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Discussion |
1 Geological Institute, Copenhagen University, Østervoldgade 10, 1350K Copenhagen, Denmark
2 Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
3 Department of Geological Sciences, University College London, Gower Street, London WCIE 6BT, UK
Scientific editing by Hugh Rollinson.
Journal, Vol. 156, 1999, pp. 149–162
H. P. Zeck writes: The recent paper Argles, Platt & Waters (1999) is in many respects a remarkable one. The paper reports the thesis work of T. W. Argles, with the main conclusion being a very fast uplift in an extensional tectonic setting during the final stage of the Alpine orogeny in the western Betic Cordilleras. Planning and execution of the work has had the benefit of knowing where the effort would land. Several years earlier 
Zeck et al. (1992; following earlier suggestions by Zeck et al. 1989) had outlined the then quite unexpected feature of extremely fast uplift/cooling in concert with extensional tectonics during the final tectogenesis of the Betie Cordilleras, putting on the record cooling rates of not less than 300°C Ma–1. Such fast uplift/cooling had not been recorded anywhere before.
Moreover, Zeck (1996) followed up the 1992 paper by spelling out the evidence and the consequences of these extreme uplift/cooling rates which were connected to slab detachment and concomitant inflow of high-temperature, low-density asthenosphere into the widening gap above the sinking lithospheric slab. Cooling rates could now be constrained to not less than 500°C Ma–1 (Fig. 1). The area of the Ronda peridotites, including the Carratraca bodies studied by Argles et al., was among the specific examples fielded to support this thesis. Zeck (1996) was published more than a year and a half before Argles et al. submitted their manuscript to Journal of the Geological Society (21.10.97).
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Zeck et al. (1992) and Zeck (1996) there is not a single word of reference in Argles et al. Curiously enough, a single reference was made to Zeck et al. (1992), but not for its main drive duplicated by Argles et al.
At places the reproduction effort reaches striking results. One example may suffice: the dating of the episode of very fast uplift/cooling in the Betic Cordilleras was discussed in detail in Zeck (1996, pp. 6–8). For the termination of the episode in the western part of the Betics an age of c. 18.5 Ma was suggested (
Fig. 1) and for its start an age of c. 24 Ma was preferred, with a possible extension to c. 26 Ma being discussed in the text. Argles et al.’s fig. 7 summarizes the geological model reached by the authors, giving ages of ‘c. 18 Ma?’ and ‘c. 27 Ma?’ for start and termination respectively. These figures are not supported by their own data and the few isotopic age refences given are not relevant as not isolated ages, but a comparison within a geologically coherent area of a series of geochronometres with different closure temperatures is required (Fig. 1; Zeck et al. 1992; Zeck 1996).
As to the part of Argles et al. that concerns original thermobarometric work, the paper does not fare much better than in the art of paying reference. Argles et al. regard the metamorphic petrological evolution of the Alpujárride crustal section as an exclusive result of Alpine tectono-metamorphism. This hypothesis serves the purpose of the authors who set out to compare the present section in the Carratraca area with the hypothetical stratigraphic starting sequence and on that basis draw conclusions on the tectono-metamorphic processes which have produced the differences.
However, this plan conveniently ignores available evidence of an older metamorphic episode in the Alpujárride nappe complex. The evidence is primarily based on the break in metamorphic grade between the Permo-Triassic section and the older basement in Betic nappe stratigraphy (e.g., Nijhuis 1964; Zeck 1968; Egeler & Simon 1969). Isotopic evidence for Hercynian ages for granitic intrusions into rock complexes corresponding to the Palaeozoic basement of Argles et al.’s lithostratigraphy (Boulin et al. 1969; Bernard-Griffith et al. 1977; see Priem et al. 1966, for an analogue from the Filábrides) is in agreement with this and suggests an Hercynian age for the pre-Alpine metamorphism.
The consequences of such a pre-Alpine metamorphic imprint for the P–T speculations of Argles et al. range from catastrophic to serious. In the worst case the older mineral grains are Hercynian, rendering the P–T interpretations of Argles et al. pointless, In the best case there has been considerable Alpine resetting of the Hercynian parageneses, implying that a careful choice of mineral grains and parts thereof might give workable results. However, one is looking here into a can of worms. Precise petrography, supported by electron microprobe mapping, and elaborate documentation is required to distil relevant and convincing information from these rocks, and chances of unequivocal results are doubtful at best. As Argles et al. do not provide petrographic descriptions of any precision, this work cannot be used to reconstruct the consequences of a pre-Alpine metamorphic episode.
