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Henry J B Dick - One of the best experts on this subject based on the ideXlab platform.

  • The Atlantis Bank Gabbro Massif, Southwest Indian Ridge
    Progress in Earth and Planetary Science, 2019
    Co-Authors: Henry J B Dick, Astri J. S. Kvassnes, Paul T. Robinson, Christopher J. Macleod, Hajimu Kinoshita
    Abstract:

    This paper presents the first detailed geologic map of in situ lower ocean crust; the product of six surveys of Atlantis Bank on the SW Indian Ridge. This combined with major and trace element compositions of primary magmatic phases in 99 seafloor Gabbros shows there are both significant vertical and ridge-parallel variations in crustal composition and thickness, but a continuity of the basic stratigraphy parallel to spreading. This stratigraphy is not that of magmatic sedimentation in a large crustal magma chamber. Instead, it is the product of dynamic accretion where the lower crust formed by episodic intrusion, large-scale upward migration of interstitial melt due to crystal mush compaction, and continuous tectonic extension accompanied by hyper- and sub-solidus, crystal-plastic deformation. Five crossings of the Gabbro-peridotite contact along the transform wall show that massive mantle peridotite is intruded by cumulate residues of moderately to highly evolved magmas, few of which could be even close to equilibrium with a primary mantle magma. This contact then does not represent the crust-mantle boundary as envisaged in the ophiolite analog for ocean crust. The residues of the magmas parental to the shallow crust must also lie beneath the center of the complex. This, and the nearly complete absence of dunites in peridotites from the transform wall, shows that melt transport through the shallow lithosphere was largely restricted to the central region of the paleo-ridge segment. There is almost no evidence for a melt lens or high-level storage of primitive melt in the upper 1500 m of Atlantis Bank. Thus, the composition of associated mid-ocean ridge basalt appears largely controlled by fractional crystallization of primitive cumulates at depth, near or at the base of the crust, modified somewhat by melt-rock reaction during transport through the overlying cumulate pile to the seafloor. Inliers of the dike-Gabbro transition show that the uppermost Gabbros crystallized at depth and were then emplaced upward, as they cooled, into the zone of diking. ODP and IODP drilling along the center of the Gabbro massif also found few primitive Gabbros that could have been in equilibrium with the original overlying lavas. Evidence of large-scale upward, permeable transport of interstitial melt through the Gabbros is ubiquitous. Thus, post-cumulus processes, including extensive reaction, dissolution, and re-precipitation within the cumulate pile have obscured nearly all evidence of earlier primitive origins. We suggest that many of the Gabbros may have started as primitive cumulates but were hybridized and transformed by later, migrating melts to evolved compositions, even as they ascended to higher levels, while new primitive cumulates were emplaced near the base of the crust. Mass balance for a likely parental melt intruded from the mantle to form the crust, however, requires that such primitive cumulates must exist at depth beneath Atlantis Bank at the center of the magmatic complex. The Atlantis Bank Gabbro Massif accreted by direct magma intrusion into the lower crust, followed by upward diapiric flow, first as a crystal mush, then by solid-state, crystal-plastic deformation, and finally by detachment faulting to the sea floor. The strongly asymmetric spreading to the south, parallel to the transform, was due to fault capture, with the bounding faults on the northern rift valley wall cut off by the detachment fault, which extended across the zone of intrusion causing rapid migration of the plate boundary to the north.

