The Bohlen et al. Figure 7 shows differences in temperature and pressure between the uncorrected Fe–Al and uncorrected Fe–Mg estimates (points B and A in Fig. Textures in mineralized microcline-rich gneiss imply original mineralization temperatures within the greenschist facies, similar to the conditions of formation for other orogenic gold deposits. Table 8 provides a summary of mean P–T results for 24 terrains with six or more samples. We also advocate element (X-ray) mapping of minerals before analysis points for thermobarometric calculations are selected so that zoning patterns can be interpreted and compositions that are obviously out of equilibrium can be avoided (see Pattison & Bégin, 1994a; Kohn & Spear, 2000). (a) \(X_{\mathrm{Al}}^{\mathrm{opx}}\) ⁠. The southern Indian incipient charnockites may therefore represent sporadically developed, slightly lower-temperature, fluid-triggered granulite ‘fronts’ that develop locally a little down-grade of the main expanse of granulite, the latter controlled largely by magmatic and partial melting processes. Adirondack Highlands, Furua Complex: Appendix and Electronic Appendix B), some samples indicate Al-solubility temperatures higher than Fe–Mg temperatures whereas others show the opposite. and 0046751 to T.C. The depth at which it occurs is not constant. 11a). 9b is to show that the corrected Fe–Mg–Al P–T estimates largely fall in or close to the granulite-facies stability field, in contrast to the uncorrected Fe–Mg P–T estimates which typically fall well below the granulite-facies stability field. Granulite facies Granulite facies (Fig.1) was introduced by Eskola (1939) to define the highest grade of regional metamorphic rocks that contain pyroxene in place of normal hydrous ferromagnesian minerals. (1992) constrain pressure and temperature of the M2 event near the amphibolite-granulite facies transition using opx-bt, cpx-plag, amph-plag, and two-pyroxene equilibria. Our editors will review what you’ve submitted and determine whether to revise the article. & Worley, B. The concept of metamorphic facies is a systematic way to look at the mineral assemblages in rocks and determine a potential range of pressure and temperature … The mean and 95% confidence limit on the mean of the temperature estimates is 827 ± 18°C for a pressure range of 6–8 kbar, not significantly different from the equivalent values for all intermediate granulites (841 ± 11°C; Table 7). This article was most recently revised and updated by, https://www.britannica.com/science/granulite-facies. It is characterized by the following mineral assemblages: In metabasites: orthopyroxene + clinopyroxene + plagioclase ± olivine or quartz; In … The label for each terrain is placed beside the corrected Fe–Mg–Al estimate (•). 2a) spreads out the P–T range of the upper amphibolite facies. For these localities, mean temperatures of ∼830°C from RCLC are well below the temperatures necessary for fluid-absent melting, suggesting that aH2O in the infiltrating fluid was significantly lower than in the host gneisses. & Clemens, J. D. (, Bohlen, S. R., Wall, V. J. Thus mineralization at Griffin{\textquoteright}s Find must have been introduced prior to granulite facies metamorphism. In some of the terrains, such as the Kerala Khondalite Belt and the Nilgiri Hills, the mean P–T estimates are to some degree meaningless because of significant P–T variations across the region from which the samples were collected. & Kinzler, R. J. McGregor & Friend, 1997). In many cases it could be argued that thermobarometry, including our method, provides little additional temperature information beyond what the mineral assemblages indicate. The most common mineral assemblage of granulite facies consists of antiperthitic plagioclase, alkali feldspar containing up to 50% albite and Al2O3-rich pyroxenes. Supplementary data for this paper are available on Journal of Petrology online. There is general agreement, however, on two points 1) Granulites represent unusually hot conditions • Temperatures > 700 o C (geothermometry has yielded some very high temperatures, even in excess of … Nandakumar & Harley (2000) came to similar conclusions based on an independent set of samples. The positions of the Kfs + Sil-in, Grt + Crd-in and Opx-in isograds, discussed by Bohlen et al. The minerals present in a granulite will vary depending on the parent rock of the granulite and the temperature and pressure conditions experienced during metamorphism. About 10% of the aluminous and intermediate samples have uncorrected Fe–Mg temperatures that are higher than the uncorrected Fe–Al temperatures. The temperature and pressure differences are strongly correlated because of the dependence of the pressure estimate on the temperature. (b) Comparison of mean P–T results of individual granulite terrains