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Exhumation of anatectic complexes through shear zones: petrological and geochemical constraints on the evolution of the Figueira de Castelo Rodrigo – Lumbrales Anatectic Complex
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Resumo(s)
The Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex (FCR-LAC) is an example of a granite-migmatite complex formed during the Variscan Orogeny located within the autochthonous terrane of the Central Iberian Zone. The FCR-LAC contacts with an Ediacaran–Cambrian metasedimentary sequence (Douro–Beiras Supergroup – DBSG) through the Huebra and Juzbado–Penalva do Castelo shear zones.
New U–Pb zircon and apatite ages constrain emplacement and migmatization ages, inherited ages and unprecedented cooling rates for the FCR–LAC lithologies. The granites emplacement ages cluster around 313–317 Ma while the migmatization age constrains the prograde metamorphic path (344 – 315 Ma) and the metamorphic peak at 316.8 ± 2.0 Ma. The migmatization/peak metamorphic ages suggest that the anatexis started during the D2 and culminated during the D3 Variscan phase, concomitantly with the emplacement of the S-type granites. The cooling (7 to 54 ºC/Ma) and exhumation rates (0.2 and 1.3 mm a-1) of the FCR–LAC imply fast cooling/exhumation conditions which are compatible with a tectonically-assisted mechanism where the shear zones have an important role.
The elemental composition of the FCR-LAC reveals the existence of two distinct diatexite groups: diatexites type-1 produced via dehydration-melting of muscovite; and the diatexites type-2 formed by a fluid-present melting reaction. These two diatexite groups evolved in different stages of the prograde metamorphic path: 343.7 ± 2.5 Ma for diatexite type-2, 328.8 ± 3.4 Ma for a diatexite type-1, culminating in the metamorphic peak at 318.8 ± 2.6 Ma with a diatexite type-1. The different melting reactions justify the isotopic variability observed in the FCR-LAC.
The U-Pb detrital zircon ages of the FCR-LAC and the DBSG reveal an important input of the Cadomian sediments. The zircon distribution ages of the Pinhão Formation (Douro Group) compared to the Beiras Group reinforce the existence of a physical barrier at south of the FCR-LAC. Younger inherited zircon from the anatectic rocks can be linked to the Ollo de Sapo magmatic rocks indicating that they melted alongside the DBSG showing the existence of source heterogeneity. This fact is also supported by the Sr-Nd isotopic compositions of the FCR-LAC and explains the existence of two groups with distinct initial isotopic compositions, both composed by metatexites, diatexites and granites.
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Palavras-chave
Orogenia Varisca anatexia crustal granito-génese migmatização complexo granito-migmatítico Variscan Orogeny crustal anatexis granitogenesis migmatization granite-migmatite complex