low-Ti Lavas Research Articles

Magmatic evolution of Low-Ti lavas in the southern Paraná-Etendeka Large Igneous Province

The southern section of the Paraná-Etendeka large igneous province exposes a sequence of heterogeneous Low-Ti lava packages that range compositionally from olivine tholeiites to rhyolites. The recent advances in the volcanic stratigraphy provide a high-resolution framework that constrains the spatial and temporal evolution of the lava sequence. The stratigraphy is formed by Low-Ti lavas, separated in four volcanic units, namely: Torres, Vale do Sol, Palmas and Esmeralda. Our data shows that individual volcanic units can be separated chemically and represent distinct batches of magma. The composition of erupted lavas is mostly controlled by a complex interplay between source composition and crystallization (±assimilation) within the crust. The magmatic plumbing system evolved gradually from the onset, towards the climax and waning phase of magmatism. LIP onset is composed by more primitive (MgO > 5%) pahoehoe basalt lava flows of the Torres Formation. These low-volume eruptions were fed by a poorly connected magmatic plumbing system focused within the lower crust. Geochemical evidence suggests that the parental basalts of Torres Formation were derived from picritic primary magmas that have evolved by fractional crystallization and assimilation of enriched lithospheric materials. The main phase of magmatism is formed by thick (>20 m), tabular, crystal-poor lavas with evolved composition, including the basaltic andesite of the Vale do Sol Formation and silicic rocks of the Palmas Formation. These large volume eruptions were formed by extensive fractional crystallization (+assimilation) of precursor basaltic magmas and directly fed by shallow magma chambers within the upper crust. The waning phase of magmatism, recorded in the Esmeralda Formation basalts-basaltic andesite, is marked by low-volume Hawaiian-like eruptions of compound lava flows at the surface and the retraction of the magmatic plumbing system at depth. These lavas also mark a shift towards more primitive (asthenospheric) compositions and the shallowing of mantle melting, reflecting the progressive extension of lithosphere during magmatism. For the Low-Ti lava sequence of the Paraná-Etendeka, the surface expression and composition of lava fields are directly controlled by crystallization (+assimilation) processes, and these petrological processes can account for broad geochemical variations within large igneous provinces.

The origins of high-Ti and low-Ti magmas in large igneous provinces, insights from melt inclusion trace elements and Sr-Pb isotopes in the Emeishan large Igneous Province

High-Ti and low-Ti lava series occur simultaneously in many large igneous provinces (LIP); however, their origins remain debated. To address this issue, we performed a detailed geochemical study of olivine and olivine-hosted melt inclusions from the Dali picrites in the Emeishan LIP. Although the Dali picrite lavas have a limited range of whole-rock Ti/Y ratios (~356–404; Wu et al., 2018), olivine-hosted melt inclusions have Ti/Y ratios that range from low-Ti to high-Ti (213–741). The Sr-Pb isotopic compositions and trace element ratios (e.g., Nb/U and Nb/La) of the melt inclusions have a restricted range and display little correlation with Ti/Y ratios, indicating that the low-Ti and high-Ti lavas share compositionally similar source. Trace element mass balance modeling of the Emeishan lavas indicates a mantle source comprising ~84% peridotite from the lower mantle, ~15% recycled MORB, and ~1% pelagic sediment. Modeling results for the Emeishan and Karoo LIPs show that partial melts formed at greater depth in the mantle have higher Ti contents and Ti/Y ratios than those formed at shallower depths. Mixing between melts formed at different depths with different degrees of partial melting can produce the continuous range of Ti/Y ratios observed. The results demonstrate the strong influence of partial melting degree and pressure on the Ti content and Ti/Y ratio of partial melts.

Identification, classification, and interpretation of boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics

AbstractBoninites are rare, high-Si, high-Mg, low-Ti lavas that have considerable tectonic significance, especially for recognizing and interpreting episodes of subduction initiation in the geologic record. Formal identification and classification of boninites may be carried out using MgO-SiO2 and MgO-TiO2 diagrams to find compositions that satisfy modified International Union of Geological Sciences (IUGS) criteria of Si8 > 52 and Ti8 < 0.5, where Si8 and Ti8 refer to concentrations of the oxides at 8 wt% MgO. However, screening of highly metasomatized rocks and accurate classification require precautions, including normalization to a 100% volatile-free basis. The MgO-SiO2 diagram can also be used for subdivision into low-Si boninites (Si8 < 57) and high-Si boninites (Si8 > 57). Satisfying one but not both of the boninite criteria are rocks with Si8 > 52 but Ti8 ≥ 0.5 (siliceous high-magnesium basalts) and rocks with Si8 ≤ 52 but Ti8 < 0.5 (low-Ti basalts). We tested the classification methodologies using ∼100 low-Ti lava suites dating from the present-day back to the Eoarchean. We conclude that, of those classifying as “boninite series,” Izu-Bonin-Mariana arc–type subduction initiation terranes provide the dominant setting only back as far as ca. 2 Ga, which marks the maximum age of extensive clinopyroxene-undersaturated melting and eruption of high-Si boninites. From 2 to 3 Ga, most boninites formed in intraplate settings by melting of refertilized, depleted cratonic roots. Prior to 3 Ga, hot, depleted mantle plumes provided the main boninite sources. Nonetheless, arc-basin boninites, though rare, do extend back to 3.8 Ga, and, together with the inherited subduction component in intracratonic boninites, they provide evidence for some form of subduction during the Archean.

