H<sub>2</sub>O and Ultrahigh-Pressure Subsolidus Phase Relations for Mafic and Ultramafic Systems

Ernst, W. G.

doi:10.1080/00206819909465176

Cited by 12 publications

(3 citation statements)

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“…This temperature is considered more reasonable than an anormously high T estimate (1132 ºC) produced using the Grt-Amp thermometer of Graham and Powell (1984) (Table 2). The temperature estimate of ~910 ºC is consistent with the hypothesis that the amphibole inclusion in garnet was mantle-derived, and may be stable up to ~900 ºC at upper mantle conditions (Ernst, 1999). Peak UHP stage (II) P-T conditions for the peak UHP metamorphic stage are calculated from the core and rim compositions of minerals of prograde growth zonation, in which reference pressures are produced using the phengite Si barometer of Massone and Schreyer (1989).…”

Section: Pre-peak Prograde Stage (I)mentioning

confidence: 49%

“…(1) Small euhedral amphibole inclusion should be part of the pre-peak assemblages rather than the retrograde assemblages, because no retrograde symplectite assemblages occur along fractures. (2) Amphibole may be stable up to ~900 ºC at upper-mantle conditions (Ernst, 1999), and the amphibole inclusion in the garnet is similar in composition to the mantle-derived amphibole reported by R. Y. , and thus it may be mantle-derived. This is also compatible with the suggestion that rutile eclogite in the CCSD main hole is mantle-derived .…”

Section: Post-peak Retrograde Stage (Iii-iv)mentioning

confidence: 93%

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Assemblages and Textural Evolution of UHP Eclogites from the Chinese Continental Scientific Drilling Main Hole

Tong

Jahn

Iizuka

et al. 2007

International Geology Review

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Coesite-bearing eclogite samples from the Chinese Continental Scientific Drilling main hole (100-2000 m) were examined. Three major evolutionary stages are distinguished from mineral assemblages and textural relations.(1) The pre-peak stage is indicated by inclusion assemblages in rutile eclogite such as amphibole and paragonite, with pre-peak P-T conditions of ~13 kbar and 910ºC. (2) The peak ultrahigh-pressure (UHP) metamorphic stage is characterized by the mineral assemblage garnet-omphacite-phengite-rutile-apatite-coesite/quartz pseudomorph ± kyanite; P-T conditions reached ~910ºC and ~37 kbar for rutile eclogite, and ~850ºC and ~35 kbar for phengite eclogite. (3) The retrograde stage produced decompression textures via back reactions, such as thin coronas of amphibole or zoisite on garnet, fine-grained amphibole-plagioclase symplectites on omphacite and biotite-plagioclase, or K-feldspar-albite on phengite, ilmenite or titanite on rutile, at P-T conditions of 530-560ºC and 7-9 kbar. The presence of K-feldspar-albite symplectite on phengite and minor interstitial K-feldspar implies that post-peak decompressional partial melting occurred locally during rapid exhumation of the subducted UHP slab.Two alternative P-T paths may be constructed: (1) the first possesses a different pre-peak stage but similar peak UHP to post-peak decompression cooling for rutile and phengite eclogite; (2) the second shows a consistent evolution from a pre-peak stage via peak UHP stage to a post-peak stage of near-isothermal decompression, followed by near-isobaric cooling for both rutile and phengite eclogite. We prefer the first P-T path, because the inferred paths show a clear increase in pressure and temperature from near-peak to peak UHP stage, and this may be associated with subductionrelated tectonism as a result of continental collision between the North China and Yangtze blocks. Rapid subduction and fast retrograde exhumation is likely responsible for preservation of some pre-peak inclusion assemblages and prograde mineral growth zoning.

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Section: Pre-peak Prograde Stage (I)mentioning

confidence: 49%

Section: Post-peak Retrograde Stage (Iii-iv)mentioning

confidence: 93%

Assemblages and Textural Evolution of UHP Eclogites from the Chinese Continental Scientific Drilling Main Hole

Tong

Jahn

Iizuka

et al. 2007

International Geology Review

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“…For an excellent review of those in UHP terranes see Compagnoni and Rolfo (1999) and, for an early description, see Smith (1980). The importance of hydrous fluid being available to speed the kinetics of reaction, particularly of retrogression during exhumation (Schreyer, 1995;Ernst, 1999;Ernst and Liou, 1999;Proyer, 2003) means that the significance of the incorporation of butter mélange into HP-UHP terranes is not just as an indicator of the mode of assembly but also as a source of water. One result is the pegmatitic necks (e.g., Krogh et al, 2003) apparently marking the attenuation of butter mélange floaters into the boudin-like shapes that characterize them in exhumed UHP terranes.…”

Section: Miles F Osmastonmentioning

confidence: 98%

Basal Subduction Tectonic Erosion (STE), Butter Mélanges, and the Construction and Exhumation of HP-UHP Belts: The Alps Example and Some Comparisons

