Stable isotope geochemistry and phase equilibria of coesite-bearing whiteschists, Dora Maira Massif, western Alps

Sharp, Zachary D.; Essene, Eric J.; Hunziker, J. C.

doi:10.1007/bf00307861

Cited by 123 publications

(80 citation statements)

References 102 publications

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“…Especially at high to ultrahigh pressure, the activity of H 2 O is often reduced (e.g., Chopin and Monié 1984;Sharp et al 1993). The effect of the water activity on the stability of whiteschist mineral assemblages is shown for clay sample 667418 (Fig.…”

Section: Effect Of H 2 Omentioning

confidence: 98%

“…At higher temperatures, talc is decomposed at lower and kyanite disappears at higher water activities. Chopin and Monié (1984) Sharp et al (1993). We therefore assumed a water activity of 0.6 and calculated an equilibrium phase diagram for sample 667418 (Fig.…”

Section: Effect Of H 2 Omentioning

confidence: 99%

“…Some investigators (cf. Sharp et al 1993) expanded the definition of whiteschist including coesite-pyrope quartzites, which they regard as ultrahigh-pressure protoliths of high-pressure whiteschists. Whiteschists have been found in numerous high-and ultrahigh-pressure metamorphic areas (see Table S1 of the electronic supplement).…”

Section: Introductionmentioning

confidence: 99%

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Protoliths and phase petrology of whiteschists

Franz

Romer

Capitani

2013

Contrib Mineral Petrol

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Whiteschists appear in numerous high-and ultrahigh-pressure rock suites and are characterized by the mineral assemblage kyanite ? talc (?-quartz or coesite). We demonstrate that whiteschist mineral assemblages are well stable up to pressures of more than 4 GPa but may already form at pressures of 0.5 GPa. The formation of whiteschists largely depends on the composition of the protolith, which requires elevated contents of Al and Mg as well as low Fe, Ca, and Na contents, as otherwise chloritoid, amphibole, feldspar, or omphacite are formed instead of kyanite or talc. Furthermore, the stability field of the whiteschist mineral assemblage strongly depends on XCO 2 and fO 2 : already at low values of XCO 2 , CO 2 binds Mg to carbonates strongly reducing the whiteschist stability field, whereas high fO 2 enlarges the stability field and stabilizes yoderite. Thus, the scarcity of whiteschist is not necessarily due to unusual P-T conditions, but to the restricted range of suitable protolith compositions and the spatial distribution of these protoliths: (1) continental sedimentary rocks and (2) hydrothermally and metasomatically altered felsic to mafic rocks. The continental sedimentary rocks that may produce whiteschist mineral assemblages typically have been deposited under arid climatic conditions in closed evaporitic basins and may be restricted to relatively low latitudes. These rocks often contain large amounts of the clay minerals palygorskite and sepiolite. Marine sediments generally do not yield whiteschist mineral assemblages as marine shales commonly have too high iron contents. Sabkha deposits may have too high CO 2 contents. Protoliths of appropriate geochemical composition occur in and on continental crust. Therefore, whiteschist assemblages typically are only found in settings of continental collision or where continental fragments were involved in subduction. Our calculations demonstrate that whiteschists can form by closed-system metamorphism, which implies that the chemical and isotopic composition of these rocks provide constraints on the development of the protoliths.

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Section: Effect Of H 2 Omentioning

confidence: 98%

Section: Effect Of H 2 Omentioning

confidence: 99%

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Protoliths and phase petrology of whiteschists

Franz

Romer

Capitani

2013

Contrib Mineral Petrol

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“…Pseudotachylyte veins formed in the chloritoid-talc-kyanitepyrope-coesite gneiss-mylonite unit of Val Gilba (Unit I in Figure 1c) [Zechmeister et al, 2007]. The host rock assemblages indicate peak pressures of 3.0 to 3.4 GPa [Schertl et al, 1991;Sharp et al, 1993] and even up to 4.2 GPa [Chopin and Schertl, 1999]. The age of metamorphism is estimated at about 35.4 AE 1.0 Ma on the basis of U-Pb SHRIMP determinations on metamorphic zircons [Gebauer et al, 1997], with peak temperatures around 800°C.…”

