Tholeiitic Suite Research Articles

Influence of subducted carbonate on the composition of basalts in the Paleo-Asian Ocean domain

The fate of subducted carbonate of the Paleo-Asian Ocean and its control on the compositional variability of Cenozoic basalts to the west of the Daxing'anling–Taihang Gravity Lineament remain uncertain. Here we report major- and trace-element contents and Zn–Sr–Nd isotopic data for Cenozoic basalts from eastern Mongolia. The studied basalt samples from eastern Mongolia are classified as nephelinite, basanite, alkaline basalt, and tholeiite. They have different Zn–Sr–Nd isotopic compositions (δ66Zn = 0.38‰ ± 0.02‰ to 0.50‰ ± 0.04‰, 87Sr/86Sr = 0.704239–0.705175, εNd = 4.72–5.67) but display trace-element signatures of sedimentary marine carbonate, reflecting mantle heterogeneity and marine carbonate imprint. Compared with the high-Si tholeiites, which were erupted over thinner lithosphere, the nephelinites–basanites–alkaline basalts erupted over thicker lithosphere have higher δ66Zn values, La/Yb, Sm/Yb, and Nb/Y ratios, and Nb contents, indicating that these low-Si basalts were the product of lower-degree melting of carbonated mantle under higher pressure. By combining our data with available data for Cenozoic basalts from central Inner Mongolia, we suggest that the compositions of the nephelinite–basanite–alkaline-basalt–tholeiite suite show systematic spatial variation with respect to the northward subduction zone of the Paleo-Asian Ocean. The tholeiites, which have lower δ66Zn values, SiO2 contents, and Sm/Yb ratios, are dominant near the subduction zone, whereas the basanites and alkaline basalts, which have higher δ66Zn values, Sm/Yb ratios, and SiO2 contents, are dominant in the region far from the subduction zone. On the basis of a mixing model for Sr–Nd–Zn isotopic compositions, we suggest that the mantle source of the basanites and alkaline basalts incorporated a higher proportion of marine carbonate, whereas the tholeiite mantle source incorporated a higher proportion of siliceous sediments. We deduce that with increasing depth of slab subduction, more sedimentary carbonates and lesser siliceous sediments were released from the subducted slab.

Evolved magmatic arcs of South Borneo: Insights into Cretaceous slab subduction

Combined U-Pb zircon data of granitic, dioritic, and gabbroic suites reveal development of arc-related magmatism in South Borneo throughout the Late Mesozoic but dominantly in the Cretaceous (144–72 Ma). Cretaceous igneous suites constitute the Schwaner-Singkawang, Kuching-LongPahangai, and Meratus-West Java magmatic arc zones. Arc-related felsic suites are low-K tholeiitic, and calc-alkaline to high-K calc-alkaline. Most of them are metaluminous (A/CNK 0.7–1.1), I-type, and have high large-ion lithophile element (LILE) contents, low high-field strength element (HFSE) contents, and low crystallization temperatures (688–769 °C). A few rocks have adakite characteristics, such as high Sr/Y and low Y. Late Cretaceous granitic suites from South Schwaner, which evolved at elevated temperature (761–852 °C), have diverse compositions that range from I-type transitional to A-type. The highly fractionated chemistry of these samples is attributed to an intraplate stretching tectonic setting. This regime is consistent with tectonic controls of the synchronous tholeiitic suites within North Schwaner. Two subduction systems beneath Sundaland converged and interacted during the Cretaceous. The Paleo-Pacific subduction in the north built arc-trench architectures, as shown by the Early Cretaceous Lupar Line mélange, the Schwaner to Singkawang magmatic arc, and the Late Cretaceous Lupar Line mélange and Kuching-LongPahangai arc. The Meso-Tethys subduction in the southeast produced the Cretaceous Schwaner-Singkawang magmatic arc, Meratus to Luk-Ulo mélanges, and Meratus-West Java magmatic arc. When entered into Late Cretaceous, slab rollback led to crustal stretching in the Schwaner to Singkawang regions, with the production of A-type transitional and tholeiitic suites. Though debatable, we infer the SW Borneo block to be a Sundaland continuation; Semitau shows an affinity for South Cathaysia. Both of the terranes would accrete onto Sundaland during the Indosinian orogeny.

Stable zirconium isotopic fractionation during alkaline magma differentiation: Implications for the differentiation of continental crust

