The Baffin Bay lavas and the value of picrites as analogues of primary magmas

The Baffin Bay picrites have been the focal point of a controversy concerning the MgO content of primary magmas derived from the upper mantle. A sample population of 48 lava chilled margins collected across the Baffin Bay volcanic succession at the northeastern tip of Padloping Island exhibits a prominent compositional mode between 14 and 16 weight percent MgO (19–22 Mg, cation units = Mg/100 cations). The petrography of these samples, however, requires that the Padloping magmas were mixtures of olivine crystals and liquid at their eruption. Olivine phenocrysts constituted 15 to 30 volume percent of these magmas and retain compositions requiring coexisting liquid compositions with only 10 to 13.5 weight percent MgO (14–18.5 Mg). However, highly magnesian, olivine xenocrysts (up to Fo 93) found in the most magnesian lavas require the former existence of liquids with at least 18 weight percent MgO (24 Mg). If these xenocrysts represent early cumulates, then the primary liquids of the Padloping suite must have been at least this MgO rich with temperatures greater than 1,425° C. Such primary liquids could have evolved by olivine crystallization to a steady state, equilibrated crystal — liquid mixtures in a shallow reservoir system prior to eruption. The compositions of the liquids of these mixtures appear to have been perched at the point of plagioclase saturation at approximately 1,275° C.Despite the complications of mechanical sorting of olivine crystals, the virtual compositional reciprocity of olivine addition and olivine fractionation requires that the bulk compositions of picritic lavas provide compositional analogues of their primary magmas. A comparison of Phanerozoic picrite suites indicates that the Fe contents of terrestrial primary magmas of tholeiitic affinity have a restricted range from 6–9 Fe. Primary magmas associated with intra-plate volcanism appear to be distinctly more Fe-rich than those associated with inter-plate volcanism. The Al/Si ratios of Phanerozoic picrite suites could suggest that the primary magmas of MORB volcanism have equilibrated with relatively Fe-poor source regions at deeper levels in the Earth's mantle than those of other tholeiitic primary magmas.     

Jurassic plume-origin ophiolites in Japan: accreted fragments of oceanic plateaus

The Mikabu and Sorachi–Yezo belts comprise Jurassic ophiolitic complexes in Japan, where abundant basaltic to picritic rocks occur as lavas and hyaloclastite blocks. In the studied northern Hamamatsu and Dodaira areas of the Mikabu belt, these rocks are divided into two geochemical types, namely depleted (D-) and enriched (E-) types. In addition, highly enriched (HE-) type has been reported from other areas in literature. The D-type picrites contain highly magnesian relic olivine phenocrysts up to Fo_93.5, and their Fo–NiO trend indicates fractional crystallization from a high-MgO primary magma. The MgO content is calculated as high as 25 wt%, indicating mantle melting at unusually high potential temperature (T _p) up to 1,650 °C. The E-type rocks represent the enrichment in Fe and LREE and the depletion in Mg, Al and HREE relative to the D-type rocks. These chemical characteristics are in good accordance with those of melts from garnet pyroxenite melting. Volcanics in the Sorachi–Yezo belts can be divided into the same types as the Mikabu belt, and the D-type picrites with magnesian olivines also show lines of evidence for production from high T _p mantle. Evidence for the high T _p mantle and geochemical similarities with high-Mg picrites and komatiites from oceanic and continental large igneous provinces (LIPs) indicate that the Mikabu and Sorachi–Yezo belts are accreted oceanic LIPs that were formed from hot large mantle plumes in the Late Jurassic Pacific Ocean. The E- and D-type rocks were formed as magmas generated by garnet pyroxenite melting at an early stage of LIP magmatism and by depleted peridotite melting at the later stage, respectively. The Mikabu belt characteristically bears abundant ultramafic cumulates, which could have been formed by crystal accumulation from a primary magma generated from Fe-rich peridotite mantle source, and the HE-type magma were produced by low degrees partial melting of garnet pyroxenite source. They should have been formed later and in lower temperatures than the E- and D-type rocks. The Mikabu and Sorachi Plateaus were formed in a low-latitude region of the Late Jurassic Pacific Ocean possibly near a subduction zone, partially experienced high P/T metamorphism during subduction, and then uplifted in association with (or without, in case of Mikabu) the supra-subduction zone ophiolite. The Mikabu and Sorachi Plateaus may be the Late Jurassic oceanic LIPs that could have been formed in brotherhood with the Shatsky Rise.     

