The P-T-X relations of the cordierite-garnet-sillimanite-quartz equilibrium

Lonker, Steven W.

doi:10.2475/ajs.281.8.1056

Cited by 72 publications

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“…In the case of the GOAL barometer of Figure 3C, Al-isoplethsof an orthopyroxene of fixed X Ug coexisting with a garnet of variable X Mg illustrate the opposite senses of A1-zoning to be expected on the IBC and ITD paths, but also demonstrate a P-T path (path b) which would not be determinable from such an assemblage in this simplified case. The X Mg values of garnet and cordierite depicted in Figure 3D (modified from Hensen & Green, 1973;Lonker, 1981) define the position of the CAGS divariant field and hence show all the P-T-X information for such an assemblage. It is clear that only minor compositional zoning will be evident in garnet and cordierite on an isobaric cooling path whereas decompressional paths would be well characterized by this assemblage as both garnet and cordierite evolve to Fe-richer compositions.…”

Section: A Geothermobarometric Constraints On P-t Pathsmentioning

confidence: 99%

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The origins of granulites: a metamorphic perspective

1989

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Although many recent reviews emphasize a uniformity in granulite pressure-temperature (P-T) conditions and paths, granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation. A thorough survey of over 90 granulite terranes or occurrences reveals that over 50% of them record P-Tconditions outside the 7.5+ 1 kbar and 8OO + 5O°C average granulite regime preferred by many authors. In particular, an increasing number of very high temperature (900-1000 °C) terranes are being recognized, both on the basis of distinctive mineral assemblages and geothermobarometry. Petrogenetic grid and geothermobarometric approaches to the determination and interpretation of P-T histories are both evaluated within the context of reaction textures to demonstrate that the large range in P-Tconditions is indeed real, and that both near-isothermal decompression (ITD) and near-isobaric cooling (IBC) P-T paths are important. Amphibolite-granulite transitions promoted by the passage of CO 2 -rich fluids, as observed in southern India and Sri Lanka, are exceptional and not representative of fluid-related processes in the majority of terranes. It is considered, on the contrary, that fluid-absent conditions are typical of most granulites at or near the time of their recorded thermal maxima.ITD granulites are interpreted to have formed in crust thickened by collision, with magmatic additions being an important extra heat source. Erosion alone is not, however, considered to be the dominant post-collisional thinning process. Instead, the ITD paths are generated during more rapid thinning (l-2mm/yr exposure) related to tectonic exhumation during moderate-rate or waning extension. IBC granulites may have formed in a variety of settings. Those which show anticlockwise P-T histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion, with or without additional crustal extension. IBC granulites at shallow levels ( < 5 kbar) may also be formed during extension of normal thickness crust, but deeper-level IBC requires more complex models. Many granulites exhibiting IBC at deep crustal levels may have formed in thickened crust which underwent very rapid (5 mm/yr) extensional thinning subsequent to collision. It is suggested that the preservation of IBC paths rather than ITD paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust, and that hybrid ITD to IBC paths should also be observed.Most IBC granulites, and probably many ITD granulites, have not been exposed at the Earth's surface as a result of the tectonic episodes which produced them, but have resided in the middle and lower crust for long periods of time (100-2000 Ma) following these events. The eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation.

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Section: A Geothermobarometric Constraints On P-t Pathsmentioning

confidence: 99%

“…Note that the Mg75 opx coexists with garnet of variable X Mg along each isopleth. (D) CAGS barometer (modified from Hensen & Green, 1973;Lonker, 1981;P. Bertrand & D. H. Green, pers.…”

Section: A Geothermobarometry Of Granulitesmentioning

confidence: 99%

The origins of granulites: a metamorphic perspective

1989

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“…Reaction (R4) has a relatively flat slope in P-T space in the KFMASH system (dP/dT =0.203 kbar/JOO"C, calculated with the P-TX software, Perkins et al, 1987) and metamorphic cordierite is not normally stable in metapelites at pressures above 6 kbar (Lonker, 1981;Aranovich & Podlesskii, 1983). However, this mineral replaces garnet in the middle and upper structural levels as a syn-D2 retrograde product of reaction (R4), indicating decompression during the initial stages of retrogression.…”