Stable Zr isotopes have shown great potential in tracing magma differentiation. However, it remains unclear if different magmatic systems have similar Zr isotopic evolution patterns. Here we report high-precision Zr isotopic compositions of the well-defined Shidao alkaline complex (SAC), eastern China, which show small but important differences from the Hekla tholeiitic volcanic suite, Iceland. The SAC comprises monzodioritic, syenitic, and granitic intrusions with high alkali contents, representing a magmatic suite produced by various degrees of cognate magma differentiation. From mafic to felsic samples, the Zr concentrations and Zr/Hf ratios increase first and then decrease significantly at the inflection of ∼65 wt% SiO2, suggesting zircon saturation started at ∼65 wt% SiO2 during magma differentiation. The δ94Zr values (permil deviation of the 94Zr/90Zr ratio from IPGP-Zr standard) remain nearly constant at ca. +0.01‰ in samples with SiO2 < 65 wt% but then gradually increase with SiO2 in more felsic samples (from +0.05 to +0.63‰), indicating that the Zr isotope fractionation also occurs at ∼65 wt% SiO2. The Shidao alkaline and Hekla tholeiitic suites show zircon saturation at SiO2 ∼ 65 wt%, but their Zr isotopic evolution paths differ. The alkaline suite increases more slowly in δ94Zr compared to the tholeiitic one, with the former reaching the same δ94Zr value at SiO2 higher by ∼5 wt%. Our findings represent the first observation of differences in Zr isotopic evolution from different igneous systems (tholeiitic versus alkaline), which should be considered when interpreting Zr isotopic data of samples out of context, such as detrital zircons or sedimentary samples. Based on the more sluggish Zr isotopic enrichment in the alkaline series than the tholeiitic one, we re-interpret the Zr isotopic composition of the upper continental crust, which may simply be not felsic enough to be isotopically heavier.

Eoarchean subduction-like magmatism recorded in 3750\xa0Ma mafic\u2013ultramafic rocks of the Ukaliq supracrustal belt (Qu\xe9bec)

Our understanding of the nature of crustal formation in the Eoarchean is limited by the scarcity and poor preservation of the oldest rocks and variable and imperfect preservation of protolith magmatic signatures. These limitations hamper our ability to place quantitative constraints on thermomechanical models for early crustal genesis and hence on the operative geodynamic regimes at that time. The recently discovered ca. 3.75 Ga Ukaliq supracrustal enclave (northern Québec) is mainly composed of variably deformed and compositionally diverse serpentinized ultramafic rocks and amphibolitized mafic schists whose metamorphic peak, inferred from phase equilibria modeling, was below 720 °C. Inferred protoliths to the Ukaliq ultramafic rocks include cumulative dunites, pyroxenites, and gabbros, whereas the mafic rocks were probably picrites, basalts, and basaltic andesites. The bulk-rock and mineral chemistry documents the partial preservation of cumulative pyroxenes and probably amphiboles and demonstrates the occurrence of a clinopyroxene-dominated, tholeiitic suite and an orthopyroxene-dominated, boninite-like suite. Together with the presence of negative μ142Nd anomalies in the boninitic basalts, two liquid lines of descent are inferred: (i) a damp tholeiitic sequence resulting from the fractionation of a basaltic liquid produced by mantle decompression; and (ii) a boninitic suite documenting the evolution of an initially primitive basaltic andesite liquid produced by flux melting. Petrographic observations, thermodynamic modeling, bulk-rock and mineral chemistry, and 142Nd isotopic compositions identify the Ukaliq supracrustal belt as the remnant of an Eoarchean arc crust produced by the recycling of Hadean crust in a similar way as modern-style subduction.

Sr‐Enriched Glassy Picrites From the Karoo Large Igneous Province are Evolved, not Primitive Magmatic Rocks

AbstractMagmatic products of the Karoo Large Igneous Province can be divided into a volumetrically dominant, compositionally uniform low‐Ti tholeiitic suite, and a subordinate, geographically restricted, compositionally diverse, incompatible‐rich high‐Ti suite. High‐Ti picrites contain up to 2,400 ppm Sr, 1,900 ppm Ba, and 550 ppm Zr, which seems unusual for olivine‐enriched rocks. We studied six Karoo picrites to determine the phase(s) in which Sr resides. Samples consist of 10–30% olivine phenocrysts in a groundmass of brown glass, augite, feldspar, ilmenite, and apatite. Glass compositions vary, but are generally evolved, ranging from basaltic trachyandesite to dacite. X‐ray intensity maps demonstrate that most of the Sr resides in the glasses, and to a lesser extent, in feldspars, if present. Some samples contain two texturally and compositionally distinct glasses, best modeled in terms of magma hybridization: variably accumulated olivine phenocrysts surrounded by evolved Sr‐rich (to 9,470 ppm) Type 2 melts formed by extensive olivine fractionation were infiltrated by chemically distinct Type 1 melts. Upon eruption, Type 2 melt quenched to minor glass around olivine phenocrysts, and the dominant Type 1 glass acquired its evolved composition by quench crystallization of groundmass mineral phases. Both glasses are rich in H2O (up to 3.8 wt. %) but are nearly devoid of CO2. Calculated parental melts have much higher K2O and incompatible trace elements (e.g., Sr or Ba &gt;1,200 ppm) relative to low‐Ti tholeiites. The unusual parental melt compositions imply derivation by small degrees of partial melting from SCLM mantle sources enriched in Sr and other incompatibles.

