Dating of zircon and monazite from diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge - hints at burial and exhumation velocities.

In order to better understand the formation and evolution processes of ultrahigh pressure (UHP) felsic rocks, we determined the ages of various domains of zircon and monazite crystals from the diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge. According to cathodoluminescence imagery and Th/U ratios, three zircon zones were distinguished. Each was dated using several spot analyses from a sensitive high-resolution ion microprobe (SHRIMP) analysing Pb, U and Th isotopes. The results were: (1) core zone - 21 analyses: Th/U less/equal 0.023 and 337.0 plus/minus 2.7 Ma (2 sigma, combined 206Pb/238U-207Pb/235U age); (2) diamond-bearing intermediate zone - 23 analyses: Th/U greater/equal 0.037 and 336.8 plus/minus 2.8 Ma; and (3) rim zone-12 analyses: Th/U = 0.0150.038 (plus one analysis of 0.164) and 330.2 plus/minus 5.8 Ma. The U-Pb obtained ages are virtually concordant. Furthermore, two oscillatory zoned zircon cores (Th/U greater/equal to 0.8) yielded (~concordant) ages of ~400 Ma. Six SHRIMP analyses of monazites gave an age of 332.4 plus/minus 2.1 Ma. In addition, Pb, Th and U contents in monazite were analysed with an electron microprobe (EMP). A mean age of 324.7 plus/minus 8.0 (2σ) Ma was acquired from 113 analyses.By combining the defined ages with previously published P-T conditions, minimum velocities for burial and exhumation were estimated. In addition, we present a likely geodynamic scenario involving age data from the literature as well as this study: beginning 340 million years ago, gneisses at the base of a thickened continental crust (~1.8 GPa, 650C) were transported to depths of at least 130 km, possibly as deep as 250 km. Here they were heated (>1050C) and partially melted and as a result began to rise rapidly. The burial and subsequent ascent back to a depth of 50 km, where zircon rims and monazite formed, took only a few million years and perhaps significantly less

Metamorphic P-T conditions of Late Jurassic rhyolites in the Magallanes fold and thrust belt, Patagonian Andes, Chile

Meta-riolitas foliadas de la Formación Tobífera (Jurásico Superior) forman dos escamas tectónicas en el cinturón de pliegues y mantos de los Andes Patagónicos. Exhiben una foliación milonítica penetrativa desarrollada a escala regional. Exceptuando a porfi - roclastos de cuarzo, la mayor parte de la asociación mineral es de origen metamórfi co. Los minerales sincinemáticos, en las sombras de presión, son similares en composición a aquellos de la matriz. La mineralogía metamórfi ca sugiere una evolución entre unos 3 kbar (aprox. 250 ºC) y unos 7 kbar (aprox. 350 ºC). La naturaleza sincinemática de los minerales permite correlacionarlos con una fase compresional del Cretácico Inferior a medio en la región, que probablemente involucró una subducción dirigida hacia el oeste del sustrato cuasi-oceánico sobre el que se depositó la Formación Tobífera

Petrology and tectonic evolution of late Paleozoic mafic-ultramafic sequences and the Leones Pluton of the Eastern Andean Metamorphic Complex (46-47°S), southern Chile

The metamorphosed mafic-ultramafic sequences of the Eastern Andean Metamorphic Complex outcropping in the Patagonian Andes are critical to disclose the late Paleozoic tectonic evolution of the southwestern margin of Gondwana. In the study area, mafic-ultramafic bodies are thrusted onto polydeformed metasedimentary rocks and intruded by the mid-Carboniferous composite Leones Pluton. The metabasalts (mostly tremolite-chlorite schists and amphibolites) show N-MORB and BABB chemical affinities pointing that formed part of an oceanic crustal section with components of the marginal basin, emplaced after the main pulse of Devonian arc magmatism, possibly in a retreating convergent margin. Interleaved serpentinites consist of serpentine polymorphs (antigorite, lizardite, and late chrysotile) and magnetite, with variably distributed minor amounts of chlorite, tremolite, and traces of ilmenite. Serpentinites have high Cr, Ni, Ti, and Yb contents, and show slightly enriched LREE and flat HREE patterns with a noticeable Eu positive anomaly. Mineralogical and geochemical features indicate that olivine-rich clinopyroxene-spinel-bearing peridotites were metamorphosed in a newly formed east-dipping subduction zone. The closure of the marginal basin continued with the tectonic underthrusting and tectonic juxtaposition of mafic-ultramafic rocks within an accretionary wedge. The tectonic cycle of the oceanic basin finished with the intrusion of mid-Carboniferous subduction-related plutons and pluton-driven thermal metamorphism.Fil: Rojo, D.. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; Argentina. Universidad Arturo Prat; ChileFil: Calderón, M.. Universidad Andrés Bello; ChileFil: Hervé, F.. Universidad Andrés Bello; ChileFil: Díaz, J.. Universidad Andrés Bello; ChileFil: Quezada, P.. Universidade Federal do Paraná; Brasil. Universidad de Aysén; ChileFil: Suárez, Rodrigo Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Ghiglione, Matias. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Fuentes, F.. Universidad Mayor; ChileFil: Theye, T.. Universität Stuttgart; AlemaniaFil: Cataldo, J.. Universidad Andrés Bello; ChileFil: Sándoval, J.. Universidad Andrés Bello; ChileFil: Viefhaus, T.. Universität Stuttgart; Alemani

Experimental data on the Tschermak substitution in the Fe-chlorite.

