Oceanic Plateau Basalts Research Articles

Paleo-plateau/-seamount Limestone of the Cambrian Accretionary Complex in the Gorny Altai Mountains, Southern Siberia

Vendian-Cambrian Baratal limestone occurs as large allochthonous blocks in the Cambrian accretionary complex of the Gorny Altai Mountains, southern Russia. We analyzed the primary stratigraphy and depositional environments of Baratal limestone in the Kurai and Akkaya areas in the eastern part of the Gorny Altai Mountains.In the Kurai and the Akkaya areas, Baratal limestone conformably overlies basaltic greenstone. Geochemistry of this greenstone is similar to that found in modern oceanic plateau basalt or oceanic island basalt. The limestone lacks terrigeneous elastic influx. These suggest that the Baratal limestone was originally deposited on and around a plateau or seamount far from the continents in a mid-oceanic environment.Baratal limestone in the study area is lithologically divided into four types; 1) massive lime mudstone, 2) massive limestone conglomerate 3) bedded lime mudstone with slump structures, and 4) laminated lime mudstone. Massive lime mudstone contains stromatolites and ooids. This evidence suggests that the massive lime mudstone was formed in a shallowmarine environment. The massive limestone conglomerate contains angular clasts of lime mudstone, greenstone and chert. Its poorly graded and poorly sorted characteristics suggest that the limestone conglomerate was formed as debris flow deposits. Some parts of bedded lime mudstone have slump structures, and are interpreted as sliding deposits. In addition, laminated lime mudstone that shares an affinity with limestone turbidite, is associated with the massive limestone conglomerate.Sedimentary environments of these four types of limestone are inferred, respectively as follows ; massive lime mudstone may have been diposited on the top of a paleo-plateau/ -seamount, while massive limestone conglomerate, bedded lime mudstone with slump structures, and laminated lime mudstone on the slope of a paleo-plateau/-seamount.

The nature and provenance of accreted oceanic terranes in western Ecuador: geochemical and tectonic constraints

Western Ecuador consists of a complex tectonic mélange of oceanic terranes accreted to the continental margin from Late Cretaceous to Eocene time. New geochemical data from these accreted terranes (arising from a 5 year British Geological Survey mapping programme) indicate that they comprise rocks from a variety of oceanic tectonic settings: from thickened (and relatively unsubductable) oceanic plateau basalts, through island-arc tholeiites, with occasional more calc-alkaline lavas, to back-arc basin basalt sequences. This study has enabled us to construct a new geodynamic model for the Cretaceous–Tertiary evolution of the Northern Andes, and has placed important new constraints on the extent of oceanic plateau sequences in Colombia and around the Caribbean. The age and nature of sediments, combined with evidence for the age of peak metamorphism, suggests that a prolonged (15–20 Ma) accretionary event occurred in Late Cretaceous time and involved the collision of an oceanic plateau (represented by the Pallatanga Unit) with the continental margin. This accreted unit can be correlated with similar oceanic plateau sequences from the Western Cordillera of Colombia and those within and around the Caribbean region. The Naranjal and Macuchi island arcs and the associated La Portada back-arc basin developed along the accreted margin from Late Campanian to Eocene time, and these arcs accreted to the continental margin along with oceanic plateau material (represented by the Piñon Unit and Pedernales–Esmeraldas sequences) during Eocene time. The development of island arcs, which separate the two accretionary events, implies that the most westerly (coastal) oceanic plateau sequences, both in Ecuador (Piñon and Pedernales–Esmeraldas) and in Colombia (Gorgona and Serranía de Baudó), cannot belong to the Caribbean–Colombian Oceanic Plateau (CCOP). It therefore appears that at least two different oceanic plateaux are preserved within the accreted oceanic terranes of the Northern Andes. It is possible that the CCOP formed over the Galápagos hotspot, as previously proposed, but the more westerly Coastal plateau was derived from a more southerly hotspot source region, such as Sala y Gomez, in the SE Pacific.

Petrology of the greenstones of the Lower Sorachi Group in the Sorachi–Yezo Belt, central Hokkaido, Japan, with special reference to discrimination between oceanic plateau basalts and mid‐oceanic ridge basalts

