No suture in the Sardinian Variscides - A structural, petrological and geochronological analysis

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Dokumentart: Book
Date: 2003
Source: Tübinger Geowissenschaftliche Arbeiten (TGA) : Reihe A ; 68
Language: English
Faculty: 7 Mathematisch-Naturwissenschaftliche Fakultät
Department: Sonstige - Geowissenschaften
DDC Classifikation: 550 - Earth sciences
Keywords: Sardinien <Nordost> , Backarc-Becken , Variskische Gebirgsbildung , Sutur <Geologie> , Abschiebung <Geologie>
Other Keywords: prä-Variskisch , tyrrhenisches Becken , metamorpher Kernkomplex , Winkeldiskordanz , Sardische Phase
Sardinia , Tyrrhenian backarc , Variscan suture , metamorphic core complex , pre-Variscan basement
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The NE Sardinian Posada fault was proposed by Cappelli et al. (1992) to mark a Variscan suture, which is refuted by this study of tectonostratigraphy, structure, texture, petrology, and geochronology. For the sequence that hosts the Posada fault, an Ordovician magmatic belt upon pre-Variscan basement is delineated by the separately occurring metavolcanic and metaplutonic rocks of basic to acidic composition. The extrusive level comprises the Orune schists, which derived from Ordovician volcanic to semipelitic protoliths and Silurian black shales. The intrusive level includes the pelitic Siniscola schists and Brunella gneisses, to which the occurrence of D1 and of pre-D2 metamorphic mineral assemblages is limited. In-situ U-Pb isotopic data obtained by laser-ablation inductively coupled plasma mass spectrometry on zircon yield for the Tanaunella orthogneiss 453 ± 8 Ma (MSWD 0,63), the Lodè orthogneiss 493 ± 10 Ma (MSWD 0,32) and for the Lula porphyroid 485 ± 10 Ma (MSWD 0,67). Besides these lower to middle Ordovician ages, Neoproterozoic ages are inherited from a Pan-African episode. Decompressional peak metamorphic conditions were achieved during the foreland directed transport of the main Variscan deformational event D2. An up to kilometre-scaled recumbent D3 folding coaxial with the D2 transport direction and the D4 Posada detachment-faulting led to the formation of a mantled gneiss dome. The dominant orogen-parallel trend is related to the Posada shearing. S4 crosscut previous foliations at a low angle and retrogressed the high-grade Brunella gneisses under medium-grade conditions and the medium-grade Siniscola schists under low-grade conditions during early faulting stages. The initial temperature gradient across the fault was overcome with progressing retrogression. The Posada fault rocks evolved from the adjacent wall rocks by retrograde shearing. In response to the tectonic denudation further exhumation was achieved by a large D5 flexure at the flank of the gneiss dome. D4 and D5 are expression of the same exhumation process. The flexure axis is coaxial with the displacement direction in the Posada fault. In the steep limb of this flexure, the Posada fault became a dextral wrench fault and the previously recumbent D3 folds became antiformal synclines and/or synformal anticlines. In the internal dome, peak metamorphic muscovite dehydration melting at about 760°C and 8 kbar led to the K-feldspar + sillimanite zone, on which the muscovite + sillimanite zone was retrogressively superimposed at the Posada fault, so that the metamorphic jump to the medium-grade Siniscola schists is less obvious. The latter expose a Barrovian metamorphic suite continuously developed from chloritoid disappearance to kyanite appearance. The garnet + albite zone results from the interference of Variscan and pre-Variscan metamorphic mineral assemblages. The andalusite + cordierite zone in the contact aureole of the Concas granite indicates that the studied sequence was already exhumed to shallow crustal level when the Variscan deformational activity ceased. Two contrasting tectonostratigraphic domains are juxtaposed in the Sardinian Variscides. The axial and nappe zones correlate with the Variscan domain presently exposed in the Alps, whereas the foreland shows an affinity to the Ibero-Aquitaine domain. The reorientation of the basement structures by Alpine faulting is estimated and set into the wider context of the Tyrrhenian basin opening.

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