Reaction textures in calc-silicate gneisses from the Tormes gneiss complex: implications for the retrogade P-T-XC02 path

[Abstract] Calc-silicate gneisses from the LowerUnit ofthe Tormes Gneiss Complex,NW Salamanca, exhibit a sequence of reaction textures that have been used to elucidate their retrograde P-T-XC02 path. The highest temperature recorded in the calc-silicates is represented by the wollastonite- and scapolite- (meíoniterich) bearing assemblages which yield at least 7500 C at 6 kbar based on experimental results. The calc-silicates have partially re-equilibrated at lower temperatures (down to 4500 C) as evidenced by the successive reactions: (1) . Wo + Scp + Cal ~ Grt + CO2 ; (2) Wo + CO2~ Cal + Qtz; (3) Scp ~ PI + Cal + Qtz; (4) Grt + CO2 + H20 ~ Ep/Clz + Cal + Qtz ±PI; y (5) Cpx + CO2 + H20 ~ Tr + Cal + Qtz. The cooling down to lower temperatures of4500 C (at P<3 kbar), in the andalusite stability field, is also suggested by the secondary andalusite replacement ofprevious fibrolitic sillimanite in adjacent metapelites. In this paper, mineral devolatilization equilibria are used to constrain the composition ofboth syn- and post-peak-D2 fluids in the deepest exposed levels of the metamorphic complexo The results indicate that during the transitional granulite facies metamorphism, large-scale CO2 advection did not occur

Tectonothermal evolution associated with crustal escale extension: an hercynian example from the Tormes gneiss dome (NW Salamanca, Iberian Massif)

[Abstract] The tectonothermal Variscan evolution ofthe Tormes Gneiss Dome is controlled by a subhorizontal ductile shear zone of crustal scale and extensional characteristics, that induced a quick exhumation ofthe deep parts ofthe metamorphic complex during crustal thinning. The shearing affected a broad band of metamorphic rocks but, as the temperature decreased, became progressively concentrated in a low-grade ductile detachment that separates two distinct units, characterized by their lithology and different tectonothermal evolution. Kinematic indicators in non-coaxial fabrics show a displacement of the hangingwall to the SE, i. e., parallel to the trend of the foldbelt. The subhorizontal shearing is related to the extensional collapse ofthe variscan crust, previously thickened during the collision tectonics

Trayectoria metamórfica P-T relacionada con subducción en eclogitas del Complejo de Basamento de Samaná, Cordillera Septentrional, República Dominicana

Lenses of eclogites from the Punta Balandra unit of the Samaná basement complex, Cordillera Septentrional, Dominican Republic, preserve information of the early metamorphic and tectonic history of subduction in the Greater Antilles island-arc system and their collision with the North America plate. For this reason, these rocks were investigated for reconstruction of quantitative metamorphic P-J paths based upon interpretation of meso and microfabrics, mineral assemblages and mineral chemistry, with the help of equilibrium phase diagrams calculated for specific bulk composition in the model system CKNFMASH and isopleths for selected solution end-members. The obtained subduction-related prograde evolution went from garnet-free first and garnet-bearing next lawsonite-blueschist facies to peak eclogue facies conditions at P=22-24 kbar and 7=6 1 0-625sC. The subsequent retrograde P-T path entered the epidote-blueschist (garnet-free) facies and ended within the greenschist facies, retracing the prograde evolution at low-P (i.e. indicates cooling during the exhumation of the rocks). Thus, Samaná basement complex eclogites and hydrated equivalents formed in a intraoceanic subduction zone in which WSW/W-migrating Atlantic lithosphere was subducted beneath the Caribbean plate. Thrust stacking of different tectonic slices can provide the geological framework for exhumation and cooling due to continuous cool underplating and explains the preservation of high-pressure assemblages

Paleostress evolution during the exhumation of high-p marbles, Samaná Complex, northern Hispaniola.

