Academic literature on the topic 'Mid-oceanic ridge basalts'

The spectrum of geochemical compositions of Oceanic Island Basalts (OIBs) and their systematic differences from Mid-Ocean Ridge Basalts (MORBs) reveal that the Earth’s mantle is chemically and isotopically heterogeneous. Two main processes, both related to plate tectonics, contribute to the creation of mantle heterogeneities: (1) partial melting generates melts enriched in incompatible elements and leaves a depleted residual rock; and (2) subduction of the oceanic lithosphere injects heterogeneous material at depth, in particular, altered oceanic crust and continental/oceanic sediments. Moreover, delamination and foundering of metasomatized subcontinental lithospheric mantle might have been important in the early Earth history, when plate tectonics did not operate as today. The fate of the subducted plate is still a matter of debate; presumably some of it is stirred by convection and some may segregate at the base of the mantle, in particular the oceanic crust, which is compositionally denser than the pyrolitic mantle. The view of the lower mantle as a “graveyard” of subducted crust prevailed for decades and was supported by the Hofmann and White (1982) observation that the geochemical fingerprint of most OIB reveals the presence of ancient recycled crust. However, recent geochemical data on short-lived systems (e.g.182Hf→182W has a half-life of 8.9 My) showed that some hotspots, namely Hawaii, Samoa, Iceland and Galápagos, have a negative µ182W anomaly. This discovery prompted a change in our view of the deep mantle because anomalies in short-lived systems require additional processes, which include, but are not limited to, the preservation of ‘pockets’ of melt from a primordial magma ocean, and/or chemical reactions between the metallic core and the silicate mantle. Exchanges at the core-mantle boundary would cause a negative µ182W anomaly, and might also add 3He to mantle material later entrained by plumes. It is now clear that some plumes probe the deepest mantle and are highly heterogeneous, as revealed by isotope ratios from long-lived radiogenic systems, noble gases and short-lived isotope systems. Here I will focus on the dynamics of plumes carrying compositional and rheological heterogeneities. This contribution attempts to be pedagogic and multi-disciplinary, spanning from seismology to geochemistry and geodynamics.

Abstract The Cretaceous Period was a time of considerable geological activity associated primarily with the disintegration of Pangaea and a considerable increase in volcanism, which had significant biogeographic consequences. As in previous chapters attention will be directed initially to the framework of major geological events before considering organic distributions. The most up-to-date plate tectonic reconstructions for the Cretaceous are those of Scotese et al. (1988), which have utilized an interactive computer graphics method. The three dimensional capabilities of this method allow the rotation and manipulation of plate outlines in ‘real time’. Two major phases of plate reorganization are recognized, in the Mid Cretaceous (95 Ma) and latest Cretaceous (65 Ma). The synchroneity of these phases across the world indicates that plate motions are interconnected and suggests to the authors that the reorganizations are triggered by the subduction of major ridge systems, or by the elimination of subduction zones due to continental collision. Furthermore the implication is that ‘slab pull’ is the dominant plate-tectonic mechanism, with oceanic spreading centres passively following lines of stress emanating from ocean trenches. Fig. 8.1 gives Scotese et al.’s global reconstruction for the Late Cretaceous, indicating the areas of new ocean floor. During the Mid Cretaceous, starting in the earliest Aptian, volcanic eruptions on a massive scale took place, registering an extraordinary upwelling of heat and deep-mantle material. Basalts were initially erupted beneath the Pacific basin and created most of the oceanic plateaus of the present-day western Pacific (Winterer 1991). Eruptions from these mantle upwellings spread to other oceans and sea-floor spreading rates increased. The overall effect was to increase the Earth’s ocean crust production by 50-100 per cent during the time interval 125 to 80 Ma. Since this time substantially coincides with a long episode of constant normal geomagnetic polarity (the so-called Quiet Zone of the ocean floor) Larsen (1991) has proposed a superplume model whereby the removal of large quantities of heat and deep-mantle material stopped the reversal process of the Earth’s magnetic field.