Abstract The Lower Sorachi Group of the Sorachi–Yezo Belt in central Hokkaido, Japan is a peculiar accretionary complex characterized by numerous occurrences of greenstones (metabasalts and diabases), which are mostly composed of aphyric basalts. Clinopyroxene‐rich phenocryst assemblage in phyric basalts is different from olivine–plagioclase assemblage in mid‐oceanic ridge basalts (MORB). The greenstones are geochemically uniform, and show a lower‐Ti trend than MORB in an FeO*/MgO‐TiO2 diagram, mostly plotting on the island arc tholeiite (IAT) field in a TiO2−10MnO−10P2O5 diagram. In a MORB‐normalized spider diagram, the greenstones show a flat pattern from P to Y, which are lower than those of normal mid‐oceanic ridge basalt (N‐MORB). These indicate that the greenstones were derived by a higher degree of partial melting from a depleted mantle similar to a N‐MORB source, and experienced olivine–clinopyroxene fractional crystallization. However, a positive spike of Nb in the spider diagram cannot be explained, and may be attributed to mantle heterogeneity. These characteristics are analogous to those of oceanic plateau basalts (OPB) such as in Ontong Java Plateau, Manihiki Plateau and Nauru Basin, suggesting that the greenstones in the Lower Sorachi Group are of oceanic plateau origin. The present study proposes new field divisions to distinguish OPB from MORB in the conventional FeO*/MgO–TiO2 and TiO2−10MnO−10P2O5 diagrams.

Compositional recycling structure of an Archean super-plume: Nb–Th–U–LREE systematics of Archean komatiites and basalts revisited

The volcanic stage of the 2.7-Ga Abitibi greenstone belt, Canada, is dominated by bimodal arc magma series and komatiite–basalt sequences. The latter represents an aerially extensive oceanic plateau erupted from an anomalously hot super-plume. Komatiites define a linear array of Nb/Th vs. Nb/U, extending from Nb/Th=8–20, and Nb/U=26–58, whereas basalts plot on a separate, but overlapping, field extending to higher Th/U but lower Nb/Th values. Inter-element ratios of Th, U, Nb, and LREE of komatiites and basalts plot with Phanerozoic and modern ocean plateau basalts. Th, U, Nb, and LREE are fractionated in subduction zones into low Nb/Th, Nb/U, and Nb/LREE arc crust, and complementary high Nb/Th, Nb/U, and Nb/LREE residual slab. Accordingly, the Archean komatiite–basalt association may be explained by a plume that likely originated from the core–mantle boundary with komatiites erupted from a hot axis containing recycled oceanic crust, and basalts erupted from the plume annulus that entrained upper mantle containing recycled oceanic and continental crust. High Nb/Th and Nb/U of plume-related volcanic sequences documented in Abitibi, Yilgarn, and Baltic Archean greenstone belts suggest that the extraction and recycling of continental crust may have occurred early in the Archean.

Geodynamic processes, continental growth, and mantle evolution recorded in late Archean greenstone belts of the southern Superior Province, Canada

An episodic continental growth model, by lateral spread of subduction–accretion complexes, is proposed for the late Archean southern Superior Province, based on structural relationships and high precision trace element data for the 2.7 Ga Wawa and Abitibi subprovinces. Two principal volcanic associations are recognized: (1) tholeiitic basalt–komatiite; and (2) tholeiitic to calc-alkaline bimodal basalt–rhyolite. Tholeiitic basalts of the former association are characterized by near-flat REE patterns and complex Th–U–Nb–LREE systematics; rare transitional to alkaline basalts and Al-depleted komatiites have fractionated REE patterns and OIB-like trace element signatures. This is interpreted as representing an oceanic plateau derived from a heterogeneous mantle plume. The latter bimodal association has fractionated REE patterns and negative Nb, Ta, P, and Ti anomalies typical of arc magmas. Turbidites plot on mixing hyperbolae between mafic and felsic end members. This association is interpreted as an arc–trench system. Tonalite plutons derived from partial melting of subducted oceanic slabs intrude the subduction–accretion complex as the magmatic arc axis migrated towards the trench. In the Abitibi belt, boninite-series low-Ti tholeiite flows stratigraphically intercalated with the basalt–komatiite association signifying coeval eruption of plume and arc lavas. Plateau tholeiites are characterized by Nb/U, Th/U, Nb/Th, Nb/La, Th/La, and La/Sm pm ratios that span the primitive mantle values. Arc associations have Nb/U, Nb/Th, and Nb/La ratios that are systematically lower than the primitive mantle ratios, but have La/Sm pm ratios that are systematically higher than the primitive mantle ratio. The variability of these ratios in oceanic plateau basalts is interpreted in terms of recycling variable quantities of oceanic crust processed through a subduction zone (high Nb/U, Nb/Th), and complementary arc crust (low Nb/U, Nb/Th) into the source of the plumes. In the Wawa and Abitibi greenstone belts of the southern Superior Province, eruption of voluminous ocean plateau and island arc volcanic sequences, deposition of kilometer-thick siliciclastic trench turbidites, and intrusion of the supracrustal units by syn- to post-kinematic TTG plutons occurred in the relatively short period of time of 2.75–2.65 Ga. All these events were accompanied by contemporaneous intense poly-phase deformation, regional greenschist to amphibolite facies metamorphism, and terrane accretion along a north–northwest-dipping subduction zone. Collectively, these events are interpreted as a part of a late Archean supercontinent cycle, involving accretion of oceanic plateaus, island arcs, continental fragments, closure of ocean basins, rifting of magmatic arcs, and plume arc interactions.

