead it invokes on the existence of an unconformity separating the basement rocks from rootless, ca 2680–2570 Ma complexly folded and faulted juvenile granite-greenstone domains, sutured together by first- to second-order fault/shear zones. Protracted periods of Archean to post-Archean tectonic uplifting and inversion were responsible for preservations and erosion of upper crustal-level granite greenstone belts, including orogenic gold depositsThe ca 2680–2570 Ma juvenile crustal addition in the Kilo Terrane occurred some 30 m.y. after the ca 2850–2600 Ma juvenile crustal addition events in well-researched Archean cratons, worldwide.

Tectonic juxtaposition of plume and subduction derived magmatic sequences in the Bababudan greenstone terrane, western Dharwar Craton, India: Constraining crustal accretion processes in a Neoarchean subduction-collision orogeny

Archean greenstone belts in the Western Dharwar Craton preserve important clues on Early Earth geodynamics. Here we investigate the ultramafic-mafic metavolcanic rock sequences from the Santaveri Formation of Bababudan greenstone terrane to address the tectonic evolution, mantle dynamics and crustal growth processes of western Dharwar Craton. The ultramafic volcanic rocks are geochemically classified as komatiite-picrite-basanites, whereas the mafic volcanic rocks correspond to magnesian andesite -Niobium-enriched basaltic andesite association. The pristine mineralogy for these ultramafic volcanic rocks is overprinted by tremolite-actinolite-biotite assemblage marking greenschist facies of metamorphism. The komatiites include Al-undepleted, Al-depleted and Ti-enriched types and share overlapping geochemical characteristics with the picrites. The basanites are distinguished as alkaline ultramafic rocks with total alkali content >3 wt% and MgO > 18 wt%. The coherent association of komatiite-picrite-basanite suggests pervasive plume-lithosphere interaction and complies with polybaric melting of a chemically heterogeneous mantle plume through a subduction-modified sub-continental lithospheric mantle at a rifted cratonic margin. Our results substantiate the role of an Archean mantle refertilized by subduction-driven recycling of Mesoarchean supracrustals concurrent with the onset of subduction event at ≥3.2 Ga. The mafic rocks were derived through melting of a hot oceanic slab and mantle hybridization. The xenocrystic zircon grains from komatiite suggest lower crustal inheritance and reworking of basement gneisses by ascending ultramafic melts. The 3177 Ma magmatic age obtained from these zircon grains indicate a Mesoarchean magmatic event in the genesis of the basement gneisses. The emplacement of komatiite-picrite-basanite association and magnesian andesite -Niobium-enriched basaltic andesite lithologies envisages a post 3.1 Ga Neoarchean crustal growth over Mesoarchean protocontinent of WDC. This crust generation episode can be translated in terms of plume-driven bottom-up and subduction-driven top-down tectono-magmatic processes at continental rift and ocean-continent convergence set-up respectively. Geochemical characteristics and petrogenetic aspects suggest that the studied volcanic supracrustals of Bababudan greenstone terrane encapsulate a Neoarchean subduction-collision orogenic system comprising plume-derived komatiite-picrite-basanite and subduction-derived magnesian andesite -niobium-enriched basaltic andesite magmatic sequences tectonically juxtaposed during Neoarchaen (~2.7 Ga) subduction-accretion-collision event.

Olivine and Cr-spinel as indicators of the petrogenesis and partial melting conditions of the high-MgO ultramafic volcanic rocks from NW Ad Dhala Province—Yemen