The geochemistry and petrogenesis of the late-Cretaceous picrites and basalts of Curaçao,Netherlands Antilles: a remnant of an oceanic plateau

The island of Curaçao in the southern Caribbean Sea is composed mainly of a thick sequence (>5?km) of pillow lavas, grading upwards from picrites at the base of the exposed section, to basalts nearer the top. Modelling suggests that picrites are related to the basalts by fractional crystallisation. Initial radiogenic isotope ratios of the picrites have a restricted compositional range: ?_Nd=+6.1 to +6.6, ⁸⁷Sr/⁸⁶Sr=0.70296–0.70319; whereas the basalts display a wider range of compositions: ?_Nd=+6.6 to +7.6, ⁸⁷Sr/⁸⁶Sr=0.70321–0.70671. This variation in isotope ratios between basalts and picrites may be due to the assimilation of altered oceanic crust (or possibly partial melts of such crust) by a picritic magma along with fractional crystallisation. The relatively narrow range of Nd and Pb isotopic compositions in the Curaçao lavas suggests either that the source region was homogeneous, or that melts from a heterogeneous mantle source were well mixed before eruption. Chondritic to slightly light rare earth element enriched patterns, combined with long-term light rare earth element depletion (positive ?_Nd), suggest that the lavas were formed by polybaric melting of spinel lherzolite, with small a contribution from garnet lherzolite melts. High-MgO lavas, the absence of a subduction related chemistry, and the chemical similarity to other oceanic plateaux, suggest a mantle plume origin for the Curaçao lava succession. The Curaçao volcanic sequence is part of an oceanic plateau formed at about 88–90?Ma, fragments of which are dispersed around the Caribbean as well as being obducted onto the western margin of Colombia and Ecuador. The occurrence of high-Mg lavas throughout this Cretaceous Caribbean–Colombian igneous province requires anomalously hot mantle (>200^°?C hotter than ambient upper mantle) over a large part of a putative plume head, which is inconsistent with some mantle plume models.     

Middle Devonian Picrites of the Southern Margin of Altay Orogenic Belt and Implications for the Tectonic Setting and Petrogenesis

INTRODUCTION Inrecentyears,greatprogressonthegeologic tec tonicevolutionandmineralresourcesofXinjianghas beenachieved.However,manyissuesarestilldebated, suchasancienttectonicpatternsandtheclosuretimeof theancientoceanicbasin(LiandXu,2004).Theseis sueshavelimitedourknowledgeoftheformationande volutionofAsiancontinents,aswellastheexploration anddevelopmentofmineralresources. Recently,theHilaketehalasuporphyrycopperde positwasdiscoveredinthestrataoftheMiddleDevoni anBeitashanFormatio…     

Constraining the potential temperature of the Archaean mantle: A review of the evidence from komatiites

The maximum potential temperature of the Archaean mantle is poorly known, and is best constrained by the MgO contents of komatiitic liquids, which are directly related to eruptive temperatures. However, most Archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid. Relatively fresh lavas from the SASKMAR suite, Belingwe Greenstone Belt, Zimbabwe (2.7 Ga) include chills of 25.6 wt.% MgO, and olivines ranging to Fo_93.6, implying eruption at around 1520°C. A chill sample from Alexo Township, Ontario (also 2.7 Ga) is 28 wt.% MgO, and associated olivines range to Fo_94.1, implying eruption at 1560°C. However, inferences of erupted liquids containing 32–33 wt.% MgO, from lavas in the Barberton Greenstone Belt, South Africa (3.45 Ga) and from the Perseverance Complex, Western Australia (2.7 Ga) may be challenged on the grounds that they contain excess (cumulate) olivine, or were enriched in Mg during alteration or metamorphism. Re-interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt.% MgO corresponding to an eruption temperature of 1580°C. This composition is the highest liquid MgO content of an erupted lava that can be supported with any confidence. The hottest modern magma, on Gorgona Island (0.155 Ga) contained 18–20% MgO and erupted at circa 1400°C.