Geochronological and geochemical constraints on the origin of the Southeast Anatolian ophiolites, Turkey

The Southeast Anatolian ophiolites outcropping in the Southeast Anatolian Orogenic Belt (Southeast of Turkey) mark the closure of the southern branch of the Neotethys Ocean associated with the collision between the Arabian Plate and Anatolian microplate. We present new geochemical, zircon U–Pb age, zircon Lu–Hf, and Sr-Nd isotopic data on the Southeast Anatolian Ophiolites to understand their formation ages, magma genesis, and geotectonic implications. The ophiolites, which are related to island arc igneous rocks, consist of mantle peridotites and crustal rocks (less dunite, gabbros, sheeted dykes, massive, and pillow basalts). The flat rare-earth element (REE) patterns, depletion in Nb and Ta, and enrichment in LILEs (Ba, Rb, Th, Sr, Pb) of gabbros suggest close similarities with very low Ti (boninitic) lavas found in the forearc regions. Using laser ablation inductively coupled plasma–mass spectrometry, zircon separated from leucogabbros, diabase dykes, and plagiogranites yield U-Pb ages of 92 and 83 Ma, which are interpreted to represent the formation ages of the ophiolites. The zircons in the gabbros and plagiogranites are dominated by positive eHf(t) values (between +3.1 and + 17.45) with a few negative eHf(t) values. High eHf(t) features are consistent with derivation from Mid-oceanic Ridge Basalt (MORB)-source mantle. The negative eHf(t) values of the zircons suggest the involvement of subducted sedimentary rocks. The southeast Anatolian ophiolites represent an SSZ-type ophiolite and are part of the Late Cretaceous oceanic lithosphere of the southern branch of the Neotethys Ocean that opened during the Late Triassic and closed during the Late Cretaceous.

Mantle sources and magma evolution of the Rooiberg lavas, Bushveld Large Igneous Province, South Africa

We report a new whole-rock dataset of major and trace element abundances and 87Sr/86Sr–143Nd/144Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with > 4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of < 4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures (~ 4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative eNdi (− 5.2 to − 9.4) and radiogenic eSri (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10–20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.

Geochemical stratigraphy and correlation within large igneous provinces: The final preserved stages of the Faroe Islands Basalt Group

The Faroe Islands Basalt Group (FIBG) comprises a gross stratigraphic thickness of over 6.5km of dominantly extrusive basaltic facies erupted during the Late Palaeocene to Early Eocene. In this study we present 140 major and trace element analyses from flow by flow field and borehole sample profiles, through the Enni Formation, which comprises the final phase of volcanism preserved on the Faroe Islands. The sample profiles target geographically spaced and overlapping stratigraphic sequences tied relative to a 3D ArcGIS surface for the regionally extensive volcaniclastic Argir Beds marker unit. From these profiles five geochemical groups including one low TiO2 (Low-Ti <1.5wt%) and four high TiO2 (High-Ti >1.5wt%) groups differentiated by Nb, Zr, Y and V variations are identified in conjunction with previous studies. The spatial and stratigraphic distribution of these groups is mapped across the islands and demonstrates a complex inter-digitated flow field evolution. Within the finer scale variations, broad spatial and temporal development trends are identified demonstrating the potential for correlation within the volcanic succession at the local, tens of kilometers scale. Low-Ti lavas formed in association with lithospheric thinning and developed extensive flow fields between the Faroe Islands and East Greenland contemporaneous to the eruption of High-Ti smaller melt fraction lava flows in both locations. The progression of High-Ti lava groups preserved on either side of the developing rift zone is very similar, but is not, however, chronostratigraphic due to multiple inter-digitations of the chemical types. We tentatively suggest that a previously proposed rift-oblique transfer zone between the Faroe Islands and East Greenland enabled non-uniform lithospheric thinning and the preservation of a near-continuous High-Ti melting region between these areas beyond the onset of Low-Ti eruptions which were initially fed from the west. This study highlights the complex nature of late stage flood basalt plumbing systems and eruption dynamics in a rift proximal setting.