The high-MgO ultramafic volcanic rocks in the NW Ad Dhala province are classified as meimechite according to the IUGS classification scheme. This province represents the southeastern outcrops of the Yemen Volcanic Group (YVG), which constitutes part of the Afro-Arabian continental large igneous province (LIP) and located within the boundary of the Afar mantle plume. In this study, we present the chemical compositions of olivine and Cr-spinel in meimechite rocks from Bagah Village in NW Ad Dhala province, aiming to characterize the genesis and partial melting conditions and to estimate the crystallization temperatures of these high-MgO rocks. Olivine crystals are characterized by high forsterite, ranges from Mg-rich core (up to Fo89.69) to relatively Fe-rich rim (down to Fo78.57), high CaO, MgO and MnO whereas Cr-spinel crystals have high TiO2 and Cr# values ranging from 0.49 to 0.63 which indicate that they are crystallized from primary magma and are typical of volcanic olivine and Cr-spinel that formed in intraplate tectonic setting. Olivine and Cr-spinel compositional data and primary melt composition (MgO ~ 23 wt.%) are compatible with the derivation of studied meimechite rocks from peridotite mantle source by small degrees of partial melting under conditions of high temperature and pressure at great depths, mostly within the garnet stability field. Also, these data provide a compelling indicator for the important role of upwelling Afar mantle plume in the genesis of these high-MgO ultramafic volcanic rocks. Finally, based on the different olivine-liquid equilibrium methods and Al-in-olivine thermometer approach the estimated crystallization temperature ranges from 1450 to 1490 °C, and mantle potential temperature (Tp) ranges from 1617 to 1677 °C, at high pressure (3–4.8 GPa). These high temperatures substantiate the existence of the Afar thermal mantle plume and its important role in the genesis of the studied meimechite rocks.

THE LOCAL SOURCES OF DETRITAL MATERIAL IN MIDDLE DEVONIAN QUARTZITES OF THE DONETS BASIN: RESULTS OF U-Pb LA-ICP-MS ZIRCON DATING

Results of U-Pb dating of zircons from the Middle Devonian sandstones of the Mykolaivka Suite, taken from the junction zone between the Azov Domain of the Ukrainian Shield and Donets basin, are reported. The rocks are taken from a drill core and they occur beneath a 210 m sequence of mafic and ultramafic volcanic rocks that belong to the Antonivka (D2-3) and Mykolaivka (D2) suites. In total, 28 U-Pb isotope analyses were performed. The predominant population (18 grains) of zircons yielded a concordant age of 2075±9 Ma (MSWD = 0.64). The second largest population consists of zircon crystals with concordant to nearly concordant ages ranging from 3130 to 2830 Ma. These results indicate a local origin for the detrital zircons. The occurrence of coarse-grained terrigenous sediments, derived from local rock sources, indicates significant erosion of the Precambrian basement that might have been caused by differential vertical movements during the Middle Devonian period. In the Donets basin, rifting and uplift started ca. 10 Myr before that of other parts of the Prypyat-Dnieper-Donets Paleorift.

Unraveling a hidden Rhyacian magmatic arc through provenance of metasedimentary rocks of the Crixás greenstone belt, Central Brazil

The Crixás-Goiás Domain, Central Brazil, comprises an association of Archean TTG (tonalite-trondhjemite-granodiorite) and Archean-Paleoproterozoic greenstone belts. The Crixás greenstone belt, located in the northern part of the domain, is constituted of Mesoarchean metakomatiites and metabasalts overlaid by Paleoproterozoic carbonaceous phyllites and metagraywackes that host important gold deposits. In this work, we present new integrated petrographical, geochemical and isotopic data to investigate the provenance and tectonic setting of the metasedimentary rocks of the Crixás greenstone belt. Mineral and textural immaturities, particularly of the metagraywackes, are consistent with a proximal source area; which is also evidenced by the chemical aspects that denote absence of significant sediment sorting and recycling. Geochemical data indicate provenance from mixed mafic and felsic sources. U-Pb detrital zircon geochronology shows unimodal age spectra, with a dominant peak at ca. 2.2–2.1 Ga and minor contribution of ca. 2.7 and 2.4 Ga grains. Concordia diagrams yielded upper intercepts between 2213.4 ± 5.6 and 2162.2 ± 9.2 Ma, interpreted as the age range of the main source area. The rocks present relatively homogeneous Sm-Nd model ages (2.44 to 2.34 Ga) and slightly negative to positive ƐNd, ranging from −0.27 to +1.25. We argue that the original sedimentary protoliths were deposited in a trench or forearc basin related to a ca. 2.2–2.1 Ga island arc formed adjacent to an Archean paleocontinent constituted of TTG-greenstone terranes. Rapid erosion of the rising intraoceanic arc resulted in syn-orogenic sedimentation that started with deep ocean black shale deposition in an anoxic environment followed by episodic turbidite currents of poorly-sorted immature sands. Sedimentary rock layers were tectonically transported to the continent and preserved in the elongated keels of Mesoarchean mafic–ultramafic volcanic rocks. This orogeny can be correlated with the amalgamation of the Goiás Massif during the Siderian-Rhyacian crustal growth related to successive collage of arc-related terranes in the formation of the western margin of the São Francisco paleocontinent. Sediment provenance of the Crixás greenstone belt proved to be crucial to the recognition and characterization of Paleoproterozoic crust that are apparently no longer preserved in outcrops.

