U-Pb dating of Ordovician felsic volcanism in the Schistose Domain of the Galicia-Trás-os-Montes Zone near Cabo Ortegal (NW Spain)

The northern termination of the Schistose Domain of the Galicia-Trás-os-Montes Zone is a tectonic slice named the Rio Baio Thrust Sheet, which is sandwiched between the Cabo Ortegal Complex and the Ollo de Sapo Domain of the Central-Iberian Zone. The Rio Baio Thrust Sheet is formed by two volcanosedimentary series, the Loiba and the Queiroga Series. The Loiba Series contains calc-alkaline dacite and rhyolite, while the overlying Queiroga Series has alkaline rhyolite. These series were considered to be in stratigraphically upwards continuity and believed to be Silurian in age. U-Pb dating of an alkaline rhyolite in the Queiroga Series provides an Arenig age of 475 ± 2 Ma. This age makes the Queiroga Series the oldest known stratigraphic unit in the Schistose Domain of the Galicia-Trás-os-Montes Zone, impeding correlation between the lithostratigraphic sequences of Ortegal and Central Galicia. As well as providing evidence of an unforeseen structural complexity within the Rio Baio Sheet, the new data supports the notion that the Schistose Domain is not parautochtonous, but a separate lithotectonic unit in thrust contact with the underlying Central-Iberian Zone

Datación U-Pb del volcanismo Siluro-Ordovícico del Sinforme de Verín (Orense, Dominio Esquistoso, Zona de Galicia-Trás-os-Montes)

El Grupo Paraño constituye la parte más alta de la estratigrafía del Dominio Esquistoso de Galicia-Trásos- Montes y está expuesto en el núcleo de la Sinforma de Verín. Como al resto del dominio, a este grupo se le ha atribuido una edad Silúrica. Con el fin de comprobar su edad se ha datado por el método U-Pb en circón una traquita félsica, de afinidades alcalinas, que ha proporcionado una edad Siluro- Ordovícica de 439,6+/-5 Ma. Esta edad demuestra que la estratigrafia del Dominio Esquistoso es más compleja de lo que se suponía, e invalida la correlación entre las rocas volcánicas en Galicia Central y las riolitas alcalinas de la Serie de Queiroga en la zona de Cabo Ortegal, datadas con una edad de 475 Ma por el método U-P

Idades preliminares U-Pb, ID-TIMS, das Ilhas Berlengas, Portugal = Preliminary ID-TIMS, U-Pb ages of the Berlengas Islands, Portugal

Apresentam-se os resultados provisórios das idades U-Pb de duas amostras das ilhas do grupo das Berlengas. No Farilhão Grande uma amostra de granito de duas micas com silimanite foi recolhida de um complexo metamórfico. Esta amostra ofereceu três fracções de monazite com idade 377+-1 Ma, interpretada como metamórfica, enquanto uma fracção de zircões de concórdia 483 Ma sugeriu uma idade Tremadoc. Esta última fracção é herdada, provavelmente do volumoso magmatismo do Ordovícico Inferior existente na Ibéria. Na Berlenga Grande o granito apresenta fracções de monazite e zircão concordantes de 307,4+-0,8 Ma.

Late Variscan, Permo-Carboniferous, Al-K plutonism in the South Portuguese Zone : El Crispinejo cordierite-bearing granite

The El Crispinejo granite forms part of a small, but distinctive late intrusive suite of cordierite-bearing peraluminous granites in the South Portuguese Zone (SPZ). This granite has the best outcrop relationships of the suite. It cross-cuts different members of the Sierra Norte Batholith of the SPZ and the Carboniferous Volcano-Sedimentary Complex of the Iberian Pyrite Belt, producing contact metamorphism. This late pluton has a high K content which results in a contrasting geophysical response (K-Th-U) with respect to the surrounding trondhjemitic granitoids of the TTG suite of the Sierra Norte Batholith. A concordant monazite-zircon U-Pb ID-TIMS age of 300.5 +0.5/-1.5Ma demonstrates Permo-Carboniferous age for this late Variscan magmatic event. The granite is associated with a series of ore showings (F-Pb-Zn and Sn-W) which are completely different from the nearby, massive sulphide and manganese ore deposits of the Iberian Pyrite Belt, indicating the unique character of this intrusion

U-Pb dating of Ordovician felsic volcanism in the Schistose Domain of the Galicia-Trás-os-Montes Zone near Cabo Ortegal (NW Spain)