Melting the lithosphere: Metasomes as a source for mantle-derived magmas

Peridotite constitutes most of the Earth's upper mantle, and it is therefore unsurprising that most mantle-derived magmas exhibit evidence of past equilibrium with an olivine-dominated source. Although there is mounting evidence for the role of pyroxenite in magma generation within upwelling mantle plumes, a less documented non-peridotite source of melts are metasomatic veins (metasomes) within the lithospheric mantle. Here we present major and trace element analyses of 66 lavas erupted from a small Miocene shield volcano located within the Ethiopian flood basalt province. Erupted lavas are intercalated with lahars and pyroclastic horizons that are overlain by a later stage of activity manifested in small cinder cones and flows. The lavas form two distinctive petrographic and geochemical groups: (A) an olivine–phyric, low Ti group (1.7–2.7 wt.% TiO2; 4.0–13.6 wt.% MgO), which geochemically resembles most of the basalts in the region. These low Ti lavas are the only geochemical units identified in the later cinder cones and associated lava flows; (B) a clinopyroxene–phyric high Ti group (3.1–6.5 wt.% TiO2; 2.8–9.2 wt.% MgO), which resembles the Oligocene HT-2 flood basalts. This unit is found intercalated with low Ti lavas within the Miocene shield. In comparison to the low Ti group, the high Ti lavas exhibit a profound depletion in Ni, Cr, Al, and Si, and significant enrichment in Ca, Fe, V, and the most incompatible trace elements. A characteristic negative K anomaly in primitive-mantle normalized diagrams, and Na2O>K2O, suggests a source rich in amphibole, devoid of olivine, and perhaps containing some carbonate and magnetite. While melt generation during rift development in Ethiopia is strongly correlated with the thermo-chemical anomalies associated with the African Superplume, thermobaric destabilization and melting of mantle metasomes may also contribute to lithospheric thinning. In regions impacted by mantle plumes, such melts may be critical to weakening of the continental lithosphere and the development of rifts.

A reappraisal of the high-Ti and low-Ti classification of basalts and petrogenetic linkage between basalts and mafic–ultramafic intrusions in the Emeishan Large Igneous Province, SW China

Based on published data, we reappraise the classification of high-Ti and low-Ti basalt from the Emeishan large igneous province (ELIP) and the correlations between basalts and mafic–ultramafic intrusions. Because of the lack of clear spatial and temporal variations of different types of basalts, we suggest that the basalts in the ELIP cannot be classified into high-Ti and low-Ti groups, by TiO2 contents and/or Ti/Y ratios. The distinctive characteristics of these high-Ti and low-Ti lavas probably result largely from the different fractionating assemblages. Whether or not fractional crystallization of the Fe–Ti oxides occurred probably is the key factor that controls the Ti abundances and Ti/Y ratios in the residual melts, e.g., lavas, although the nature of the mantle sources, variable degrees of partial melting of mantle and crustal contamination also influence the geochemical signatures of the lavas. Therefore, neither Ti abundance nor Ti/Y ratios in basalts can reflect the nature of their mantle source. Moreover, the different types of mafic–ultramafic intrusions in the ELIP cannot simply be attributed to be genetically related special types of basalts, either high-Ti or low-Ti basalts. It is likely that they are merely the cumulus phases, i.e. chamber or conduit of the basaltic lavas. Hence, caution should be exercised in the use of high-Ti or low-Ti basalts as prospecting vectors for ore deposits in the region. Potential implications are proposed that both the Fe–V–Ti oxide and Cu–Ni–(PGE) sulfide mineralization in the ELIP intrusions is largely due to the variable differentiation and crustal contamination during magmatic processes.

Extension-related volcanism in the Middle to Late Devonian of the Lachlan Orogen: geochemistry of mafic rocks in the Comerong Volcanics

The Comerong Volcanics are a Middle to Late Devonian bimodal sequence located in the southeastern Lachlan Orogen of NSW, Australia. Magmatism related to subduction was ongoing and located to the east of the continent during this period placing the Comerong Volcanics in a continental back arc setting. Mafic rocks in the Comerong Volcanics occur in three stratigraphically distinct units and range in composition from tholeiitic andesite to basalt. An inverse relationship between volume of erupted lava and degree of fractionation of the magma is evident from the base of the volcanic complex to its top. The lowermost mafic unit is the least voluminous and most fractionated and consists of flows with low Mg#, Ni and Cr and high incompatible trace element abundances. The overlying unit comprises both moderately and extremely fractionated basalt and is characterised by flows with a relatively high TiO2 content. Lavas in this unit can be divided into higher and lower-Ti types. The uppermost unit is the most voluminous and the least fractionated with relatively high MgO, Ni and Cr and low incompatible trace element abundances. Compositions are similar to the low-Ti lavas in the unit below. Trace element characteristics of all lavas suggest they were derived from a heterogeneously enriched source in the subcontinental lithospheric mantle. Enrichment of the source in LILE and depletion in Ti and Nb likely reflect earlier subduction in the orogen. The mafic rocks were erupted in a continental within-plate setting; a setting consistent with the field relationships.