Section: Discussionmentioning

confidence: 99%

P‐T path determinations in the Tormes Gneissic Dome, NW Iberian Massif, Spain

Viruete¹,

Indares

Arenas³

1997

Journal Metamorphic Geology

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The Tormes Oneissic Dome (TOO, NW sector of the Iberian Massif, Spain) is a high-grade metamorphic complex affected by a major episode of extensional deformation (02). The syn-D2 P-T path of the Lower Unit of the TOO was deduced from the analysis of reaction textures related to superimposed fabrics developed during exhumation, analysis of mineral zoning and thermobarometric calculations. It comprises an initial phase of decompression, determined using the TWEEQU thermobarometric technique, from 6.4-8. 1 kbar at 735-750°C (upper structural levels) and 7. 2 kbar at 770°C (lower structural levels) to 3. 3-3.9 kbar and 645-680 0c. This evolution is consistent with the observed sequence of melting reactions and the generation of garnet-and cordierite-bearing anatectic granitoids. The later part of the syn-D2 P-T path consisted of almost isobaric cooling associated with the thermal re-equilibration of the unit in the new structural position. This segment of the P-T path is recorded by assemblages with And + Bt + Ms and Ms + Chi +Ab related to the later mylonitic S2 fabrics, which indicate retrogression to low-amphibolite and greenschist facies conditions.

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“…The stability fields of andalusite (Holdaway, 1971) and sekaninaite (Fe-cordierite) (Holdaway and Lee, 1977;Lonker, 1981) show that these two minerals can crystallize in hydrothermal veins at temperatures between about 480 and 780 ~ and at a pressure less than 3 kbar. Hellingwerf et al (1987) estimated the temperature of solidification of granites similar to the Ostra H6jden granite occurring elsewhere in the Bergslagen district at 650 ~ and the temperature of formation of pegmatites and high-temperature garnet-pyroxene skarns associated with these granites at 550-600~ In the Gfisborn area the formation of garnet-pyroxene skarns is spatially related to the hydrothermal vein system (Damman, 1985), limiting the temperature of formation of the hydrothermal vein system to 550-600~ Because none of the breakdown reactions of Fe-cordierite given by Holdaway and Lee (1977) and Lonker ,1975;(3) Stout, 1972;(4) Spear, 1980;(5) James et al, 1978;(6) Robinson and Jaffe, 1971;(7) Berg, 1985 (data from Schumacher, t980); (8) Lal and Moorehouse, 1969;(9) Deer et al, 1963;(10) Beeson, 1978;(11) Papike and Ross, 1970;(12) Rabbitt, 1948;(13) Zotov and Siderenko, 1967;(14) Otten, 1984;(15) Berg, 1985(15) Berg, .…”

Section: Estimation Of P T Conditions During the Formation Of The Hymentioning

confidence: 99%

Hydrothermal subsilicic sodium gedrite from the Gåsborn area, West Bergslagen, central Sweden

Damman¹

1988

Mineral. mag.

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Subsilicic sodium gedrite has been found in a hydrothermal vein together with sekaninaite (Fe-cordierite), andalusite, plagioclase, topaz, sillimanite, quartz, biotite, magnetite, ilmenite, hercynite, wolframite, pyrrhotite, chalcopyrite and pyrite. This vein is part of a hydrothermal vein system consisting predominantly of quartz-feldspar veins, some of which contain andatusite, sekaninaite, biotite, musco-9 cite, fuorite and accessory oxides and sulphides, which was formed during the later stages of crystallization of a high-level anorogenic Svecofennian granite. Petrographic observations suggest the following crystallization sequence for the quartz-feldspar veins: plagioclase-quartz-andalusite-sekaninaitemicrocline-biotite-albite-oxides and sulphides-muscovite-fluorite, and for the subsilicic sodium gedrite-bearing vein: andalusite-sekaninaite-subsilicic sodium gedrite-biotite-quartz-albite-sillimanitetopaz-oxides and sulphides. Electron microprobe analysis revealed that all subsilicic sodium gedrite is relatively homogeneous with only the following compositional variation: Na(A) 0.57-0.81, AI iv 2.31-2.57, XMg 0.15-0.21. The temperature (T) for the formation of the hydrothermal vein system is estimated at 550-600 ~ and the pressure (P) is estimated to be less than 3 kbar.

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The P-T-X relations of the cordierite-garnet-sillimanite-quartz equilibrium

Cited by 72 publications

References 0 publications

The origins of granulites: a metamorphic perspective

The origins of granulites: a metamorphic perspective

P‐T path determinations in the Tormes Gneissic Dome, NW Iberian Massif, Spain

Hydrothermal subsilicic sodium gedrite from the Gåsborn area, West Bergslagen, central Sweden

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