New constraints on the age, geochemistry, and environmental impact of High Arctic Large Igneous Province magmatism: Tracing the extension of the Alpha Ridge onto Ellesmere Island, Canada

Abstract The High Arctic Large Igneous Province (HALIP) represents extensive Cretaceous magmatism throughout the circum-Arctic borderlands and within the Arctic Ocean (e.g., the Alpha-Mendeleev Ridge). Recent aeromagnetic data shows anomalies that extend from the Alpha Ridge onto the northern coast of Ellesmere Island, Nunavut, Canada. To test this linkage we present new bulk rock major and trace element geochemistry, and mineral compositions for clinopyroxene, plagioclase, and olivine of basaltic dykes and sheets and rhyolitic lavas for the stratotype section at Hansen Point, which coincides geographically with the magnetic anomaly at northern Ellesmere Island. New U-Pb chronology is also presented. The basaltic and basaltic-andesite dykes and sheets at Hansen Point are all evolved with 5.5–2.5 wt% MgO, 48.3–57.0 wt% SiO2, and have light rare-earth element enriched patterns. They classify as tholeiites and in Th/Yb vs. Nb/Yb space they define a trend extending from the mantle array toward upper continental crust. This trend, also including a rhyolite lava, can be modeled successfully by assimilation and fractional crystallization. The U-Pb data for a dacite sample, that is cut by basaltic dykes at Hansen Point, yields a crystallization age of 95.5 ± 1.0 Ma, and also shows crustal inheritance. The chronology and the geochemistry of the Hansen Point samples are correlative with the basaltic lavas, sills, and dykes of the Strand Fiord Formation on Axel Heiberg Island, Nunavut, Canada. In contrast, a new U-Pb age for an alkaline syenite at Audhild Bay is significantly younger at 79.5 ± 0.5 Ma, and correlative to alkaline basalts and rhyolites from other locations of northern Ellesmere Island (Audhild Bay, Philips Inlet, and Yelverton Bay West; 83–73 Ma). We propose these volcanic occurrences be referred to collectively as the Audhild Bay alkaline suite (ABAS). In this revised nomenclature, the rocks of Hansen Point stratotype and other tholeiitic rocks are ascribed to the Hansen Point tholeiitic suite (HPTS) that was emplaced at 97–93 Ma. We suggest this subdivision into suites replace the collective term Hansen Point volcanic complex. The few dredge samples of alkali basalt available from the top of the Alpha Ridge are akin to ABAS in terms of geochemistry. Our revised dates also suggest that the HPTS and Strand Fiord Formation volcanic rocks may be the hypothesized subaerial large igneous province eruption that drove the Cretaceous Ocean Anoxic Event 2.

Permian post-collisional basic magmatism from Corsica to the Southeastern Alps

Post-Variscan Early Permian magmatism is widespread from Corsica to the Eastern Alps. After the formation of the Variscan orogenic belt between Laurussia and Gondwana, felsic and mafic bodies were emplaced during an extensional tectonic phase. This study focuses on a mafic dyke swarm that intruded in the region of Ajaccio (Corsica, France) and on a gabbroic intrusive complex outcropping in the Eastern Alps (Bressanone/Brixen, South Tyrol, Italy) both of which were emplaced during this extensional event. New U-Pb data from zircon show that both of these intrusions were emplaced at ca. 282 Ma. Most Ajaccio dykes display a calc-alkaline affinity and are characterized by enrichment of LILE (Large Ion Lithophile Elements) and LREE over HREE (Light and Heavy Rare Earth Elements) and HFSE (Nb, Ta, Hf, Zr; High Field Strength Elements). Two dykes show tholeiitic affinity and are characterized by marked LREE and Th-U depletion and by minor negative Nb-Ta anomalies on multi-element diagrams, which is typical for within-plate basalts. Bressanone/Brixen gabbros display similar geochemical features to the Ajaccio calc-alkaline samples with LREE enrichment over MREE (Medium Rare Earth Elements) and HREE, and HFSE depletion. Such characteristics are also confirmed by trace element data obtained by LA-ICP-MS on clinopyroxene and plagioclase crystals.The calc-alkaline to tholeiitic dykes are characterized by enriched to depleted Sr-Nd-Pb isotopic compositions, respectively. Some Ajaccio dykes and the Bressanone/Brixen gabbros are clearly affected by crustal contamination. However, the combination of trace element and isotopic variations suggests that mantle source heterogeneities were also involved. Isotopic compositions of the Ajaccio dykes reveal the presence of two distinct mantle source signatures: an enriched mantle (EM) component, which dominates the composition of the calc-alkaline-like suite, while a DMM (Depleted MORB Mantle) component dominates the source of the tholeiitic suite. The EM mantle source likely formed during Variscan subduction, however the composition of the tholeiitic suite requires a source composed dominantly of depleted mantle, with a minor contribution from both the subducting slab and supra-subduction mantle wedge. The mantle source of the tholeiitic magmas possibly originated from greater depths (compared to the calc-alkaline-like suite) during the post-orogenic extensional phase potentially due to break-off and/or roll-back of the Variscan slab or mantle flow from the front or margins of the subducted slab.Notably, coeval Permian mafic intrusive bodies from throughout Corsica (Bocca di Tenda, Porto, Pila Canale) and from the Australpine and Southalpine Alps intruded at a similar time and have geochemical features that are very similar to the Ajaccio dyke swarm and Brixen gabbro. This indicates that a widespread Permian magmatic province developed in a post-orogenic extensional tectonic setting at the margin of the former Variscan belt. During the Middle-Late Triassic magmatism with similar enriched geochemical signatures occurred in the Southern Alps, Dinarides, Australpoine domains and at the end of the Triassic over central Pangea (Central Atlantic Magmatic Province). This would highlight the importance that the Variscan orogeny had in the development of a more fertile mantle.