Changing compositions in the Iceland plume; Isotopic and elemental constraints from the Paleogene Faroe flood basalts

Elemental and Sr, Nd, Hf and high precision Pb isotopic data are presented from 59 low-Ti and high-Ti lavas from the syn-break up part of the Faroe Flood Basalt Province. The depleted MORB-like low-Ti lavas erupted in the rift zone between the Faroe Islands and central East Greenland around the time of break up of the North Atlantic have isotopic end-member compositions different from the depleted Iceland lavas. We suggest that the main low-Ti mantle component is NAEM (North Atlantic End-Member (Ellam and Stuart, 2000, J. Petrol. 41, 919) and that the 207Pb/ 204Pb value of the component should be 15.35 and ε Hf = + 16.5. NAEM is the main depleted component in the early Iceland plume. This is supported by high mantle potential temperatures (up to 1550 °C) calculated for the source of the low-Ti basalts. The unique mantle isotopic composition of NAEM with low 206Pb/ 204Pb (17.5) and Δ7/4Pb (− 3.8) precludes a derivation from recycled MORB lithosphere. Instead we suggest that NAEM represents a plume component of recycled depleted Archean lithospheric mantle that was further depleted ~ 500 Ma ago, possibly in connection with the recycling process. Two other isotopic end-members are required to explain the variation of the Faroe low-Ti basalts: (1) The Faroe depleted component (FDC), with 87Sr/ 86Sr = 0.7025, ε Nd = + 11, ε Hf = + 19.5, 206Pb/ 204Pb = 18.2, 207Pb/ 204Pb = 15.454 and 208Pb/ 204Pb = 37.75, which is similar in composition to some Atlantic MORB and is regarded as a local upper mantle source. (2) An enriched EM-type component similar in geochemistry to the Icelandic Öræfajökull lavas. This component is believed to be recycled pelagic sediments in the plume but it can alternatively be a local crustal or lithospheric mantle component. The enriched Faroe high-Ti lavas erupted inland from the rift have isotopic compositions very similar to the enriched Icelandic neo-volcanics and these lava suites apparently share the two enriched plume end-members IE1 and IE2 (Geochim. Cosmochim. Acta 68, 2, 2004). The lack of mixing between high and low-Ti melts at the time of break up, is explained by a zoned plume where only low-Ti sources were present beneath the rift zone surrounded by high-Ti sources on both sides of the rift. The enriched plume components in the high-Ti lava sequences on the Faroe Islands and central East Greenland changed rapidly on a ka-scale which implies, from geophysical modelling, that this area was positioned above the center of the plume, and that the Iceland plume was centered under the Atlantic ridge already from the Paleocene.

Geochemistry and genesis of behind-arc basaltic lavas from eastern Nicaragua

The petrology and chemistry of the Behind the Volcanic Front (BVF) lavas from eastern mainland Nicaragua and the adjacent Great Corn Island in the Caribbean Sea illustrate the complex nature of sources and processes operating in such a tectonic setting. The older, Early Miocene (∼ 17 Ma) group of low-Ti (< 1 wt.%) basalts–andesites is characterized by a strong LILE/HFSE depletion. The low-Ti lavas from El Rama and El Bluff areas are interpreted as relics of Early Miocene volcanic arc, largely analogous to the nowadays extinct Coyol arc further west. However, these rocks differ in some parameters from the modern volcanic front lavas, most notably in having lower δ 7Li values, Ba/Yb ratios and lower U contents. The younger high-Ti (Ti > 1.5%) lavas, rich in other HFSE as well, are represented both by alkaline (Quaternary trachybasalts: Volcán Azul and Kukra Hill) and subalkaline (basalts–basaltic andesites: Late Miocene, ∼ 11 Ma Great Corn Island and Quaternary, Pearl Lagoon) volcanic rocks. The Late Miocene and Quaternary high-Ti BVF lavas probably represent small-volume decompression melts of a source similar to that of the OIB-like magmas, most likely upwelling asthenosphere having a strong Galápagos mantle imprint. The positive Sr–Nd isotopic correlation indicates an interaction between this OIB component and a depleted lithospheric mantle modified by a subduction-related influx of Sr and, to a lesser extent, other hydrous fluid-mobile elements. However, the rocks show no recognizable influence of the modern subduction. The feeble trace-element (e.g., slightly elevated Ba, K, and Sr at some localities) and a more pronounced Sr–Li isotopic subduction-related signal stems most likely from the Miocene convergence episode. Subduction of the Galápagos hot-spot tracks in Costa Rica produces magmas that can be readily recognized by their elevated Sr isotopic ratios due to seafloor alteration; the Nd isotopic signature remains unaffected. Such a component with relatively unradiogenic Nd and radiogenic Sr is required in the source of the modern volcanic front lavas but is not needed to explain the variation in the studied BVF dataset. Terrains with multiepisodic subduction history should be considered with caution, as the lavas generated by decompression melting of the asthenospheric source in the back-arc region may bear a geochemical imprint of the fossil and not the modern subduction component stored in the local lithospheric mantle.