If 1580°C is taken as the temperature of the most magnesian known eruption, then the source mantle from which the liquids rose would have been at up to 2200°C at pressures of 18 GPa corresponding to a mantle potential temperature of 1900°C. These temperatures are in excess of the mantle temperatures predicted by secular cooling models, and thus komatiites can only be formed in hot rising convective jets in the mantle. This result requires that Archaean mantle jets may have been 300°C hotter than the Archaean ambient mantle temperature. This temperature difference is similar to the 200–300°C temperature difference between present day jets and ambient mantle temperatures. An important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions.     

Composition and age of tertiary sills and dykes, Jameson Land Basin, East Greenland: relation to regional flood volcanism

The Paleozoic–Mesozoic Jameson Land Basin (East Greenland) is intruded by a sill complex and by a swarm of ESE trending dykes. Together with dykes of the inner Scoresby Sund fjord, they form a regional Early Tertiary intrusive complex located 200–400 km inland of the East Greenland rifted continental margin. Most of the intrusive rocks in the Jameson Land Basin are geochemically coherent and consist of evolved plagioclase–augite–olivine saturated, uncontaminated high-Ti basalt with 48.5–50.2 wt.% SiO₂, 2.2–3.2 wt.% TiO₂, 5.1–7.4 wt.% MgO, 9–17 ppm Nb and La/Yb_N=2.8–3.6. Minor tholeiitic rock types are: (a) low-Ti basalt (49.7 wt.% SiO₂, 1.7 wt.% TiO₂, 6.8 wt.% MgO, 2.6 ppm Nb and La/Yb_N=0.5) akin to oceanic basalts; (b) very-high-Ti basalt (48.6 wt.% SiO₂, 4.1 wt.% TiO₂, 5.1 wt.% MgO and 21 ppm Nb); and (c) plagioclase ultraphyric basalt. The tholeiitic dolerites are cut by alkali basalt (43.7–47.3 wt.% SiO₂, 4.1–5.1 wt.% TiO₂, 4.9–6.2 wt.% MgO, 29–46 ppm Nb and La/Yb_N=16–17) sills and dykes.Modelling of high-field-strength and rare-earth elements indicate that the high-Ti basalts formed from 6–10% melting of approximately equal proportions of garnet- and spinel-bearing mantle of slightly depleted composition beneath thick continental lithosphere. Conversely, dolerite intrusions and flood basalts of similar compositional kindred from adjacent but more rift-proximal occurrences in Northeast Greenland formed from shallower melting of dominantly spinel-bearing mantle beneath extended and thinned continental lithosphere. These variations in lithospheric thickness suggest the continent–ocean transition of the East Greenland rifted volcanic margin is sharp and narrow.⁴⁰Ar–³⁹Ar dating and paleomagnetism show that the high-Ti dolerites were emplaced at 53–52 Ma (most likely during C23r) and hence surprisingly postdate the main flood volcanism by 2–5 Ma and the inception of seafloor spreading between Greenland and Europe by 1–2 Ma. The formation of tholeiitic and alkaline magmas emplaced into the Jameson Land Basin corroborates to the importance of post-breakup magmatism along the East Greenland volcanic rifted margin. Upwelling of the ancestral Iceland mantle plume under central Greenland at 53–52 Ma (rather than under the active rift), perhaps accompanied by a failed attempt to shift the rift zone westward towards the plume axis, may have triggered post-breakup continental magmatism of the Jameson Land Basin and the inner Scoresby Sund region, along preexisting structural lineaments.     

Petrogenesis and tectonic significance of mafic-ultramafic rocks from southwestern Yunnan,China

This study focuses on the mafic-ultramafic lavas of the Early Carboniferous outcrop in Mangxin, southwestern Yunnan, China. Picrites with 26–32 wt% MgO and a quenched texture are the most significant components of this rock association. This article divides the Mangxin picrites into two types. The mantle potential temperature (T_p) of these picrites is higher than the T_p range of mid-ocean ridges and reaches that of mantle plumes. According to their geochemical characteristics, type-1 picrites probably formed from the melting of the mantle plume head and were contaminated by the ambient depleted mantle, whereas type-2 picrites formed from the melting of mantle plume tails. These plume-related mafic-ultramafic rocks in Mangxin and the ocean island basalt (OIB)-carbonate rock associations in many areas of the Changning–Menglian belt provide significant evidence for the improvement of previous models of the Palaeotethyan oceanic plateau.     