Section: Geology Of the Sampling Localitiesmentioning

confidence: 99%

Magnetic properties of fault pseudotachylytes in granites

Ferré¹,

Geissman²,

Zechmeister³

2012

J. Geophys. Res.

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[1] We investigate the petrographic, geochemical and magnetic properties of fault pseudotachylytes formed by frictional melting in granitic rocks from Southern California, the Italian Alps and Kyushyu, Japan. The main magnetic remanence carriers are mixtures of grain sizes of fine grained magnetite. These ferrimagnetic grains record a stable, multicomponent magnetization that consists of one or more of the following: coseismic thermal remanent magnetization, coseismic lightning isothermal remanent magnetization and post-seismic chemical remanent magnetization. Fault pseudotachylytes from the three localities display contrasting magnetic properties, which suggests that oxygen fugacity and host rock composition ultimately control the magnetic assemblage.

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“…The high radiogenic heat production typical of continental rocks should lead to thermally-induced melting since ultrahigh-pressure metamorphism is generally at conditions above the 'wet' solidus for granitic melts (Huang & Wyllie 1975). The local presence of melt has been discussed by Schreyer et aL (1987), Schreyer (1995), Phillipot (1993 and Sharp et al (1993) but clear evidence for melting remains scarce. Most people argue that the exhumation path of ultrahigh-pressure rocks is characterized by cooling during decompression (Roberto Compagnoni, pers.…”

Section: Other Unresolved Issuesmentioning

confidence: 99%

Exhumation processes

et al. 1999

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Deep-seated metamorphic rocks are commonly found in the interior of many divergent and convergent orogens. Plate tectonics can account for high-pressure metamorphism by subduction and crustal thickening, but the return of these metamorphosed crustal rocks back to the surface is a more complicated problem. In particular, we seek to know how various processes, such as normal faulting, ductile thinning, and erosion, contribute to the exhumation of metamorphic rocks, and what evidence can be used to distinguish between these different exhumation processes.In this paper, we provide a selective overview of the issues associated with the exhumation problem. We start with a discussion of the terms exhumation, denudation and erosion, and follow with a summary of relevant tectonic parameters. Then, we review the characteristics of exhumation in different tectonic settings. For instance, continental rifts, such as the severely extended Basin-and-Range province, appear to exhume only middle and upper crustal rocks, whereas continental collision zones expose rocks from 125 km and greater. Mantle rocks are locally exhumed in oceanic rifts and transform zones, probably due to the relatively thin crust associated with oceanic lithosphere.Another topic is the use of P-T-t data to distinguish between different exhumation processes. We conclude that this approach is generally not very diagnostic since erosion and normal faulting show the same range of exhumation rates, reaching maximum rates of >5-10 km Ma -1 for both processes. In contrast, ductile thinning appears to operate at significantly slower rates. The pattern of cooling ages can be used to distinguish between different exhumation processes. Normal faulting generally shows an asymmetric distribution of cooling ages, with an abrupt discontinuity at the causative fault, whereas erosional exhumation is typically characterized by a smoothly varying cooling-age pattern with few to no structural breaks. Last, we consider the challenging problem of ultrahigh-pressure crustal rocks, which indicate metamorphism at depths greater than 100-125 kin. Understanding the exhumation of these rocks requires that we first know where and how they were formed. One explanation is that metamorphism occurred within a thickened crustal root, but it does seem unlikely that the crust, including an eclogitized mafic lower crust, could get much thicker than c. 110 km while maintaining a reasonable Moho depth (<70 km, assuming that the seismically defined Moho would be observed to lie above the eclogitized lower crust). Diamondbearing crustal rocks cannot be explained by this scheme. The alternative is to accrete the upper 10-40 km of lithospheric mantle into the orogenic root. This scenario will provide sufficient pressures for both coesite-and diamond-bearing eclogite-facies metamorphism, while maintaining a reasonable Moho depth (<70 km) and reasonable mean topography (<3 kin). We speculate that the detachment and foundering of the mantle root may contribute to the exhumation of any crust...

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Stable isotope geochemistry and phase equilibria of coesite-bearing whiteschists, Dora Maira Massif, western Alps

Cited by 123 publications

References 102 publications

Protoliths and phase petrology of whiteschists

Protoliths and phase petrology of whiteschists

Magnetic properties of fault pseudotachylytes in granites

Exhumation processes

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