In conclusion: Argles et al. presented as its original results conclusions which had been reached c. 7 years (Zeck et al. 1992) and c. 3 years (Zeck 1996) earlier. No reference is given in Argles et al., although it is beyond doubt that the authors have been aware both of the existence of these papers and their relevance for their investigations. Ironically, the thermobarometric investigations which Argles et al. pursued to duplicate these earlier results are based on highly doubtful simplifications and therefore could hardly have given the desired results had these not been known beforehand.
29 March 1999
T. W. Argles & J. P. Platt reply: We regret that Zeck feels that we have given insufficient credit to his published work, and we agree that his paper documenting high cooling rates in the Alpujarrides (Zeck et al. 1992) was a significant contribution to our understanding of the metamorphic evolution of the Betic Cordillera. As indicated by its title, however, our paper was mainly concerned with the amount and the mechanism of exhumation, not with its timing. The methodology we employed to study the exhumation history is radically different, and complementary, to that of Zeck et al. (1992), which documents cooling via dating of the closure of isotopic systems in a series of minerals. We, in contrast, use pressure estimates to constrain the exhumation history, which was isothermal for a significant part of the process. The core of our paper consists of original structural and petrological data on the metamorphic rocks surrounding the Carratraca peridotite body. This provided the framework for detailed petrographic study of nearly 500 thin sections, and selection of the most appropriate assemblages for thermobarometry, a painstaking process that clearly cannot be documented fully in a publication of this nature: the thesis Zeck refers to above (Argles 1996) contains abundant petrographical detail. Our principal conclusions in the paper were that the zoned low P/T ratio sequence above the peridotite was produced by attenuation of an originally high P/T ratio sequence by a factor of five or more; that this attenuation was a result of early coaxial flattening followed by top-to-NE shear, producing the dominant foliation and lineation in the rocks; and that the emplacement of the hot peridotite slab was a subordinate factor in the thermal evolution of the rocks.
The question of pre-Alpine metamorphism in Betic rocks deserves consideration in more detail. Firstly, it should be stressed that few Palaeozoic sections in the Betic Cordillera preserve evidence for pre-Alpine metamorphism. A notable exception is andalusite–biotite assemblages in the Mulhacen nappe of the Nevado–Filabride complex, though even these are largely replaced by Alpine kyanite, staurolite and garnet (Puga & Diaz de Federico 1976). There is no evidence for a ‘break in grade between the Permo-Triassic and Palaeozoic’ in the Nevado–Filabride section. Both show upper greenschist to epidote amphibolite facies metamorphism, with a slight upward increase in metamorphic temperature. Relicts of early eclogite facies metamorphism are essentially confined to the Mesozoic metabasic rocks, where they are clearly overprinted by the epidote amphibolite facies metamorphism (e.g. Gomez-Pugnaire & Fernandez-Soler 1987). The Palaeozoic of the Malaguide complex is virtually unmetamorphosed, with only slaty cleavage developed.
There is no actual documentation of pre-Alpine metamorphism in the Alpujarride complex, though its existence has been suggested by several workers (e.g. Aldaya 1969; Egeler & Simon 1969; Kornprobst 1976, among others). As with the Nevado–Filabride, there is in most cases no evidence of a break in grade between the Permo-Trias and Palaeozoic, although their compositional differences (e.g. graphite content, calcareous component) result in markedly different assemblages, even in superficially similar rocks. Exceptions have been interpreted as a result of late Alpine extension (e.g. Sierra Alhamilla; Platt et al. 1983). Where the Permo-Triassic is low-grade (i.e. chlorite, chloritoid, carpholite assemblages), so in general is the Palaeozoic (chloritoid, biotite assemblages); for example the upper levels of the Jubrique Unit, in the Sierra de Gador (Orozco et al. 1998), and in the northern Sierra de Las Estancias (Akkerman et al. 1980). Conversely, in several sections analogous to the Carratraca Alpujarride from the western Betics, both Palaeozoic and Permo-Trias rocks have been metamorphosed to sillimanite grade; clearly this cannot represent a Hercynian event. These sections, in common with the Carratraca Alpujarride, show a gradual temperature decrease up-section punctuated only by metamorphic breaks across late extensional fault zones (Argles et al. 1999a) and late imbrication (Balanyá et al. 1997).
Late Alpine reworking of the Carratraca section during near-isothermal decompression recrystallized all but the most robust phases. Temperatures of 500–800°C allowed most porphyroblast rims to maintain equilibrium with matrix phases, and P-T estimates (allowing for retrograde diffusion) are both robust, and consistent through the section. Homogenized garnet profiles in much of the section further demonstrate the degree of Alpine chemical re-setting. Numerous authors have shown this low P/T episode represents the latter stages of the Alpine event (e.g. Loomis 1975; Priem et al. 1979; Zeck et al. 1989; Monié et al. 1991). Moreover, the dominantly isothermal decompression paths of Alpujarride rocks suggest that high T during the late low P/T stage correlates with deep burial during the early high P/T stage. This correlation would be unlikely if the early stage were Hercynian, and the rocks were already exhumed by the Permian.