Comment on “Variability of Nb/U and Th/La in 3.0 to 2.7 Ga Superior Province ocean plateau basalts: implications for the timing of continental growth and lithosphere recycling”

Comment on “Variability of Nb/U and Th/La in 3.0 to 2.7 Ga Superior Province ocean plateau basalts: implications for the timing of continental growth and lithosphere recycling”

Reply to the Comment by Kamber and Collerson on “Variability of Nb/U and Th/La in 3.0 to 2.7 Ga Superior Province ocean plateau basalts: implications for the timing of continental growth and lithosphere recycling”

Reply to the Comment by Kamber and Collerson on “Variability of Nb/U and Th/La in 3.0 to 2.7 Ga Superior Province ocean plateau basalts: implications for the timing of continental growth and lithosphere recycling”

Contrasting arc and MORB-like assemblages in the Paleoproterozoic Flin Flon Belt, Manitoba, and the role of intra-arc extension in localizing volcanic-hosted massive sulphide deposits

The Flin Flon Belt (Trans-Hudson Orogen, Manitoba and Saskatchewan) is the largest Paleoproterozoic volcanic-hosted massive sulphide (VMS) district in the world, with 118.7 million tonnes (Mt) of Zn-Cu-(Au-Ag) sulphide ore in 25 past or presently producing mines and 64.3 Mt in subeconomic deposits. The orebodies are restricted to isotopically juvenile volcanic-arc sequences, dated at 1.903-1.881 Ga at Flin Flon. Sequences of ca. 1.904-1.901 Ga back-arc and ocean-plateau basalts and related plutonic rocks, structurally juxtaposed with the arc assemblages at 1.880-1.870 Ga, are not known to contain economic base metal deposits. The juvenile arc tectono-stratigraphic assemblages are generally marked by older and stratigraphically lower tholeiitic submarine volcanic packages (ca. 1.903-1.886 Ga) that are observed or interpreted to be overlain by extensive and lithologically varied sequences of calc-alkaline and alkaline (shoshonitic) arc rocks and arc rift deposits (ca. 1.888-1.881 Ga). VMS deposits occur in both the tholeiitic and calc-alkaline arc sequences, but the 62 Mt Flin Flon deposit occurs in a 1.903 Ga tholeiitic primitive arc package. It can be demonstrated that for the Flin Flon - Callinan - Triple 7, Cuprus, and White Lake VMS deposits, whose stratigraphic context is preserved, deposition of the massive sulphides was temporally associated with inferred arc rifting processes. Critical observations for arc rifting include evidence for extensional faulting, erosion, and development of unconformities; extrusion of MORB-like basalts and associated rhyolites; and development of depositional basins with thick sequences of shoshonitic turbidites. As has been proposed for other major VMS camps (e.g., Kuroko, Kidd Creek, Bathurst), arc rifting can generate the loci of sustained high heat flow and fluid pathways required for the development of long-lived hydrothermal convection systems.

Formation of an Archean tectonic mélange in the Schreiber‐Hemlo greenstone belt, Superior Province, Canada: Implications for Archean subduction‐accretion process

The late Archean (circa 2750–2670 Ma) Schreiber‐Hemlo greenstone belt, Superior Province, Canada, is composed of tectonically juxtaposed fragments of oceanic plateaus (circa 2750–2700 Ma), oceanic island arcs (circa 2720–2695 Ma), and siliciclastic trench turbidites (circa 2705–2697 Ma). Following juxtaposition, these lithotectonic assemblages were collectively intruded by synkinematic tonalite‐trondhjemite‐granodiorite (TTG) plutons (circa 2720–2690 Ma) and ultramafic to felsic dikes and sills (circa 2690–2680 Ma), with subduction zone geochemical signatures. Overprinting relations between different sequences of structures suggest that the belt underwent at least three phases of deformation. During D1 (circa 2695–2685 Ma), oceanic plateau basalts and associated komatiites, arc‐derived trench turbidites, and oceanic island arc sequences were all tectonically juxtaposed as they were incorporated into an accretionary complex. Fragmentation of these sequences resulted in broken formations and a tectonic mélange in the Schreiber assemblage of the belt. D2 (circa 2685–2680 Ma) is consistent with an intra‐arc, right‐lateral transpressional deformation. Fragmentation and mixing of D2 synkinematic dikes and sills suggest that mélange formation continued during D2. The D1 to D2 transition is interpreted in terms of a trenchward migration of the magmatic arc axis due to continued accretion and underplating. The D2 intra‐arc strike‐slip faults may have provided conduits for uprising melts from the descending slab, and they may have induced decompressional partial melting in the subarc mantle wedge, to yield synkinematic ultramafic to felsic intrusions. A similar close relationship between orogen‐parallel strike‐slip faulting and magmatism has recently been recognized in several Phanerozoic transpressional orogenic belts, suggesting that as in Phanerozoic counterparts, orogen‐parallel strike‐slip faulting in the Schreiber‐Hemlo greenstone belt played an important role in magma emplacement.