  • formation of the lower ocean crust and the crystallization of Gabbroic cumulates at a very slowly spreading ridge
    Journal of Volcanology and Geothermal Research, 2001
    Co-Authors: James H. Natland, Henry J B Dick
    Abstract:

    Abstract Ocean Drilling Program Hole 735B was extended to 1508 m below the sea floor during Leg 176, atop a shallow bank near Atlantis II Fracture Zone on the very slowly spreading Southwest Indian Ridge. All the drilling was in Gabbro, and recovery averaged nearly 87%. The drill penetrated a series of stacked plutons consisting mostly of olivine Gabbro, but some with troctolite. Each pluton is some 200–500 m thick, each has its own internally coherent stratigraphy, and each apparently represents an individual event of significant magma inflation and addition to the crust. The entire column was extensively deformed along inclined zones of distributed shear before it was completely frozen, this marking the onset of unroofing of the rocks and their ascent to high rift mountains. The deformation mobilized late-stage melts into flow patterns which led to concentration of ilmenite and magnetite in hundreds of seams of oxide Gabbro along or near zones of strong crystal–plastic deformation, the highly differentiated melts overall being concentrated by buoyancy forces toward the top of the section, especially in one zone nearly 70 m thick. However, upward flow was ultimately blocked or deflected by zones of impermeable rock resulting either from downward freezing or grain-size reduction during shear. A melt lens probably did not form at the base of sheeted dikes, as it does at the East Pacific Rise. Despite this, the rocks are cumulates, and most are adcumulates, with very low residual melt porosities. Cumulate theory based on stratiform, layered intrusions does not entirely apply to these rocks. Instead, all Gabbros, including the oxide Gabbros, crystallized in a dense crystal mush in patterns dominated by fractures, channelized flow, and intergranular porous flow. Most Gabbros are not layered; weak modal layering of uncertain origin is present in As the block was lifted from beneath the rift-valley floor, crystal–plastic deformation gave way to brittle fracture, and the now dominantly subsolidus metamorphism shifted from high-temperature assemblages characterized by amphibole near the top of the section, to low-temperature assemblages with smectite–chlorite and zeolite near the bottom. The rocks acquired their stable magnetization during formation of amphiboles and secondary magnetite during this metamorphism. The entire body of rock is reversely stably magnetized at a consistent inclination, and it is rotated to the south, perhaps along a curving detachment surface, away from the ridge segment where it formed, by about 20°. It has an intensity of magnetization sufficient to account for the magnetic anomaly observed over the site. Drilling did not reach ultramafic rock, but dredging indicates that peridotite is probably within a few hundred meters of the bottom of the hole. Seismic Moho, however, is placed at 5 km beneath the summit of Atlantis Bank. Much of the rock between Moho and the bottom of Hole 735B must therefore be partially serpentinized peridotite.

  • a long in situ section of the lower ocean crust results of odp leg 176 drilling at the southwest indian ridge
    Earth and Planetary Science Letters, 2000
    Co-Authors: Henry J B Dick, Benoit Ildefonse, Wolfgang Bach, James Natland, Daniel Bideau, Sarah Haggas, Jan Gh Hertogen, Greg Hirth, Paul Martin Holm, Gerardo J Iturrino
    Abstract:

    Ocean Drilling Program Leg 176 deepened Hole 735B in Gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine Gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferroGabbro intrusions, often associated with shear zones that cross-cut the olivine Gabbros. The ferroGabbros dramatically increase upward in the section. Whereas there are many small patches of ferroGabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine Gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the Gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle^ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective

  • the fingerprint of seawater circulation in a 500 meter section of ocean crust Gabbros
    Geochimica et Cosmochimica Acta, 1999
    Co-Authors: Stanley R. Hart, Jerzy S Blusztajn, Henry J B Dick, Peter S Meyer, Karlis Muehlenbachs
    Abstract:

    A novel strip-sampling technique has been applied to the 500-m Gabbroic section drilled at site 735 during Leg 118. Twenty-two continuous strips of 1.1- to 4.5-m length were cut longitudinally from the core, allowing for a more representative sampling of this section of the deep ocean crust. A full suite of trace element and isotopic (Sr, Nd, Pb, Os, δ18O) analyses were conducted on these strip samples; for comparison, analyses were conducted on a small suite of protolith samples, selected for their fresh and unaltered appearance. Amphibole, diopside, and plagioclase from 18 vein samples were also analyzed for Sr and Nd isotopes. Although the evidence for a seawater component in these Gabbros is clear (87/86 Sr up to 0.70316; 206/204 Pb up to 19.3; δ18O down to 2.0‰; 187/188 Os up to 0.44), the trace element signatures are dominated by magmatic effects (infiltration and impregnation by late-stage melts derived locally or from deeper levels of the crust). The average upper 500 m 735B Gabbro section is somewhat lower than average N-MORB in trace elements such as Ba (30%), Nb (50%), U (40%), and heavy REE (Yb and Lu, 30%), but somewhat enriched in others such as La (23%), Ce (24%), Pb (23%), and Sr (40%). Although the section is largely comprised of cumulate Gabbros (Natland et al., 1991), and many of the strip samples show marked Sr and Eu anomalies (plagioclase cumulation), the average composition of the total 500 m section shows no Sr or Eu anomalies (<1%). This implies that there has been local separation of melt and solids, but no large scale removal of melts from this 500-m Gabbro section.

Riccardo Tribuzio - One of the best experts on this subject based on the ideXlab platform.

  • Tectono-magmatic Interplay and Related Metasomatism in Gabbros of the Chenaillet Ophiolite (Western Alps)
    Journal of Petrology, 2019
    Co-Authors: Riccardo Tribuzio, Gianreto Manatschal, Maria Rosaria Renna, Luisa Ottolini, Alberto Zanetti
    Abstract:

    Abstract The Jurassic Chenaillet ophiolite in the Western Alps consists of a Gabbro–mantle association exhumed to the seafloor through detachment faulting and partly covered by basaltic lavas. One of the Chenaillet Gabbroic bodies includes mylonites that are transected by a network of felsic veins, thereby testifying to the interplay of ductile shearing and magma emplacement. The deformed Gabbros preserve clinopyroxene porphyroclasts of primary magmatic origin, which are typically mantled by amphibole (titanian edenite) and minor secondary clinopyroxene. Titanian edenite and secondary clinopyroxene also occur as fine-grained syn-kinematic phases locally associated with fine-grained plagioclase. The felsic veins are made up of anorthite-poor plagioclase and minor titanian edenite. Geothermometric investigations document that the ductile Gabbro deformation and the crystallization of the felsic veins occurred at 765 ± 50 °C and 800 ± 55 °C, respectively. With respect to undeformed counterparts, the deformed Gabbros are variably enriched in SiO2 and variably depleted in Mg/(Mg + Fetot2+) and Ca/(Ca + Na). In addition, the deformed Gabbros show relatively high concentrations of incompatible trace elements such as rare earth elements (REE), Y, Zr and Nb. The felsic veins are characterized by low Mg/(Mg + Fetot2+) and Ca/(Ca + Na), high SiO2 and high concentrations of incompatible trace elements. Relict clinopyroxene porphyroclasts from the deformed Gabbros display a rather primitive, mid-ocean ridge-type geochemical signature, which contrasts with the trace element fingerprint of titanian edenite from both the deformed Gabbros and the felsic veins. For instance, titanian edenite typically has relatively high REE abundances, with chondrite-normalized REE patterns characterized by a pronounced negative Eu anomaly. A similar trace element signature is shown by secondary clinopyroxene from the deformed Gabbros. Amphibole from both the deformed Gabbros and the felsic veins displays high F/Cl values. We show that the SiO2-rich hydrous melts feeding the felsic veins were involved in the high-temperature Gabbro deformation and that melt–Gabbro reactions led to major and trace element metasomatism of the deforming Gabbros.

