Rift Paleozoico temprano en el Complejo de Ossa Morena (SO del Macizo Ibérico): geoquímica y geocronología de un evento pre-Varisco en el Dominio de Sierra Albarrana y el Plutón de La Cardenchosa

Recognition of preorogenic geological features in the internal zones of polyorogenic areas, like the Sierra Albarrana Domain and La Cardenchosa Pluton, is difficult. New SHRIMP ages indicate that the main tectonomagmatic event recorded in these areas occurred between Late Cambrian (granite emplacement; 481±2 Ma) and Lower Ordovician (migmatization; 478±2 Ma). These ages also suggest that the intrusion of the igneous body is closely related with the low-pressure partial melting and the deformation of the structurally lower detrital formations. Geochemical data show that the granite from La Cardenchosa Pluton is peraluminous with a slight tendency towards an alkaline geochemical character, and a within-plate setting for the magmatism. Sr-Nd isotope data indicate an igneous origin with a strong upper crustal component, compatible with an extensional environment in the middle-upper crustal levels that marked the early stages of a continental rift development. These data are in agreement with the magmatism reported in other zones from the Iberian Peninsula, and are compatible with the latest magmatic event of the Early Paleozoic rifting. Our results, together with the main regional fabrics, suggest that the granitic magmatism and the migmatization are part of a single event, only slightly modified by Variscan reworking.Reconocer los rasgos geológicos preorogénicos en las zonas internas de áreas poliorogénicas, como el Dominio de Sierra Albarrana y el Plutón de la Cardenchosa, es complejo. Nuevas edades U-Pb confirman que el principal evento tectonomagmático registrado en esta zona ocurrió entre el Cámbrico tardío (emplazamiento del granito; 481±2 Ma) y el Ordovícico inferior (migmatización; 478±2 Ma). También sugieren que existe relación entre la intrusión del cuerpo ígneo, la fusión parcial y la deformación a baja presión de las migmatitas de las formaciones detríticas estructuralmente inferiores. El granito de La Cardenchosa es peralumínico con tendencia alcalina y afinidad de intraplaca. Los datos isotópicos Sr-Nd indican un origen ígneo con una fuerte componente de la corteza superior, en un ambiente geodinámico compatible con un entorno extensio- nal en los niveles corticales medios y superiores, que marcaron los momentos iniciales del desarrollo de un rift continental. Estos datos coinciden con el magmatismo descrito en otras zonas de la Península Ibérica y son compatibles con el último evento magmático del rifting Paleozoico. Nuestros resultados, junto con las principales fábricas regionales, sugieren que el magmatismo granítico y la migmatización son parte de un único evento, solo ligeramente modificado por la orogenia Varisca.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEMinisterio de Ciencia e Innovación (MICINN)Universidad Complutense de Madrid/Banco de SantanderRussian FederationUniversidad de Salamancapu

Metagranites of the Basal Units of the allochthonous complexes of Galicia (NW Iberian Massif): Geochemical evolution and geodynamic setting

Las Unidades Basales de los complejos alóctonos de Galicia incluyen rocas metamórficas paraderivadas que alternan con abundantes rocas ortoderivadas con edades comprendidas entre 500 y 470 Ma. Por su composición, tanto mineralógica como química de roca total, las rocas ortoderivadas se dividen en dos grandes grupos: uno de gneises graníticos s.l. (tonalíticos, granodioríticos y graníticos), con una composición química que transita de tipos magnesianos-cálcicos meta-aluminosos, hasta tipos férricos calcoalcalinos y alcalino-cálcicos peralumínicos; y otro segundo grupo de gneises alcalinos y peralcalinos con una composición férrico alcalina que varía de términos peralcalinos a meta-aluminosos, llegando a tener un carácter per-alumínico los tipos más diferenciados. Los gneises graníticos s.l. presentan anomalías negativas en Nb y Ta, sugiriendo su relación con un margen continental activo, mientras que las rocas alcalinas y peralcalinas están enriquecidas en elementos traza incompatibles y presentan altos contenidos en Th, Ga y Nb, característicos de magmas asociados a un régimen extensional. Esta evolución geoquímica probablemente refleja el tránsito desde un régimen geodinámico convergente caracterizado por un sistema de arcos magmáticos hacia un rifting intracontinental.The Basal Units of the allochthonous complexes of Galicia include para-derived metamorphic rocks alternating with abundant ortho-derived rocks with protolith ages ranging between 500 and 470 Ma. According to their whole-rock chemical and mineralogical composition, the ortho-derived rocks are divided in two major groups: one consisting of granitic gneisses s.l. (tonalitic, granodioritic, and granitic) with a chemical composition ranging from magnesian calcic metaluminous types to ferroan calc-alkalic and alkali-calcic peraluminous types, and a second group including alkaline and peralkaline terms, with ferroan alkalic and peralkaline to metaluminous composition, some of the most differentiated terms reaching also a peraluminous composition. The granitic gneisses s.l. show negative Nb and Ta anomalies, typical for rocks formed in an active margin, whereas the alkaline and peralkaline rocks are rich in incompatible trace elements and show a high content of Th, Ga and Nb, as expected for crustal extension-related magmas. This geochemical evolution probably reflects the switch from a convergent geodynamic setting featured by magmatic arcs to an intracontinental rifting.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEMinisterio de Economía y Competitividad (MINECO)pu