Oceanic plateau and island arcs of southwestern Ecuador: their place in the geodynamic evolution of northwestern South America

Coastal Ecuador is made up of an oceanic igneous basement overlain by Upper Cretaceous to Lower Paleocene (≈98–60 Ma) volcaniclastic and volcanic rocks of island-arc affinities. The igneous basement, known as the Piñón Formation, locally dated at 123 Ma, consists of olivine-free basalts and dolerites. Relative to N-MORB, both types of rocks exhibit high concentrations in Nb (0.3–10.75 ppm), Ta (0.03–0.67 ppm), Th (0.11–1.44 ppm), light and medium rare earth elements, and low Zr (22–105 ppm) and Hf (0.59–2.8 ppm) contents, thus showing oceanic plateau basalts affinities. Most of these oceanic plateau basalts tholeiites display rather homogeneous ε Nd ( T = 123 Ma) ratios (∼+7), with the exception of two rocks with higher (+10) and lower (+4.5) ε Nd ( T = 123 Ma) , respectively. All these basalts plot, with one exception, within the ocean island basalts field. Their ( 87Sr/ 86Sr) i ratios are highly variable (0.7032–0.7048), probably due to hydrothermal oceanic alteration or assimilation of altered oceanic crust. The rocks of the Piñón Formation are geochemically similar to the oceanic plateau tholeiites from Nauru and Ontong Java Plateaus and to the Upper Cretaceous (92–88 Ma) Caribbean Oceanic Plateau lavas. The basalts and dolerites of the Upper Cretaceous–Lower Paleocene island arcs show calc-alkaline affinities. The ε Nd ratios (+6.1 to +7.1) of these arc-rocks are very homogenous and fall within the range of intra-oceanic island-arc lavas. The Upper Cretaceous–Lower Paleocene calc-alkaline and tholeiitic rocks from coastal Ecuador share similar high ε Nd ratios to Cretaceous intra-oceanic arc rocks from north, central and South America and from the Greater Antilles. Since the Piñón oceanic plateau tholeiites are locally overlain by early-Late Cretaceous sediments (∼98–83 Ma) and yielded locally an Early Cretaceous age, they do not belong to the Late Cretaceous Caribbean Oceanic Plateau. The basement of coastal Ecuador is interpreted as an accreted fragment of an overthickened and buoyant oceanic plateau. The different tectonic units of coastal Ecuador cannot be easily correlated with those of western Colombia, excepted the Late Cretaceous San Lorenzo and Ricaurte island arcs. It is suggested that northwestern South America consists of longitudinally discontinuous terranes, built by repeated accretionary events and significant longitudinal displacement of these terranes.

Variability of Nb/U and Th/La in 3.0 to 2.7 Ga Superior Province ocean plateau basalts: implications for the timing of continental growth and lithosphere recycling

Extensive volcanic sequences of tholeiitic basalts, having near-flat REE patterns and spatially associated with komatiites, occur in many Archean Superior Province greenstone belts; they are considered to be fragments of intraoceanic volcanic plateaus derived from a plume. Basalts from the 2.9–3.0 Ga Lumby Lake greenstone belt, carefully screened for minimum alteration, have variable Nb/U ratios of 36 to 58. Least-altered basalts from the Abitibi and Wawa greenstone belts also have variable Nb/U ratios of 25 to 50 and 28 to 42, respectively, compared to an average value of 47 for modern ocean basalts. In the Abitibi suite Nb/U correlates positively with Nb/Lapm but negatively with Th/Lapm. Alteration can be ruled out as the cause of Nb/U variation, as there are no correlations of Nb/U with LOI or Eu/Eu*, and Nb/U correlates with Nb/Th in all three suites of basalts. Numerous lines of evidence indicate that crustal contamination can be eliminated as the cause of Nb/U variability, especially for samples with Nb/U > 36. High Nb/U ratios can be explained by recycling ocean crust processed through a subduction zone (high Nb/U and Nb/Lapm, low Th/Lapm) into the mantle source of the basalts, whereas low Nb/U ratios can be accounted for by recycling complementary subarc mantle lithosphere, or continental crust (low Nb/U and Nb/Lapm, high Th/Lapm) into the mantle source. For the entire population of basalts, Th/Lapm ratios generally <1 may result from recycling the residue of slab-derived Archean-type tonalites having high Th/La ratios. Nb/U ratios as high as the 47 ± 10 range of modern ocean basalts have been found in a 2.7 Ga volcanic belt of the Yilgarn craton. Taken together, the results signify very early growth of the continental crust, rather than episodic growth over several Ga.

