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Metre to tens‐of‐metre wide, steeply dipping, greenschist facies shear zones that cut blueschists and eclogites of the Combin and Zermatt–Saas Zones at Täschalp and in adjacent areas of the western Alps were sites of extensive recrystallization driven by fluid flow and deformation. Rb–Sr data imply that these shear zones formed at 42–37 Ma with a systematic younging of structures northward toward, and into, the hangingwall of the Mischabel Structure. Shearing commenced at 400–475 °C and 400–500 MPa and continued as pressures and temperatures fell to 300–350 °C and 300–350 MPa. Individual shear zones were active for 2–3 Myr with later lower grade stages of shearing concentrated into narrow zones. Fluids that infiltrated the shear zones were water rich (XH2O > 0.9). Alteration zones around albite veins and at the margins of serpentinite bodies are penecontemporaneous with these shear zones and formed at approximately the same conditions. The eclogites were exhumed from c. 64 km at 44 Ma to 14–16 km at 42–41 Ma implying exhumation rates of 2–5 cm yr?1. Rapid exhumation was probably achieved by extension aided by buoyancy, following subduction of continental crust, and rapid erosion. The shear zones form part of a regional‐scale extensional system responsible for a significant portion of the exhumation of the subducted oceanic crust. 相似文献
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The subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle; however,
despite its potential importance, fluid–rock interaction during high-pressure metamorphism is relatively poorly understood.
The stable isotope and major element geochemistry of Pennine ophiolite rocks from Italy and Switzerland that were metamorphosed
at high pressures are similar to that of unmetamorphosed ophiolites, suggesting that they interacted with little pervasive
fluid during high-pressure metamorphism. Cover sediments also have oxygen isotope ratios within the expected range of their
protoliths. In the rocks that escaped late greenschist-facies retrogression, different styles of sub-ocean-floor alteration
may be identified using oxygen isotopes, petrology, and major or trace element geochemistry. Within the basalts, zones that
have undergone high- and low-temperature sub-ocean-floor alteration as well as relatively unaltered rocks can be distinguished.
Serpentinites have δ18O and δ2H values that suggest that they were formed by hydration on or below the ocean floor. The development of high-pressure metamorphic
mineralogies in metagabbros occurred preferentially in zones that underwent sub-ocean-floor alteration and which contained
hydrated, fine-grained, reactive assemblages. Given that the transformation of blueschist-facies metabasic rocks to eclogite-facies
assemblages involves the breakdown of hydrous minerals (e.g. lawsonite, zoisite, and glaucophane), and will thus liberate
considerable volumes of fluids, metamorphic fluid flow must have been strongly channelled. High-pressure (quartz+calcite±omphacite±glaucophane±titanoclinohumite)
veins that cut the ophiolite rocks represent one possible channel; however, stable isotope and major element data suggest
that they were not formed from large volumes of exotic fluids. Fluids were more likely channelled along faults and shear zones
that were active during high-pressure metamorphism. Such strong fluid channelling may cause fluids to migrate toward the accretionary
wedge, especially along the slab–mantle interface, which is probably a major shear zone. This may preclude all but a small
fraction of the fluids entering the mantle wedge to flux melting. Additionally, because fluids probably interact with relatively
small volumes of rock in the channels, they cannot "scavenge" elements from the subducting slab efficiently.
Received: 28 January 1999 / Accepted: 2 February 1999 相似文献
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S. Meffan-Main R. A. Cliff A. C. Barnicoat B. Lombardo R. Compagnoni 《Journal of Metamorphic Geology》2004,22(4):267-281
The timing of high‐pressure (HP) metamorphism in the internal basement massifs of the Western Alps has been contentious. In the Gran Paradiso massif silvery micaschists, thought to have developed from granitic precursors, contain assemblages indicative of pressures in excess of 18 kbar at 500–550 °C. This paper presents unique geochronological data for the paragenesis of the silvery micaschist HP assemblage. Rb–Sr microsampling of an apatite–phengite pair thought to have remained closed to Rb–Sr exchange since the HP paragenesis formed has yielded an age of 43.0 ± 0.5 Ma. Greenschist retrogression occurred after 36.3 ± 0.4 Ma, probably in the interval 36–34 Ma. The localised disturbance of the Rb–Sr system in phengite, apatite and allanite during retrogression means that only in situ microsampling could obtain meaningful ages from these rocks. The new data indicating a Tertiary age for HP metamorphism in the Gran Paradiso massif agree with recent data for other internal basement massifs in the Western Alps. A model fitting the Gran Paradiso massif into the Western Alpine framework is presented. 相似文献
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Fluid flow at greenschist facies conditions during exhumation of the western Alps occurred in several penecontemporaneous systems, including shear zones at lithological contacts, deformed contacts between serpentinite bodies and metabasalts, albite veins within metabasalts, and calcite + quartz veins within calcareous schists. Fluid flow in shear zones that juxtapose metasediments and ophiolitic rocks within the Piemonte Unit reset O and H isotope ratios. δ18O values are buffered by the wall rocks; however, calculated fluid δ2H values are similar within all the shear zones suggesting that they formed an interconnected network. The similarity of δ2H values of the sheared rocks and those of unsheared calcareous schists suggests that the fluids were derived from, or had equilibrated with, the schists that envelop the ophiolite rocks. Time‐integrated fluid fluxes at the sheared contacts estimated from changes in Si in metabasalts were up to 105 m3 m?2, with the fluid flowing up temperature driven either by topography or seismic pumping. Individual shear zones were active for c. 2–3 Myr, implying average fluid fluxes of up to 10?9 m3 m?2 s?1. Rocks in shear zones within the ophiolite away from contacts with the metasediments show much less marked isotopic and geochemical changes, implying that fluid volumes decreased into the ophiolite unit, consistent with the source of fluids being the metasediments. Fluids were generated by dehydration reactions that were intersected during exhumation and, while many rocks show the affects of fluid–rock interaction, large‐scale fluid flow between major units was not common. 相似文献
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Faults in sedimentary rocks can act as fluid pathways or barriers to flow and display a range of deformation styles. These
features can be explained by behaviours observed in deformation experiments on sedimentary rocks that reveal a transition
from dilatant brittle faulting and permeability enhancement to cataclasis and permeability reduction, with increasing porosity,
grain size and confining pressure. This transition implies that faults in sedimentary rocks are unlikely to act as fluid pathways
shallower than ~3 km, unless the sediments have undergone early cementation, or have been exposed following burial and uplift.
This has important implications for many geological processes, including fluid circulation in geothermal systems, formation
of sediment-hosted mineral deposits and earthquakes in subduction zones. Stratiform Zn–Pb deposits that have been interpreted
as syngenetic, seafloor deposits could instead be interpreted as early epigenetic deposits representing the depth at which
faults change from fluid pathways to barriers. 相似文献
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