The marble of the Samaná complex presents a widespread foliation formed during its exhumation following a general decompressive strain path from high pressure (2.0>P>0.7 GPa) and low temperature (350 MPa during deformation. In contrast, mean flow stress during grain-boundary migration is estimated in |σ1-σ3| 110 MPa Ma-1). All of these data suggest that exhumation always occurred near the brittle-ductile regime of deformation. © 2017, Instituto Geologico y Minero de Espana. All rights reserved.El trabajo ha sido financiado por los proyectos de investigación CGL2010-14890 y CGL2011-23628, auspiciados por el plan nacional I+D+i del Ministerio de Economía y Competitividad del Gobierno de España.Peer reviewe

Subdivisión geoquímica del Arco Isla Circum-Caribeño, Cordillera Central Dominicana: Implicaciones para la formación, acrecion y crecimineto cortical en un ambiente intraoceánico

The Upper Cretaceous-Eocene Circum-Caribbean island-arc system (AICC) is a complex collage of crustal units or terranes s.l. which have formed and accreted within an intra-oceanic environment since Late Jurassic-Early Cretaceous times. In the Cordillera Central of the Dominican Republic these terranes are represented by several tectonostratigraphic units, that define a pre-Aptian-Albian deformed and metamorphosed basement, over and into which the igneous rocks of the AICC were extruded or intruded. Basement sequences can be subdivided into: (1) a plume-related Duarte terrane (plateau I); (2) a primitive arc-related Maimón-Amina terrane, which includes intra- or back-arc N-MORB units (Río Verde Complex) with a subduction geochemical imprint; and (3) the Loma Caribe peridotite terrane of mantle provenance. Two successive stages of the arc growth are superposed: an Early Cretaceous arc tholeiite stage with boninitic affinities (arc I), and a Late Cretaceous-Eocene calc-alkaline stage (AICC; arc II). The arc I stage is not recorded in the Duarte terrane but is represented in the Maimón-Amina terrane by the Río Verde Complex, the Maimón Forma-tion and the Peralvillo Norte Formation. In the Duarte terrane, the arc II growth stage include the Siete Cabezas Formation (Cenomanian- Maastrichtian, plateau II) and the fill of an intra-arc basin by the Tireo Formation (Cenomanian-Maastrichtian). In the Maimón-Amina terrane, the arc II growth is represented by the Las Lagunas and Peralvillo Sur Formations. The former lies unconformably on rudist-bearing limestone of Albian age (Hatillo Limestone). The Jautía gabbro-norite batholith and the calc-alkaline foliated and non-foliated tonalitic plutons of Late-Cretaceous-Early Eocene age intrude the rocks of the Duarte terrane except the Siete Cabezas Formation. Deformation and metamorphism of the pre-Aptian-Albian basement is a consequence of the collision of the Duarte plateau terrane with the Caribbean primitive island arc (Maimón-Amina terrane), that caused the emplacement of the Duarte terrane with a fragment of oceanic lithosphere (ophiolite). The interpretation of the geological and geochemical data support a tectonomagmatic model, which relates the compositional evolution of the intruded and extruded magmas in this segment of the AICC, with a flip in the subduction polarity under the primitive arc after the Aptian/Albian collision

Datos geocronológicos preliminares de la colisión entre el Gran Arco-Isla Caribeño y la Plataforma Continental de Bahamas en el Segmento de La Española

In parallel to structural and metamorphlc studies, geochronologlcal Investigations have been initiated in eclogites and blueschists from the Samaná Peninsula complex, northern Hispaniola, using Sm-Nd, UPb, Rb-Sr and 40Ar/39Ar methods, to constraint subduction, collision and exhumation processes in the Caribbean-North America plate boundary zone. A garnet-omphacite- whole rock Sm-Nd isochron from eclogite yields an age of 86±47 Ma (l43Nd/l44Ndinitial= 0,512894±0,000057; MSWD=0,00036). Although imprecise, we interpret this age as dating the peak of eclogitic metamorphism and allows us to calculate an initial eNd value of +7,2, which suggests a relatively depleted source for the protolith and that the rock formed in an intra-oceanic island-arc setting without significant influence from continental crust. Whole rock trace elements and REE data and preliminary Pb and Rb-Sr analyses also support this interpretation. The 40Ar/39Ar analyses of phengite mineral separates from eclogite and blueschist were made to record the cooling age during retrograde metamorphism (Tc»325-3009C). Phengites of eclogite and blueschist yields average plateau ages of 36,30±0,l 3 Ma and 33,85±0,12 Ma, respectively. These Late Eocene to Early Oligocène ages are related to the regional exhumation of the high-P basement complex and are attributed to the initial oblique collision of the Bahama Platform beneath the Hispaniola arc