Geochemical diversity in oceanic komatiites and basalts from the late Archean Wawa greenstone belts, Superior Province, Canada: trace element and Nd isotope evidence for a heterogeneous mantle

Diverse types of komatiites and basalts are present in the 2.7 Ga Schreiber–Hemlo and White River–Dayohessarah greenstone belts of the Wawa subprovince. Al-undepleted and Al-depleted komatiites coexist. The least altered Al-undepleted komatiites are characterized by variably depleted LREE and flat to slightly fractionated HREE patterns (La/Sm n=0.5–1.2, Gd/Yb n=0.90–1.3). They have small positive Zr (Hf) anomalies relative to MREE. Al-depleted komatiites and komatiitic basalts have fractionated REE (La/Sm n=2.2–2.4), and variable Nb and Zr (Hf) anomalies (Nb/La pm=0.8–1.4; Zr/Zr*=0.8–1.0). They feature pronounced Fe-enrichment in comparison with Al-undepleted komatiites (Fe 2O 3=15–22 vs. 11–16 wt%). Tholeiitic basalts are the prevalent volcanic rock type. Mg# varies from 65 to 33 over a relatively narrow range of SiO 2 (47–53 wt%). Chromium and Ni decrease erratically with Mg#, whereas Fe, Ti, Zr and all REE increase. Compositionally, these volcanic rocks are similar to Phanerozoic ocean plateau basalts such as at Ontong Java. In suites of basalts from each of four areas, Nb and Th anomalies relative to La vary continuously from negative to positive (Nb/La pm=0.56–1.10, Th/La pm=0.32–1.23). Mantle melting processes, crustal assimilation, alteration and metamorphism, or fractional crystallization can all be ruled out as the cause of anomalies. Rather, the anomalies are interpreted to be recycling of lithosphere into the mantle: recycled ocean crust processed though a subduction zone having positive Nb anomalies, and complementary recycled subarc mantle or continental crust having negative anomalies. Alkali basalts show strong REE fractionation (La/Yb n=14–16), and large positive Nb anomalies (Nb/La pm=1.32–1.42) like Phanerozoic HIMU ocean island basalts, whereas transitional counterparts have less fractionated REE (La/Yb n=2.6–5.3), and higher Al 2O 3 contents with convex-up patterns through La–Nb–Th. Both variants share the negative Zr and Hf anomalies with Al-depleted komatiites and komatiitic basalts, and generally have high Zr/Hf ratios. Al-depleted komatiites and associated alkali basalts tend to have more positive ε Nd (+3.3 to +3.5) than Al-undepleted komatiites and associated tholeiites (+0.7 to +0.5). The former association has generally flat trends of Nb/La pm and Th/Nb pm with ε Nd consistent with mixing of more and less depleted mantle components. The latter association shows negative trends of Th/La pm and Th/Nb pm with ε Nd, consistent with mixing between a depleted mantle component and a less depleted or enriched component of recycled crust. The komatiites and spatially associated tholeiitic to alkaline basalts are interpreted to have been an ocean plateau derived from a heterogeneous multi-component plume. Negative Zr (Hf) anomalies are consistent with melt segregation with residual majorite garnet at a depth of >400 km, whereas the tholeiitic basalts melted at shallower depths either in the plume or in entrained upper mantle. The plateau was fragmented in a subduction–accretion complex and tectonically mixed with primitive arc tholeiites.

Late Jurassic Oceanic Crust and Upper Cretaceous Caribbean Plateau Picritic Basalts Exposed in the Duarte Igneous Complex, Hispaniola

Four distinct rock units have been recognized near El Aguacate, in the Janico–Juncalito–La Vega area of the Duarte complex (Dominican Republic): (1) serpentinites crosscut by numerous diabasic dikes, (2) basalts interbedded with Late Jurassic ribbon cherts, (3) picrites and ankaramites relatively enriched in incompatible trace elements, and (4) amphibolites and gneissic amphibolites chemically similar to Oceanic Plateau Basalts. Similar Ar‐Ar ages of late magmatic amphibole from a picrite, and hornblende from an amphibolite ($$86.1\pm 1.3$$ Ma and $$86.7\pm 1.6$$ Ma, respectively), suggest that the Duarte picrites are contemporaneous with the Deep Sea Drilling Program Leg 15 and Ocean Drilling Program Leg 126 basalts drilled from the Caribbean oceanic plateau. These basalts are associated with sediments containing Late Cretaceous faunas. Sr, Nd, and Pb data show that enriched picrites and amphibolites are isotopically similar to mafic lavas from previously described Caribbean plateau and Galápagos hotspot basalts. Major element, trace element, and lead isotopic features of Late Jurassic basalts and diabases are consistent with those of normal oceanic crust basalt. However, these basalts differ from typical N‐MORB because they have lower ϵNd ratios that plot within the range of Ocean Island Basalts. These rocks appear to represent remnants of the Caribbean Jurassic oceanic crust formed from an oceanic ridge possibly close to a hotspot. Later, they were tectonically juxtaposed with Late Cretaceous slices of the Caribbean‐Colombian plateau.