峨眉山大火成岩省平川苦橄岩地幔源区性质——来自橄榄石微量元素的制约

橄榄石是基性岩浆中最早期结晶的硅酸盐矿物之一,其主量、微量元素特征可以反映出岩浆演化环境、岩浆源区岩性和再循环组分性质等重要信息.本次研究通过对峨眉山大火成岩省平川苦橄岩中橄榄石主量和微量元素分析,以及橄榄石内尖晶石包裹体分析,并与大理苦橄岩中橄榄石和尖晶石成分进行对比,来探讨不同苦橄岩母岩浆氧逸度及源区性质的异同.橄...     

Experimental study of the phase and melting relations of homogeneous basalt + peridotite mixtures and implications for the petrogenesis of flood basalts

Flood basalt provinces may constitute some of the most catastrophic volcanic events in the Earth's history. A popular model to explain them involves adiabatic ascent of plumes of anomalously hot peridotite from a thermal boundary layer deep in the mantle, across the peridotite solidus. However, peridotitic plumes probably require unreasonably high potential temperatures to generate sufficient volumes of magma and high enough melting rates to produce flood volcanism. This lead to the suggestion that low melting eclogitic or pyroxenitic heterogeneities may be present in the source regions of the flood basalts. In order to constrain petrogenetic models for flood basalts generated in this way, an experimental investigation of the melting relations of homogeneous peridotite + oceanic basalt mixtures has been performed. Experiments were conducted at 3.5 GPa on a fertile peridotite (MPY90)–oceanic basalt (GA1) compositional join. The hybrid basalt + peridotite compositions crystallised garnet lherzolite at subsolidus temperatures plus quenched ne-normative picritic liquids at temperatures just above the solidus, over the compositional range MPY90 to GA1₅₀MPY90₅₀. The solidus temperature decreased slightly from ∼1500 °C for MPY90 to ∼1450 °C for GA1₅₀MPY90₅₀. Compositions similar to GA1₃₀MPY90₇₀ have 100% melting compressed into a melting interval which is approximately 50–60% smaller than that for pure MPY90, due to a liquidus minimum. During adiabatic ascent of hybrid source material containing a few tens of percent basalt in peridotite, the lower solidus and compressed solidus–liquidus temperature interval may conspire to substantially enhance melt productivity. Mixtures of recycled oceanic crust and peridotite in mantle plumes may therefore provide a viable source for some flood volcanics. Evidence for this would include higher than normal Fe/Mg values in natural primary liquids, consistent with equilibration with more Fe-rich olivine than normal pyrolitic olivine (i.e. <Fo_89–92). Modelling of fractionation trends in West Greenland picrites is presented to demonstrate that melts parental to the Greenland picrites were in equilibrium at mantle P–T conditions with olivine with Fo_84–86, suggesting an Fe-enriched source compared with normal peridotite, and consistent with the presence of a basaltic component in the source. Received: 29 October 1999 / Accepted: 3 February 2000     

Shallow submarine volcano group in the early stage of island arc development: Geology and petrology of small islands south off Hahajima main island,the Ogasawara Islands

Small Islands south off Hahajima, the southernmost of the Ogasawara Archipelago, consist of primitive basalts (<12 wt.% MgO) to dacite erupted during the transitional stage immediately following boninite volcanism on the incipient arc to sustained typical oceanic arc. Strombolian to Hawaiian fissure eruptions occurring on independent volcanic centers for the individual islands under a shallow sea produced magnesian basalt to dacite fall-out tephras, hyaloclastite and a small volume of pillow lava, which were intruded by NE-trending dikes. These volcanic strata are correlated to the upper part (<40 Ma) of the Hahajima main island. Volcanic rock samples have slightly lower FeO^*/MgO ratios than the present volcanic front lavas, and are divided into three types with high, medium and low La/Yb ratios. Basalt to dacite of high- and medium-La/Yb types show both tholeiitic (TH) and calc-alkaline (CA) differentiation trends. Low-La/Yb type belongs only to TH basalt. The multiple magma types are coexistence on the each island. TH basalts have phenocrysts of olivine, clinopyroxene and plagioclase, while CA basalts are free from plagioclase phenocrysts.     