Metamorfismo y estructura de la Formación Maimón y los Complejos Duarte y Río Verde, Cordillera Central Dominicana: implicaciones en la estructura y la evolución del primitivo Arco Isla Caribeño

The mainly metabasaltic pre-Aptian/Albian basement of the Median Belt of Hispaniola includes the Duarte Complex, the Río Verde Complex and the Maimón-Los Ranchos Formations. In base to mineral assemblages present in metabasic rocks and P-T conditions estimated from thermobarometry, the Río Verde Complex is divided into four metamorphic zones and Zone IV is further subdivided into two. The metamorphic grade increase upward in the structural sequence, from prehnite-pumpellyte facies (Zone I), through greens-chist facies (Zones II and III) and amphibolite facies (Zone IVa), to upper amphibolite facies (Zone IVb), only restricted to Cpx-bearing amphibolites just below the contact with the overlying Loma Caribe Peridotite. The metamorphic field gradient is inverse and of low-P type. The P-T paths documented for Zones IVa and IVb of the Río Verde Complex involve a two-stage prograde evolution: a first event of near isobaric heating in the low-pressure field, typical of sub-ophiolite metamorphic sole rocks and characterized by critical high-grade assemblages; and a second event marked by a medium-pressure overprint of the first-stage metamorphic assemblages following a high-P gradient. These P-T paths are interpreted to result from intra-oceanic thrusting during the closure of a back-arc basin related with the Primitive Caribbean Island Arc and the onset of subduction of arc units in the Aptian/Albian time, which formed the high-pressure metamorphic overprint. The heating and development of an inverted metamorphic gradient in the sub-ophiolite Río Verde Complex, can be genetically related with the hanginwall emplacement of the hot peridotitic slice and the conductive heat transfer downward. The studied sector of the Duarte Complex (metamorphosed oceanic plateau) is divided into three metamorphic zones. Their distribution suggest that there is a temperature increase westward and downward in the structural sequence, from the upper greenschist facies (zone A), through Ep-amphibolite and amphibolite facies (zone B), to upper amphibolite transitional to lower granulite facies (zone C). The metamorphic field gradient is normal and the mid-P type (25-30º C/km). The lowest structural levels of the complex are occupied by the gabro-norites of the La Jautía batholith, which formed in ductile shear zones Grt+Opx-bearing granulites metamorphic assemblages. The 89 Ma date obtained from foliated tonalites (U-Pb in zircons) established an Upper Cretaceous age for the main ductile shearing deformation. However, the deformation is very heterogeneous in the complex, existing regionally wide metamorphic sectors without related foliation development. Therefore, the prograde metamorphism of the Duarte Complex is interpreted to result from moderated thickening of a previously thick oceanic crust, due to the great accumulation of plateau-basalts (30 km). The post-thermal peak P-T paths suggest the unloading and cooling of the complex, during the continuous retrograde development of deformative and mylonitic non-coaxial Sp fabrics (84,6±0,5 Ma; 40 Ar/39 Ar cooling age in syn-Sp muscovite). In summary, the metamorphic rocks of the pre-Aptian/Albian basement units of the Median Belt record different stages in the history of the acretion-obduction of the Duarte plateau with the Caribbean Primitive Island Arc. All these tectonothermal events pre-date the final arc-continent collision between the Caribbean island arc and the Bahamas platform during the Late Cretaceous

Geological, geophysical and geochemical structure of a fault zone developed in granitic rocks: Implications for fault zone modeling in 3-D