Radiometric dating of metamorphic episodes is difficult, as few dateable phases can be related directly to metamorphism (e.g. Christensen et al. 1989; Mezger et al. 1989). Indeed, the only Hercynian radiometric data from the Betics come from magmatic zircons in granites (ef. Zeck & Whitehouse 1999), although Montel et al. (1995) reported Hercynian ages preserved in monazites armoured by garnet from the Beni Bousera kinzigites. Matrix monazites in these samples are Alpine (<30 Ma), however, and recent work around Carratraca supports the dominance of the Alpine metamorphism (Platt & Whitehouse 1999). Although zircon cores from related Alpujarride parageneses yield pre-Alpine ages with various origins (Sánchez-Rodríguez et al. 1996), rim ages are mostly Alpine. Similarly, Argles et al. (1999b) dated pre-Alpine garnet core growth with the Sm–Nd method at c. 235 Ma (i.e. post-Hercynian). These distinctive garnets, in a staurolitegrade Alpujarride schist, were omitted from Carratraca thermobarometry because their cores were out of equilibrium with the matrix phases both texturally and chemically; as in the rest of the section, the matrix was entirely recrystallized during the latter part of the Alpine orogeny.
1 July 1999
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References |
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 Top References |
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Akkerman, J.H., Maier, G. & Simon, O.J. 1980. On the geology of the Alpujarride complex in the western Sierra de las Estancias (Betic Cordilleras, SE Spain). Geologie en Mijnbouw 59, 363-374.[GeoRef]
Aldaya, F. 1969. Los Mantos Alpuárrides al Sur de Sierra Nevada. Doctoral Thesis. University of Granada.
Argles, T.W. 1996. Tectonometamorphic studies in the crustal envelope of mantle peridotites in the western Betic Cordillera, Southern Spain. Doctoral Thesis, Oxford.
Argles, T.W., Platt, J.P. & Waters, D.J. 1999a. Attenuation and Excision of a Crustal Section During Extensional Exhumation: The Carratraca Massif, Betic Cordillera, Southern Spain. Journal of the Geological Society, London 156, 149-162.
Argles, T.W., Prince, C.I, Foster, G.L. & Vance, D. 1999b. New Garnets for Old: Cautionary Tales from Young Mountain Belts. Earth and Planetary Science Letters 172, 301-309.[CrossRef][Web of Science][GeoRef]
Balanyá, J.C., García-Dueñas, V, Azañon, J.M. & Sánchez-Gómez, M. 1997. Alternating contractional and extensional events in the Alpujarride nappes of the Alboran Domain (Betics, Gibraltar Arc). Tectonics 16, 226-238.[CrossRef][Web of Science][GeoRef]
Bernard-Griffith, J., Cantagrel, J.M. & Kornprobst, J. 1977. Age des gneiss du Hacho de Ceuta: un événement thermique Hercynien dans le zone interne du Rif (abstract). RAST Rennes64.
Boulin, J., Ledent, D. & Pasteels, P. 1969. Repères géochronologiques dans les zones internes des Cordillères Bétiques au sud-ouest de la Sierra Nevada (Espagne). Annales de la Société Géologigue de la Belgique 92, 377-381.
Christensen, J.N., Rosenfeld, J. & DePaolo, D.J. 1989. Rates of tectonometamorphic processes from rubidium and strontium isotopes in garnet. Science 244, 1465-1469.[Web of Science][GeoRef]
Egeler, C.G. & Simon, O.J. 1969. Sur la tectonique de la zone Bétique (Cordillères Bétiques, Espagne). Verhandelingen der Koninklijke Nederlandse Akademie van Wetenschappen, Natuurkunde 25, (Series 1).
Gomez-Pugnaire, M.T. & Fernandez-Soler, J.M. 1987. High-pressure metamorphism in metabasites from the Betic Cordilleras and its evolution during the Alpine Orogeny. Contributions to Mineralogy and Petrology 95, 231-244.[CrossRef][Web of Science][GeoRef]
Kornprobst, J. 1976. Signification structurale des péridotites dans l’orogène bético-rifain: arguments tirés de l’etude des detritus observés dans les sédements paléozoiques. Bulletin de la Societé Géologique de la France 18, 607-618.[GeoRef]
Loomis, T.P. 1975. Tertiary mantle diapirism, orogeny, and plate tectonics east of the Strait of Gibraltar. American Journal of Science 275, 1-31.