The Stonyford Volcanic Complex: a Forearc Seamount in the Northern California Coast Ranges

Jurassic age volcanic rocks of the Stonyford volcanic complex (SFVC) comprise three distinct petrological groups based on their whole-rock geochemistry: (1) oceanic tholeiites; (2) transitional alkali basalts and glasses; (3) high-Al, low-Ti tholeiites. Major and trace element, and Sr–Nd–Pb isotopic data indicate that the oceanic tholeiites formed as low-degree partial melts of normal mid-ocean ridge basalt (N-MORB)-source asthenosphere similar in isotope composition to the East Pacific Rise today; the alkalic lavas were derived from an enriched source similar to that of E-MORB. The high-Al, low-Ti lavas resemble second-stage melts of a depleted MORB-source asthenosphere that formed by melting spinel lherzolite at low pressures. Trace element systematics of the high-Al, low-Ti basalts show the influence of an enriched component, which overprints generally depleted trace element characteristics. Tectonic discrimination diagrams show that the oceanic tholeiite and alkali suites are similar to present-day basalts generated at mid-oceanic ridges. The high-Al, low-Ti suite resembles primitive arc basalts with an enriched, alkali basalt-like overprint. Isotopic data show the influence of recycled components in all three suites. The SFVC was constructed on a substrate of normal Coast Range ophiolite in an extensional forearc setting. The close juxtaposition of the MORB-like olivine tholeiites with alkali and high-Al, low-Ti basalts suggests derivation from a hybrid mantle source region that included MORB-source asthenosphere, enriched oceanic asthenosphere, and the depleted supra-subduction zone mantle wedge. We propose that the SFVC formed in response to collision of a mid-ocean ridge spreading center with the Coast Range ophiolite subduction zone. Formation of a slab window beneath the forearc during collision allowed the influx of ridge-derived magmas or the mantle source of these magmas. Continued melting of the previously depleted mantle wedge above the now defunct subduction zone produced strongly depleted high-Al, low-Ti basalts that were partially fertilized with enriched, alkali basalt-type melts and slab-derived fluids.

Melt viscosity, temperature and transport processes, Troodos ophiolite, Cyprus

The lava section in the Troodos ophiolite, Cyprus, is chemically stratified and divided into a shallow lava sequence with low TiO 2 content and a deeper lava sequence with high TiO 2 content. We calculate the viscosity at magmatic temperature based on major element chemistry of lavas in Cyprus Crustal Study Project (CCSP) Holes CY-1 and 1A. We find that typical shallow low-Ti lavas have a magmatic viscosity that is two to three orders of magnitude lower than that of the deeper high-Ti lavas. This implies that, after eruption on-axis, Troodos low-Ti lavas would have been able to flow down the same slope faster and farther than high-Ti lavas. The calculated lava viscosity increases systematically from the lava–sediment interface to the bottom of the composite Hole CY-1/1A. This suggests that an efficient process of lava segregation by viscosity on the upper flanks of the paleo Troodos rise may have been responsible for the chemical stratification in the Troodos lava pile. Calculated magmatic temperature and molar Mg/(Mg+Fe), or Mg#, decrease systematically down-section, while SiO 2 content increases. Correlation of Mg# in the lavas with Mg# in the underlying, lower crustal plutonic rocks sampled by CCSP Hole CY-4 shows that the shallow lavas came from a high-temperature, lower crustal magma reservoir which is now represented by high-Mg# pyroxenite cumulates, while the deeper lavas were erupted from a lower-temperature, mid-crustal reservoir which is now represented by gabbroic cumulates with lower Mg#.