Archaean basic volcanism in the Eastern Goldfields Province,Yilgarn Block,Western Australia

Petrographic, petrological and geochemical studies have demonstrated the presence of three distinctive basic volcanic suites in the Eastern Goldfields Province, Yilgarn Block, Western Australia. These are termed the high magnesian series basalts (HMSB), the low magnesian series basalts (LMSB) and the siliceous high magnesian series basalts (SHMSB).The HMSB and SHMSB constitute differentiation series which contain both high MgO (9.5–14 wt.%) and low MgO (<9.5 wt.%) members. These suites are commonly characterized by igneous textures indicative of very rapid crystallization suggesting high eruption temperatures. This feature clearly distinguishes those low MgO members of HMSB which contain amphibole pseudomorphs after spherulitic-textured pyroxene from compositionally similar LMSB. The LMSB are generally characterized by an intergranular texture consisting of plagioclase laths and interstitial amphibole pseudomorphs after pyroxene grains. Variolitic-textured basalts are common and appear to be restricted to the SHMSB suite of basic volcanics.The HMSB and LMSB were derived from source mantle regions which were variably depleted in the incompatible elements. Archaean komatiites were derived from similarly depleted source regions and it is argued that the main petrogenetic difference between these three volcanic suites was the degree of partial melting from which they were derived. The depleted nature of the source regions may have been induced by earlier small degree (< 5%) partial melting events with subsequent extraction of a proportion of that melt. Variations in both the degree of such melting, and the proportions of melt removal, could induce considerable heterogeneity of incompatible elements in the Archaean upper mantle.Source mantle regions of the SHMSB were variably enriched in the incompatible elements and water and parental magmas of the SHMSB were derived from moderately hydrous conditions of partial melting.The relative proportions of each basalt suite varies considerably between the layered successions examined. For example, the basic volcanics overlying the komatiites at Kambalda are SHMSB, while the footwall volcanics consist predominantly of HMSB with subordinate LMSB. However, the Norseman succession, where no ultramafic volcanics are known to occur, is comprised mainly of LMSB with a smaller proportion of HMSB.     

Petrology of the parental melts and mantle sources of Siberian trap magmatism

Based on the investigation of olivine phenocrysts and melt and spinel inclusions in them from the picrites of the Gudchikhinsky Formation and olivine phenocrysts and the whole-rock geochemistry from the Tuklonsky and Nadezhdinsky formations of the Noril’sk region, the compositions and conditions of formation and evolution of the parental melts and mantle sources of Siberian trap magmatism were evaluated. Olivine phenocrysts from the samples studied are enriched in Ni and depleted in Mn compared with olivines equilibrated with the products of peridotite melting, which suggests a considerable role of a nonperidotitic component (olivine-free pyroxenite) in their mantle source. The onset of Siberian trap magmatism (Gudchikhinsky Formation) was related to the melting of pyroxenite produced by the interaction of ancient recycled oceanic crust with mantle peridotite. During the subsequent evolution of the magmatic system (development of the Tuklonsky and Nadezhdinsky formations), the fraction of the pyroxenite component in the source region decreased rapidly (to 40 and 60%, respectively) owing to the entrainment of peridotite material into the melting zone. The formation of magmas was significantly affected by the contamination by continental crustal material. The primitive magmas of the Gudchikhinsky Formation crystallized under near-surface conditions at temperatures of 1250–1170°C and oxygen fugacities 2.5–3.0 orders of magnitude below the Ni-NiO buffer. Simultaneously, the magmas were contaminated by continental silicic rocks and evaporites. The parental magmas of the Gudchikhinsky rocks corresponded to tholeiitic picrites with 11–14 wt % MgO. They were strongly undersaturated in sulfur, contained less than 0.25 wt % water and carbon dioxide, and were chemically similar to the Hawaiian tholeiites. They were produced by melting of a pyroxenite source at depths of 130–180 km in a mantle plume with a potential temperature of 1500–1580°C. The presence of low melting temperature pyroxenite material in the source of Siberian trap magmas promoted the formation of considerable volumes of melt under the thick continental lithosphere, which could trigger its catastrophic collapse. The contribution of pyroxenite-derived melt to the magmas of the Siberian trap province was no less than 40–50%. This component, whose solid residue was free of sulfides and olivine, played a key role in the origin of high contents of Ni, Cu, and Pt-group elements and low sulfur contents in the parental trap magmas and prevented the early dispersion of these elements at the expense of sulfide melt fractionation. The high contents of Cl in the magmas resulted in considerable HCl emission into the atmosphere and could be responsible for the mass extinction at the Paleozoic-Mesozoic boundary.     