  • New constraints on the origin and age of Variscan eclogitic rocks (Ligurian Alps, Italy)
    Contributions to Mineralogy and Petrology, 2007
    Co-Authors: Folco Giacomini, Massimo Tiepolo, Roberto Braga, Riccardo Tribuzio
    Abstract:

    Gabbro and eclogite boudins are preserved within the amphibolites of the composite para- and ortho-gneiss Variscan basement of the Savona Crystalline Massif (Ligurian Briançonnais, Italy). Whole rock trace element patterns, low initial εNd (+5.4 to +8.8) data and trace element analyses on relict igneous clinopyroxene revealed that the mafic rocks were derived from depleted mantle melts, which most likely underwent crustal contamination during emplacement. Gabbros have a cumulus origin controlled by clinopyroxene and plagioclase segregation, whereas the eclogites represent evolved melts. U-Pb and trace element micro-analyses on zircons separated from one amphibolitised Gabbro and one eclogite help to constrain coeval ages at ~468 Ma for their igneous protoliths. The occurrence of a few inherited zircons confirms the involvement of a crustal component in the petrogenesis of the mafic rocks. In the eclogite, concordant zircon ages younger than the protolith age testify to metamorphic re-crystallisation (or new growth) from about 420 to 305 Ma. Zircon textures and trace element compositions indicate that eclogite facies metamorphism occurred 392–376 Ma ago. The younger zircon portions yielding a mean Concordia age of 333 ± 7 Ma are related to equilibration or new growth during the post-eclogite, amphibolite-facies equilibration.

  • origin of the Gabbro peridotite association from the northern apennine ophiolites italy
    Journal of Petrology, 2004
    Co-Authors: Riccardo Tribuzio, M F Thirlwall, Riccardo Vannucci
    Abstract:

    The Northern Apennine ophiolites are remnants of the Middle Jurassic---Early Cretaceous lithosphere from the Ligurian Tethys. New trace element and Nd---Sr isotope investigations were performed on: (1) the rare Gabbros associated with the subcontinental mantle rocks from the External Liguride ophiolites; (2) the Gabbro--peridotite association from the poorly known ophiolitic bodies from Cecina valley (Southern Tuscany). Clinopyroxenes from the External Liguride and Cecina valley Gabbros have similar trace element compositions, which are consistent with formation from normal mid-ocean ridge basalt (N-MORB) magmas. Sm---Nd mineral isochron ages are 179 9Ma for an External Liguride Gabbro and 170 13Ma and 173 5 4 8Ma for two different Gabbroic bodies from the Cecina valley ophiolites. These ages are interpreted to date the igneous crystallization of the Gabbros and are slightly older than the oldest pelagic sediments of the Ligurian Tethys. Initial eNd (þ8 5 toþ8 9) and Sr/Sr of clinopyroxene are consistent with the interpretation that the studied Gabbros were derived from N-MORB magmas. The least serpentinized mantle rocks from the Cecina valley ophiolites are porphyroclastic spinel lherzolites displaying a residual geochemical signature. They are similar to the least depleted residual peridotites from modern oceans. Nd and Sr isotopic ratios for separated mantle clinopyroxene are respectively higher (e.g. eNd 1⁄4 þ11) and lower than those of clinopyroxene from associated Gabbros at the time of the Gabbro intrusion. The Gabbro---peridotite associations from the Northern Apennine ophiolites record the progression of the rifting process that led to opening of the Ligurian Tethys.

  • Origin of the Gabbro–Peridotite Association from the Northern Apennine Ophiolites (Italy)
    Journal of Petrology, 2004
    Co-Authors: Riccardo Tribuzio, M F Thirlwall, Riccardo Vannucci
    Abstract:

    The Northern Apennine ophiolites are remnants of the Middle Jurassic---Early Cretaceous lithosphere from the Ligurian Tethys. New trace element and Nd---Sr isotope investigations were performed on: (1) the rare Gabbros associated with the subcontinental mantle rocks from the External Liguride ophiolites; (2) the Gabbro--peridotite association from the poorly known ophiolitic bodies from Cecina valley (Southern Tuscany). Clinopyroxenes from the External Liguride and Cecina valley Gabbros have similar trace element compositions, which are consistent with formation from normal mid-ocean ridge basalt (N-MORB) magmas. Sm---Nd mineral isochron ages are 179 9Ma for an External Liguride Gabbro and 170 13Ma and 173 5 4 8Ma for two different Gabbroic bodies from the Cecina valley ophiolites. These ages are interpreted to date the igneous crystallization of the Gabbros and are slightly older than the oldest pelagic sediments of the Ligurian Tethys. Initial eNd (þ8 5 toþ8 9) and Sr/Sr of clinopyroxene are consistent with the interpretation that the studied Gabbros were derived from N-MORB magmas. The least serpentinized mantle rocks from the Cecina valley ophiolites are porphyroclastic spinel lherzolites displaying a residual geochemical signature. They are similar to the least depleted residual peridotites from modern oceans. Nd and Sr isotopic ratios for separated mantle clinopyroxene are respectively higher (e.g. eNd 1⁄4 þ11) and lower than those of clinopyroxene from associated Gabbros at the time of the Gabbro intrusion. The Gabbro---peridotite associations from the Northern Apennine ophiolites record the progression of the rifting process that led to opening of the Ligurian Tethys.

Othmar Muntener - One of the best experts on this subject based on the ideXlab platform.

  • the transition from rifting to sea floor spreading within a magma poor rifted margin field and isotopic constraints
    Terra Nova, 2002
    Co-Authors: Urs Schaltegger, Laurent Desmurs, Martin Frank, Othmar Muntener, Gianreto Manatschal, Martin Meier, Daniel Bernoulli
    Abstract:

    We provide new geological and isotope geochemical constraints on the evolution from continental rifting to sea-floor spreading along a segment of the Jurassic Tethyan margin exposed in the Platta and Err nappes (eastern Central Alps). Field observations show that the ocean–continent transition zone is characterized by oceanward-dipping detachment faults leading to the exhumation of subcontinental mantle rocks subsequently intruded by Gabbro bodies and dolerite dikes, and covered by pillow basalts and radiolarites. Zircons extracted from Gabbros and albitite yield concordant U–Pb ages of 161 ± 1 Ma; their initial ɛHf (+ 14.4 to + 14.9) as well as bulk rock ɛNd values of from Gabbros and basalts (+ 7.3 to + 9.5) point to a MORB-type depleted mantle source. These data suggest that the onset of magmatic activity coincides with the latest phase of mantle exhumation along low-angle detachment faults and may be controlled by upwelling asthenosphere beneath a zone of exhumed continental mantle.

  • U-Pb zircon geochronology of a tholeiitic intrusion and associated migmatites at a continental crust-mantle transition, Val Malenco, Italy
    Schweizerische Mineralogische Und Petrographische Mitteilungen, 2001
    Co-Authors: W Hansmann, Othmar Muntener, J. Hermann
    Abstract:

    This paper provides new constraints on the crystallisation age of the Gabbroic rocks and associated migmatites that are exposed now along the boundary of the Penninic and Austroalpine nappes in the Eastern Central Alps (Val Malenco, N Italy). The Gabbros intruded at the crust-to-mantle boundary and caused granulite facies metamorphism in the country rocks. Zircons were extracted from the Gabbros and metaGabbros as well as from an associated leucogranite and single crystals were dated by the U-Pb method. In addition the lead isotopic composition was determined on plagioclase separated from the same samples. One Fe-Gabbro with preserved primary mineral assemblage yielded sufficient zircons suited for U/Pb dating. Additionally a Zr-rich Ti-Fe-Gabbro which is overprinted by Alpine metamorphism and a leucogranite representing local anatexis of country rocks during the Gabbro intrusion were selected for dating. 15 U-Pb data points of the U-poor (30-150 ppm) zircons from both Gabbros yielded an upper concordia intersection age of 281 +/- 19 Ma, whereas 10 U-rich (600-4000 ppm) zircons from the leucogranite define a very similar upper concordia intersection at 278 -2.5/+2.6 Ma corresponding to an Early Permian age. On a regional scale, the Early Permian age of the Gabbros is consistent with a major phase of continental crustal growth during the late Paleozoic. Plagioclase of all but one sample of the tholeiitic Gabbro suite is characterised by a Pb isotopic composition typical for average crust. The relatively homogeneous Pb isotopic composition of these rocks, which is independent of the degree of differentiation, suggests a contaminated mantle source of their parental magma rather than crustal contamination during the emplacement at the crust-to-mantle boundary. One Ti-Fe-Gabbro dike showed elevated mu and omega values (mu = 9.98; omega = 40.3) very similar to those of the anatectic leucogranite. Contamination of this rock most likely took place during its intrusion into metasedimentary country rocks. U/Pb results of some zircons of an other Ti-Fe-Gabbro suggest the presence of inherited crustal components. Abundant zircon and a high Zr content (603 ppm) indicate that the magma of this sample was saturated with zircon and thus xenocrystic zircon may have survived. However, the presence of cores or xenocrysts has not been confirmed by optical means yet. Observed cores and the presence of inherited lead in zircons of the leucogranite are in agreement with an anatectic origin of this rock.