Osmaston

2008

International Geology Review

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Studies of high-pressure-ultrahigh-pressure (HP-UHP) belts stand to benefit from having a better idea of how subduction constructs them. Their exhumation, too, merits a closer look at epeirogenic processes. Progress in both these matters now appears possible and their relevance tested, using the Alps, the world's most comprehensively studied such belt, as the prime example. Its comparative youth means that less of the potentially pertinent surface detail has been lost by erosion or burial.Much evidence suggests that subduction downbend is actually not a flexure but is a seismically evident, escalator-like, through-plate step-faulting process. Especially where the subducting plate is younger than about 90 Ma, this gives it the property (STE) of incorporating upper-plate material in the step-angles, thus moving forward the position of the downbend surface that it has carved in the upper plate rapidly, and at a shallow angle for hundreds of kilometers. This yields a nominally "flat" interface profile, above which, at the shallow end of the system, the upper-plate crust will have been chilled and perhaps had its lower part removed, thus providing an origin for basement thrust sheets and (as subducted imbricate slices) those seen in HP-UHP belts.Seismology shows that "flat" interface profiles often pass through an inflection nearer the trench. This means that STE is "taking a second cut" and then disgorges the step-angle material along the succeeding flat part of the interface. When a STE-thinned margin imbricates, this diagnostic material appears as a fluid-overpressured tectonic mélange "buttered" (no shearing at the contact) onto the underside of each. Included floaters (up to >5 km in size) exhibit many lithologies, even slickensided, in an ex-oceanic, usually pelitic matrix that protected them from deformation. This "butter" is a focus of attention in HP-UHP belts, especially for its mafic-ultramafic floaters, while the water in its matrix seems important for reaction kinetics, for eventual epeirogenic action, and even to flux partial melting.Subduction of imbricate slices, already cooled by STE beneath them, and their successive lodgement beneath the "roof" and angle of the distant downbend profile carved by STE into the hanging wall, builds a subcreted triangular section extending far down the back wall, much further than hitherto thought possible. So this part exhumes the most, breaking through the "roof" above as a fault-line prone to strike-slip (Insubric, Møre-Trøndelag, Median Tectonic Line).In the Alps, and following extensive STE from the north, successive imbrication marked by Cretaceous flysch sequences transposed the paleogeographic order, so the Piemont ophiolites were from an oceanic-crusted northern forearc, not a separate ocean. Such transpositions cannot happen during collision; they require too much transport. During Eocene collision, the old block-and-basin structure in the European autochthon was epeirogenically rejuvenated (external massifs, Tauern window) by deep-crust heating and petrologic...

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Fluid activity during exhumation of deep-subducted continental plate

Zheng

2004

Chin. Sci. Bull.

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It is well known that a great deal of fluid was released during subduction of oceanic crust, resulting in arc magmatism, quartz veining and metamorphic mineralization of syn-subduction. In contrast, the process of continental subduction is characterized by the relative lack of fluid and thus no arc magmatism has been found so far. During exhumation of deep-subducted continental crust, nevertheless, significant amounts of aqueous fluid became available from the decomposition of hydrous minerals, the decrepitation of primary fluid inclusions, and the exsolution of structural hydroxyIs. This kind of metamorphic fluid has recently attracted widespread interests and thus been one of the most important targets in deciphering the geological processes concerning metamorphism, magmatism and mineralization in collisional orogens. A large number of studies involving stable isotopes, fluid inclusions and petrological phase relationships have been accomplished in past a few years with respect to the mobility and amount of metamorphic fluid in UHP metamorphic rocks from the Dabie-Sulu orogenic belt. The results demonstrate that the fluid activity during the exhumation of deep-subducted continental crust has the following effects: (1) amphibolite-facies retrogression due to pervasive fluid flow; (2) formation of HP quartz veins within eclogites due to channelized fluid flow; and (3) partial melting of overlying crustal rocks due to focused fluid flow, producing syn-exhumation magmatism within the orogenic belt. In particular, the aqueous fluid released by decompression exsolution of hydroxyl from UHP minerals is characterized by low salinity and is capable of resulting in pervasive and channellized flow. Therefore, the intensive study of fluid activity during exhumation of UHP metamorphic rocks can not only provide insight into geodynamic processes that occurred in continental collisional belts, but also shed light on understanding of crust-mantle recycling and relevant magmatism in subduction zone setting.Fluid activity in the processes of plate subduction and exhumation is one of the most important concerns in the study of metamorphism, magmatism and mineralization in collisional orogens. Mobility and amount of metamorphic fluid can exert significant influences not only on geodynamic processes but also on crust-mantle interaction and syn-subduction arc magmatism that occurred in colli-FRONTIERS sional orogens. A great deal of studies have been devoted to the composition and flow of fluid in high pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks that formed during subduction of oceanic crust[1-6\ and some attentions have also been paid to fluid activity in HP to UHP metamorphic rocks that formed during subduction of continental crust[7-111. However, knowledge concerning the origin of retrograde fluid is still scarce lDn , and sometimes misleading because it was simply ascribed to external origin by ignoring exsolution of internal fluids. With the advance in the study of stable isotopes, fluid inclusions and petrolog...

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H₂O and Ultrahigh-Pressure Subsolidus Phase Relations for Mafic and Ultramafic Systems

Cited by 12 publications

References 57 publications

Assemblages and Textural Evolution of UHP Eclogites from the Chinese Continental Scientific Drilling Main Hole

Assemblages and Textural Evolution of UHP Eclogites from the Chinese Continental Scientific Drilling Main Hole

Basal Subduction Tectonic Erosion (STE), Butter Mélanges, and the Construction and Exhumation of HP-UHP Belts: The Alps Example and Some Comparisons

Fluid activity during exhumation of deep-subducted continental plate

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H2O and Ultrahigh-Pressure Subsolidus Phase Relations for Mafic and Ultramafic Systems

Cited by 12 publications

References 57 publications

Assemblages and Textural Evolution of UHP Eclogites from the Chinese Continental Scientific Drilling Main Hole

Assemblages and Textural Evolution of UHP Eclogites from the Chinese Continental Scientific Drilling Main Hole

Basal Subduction Tectonic Erosion (STE), Butter Mélanges, and the Construction and Exhumation of HP-UHP Belts: The Alps Example and Some Comparisons

Fluid activity during exhumation of deep-subducted continental plate

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H₂O and Ultrahigh-Pressure Subsolidus Phase Relations for Mafic and Ultramafic Systems