Processes in High-Mg, High-T Magmas: Evidence from Olivine, Chromite and Glass in Palaeogene Picrites from West Greenland

Uncontaminated volcanic rocks from the 60 Ma Vaigat Formation,West Greenland, contain 6·5–30 wt % MgO, averaging15·5 wt % MgO. Olivine (mg-number 77·4–93·3)forms diverse assemblages of zoned phenocrysts and xenocrystsshowing evidence for equilibrium and fractional crystallization,oxidation, partial to complete re-equilibration, as well asmagma mixing. The olivine crystals contain glass inclusionsand have high contents of Ca and Cr, indicating that all olivineswith up to mg-number 93·0 crystallized from melts. Associatedchromites (mg-number 45·4–77·2) are essentiallyunzoned and in equilibrium with the olivines. Matrix glassesfrom pillow breccias have 6·7–8·8 wt % MgOand quenched close to 1200°C with oxidation states one log-unitabove the NNO (nickel–nickel oxide) buffer. Compositionaldifferences between the glasses from different volcanic membersare inherited from the primary melts. The magmas erupted ascrystal-charged melts, and liquids with more than     

Cotectic Proportions of Olivine and Spinel in Olivine-Tholeiitic Basalt and Evaluation of Pre-Eruptive Processes

The volume %, distribution, texture and composition of coexistingolivine, Cr-spinel and glass has been determined in quenchedlava samples from Hawaii, Iceland and mid-oceanic ridges. Thevolume ratio of olivine to spinel varies from 60 to 2800 andsamples with >0·02% spinel have a volume ratio ofolivine to spinel of approximately 100. A plot of wt % MgO vsppm Cr for natural and experimental basaltic glasses suggeststhat the general trend of the glasses can be explained by thecrystallization of a cotectic ratio of olivine to spinel ofabout 100. One group of samples has an olivine to spinel ratioof approximately 100, with skeletal olivine phenocrysts andsmall (<50 µm) spinel crystals that tend to be spatiallyassociated with the olivine phenocrysts. The large number ofspinel crystals included within olivine phenocrysts is thoughtto be due to skeletal olivine phenocrysts coming into physicalcontact with spinel by synneusis during the chaotic conditionsof ascent and extrusion. A second group of samples tend to havelarge olivine phenocrysts relatively free of included spinel,a few large (>100 µm) spinel crystals that show evidenceof two stages of growth, and a volume ratio of olivine to spinelof 100 to well over 1000. The olivine and spinel in this grouphave crystallized more slowly with little physical interaction,and show evidence that they have accumulated in a magma chamber. KEY WORDS: olivine; spinel; basalt glass; volume %; cotectic     

Picritic primary magma and its source mantle for Oshima-Ōshima and back-arc side volcanoes,Northeast Japan arc

Oshima-shima volcano is an endmember of a geochemical variation which is characterized by a low FeO content toward the back-arc side across the NE Japan arc. Analyses of the basalts show primitive characteristics. Variation trends of the chemical compositions indicate initial olivine control then olivine+clinopyroxene control from a picritic to a differentiated basalt. The more magnesian basalts have the more magnesian olivine phenocrysts. The most magnesian (MgO 15%) of all rock samples, contains olivine phenocrysts with a composition of Fo 93.7 as a liquidus phase and is considered a product of a mantle-derived magma. The possible range in FeO and MgO content of source mantle for the Oshima-shima magma can be demonstrated. Ichinomegata lherzolite inclusions, also from the back-arc side of NE Japan, is unlikely to be a candidate for the source mantle for high FeO. The upper mantle beneath the back-arc side is considered to be compositionally zoned; a Fe-rich mantle (Ichinomegata lherzolite) at shallower place and a Fe-poor mantle (the source mantle for back-arc side volcanoes).     