Mafic crustal xenoliths and the origin of the lower continental crust

The La/Nb ratio (<1.4) and the Ni concentration (>30 ppm at a Mg number of 40) in oceanic plateau basalts and MORB provide a useful index to distinguish them from arc basalts. MORB can generally be distinguished from plateau basalts by their lower Nb contents (<3 ppm) and lower Nb/Y ratios (<0.1). The La/Nb ratio and Ni contents of mafic lower crustal xenoliths indicate that at most xenolith sites, both arc and plume-related components occur in the lower crust. Considered as a whole, mafic xenolith data suggest that about one third of the post-Archean lower continental crust is composed of mafic rocks from mantle plume sources (accreted oceanic plateaus or mafic underplate), and the remainder come chiefly from arc sources. Because both crustal and host magma contamination should raise La/Nb ratios in xenoliths, the value of one third is a minimum for the plume component in the lower crust.

Climate change during the last 150 million years: reconstruction from a carbon cycle model

Variations of the atmospheric CO 2 level and the global mean surface temperature during the last 150 Ma are reconstructed by using a carbon cycle model with high-resolution input data. In this model, the organic carbon budget and the CO 2 degassing from the mantle, both of which would characterize the carbon cycle during the Cretaceous, are considered, and the silicate weathering process is formulated consistently with an abrupt increase in the marine strontium isotope record for the last 40 Ma. The second-order variations of the atmospheric CO 2 level and the global mean surface temperature in addition to the first-order cooling trend are obtained by using high-resolution data of carbon isotopic composition of marine limestone, seafloor spreading rate, and production rate of oceanic plateau basalt. The results obtained from this model are in good agreement with the previous estimates of palaeo-CO 2 level and palaeoclimate inferred from geological, biogeochemical, and palaeontological models and records. The system analyses of the carbon cycle model to understand the cause of the climate change show that the dominant controlling factors for the first-order cooling trend of climate change during the last 150 Ma are tectonic forcing such as decrease in volcanic activity and the formation and uplift of the Himalayas and the Tibetan Plateau, and, to a lesser extent, biological forcing such as the increase in the soil biological activity. The mid-Cretaceous was very warm because of the high CO 2 level (4–5 PAL) maintained by the enhanced CO 2 degassing rate due to the increased mantle plume activities and seafloor spreading rates at that time, although the enhanced organic carbon burial would have a tendency to decrease the CO 2 level effectively at that period. The variation of organic carbon burial rate may have been responsible for the second-order climate change during the last 150 Ma.

Komatiite-Tholeiite and Boninite Series Volcanic Associations of the Abitibi Greenstone Belt: Plume-Protoarc Interaction, and Crustal Growth

