Late stage magmatic evolution of oceanic gabbros as a result. Signatures of residual melts, magmatic and seawater. It is therefore important to understand the processes and conditions under which magma differentiates at mid ocean ridges. Taken together the oceanic ridge system of the earth is about 65000 km long and extend all around the globe.
Insights from amphibole geochemistry the near solidus evolution of plutonic rocks formed at slowspreading ridges is investigated using. A record of tectonic and surface processes in central anatolia. Seismic attenuation structure of fastspreading midocean. Magmatic and tectonic controls on the evolution of oceanic magma chambers 89 figure 2. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Gabbro is a medium or coarsegrained rock that consists primarily of plagioclase feldspar and pyroxene. They can vary in size from 10 to 150 km in length, 5 to 15 km in width, and 500 to 1500 m in height. Evidence from the ocean drilling program odp leg 153 drilling at the mid. The crust overlies the solidified and uppermost layer of the mantle. Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate. Geology of the oceanic crust as a setting for chemoautotrophy. Essentially, gabbro is the intrusive plutonic equivalent of basalt, but whereas basalt is often remarkably homogeneous in mineralogy and composition, gabbros are exceedingly variable. Deter mination of the temporal characteristics of sub solidus events in the lower crust i.
In particular, chemical changes in response to deformation and the occurrence of very high. The structure revealed by seismic studies has a close relationship to the composition and structure of rocks found in these layers. The near solidus evolution of plutonic rocks formed at slowspreading ridges is investigated using the major and trace element compositions of amphiboles in a suite of gabbros from the midatlantic ridge. A pluton is an igneous structure that crystallized at very shallow depths. In the other l2612 gabbros this transition does not show a definitive general trend. The relatively short, simple history of young ocean lithosphere makes it an ideal place to determine the property that defines a plate, yet the remoteness and harshness of the seafloor have made precise imaging challenging. Alternatively, these features have been attributed to sources containing veined, metasomatized lithosphere. Minerals in gabbros from atlantis bank, southwest indian ridge, a modern, insitu example of lower oceancrust, were analyzed for major and traceelements. The mantle peridotites of the lizard complex, southwest england, preserve direct and indirect evidence for several deformation episodes. Pdf the reaction mechanism of fluidinduced partial melting of.
Formation of the lower ocean crust and the crystallization of. Such ophiolites are better models for the structural evolution than the geochemical evolution of the lower oceanic crust at mid ocean ridges. Minerals in gabbros from atlantis bank, southwest indian ridge, a modern, insitu example of lower ocean crust, were analyzed for major and traceelements. Frontiers partial melting of lower oceanic crust gabbro. Emplacement and hightemperature evolution of gabbros of. Uniformly mantlelike d18o in zircons from oceanic plagiogranites and gabbros. Successive magma batches underplate, ascend, stall and erupt along spreading ridges, building the oceanic crust.
Occurrence of felsic rocks in oceanic gabbros from iodp hole. Scattered wave imaging of the oceanic plate in cascadia. Pdf hydrous partial melting of oceanic gabbro as a consequence of fluidrock interaction at. Although fractional crystallization is considered to be the dominant mechanism for magma differentiation, opensystem igneous complexes also experience melting. A reactive porous flow control on mid ocean ridge magmatic evolution. Structure and evolution of an oceanic megamullion on the midatlantic ridge at 27 0n by amy r. In particular, the geological and thermal structures of subduction zones are linked to the petrological and geochemical records of orogenic rocks. Such ophiolites are better models for the structural evolution than the geochemical evolution of the lower oceanic crust at midocean ridges. Na recycled oceanic crust encounters its solidus at a depth. Most oceanic spreading centers are not in the middle of their hosting ocean basis but regardless, are traditionally called midocean ridges. Samples originate primarily from drill core acquired during ocean drilling program and integrated ocean drilling program operations near. Emplacement and hightemperature evolution of gabbros of the 16.
