1,721,011 research outputs found
A quantitative model to explain the bimodal distribution of gabbros from the 16.50oN core complex: an attempt to explain the compositional heterogeneity of the lower oceanic crust.
No full textAbout the origin of the olivine-rich troctolites from the ocean lithosphere: remnants of a reactive MOHO?
No full textBuilding of the deepest crust at a fossil slow-spreading centre (Pineto gabbroic sequence, Alpine Jurassic ophiolites)
Full text linkThis work presents new field and petrological data on a poorly known lower crustal section from the Alpine Jurassic ophiolites, the Pineto gabbroic sequence from Corsica (France). The Pineto gabbroic sequence is estimated to be ~1.5 km thick and mainly consists of clinopyroxene-rich gabbros to gabbronorites near its stratigraphic top and of troctolites and minor olivine gabbros in its deeper sector. The sequence also encloses olivine-rich troctolite and mantle peridotite bodies at different stratigraphic heights. The composition and the lithological variability of the Pineto gabbroic sequence recall those of the lower crustal sections at slow- and ultra-slow spreading ridges. The gabbroic sequence considered in this study is distinct in the high proportion of troctolites and olivine gabbros, which approximately constitute 2/3 of the section. In particular, the lower sector of the Pineto gabbroic sequence shows the existence of large-scale fragments of the deepest oceanic crust displaying a highly primitive bulk composition. The mineral chemical variations document that the origin and the evolution of the Pineto gabbroic rocks were mostly constrained by a process of fractional crystallisation. The clinopyroxenes from the olivine gabbros and the olivine-rich troctolites also record the infiltration of olivine-dissolving, Cr2O3-rich melts that presumably formed within the mantle, into replacive dunite bodies. Cooling rates of the troctolites and the olivine gabbros were evaluated using the Ca in olivine geospeedometer. We obtained high and nearly constant values of -2.2 to -1.7 °C/year log units, which were correlated with the building of the Pineto gabbroic sequence through multiple gabbroic intrusions intruded into a cold lithospheric mantle
A conceptual model for the formation of a km-scale gabbroic body in a fossil slow spreading center (Corsica ophiolites, France)
No full textBuilding of the deepest gabbroic crust at a fossil slow spreading centre (Pineto gabbroic sequence, Alpine Jurassic ophiolites)
No full textLower crust generation in the Jurassic Ligurian-Piedmontese Basin (Pineto gabbroic complex, Corsica)
No full textOrigin of olivine-rich troctolites from the oceanic lithosphere: a comparison between the Alpine Jurassic ophiolites and modern slow spreading ridges
No full textOlivine-rich troctolite bodies occur within lower crust and mantle sections of the Jurassic oceanic lithosphere exposed along the Alpine-Apennine belt. These rocks bear structural and compositional resemblances to the olivine-rich troctolites from slow spreading ridges. The olivine-rich troctolites from the Alpine-Apennine belt contain olivines (Fo = 89-87 mol%) with rounded to embayed morphology and clinopyroxene oikocrysts with high Mg# (90-88). The clinopyroxene oikocrysts have higher Cr2O3 (1.6-1.3 wt%) and lower Ti/Yb than clinopyroxenes in equilibrium typical MORB-type melts. These chemical characteristics were most likely acquired by reaction between an olivine-rich matrix and migrating melts crystallizing clinopyroxene and plagioclase. The plagioclases from the olivine-rich troctolites of the Alpine-Apennine belt are commonly poorer in anorthite component (71-61 mol%) than the plagioclases from slow spreading ridge olivine-rich troctolites. The migrating melts involved in the formation of the olivine-rich troctolites from the Alpine-Apennine belt were most likely slightly enriched in Na2O with respect to the basalts normally produced at slow spreading ridges. We attribute this Na2O enrichment to a low degree of melting of asthenospheric sources. The olivine-rich troctolites from fossil and modern oceanic lithosphere probably formed at the mantle-crust transition. The occurrence of olivine-rich troctolite bodies within gabbroic sequences is reconciled with a process of dissection and entrapment of the mantle-crust transition during the growth of the lower crust
Origin of the gabbroic sequences from the Ligurian Jurassic ophiolites: implications for lower crust generation at slow spreading settings
No full textMelt transport and deformation history in a nonvolcanic ophiolitic section, northern Apennines, Italy: Implications for crustal accretion at slow spreading settings
No full textField observations and petrological and geochemical data are used to constrain a conceptual model for the formation of a gabbro-peridotite section from Ligurian ophiolites (Italy). The studied section is attributed to an intraoceanic domain of the Jurassic Ligurian-Piedmontese basin and is characterized by the lack of a basalt layer, similar to nonvolcanic segments from (ultra)slow spreading ridges. The proposed model shows a “hot” lithospheric evolution in which melt transport in the mantle under spinel to plagioclase facies conditions occurred mostly in the form of grain-scale porous flow. We recognize a series of melt/peridotite interaction events, either diffuse or channeled, which modified the composition of the moderately depleted precursor mantle. In particular, localized infiltrations of MORB-type melts gave rise to formation of spinel websterite layers close to the lithosphere-asthenosphere boundary. The peridotite-websterite association was involved in a spinel facies deformation attributed to emplacement of asthenospheric material at the base of the lithosphere. The “hot” lithospheric evolution is followed by an evolution characterized by melt transport through fractures, which started with crystallization of melt into troctolite to olivine gabbro dikes. Both mantle structures and gabbroic dikes are locally crosscut by gabbroic sills. As the mantle section cooled significantly, the dip of the melt migration structures evolved from subvertical to subhorizontal. The growth of a gabbroic pluton (up to ~400 m thick) that is intruded into the mantle sequence is attributed to accretion of gabbroic sills. The tectonomagmatic history recorded by the gabbroic pluton after its solidification is characterized by ductile shearing developed from near-solidus to amphibolite facies conditions
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