Geochemistry and petrogenesis of high-MgO ultramafic tertiary volcanic rocks from Bagah area, NW Ad Dhala Province, Yemen

The studied high-MgO ultramafic Tertiary volcanic rocks are recorded in Bagah Village at the NW Ad Dhala province within the southeastern exposures of the volcanic rocks of the late Oligocene-early Miocene Yemen Trap Series (YTS) that belong to the Yemen Volcanic Group (YVG) which constitute part of the Afro-Arabian rift-related continental large igneous province. These rocks are strongly porphyritic composed of olivine phenocrysts set in hypocrystalline groundmass composed of olivine, pyroxene, opaque, rarely plagioclase and glass materials. The Bagah high-MgO volcanic rocks characterized by their high MgO (23.98–25.47 wt.%) and TiO2 (1.32–1.48 wt.%) with low Na2O + K2O (1.15–1.60 wt.%) and SiO2 (43.71–44.26 wt.%) and are classified as meimechite, and this is the first reported meimechite rocks throughout the volcanic rocks of the western Yemen continental volcanic province and, upon our knowledge, throughout the Afro-Arabian continental flood province. These rocks are classified to be of the high-Ti type and assigned to HT1 type corresponding to the Ethiopian-Yemeni CFB zonal arrangement reported in the literatures. They have high Mg number (79.4–80.0) and high concentrations of Cr (up to 2976 ppm), Ni (up to 1115 ppm) and other compatible elements with overall enrichment in an incompatible and LREEs and depletion in Y and other HREEs with variable anomalies of LIL and HFS elements. These characteristics are similar to meimechite rocks in many publications particularly those from the Vestfjella area of the Karoo LIP and Yanbian Terrane of the Emeishan LIP, but slightly differ from those of Meimecha-Kotui region of the Siberian LIP. The genesis of these rocks implies the incorporation of four mantle sources closely related to the interaction of the Arabian lithospheric mantle with the upwelling thermal Afar plume beneath study area and include OIB, N-MORB and E-MORB sources with participation of HIMU source as an additional component. The OIB-like mantle source derived directly from Afar plume. The N-MORB and E-MORB sources are formed by partial melting of normal and metasomatized Arabian lithospheric mantle due to the continuous raising of Afar mantle plume, and these mantle lithosphere-derived magmas had introduced significant contributions to the genesis of Bagah ultramafic rocks than those derived from Afar plume. Accordingly, the geochemical characteristics of these rocks can be interpreted in terms of mixing between these mantle-derived magma sources and the conditions that were operated during the creation of such magmas. The Bagah ultramafic volcanic (supposed meimechite) rocks were formed by small degrees of partial melting of garnet-bearing peridotite mantle at great depths under high melting pressure and temperatures above the amphibole stability due to the thermal effects of Afar plume. Such small degrees of partial melting, under similar melting conditions of either normal and/or metasomatized mantle peridotite are suggested for the formation of meimechites rocks from the Meimecha-Kotui province of the Siberian LIP, the Vestfjella area of the Karoo LIP and the Yanbian Terrane of the Emeishan LIP. Finally, these meimechite-forming magmas are rapidly ascent through the Arabian lithosphere with no or negligible crustal assimilation in rout to the surface.

