Skip to main content
Log in

Petrology of garnet — cordierite — sillimanite gneisses from the El Tormes thermal dome, Iberian Hercynian foldbelt (W Spain)

Contributions to Mineralogy and Petrology Aims and scope Submit manuscript

Abstract

The core of the El Tormes thermal dome, situated in the central part of one of the main metamorphic belts of the Iberian Peninsula, is formed by garnet-cordierite-biotite-sillimanite pelitic gneisses. These rocks, that very often are cut by minor intrusions of Al-rich S-type granites, are metatexitic gneisses in which there exists garnet showing different stages of resorption and transformation into an aggregate of cordierite±plagioclase±biotite. The garnet, mantled and corroded mainly by cordierite, has never been found to occur in contact with the prismatic sillimanite of the matrix, thus indicating that the continuous reaction Gr+Sill+Q = Cd has taken place. The presence of corroded biotite inside the garnet-rimming cordierite of the aggregates as well as inside the cordierite of the matrix, which usually includes remmants of sillimanite, indicates that the continuous reaction Bi+Sill+Q = Cd+FK+H2O has occurred too. Therefore, a realistic net reaction for these aggretates should be represented by the univariant, at a given \(X_{{\text{H}}_{\text{2}} {\text{O}}} \), equilibrium: Biotite+Sillimanite+Garnet+Quartz = Cordierite+K-feldspar+H2O (1)

The important garnet resorption near the anatectic granitic veins implies that this process is favoured by a decrease in \(X_{{\text{H}}_{\text{2}} {\text{O}}} \), this factor being otherwise buffered by the reaction (1) assemblage.

The most probable P-T path, assuming these conditions, consistent with the AFM projection of the former (inferred) and present assemblages in the aggregates and in the matrix, implies a decrease in P coeval with a decrease in T (Fig. 4, path 2).

The most reliable P-T determination for the final stage of garnet breakdown through reaction (1), based on the coexistence of the seven phase assemblage garnet — cordierite — biotite — sillimanite — plagioclase — potash feldspar — quartz plus melt, gives 695° C, 4.3 kbar, \(X_{{\text{H}}_{\text{2}} {\text{O}}} \)= 0.5, The maximum pressure for this process, obtained from the garnet — plagioclase equilibrium, is 6.5±1 kbar at the same temperature.

The estimates of the T for the garnet core-garnet included biotite pairs are consistently lower, ca. 550° C, than those obtained for the garnet rim-biotite in aggregates, ca. 645° C, or garnet rim-adjacent cordierite pairs, ca. 695° C.

It may, therefore, be supposed that, during their evolution these rocks underwent first an increase in T and then, during the last stages, as garnet and biotite brokedown, a decrease in P and T. This represents an uplift of the core of El Tormes dome under high grade amphibolite to low pressure granulite facies conditions, accompanied by a process of partial melting with local decrase in \(X_{{\text{H}}_{\text{2}} {\text{O}}} \). It is suggested, from mineral growth-deformation relationships, that this process took place during the late hercynian deformation phases, P-3 or doming stage.

This is a preview of subscription content, log in via an institution to check access.

Access this article

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Instant access to the full article PDF.

Institutional subscriptions

Similar content being viewed by others

References

  • Blümel P, Schreyer W (1977) Phase relationships in pelitic and psammitic gneisses of the sillimanite-potash feldspar and cordierite-potash feldspar zones in the Moldanubicum of the Lam-Bodenmais area, Bavaria. J Petrol 18:431–459

    Google Scholar 

  • Currie KL (1971) The reaction 3 cordierite = 2 garnet+4 sillimanite+5 quartz as a geological thermometer in the Opinicon Lake region, Ontario. Contrib Mineral Petrol 33:215–226

    Google Scholar 

  • Ferry JM, Spear FS (1978) Experimental calibration of Fe and Mg between biotite and garnet. Contrib Mineral Petrol 66:113–117

    Google Scholar 

  • Ghent ED, Robbins DB, Stout MZ (1979) Geothermometry, geobarometry and fluid compositions of metamorphosed calc-silicates and pelites, Mica Creek, British Columbia. Am Mineral 64:874–885

    Google Scholar 

  • Ghent ED, Stout MZ (1981) Geobarometry and geothermometry of plagioclase-biotite-garnet-muscovite assemblages. Contrib Mineral Petrol 76:92–97

    Google Scholar 

  • Gil Ibarguchi JI (1982) The metamorphic evolution of the MuxiaFinisterre region (Galice, NW Spain) during the Hercynian Orogenesis. Geol Rundsch 71:657–686

    Google Scholar 

  • Grant JA, Weiblen PW (1971) Retrograde zoning in garnet near the second sillimanite isograd. Am J Sci 270:281–296

    Google Scholar 

  • Griffin WL, Jensen BB, Misra SN (1971) Anomalously elongated rutile in eclogite-facies pyroxene and garnet. Norsk Geol Tidssk 51:177–185

    Google Scholar 

  • Hensen BJ (1971) Theoretical phase relations involving cordierite and garnet in the system MgO-FeO-Al2O3-SiO2. Contrib Mineral Petrol 33:191–214