Petrologic and geochemical constraints on the petrogenesis of Permian–Triassic Emeishan flood basalts in southwestern China

The Emeishan flood basalt is a large igneous province erupted during the Permian–Triassic period in southwestern China. Based on petrographic, major and trace element, and Sr–Nd isotope data, the Emeishan basalts can be classified into two major magma types. These are: (1) a low-Ti (LT) type that exhibits low Ti/Y (<500), Fe 2O 3* (<12%), Nb/La (0.6–1.4), ε Nd( t) (−4.8 to +1.4) and relatively high SiO 2 (48–53%) and Mg# (0.52–0.64); (2) a high-Ti (HT) type that has high Ti/Y (>500). The HT lavas can be further divided into three subtypes. HT1 lavas exhibit significantly high TiO 2 (3.65–4.7%), Fe 2O 3* (12.7–16.4%), Nb/La (0.75–1.1), coupled with higher ε Nd( t) (1.1–4.8) and lower SiO 2 (45–51%); HT2 lavas are compositionally similar to the HT1 lavas but show conspicuous depletion in U and Th. The HT3 type has higher Mg# (0.51–0.61) than the HT1 and HT2 lavas. It differs from the LT type in having higher TiO 2 (∼3%) at comparable Mg#. Elemental and isotopic data suggest that the chemical variations of the LT and HT lavas cannot be explained by crystallization from a common parental magma. Instead, they may originate from different mantle sources under various melting conditions and underwent distinct differentiation and contamination processes. REE inversion calculations indicate that the HT magmas were generated by low degrees of partial melting (1.5%) of a mantle source that has ε Nd( t) of ∼+5 and 87Sr/ 86Sr( t) of ∼0.704 within the garnet stability field. These magmas were then subjected to shallow level gabbroic fractionation, which led to larger chemical variations. In contrast, parental magmas of the LT type were generated by higher degree of partial melting (16%) of a distinct mantle source ( ε Nd( t)≈+2, 87Sr/ 86Sr( t)≈0.705) around the spinel–garnet transition zone. The chemical evolution of the LT lavas is controlled by an olivine (ol)+clinopyroxene (cpx) fractionation. The Emeishan flood basalts may result from a starting mantle plume. The petrogenesis of both the LT and HT magmas was further complicated by contamination of upper crust and lithospheric mantle. While the HT1 lavas have experienced an AFC style of contamination in the upper crust, the HT2 lavas that mark with U–Th depletions may result from additional interaction with melts derived from a gabbroic layer near the crust–mantle boundary. In contrast, a temperature-controlled style of contamination was associated with the LT lavas. Our data show that both temporal and spatial geochemical variations exist in the Emeishan flood basalt province. The occurrence of thick LT lavas in the western part of the province may record the main episode of the flood basalt emplacement. In contrast, the less abundant overlying HT basalts may imply a waning activity of the plume. In fact, the HT basalts are the dominant magma type in the periphery of the province. The lower degrees of mantle melting of the HT lavas may be a result of relatively thicker lithosphere and lower geotherm.

Helium isotopic variations in Ethiopian plume lavas: nature of magmatic sources and limit on lower mantle contribution

Oligocene Continental Flood Basalts (CFB) from Ethiopia exhibit a wide range of 3He/ 4He ratios, from 0.035 Ra in crust-contaminated Low-Ti lava to 19.6 Ra in plume-derived high-Ti lava (where Ra is the atmospheric ratio of 1.38 × 10 −6). Quaternary basalts sampled in the Main Ethiopian Rift and in Afar also display dramatic 3He/ 4He variations from 0.009 Ra to 16.9 Ra. Low isotopic ratios partly reflect crustal assimilation whereas 3He/ 4He values higher than the mean isotopic ratio of the upper mantle (8 ± 1 Ra as measured in Normal Mid-Ocean Ridge Basalts [N-MORB]) indicate the contribution of a lower mantle component. The geographical extension of plume-type He is consistent with an approximate radius of ∼ 1000 km for the flattened plume head. Helium isotopic data show that strong lower mantle signals were already apparent during early emissions of CFB and pre-dated by more than 15 Ma the major phases of rifting in the African Horn region, contrary to the view of passive decompression melting contemporary with large-scale rifting. The He-Sr composition of the plume component cannot result from a simple binary mixing between lower mantle and upper mantle end-members and requires the contribution of other component(s) such as recycled crust. The proportion of the lower mantle contribution to the total mass of material involved in the building of the Ethiopian magmatic province, as estimated from He contents and isotopic ratios in the respective mantle sources, is found to be small (< 5%). Except for He (and Ne), such contribution has no impact on the trace element and isotopic compositions of plume basalts, which are dominated by mixing between upper mantle, continental crust and recycled sources. If the thermal anomaly necessary to produce CFB originates in the lower mantle, as generally proposed, then there exists a dramatic decoupling between mass and heat transfers across the lower mantle-upper mantle boundary layer.