Effects of superheating magnitude on olivine growth

Magmatic superheating is a condition with relevance to natural systems as well as experimental studies of crystallization kinetics. Magmas on Earth and other planetary bodies may become superheated during adiabatic ascent from the mantle or as a consequence of meteorite impact-generated crustal melting. Experimental studies of igneous processes commonly employ superheating in the homogenization of synthetic starting materials. We performed 1-atmosphere dynamic crystallization experiments to study the effects of superliquidus thermal history on the morphologies and compositions of subsequently grown olivine crystals. An ultramafic volcanic rock with abundant olivine was fused above the experimentally determined liquidus temperature (1395 °C), held for 0, 3, or 12 h, cooled at 25 °C h−1, and quenched from 200 °C below the liquidus, all at constant fO2, corresponding to FMQ-2 ± 0.2 log units. An increase in olivine morphologic instability is correlated with superheating magnitude, parameterized as the integrated time the sample is held above the liquidus (“TtL”; °C h). We infer that a delay in nucleation, which intensifies monotonically with increasing TtL, causes crystal growth to be increasingly rapid. This result indicates that the structural relaxation time scale controlling the formation of crystal nuclei is (a) far longer than the time scale associated with viscous flow and (b) exceeds the liquidus dwell times typically imposed in crystallization experiments. The influence of magmatic superheating on crystal morphology is similar in sense and magnitude to that of subliquidus cooling rate and thus, both factors should be considered when interpreting the thermal history of a volcanic rock containing anhedral olivine.

Petrological, mineralogical and geochemical peculiarities of Archaean cratons

The most outstanding features of Archaean cratons are their extraordinary thickness and enduring longevity. Seismically, Archaean cratonic fragments are sharply-bounded deep roots of mechanically strong, cold lithospheric mantle, clearly distinguishable from non-cratonic lithosphere. Rhenium-depletion of deep cratonic xenolith whole rocks and sulphide inclusions in diamond indicate that melting was broadly coeval with formation of the overlying proto-cratonic crust, which was of limited mechanical strength.A very important process of proto-cratonic development was vertical crustal reorganisation that eventually yielded a thermally stable, cratonised crust with a highly K-U-Th-rich uppermost crust and much more depleted deeper crust. Clastic sedimentary rocks available for geochemical study are predominantly found in the youngest parts of supracrustal stratigraphies and over-represent the highly evolved rocks that appeared during cratonisation. Vertical crustal reorganisation was driven by crustal radiogenic heat and emplacement of proto-craton-wide, incubating and dense supracrustal mafic and ultramafic volcanic rocks. Statistical analysis of these cover sequences shows a preponderance of basalt and a high abundance of ultramafic lavas with a dearth of picrite. The ultramafic lavas can be grouped into Ti-enriched and Ti-depleted types and high pressure and temperature experimental data indicate that the latter formed from previously depleted mantle at temperatures in excess of 1700 °C.Most mantle harzburgite xenoliths from cratonic roots are highly refractory, containing very magnesian olivine and many have a high modal abundance of orthopyroxene. High orthopyroxene mode is commonly attributed to metasomatic silica-enrichment or a non-pyrolitic mantle source but much of the excess silica requirement disappears if melting occurred at high pressures of 4–6 GPa. Analysis of experimental data demonstrates that melting of previously depleted harzburgite can yield liquids with highly variable Si/Mg ratios and low Al2O3 and FeO contents, as found in komatiites, and complementary high Cr/Al residues. In many harzburgites, there is an intimate spatial association of garnet and spinel with orthopyroxene, which indicates formation of the Al-phase by exsolution upon cooling and decompression. New and published rare earth element (REE) data for garnet and orthopyroxene show that garnet has inherited its sinusoidal REE pattern from the orthopyroxene. The lack of middle-REE depletion in these refractory residues is consistent with the lack of middle- over heavy-REE fractionation in most komatiites. This suggests that such pyroxene or garnet (or precursor phases) were present during komatiite melting. In the Kaapvaal craton, garnet exsolution upon significant cooling occurred as early as 3.2 Ga and geobarometry of diamond inclusions from ancient kimberlites also supports cool Archaean cratonic geotherms. This requires that some mantle roots have extended to 300 to possibly 400 km and that early cratons must have been much larger than 500 km in diameter.We maintain that the Archaean-Proterozoic boundary continues to be of geological significance, despite the recognition that upper crustal chemistry, as sampled by sedimentary rocks, became more evolved from ca. 3 Ga onwards. The boundary coincides with the disappearance of widespread komatiite and marks the end of formation of typical refractory cratonic lithosphere. This may signify a fundamental change in the thermal structure of the mantle after which upwellings no longer resulted in very high temperature perturbations. One school of thought is that the thermal re-ordering occurred at the core-mantle boundary whereas others envisage Archaean plumes to have originated at the base of the upper mantle. Here we speculate that Archaean cratonic roots may contain remnants of older domains of non-convecting mantle. These domains are potential carriers of isotope anomalies and their base could have constituted a mechanical and thermal boundary layer. Above laterally extensive barriers, emerging proto-cratons were protected from the main mantle heat loss. The eventual collapse of these mechanical barriers terminated very high temperature upwellings and dismembered portions of the barrier were incorporated into the cratonic mantle during the final Neoarchaean ‘superplume’ event. The surviving cratons may therefore preserve biased evidence of geological processes that operated during the Archaean.

