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Ernesto Paolo Prinzi - One of the best experts on this subject based on the ideXlab platform.
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Fault rocks within the blueschist Metabasalts of the Diamante–Terranova unit (southern Italy): potential fossil record of intermediate-depth subduction earthquakesS Vitale et al.Fault rocks within blueschist Metabasalts
Geological Magazine, 2019Co-Authors: Stefano Vitale, Lorenzo Fedele, Francesco D'assisi Tramparulo, Ernesto Paolo PrinziAbstract:AbstractWe report the first evidence of fault rocks (FRs) developed during high-pressure/low-temperature (HP/LT) subduction-related metamorphism, within quartz+epidote pods embedded in the glaucophane–lawsonite-bearing ophiolitic Metabasalts of the Diamante–Terranova unit (Calabria, Italy). FRs occur as relic injections appearing as thin dark seams, locally showing an internal foliation characterized by tabular, curvilinear and meander-like shapes, and consist of very fine grains of glaucophane and titanite, locally including survivor clasts of epidote and lawsonite. Some boudinaged veins show glaucophane fibres in the boudin necks, marking a clear HP/LT syn-metamorphic origin at c. 30 km depth. The injected FRs can be alternatively interpreted either as pseudotachylytes or as fluidized ultracataclasites. Although subsequent recrystallization largely obliterated primary diagnostic features, the occurrence of (i) different-coloured flow streaks, characterized by alternating layers of glaucophane and titanite, (ii) well-developed flow folds and (iii) corroded epidote survivor crystals could indicate a viscous flow of molten material characterized by a non-uniform chemical composition. With this in mind, we support the hypothesis that these fine-grained veins were originally pseudotachylytes generated by the frictional melting of the glaucophane-rich layers of the Diamante–Terranova Metabasalts, likely related to seismic events occurring during the Eocene along thrust faults within the subducting oceanic Ligurian lithosphere. The lack of evidence for pseudotachylyte relics in the Metabasalt source rock argues for a selective preservation, largely dependent on the efficient mechanical shielding action of the stiffer quartz+epidote pods.
Harald Furnes - One of the best experts on this subject based on the ideXlab platform.
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the origin of large varioles in flow banded pillow lava from the hooggenoeg complex barberton greenstone belt south africa
Contributions to Mineralogy and Petrology, 2011Co-Authors: Nils Rune Sandsta, Harald Furnes, Brian Robins, Maarten J. De WitAbstract:Exceptionally well-preserved pillowed and massive phenocryst-free Metabasaltic lava flows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex of the Barberton Greenstone Belt exhibit both flow banding and large leucocratic varioles. The flow banding is defined by blebs and bands of pale and dark green Metabasalt and was the result of mingling of two types of basalt (Robins et al. in Bull Volcanol 72:579–592, 2010a). Varioles occur exclusively in the dark chlorite-, MgO- and FeO-rich Metabasalt. Varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows. In the central parts of some pillows, they impinge to form homogeneous pale patches, bands or almost homogenous cores. Individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite. Many varioles exhibit concentric zones and finer-grained rims. Some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer-grained rim. The matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain-like, skeletal Ca-rich pyroxenes that are partly overgrown by the margins of varioles. Varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile TiO2, Cr, Zr and REE that are similar to the host Metabasalts. The shape, distribution, texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass. Nucleation of plagioclase was strongly inhibited and took place on vesicles, on the bases of drainage cavities and along early fractures. Eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows.
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Evidence for refilling of previously emptied basaltic pillows in the Hooggenoeg Complex, Barberton Greenstone Belt
Geological Magazine, 2010Co-Authors: Brian Robins, Harald Furnes, Nils Rune Sandsta, Maarten J. De WitAbstract:Some large Metabasaltic pillows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex in the Barberton Greenstone Belt, South Africa, exhibit flow-banded margins and homogeneous cores that have a different texture and compositional variation. The margins consist of millimetre to several centimetre thick alternating bands of pale green spherulitic and darker, conspicuously variolitic varieties of non-vesicular and aphyric Metabasalt, previously inferred to be due to mingling of two different types of lava. The dark cores have sharp, aphanitic contacts with the flow-banded carapaces. They lack flow banding, have coarse-grained interiors and exhibit well-preserved primary textures with pseudomorphs after prismatic pyroxene set in a groundmass containing skeletal plagioclase. The compositional range of samples from these cores is unlike that of the flow-banded Metabasalt but is similar to a 19 m thick lobate Metabasalt flow ~150 m stratigraphically further up, at the local top of the Hooggenoeg volcanic sequence. The pillow cores are inferred to result from the later refilling of drained hollow pillows.