Petrogenesis of post-collisional Late Paleozoic volcanic rocks of the Bohemian Massif (Central Europe): Isotopic variations of the lithospheric mantle related to Variscan orogeny

Post-collisional volcanic rocks associated with the Variscan orogeny are preserved in relicts of Late-Paleozoic volcano-sedimentary basins in the Bohemian Massif (central Europe). They include both the Late Carboniferous calc-alkaline and the Early Permian tholeiitic suites, which were emplaced in an extensional intracontinental setting. Both suites were derived from an enriched continental lithospheric mantle and underwent early fractionation of mafic silicate minerals. A second period of fractionation occurred within the crust. Modeling indicates that the calc-alkaline rocks fractionated at a greater depth (0.55 GPa), under oxidizing (ΔFMQ +1) and hydrous conditions whereas the tholeiitic rocks fractionated at shallow depth (0.1 GPa), under moderately oxidizing (ΔFMQ +0.7) and drier conditions. During the Variscan orogeny, the lithospheric mantle under the Bohemian Massif underwent wholesale modification, where the mantle was contaminated by subducted Precambrian sediment admixture. The Carboniferous-Permian mantle-derived rocks typically have negative ɛNd(t) and Precambrian depleted mantle model ages (TDM) compared to the equivalent Early Paleozoic rocks of the Bohemian Massif, which have positive ɛNd(t) and younger TDM. Subsequently, prior to the Cenozoic, the lithospheric mantle under the whole Bohemian Massif was rejuvenated as the Cenozoic mantle-derived rocks have positive ɛNd(t) and young TDM.

Mineral and bulk–rock chemistry of Shadli bimodal metavolcanics from Eastern Desert of Egypt: Implication for tectonomagmatic setting and Neoproterozoic continental growth in the Arabian–Nubian Shield

The Neoproterozoic Shadli metavolcanics in the Eastern Desert of Egypt have been conventionally categorized as a bimodal island-arc association. The present study deals with Shadli metavolcanics of three areas: Hamama in the central and Darhib and Atshan in the southern Eastern Desert of Egypt. They contain both mafic and felsic compositions and display a bimodal nature. Geochemical classification indicates that the mafic end-members are tholeiitic basalt and basaltic andesite whereas the felsic ones comprise calc-alkaline dacite and rhyolite.Mafic end-members show flat to slightly LREE-depleted patterns comparable to normal MORB produced from a depleted mantle source. Major element data indicates affinity to arc-related non-cumulative mafic rocks. The obvious negative Nb and positive Pb anomalies, the nearly flat REE patterns together with pyroxene chemistry are characteristic of island-arc tholeiitic suite. Accordingly, we suggest that their primary magma was derived from partial melting of a depleted mantle source metasomatized by subduction-related slab fluids/melts.Felsic end-members exhibit magnesian character consistent with a relatively hydrous, oxidizing magma and subduction-related source. Their weakly peraluminous character and calc-alkalic to calcic compositions are characteristics of arc-related magmas. Moreover, the negative Nb and Ti anomalies suggest an influx of subduction-related slab fluids/melts. Their SiO2 contents are too high to represent magmas directly derived by partial melting of a mantle source. So, the high SiO2 and Al2O3 and the low MgO contents suggest melting crustal source. The high Y/Nb ratios (> 1.2) confirm that they were generated from crustal source. Actually, their Ce/Pb ratios (average of 2.66) are affiliated to those of continental crust and contrast typical mantle values. The weakly peraluminous and sodic nature is a typical feature of I-type magma. Therefore, their magmas were most likely derived by partial melting of thickened low-K mafic lower crust.Several lines of evidence let us speculate a genetic relationship between mafic and felsic end-members and that the felsic rocks were possibly evolved from the mafic melts that derived from metasomatized mantle. These comprise the close proximity and a coeval nature; the subalkaline character with approximately continuous trend; the similarity of trace element patterns; the arc-like signature, and the well fitness of major element data of the felsic rocks with melts derived from low-K mafic lower crust. The general tendency of silica increase with alkalis from mafic to felsic end-members may indicate that the felsic rocks were generated by fractional crystallization processes of the mafic magmas in the area.