Mashhad komatiitic rocks in NE Iran: Origin and implications for the evolution of the Paleo‐Tethyan Ocean

AbstractMashhad ultramafic volcanic rocks, located within the Paleo‐Tethys suture zone in the north‐eastern Iran, are composed mainly of komatiitic rocks with interbedded picrobasalts. The Mashhad komatiitic rocks are characterized by low Al2O3/TiO2 ratios (9.85–11.44) and depleted HREE patterns with slightly high (Gd/Yb)N ratios (1.48–1.74), similar to those of the Al‐depleted Barberton‐type komatiites. These rocks have ƐNd (t) values ranging from +9.0 to +10.4 and initial 87Sr/86Sr ratios ranging from 0.7039 to 0.7072, indicating they were derived from a depleted mantle source. They have a total 187Re/188Os and 187Os/188Os ratios ranging from 0.278 to 72.254 and from 0.12741 to 0.14012, respectively, and display consistent PGE patterns similar to that of the primitive mantle. The picrobasalts have high contents of SiO2 (45.08–48.87 wt.%), MgO (7.28–11.51 wt.%), nearly flat REE pattern, and ƐNd (t) values ranging from +6.5 to +7.9 and initial 87Sr/86Sr ratios focused on 0.7040 and 0.7070, illustrating that they were derived from a relatively enriched source compared with the komatiitic rocks. The thermobarometric calculations suggest that the Mashhad komatiitic rocks were generated by high degree partial melting between 20% and 30% at 1,604–1,681°C and 3.20–4.65 Gpa, essentially consistent with those of the Hawaiian mantle plume (1,500–1,600°C). These new data demonstrate that the Mashhad komatiitic rocks are of a mantle plume origin, which might have caused the opening of the Paleo‐Tethyan Ocean, and the mantle plume was likely located beneath or nearby the mid‐ridge of the Paleo‐Tethyan Ocean when the Mashhad komatiitic lava erupted, a scenario resembling the present Iceland and Ontong Java.