The northern termination of the Schistose Domain of the Galicia-Trás-os-Montes Zone is a tectonic slice named the Rio Baio Thrust Sheet, which is sandwiched between the Cabo Ortegal Complex and the Ollo de Sapo Domain of the Central-Iberian Zone. The Rio Baio Thrust Sheet is formed by two volcanosedimentary series, the Loiba and the Queiroga Series. The Loiba Series contains calc-alkaline dacite and rhyolite, while the overlying Queiroga Series has alkaline rhyolite. These series were considered to be in stratigraphically upwards continuity and believed to be Silurian in age. U-Pb dating of an alkaline rhyolite in the Queiroga Series provides an Arenig age of 475 ± 2 Ma. This age makes the Queiroga Series the oldest known stratigraphic unit in the Schistose Domain of the Galicia-Trás-os-Montes Zone, impeding correlation between the lithostratigraphic sequences of Ortegal and Central Galicia. As well as providing evidence of an unforeseen structural complexity within the Rio Baio Sheet, the new data supports the notion that the Schistose Domain is not parautochtonous, but a separate lithotectonic unit in thrust contact with the underlying Central-Iberian Zone

Influence of deformation and fluids on Ar retention in white mica: Dating the Dover Fault, Newfoundland Appalachians

White mica 40Ar/39Ar analyses may provide useful constraints on the timing of tectonic processes, but complex geological and thermal histories can perturb Ar systematics in a variety of ways. Ductile shear zones represent excellent case studies for exploring the link(s) between dynamic re-/neo-crystallization of white mica and coeval enhanced fluid flow, and their effect on 40Ar/39Ar dates. White mica 40Ar/39Ar dates were collected from compositionally similar granites that record different episodes of deformation with proximity to the Dover Fault, a terrane-bounding strike-slip shear zone in the Appalachian orogen, Newfoundland, Canada. 40Ar/39Ar data were collected in situ by laser ablation and by step heating single crystals. Results were compared to each other and against complementary U-Pb zircon and monazite, and K-Ar fault gouge analysis. Although step-heat 40Ar/39Ar is a widely applied method in orogenic settings, this dataset shows that relatively flat step-heat 40Ar/39Ar spectra are in contradiction with wide spreads in in-situ 40Ar/39Ar dates from the same samples, and that plateau dates in some cases yielded mixed dates of equivocal geological significance. This result indicates that the step-wise release of Ar from white mica likely homogenizes and obscures spatially-controlled Ar isotope reservoirs in white mica from sheared rocks. In contrast, in situ laser ablation 40Ar/39Ar analysis preserves the spatial resolution of 40Ar reservoirs that have been variably reset by deformation and fluid interaction. This study therefore suggests that laser ablation is the best method for dating the timing of deformation recorded by white mica. Final interpretation of results should be guided by microstructural analysis, estimation of deformation temperature, chemical characterization of white mica, and complementary chronometers. Overall the dataset shows that granitic protoliths were emplaced between 430-422 Ma (U-Pb zircon). High strain deformation along the Wing Pond Shear Zone occurred between ca. 422-405 Ma (U-Pb monazite and 40Ar/39Ar). Subsequent patchy Ar loss in white mica occurred locally during low T shear (40Ar/39Ar). K-Ar dating of authigenic illite in fault gouge from the broadly co-linear brittle Hermitage Bay Fault indicates that slip along the terrane boundary persisted until at least the Mississippian

U-Pb dating of Siluro-Ordovician volcanism in the Verín Synform (Orense; Schistose Domain, Galicia-Trás-os-Montes Zone)

El Grupo Paraño constituye la parte más alta de la estratigrafía del Dominio Esquistoso de Galicia-Trásos-Montes y está expuesto en el núcleo de la Sinforma de Verín. Como al resto del dominio, a este grupo se le ha atribuido una edad Silúrica. Con el fin de comprobar su edad se ha datado por el método U-Pb en circón una traquita félsica, de afinidades alcalinas, que ha proporcionado una edad Siluro-Ordovícica de 439,6+/-5 Ma. Esta edad demuestra que la estratigrafia del Dominio Esquistoso es más compleja de lo que se suponía, e invalida la correlación entre las rocas volcánicas en Galicia Central y las riolitas alcalinas de la Serie de Queiroga en la zona de Cabo Ortegal, datadas con una edad de 475 Ma por el método U-Pb

U/Pb rutile cooling agefrom the Chímparra Gneiss (Cabo OrtegalNW Spain)

The Chfmparra gneisses are characterized by a HP-HT (800°C, ISOO MPa) mylonitic fabric reequilibrated under amphibolite facies (600°C, 900 MPa). This fabric is formed by Qtz+Plag+Bt+white mica + Grt+Ky. Rutiles are preserved as inclusions whithin garnet. These rutiles have been dated at 382+1-3 Ma C0GPb 123SU age). This cooling age is consistent with a reported age of peak melamorphism in the same unit (392+1-27 Ma, Ordonez-Casado et al., 1995) suggesting a fast cooling rate during the exhumation process responsible for the mylonitic foliatio