Lithospheric to asthenospheric transition in Low-Ti flood basalts from southern Paraná, Brazil

Two geochemically distinct Low-Ti magma types (Gramado and Esmeralda) are distinguished within the flood basalt sequences of southern Paraná on the basis of certain element abundances and ratios. Esmeralda magmas have lower Ce/Sm and ( 87 Sr 86 Sr ), i and higher Ti Zr and ϵ Nd, than Gramado magmas. Detailed stratigraphical work indicates that there was a temporal progression from Gramado- to Esmeralda-type magmas. This compositional shift cannot simply be explained in terms of a declining extent of crustal contamination of an asthenosphere-derived melt with time, and instead it seems that the two magma types evolved from distinct parental magmas. Although compositional variations of Gramado magmas are dominated by crustal assimilation and fractional crystallisation, geochemical features of the inferred parental magmas [low Ti Y of < 300 and low Nb La of < 0.8, negative ϵ Nd-values ∼ −3 and high ( 87 Sr 86 Sr ) i 0.708−0.710] are indicative of a significant contribution from the continental mantle lithosphere. Gramado magmas also show a marked regional variation which might imply that the mantle source was compositionally heterogeneous over a length scale of a few hundred km. It is suggested that variations within the Esmeralda magma type are controlled by mixing between an asthenospheric melt with ϵ Nd > + 4 and a Gramado-type magma, at relatively shallow crustal levels, followed by fractional crystallisation. The asthenospheric component has relatively depleted incompatible element characteristics comparable to MORB. Thus, Low-Ti lava sequences of southern Paraná record a shift from a predominantly lithospheric to an asthenospheric signature with time. Paraná flood basalts appear to have been generated in response to lithospheric extension associated with the opening of the South Atlantic over a region of anomalously hot mantle, attributed to the presence of the Tristan mantle plume. However, the distinctive geochemical signature of the modern Tristan plume, seen on the islands of Tristan da Cunha, Gough and Inaccessible, is not evident in the compositions of Low-Ti magmas in southern Paraná.

Isotope and trace-element heterogeneities in high-grade basic metamorphic rocks of Marvejols: Tectonic implications for the hercynian suture zone of the French Massif Central

The Leptyno-Amphibolite Group, which occurs at the major suture of the Variscan orogen in the French Massif Central, contains abundant meta-basic rocks in the form of granulites, amphibolites and eclogites. Mafic granulite boudins from the Marvejols area generally have geochemical characteristics similar to oceanic tholeiites of MORB-affinity. They have flat or slightly LREE-depleted patterns, high ϵ Nd i and low ϵ Sr i. They lie very close the mantle array at the probable time of their formation (480 Ma). One granulitic boudin is LREE-enriched with low HREE and HFSE and appears to be a low-Ti lava with island-arc tholeiite affinities. Similar material also forms much of the amphibolites surrounding the boudins. The close association of low-Ti lavas and MORB tholeiites suggests formation in a marginal basin or back-arc environment.

Boninitic and low-Ti subduction-related lavas from intraoceanic arc-backarc systems and low-Ti ophiolites: a reappraisal of their petrogenesis and original tectonic setting

A comprehensive review of boninitic and other low-Ti subduction-related lavas from modern intraoceanic arc-backarc systems, and of low-Ti ophiolites shows a complete compositional range from boninite to island arc basalts. Transitional low-Ti types distinguishable from the main magma types are well represented and defined here as (1) transitional boninites (mainly from low-Ti ophiolites) and (2) low-Ti island arc basalts. Major-compatible and incompatible elements are clearly decoupled in all these low-Ti subduction-related magmas. In general, their petrogenesis reflects a high degree of hydrous but not necessarily water-saturated melting (at pressures lower than 15–20 kbar) of severely depleted mantle sources enriched in LILE and H2O which are residual after the earlier extraction of basalt (s). As the residues after the extraction of low-Ti subduction-related magmas very likely have a dunitic-clinopyroxenefree harzburgitic composition, it is proposed that the CaO/Al2O3 ratios of the primary melts reflect those of their mantle sources, particularly for the boninites and transitional boninites. In this model, the source of boninites sensu stricto would show a variable degree of depletion from clinopyroxene-bearing to clinopyroxene-free harzburgite, whereas those of transitional boninites-low-Ti island arc basalts would have clinopyroxene-bearing harzburgite-clinopyroxene-poor lherzolite compositions. The metasomatizing components involved in the genesis of low-Ti island arc basalts have a very close affinity with those involved with the sources of normal island arc tholeiites. The common eruptive setting indicates that these low-Ti lavas represent extreme magma types of an otherwise normal island arc tectonomagmatic regime. The petrogenesis of Tertiary boninites and transitional boninites and the comparable magma types of the low-Ti ophiolites require at least these components: (1) A variably depleted mantle source, probably a subarc mantle residual after island arc basalt extraction; (2) a hydrous fluid, probably derived from dehydration of the subducted, altered oceanic crust, which carries most of Sr, Ba, K and Rb in the subarc source and (3) a LREE ± Zr and Nb-enriched component with a low 143Nd/144Nd ratio, probably derived from an “ocean island-type” source. It is proposed that this third component also carried significant amounts of Rb (and K) which totally obliterated the Ba spike typical of island arc tholeiites (IATs) and produced the distinctive Rb spike typical of the boninite-transitional boninite magmas. This component is hypothetically identified in liquids produced by partial melting of small scale, large magnitude K richerite ± phlogopite-bearing heterogeneities within the hot rising asthenospheric diapirs required in the model of Crawford et al. (1981) for the genesis boninite-transitional boninite magmas. Geochemical, thermal and geological constraints are best explained in a model which restricts the genesis of boninite-transitional boninite magmas to the early stages of intraoceanic interarc spreading and to important rifting phases of the arc and/or early interarc basin structures connected with hot rising asthenospheric mantle diapirs.