  • Fossil crust-to-mantle transition, Val Malenco (Italian Alps)
    Journal of Geophysical Research-Solid Earth, 1997
    Co-Authors: J. Hermann, W Hansmann, Volker Trommsdorff, Othmar Muntener, Giovanni B. Piccardo
    Abstract:

    An exhumed, undisturbed fossil lower crust to upper mantle section is preserved in Val Malenco, Italian Alps, and is now exposed along the boundary between Penninic and Austroalpine nappes. Lower-crustal metapelitic rocks are welded to upper-mantle ultramafic rocks by a mid-Permian Gabbro intrusion. The underplating of Gabbro caused granulite metamorphism and partial melting of the metapelites. In the crust-to-mantle transition zone of at least 1 km thickness, Gabbros, large xenoliths of restitic metapelites and ultramafic rocks occur, with densities of 2.95-3.14, 3.25 and 3.27 g/cm(3), respectively. The seismic Moho therefore did not coincide with the boundary between peridotites and crustal rocks but was situated above the upper limit of the peridotitic mantle. The whole complex underwent cooling with only moderate decompression within the kyanite field. This process started at 1 GPa and similar to 800 degrees C and ended at 0.85 GPa and 600 degrees C and is interpreted as thermal relaxation after the Gabbro intrusion. Later, during Jurassic rifting, the crust-to;mantle section was exhumed at the Adria margin of the Tethys ocean.

Peter B Kelemen - One of the best experts on this subject based on the ideXlab platform.

  • origin of Gabbro sills in the moho transition zone of the oman ophiolite implications for magma transport in the oceanic lower crust
    Journal of Geophysical Research, 1997
    Co-Authors: Jun Korenaga, Peter B Kelemen
    Abstract:

    The Moho transition zone (MTZ) of the Oman ophiolite commonly includes a number of Gabbro sills surrounded by dunites. The petrology and geochemistry of these sills are investigated to provide constraints on how magma migrates from the subridge mantle to the oceanic crust. The Gabbro sills have millimeter-scale to tens of centimeter-scale modal layering that closely resembles layering in lower crustal Gabbros of the ophiolite. Variations in mineral compositions correlate with the modal layering, but there are no overall trends within the sills. The Gabbroic sills and the layered Gabbros have clear covariations among mineral compositions, which can be interpreted as a fractional crystallization path from a common parental magma. Together with constraints from mid-ocean ridge thermal evolution and crustal accretion dynamics, the petrological and geochemical observations on the Gabbro sills indicate that they formed from small, open-system, melt-filled lenses within the MTZ. The thermal evolution of the MTZ melt lenses, buffered by the ambient mantle, is characterized by a slow cooling rate (<10−3°C/yr) and a small temperature difference (∼0.1–1°C) within lenses. Internal origins for modal layering, such as gravity currents and oscillatory nucleation, are unlikely in such a thermal environment, and we propose that open-system evolution of the melt lenses is essential to produce the observed layering. The formation of the MTZ melt lenses may be a consequence of porous flow with low Peclet number entering a conductively cooling regime, where porosity becomes “clogged” by crystallized plagioclase. Preservation of fine-scale vertical variation in mineral composition, together with correlated compositions of different minerals, rules out diffuse porous flow as the primary mechanism of melt transport above these melt lenses. Instead, melt extraction must have been focused into porous channels or melt-filled fractures. Melt lenses drained by fractures would experience repetitious expulsion with continuous melt replenishment. Modal layering could develop through the expulsion cycles, probably via in situ crystallization at the margins of melt lenses.