A large proportion of the dredged gabbros reveal evidence for deformation at hightemperature conditions. Instead, geological observations have shown that this magmatic. Carbondioxiderich silicate melt in the earths upper. Geological setting the thompson nickel belt tnb forms much of the eastern external zone of the transhudson orogen e. Partial melting experiments on a morblike pyroxenite. Mantle composition several approaches have been used to try to determine the bulk composition of the earth, or more speci. The trace element abundances of highti brown amphibole, plagioclase, and zircon in veins, as well as the presence of myrmekitic texture in the studied felsic rocks support crystallization origin from highlyevolved melts, probably controlled by fractional. These are the youngest portions of the ocean basins where new ocean crust is generated through mantle upwelling and plate divergence. Request pdf near solidus evolution of oceanic gabbros. Formation of oceanic layered gabbros in sills has been previously suggested for oman by browning 16,17 and benn et al.
Fifty years after plate tectonic theory was developed, the defining mechanism of the plate is still widely debated. From 32 detailed cross sections through the gabbro unit of the oman ophiolite, it is concluded that the thickness of this unit is on average 3. Rapid hightemperature metamorphism of east pacific rise. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumulates. Describe how the bowens reaction series relates mineral crystallization and melting temperatures. These data support the hypothesis that the lower gabbros formed in lower crustal sills.
Feb 01, 2005 read late stage magmatic evolution of oceanic gabbros as a result of hydrous partial melting. Gabbro, any of several medium or coarsegrained rocks that consist primarily of plagioclase feldspar and pyroxene. Melts undergoing these reactive processes change in composition, which can explain both the major. The nearsolidus evolution of oceanic gabbros from the slowspreading mar has been investigated using amphibole compositions and textures. Jan 16, 2003 the archaean oceanic crust was probably thicker than presentday oceanic crust owing to higher heat flow and thus higher degrees of melting at midocean ridges1. Pdf emplacement and hightemperature evolution of gabbros. The first discovered midocean ridge was the midatlantic ridge, which is a spreading center that bisects the north and south atlantic basins. Geochemical data define three magmatic series in the core. Recycled gabbro signature in hotspot magmas unveiled by plume. Atlantic ridge, geochemistry, geophysics, geosystems on deepdyve, the largest online rental service for scholarly research with thousands of academic publications available at your fingertips. This study is a geochemical investigation into the accretion of lower oceanic crust and processes of shallow meltrock reaction at midocean ridges.
Slowcooling, coarsegrained gabbro is chemically equivalent to rapidcooling, finegrained basalt. Dec 01, 2001 read near solidus evolution of oceanic gabbros. Igneous rocks school of ocean and earth science and. Components of the oceanic lithosphere and their physical properties 4. Borisovskiy, signatures of residual melts, magmatic and seawaterderived fluids in oceanic lowercrust gabbro from the vema lithospheric section, central atlantic, journal of petrology, volume 56, issue 6, june 2015. These new data allow unambiguous geochemical discrimination between amphiboles of magmatic and hydrothermal origin. Much of the earths oceanic crust is made of gabbro, formed at mid ocean. Fluid evolution in slowspreading environments1 deborah s. Illustrate and label an ideal sequence of rocks in an ophiolite. Deep and hightemperature hydrothermal circulation in the oman. The recovered core consists dominantly of olivine gabbro, with subordinate gabbro, feti oxide gabbro and felsic veins. Layers are defined based on seismic studies of reflection, refraction, and speed of seismic waves.
This paper aims to highlight the most important aspects of continental versus oceanic subduction zones, such as their structures, inputs, processes and products. Carbondioxiderich kimberlitic melt explains the low velocity and high electrical conductivity of the mantle asthenosphere and controls the flux of incompatible elements at oceanic. Apr 24, 2010 lower ocean crust is primarily gabbroic, although 12% felsic igneous rocks that are referred to collectively as plagiogranites occur locally. Interaction between seawater and oceanic basement is important for. Iodp hole u1473a, sited at the top of the atlantis bank oceanic core complex occ on the southwest indian ridge, extends 809m into gabbroic lower ocean crust. The calculated near solidus adiabatic productivity for g2 is %gpa and averages 59%gpa through the melting interval, suggesting substantial partial melting deep in basalt source regions. Start studying geology chapter divergent boundaries. Ocean drilling program scientific results volume 118. A total of 197 zircons from 43 plagiogranite, evolved gabbro, and hydrothermally altered fault rock samples have been analyzed. Igneous rocks occur in a wide range of geological settings. Phd dissertation, stanford university, stanford, california, 1997.