Late Cambrian magmatic arc activity in peri-Gondwana : geochemical evidence from metagranitoid rocks of the Basal Allochthonous Units of NW Iberia

The North African section of the Gondwana margin was the site of voluminous, arc-related magmatism during the Late Neoproterozoic (Avalonian-Cadomian orogen). The lower (and older) metasedimentary sequence that constitutes the Basal Units of the llochthonous Complexes of NW Iberia was deposited in that setting. In these units, sedimentation was followed by the intrusion of tonalites and granodiorites in the late Cambrian (ca. 493-489Ma). In the Late Paleozoic, the collision of Gondwana and Laurussia (Variscan orogeny) deformed and metamorphosed the whole ensemble. New whole rock geochemical analysis performed in seven samples of metatonalites and fourteen samples of metagranodiorites are characterized by: i) slight enrichment in incompatible elements (Rb, Ba, Th, U), ii) negative anomalies in Nb, Ta, P, and Ti, and iii) negative anomalies in Eu. These chemical features are in agreement with a subduction-related setting for the genesis of both types of magma, which is also supported by chemical discrimination using tectonic setting diagrams. Positive anomalies of Pb suggest a crustal component. The new geochemical data reveal that the convergent orogen that ruled the paleogeography of the Gondwana periphery during the Neoproterozoic (Cadomian orogen) remained active beyond the Neoproterozoic-Cambrian transition, up to at least late Cambrian times

Los metagranitos de las Unidades Basales de los complejos alóctonos de Galicia (NO del Macizo Ibérico): Evolución geoquímica y contexto geodinámico

Las Unidades Basales de los complejos alóctonos de Galicia incluyen rocas metamórficas paraderivadas que alternan con abundantes rocas ortoderivadas con edades comprendidas entre 500 y 470 Ma. Por su composición, tanto mineralógica como química de roca total, las rocas ortoderivadas se dividen en dos grandes grupos: uno de gneises graníticos s.l. (tonalíticos, granodioríticos y graníticos), con una composición química que transita de tipos magnesianos-cálcicos meta-aluminosos, hasta tipos férricos calcoalcalinos y alcalino-cálcicos peralumínicos; y otro segundo grupo de gneises alcalinos y peralcalinos con una composición férrico alcalina que varía de términos peralcalinos a meta-aluminosos, llegando a tener un carácter per-alumínico los tipos más diferenciados. Los gneises graníticos s.l. presentan anomalías negativas en Nb y Ta, sugiriendo su relación con un margen continental activo, mientras que las rocas alcalinas y peralcalinas están enriquecidas en elementos traza incompatibles y presentan altos contenidos en Th, Ga y Nb, característicos de magmas asociados a un régimen extensional. Esta evolución geoquímica probablemente refleja el tránsito desde un régimen geodinámico convergente caracterizado por un sistema de arcos magmáticos hacia un rifting intracontinental

Supporting info item, In: "Using SHRIMP zircon dating to unravel tectonothermal events in arc environments. The early Palaeozoic arc of NW Iberia revisited"