  • geochemistry of Gabbro sills in the crust mantle transition zone of the oman ophiolite implications for the origin of the oceanic lower crust
    Earth and Planetary Science Letters, 1997
    Co-Authors: Peter B Kelemen, Kenneth T Koga, N Shimizu
    Abstract:

    Gabbroic sills intruding dunite in the crust-mantle transition zone (MTZ) of the Oman ophiolite have textures and compositions very similar to those in modally layered Gabbros that form the lower part of the Gabbro section in the ophiolite, and different from those in non-layered Gabbros near the dike-Gabbro transition. The presence of Gabbroic sills in the MTZ indicates that modally layered Gabbros can form far below the level of magmatic neutral buoyancy and far below the dike-Gabbro transition. Minerals in the sills and lower, layered Gabbros are in FeMg and trace element exchange equilibrium with liquids identical to those that formed the sheeted dikes and lavas in the ophiolite. In contrast, many of the upper, non-layered Gabbros resemble crystallized liquid compositions, similar to the dikes and lavas. The lower, layered Gabbros probably formed in sills similar to those in the MTZ. Mantle-derived magmas cooled in these sills, where they crystallized from a few percent to 50% of their mass. Residual liquids then rose to form upper Gabbros, dikes and lavas. Sills may form beneath permeability barriers created by the crystallization of cooling liquid migrating by porous flow. Once permeability barriers are present, however, porous flow becomes a less important mode of magma ascent, compared to ponding in sills, gradual increase in magma pressure, and periodic ascent in hydrofractures. Thus, Gabbroic sills in the MTZ may represent the transition in fast-spreading ridge environments from continuous porous flow in the mantle to periodic diking in the crust.

Giovanni B. Piccardo - One of the best experts on this subject based on the ideXlab platform.

  • Fossil crust-to-mantle transition, Val Malenco (Italian Alps)
    Journal of Geophysical Research-Solid Earth, 1997
    Co-Authors: J. Hermann, W Hansmann, Volker Trommsdorff, Othmar Muntener, Giovanni B. Piccardo
    Abstract:

    An exhumed, undisturbed fossil lower crust to upper mantle section is preserved in Val Malenco, Italian Alps, and is now exposed along the boundary between Penninic and Austroalpine nappes. Lower-crustal metapelitic rocks are welded to upper-mantle ultramafic rocks by a mid-Permian Gabbro intrusion. The underplating of Gabbro caused granulite metamorphism and partial melting of the metapelites. In the crust-to-mantle transition zone of at least 1 km thickness, Gabbros, large xenoliths of restitic metapelites and ultramafic rocks occur, with densities of 2.95-3.14, 3.25 and 3.27 g/cm(3), respectively. The seismic Moho therefore did not coincide with the boundary between peridotites and crustal rocks but was situated above the upper limit of the peridotitic mantle. The whole complex underwent cooling with only moderate decompression within the kyanite field. This process started at 1 GPa and similar to 800 degrees C and ended at 0.85 GPa and 600 degrees C and is interpreted as thermal relaxation after the Gabbro intrusion. Later, during Jurassic rifting, the crust-to;mantle section was exhumed at the Adria margin of the Tethys ocean.