Explain the origin of magma it relates to plate tectonics. Small percentages of pyroxenite in the source significantly affect oceanic crust production and. From right to left, a coarsegrained poikilitic gabbro with clinopyroxene oikocrysts in contact with layers of finegrained. A large proportion of the dredged gabbros reveal evidence for deformation at high. Insights into the compositional variability of the lower oceanic crust. Direct observations of gabbro deformation within and near the paleoridge are complementary to the known deformation in the upper crust i. Interpretive general cross section across the slowspreading midatlantic ridge near the kane fracture zone 238209n, mark area. The magmatic reaction, the comparison of tectonic fabrics, size of neoblasts and subgrains in olivine with experimental data, suggest that the harzburgite sequence was deformed at or near solidus temperatures and under a deviatoric stress increasing from 300 to 400 bars in the upper part of the sequence and from 600 to 700 bars in the lower part. Abstract the near solidus evolution of plutonic rocks formed at slowspreading ridges is investigated using the major and trace element compositions of amphiboles in a suite of gabbros from the midatlantic ridge. In melting experiments on natural amphibolerich veins at 1. An oceanic core complex, or megamullion, is a seabed geologic feature that forms a long ridge perpendicular to a midocean ridge. This event was likely associated with the expulsion of interstitial, evolved magmas from the crystal mush, a mechanism that enhanced the formation of. In contrast, near axis lavas show no primitive noble gas isotope signatures, but are enriched in strontium and lead, indicative of subducted former oceanic lower. Chapter 10 origin and evolution of the ocean floor 1.
Metasomatized lithosphere and the origin of alkaline lavas. Apr 10, 2011 in contrast, near axis lavas show no primitive noble gas isotope signatures, but are enriched in strontium and lead, indicative of subducted former oceanic lower crust melting within the plume. The tectonic evolution of peridotites in the lizard ophiolite complex, southwest england. Ninety samples of evolved gabbro and felsic veins were. Cross section through the peridotite in the samail.
Recycled oceanic crust, with or without sediment, is often invoked as a source component of continental and oceanic alkaline magmas to account for their traceelement and isotopic characteristics. Felsic rocks are minor in abundance but occur ubiquitously in international ocean discovery program hole u1473a, southwest indian ridge. Evolution of the archaean crust by delamination and shallow. Learn vocabulary, terms, and more with flashcards, games, and other study tools. It contains smooth domes that are lined with transverse ridges like a corrugated roof. On the evolution of the water ocean in the platemantle.
Geochemistry of gabbro sills in the crustmantle transition zone of the oman ophiolite. Magmatic and tectonic controls on the evolution of oceanic. Models of the generation and early evolution of oceanic crust must be constrained by observations of the structure and physical state of crustal material near mid ocean ridge axes. Continental versus oceanic subduction zones national. Mid oceanic ridges marked with white arrows are extensive. This talk aims at presenting field, structural and petrological data from alpineapennine ophiolitic peridotites that evidence, froma mantle perspective, the tectonic and magmatic processes which.
Mid ocean ridges dominate the earths global volcanic flux and have been the locus of formation of the crust covering twothirds of the earths surface. Pdf the solidus for a fertile peridotite composition hawaiian pyrolite in the presence of co2 has been determined from 14 to 35 kbar. Recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawaterderived fluids, and thus they may indicate early, hightemperature hydrothermal fluid circulation. Mineralogical and geochemical investigation of layered. Signatures of residual melts, magmatic and seawaterderived. Tracking the evolution of magmas from heterogeneous mantle. Geochemistry of gabbro sills in the crustmantle transition. In the dashed area the same as in panel a, partial melt derived from recycled oceanic crust undergoes reactive infiltration through the surrounding subsolidus peridotite. By the end of this chapter, students should be able to. The oceanic lithosphere, which accounts for more than 65% of the earths surface, is. Amphibole begins to form as interstitial crystals within a crystal mush at.
Kelley2 abstract variably altered and deformed gabbroic and intercalated trondhjemitic to dioritic rocks, recovered from ocean drilling program odp sites 921 to 923 near the eastern intersection of the kane fracture zone and the midatlantic ridge mark, dis. Active continental margins are characterized by a narrow shelf and a slope that. Abstract this study reports the composition of the oceanic crust from the. Recycled gabbro signature in hotspot magmas unveiled by. Structural investigation of the thompson mine south pit. The crust and the solid mantle layer together constitute oceanic lithosphere.