An isotopically depleted lower mantle component is intrinsic to the Hawaiian mantle plume

Most ocean island basalts sample an isotopically depleted mantle component, but the origin of this component is unclear. It may come from either the entrained upper mantle or from a reservoir intrinsic to the plume, sourced from the lower mantle. For Hawaii, the isotopically depleted component is primarily sampled during the secondary rejuvenated-stage volcanism, 0.5–2 million years after the initial shield-stage volcanism. However, it is also inferred in shield and post-shield lavas. We analyse the radiogenic isotopic and trace element compositions of a suite of Mauna Kea shield-stage tholeiites, and found that they have the same isotopic compositions as rejuvenated-stage lavas. We use trace element models to show that these shield-stage basalts can be explained as higher degree partial melts of a rejuvenated-stage source. Our data, therefore, show that the depleted rejuvenated-stage component was directly sampled during shield-stage volcanism. The common source for both shield-stage and secondary rejuvenated volcanism implies that the depleted rejuvenated component is intrinsic to the Hawaiian mantle plume. It is further inferred that the mantle region from which the Hawaiian plume originates, probably in the lower mantle, is also isotopically depleted, similar but not identical to the upper mantle. A mantle component that is isotopically depleted is intrinsic to the Hawaiian mantle plume, and is probably sourced from the lower mantle, according to radiogenic isotope analyses on shield-stage tholeitic basalts from Mauna Kea.

The Pan-African post-collision Hosséré Mana plutonic complex and associated Gapi Stock (Western Cameroon Domain, Cameroon): Petrology, mineralogy and geochemistry

In the Cameroon Pan-African orogenic belt, the West Cameroon Domain (WCD) is characterized by the apparent lack of Archean-Paleoproterozoic relicts, the development of Neoproterozoic basins and abundant plutonic massifs. The Hosséré Mana plutonic complex (HMPC) and the Gapi gabbroic stock are located in the vicinity of the Cretaceous Ngaou Boh granite complex and the Neogene Tchabal Gangbada basanite-trachyte-rhyolite volcanic plateau. They comprise clinopyroxene + amphibole ± olivine leucogabbro, clinopyroxene + amphibole monzogabbro, biotite + amphibole ± clinopyroxene monzodiorite, porphyritic and foliated biotite ± muscovite monzogranite. Two parallel trends of Mg#, displayed by leucogabbro (0.66–0.61) - monzogabbro (0.57–0.37), and by monzodiorite (0.66–0.55) -monzogranite (0.47–0.33), evidence two contrasting igneous suites, magnesian tholeiitic and magnesian high-K alkali-calcic to calc-alkaline (I-type). Primitive mantle-normalized patterns of the tholeiitic suite are fractionated, with no significant HFSE anomalies, suggesting a garnet-bearing enriched peridotite source. Primitive mantle-normalized patterns of the high-K alkali-calcic to calc-alkaline suite include more fractionated felsic types and display NbTa negative anomalies, suggesting another source with amphibole - garnet in the residue and possible origin of felsic rocks by partial melting of mafic lower crust with garnet and titanite-rutile in the residue. In leucogabbro, fairly low Sri (0.70538) and little negative εNd(750 Ma) (+0.2), with a TDM single-stage Nd model age of 1.42 Ga, are consistent with magmas extracted near the lithosphere – asthenosphere boundary from an enriched mantle source close to BSE (Bulk Silicate Earth). In monzodiorite, little higher Sri (0.70628) and strongly negative εNd(560 Ma) (−8.8), with a TDM single-stage Nd model age of 1.64 Ga, point to a supplementary (lithospheric?) mantle source metasomatized by subduction products. HMPC and GS massifs, emplaced in a syn-to post-kinematic environment after the major collision stage, illustrate coeval magnesian tholeiitic and magnesian high-K alkali-calcic calc-alkaline (I-type) igneous suites, in which primary magmas were tapped from different mantle sources, followed by younger felsic suite issued from garnet-bearing lower crustal source.

The historical basanite - alkali basalt - tholeiite suite at Lanzarote, Canary Islands: Carbonated melts of heterogeneous mantle source?

For the last three decades recycled oceanic lithosphere has been proposed as the principal mantle source for mafic magma erupting on oceanic islands. However, the composition of this lithosphere, as well as how and what part of it melts, is a matter of debate. Several melting experiments show that a mafic pyroxenite/eclogite lithology melts at higher pressure than that of ultramafic peridotite/lherzolite. In addition, the role of carbonate recycling and a carbon-rich fluxing agent producing carbonated silicate melt is increasingly being discussed. The historical Timanfaya eruption (1730–1736) at Lanzarote, Canary Islands, produced a range of mafic magmas that evolved with time from basanite trough alkali basalt to silica-saturated tholeiite, whereas only basanite was erupted in the smaller 1824 eruption. Mantle nodules are common and most lavas and tephra are characterised by abundant olivine macrocrysts. Variable trace element ratios of whole-rock tephra (Rb/Th: 3.3–7.7; La/Yb: 12–44; Nb/Ta: 16–21), and more especially those of olivine-bearing melt inclusions (Rb/Th: 2.5–12; La/Yb: 9–77; Nb/Ta: 14–22) suggest rapid melt extraction from a heterogeneous mantle source beneath the island. Increasing (Nb,Ta)/Th, Zr/Hf and decreasing Nb/Ta from basanite through alkali basalt to tholeiite reflect decreasing carbon-rich fluxing on a time scale of less than a few years. Increasing K, Rb, Cs, and Ba over Th with time during the Timanfaya eruption most likely result from diminishing phlogopite control during the production of increasingly silica-saturated magma. Highly variable melt inclusion compositions are compatible with fractional melting of carbonated pyroxenite, with the whole-rock composition most likely representing the amalgamation of these fractional melts at the asthenosphere-lithosphere boundary beneath the eastern Canary Islands.

