3D Analysis of an Ordovician igneous ensemble: A complex magmatic structure hidden in a polydeformed allochthonous Variscan unit

[EN]The basal units of the allochthonous complexes of NW Iberia represent a fragment of the external edge of Gondwana subducted and subsequently exhumed during the Variscan collision. The structural analysis carried out in orthogneissic massifs of the southern part of one of these, the Malpica–Tui Unit, reveals the generation of recumbent folds and associated axial planar foliation during their exhumation. These folds nucleated in irregular igneous bodies that were initially deformed during the subduction event and show east to southeast vergence. Down-plunge projection of surface data and a series of regularly spaced cross-sections have been used to build 3D models of the two main bodies of orthogneiss, of calc-alkaline and peralkaline compositions respectively. The first is presently a lens-shaped body folded in a recumbent syncline, whereas the peralkaline gneiss, also affected by a train of asymmetric recumbent folds in the south, exhibits in the north a fold-like structure which is interpreted as inherited from its primary geometry, that of a ring dike pluton

One- and two- dimensional models are equally effective inmonitoring the crust’s thermal response to advection bylarge-scale thrusting during orogenesis

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Careón ophiolite, NW Spain: Suprasubduction zone setting for the youngest Rheic Ocean fl oor

The Careón ophiolite (Galicia, NW Iberian Massif) shows lithological and geochemical features suggestive of an origin in a suprasubduction zone setting. As with other Devonian ophiolites in the European Variscan belt, it was generated within a contracting Rheic Ocean. This setting and the general absence of large Silurian-Devonian volcanic arcs on both of the Rheic Ocean margins strongly suggest that this ocean was closed by intraoceanic subduction directed to the north. This subduction removed the older normal (N) mid-oceanic-ridge basalt (MORB) oceanic lithosphere and gave rise to a limited volume of new suprasubduction zone oceanic lithosphere. The Careón ophiolite is a key element in understanding the evolution of the Rheic Ocean, which was the main oceanic domain that closed during the Paleozoic convergence of Gondwana and Laurussia, preceding the assembly of Pange

Use of thermal modeling to assess the tectono-metamorphic history of the Lugo and Sanabria gneiss domes, Northwest Iberia

The Lugo and Sanabria domes in Northwest Iberia have well constrained metamorphic and structural histories. Both occur in the Iberian autochthon and resulted from late-Variscan extensional collapse following crustal thickening related to the Variscan collision. The two domes developed beneath large thrust sheets, are cored by sillimanite-orthoclase anatectic gneiss, preserve evidence of a steep thermal gradient (1 oC MPa-1), and exhibit a distinct decrease in metamorphic grade to the east in the direction of nappe movement. Geochronological evidence indicates that the lower crust melted within 30 Ma of initial crustal thickening and that dome formation occurred within 50 Ma. The histories of the two domes are considered as the basis for one-dimensional finite-difference models of thermal response to changes in crustal thickness. Results from thermal models suggest that thickening was limited to the crust, provide a numeric explanation for timing and nature of granite magmatism, and indicate that high-temperature metamorphism and crustal anatexis may result directly from thermal relaxation, eliminating the need for significant mantle thermal contribution. Also, the models show that small differences in thickness of large, wedge-shaped thrust sheets can explain distinct P-T paths experienced by different limbs of the domes

Magnetic Anomalies in Extensional Detachments: The Xistral Tectonic Window of the Lugo Dome (NW Spain)

[EN]Magnetic anomalies in the Lugo gneiss dome of the NW Iberian Massif are related with outcropping late Variscan extensional detachments The sources of magnetic anomalies are high‐grade metamorphic rocks, including migmatites and inhomogeneous granites The Lugo Dome is one of a series that coincides with a broad, curved magnetic anomaly that delineates an orocline in NW Iberi

Characterization of the Eastern Galicia Magnetic Anomaly. A warning regarding the interpretation of aeromagnetic data

[EN]Caution should be applied to the interpretation of aeromagnetic data since often they use unknown geological bodies of unknown characteristics to explain the anomalies. A case study in the Lugo Sanabria Extensional Dome (NW Spain) shows that what was interpreted as an anomaly sourced by a basic/ultrabasic magnetic body from the lower crust/mantle, seems to be related to magnetization along extensional detachments in relation to the orogenic evolution of a thickenend Variscan crust

Magnetic anomalies associated to extensional detachments? An example at Xistral TectonicWindow of the Lugo Dome (NW Spain)

[EN]Magnetic anomalies in the Lugo gneiss dome of the NW Iberian Massif are related with outcropping late Variscan extensional detachments The sources of magnetic anomalies are high‐grade metamorphic rocks, including migmatites and inhomogeneous granites The Lugo Dome is one of a series that coincides with a broad, curved magnetic anomaly that delineates an orocline in NW Iberi