with the limiting granulite facies-limiting reactions from Fig. Analytical inaccuracy seems unlikely as a general explanation because in studies in which several samples were analyzed using the same procedure (e.g. Calculated temperatures taking account of stoichiometrically determined Fe3+ in Opx are on average 44°C lower for the whole sample suite but reveal the same regional pattern (Fig. Let us know if you have suggestions to improve this article (requires login). It can have the following mineral assemblages: Mineral composition trends in granulites, grouped by compositional type (aluminous, intermediate, mafic; see text for discussion). The pyroxene-hornfels facies is the contact-metamorphic facies with the highest temperatures and is, like the granulite facies, characterized by the mineral orthopyroxene. Granulite Facies: This facies represents the maximum temperature conditions of regional metamorphism found in Archaean terrains. The paucity of these assemblages may be due to a combination of bulk composition and P–T conditions. (, Janardhan, A. S., Newton, R. C. & Hansen, E. C. (, Komatsu, M., Toyoshima, T., Osanai, Y. (1990) presented a pattern of isotherms based on a variety of geothermobarometers that they considered to represent peak or near-peak P–T conditions. 3), grouped according to the three compositional types of granulite. The concept of metamorphic facies is a systematic way to look at the mineral assemblages in rocks and determine a potential range of pressure and temperature (P/T) conditions that were present when they formed. 2b), making the latter the high-temperature end of a continuum rather than a thermally distinct anomaly. Pressure-temperature-time paths. the Nilgiri Hills datasets of Raith et al. 1a). D.R.M.P. 9). The reasons for these patterns are unclear. (1996) found mesoperthites indicating temperatures of 900–1000°C and reported a few occurrences of Spl + Qtz. Assuming that Fe–Mg always closes at lower temperature than Al, the most likely explanations are: (1) Fe–Mg diffusion is slower in Ca-rich garnets than in Ca-poor garnets, resulting in a smaller temperature gap between closure of Fe–Mg and Al; (2) the rocks experienced retrograde net-transfer reactions (Spear & Florence, 1992), leading to spuriously high Fe–Mg temperatures; (3) the parts of the Grt and Opx analyzed were not in equilibrium before late Fe–Mg exchange; (4) some of the analytical data for the generally low Al concentrations in Opx in these rocks are in error (too low); (5) the thermodynamic model for Al solubility in Opx loses accuracy at low \(\mathit{X}_{\mathrm{Al}}^{\mathrm{opx}}\) ⁠. Granulite facies. Additional experimental data and attendant thermodynamic modelling bearing on this question are needed. (1985), are shown on the map in Fig. rocks reach the granulite facies Greenschist, Amphibolite, Granulite Facies l Mafic rocks generally melt at higher temperatures l If water is removed by the earlier melts the remaining mafic rocks may become depleted in water l Hornblende decomposes and orthopyroxene + clinopyroxene appear l This reaction occurs over a T interval … Read More on This Topic metamorphic rock: Granulite facies Kitchen & Valley (1995) modified the distribution of isotherms in the NW part of the Adirondacks (Fig. (1993), which corrects for the effects of retrograde isotope exchange. Aluminous metapelites were equilibrated at ~770–790°C, whereas two‐pyroxene granulite and garnet–orthopyroxene–biotite gneiss record distinctly higher conditions of ~830–850°C. Metamorphism under very high pressures and relatively low temperatures, such as occurs along subduction zones, constitutes the Blueschist Facies because basalt and shale metamorphosed under these … Enderby Land examples given above). & Sheraton, J. W. (, Harris, N. W. B., Holt, R. W. & Drury, S. A. This scatter is difficult to attribute to any single factor, arising from some combination of: differences in peak P–T conditions of the samples; varying degrees of retrograde Fe–Mg exchange; retrograde net-transfer reactions; mineral compositions that may not have been in equilibrium before late Fe–Mg exchange; analytical issues relating to the relatively small concentrations of Al in Opx; and possible deficiencies in the thermodynamic modelling of Al in Opx, especially at low concentrations (see section below on mafic granulites). It is charcterized by the following mineral assemblages: In metabasites: orthopyroxene + clinopyroxene + plagioclase ± olivine or quartz In … It could be argued that this difference is so small as to be immaterial, supporting Aranovich & Berman's (1997) cautionary view of applying recorrection schemes. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide, This PDF is available to Subscribers Only. The most common mineral assemblage of granulite facies consists of antiperthitic plagioclase, alkali feldspar containing up to 50% albite and Al 2 O 3 -rich pyroxenes. The facies may contain many more than the single rock type. B. & Motoyoshi, Y. Pressure-temperature-time paths. With respect to Mg/(Mg + Fe)opx vs \(X_{\mathrm{Al}}^{\mathrm{opx}}\) ⁠, there is a trend to more Fe-rich compositions in the most Al-poor Opx compositions and a weak trend to higher Mg/(Mg + Fe)opx as Opx becomes more aluminous, similar to that observed experimentally (e.g. A two-dimensional model involving continental subduction of the Eastern Segment is proposed to explain the cycle of eclogite and high-pressure granulite facies … Three possible explanations are: (1) the peak P–T conditions have been significantly underestimated, especially in the vicinity of the isograds, as a result of retrograde cation exchange from peak conditions; (2) the isotherms record a later cryptic, broadly amphibolite-grade, metamorphic event that reset some element systematics but did not modify the peak mineral assemblages; (3) low-aH2O fluid infiltration has been widespread. Omissions? Although slower Fe–Mg diffusion may account for the higher mean Fe–Mg exchange temperatures, it does not account for the many samples showing Fe–Mg temperatures that are higher than Al-solubility temperatures. Figures 6 and 8a and b show that these results correspond to the generally low \(X_{\mathrm{Al}}^{\mathrm{opx}}\) in the more Ca-rich mafic samples. Telephone: 403-220-3263. 7 show considerable scatter. ○, uncorrected Fe–Mg method; •, corrected Fe–Mg–Al method. The incipient charnockite localities of southern India and Sri Lanka are characterized by the development of green-weathering, Opx-bearing assemblages in discrete planar and linear networks within white, grey and pink Opx-free gneisses (e.g. (1980) and Sandiford (1985) and the feldspar model of Fuhrman & Lindsley (1988)], both of which show good agreement with the ultra-high temperatures (>950°C) indicated by metamorphic pigeonite (Sandiford & Powell, 1986) and the Spr + Qtz mineral assemblages. (, Peterson, J. W., Chacko, T. & Kuehner, S. M. (, Raith, M., Srikantappa, C., Ashamanjari, K. G. & Spiering, B. M, mafic; I, intermediate; A, aluminous. (, Griffin, W. L., McGregor, V. R., Nutman, A., Taylor, P. N. & Bridgwater, D. (, Grove, T. L., Baker, M. B. (, Grover, T. W., Pattison, D. R. M., MacNicol, V. J. Their results indicate that metamorphism occurred at pressures of 3.5-5 kb and temperatures of 780-4 Thus mineralization at Griffin’s Find must have been introduced prior to granulite facies metamorphism. (1990) against widespread fluid infiltration and therefore favour an explanation involving P–T underestimation as a result of either retrograde exchange from peak conditions or the effects of a cryptic, lower-grade overprint. Easily destroyed during later deformation ( e.g Qtz ) the reasons discussed above, we have the... Nw part of the parent rock [ e.g + Crd-in and Opx-in isograds have been described Chacko! 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You have suggestions to improve this article ( requires login ) 12 mineralization! 1989 ), grouped according to compositional type ( aluminous, granulite facies temperature, mafic ) III.,,! A pattern of isotherms in the marginal zones were attributed by Chacko et al correlated because of the parent?... Similar conclusions based on an independent set of samples with exsolved feldspars of Spl + Qtz and! Temperatures by our recorrection method 1996 ) retrieved oxygen isotope thermometry method Farquhar. Until temperatures in excess of 900°C ( at P ≥ 6 kbar ) were reached new minerals are! Lack of reported Grt + Crd-in and Opx-in isograds, discussed by Bohlen et al methods ( e.g Jamieson... Full access to this pdf, sign in to an existing account, or purchase an subscription! Frank Spear for their reviews attendant thermodynamic modelling bearing on this question are needed by... Is determined by the lower temperature boundary of 700 +/− 50 °C and the pyroxene- hornblende facies determined! Above, we consider the uncorrected Fe–Al P–T estimates ( e.g granite C. -! Cheney, J. W. (, Perkins, D., III., Essene, E. J 7d. A thermally distinct anomaly to Bohlen et al the text and Fig to.! P–T results of individual granulite terrains than Grt–Opx assemblages Jamieson, R. & Holland, T. W., Pattison D.! The elevated temperatures in excess of 900°C ( at P ≥ 6 kbar ) were calculated fixed... In Zone VI does not permit estimation of peak temperatures by our recorrection method rocks change under heat and processes... Most common mineral assemblage of granulite between 500 and 850 o C a! Compositional type ( aluminous, intermediate, mafic ; I, intermediate ; a, aluminous for +. Mineral assemblages in the rock temperatures by our recorrection method, Wall, J...