    Google Scholar 

  • Hensen BJ, Green DH (1973) Experimental study of the stability of cordierite and garnet in pelitic compositions at high pressures and temperatures, III. Synthesis of experimental data and geological applications. Contrib Mineral Petrol 38:151–166

    Google Scholar 

  • Holdaway MJ (1971) Stability of andalusite and the aluminium silicate phase diagram. Am J Sci 271:97–131

    Google Scholar 

  • Holdaway MJ, Lee SM (1977) Fe-Mg cordierite in high grade pelitic rocks based on experimental, theoretical and natural observations. Contrib Mineral Petrol 63:175–198

    Google Scholar 

  • Hollister LS (1969) Contact metamorphism in the Kwoiek area of British Columbia: an end member of the metamorphic process. Geol Soc Am Bull 80:2465–2494

    Google Scholar 

  • Hollister LS (1977) The reaction forming cordierite from garnet in the Khtada Lake metamorphic complex, British Columbia. Can Mineral 15:219–229

    Google Scholar 

  • Julivert M, Fontboté JM, Ribeiro A, Conde L (1974) Mapa tectónico de la Península Ibérica y Baleares. Inst Geol Miner Esp, Madrid, pp 1–113

    Google Scholar 

  • Julivert M, Martinez FJ, Ribeiro A (1980) The Iberian segment of the European Hercynian foldbelt. Mem BRGM 108:133–158

    Google Scholar 

  • Kerrick DM (1972) Experimental determination of muscovite+quartz stability with \(X_{{\text{H}}_{\text{2}} {\text{O}}} \) Ptotal. Am J Sci 272:946–958

    Google Scholar 

  • Lee SM, Holdaway MJ (1977) Significance of Fe-Mg cordierite stability relations on temperature, pressure and water pressure in cordierite granulites. In: Geophys Monogr 20, AGU, Washington, DC, pp 79–94

    Google Scholar 

  • Lonker SW (1981) The P-T-X relations of the cordierite — garnet — sillimanite — quartz equilibrium. Am J Sci 281:1056–1090

    Google Scholar 

  • Loomis TP (1975) Reaction of zoning of garnet. Contrib Mineral Petrol 52:285–305

    Google Scholar 

  • López Ruiz J, Aparicio A, García Cacho L (1975) El metamorfismo de la Sierra de Guadarrama, Sistema Central Español. Inst Geol Miner Esp, Madrid pp 1–127

    Google Scholar 

  • Martinez FJ (1974) Estudio del área metamórfica del NW de Salamanca (Cordillera Herciniana, España). Trab Geol, Univ Oviedo, 7:3–60

    Google Scholar 

  • Martinez FJ, Gil Ibarguchi JI (1982) El metamorfismo en el Macizo Ibérico. Libro Jub JM Rios (in press)

  • Miyashiro A (1961) Evolution of metamorphic belts. J Petrol 2:277–311

    Google Scholar 

  • Newton RC, Wood BJ (1979) Thermodynamics of water in cordierite and some petrologic consequences of cordierite as a hydrous phase. Contrib Mineral Petrol 68:391–405

    Google Scholar 

  • O'Hara MA, Yoder HS (1967) Formation and fractionation of basic magmas at high pressures. Scottish J Geol 3:67–117

    Google Scholar 

  • Oen IS (1970) Granite intrusion, folding and metamorphism in Central Northern Portugal. Bol Geol Miner Esp 81:271–298

    Google Scholar 

  • Olsen SN (1977) Origin of the Baltimore gneiss migmatites at Piney Creek, Maryland. Geol Soc Am Bull 88:1089–1101

    Google Scholar 

  • Portugal Ferreira MR (1972) Rochas metamórficas. Publ Univ Coimbra pp 1–203

  • Rucklidge S, Gasparrini EL (1969) Empadr VII, electron microprobe analytical reduction. Univ Toronto

  • Selverstone J, Hollister LS (1980) Cordierite bearing granulites from the Coast Ranges, British Columbia: P-T conditions of metamorphism. Can Mineral 18:119–129

    Google Scholar 

  • Smellie JAT (1974) Formation of atoll garnets from the aureole of the Ardara pluton, Co. Donegal, Ireland. Mineral Mag 39:878–888

    Google Scholar 

  • Thompson AB (1976) Mineral reactions in pelitic rocks: II, calculation of some P-T-X (Fe-Mg) phase relations. Am J Sci 276:425–454

    Google Scholar 

  • Touret J (1971) Le facies granulite en Norvège Méridionale. II. Les inclusions fluides. Lithos 4:423–436

    Google Scholar 

  • White AJR, Chappell BW (1977) Ultrametamorphism and granitid genesis. Tectonophysics 43:7–22

    Google Scholar 

  • Wood BJ, Fraser DG (1976) Elementary thermodynamics for geologists. Oxford Univ Press, London, pp 1–303

    Google Scholar 

Download references

Author information

Authors and Affiliations

Authors

Rights and permissions

Reprints and permissions

About this article

Cite this article

Gil Ibarguchi, J.I., Martinez, F.J. Petrology of garnet — cordierite — sillimanite gneisses from the El Tormes thermal dome, Iberian Hercynian foldbelt (W Spain). Contr. Mineral. and Petrol. 80, 14–24 (1982). https://doi.org/10.1007/BF00376731

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI: https://doi.org/10.1007/BF00376731

Keywords

Navigation