Komatiite-tholeiite sequences and boninite series volcanic rocks are spatially associated at several localities along a 300 km strike sector of the late Archaean (2.7 Ga) Abitibi greenstone belt, Canada. Examples include the Kidd volcanic complex (KVC) and volcanic sequences in Tisdale and Whitney Townships near Timmins, Ontario, as well as volcanic sequences near Val d'Or, Quebec. In the field, the different types of flows interdigitate on a scale of 10's to 100's metres. Komatiites include Munro, or Al-undepleted type, and Barberton, or Al-depleted type. Spinifex textured Al-undepleted komatiites at the KVC and Whitney localities, like Munro counterparts, are characterized by near-chondritic A1203/TiO2, Ti/Zr, and Zr/Y ratios. They have (1) near-fiat HREE, (2) variable L R E E depletion (La/Smn 0.4 to 0.8), and (3) small positive Hf anomalies relative to Sm, with Zr/Hf 400 km, whereas Al-undepleted komatiites reflect melt segregation at shallow levels. Mg-, Fe-tholeiites are spatially associated with the komatiites. They have near-flat R E E patterns over a continuous range from 4 to 20x chondritic abundance. Mg#, Cr, and Ni decrease continuously with increasing REE, Th, Nb, Zr, Hf and Y. The most primitive have near-chondritic interelement ratios. This magma series is interpreted to represent dynamic partial melting in the outer envelope of the mantle plume from which the komatiitic liquids were derived. Collectively, the tholeiites have a continuous range from negative to positive anomalies of Nb relative to Th and La, similar to Phanerozoic and Recent ocean plateau basalts and some ocean island basalts (Saunders et al., 1988). Alteration and crustal contamination can be ruled out as the source of the anomalies. The negative anomalies are consistent with recycling of subduction-influenced subarc mantle, or continental crust, into the mantle, and the positive anomalies recycling of ocean crust processed through a subduction zone (Saunders et al., 1988; McDonough, 1991). Consequently, the plume from which the komatiites and tholeiitic basalts were derived was multi-component, as demonstrated from trace element and isotope systematics on Phanerozoic and Recent plume-related basalts (Mahoney et al., 1993). Compositionally, the boninite series flows range from Mg# = 81-45, TiO2 = 0.17-0.41, and Ni = 1020-310 ppm. They are characterized by high A1203 (12-16 wt.%) contents, and A1203/TiO2 (60-84) ratios. In common with Phanerozoic boninites, these flows have negatively fractionated H R E E (Gd/Ybn = 0.4-0.6), Zr/Y (1.4-2.2) less than the primitive mantle value of 2.4, and variably positive Zr and Hf anomalies, where Zr/Hf 53% and Mg# > 60 given by Crawford et al. (1989). Collectively, the boninite series overlap spinifex texatred Al-undepleted komatiites in terms of TiO2 and Zr contents, but are much more aluminous, possess higher A1203/ TiO2 ratios and Sc and Yb contents, but lower Gd/ Ybn and Zr/Y ratios. The boninites are interpreted to have formed in a protoarc from second stage metasomatic enrichment of a mantle source highly

Accreted Eclogites with Oceanic Plateau Basalt Affinities in Ecuador

High-pressure, regionally-metamorphosed rocks (the Raspas Formation) outcrop extensively (more than 10 km long and 3 km wide) within a terrane including extensively serpentinized harzburgite in E 1 0 r o Province, Ecuador. The high-pressure rocks consist of pelitic schists, eclogite, eclogite amphibolite and mafic rocks retrograded into amphibolites and greenschists. Exhumation of these high-pressure rocks would have occurred at about 132 Ma, based on K/Ar ages obtained from phengite (Feininger, 1980). Eclogites are composed chiefly of euhedral garnet, omphacite, black stubby amphibole and quartz. The garnet is peppered with abundant rutile inclusions, pyroxene and tiny quartz crystals. This metamorphic assemblage equilibrated under conditions of about P(total) = 1.3 GPa 1.5-2 GPa ( 45-60 km depth) and a T of 600 ~ On the basis of the major element compositions, the protoliths of the eclogites were basaltic (e.g. 97CE1:SiO2 = 49%; MgO = 7.58%; TiO2 = 1.48%; Fe203 =12.44%) and/or cumulate gabbroic (e.g. 97CE4A: SiO2 = 47%, MgO = 10.36%; TiO 2 = 0.46%; Fe203 = 8.12%). Trace element abundances have been determined both by bulk solution ICP-MS as well as laser ablation-ICP-MS analysis of individual ( 50 gm spots) mineral grains. The metabasalt has slightly enriched light rare earth element (LREE) abundances relative to heavy (H)REE [chondrite-normalised (cN) (La/Yb) CN = 1.3] with a slight negative Eu anomaly. In contrast, the metagabbro is characterised by depleted LREE abundances relative to HREE [(La/Yb)cN = 0.54] and a marked positive Eu anomaly. Relative to N-MORB, the metabasalt is Nb-, Ta-, Th-, U-, La-, and Ceenriched, and Srand Rbdepleted. In contrast, the gabbro is depleted in most of these the elements, additionally in Nb, Zr, and Hf, but has significant enrichments in Ba and Sr. Omphacite and amphibole separates have complementary REE abundance patterns, i.e. amphibole is depleted in LREE relative to the HREE [(La/Yb) cN = 0.41] while omphacite is enriched [(La/Yb)cN = 5.95]. Absolute REE concentrations of amphibole and omphacite are low (about 10 times chondritic abundances). In terms of isotopic characteristics, omphacite has an eNd(T=lSOMa) = +7.96, which is slightly higher than that of the amphibole (+6.82), but both ratios fall within the range of OIB. The metabasalt has a relatively high (87Sr/86Sr)i [0.70649; eSr(T-lS0Ma) = +30] which is likely related to oceanic hydrothermal alteration. Remarkably, the trace element abundances (absolute and relative) and isotopic characteristics of both eclogitised basalt and gabbro do not differ significantly from those of unmetamorphosed equivalents of an oceanic plateau origin, elsewhere in Ecuador (e.g. basalts and diabases of the Pifidn Formation, San Juan cumulate gabbro; refer to figure). We note further that the subcreted portion of the accreted