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Flow banding in basaltic pillow lavas from the Early Archean Hooggenoeg Formation, Barberton Greenstone Belt, South Africa
Bulletin of Volcanology, 2010Co-Authors: Brian Robins, Harald Furnes, Nils Rune Sandsta, Maarten J. De WitAbstract:Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the Hooggenoeg Formation in the Barberton Greenstone Belt exhibit pronounced flow banding. The banding is defined by mm to several cm thick alternations of pale green and a dark green, conspicuously variolitic variety of aphyric Metabasalt. Concentrations of relatively immobile TiO2, Al2O3 and Cr in both varieties of lava are basaltic. Compositional differences between bands and variations in the lavas in general have been modified by alteration, but indicate mingling of two different basalts, one richer in TiO2, Al2O3, MgO, FeOt and probably Ni and Cr than the other, as the cause of the banding. The occurrence in certain pillows of blebs of dark Metabasalt enclosed in pale green Metabasalt, as well as cores of faintly banded or massive dark Metabasalt, suggest that breakup into drops and slugs in the feeder channel to the lava flow initiated mingling. The inhomogeneous mixture was subsequently stretched and folded together during laminar shear flow through tubular pillows, while diffusion between bands led to partial homogenisation. The most common internal pattern defined by the flow banding in pillows is concentric. In some pillows the banding defines curious mushroom-like structures, commonly cored by dark, variolitic Metabasalt, which we interpret as the result of secondary lateral flow due to counter-rotating, transverse (Dean) vortices induced by the axial flow of lava towards the flow front through bends, generally downward, in the tubular pillows. Other pillows exhibit weakly-banded or massive, dark, variolitic cores that are continuous with wedge-shaped apophyses and veins that intrude the flow banded carapace. These cores represent the flow of hotter and less viscous slugs of the dark lava type into cooled and stiffened pillows.
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Geochemistry of the Sunnfjord Melange: sediment mixing from different sources during obduction of the Solund–Stavfjord Ophiolite Complex, Norwegian Caledonides
Geological Magazine, 1994Co-Authors: Einar Alsaker, Harald FurnesAbstract:AbstractThe Sunnfjord Melange is a strongly tectonized sedimentary assemblage occurring tectonostratigraphically below the Solund–Stavfjord Ophiolite Complex of Ashgill age, and upon continental margin deposits of Wenlock or older age. The Sunnfjord Melange, which probably started to form within an oceanic transform fault, developed further during obduction of the Solund–Stavfjord Ophiolite Complex. The Sunnfjord Melange is divided into two units, the lower Øyravatn Unit of predominantly fine grained calcareous quartz-bearing chlorite–muscovite schists, and the upper Markavatn Unit, mainly metagreywackes. Both units contain blocks of Metabasalts, marble and meta-arkose. Serpentinite blocks are restricted to the Markavatn Unit. The appearance of serpentinite detritus is marked by much higher contents of Cr and Ni in the metasediments of the Markavatn Unit than in those of the Øyravatn Unit. The geochemistry of the metasediments from the Øyravatn Unit and the Markavatn Unit is compatible with mixing of detritus from continental and ophiolitic sources. Despite the strong and pervasive deformation of the Sunnfjord Melange, its geochemical composition from the lowest to the highest tectonostratigraphic levels reflects that of the inverse pseudostratigraphy of the Solund–Stavfjord Ophiolite Complex. Thus, only Metabasalts of the Solund–Stavfjord Complex and continental meta-arkoses yielded detritus to the schists of the Øyravatn Unit. The compositions of the schists and metagreywackes of the Markavatn Unit, on the other hand, show that a multicomponent detritus derived from Metabasalt and serpentinite of the Solund–Stavfjord Ophiolite Complex, mixed with the detritus from continental meta-arkoses.
Derrill M Kerrick - One of the best experts on this subject based on the ideXlab platform.
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metamorphic controls on seismic velocity of subducted oceanic crust at 100 250 km depth
Earth and Planetary Science Letters, 2002Co-Authors: J A D Connolly, Derrill M KerrickAbstract:Abstract Most circum-Pacific subduction zones at 100–250 km depth contain layers in which seismic velocities are ca. 5% slower than in the adjacent mantle. We compute seismic velocities from thermodynamic data for equilibrium Metabasalt mineralogies, determined by free energy minimization, at subduction zone conditions. Lawsonite stability has a profound effect on seismic velocities of subducted oceanic Metabasalts. Velocity reductions of 3–7% are estimated for lawsonite–eclogites derived by metamorphism of hydrothermally altered oceanic basalt subducted along relatively cool geotherms, whereas a 2–4% velocity increase is characteristic of anhydrous eclogites within the coesite stability field. The restricted depth extent of low-velocity layers is explicable through the influence of the coesite–stishovite transition, which reduces lawsonite stability at high pressure. This transition also increases the positive velocity anomaly in anhydrous eclogites to 4–6%, an effect that may account for deep high-velocity layers. The quality of the match between the properties of lawsonite–eclogite and low-velocity layers supports the contention that significant quantities of volatiles are retained within the oceanic crust beyond sub-arc depths. Because the velocity anomalies are explicable in terms of equilibrium phase relations, we find no reason to invoke metastability of metamorphic reactions to explain the low-velocity layers.