Petrological constraints on the origin of arc picrites,New Georgia Group,Solomon Islands

Subduction related picrites from the New Georgia archipelago (Solomon Islands) range in bulk MgO from 13 to 30 wt%. Two generations of olivine are identified based on CaO contents: High-CaO olivine phenocrysts from the picritic parental melt and low-CaO olivine xenocrysts incorporated from either lithospheric or asthenospheric upper mantle. There is also evidence that some of the low-CaO olivines are boninitic in origin. The bulk MgO range in the picrites is largely controlled by assimilation of low-CaO olivine xenocrysts. Oxidation states of the melt (FMQ+2.2), calculated from magnetite activities in liquidus chromites, constrain the MgO content of the parental melt to 13 wt%, assuming Fe–Mg exchange equilibrium between melt and liquidus olivine composition. The dry liquidus temperature of the parent melt based on this MgO content is 1340°C, about 80°C above the temperature obtained with the olivine–clinopyroxene Ca-exchange thermobarometer. The residence time of the low-CaO olivine xenocrysts in the magma, estimated from Ca- and Fe–Mg interdiffusion profiles, did not exceed 1 year.     

Siberian meimechites: origin and relation to flood basalts and kimberlites

Here we combine petrological-geochemical and thermomechanical modeling techniques to explain origin of primary magmas of both Maimecha–Kotui meimechites and the Gudchikhinskaya basalts of Norilsk region, which represent, respectively, the end and the beginning of flood magmatism in the Siberian Trap Province.We have analyzed the least altered samples of meimechites, their olivine phenocrysts, and melt inclusions in olivines, as well as samples of dunites and their olivines, from boreholes G-1 and G-3 within the Guli volcanoplutonic complex in the Maimecha–Kotui igneous province of the northern Siberian platform. The Mn/Fe and Ni/MgO ratios in olivines indicate a mantle peridotite source of meimechites. Meimechite parental magma that rose to shallow depths was rich in alkalis and highly magnesian (24 wt.% MgO), largely degassed, undersaturated by sulfide liquid and oxidized. At greater depths, it was, likely, high in CO₂ (6 wt.%) and H₂O (2 wt.%) and resulted from partial melting of initially highly depleted and later metasomatized harzburgite some 200 km below the surface. Trace-element abundances in primary meimechite magma suggest presence of garnet and K-clinopyroxene, in the mantle source and imply for genetic link to the sources of the early Siberian flood basalts (Gudchikhinskaya suite) and kimberlites. The analyzed dunite samples from the Guli complex have chemistry and mineralogy indicating their close relation to meimechites.We have also computed thermomechanical model of interaction of a hot mantle plume with the shield lithosphere of variable thickness, using realistic temperature- and stress-dependent visco-elasto-plastic rocks rheology and advanced finite element solution technique.Based on our experimental and modeling results we propose that a Permian–Triassic plume, with potential temperature of about 1650 °C transported a large amount of recycled ancient oceanic crust (up to 15%) as SiO₂-supersaturated carbonated eclogite. Low-degree partial melting of eclogite at depths of 250–300 km produced carbonate-silicate melt that metasomatized the lithospheric roots of the Siberian shield. Further rise of the plume under relatively attenuated lithosphere (Norilsk area) led to progressive melting of eclogite and formation of reaction pyroxenite, which then melted at depths of 130–180 km. Consequantly, a large volume of melt (Gudchikhinskaya suite) penetrated into the lithosphere and caused its destabilization and delamination. Delaminated lithosphere that included fragments of locally metasomatized depleted harzburgite subsided into the plume and was heated to the temperatures of the plume interior with subsequent generation of meimechite magma. Meimechites showed up at the surface only under thicker part of the lithosphere aside from major melting zone above because otherwise they were mixed up in more voluminous flood basalts. We further suggest that meimechites, uncontaminated Siberian flood basalts and kimberlites all shear the same source of strongly incompatible elements, the carbonated recycled oceanic crust carried up by hot mantle plume.     