Compositional variation of olivine-chromian spinel in Mg-rich magmas as a guide to their residual spinel peridotites

Peridotite restites or last equilibrated peridotites for main magma groups are estimated on the basis of compositional relations between olivine (Fo content) and chromian spinel (Cr#=Cr/(Cr+A1) atomic ratio). A particular magma or magma group develops a particular Fo-Cr# variation trend (=Fo-Cr# fractionation line) during magmatic differentiation. A restite for the magma can be assessed by extrapolating the Fo-Cr# fractionation line back to the olivine-spinel mantle array (=a Fo-Cr# trend for residual mantle peridotites). Cr#'s of chromian spinel in peridotite restites for magma types are estimated as follows: 0.2–0.6 (mostly 0.4–0.6) for MORB, 0.2–0.5 for intraplate alkali basalts, ca. 0.7 for intraplate tholeiites, ca. 0.8 for oceanic plateau basalts, 0.3–0.5 (mostly 0.4–0.5) for back-arc basin basalts, >0.9 for boninites, ca. 0.9 for high-Mg arc tholeiites and high-Mg andesites, 0.1–0.9 (mostly 0.1–0.7) for Quaternary arc basalts of the Northeast Japan arc, and 0.7–0.8 for magmas which produced Precambrian layered intrusions. Arc magmas from a single-arc system can produce various peridotite restites within the upper mantle, which is due to a wide range of melting conditions (depth and degree of melting, hydrous to anhydrous melting) and of source peridotites (fertile to refractory).

The Changing Nature of Mantle Hotspots through Time: Implications for the Chemical Evolution of the Mantle

The melting products of mantle plumes can place constraints on the geochemical structure of the mantle and show how it has varied through time. The highest-temperature melts (komatiites and picrites) derived from each plume are assumed to represent the best available sample of the thermal boundary layer from which the plume originates. A survey of the geochemistry of picrites and komatiites shows that they underwent a fundamental change at about the Archean-Proterozoic boundary. Archean komatiites have depleted or neutral geochemistry, whereas most Protero-zoic and younger picrites have originated from enriched (OIB-type) mantle, although rare depleted plumes persist to the Tertiary (e.g., Gorgona). If plumes originate from a thermal boundary layer near the core-mantle interface, the composition of this layer must have changed from a mixture of depleted and neutral mantle during the Archean to mainly enriched mantle in the Proterozoic. This change coincides with a drop in the maximum MgO content of the magmas from >30% in the Archean to <22% in younger magmas, indicating a decrease in plume source temperature. We suggest that the change in the temperature and composition of the hotspot sources reflects a fundamental change in the dominant form of the component of the earth's convection, driven by surface heat loss: from cold plumes or drips breaking away from beneath a stable lithosphere during the pre-Archean to lithospheric subduction in the Archean and post-Archean mantles. The absence of enriched or OIB-type plumes in the Archean is interpreted to reflect the length of time for oceanic crust to become unstable, sink to the core-mantle boundary, heat up, and return to the upper mantle in plumes. The fall in the maximum MgO content of plume-related magmas between the Archean and Proterozoic may be due to the accumulation of a compositional boundary layer above the core that lowers the temperature of the plume source region by partially insulating it from the core, or to the material in OIB-type plumes being slightly compositionally lighter than the rest of the mantle. Continental flood basalts and oceanic plateau basalts have less extreme geochemical characteristics than those of the associated komatiites or picrites. If these magmas form by melting in the head of a starting plume that is a mixture of mantle from the plume source and entrained lower mantle, the modern lower mantle must have geochemical and isotopie characteristics closer to chondritic than the depleted or enriched mantle reservoirs.

Chemistry of chromian spinel in volcanic rocks as a potential guide to magma chemistry

AbstractChromian spinel in volcanic rocks is a potential discriminant for magma chemistry. The TiO2 content of spinel, compared at similar Fe3+/(Cr + Al + Fe3+) ratios, can distinguish island arc basalts from intraplate basalts. MORB spinels are low in this ratio and are intermediate for the TiO2 level at comparable Fe3+ ratios. Spinels from back-arc basin basalts, although similar in TiO2/Fe3+ ratio, are more enriched in Fe3+ than the MORB spinels. Spinels in the oceanic plateau basalts are distinctly lower in TiO2 than other intraplate basalt spinels and even slightly lower in TiO2 than the MORB spinels. The data were successfully applied to estimate the kind of the magma from which spinelbearing cumulates, especially dunites, were formed. Original magma chemistry of altered or metamorphosed volcanics in which spinels survive can also be estimated by the chemistry of relict spinel alone. It is possible to estimate the magma type of source volcanics for detrital spinel particles of volcanic derivation.