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Granulitos máficos de Alta pressão na Zona de Cisalhamento de Coimbra-Cordova (Unidade de Campo Maior, Nordeste Alentejano): Caracterização metamórfica e geotermobarométrica

High pressure mafic granulites in the Coimbra-Cordoba Shear Zone (Campo Maior Unit,

Northeast Alentejo): Metamorphic and geothermobarometric characterization

Pereira, M.F. (1), Apraiz, A. (2) (1) Departamento de Geociências, Centro de Geofísica de Évora, Universidade de Évora, Apt. 94, 7001-554 Évora, Portugal. (2) Geodinamika Saila, Zientzia eta Teknologia Fak., Euskal Herriko Unibertsitatea. Apt. 644, 48080 Bilbo, Spain. E-mail (s): mpereira@uevora.pt, arturo.apraiz@ehu.es

SUMÁRIO

No Nordeste Alentejano (zona de cisalhamento de Coimbra-Córdoba, Zona de Ossa-Morena, Portugal) foram amostradas rochas granulíticas máficas de alta pressão para estudos de caracterização petrográfica e de geotermobarometria. Os resultados obtidos permitem mostrar que a descompressão teve início sob condições metamórficas na facies granulítica. A paragénese mineral que indica o pico metamórfico (14 - 18 kbar e 830 – 890 °C) foi afectada por posterior simplectitização (10 – 13 kbar e 740 – 850 °C). Subsequentemente, a temperatura e pressão decresceram sob condições metamórficas da facies anfibolítica.

Palavras-chave: rochas granulíticas, alta pressão, zona de cisalhamento de Coimbra-Córdova, Unidade de Campo Maior

SUMMARY In the Northeast Alentejo (Coimbra-Cordoba shear zone, Ossa-Morena zone, Portugal) high-pressure mafic granulitic rocks were sampled for petrographic and geothermobarometric studies. The obtained results shown that descompression began under still granulitic metamorphic conditions. Peak mineral assemblage (14 - 18 kbar and 830 – 890 °C) was postdated by symplectitization (10 – 13 kbar and 740 – 850 °C). Subsequently, temperature and pressure decreased under amphibolitic facies metamorphism Key-words: granulitic rocks, high pressure, Coimbra-Cordoba shear zone, Campo Maior Unit

Introduction

A common feature of intra-continental transcurrent shear zones is that, within high-grade tectonic units, high-pressure rocks occur associated with migmatites and gneisses. These host rocks generally present lower pressure mineral assemblages. This can be explained by the fact that following the peak metamorphic equilibration at a deep crustal level, the high-pressure rocks were subsequently tectonically justaposed with the lower pressure

migmatitic host rock, at a shallower crustal level, due to exhumation processes [1]. Their close spatial association to eclogites (high-pressure granulites might have taken place due to retrogressive thermobaric evolution after eclogite facies metamorphism) has been used to establish a link with convergent plate tectonics settings [2-4]. The Campo Maior unit defined within the Coimbra-Córdoba shear zone [5] offer a good example for studying decompression processes associated with mafic high-pressure granulites.

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Geological setting The Campo Maior unit [5], located in the Northeast Alentejo belongs to the Coimbra-Cordoba shear zone [6], a major Variscan transcurrent intra-continental crustal structure located at the northern domains of Ossa-Morena Zone (SW Iberian Massif). This tectonic unit extends in Portugal for approximately 60 km from Campo Maior (6 km of width) to Crato (500m of width), defining the tail of a major kilometre-scale sigmoidal shape structure represented in Spain by the Blastomylonitic migmatitic gneisses unit [7]. It consists of strongly sheared gneisses and migmatites with N300-310º-trending and steeply dipping foliation. The biotite-rich gneisses with sillimanite are dominant and associated with amphibolitic and/or pyroxenitic gneisses and migmatites with different textures. Shearing is linked to a sinistral transcurrent movement along a slightly plugging stretching lineation. Despite of the strong deformation developed in the host rocks, boudins of more competent rocks were protected from the widespread mylonitization. An example of such relics, that preserved earlier temperature and pressure conditions, were the previously studied retrogressed eclogites (with unknown precise location) [8] and the high-pressure mafic granulites presented in this study, which were mapped at the rio Caia cross-section (nearby Monte Sarnadas) [5]. Field relations In the field, the high-pressure mafic granulites outcrop as metric-scale boudins, as well as, coarse-grained garnet bearing felsic granulites, within heterogeneous migmatites and strongly sheared biotite-rich blastomylonitic gneisses. A well-developed N310-320º-trending and steeply dipping foliation surrounds these sigmoidal shape rock fragments without internal structures. Shear criteria found in association with the slightly plugging (2-14º) to northwest stretching lineation (S-C structures, C’-extensional shear planes and asymmetric tails in feldspar porphyroblasts) indicate sinistral transcurrent movement. Isoclinal folds with axis parallel to the stretching lineation are also present. Petrography The mineral assemblage of the high-pressure mafic granulites is represented garnet + clinopyroxene + plagioclase + quartz + amphibole in a porphydonematoblastic texture (Fig.1). Acessories minerals are rutile, ilmenite and sphene.

Figure 1- Photomicrograph of the texture of the studied mafic granulites. Note: Garnet (Grt) grains are surrounded by plagioclase (Pl) coronas and clinopyroxene is overprinted by amphibole (Amph).