Geochemical fingerprinting of ∼2.5 Ga forearc-arc-backarc related magmatic suites in the Bastar Craton, central India

The Pitepani volcanic suite of the Dongargarh Supergroup, central India comprises of a calc-alkaline suite and a tholeiitic suite, respectively. The rare earth element (REE) patterns, mantle normalized plots and relict clinopyroxene chemistry of the Pitepani calc-alkaline suite are akin to high-Mg andesites (HMA) and reveal remarkable similarity to the Cenozoic Setouchi HMA from Japan. The Pitepani HMAs are geochemically correlated with similar rocks in the Kotri-Dongargarh mobile belt (KDMB) and in the mafic dykes of the Bastar Craton. The rationale behind lithogeochemical correlations are that sanukitic HMAs represent fore-arc volcanism over a very limited period of time, under abnormally high temperature conditions and are excellent regional and tectonic time markers. Furthermore, the tholeiitic suites that are temporally and spatially associated with the HMAs in the KDMB and in the mafic dykes of the Bastar Craton are classified into: (a) a continental back-arc suite that are depleted in incompatible elements, and (b) a continental arc suite that are more depleted in incompatible elements, respectively. The HMA suite, the continental back-arc and continental arc suites are lithogeochemically correlated in the KDMB and in the mafic dykes of the Bastar Craton. The three geochemically distinct Neoarchaean magmatic suites are temporally and spatially related to each other and to an active continental margin. The identification of three active continental margin magmatic suites for the first time, provides a robust conceptual framework to unravel the Neoarchaean geodynamic evolution of the Bastar Craton. We propose an active continental margin along the Neoarchaen KDMB with eastward subduction coupled with slab roll back or preferably, ridge-subduction along the Central Indian Tectonic Zone (CITZ) to account for the three distinct magmatic suites and the Neoarchean geodynamic evolution of the Bastar Craton.

Tholeiitic to calc-alkaline metavolcanic transition in the Archean Nigerlikasik Supracrustal Belt, SW Greenland

We present bulk-rock geochemical and U-Pb zircon age constraints on a ∼580m thick sequence of Mesoarchean metavolcanic rocks from SW Greenland. The rocks were arguably deformed into a tight synform and metamorphosed under amphibolite facies conditions, where relict volcanic structures testify to their igneous origins. The sequence includes picrites, tholeiitic basalts and calc-alkaline andesitic to dacitic schists, interbedded with syn-volcanic mafic and felsic feeder intrusions. Amongst late aplitic intrusions, four conform to a minimum U-Pb age for the entire succession of 2929±5Ma, which is slightly older than a protolith intrusion age of 2902±4Ma for the regional grey orthogneisses. The metavolcanic section describes an overall inward increase in SiO2 and Al2O3, coupled with a decrease in MgO, total iron (FeOT) and CaO, across a major discontinuity that separates: (1) A tholeiitic suite with high FeOT, low Al2O3/TiO2 coupled with flat REE-patterns, interpreted as mantle-derived signatures; and (2) a more evolved calc-alkaline suite with relatively low FeOT, high Al2O3/TiO2 and steep REE-patterns, yet relatively high Mg, Ni and Cr, which resemble adakites (or high-Mg andesites). A systematic greater negative Nb-anomaly (Nb/Nb∗) and LREE-enrichment (La/Sm)N up through the metavolcanic section is consistent with an increasing subduction zone signature, and thereby a likely arc setting for all rocks. The overall field relationships argue strongly for the deposition of (i) enriched tholeiites (including basal picrites that could be either lavas or cumulates), (ii) more depleted tholeiites that also intruded the enriched tholeiites, before (iii) an abrupt transition into distinctly different calc-alkaline and adakite-like volcanics that evolved from andesites to dacites. Thus, the Nigerlikasik section records tholeiitic magmatism, succeeded by calc-alkaline adakitic magmatism during the evolution of an Archean island arc. In our single arc model, we discuss the possibilities of the early tholeiitic suite being formed through progressively more hydrous melting of a juvenile mantle wedge and subsequent low-P fractionation of plagioclase-bearing cumulates; whereas the adakitic suite more likely formed through high-P hornblende+garnet fractionation of an enigmatic parent that was possibly sourced from a crustally contaminated mantle.

The Windimurra Igneous Complex: an Archean Bushveld?