Dating of zircon cores and rims from granulites developed in a shear zone provides insights into the complex relationship between magmatism and metamorphism in the deep roots of arc environments. The granulites belong to the uppermost allochthonous terrane of the NW Iberian Massif, which forms part of a Cambro-Ordovician magmatic arc developed in the peri-Gondwanan realm. The obtained zircon ages confirm that voluminous calc-alkaline magmatism peaked around 500 Ma and was shortly followed by granulite facies metamorphism accompanied by deformation at c. 480 Ma, giving a time framework for crustal heating, regional metamorphism, deformation and partial melting, the main processes that control the tectonothermal evolution of arc systems. Traces of this arc can be discontinuously followed in different massifs throughout the European Variscan Belt, and we propose that the uppermost allochthonous units of the NW Iberian Massif, together with the related terranes in Europe, constitute an independent and coherent terrane that drifted away from northern Gondwana prior to the Variscan collisional orogenesis

40Ar/39Ar laserprobe dating of mylonitic fabrics in a polyorogenic terrane of NW Iberia

The tectonothermal evolution of a polyorogenic terrane in the Variscan belt of NW Spain has been constrained by 40Ar/39Ar laserprobe incremental heating experiments on mylonitic fabrics developed in major structures. Transitional levels between HP–HT and IP upper units in the O´ rdenes Complex where metamorphic and structural records demonstrate two cycles of burial and exhumation were selected for dating. Two groups of ages have been defined: (1) Silurian–Early Devonian, obtained from mylonites of the Forna´s extensional detachment, here considered as the minimum age for the start of tectonic exhumation of the HP– HT units and an upper age-limit for the HP–HT event itself; (2) Early to Mid-Devonian, from structures related to the Variscan convergence in the area, which include top-to-the-east thrusts and extensional detachments. A single, younger Carboniferous age obtained from the uppermost allochthonous sequences possibly reflects the final stages of emplacement of the allochthonous complexes. Our data indicate a polyorogenic character for a part of the Iberian allochthonous complexes, including Variscan (sensu stricto) and Early Variscan convergence, as well as an older, Early Palaeozoic cycle

Magmatism and early-Variscan continental subduction in the northern Gondwana margin recorded in zircons from the basal units of Galicia, NW Spain

In situ uranium-lead dating (LA-SF-ICPMS and SIMS) and Lu-Hf isotope analyses (LA-MC-ICP-MS) of zircon from eclogite facies rocks from the basal units of the Variscan Belt in Galicia constrain their magmatic and metamorphic evolution and give some clues about the nature and origin of the involved basement. The samples studied are two felsic gneisses, two eclogites, and one eclogitic gneiss of intermediate composition (metatonalite). Oscillatory-zoned zircon cores from the felsic samples gave a main clustering of U-Pb ages at 493 ± 2 and 494 ± 2 Ma, and some older ages that represent inherited cores. Zircon grains from the intermediate and one of the mafi c rocks show no inherited cores and yielded ages of 494 ± 3 and 498 ± 6 Ma, respectively, interpreted as time of protolith crystallization. Variably developed homogeneous zircon rims in one felsic gneiss yielded an age of 372 ± 3 Ma, and very tiny zircons of one eclogite gave 350 ± 2 Ma, both of which we interpret as metamorphic ages. The new age data demonstrate that the calc-alkaline magmatic suite described in the basal unit is ca. 20 Ma older than the alkaline to peralkaline plutonic suite of the same unit (dated at 472 ± 2 Ma; Rodríguez et al., 2007), and thus probably represents a distinct geologic event. Overgrowth rims are interpreted as metamorphic on the basis of their Lu/Hf and Th/U ratios. The 372 ± 3 age is considered as dating the high-pressure (high-P) metamorphism, and is essentially in agreement with previous Ar-Ar and Rb-Sr data. This high-P metamorphism marks the initial early-Variscan subduction of the Gondwana margin. The inherited zircon ages and Hf isotopic composition of zircons point to a considerable input of crustal material with West African Craton provenance to the felsic magma

Subduction and syn-collisional exhumation of a Paleozoic continental margin: an integrated study based on structures and P-T paths of the Basal Units in the Ordenes Complex (Galicia, Spain)

According to their metamorphic evolution, the Basal Units of the Galician Allochthonous Complexes are thought to represent a part of the margin of the Paleozoic Gondwana, subducted at the onset of the Variscan Orogeny. Variations in the P-T conditions of the first HP metamorphic event along the units suggest an important westward component for the direction of sucduction. Subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the Basal Units whereas the convergence continued. Compressional and extensional structures were synchronous or alternated in time and, together, induced the thinning and tapering of the orogenic wedge. 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