ABSTRACT The Maksyutov complex is a mid- to late-Paleozoic high- to ultrahigh-pressure (HP-UHP) eclogite-bearing subduction zone terrane in the south Ural Mountains. Previous reports of radial fractures emanating from quartz inclusions in garnet, omphacite, and glaucophane, cuboid graphite pseudomorphs after matrix diamond, and microdiamond aggregates preserved in garnet identified by Raman spectroscopy indicate that parts of the complex were subjected to physical conditions of ∼600 °C and >2.8 GPa for coesite-bearing rocks, and >3.2 GPa for diamond-bearing rocks. Peak UHP eclogite-facies metamorphism took place at ca. 385 Ma, and rocks were exhumed through retrograde blueschist-facies conditions by ca. 360 Ma. Bulk analyses of 18 rocks reflect the presence of mid-oceanic-ridge basalt (MORB), oceanic-island basalt (OIB), and island-arc tholeiite (IAT) basaltic and andesitic series plus their metasomatized equivalents. To more fully constrain the petrotectonic evolution of the complex, we computed isochemical phase equilibria models for representative metabasites in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 based on our new bulk-rock X-ray fluorescence (XRF) data. Both conventional Fe-Mg exchange thermometry and phase equilibrium modeling result in higher peak equilibrium temperatures than were previously reported for the complex. Pseudosection analysis provides minimum P-T conditions of 650–675 °C and 2.4–2.6 GPa for peak assemblages of the least retrogressed Maksyutov eclogites, whereas Fe-Mg exchange thermometry yields temperatures of 750 ± 25 °C for a pressure of 2.5 GPa. We interpret our new P-T data to reflect a thermal maximum reached by the eclogites on their initial decompression-exhumation stage, that defines a metamorphic field gradient; the relict coesite and microdiamond aggregates previously reported testify to pressure maxima that define an earlier prograde subduction zone gradient. The eclogitic Maksyutov complex marks underflow of the paleo-Asian oceanic plate and does not represent subduction of the Siberian cratonal margin.

ABSTRACT The southeast Ladakh (India) area displays one of the best-preserved ophiolite sections in this planet, in places up to 10 km thick, along the southern bank of the Indus River. Recently, in situ, ultrahigh-pressure (UHP) mineralogical evidence from the mantle transition zone (MTZ; ∼410–660 km) with diamond and reduced fluids were discovered from two peridotite bodies in the basal mantle part of this Indus ophiolite. Ultrahigh-pressure phases were also found by early workers from podiform chromitites of another coeval Neo-Tethyan ophiolite in southern Tibet. However, the MTZ phases in the Indus ophiolite are found in silicate peridotites, but not in metallic chromitites, and the peridotitic UHP phases show systematic and contiguous phase transitions from the MTZ to shallower depth, unlike the discrete UHP inclusions, all in Tibetan chromitites. We observe consistent change in oxygen fugacity (fO2) and fluid composition from (C-H + H2) to (CO2 + H2O) in the upwelling peridotitic mantle, causing melting to produce mid-ocean-ridge basalt (MORB). At shallow depths (<100 km) the free water stabilizes into hydrous phases, such as pargasitic amphibole, capable of storing water and preventing melting. Our discoveries provide unique insights into deep sub-oceanic-mantle processes, and link deep-mantle upwelling and MORB genesis. Moreover, the tectonic setting of Neo-Tethyan ophiolites has been a difficult problem since the birth of the plate-tectonics concept. This problem for the origin of ophiolites in mid-ocean-ridge versus supra-subduction zone settings clearly confused the findings from Indus ophiolites. However, in this contribution, we provide arguments in favor of mid-ocean-ridge origin for Indus ophiolite. In addition, we venture to revisit the “historical contingency” model of E.M. Moores and others for Neo-Tethyan ophiolite genesis based on the available evidence and have found that our new results strongly support the “historical contingency” model.

The earth is divided into three layers: the crust, the mantle, and the core. There are two principle regions within the crust: continents and ocean basins. The rocks that make up these layers differ from one another in chemical composition and density. The mantle is composed of dense ultramafic rocks, rich in magnesium-iron silicate minerals such as olivine and pyroxene. Ultramafic rock is the main source of serpentine soil in the continental crust. Most of the lighter crustal rocks are made up of silicate minerals that are enriched in the lighter elements sodium, calcium, and potassium, which have large cations, rather than magnesium (also a light element) and iron, which have smaller cations (table 2-1, appendix A). Over geological time living organisms have evolved on continents or in oceans with elemental concentrations dependent more on the crust than on the mantle. In the oceanic realm, new oceanic crust forms at spreading centers between active plates where hot, decompressed mantle rock rising toward the surface partially melts to form basaltic magma. The spreading centers develop at mid-ocean ridges, behind volcanic arcs (back-arc basins), in front of volcanic arcs (forearc basins), or as continents rift apart, as with the Red Sea. Cracks formed between the spreading plates are intruded by basaltic magma that forms thin vertical sheets (sheeted dikes). New cracks and dikes are continually forming as the plates spread apart. Some of the magma rising into the cracks reaches the ocean floor and, as the hot lava is quenched by ocean water, it solidifies to form distinctive rounded, pillowlike structures. As magma above the partially melted mantle cools, some of the first crystals to form settle to the bottoms of liquid magma chambers, producing layered gabbros—a process called differentiation. The layered sequence of pillow lava, diabase dikes, and gabbro built upon the ultramafic mantle is typical of new ocean crust. New crust formed at spreading centers slowly migrates away from the spreading center and cools into a rigid oceanic crust that ranges in thickness from 6 to 12 km.