Abstract The Archean Windimurra Igneous Complex consists of distinct components, including a thick layered series, with a cumulate mineral stratigraphy similar to the zones identified in the well-studied Bushveld Complex, South Africa. The complex is part of the plume-related and laterally extensive 2.81 Ga Meeline Suite, the intrusive component of a large igneous province. It is an anhydrous tholeiitic suite consisting of five layered mafic–ultramafic intrusions 25–85 km in the long dimension. These intrusions host significant V–Ti mineralization in their fractionated, Fe-rich upper zones. Recent mapping, combined with aeromagnetic, gravity and seismic surveys, has provided unparalleled three-dimensional constraints on the largest of these intrusions. The results of three-dimensional modelling show that it is thicker than previously recognized. At c. 11 km, it is the thickest layered mafic–ultramafic intrusion identified globally and one of the largest such intrusions volumetrically. The mineral zone stratigraphy and many other features associated with this complex share similarities with the c. 800 myr younger Bushveld Complex. On a large scale, three discordant units are delineated geometrically, providing fundamental constraints on a multi-stage genetic model for magma emplacement. The indication of a thick, subsurface Ultramafic Zone provides a potential target for Ni–Cr–platinum group element mineralization.

Geochemical Characteristics and Tectonic Setting of Amphibolites in Ifewara Area, Ife-Ilesha Schist Belt, Southwestern Nigeria

Trace and Rare Earth Elements (REEs) data are used to constrain the geochemical evolution of the amphibolites from Ifewara in the Ife-Ilesha schist belt of southwestern Nigeria. The amphibolites can be grouped into banded and sheared amphibolites. Major element data show SiO2 (48.34%), Fe2O3 (11.03-17.88%), MgO (5.76-9.90%), CaO (7.76-18.6%) and TiO2 (0.44-1.77%) contents which are similar to amphibolites in other schist belts in Nigeria. The Al2O3 (2.85-15.55%) content is varied, with the higher values suggesting alkali basalt protolith. Trace and rare earth elements composition reveal Sr (160-1077ppm), Rb (0.5-22.9ppm), Ni (4.7-10.2ppm), Co (12.2-50.9 ppm) and Cr (2-7ppm). Chondrite-normalized REE patterns show that the banded amphibolites have HREE depletion and both negative and positive Eu anomalies while the sheared variety showed slight LREE enrichment with no apparent Eu anomaly. The study amphibolites plot in the Mid Oceanic Ridge Basalts (MORB) and within plate basalt fields on the Zr/Y vs Zr discriminatory diagrams. They are further classified as volcanic arc basalt and E-type MORB on the Th- Hf/3- Ta and the Zr-Nb-Y diagrams. The amphibolites precursor is considered a tholeiitic suite that suffered crustal contamination, during emplacement in a rifted crust.

Geochemical characteristics and petrogenesis of four Palaeoproterozoic mafic dike swarms and associated large igneous provinces from the eastern Dharwar craton, India

Palaeoproterozoic mafic dike swarms of different ages are well exposed in the eastern Dharwar craton of India. Available U-Pb mineral ages on these dikes indicate four discrete episodes, viz. (1) ~2.37 Ga Bangalore swarm, (2) ~2.21 Ga Kunigal swarm, (3) ~2.18 Ga Mahbubnagar swarm, and (4) ~1.89 Ga Bastar-Dharwar swarm. These are mostly sub-alkaline tholeiitic suites, with ~1.89 Ga samples having a slightly higher concentration of high-field strength elements than other swarms with a similar MgO contents. Mg number (Mg#) in the four swarms suggest that the two older swarms were derived from primary mantle melts, whereas the two younger swarms were derived from slightly evolved mantle melt. Trace element petrogenetic models suggest that magmas of the ~2.37 Ga swarm were generated within the spinel stability field by ~15–20% melting of a depleted mantle source, whereas magmas of the other three swarms may have been generated within the garnet stability field with percentage of melting lowering from the ~2.21 Ga swarm (~25%), ~2.18 Ga swarm (~15–20%), to ~1.89 Ga swarm (~10–12%). These observations indicate that the melting depth increased with time for mafic dike magmas. Large igneous province (LIP) records of the eastern Dharwar craton are compared to those of similar mafic events observed from other shield areas. The Dharwar and the North Atlantic cratons were probably together at ~2.37 Ga, although such an episode is not found in any other craton. The ~2.21 Ga mafic magmatic event is reported from the Dharwar, Superior, North Atlantic, and Slave cratons, suggesting the presence of a supercontinent, ‘Superia’. It is difficult to find any match for the ~2.18 Ga mafic dikes of the eastern Dharwar craton, except in the Superior Province. The ~1.88–1.90 Ga mafic magmatic event is reported from many different blocks, and therefore may not be very useful for supercontinent reconstructions.

The “eye of Africa” (Richat dome, Mauritania): An isolated Cretaceous alkaline–hydrothermal complex

The Richat dome is a spectacular circular structure located in the Mauritanian part of the Sahara Desert. The current erosion level of this igneous complex presents a wide variety of contrasting extrusive and intrusive rocks from shallow to deep source regions providing insight into the magmatic process at the origin of the complex.The Richat is the superposition of a bimodal tholeiitic suite crosscut by carbonatitic and kimberlitic magmatic rocks. The bimodal series is characterized by two concentric gabbroic ring dikes and two extrusive rhyolitic centers representing the remnant of two maar systems. Silica undersaturated magmas occur as carbonatite dikes, a kimberlite plug, and kimberlite sills extruded along the old regional anisotropies filling NNE–SSW dextral strike-slip faults and en-echelon tension gashes. An intense low-temperature hydrothermal event affected the Richat area. It is responsible, notably, for the karst-collapse central mega-breccia, the alteration of the rhyolites, the potassic alteration of the gabbros and the stable isotope enrichment in the carbonatites. A piston-like collapse is proposed to explain the contrast existing between the central and outer part of the Richat.Structural inheritance played an important role in the history of the Richat complex. Pre-existing anisotropies acted as a pathway for the ascent of asthenospheric and sub-continental melts and allowed the coexistence of alkaline and tholeiitic magmas within the same igneous complex.