The structure of a fault zone developed in granitic rocks can be established on the basis of the spatial variability of geological, geophysical and geochemical parameters. In the North Fault of the Mina Ratones area (SW Iberian Massif, Spain), fault rocks along two studied traverses (SR-2 and SR-3 boreholes) exhibit systematic changes in mineralogy, geochemistry, fabrics and microstructures that are related to brittle deformation and alteration of granite to form cataclasite and subsequent gouge. The spatial distribution and intensity of these changes suggest a North Fault morphology that is consistent with the fault-core/damage-zone model proposed by Chester et al. (1993) to describe a fault zone architecture. North Fault damage zone thickness can be defined by the development of mechanically related mesoscopic faults and joints, that produce a Fracture Index (FI)>10. High FI values are spatially correlated with relative low seismic velocity zones (VP<5 km/s and VS<2.5 km/s in the well-logs), more probably related to a high concentration of fractures and geochemical alteration produced by meteoric water-granite interaction along fault surfaces. This correlation is the base of a geostatistical model proposed in the final part of this study to image the fault zone architecture of a granitic massif

Caracterización sísmica del entorno de Mina Ratones en el granito de Albalá

A detailed characterization o f the physical properties of a granitic pluton, as well as a three dimensional reconstruction of the fractures network and its surrounding are obtained by high resolution seismic tomography. The input travel times were picked from offset and azimuth variable seismic profile (OVSP) acquired in a 500 m deep vertical borehole. The three-component seismic data sets recorded allowed to test the methodology for P- and S-waves and to compare the velocity models derived from the first arrivals tomographic inversion of the two different kind of seismic waves. Furthermore, the combination of the two velocity models (P- and S-waves) provides a 3D Poisson's ratio diagram of the rock massif. This coefficient allows o differentiate volumes of fractured and altered granite from fresh rock. Poisson's ratios close to 0.3 mean fractured areas and correlate with fracture index (FI) and sonic logs, otherwise Poisson's ratio close to 0.25 is related to unaltered and unfractured rock

Evolucion tectonotérmica del área metamórfica del SO de Salamanca (Zona Centroibérica O de España)

Los estudios realizados en el sector suroccidental de la provincia de Salamanca en el marco del Proyecto MAGNA, han permitido obtener nuevos datos sobre la estratigrafia, estructura y metamorfismo del potente y monotono conjunto de metasedimentos del Complejo Esquisto-grauvaquico que afloran en el Area. Estos materiales fueron sometidos a une deformaci6n hercinica polifasica cuyas estructuras pueden agruparse en tres fases principales de deformacion, ademas de otras tardias. La primera fase produce un tren de pliegues de longitud de onda kilometrica y de direccion NO-SE. Durante la segunda fase se desarrolla una zona de cizalla subhorizontal, de espesor kilometrico y con sentido de movimiento de bloque de techo hacia el Sureste. aue tiene caracter extensional. La tercera fase oroduce un re.o le-a emiento de las estructuras o r e vias, practicamente coaxial con la primera fase. Esta deformacion esta acompariada de un metamorfismo inicial de presiones intermedias que evoluciona posteriormente a otro de baja presión. El conjunto de los datos permite ser integrado en un modelo de evolución tectonotermal en el que el engrosamiento cortical, al que va asociado el metamorfismo de presiones intermedias, se ve interrumpido por un colapso extensional controlado por la zona de cizalla de segunda fase, responsable del gradiente metamórfico de bajas presiones. [ABSTRACT] A new set of data concerning the stratigraphy, structure and metamorphism of the monotonous Schist and Graywacke Complex, exposed in the southwestern Salamanca province, have been made available as an outgrowh of studies undertaken in the framework of the MAGNA Project. In structural terms, these allow characterization of the Hercynian orogeny as a polyphase event with three main phases of deformation and some late phases of less importance. The first phase was responsible forthe formation of NW-SE striking folds with kilometric wavelengths. A kilometre scale thick, extensional shear zone with downthrown to the SE of the hangingwall characterizes the second phase. The third phase in turn, roughly coaxial with the first one. resulted in a refoldino of the oreviouslv formed structures. This deformation seouence devolooed under initial metamordhic conditions of an iniermediaie pressuie type and evolved subsequently to a lowi pressure regime. The tectonothermal evolution of this area is thus envisaged in response to an initial crustal thickening event. The one associated to the intermediate pressure metamophic regimen, subsequently interrumpted by an extensional collapse of the thickened crust, controlled by the second phase shear zone. This is held responsible for the shift to the lower pressure metamorphic conditions which dominated the second part of the evolution