Precious metals in magnesian low-Ti lavas: Implications for metallogenesis and sulfur saturation in primary magmas

Boninites and related magnesian low-Ti magmas are generally regarded as partial melts of a moderately to severely depleted peridotite source. Incompatible lithophile element abundances indicate that this source was variably enriched in LREE, Zr, Sr, Ba and alkalis by some mantle metasomatic process. Low-Ti lavas from the Bonin-Mariana arc system, Cape Vogel, New Caledonia, Cyprus, Newfoundland and SE Australia have been analysed for Pd, Ir, Au, Cu, S and Se. Comparison of fresh glassy material with variably altered samples suggests sporadic loss of Au and Cu and essentially inert behaviour for Pd, Ir and Se during seawater and subsequent alteration. They are uniformly enriched in Pd (mean 15 ppb) and depleted in Cu (mean 20 ppm), S (mean < 54 ppm) and Se (mean 53 ppb) compared to average MORB (<0.8 ppb Pd, 72 ppm Cu, 800 ppm S and 196 ppb Se) and exhibit incompatible-like behaviour for these elements and Au.The data are compatible with fractionation of the chalcophile elements during multi-stage mantle melting. Primary MORB liquids are S-saturated in their mantle source and an immiscible sulfide component is retained in the mantle residue. This results in the preferential removal of metals having low DS/L- values (base metals) and concentration of those metals with high DS/L values (precious metals) in the residual mantle sulfide fraction. Subsequent remelting of this refractory source produces S-deficient precious metal-enriched magmas, as exemplified by boninites. The absence of correlation between incompatible lithophile element enrichment and chalcophile element abundances suggests that the latter were not added to the source during mantle metasomatism.The constraints imposed by the nature of the source region result in two fundamentally contrasting patterns of behaviour for exclusively chalcophile elements. Magmas generated in mildly depleted to undepleted source regions by low to moderate degrees of partial melting (e.g. MORB) are S-saturated and become rapidly impoverished in precious metals during the early stages of silicate fractionation, owing to the co-precipitation of an immiscible sulfide component. Magmas generated from a strongly depleted source are initially S-undersaturated and concentrate chalcophile metals in their liquid residua.The contrasting behaviour of chalcophile metals during the early crystallisation stage of MORB and low-Ti magmas lead to divergent predictions concerning the primary distribution of these metals in oceanic crust generated by these magmas. The similarity in composition of early Bushveld magmas and boninites suggests that these S-deficient, PGE-enriched magmas may be essential to the formation of platiniferous horizons in layered intrusions.

Nd-Sr isotopic and geochemical systematics in Cambrian boninites and tholeiites from Victoria, Australia

Rocks with boninitic affinities have been recognised in a number of “ophiolites”, including the Cambrian Heathcote and Mt Wellington Greenstone Belts of Victoria. Boninites and high-Mg andesites from the Heathcote Greenstone Belt show a restricted range of initial ɛ Nd values of between +3.3 to +5.8. Extremely refractory boninites from the Mt Wellington Greenstone Belt have ɛ Nd ranging from +1.3 to −9. Ti/Zr is positively correlated with Sm/Nd with the Heathcote lavas generally possessing greater depletion of Ti and enrichment of Zr relative to the middle and heavy REE with increasing LREE/HREE. These data are consistent with the generation of boninites by partial melting of refractory peridotite following invasion by LREE- and Zr-enriched, low ɛ Nd fluids. Tholeiites overlying the boninites in both greenstone belts have flat REE patterns and ɛ Nd∼+5, lower than that anticipated for lavas derived from depleted MORB source reservoirs in the Cambrian, suggesting that their source was also contaminated by a LREE-enriched, low ɛ Nd component similar to that involved in the generation of the Howqua boninites. The added components have characteristics compatible with their derivation from subducted altered oceanic crust and/or from wet subducted sediments. The identification of boninites and other low-Ti lavas in the Victorian greenstone belts is strong evidence for island arc development in southeastern Australia during the Lower Cambrian and provides further support for a subduction-related origin for many ophiolites.