Geochemistry of the 130 to 80 Ma Canadian High Arctic Large Igneous Province (HALIP) Event and Implications for Ni-Cu-PGE Prospectivity

The geochemistry of a suite of lava flows, sills, and dikes from Axel Heiberg and Ellesmere islands in Arctic Canada has been examined to determine the Ni-Cu-platinum group element (PGE) sulfide prospectivity of the Canadian portion of the 130 to 80 Ma High Arctic large igneous province (HALIP). This event can be split into mildly alkaline or transitional and tholeiitic suites, each with distinct provenances, S saturation histories, and Ni-Cu-PGE prospectivity. The mildly alkaline magmas of the HALIP were sourced from a deep, variably enriched part of the mantle, and most probably represent lower-degree partial melts than the tholeiitic parts of the HALIP that were formed from magmas generated by shallower, higher-degree partial melting of a region of unenriched mantle. The two types of magma also differ in their Ni-Cu-PGE magmatic sulfide prospectivity, in that the geochemistry of the majority of the mildly alkaline samples is indicative of unfertile or poorly fertile S-saturated magmas which did not sequester significant amounts of chalcophile elements from the mantle during partial melting, and therefore may be unprospective. In comparison, the presence of chalcophile element-undepleted tholeiitic samples suggests that the magmas forming these rocks were S undersaturated and fertile, and sequestered significant amounts of Cu and PGEs from the mantle during partial melting. Furthermore, the presence of chalcophile-depleted and crustally contaminated samples within the tholeiitic suite indicates that the magmas that formed these samples assimilated crustal material and became sulfur saturated prior to emplacement, segregating magmatic sulfides that may have been deposited and left behind within ultramafic or mafic intrusive rocks associated with the tholeiitic segments of the Canadian HALIP. Interestingly, some tholeiitic flows are crustally contaminated but are only weakly Pd depleted; this suggests that these flows may have locally assimilated crustal material, which led to only minor S saturation and in situ formation of voluminously insignificant magmatic sulfides. These observations suggest that the tholeiitic portion of the Canadian HALIP is prospective for Ni-Cu-PGE sulfide mineralization, and that mafic-ultramafic sequences associated with this suite should be targeted for mineral exploration. Identification of parts of the Canadian HALIP where magmas assimilated S-bearing sedimentary material could also be a useful tool for exploration. The data and interpretations presented here provide a number of scientific hypotheses that could be tested for other parts of the circum-Arctic HALIP event.

Geochemistry of anorthositic xenolith and host tholeiite basalt from Jeju Island, South Korea

Volcanic rocks on Jeju Island consist mainly of intraplate alkaline lavas with subordinate intercalated tholeiite basaltic rocks. The alkaline suite can be divided into high-Al alkali (HAA) and low-Al alkali (LAA) sub-suites. We determined the 87Sr/86Sr ratios of anorthositic xenolith plagioclases and the Sr-Nd-Pb isotope compositions of the host tholeiite basalt (Taeheungri basalt). The anorthositic xenoliths are considered to represent buoyancy-induced floating cumulates from previous tholeiite magmatism on Jeju Island. The 87Sr/86Sr ratios of the xenolith plagioclases are highly elevated compared with those of the Jeju tholeiite lavas, including the host basalt. The 87Sr/86Sr ratios of the Jeju volcanic rocks increase from the LAA through the HAA to the tholeiite suite. The tholeiite suite generally exhibits less radiogenic 143Nd/144Nd ratios than the alkaline suite. In Pb-Pb space, the tholeiite suite has, in general, elevated 207Pb/204Pb ratios compared with those of the alkaline suite at a given 206Pb/204Pb ratio. In contrast to the 87Sr/86Sr ratios, the two alkaline sub-suites (HAA and LAA) display indistinguishable Nd and Pb isotope compositions. Notably, some of the tholeiite and HAA suites exhibit distinctive positive anomalies in Eu and Sr in the trace element distribution pattern and the 87Sr/86Sr ratios increase as a function of the degree of Sr or Eu enrichment. These observations, along with the relatively fractionated nature of the tholeiite and especially the HAA suites, indicate that the suites might be products of assimilation of plagioclase-rich cumulates, possibly at lower crustal depth, during parent magma evolution. Jeju volcanic rocks exhibit a mixing line between Indian mid-ocean ridge basalt-like depleted mantle and enriched mantle type 2 (EM2) components in Sr-Nd and Pb-Pb isotope correlation diagrams. The enriched component, the source of which may have been the asthenosphere, is supposed to have been higher in the Jeju tholeiite magmatism than in the alkaline magmatism.