ABSTRACT Ophiolitic rocks occupy different positions relative to paleosubduction zones and have a variety of origins. Distinguishing the structural-tectonic positions of such ophiolitic assemblages is important for relating the ophiolitic rocks to tectonic processes and orogenic history. The California Coast Ranges facilitate recognition of the original structural-tectonic setting of the ophiolitic rocks in the area because the subduction episode associated with them terminated by conversion to a transform plate margin, rather than continental collision, and exhumation since subduction termination has been minimal (<3 km in most areas). Ophiolitic rocks in the Coast Ranges can be classified as upper-plate assemblages that formed above a coeval subduction zone, contrasted with lower-plate assemblages derived from the subducting oceanic plate by the process of subduction-accretion to form the Franciscan subduction complex. Upper-plate ophiolitic rocks include the Coast Range ophiolite, which forms an exposure belt ~750 km in length (~1000 km with restoration of postsubduction slip) with individual remnants up to 5 km thick and exposure lengths up to 20 km. The largely coherent Coast Range ophiolite remnants show variable pseudostratigraphy with a range of proportions and components, including serpentinized ultramafic rocks, mafic plutonic and volcanic rocks, rare sheeted intrusive rocks, and pelagic sedimentary rocks. The Coast Range ophiolite exhibits a suprasubduction zone geochemical affinity, and the remnants span a relatively small age range (172–161 Ma) along the length of the exposure belt. The Coast Range ophiolite largely lacks burial metamorphism, whereas sub-seafloor metamorphism increases from zeolite facies in the uppermost volcanic units to greenschist and amphibolite grade in the plutonic horizons. The Coast Range ophiolite mafic rocks lack penetrative deformation, except for some gabbros that apparently display fabrics developed beneath a spreading center. Largely siliciclastic forearc basin strata of the Great Valley Group depositionally overlie the Coast Range ophiolite, and the basal horizons include sedimentary (olistostromal) serpentinite mélange horizons that reach 1 km in thickness and crop out over a strike length of up to 40 km. The length of the (collective, not continuous) exposure belt of such mélanges is at least 200 km, and this increases to ~400 km with restoration of postsubduction dextral slip. Most of the Great Valley Group is unmetamorphosed, except for some basal horizons with zeolite assemblages and metamorphic blocks-in-olistostromes. The Great Valley Group lacks penetrative deformation except for a brittle foliation in the olistostromal units and metamorphic foliation in some of the blocks-in-olistostromes. Great Valley Group serpentinite mélanges have abundant blocks reaching hundreds of meters in size composed of volcanic and plutonic rocks similar to Coast Range ophiolite in lithologic and geochemical character. Rare blocks of high-pressure metamorphic rocks were presumably derived from exhumed lower-plate (subduction complex) rocks. The lower-plate Franciscan Complex comprises mostly siliciclastic lithologies that accreted to the upper plate from ca. 176 to 12 Ma. Accretion ages young structurally downward. About a fourth of the Franciscan Complex has undergone high-pressure metamorphism of lawsonite-albite facies or higher grade, and the remainder has undergone prehnite-pumpellyite or zeolite facies metamorphism. Approximately a fourth of the Franciscan Complex displays penetrative brittle-ductile to ductile fabrics, and mélange horizons, including those of zeolite and prehnite-pumpellyite grade, commonly display a brittle foliation or cleavage. Lower-plate coherent ophiolitic rocks include slivers of basalt, chert, serpentinite, and rare limestone, which are subordinate components of the largely siliciclastic subduction complex. Coherent mafic slivers generally lack plutonic rocks, and most are <500 m in thickness and extend <10 km along strike. An exception is the Snow Mountain volcanic complex, which forms a sheet spanning 20 × 12 km in map view with a thickness of ~2 km. Chert and/or limestone horizons overlying mafic rocks are ≤300 m in thickness. These coherent mafic slices exhibit mid-ocean-ridge basalt (MORB) or ocean-island basalt (OIB) geochemical affinity, with the exception of very rare, coherent, largely metabasite high-grade slabs up to ~2 km in outcrop length and 300 m in thickness that have a suprasubduction zone geochemical affinity. Coherent lower-plate serpentinite slabs reach up to 1.5 km in thickness with an outcrop length up to 30 km and have an abyssal geochemical affinity. The intact oceanic slabs have a wide collective range of igneous formational ages at mid-ocean ridge and off-axis oceanic settings from ca. 190 to 50 Ma. Mélanges with exotic blocks include siliciclastic matrix units up to ~1 km thick that extend for up to 10 km in outcrop length, and serpentinite matrix horizons up to 200 m thick and cropping out over a length of up to 5 km. Blocks in such mélanges include ophiolitic lithologies, such as serpentinite, mafic-felsic volcanic and plutonic rocks, and pelagic sedimentary rocks, so even the siliciclastic mélanges may be considered “ophiolitic mélanges” by some researchers. Serpentinite blocks in such mélanges display a suprasubduction zone geochemical affinity, and suprasubduction zone geochemical affinity is common in volcanic-plutonic blocks as well. Whereas it is recognized that continental collision overprints and attenuates upper-plate and lower-plate units, making them harder to distinguish, bedrock units in the northern Sierra Nevada of California (USA) show such attenuation and overprinting of primary relationships without a history of continental collision. Subduction erosion as well as thermo-tectonic overprinting from arc-arc collisional processes have apparently profoundly obscured the primary orogenic components in the Sierra Nevada.