Garnets (Alm51-60Grs22-29Pyr8-22) with plagioclase coronas (Fig. 2) show a well-developed zonation with a core-to-rim decrease in Fe, Mn and XFe and increase in Mg and Ca. This pattern represents a prograde zoning, probably related with the growth of garnet by the follow reaction:

Cpx + Pl1 - Grt + Pl2 + Qtz

Figure 2- Detail view of a plagioclase rim around garnet and exsolution lamellae of Na-rich Plagioclase in an amphibole-bearing mafic granulite. Amph- amphibole, Cpx, clinopyroxene, Grt- garnet, Pl-plagioclase.

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In the analyzed garnets, the Ca content drop at the extreme rim. The decompression-driven loss of Ca in the rims is related with the formation of coronitic plagioclase. Primary clinopyroxenes form prismatic crystals with diopside composition (XMg=0.69-0.72). The secondary ones present augite composition (XMg=0.61) and constitute with plagioclase simplectitic intergrowths. This texture represents a post-peak metamorphic evolution characterised by the follow decompression-driven mineral reaction: Cpx1 + Qtz ----> Cpx2+ Pl Primary clinopyroxenes were progressively replaced from core to rim by amphibole. This amphibolitization also affects the secondary clinopyroxenes. The amphibole (magnesian hastingsite - iron pargasite) and the coronitic plagioclase were probably produced by destabilization of garnet and clynopiroxene as result of the reaction: Grt + Cpx -- Pl + Anf + Qtz The anorthite component proportion of the coronitic plagioclases (An24-43) is higher than the plagioclase that constitutes the matrix (An15-25). Geothermobarometry The studied mineral assemblage garnet + clinopyroxene + plagioclase + quartz is characteristic of the high-pressure granulitic facies [9-12] and signs the transition with the eclogitic facies of metamorphism [13]. Maximum temperatures and pressures, probably near the peak metamorphic conditions, were calculated using the garnets composition of the rim-growth zoning pattern (lowest XFe relation and highest Ca content) combined with core analyses of primary clinopyroxenes and the lowest An matrix plagioclase core compositions. On contrary, the garnet and clynopyroxene desestabilization to form plagioclase and amphibole indicates the transition between the granulitic and the amphibolitic facies of metamorphism. P-T data related with this evolution were obtained using the composition of garnet extreme rims, coronitic plagioclase and amphibole. The suggested textural evolution was corroborated by convectional geothermobarometry, using different calibrations of geothermobarometers garnet - clinopyroxene, garnet - hornblende and amphibole - plagioclase and geothermobarometers garnet – clinopyroxene - plagioclase and garnet – amphibole - plagioclase. Calculations yield P-T conditions of 14 - 18 kbar and 830 – 890 °C for the peak mineral assemblage. The post-peak metamorphic conditions were estimate at 10 – 13 kbar and 740 – 850 °C, indicating amphibolitic facies of metamorphism under considerable pressures.

Discussion This preliminary petrographic and geo-thermobarometric study of the high-pressure granulites from the Campo Maior unit suggests that these rocks may have resulted from eclogites. The preserved growth zonation of garnets under the estimated temperatures might indicate a short tectonic event and a rapid exhumation process [10-11]. The preserved textures and mineral assemblages show that retrogression began under still high-pressure granulite conditions (14 - 18 kbar and 830 – 890 °C) and is postdated by symplectitization (10 – 13 kbar and 740 – 850 °C). Subsequently, temperature and pressure decreased under amphibolitic facies metamorphism as suggested by the mineral assemblage sillimanite + garnet + biotite + perthitic feldspar + quartz of the blastomylonitic gneisses host rocks. These high-pressure granulites are also associated with strongly sheared migmatitic and gneissic host rocks under lower pressures and anfibolitic facies metamorphism (sillimanite stability field). Relatively rapid exhumation could explain the transition from granulitic to amphibolitic facies metamorphism. As pointed in previous studies [14-15] there are relevant metamorphic gaps between the different tectonic units defined in this segment of the Coimbra-Córdoba shear zone. High-temperature medium-pressure tectonites from the central units (amphibolitic facies metamorphism- garnet bearing quartz-feldspar gneisses from the Ouguela unit: 600-700 ºC and 7-9 kbar; garnet bearing amphibolites from the Contenda-Barragem do Caia unit: 650-700ºC and 8.5-9.5 kbar) contact with low-temperature low-pressure tectonites (greenschist facies metamorphism- 300-350ºC and 2-3 kbar) from the borders. Combined with the shearing related structures it is evident that transcurrent movements were responsible for the recognizable complexity of this major intra-continental crustal weakness zone. We therefore consider that the exhumation of these high-pressure granulites was due to orogen-parallel fast displacements of hundred kilometres, which moved these deep crustal rocks far way from the original convergent plate tectonics setting. Acknowledgements This work is a contribution for the IGCP Project 497. References [1] Cooke, R.A., O´Brien, P.J. 2001. Resolving the relationship between high P-T rocks and gneisses in collisional terranes: an example from the Ghöhl gneiss-granulite association in the Moldanubian Zone, Austria. Lithos, 58: 33-54. [2] Carswell, D.A. 1990. Eclogites and the eclogite facies: definitins and classifications. In: Eclogite Facies Rocks (ed. Carswell, D.A.). Blackie, Glasgow, 1-13.

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