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metamorphic devolatilization of subducted oceanic Metabasalts implications for seismicity arc magmatism and volatile recycling
Earth and Planetary Science Letters, 2001Co-Authors: Derrill M Kerrick, James A D ConnollyAbstract:Subducted oceanic Metabasalts are believed to be a primary source of volatiles for arc magmatism and fluid-induced seismicity. From phase equilibria computed for an average oceanic Metabasalt we present a model for subduction zone devolatilization for pressures up to 6 GPa (∼180 km). Along high temperature geotherms complete dehydration occurs under forearcs, whereas dehydration does not occur along low temperature geotherms. For intermediate geotherms, major dehydration occurs under subarcs and provides a subjacent H2O source for arc volcanism. Decarbonation is negligible along cold and intermediate geotherms and limited along high temperature geotherms. Because decarbonation is limited for all subducted carbonate-bearing lithologies, transfer of CO2 from subducted slabs to arc magmas may be triggered by aqueous fluid infiltration. Metabasalt devolatilization could induce seismicity in forearcs (high temperature geotherms) and subarcs (intermediate geotherms); however, because of the lack of devolatilization, Metabasalts would not be a fluid source for seismicity with low temperature geotherms. Along low temperature geotherms, limited devolatilization of subducted oceanic Metabasalts and marine sediments in forearcs and subarcs provides a mechanism for return of volatiles to the deeper mantle.
Stefano Vitale - One of the best experts on this subject based on the ideXlab platform.
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Fault rocks within the blueschist Metabasalts of the Diamante–Terranova unit (southern Italy): potential fossil record of intermediate-depth subduction earthquakesS Vitale et al.Fault rocks within blueschist Metabasalts
Geological Magazine, 2019Co-Authors: Stefano Vitale, Lorenzo Fedele, Francesco D'assisi Tramparulo, Ernesto Paolo PrinziAbstract:AbstractWe report the first evidence of fault rocks (FRs) developed during high-pressure/low-temperature (HP/LT) subduction-related metamorphism, within quartz+epidote pods embedded in the glaucophane–lawsonite-bearing ophiolitic Metabasalts of the Diamante–Terranova unit (Calabria, Italy). FRs occur as relic injections appearing as thin dark seams, locally showing an internal foliation characterized by tabular, curvilinear and meander-like shapes, and consist of very fine grains of glaucophane and titanite, locally including survivor clasts of epidote and lawsonite. Some boudinaged veins show glaucophane fibres in the boudin necks, marking a clear HP/LT syn-metamorphic origin at c. 30 km depth. The injected FRs can be alternatively interpreted either as pseudotachylytes or as fluidized ultracataclasites. Although subsequent recrystallization largely obliterated primary diagnostic features, the occurrence of (i) different-coloured flow streaks, characterized by alternating layers of glaucophane and titanite, (ii) well-developed flow folds and (iii) corroded epidote survivor crystals could indicate a viscous flow of molten material characterized by a non-uniform chemical composition. With this in mind, we support the hypothesis that these fine-grained veins were originally pseudotachylytes generated by the frictional melting of the glaucophane-rich layers of the Diamante–Terranova Metabasalts, likely related to seismic events occurring during the Eocene along thrust faults within the subducting oceanic Ligurian lithosphere. The lack of evidence for pseudotachylyte relics in the Metabasalt source rock argues for a selective preservation, largely dependent on the efficient mechanical shielding action of the stiffer quartz+epidote pods.
Francesco D'assisi Tramparulo - One of the best experts on this subject based on the ideXlab platform.
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Fault rocks within the blueschist Metabasalts of the Diamante–Terranova unit (southern Italy): potential fossil record of intermediate-depth subduction earthquakesS Vitale et al.Fault rocks within blueschist Metabasalts
Geological Magazine, 2019Co-Authors: Stefano Vitale, Lorenzo Fedele, Francesco D'assisi Tramparulo, Ernesto Paolo PrinziAbstract:AbstractWe report the first evidence of fault rocks (FRs) developed during high-pressure/low-temperature (HP/LT) subduction-related metamorphism, within quartz+epidote pods embedded in the glaucophane–lawsonite-bearing ophiolitic Metabasalts of the Diamante–Terranova unit (Calabria, Italy). FRs occur as relic injections appearing as thin dark seams, locally showing an internal foliation characterized by tabular, curvilinear and meander-like shapes, and consist of very fine grains of glaucophane and titanite, locally including survivor clasts of epidote and lawsonite. Some boudinaged veins show glaucophane fibres in the boudin necks, marking a clear HP/LT syn-metamorphic origin at c. 30 km depth. The injected FRs can be alternatively interpreted either as pseudotachylytes or as fluidized ultracataclasites. Although subsequent recrystallization largely obliterated primary diagnostic features, the occurrence of (i) different-coloured flow streaks, characterized by alternating layers of glaucophane and titanite, (ii) well-developed flow folds and (iii) corroded epidote survivor crystals could indicate a viscous flow of molten material characterized by a non-uniform chemical composition. With this in mind, we support the hypothesis that these fine-grained veins were originally pseudotachylytes generated by the frictional melting of the glaucophane-rich layers of the Diamante–Terranova Metabasalts, likely related to seismic events occurring during the Eocene along thrust faults within the subducting oceanic Ligurian lithosphere. The lack of evidence for pseudotachylyte relics in the Metabasalt source rock argues for a selective preservation, largely dependent on the efficient mechanical shielding action of the stiffer quartz+epidote pods.