Tectonics and plate tectonics model for the Variscan belt of Europe

https://eprints.ucm.es/17386/1/1000358-_04.pdf

Diachronous Variscan tectonothermal activity in the NW Iberian Massif: Evidence from 40Ar/39Ar dating of regional fabrics

Trace element fractionation during dehydration of eclogites from high-pressure terranes and the implications for element fluxes in subduction zones

Geochemical and Nd-Sr isotope tracer studies of ultrahigh-pressure (UHP) eclogites and associated ultramafic rocks from the Su—Lu and Dabie terranes of central China provide important constraints on tectonic models of continental subduction, collision, and subsequent exhumation in the Dabie orogen. UHP eclogites have three types of occurrence: I) enclaves in ortho- and paragneisses gneisses of granitic composition, II) enclaves in or interlayers with marbles or metaclastic sediments (e.g., garnet-mica schists), and III) enclaves and interlayers with ultramafic rocks. The eclogites are mainly of basaltic composition, but show a wide range of major and trace element abundances, suggesting multiple origins and heterogeneous sources, and in some cases, metamorphic segregation.

Geochemical and Isotopic Characteristics of UHP Eclogites and Ultramafic Rocks of the Dabie Orogen: Implications for Continental Subduction and Collisional Tectonics

A structural and metamorphic study was carried out in the basal units of the Ordenes Complex in Spain, thought to represent a subducted part of the Paleozoic margin of Gondwana. According to their metamorphic evolution, this part of the margin was subducted at the onset of the Variscan Orogeny, becoming part of an accretionary complex developed below a colliding element built previously. Variations in the PT conditions of the first high-pressure metamorphic event along the units indicate a polarity of the subduction to the west. Subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units, whereas the convergence continued. Recumbent folds and thrusts developed along with successive normal detachments. Compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading. The unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units.

Variscan exhumation of a subducted Paleozoic continental margin: The basal units of the Ordenes Complex, Galicia, NW Spain

REE abundances and Nd isotopic compositions were determined on representative samples of eclogite from the Cabo Ortegal Complex of northern Spain. Zircons were also separated from a whole-rock eclogite and analysed by the UPb radiometric method. Results indicate that eclogite facies metamorphism occurred between 480 and 420 Ma ago, but no precise constraint can be placed on the protolith age. The REE patterns observed suggest that there has been no significant alteration of the protolith whole-rock systems and that high-grade metamorphism has had little effect on the more mobile LREE. The eclogite protoliths were probably derived from ancient mantle sources with geochemical characteristics very similar to present-day MORB sources. This implies that LREE-depleted (N-type) tholeiites have been erupted at constructive plate margins since at least the early Palaeozoic and possibly long before. The Cabo Ortegal eclogites are allochthonous. They have been thrust up on to the continent and thus they can be compared to other eclogites which also show MORB-like characteristics (e.g., 90% of the eclogites of Vendee area of western France).

UPb, Nd Isotope and REE geochemistry in eclogites from the Cabo Ortegal Complex, Galicia, Spain: An example of REE immobility conserving MORB-like patterns during high-grade metamorphism

Jean Cornichet

Papers

UPb, Nd Isotope and REE geochemistry in eclogites from the Cabo Ortegal Complex, Galicia, Spain: An example of REE immobility conserving MORB-like patterns during high-grade metamorphism