Geochemistry of Mesoarchean felsic to ultramafic volcanic rocks of the Lac Guyer area, La Grande Subprovince (Canada): Evidence for plume-related magmatism in a rift setting

The Lac Guyer area of the La Grande Subprovince (Superior Province) hosts Mesoarchean east-trending volcano-sedimentary sequences subdivided into 3 stratigraphic units (Mintisch and Rouget Formations and Guyer Group), which were emplaced between 2847 and 2806 Ma on a gneissic-tonalitic basement. These approximately 35 to 175 km-long sequences comprise felsic to ultramafic volcanic rocks that have been metamorphosed to the amphibolite and granulite facies and record mainly two Archean episodes of ductile deformation.The Mesoarchean tholeiitic basalt, komatiitic basalt, and komatiite appear to be genetically related based on petrologic modeling, their close spatial association, and their major and trace element signatures. They are characterized by the following: depletion to enrichment in LREE relative to HREE; Zr/Ypm, Ti/Vpm, and La/Smpm ratios higher than or close to 1; Ti/Zrpm and Nb/Thpm ratios lower than or close to 1; common negative anomalies in Nb-Ta ± Ti; and local positive to negative anomalies in Hf-Zr. Komatiitic basalt and komatiite are mostly Al-undepleted and rarely Al-depleted. According to current komatiite melting models, this suggests that these rocks formed by a high degree of partial melting of a garnet-bearing lherzolite source that consumed any residual garnet and progressively depleted the primitive mantle source in incompatible elements. Basalt and komatiitic basalt compositions were likely produced by assimilation and fractional crystallization (AFC) processes from komatiitic magmas (∼21–23 wt% MgO), as suggested by AFC modeling and their geochemical characteristics, whereas komatiite compositions were controlled by partial melting processes and crustal contamination.The Mesoarchean calc-alkaline rhyolite, dacite, and andesite are enriched in LREE relative to HREE, enriched in Th relative to LREE, and have negative anomalies in Nb-Ta-Ti, negative to positive anomalies in Hf-Zr, as well as high Zr/Y and Ti/V ratios and low Ti/Zr ratios. Such geochemical signatures could be explained either by concurrent assimilation and fractional crystallization of the basaltic magmas in the lower crust or by magma mixing between basalt and lower crustal anatectic magmas.The spatial and temporal distribution of Mesoarchean volcanic rocks within the Lac Guyer area and their major and trace element characteristics suggest that they were emplaced into a continental rift associated with a mantle plume. The geochemical similarities between these rocks and the rift-related volcanic rocks of the younger Yasinski Group (2751–2732 Ma), and the presence of other Mesoarchean volcanic sequences with rift affinity indicate that the La Grande Subprovince was affected by two main plume-related rifting events during the Archean (ca. 2.88–2.81 and 2.75–2.71 Ga), each produced by a single or multiple magmatic plumes.

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.

Mantle Potential Temperature Estimates and Primary Melt Compositions of the Low-Ti Emeishan Flood Basalt

The Late Permian Emeishan large igneous province (ELIP) is considered to be one of the best examples of a mantle plume derived large igneous province. One of the primary observations that favour a mantle plume regime is the presence of ultramafic volcanic rocks. The picrites suggest primary mantle melts erupted and that mantle potential temperatures (TP) of the ELIP were > 200oC above ambient mantle conditions. However, they may represent a mixture of liquid and cumulus olivine and pyroxene rather than primary liquids. Consequently, temperature estimates based on the picrite compositions may not be accurate. Here we calculate mantle potential temperature (TP) estimates and primary liquids compositions using PRIMELT3 for the low-Ti (Ti/Y < 500) Emeishan basalt as they represent definite liquid compositions. The calculated TP yield a range from ~1400oC to ~1550oC, which is consistent with variability across a mantle plume axis. The primary melt compositions of the basalts are mostly picritic. The results of this study indicate that the Emeishan basalt was produced by a high temperature regime and that a few of the ultramafic volcanic rocks may be indicative of primary liquids.