ABSTRACT The Arteaga-Tumbiscatío region of the Zihuatanejo terrane forms the westernmost part of the Guerrero composite terrane. This region contains one of the most complete stratigraphic columns of the terrane. Its evolution is divided into nine main tectonic stages characterized by alternating deformational and volcanic-arc events. The oldest rocks in the region belong to the Arteaga Complex, which is made up of highly sheared and locally metamorphosed turbidites (shale, quartzose sandstone, and black chert) and volcanic siltstone that forms a sedimentary matrix with blocks of mid-ocean-ridge basaltic lavas, banded gabbro, green chert, and limestone. The sedimentary matrix is mostly made up of continent-derived quartzose sandstone that yields 1.2 Ga Nd model ages. The age of deposition of the Arteaga Complex is constrained by Triassic (Norian) radiolarians from chert, Permian–Triassic maximum detrital zircon depositional ages, and a U/Pb zircon age of 180 Ma from a block of gabbro. Lithologic associations, geochemical affinities, and contact relations suggest that rocks of the Arteaga Complex were originally deposited in an oceanic basin that evolved near the paleocontinental margin of Mexico-Gondwana (stage 1). The Arteaga Complex was deformed as a subduction-related accretionary prism sometime between the Late Triassic and Early Jurassic (stage 2, D1 and D2) and was intruded by Upper Jurassic granodiorite (stage 3, VA-I) that shows more evolved geochemical and isotopic signatures (peraluminous) than the subsequent magmatic events. These granitoids yielded a 40Ar/39Ar age of 152 ± 0.07 Ma and Nd model ages of 1.2 Ga and are interpreted as related to subduction initiation. Rocks of Lower Cretaceous mafic volcanic-arc affinities (stage 4, VA-II) are not exposed in the study area. Jurassic granodiorite intrusive units were deformed, mylonitized, and exhumed (stage 5, D3) previous to the development of a Cretaceous (Aptian–Albian) volcanic arc (stage 6, VA-III). Four Cretaceous lithostratigraphic units (stage 6, VA-III), the Agua de Los Indios, Pinzán, Resumidero, and Playitas Formations, unconformably overlie the Arteaga Complex and deformed Jurassic intrusive units. All were deposited in an intra-arc basin and record two alternating sea-level transgression/regression cycles. The Cretaceous succession was deformed, causing wide regional folds (stage 7, D4) that are interpreted as initial pulses of the contractional event that originated the Mexican orogen of the Sierra Madre Oriental fold-and-thrust belt of eastern Mexico. Other areas of the Zihuatanejo terrane recorded Santonian–Maastrichtian arc volcanism (stage 8, VA-IV) that was coeval to late pulses of the Late Cretaceous contractional event that formed the Mexican orogen. This was followed by Eocene magmatism and strike-slip deformation (stage 9, VA-V).