Petrology of peridotite xenolith-bearing basaltic to andesitic lavas from the Shiribeshi Seamount,off northwestern Hokkaido,the Sea of Japan

The Shiribeshi Seamount off northwestern Hokkaido, the Sea of Japan, is a rear-arc volcano in the Northeast Japan arc. This seamount is composed of calc-alkaline and high-K basaltic to andesitic lavas containing magnesian olivine phenocrysts and mantle peridotite xenoliths. Petrographic and geochemical characteristics of the andesite lavas indicate evidence for the reaction with the mantle peridotite xenoliths and magma mixing between mafic and felsic magmas. Geochemical modelling shows that the felsic end-member was possibly derived from melting of an amphibolitic mafic crust. Chemical compositions of the olivine phenocrysts and their chromian spinel inclusions indicate that the Shiribeshi Seamount basalts in this study was derived from a primary magma in equilibrium with relatively fertile mantle peridotites, which possibly represents the mafic end-member of the magma mixing. Trace-element and REE data indicate that the basalts were produced by low degree of partial melting of garnet-bearing lherzolitic source. Preliminary results from the mantle peridotite xenoliths indicate that they were probably originated from the mantle beneath the Sea of Japan rather than beneath the Northeast Japan arc.     

Lamproites as indicators of accretion and/or shallow subduction in the assembly of south‐western Anatolia,Turkey

The geochemistry and mineralogy of lamproites from south‐western Anatolia can be used as a snapshot of the lithospheric composition beneath the Menderes Massif. High and near‐constant K₂O contents, the presence of mantle xenocrystic phlogopite and olivine, highly magnesian olivine phenocrysts and Cr‐rich spinel inclusions all indicate that the lithospheric mantle was phlogopite‐bearing ultradepleted harzburgite at the time of lamproite eruption (20–4 Ma). This mantle assemblage most probably originated in a complex multistage process, including (intra‐oceanic) supra‐subduction zone depletion during the final stages of southern Neotethyan ocean closure, and accretion of the forearc oceanic lithosphere as shallowly subducted material to the already assembled Anatolia. The data presented here support shallow subduction of the oceanic lithosphere as a cause of the uplift of the Menderes Massif, in contrast to the traditional core‐complex model. Terra Nova, 00, 000–000, 2010     

Mantle plume or slab window?: Physical and geochemical constraints on the origin of the Caribbean oceanic plateau

The Caribbean oceanic plateau formed in the Pacific realm when it erupted onto the Farallon plate from the Galapagos hotspot at ～ 90 Ma. The plateau was subsequently transported to the northeast and collided with the Great Arc of the Caribbean thus initiating subduction polarity reversal and the consequent tectonic emplacement of the Caribbean plate between the North and South American continents. The plateau represents a large outpouring of mafic volcanism, which has been interpreted as having formed by melting of a hot mantle plume. Conversely, some have suggested that a slab window could be involved in forming the plateau. However, the source regions of oceanic plateaus are distinct from N-MORB (the likely source composition for slab window mafic rocks). Furthermore, melt modelling using primitive (high MgO) Caribbean oceanic plateau lavas from Curaçao, shows that the primary magmas of the plateau contained ～ 20 wt.% MgO and were derived from 30 to 32% partial melting of a fertile peridotite source region which had a potential temperature (T_p) of 1564–1614 °C. Thus, the Caribbean oceanic plateau lavas are derived from decompression melting of a hot upwelling mantle plume with excess heat relative to ambient upper mantle. Extensional decompression partial melting of sub-slab asthenosphere in a slab window with an ambient mantle T_p cannot produce enough melt to form a plateau. The formation of the Caribbean oceanic plateau by melting of ambient upper mantle in a slab window setting, is therefore, highly improbable.