Mezger, K., Hanson, G.N. & Bohlen, S.R. 1989. U-Pb systematics of garnet: Dating the growth of garnet in the Late Archaean Pikwitonei granulite domain at Cauchon and Natawahuna Lakes, Manitoba, Canada. Contributions to Mineralogy and Petrology 101, 136-148.[CrossRef][Web of Science][GeoRef]
Monié, P., Galindo-Zaldivar, J, Gonzalez-Lodeiro, F, Goffé, B. & Jabaloy, A. 1991. 40Ar/39Ar geochronology of Alpine tectonism in the Betic Cordilleras (southern Spain). Journal of the Geological Society, London 148, 289-297.
Montel, J.M., Kornprobst, J, Vielzeuf, D. & Veschambre, M. 1995. Shielding effect of garnet for the U-Th-Pb system in monazite: an e-probe study at Beni Bousera (Morocco). Terra Abstracts (EUG) 8, 348.
Nijhuis, M. 1964. Plurifacial Alpine metamorphism in the SE Sierra de los Filabres S of Lubrín, SE Spain. Proefschrift, Amsterdam University.
Orozco, M., Alonso-Chaves, F.M. & Nieto, F. 1998. Development of large north-facing folds and their relation to crustal extension in the Alboran Domain. Tectonophysics 298, 271-295.[CrossRef][Web of Science][GeoRef]
Platt, J.P., van den Eeckhout, B, Janzen, E, Konert, G, Simon, O.J. & Weijermars, R. 1983. The structure and tectonic evolution of the Aguilon fold-nappe, Sierra Alhamilla Betic Cordilleras, SE Spain. Journal of Structural Geology 5, 519-538.[CrossRef][Web of Science][GeoRef]
Platt, J.P. & Whitehouse, M. 1999. Early Miocene high-temperature metamorphism and rapid exhumation in the Betic Cordillera (Spain): evidence from U-Pb Zircon Ages. Earth and Planetary Science Letters 172, 591-605.
Priem, H.N.A., Boelrijk, N.A.I.M, Hebeda, E.H. & Verschure, R.H. 1966. Isotopic age determination on tourmaline granite-gneisses and a metagranite in the eastern Betic Cordilleras (southeastern Sierra de Los Filabres), S.E. Spain. Geologie en Mijnbouw 45, 184-187.
Priem, H.N.A., Hebeda, E.H, Boelrijk, N.A.I.M, Verdumen, T.E.A. & Oen, I.S. 1979. Isotopic dating of the emplacement of the ultramafic masses in the Serrania de Ronda, Southern Spain. Contributions to Mineralogy and Petrology 70, 103-109.[CrossRef][Web of Science][GeoRef]
Puga, E. & Diaz de Federico, A. 1976. Pre-Alpine metamorphism in the Sierra Nevada Complex, Betic Cordillera, Spain. Cuadernos de Geologia 7, 161-171.
Sánchez-Rodríguez, L., Gebauer, D, Tubía, J.M, Gil-Ibarguchi, J.I. & Rubatto, D. 1996. First shrimp-ages on pyroxenites, eclogites and granites of the Ronda Complex and its country rocks. Geogaceta 20, 487-488.
Zeck, H.P. 1968. Anatectic origin and further petrogenesis of almandine-bearing biotite-cordierite-labradorite-dacite with many inclusions of restite and basaltoid material, Cerro del Hoyazo, SE Spain (with data on the adjoining part of the Sierra Alhamilla). Proefschrift, Amsterdam University.
Zeck, H.P. 1996. Betic-Rif orogeny: subduction of Mesozoic Tethys under E-ward drifting Iberia, slab detachment shortly before 22 Ma, and subsequent uplift and extensional tectonics. Tectonophysics 254, 1-16.[CrossRef][Web of Science][GeoRef]
Zeck, H.P. & Whitehouse, M.J. 1999. Hercynian, Pan-African, Proterozoic and Archaean ion-microprobe zircon ages for a Betic-Rif core complex, Alpine belt, Mediterranean – consequences for its P-T-t path. Contributions to Mineralogy and Petrology 134, 134-149.[CrossRef][Web of Science][GeoRef]
Zeck, H.P., Albat, F, Hansen, B.T, Torres-Roldan, R.L, García-Gisco, A. & Martín-Algarra, A. 1989. A 21 ± 2 Ma age for the termination of the ductile Alpine deformation in the internal zone of the Betic Cordilleras S. Spain. Tectonophysics 169, 215-220.[CrossRef][Web of Science][GeoRef]
Zeck, H.P., Monié, P, Villa, I.M. & Hansen, B.T. 1992. Very high rates of cooling and uplift in the Alpine belt of the Betic Cordilleras, southern Spain. Geology 20, 79-82.
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