International audienceIron chlorite with compositions intermediate between the two end-members daphnite [Fe5Al2Si3O10(OH)8] and Fe-amesite [Fe4Al4Si2O10(OH)8] were synthesized from gels, under fO2 conditions deÞ ned by the solid oxygen buffer Fe-FeO. The unit-cell parameters and volume of chlorite with Si-content ranging from 2.3 to 2.7 were calculated. A least-squares Þ t of these data yields V0 Fe-ames = 213.06 cm3 and V0 daph = 216.50 cm3. The molar volume of daphnite is similar to that estimated by Vidal et al. (2001), but the volume difference between Fe-amesite and daphnite is too low. The experimental data were also Þ tted for reasonable values of V0 Daph V0 Fe-ames and VFeMg-1, with linear (ideal) or non-ideal volume models involving a positive excess volume. With these models we obtain V0 Daph between 216 cm3 and 217.49 cm3, and V0 Fe-ames between 209 and 211.35 cm3. Equilibration experiments involving chlorite with almandine-hercynite/fayalite or chloritoidhercynite/fayalite provide data on the chlorite composition as a function of T and P at temperatures between 420 and 520 °C and pressures between 3 and 20 kbar, at fO2 buffered by the assemblage Fe-FeO. Initial Si-rich and Si-poor chlorite compositions converged in most cases toward an equilibrium composition during the experiments. The results show that the Si-content of chlorite is sensitive to temperature for the various divariant assemblages. The most deÞ nitive results, obtained for the assemblage chlorite-almandine-fayalite, were used to estimate Hf 0 Fe-amesite and the Al-Fe Margules parameter for the various sets of daphnite and Fe-amesite molar volumes constrained from the synthetic chlorites. The results indicate that Hf 0 Fe-amesite = 7616 ± 3 kJ and WG AlFe ~ 10 kJ

Metamorphic history and geodynamic significance of the Early Cretaceous Sabzevar granulites (Sabzevar structural zone, NE Iran)

The Iranian ophiolites are part of the vast orogenic suture zones that mark the Alpine-Himalayan convergence zone. Few petrological and geochronological data are available from these ophiolitic domains, hampering a full assessment of the timing and regimes of subduction zone metamorphism and orogenic construction in the region. This paper describes texture, geochemistry, and the pressuretemperature path of the Early Cretaceous mafic granulites that occur within the Tertiary Sabzevar ophiolitic suture zone of NE Iran. Whole rock geochemistry indicates that the Sabzevar granulites are likely derived from a MORB-type precursor. They are thus considered as remnants of a dismembered dynamo-thermal sole formed during subduction of a back-arc basin (proto-Sabzevar Ocean) formed in the upper-plate of the Neotethyan slab. The metamorphic history of the granulites suggests an anticlockwise pressuretemperature loop compatible with burial in a hot subduction zone, followed by cooling during exhumation. Transition from a nascent to a mature stage of oceanic subduction is the geodynamic scenario proposed to accomplish for the reconstructed thermobaric evolution. When framed with the regional scenario, results of this study point to diachronous and independent tectonic evolutions of the different ophiolitic domains of central Iran, for which a growing disparity in the timing of metamorphic equilibration and of pressuretemperature paths can be expected to emerge with further investigations. \ua9 2011 Author(s)

Feedback between fluid infiltration and rheology along a regional ductile-to-brittle shear zone: The East Tenda Shear Zone (Alpine Corsica)

The East Tenda Shear Zone (ETSZ) is themajor Alpine tectonic boundary marking the overthrusting of the ocean-derived Schistes Lustrés nappe onto the Variscan crystalline basement of Corsica. New structural, mineralogical, and geochemical investigations along a transect ranging from the Variscan basement to the contact with the Schistes Lustrés are used to construct a rheological model for the ETSZ during its polyphase deformation history. The progressive transformation of the isotropic granitoid protolith into gneisses, shear zones, and, locally, phyllonites is described. The textural/mineralogical change suggests a concurrent increasing metasomatism associated with structurally controlled fluid flow. The effect of such textural/mineralogical evolution on the bulk rheology of the ETSZ is estimated. Rheological flow laws are obtained using an averaging procedure based on the integration of single-phase rheological behavior. The flow laws are used to infer strain rates and construct strength envelopes for each structural domain during progressive deformation, which highlight the relative rheological differences of the main textural components of the ETSZ and the variations in their brittle/ductile transition depths. Two competing processes are inferred. On the one hand, the combined effects of the feldspar-to-mica reaction and the development of a strong planar fabric induce weakening and strain localization along shear zones. On the other hand, fluid channeling along these shear zones enhances alkali feldspar neoblastesis, inducing strain hardening. Among the possible consequences of such feedback processes between strain localization andfluid-rock interaction are episodes of rheological transitions fromductile to brittle behavior during the polyphase tectonic evolution of the ETSZ