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1.
Peter H. Thompson 《Contributions to Mineralogy and Petrology》1976,57(3):277-295
In rocks undergoing regional metamorphism, pressure and temperature gradients are not everywhere parallel, that is, isotherms are not everywhere parallel to isobars. Temperature gradients vary across the metamorphic terrain and decrease with increasing temperature. The angle between the temperature and the pressure gradients and the magnitude of the temperature gradient must be considered when isograd patterns are used to determine the distribution of pressure and temperature in the rocks when the isograds formed.In Val Mesolcina, southeastern Switzerland, post-tectonic isograds corresponding to mineral dehydration reactions dip steeply northward intersecting the more gently dipping isograd corresponding to the kyanite-andalusite transition. Distributions of pressure and temperature compatible with the isograd pattern have values of α (the angle between the pressure and the temperature gradients) of 50–70° and temperature gradients of 12–15 °C/km. These results indicate that at or near the peak of the Tertiary (Lepontine) phase of Alpine metamorphism isothermal surfaces in these rocks dipped steeply northward and isobaric surfaces were horizontal.The westward divergence of Lepontine mineral zone boundaries can be explained by decreasing values of α; that is, the flattening toward the west of a steep-sided thermal dome present in the eastern part of the Lepontine terrain at the time of metamorphism. 相似文献
2.
D. L. WHITNEY C. TEYSSIER E. TORAMAN N. C. A. SEATON A. K. FAYON 《Journal of Metamorphic Geology》2011,29(2):193-212
Metamorphic terranes comprised of blueschist facies and regional metamorphic (Barrovian) rocks in apparent structural continuity may represent subduction complexes that were partially overprinted during syn‐ to post‐subduction heating or may be comprised of unrelated tectonic slices. An excellent example of a composite blueschist‐to‐Barrovian terrane is the southern Sivrihisar Massif, Turkey. Late Cretaceous blueschist facies rocks are dominated by marble characterized by rod‐shaped calcite pseudomorphs after aragonite and interlayered with blueschist that contains eclogite and quartzite pods. Barrovian rocks, which have 40Ar/39Ar white mica ages that are >20 Myr younger than those of the blueschists, are also dominated by marble, but rod‐shaped calcite has been progressively recrystallized into massive marble within a ~200‐m transition zone. Barrovian marble is interlayered with quartzite and schist in which isograds are closely spaced and metamorphic conditions range from chlorite to sillimanite zone over ~1 km present‐day structural thickness. Andalusite, kyanite and prismatic sillimanite are present in muscovite‐rich quartzite; in one location, all three are in the same rock. Andalusite pre‐dates Barrovian metamorphism, kyanite is both pre‐ and syn‐Barrovian and sillimanite is entirely Barrovian. Muscovite with phengitic cores and relict kyanite in quartzite below the staurolite‐in isograd are evidence for pre‐Barrovian subduction metamorphism preserved at the low‐T end of the Barrovian domain; above the staurolite isograd, all evidence for subduction metamorphism has been erased. Some regional metamorphism may have occurred during exhumation, as indicated by syn‐kinematic high‐T minerals defining the fabric of L‐tectonite. Quartz microstructures in lineated quartzite reveal a strong constrictional fabric that may have formed in a transtensional bend in the plate boundary. Transtension accounts for the closely spaced isograds and development of a strong constrictional fabric during exhumation. 相似文献
3.
G. T. R. DROOP 《Journal of Metamorphic Geology》1985,3(4):371-402
Abstract The Pennine rocks exposed in the south-east Tauern Window, Austria, contain mineral assemblages which crystallized in the mid-Tertiary ‘late Alpine’regional metamorphism. The pressure and temperature conditions at the thermal peak of this event have been estimated for rocks at four different structural levels using a variety of published and thermochemically derived geobarometers and geothermometers. The results are: (a) In the garnet+chlorite zone, 2–5 km structurally above the staurolite+biotite isograd: T= 490.50°C, P= 7° 1 kbar; (b) Within 0.5 km of the staurolite+biotite isograd: T= 560±300C, P=7.1 kbar; (c) In the staurolite+biotite zone, c. 2.5 km structurally below the staurolite+biotite isograd: T= 610±30°C, P=7.6±1.2 kbar; (d) In the staurolite+biotite zone, 3–4 km structurally below the staurolite+biotite isograd: T= 630±40°C, P= 6.6±1.2 kbar. The pressure estimates imply that the total thickness of overburden above the basement-cover interface in the mid-Tertiary was c. 26.4 km. This overburden can only be accounted for by the Austro-Alpine units currently exposed in the vicinity of the Tauern Window, if the Altkristallin (the ‘Middle Austro-Alpine’nappe) was itself buried beneath an ‘Upper Austro-Alpine’nappe or nappe-pile which was 7.4 km thick at that time. The occurrence of epidote + margarite + quartz pseudomorphs after lawsonite in garnet, indicates that part of the Mesozoic Pennine cover sequence in the south-east Tauern experienced blueschist-facies conditions (T<450°C, P<12 kbar) in early Alpine times. Evidence from the central Tauern is used to argue that the blueschist-facies imprint post-dated the main phase of tectonic thickening (D1A) and was thus a direct consequence of continental collision. Combined oxygen-isotope and fluid-inclusion studies on late-stage veins, thought to have been at lithostatic pressure and in thermal equilibrium with their host rocks during formation, suggest that they crystallized from aqueous fluids at 1.1±0.4 kbar and 420.20°C. Early Alpine, late Alpine and vein-formation P–T constraints have been used to construct a P–T path for the base of the Mesozoic cover sequence in the south-east Tauern Window. The prograde part of the P–T path, between early and late Alpine metamorphic imprints, is unlikely to have been a smooth curve and may well have had a low dP/dT overall; the decompression (presumably due to erosion) which occurred immediately before the thermal peak and possibly also earlier in the Tertiary, was probably partly or completely cancelled by the effects of early- to mid-Tertiary (D2A) tectonic thickening. The thermal peak of metamorphism was followed by a phase of almost isothermal decompression, which implies a period of rapid uplift in the middle Tertiary. The peak metamorphic P–T estimates are compared with the solutions of England's (1978) one-dimensional conductive thermal model of the Eastern Alps, and are shown to be consistent with the idea that the late Alpine metamorphism was caused by tectonic burial of the Pennine Zone beneath the Austro-Alpine nappes in the absence of extraneous heat sources, such as large intrusions, at depth. 相似文献
4.
Brad E. Rosenheim Peter K. Swart Philippe Willenz 《Geochimica et cosmochimica acta》2009,73(18):5308-5319
A revised calibration is presented relating the oxygen isotope composition of the aragonite-secreting sclerosponge Ceratoporella nicholsoni, oxygen isotope composition of seawater, and ambient water temperature. This new relationship has been obtained using high-resolution δ18O data measured in sclerosponges from the Bahamas and Jamaica compared to ambient temperature measurements and δ18O values of seawater from the two locations, both measured and published. New data improve an existing calibration which was determined using measurements of salinity rather than directly measured δ18O values of the seawater and was composed of measurements from different species of sclerosponge and other aragonite-secreting organisms. The updated calibration (n = 12, r2 = 0.95) is:
T(°C)=16.1(±3.1)-[6.5(±1.1)](δarag-δsw),