Search results for "Metamorphic facies"
showing 10 items of 41 documents
The effect of Mn on mineral stability in metapelites revisited: new a -x relations for manganese-bearing minerals
2014
The a-x relations recently presented in White et al. (, Journal of Metamorphic Geology, 32, 261-286) are extended to include MnO. This provides a set of internally consistent a-x relations for metapelitic rocks in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (MnNCKFMASHTO) system. The mixing parameters for the Mn-bearing minerals were estimated using the micro-ϕ{symbol} approach of Powell et al. (, Journal of Metamorphic Geology, 32, 245-260). Then the Mn-end-member thermodynamic properties were calibrated using a database of co-existing minerals involving literature data from rocks and from experiments on natural materials. Mn-end-members were calibrated for orthopyroxene, cordierit…
On equilibrium in non-hydrostatic metamorphic systems
2018
Metamorphic geology has accumulated a huge body of observation on mineral assemblages that reveal strong patterns in occurrence, summarised for example in the idea of metamorphic facies. On the realisation that such patterns needed a simple explanation, there has been considerable a posteriori success from adopting the idea that equilibrium thermodynamics can be used on mineral assemblages to make sense of the patterns in terms of, for example, the pressure and temperature of formation of mineral assemblages. In doing so, a particularly simple implicit assumption is made, that mineral assemblages operate essentially hydrostatically. Structural geologists have studied the same rocks for diff…
Fluid migration above a subducted slab - Thermodynamic and trace element modelling of fluid-rock interaction in partially overprinted eclogite-facies…
2011
Abstract The amount and composition of subduction zone fluids and the effect of fluid–rock interaction at a slab–mantle interface have been constrained by thermodynamic and trace element modelling of partially overprinted blueschist-facies rocks from the Sesia Zone (Western Alps). Deformation-induced differences in fluid flux led to a partial preservation of pristine mineral cores in weakly deformed samples that were used to quantify Li, B, Sr and Pb distribution during mineral growth, -breakdown and modification induced by fluid–rock interaction. Our results show that Li and B budgets are fluid-controlled, thus acting as tracers for fluid–rock interaction processes, whereas Sr and Pb budge…
Partial melting of metagreywacke: a calculated mineral equilibria study
2008
Greywacke occurs in most regionally metamorphosed orogenic terranes, with depositional ages from Archean to recent. It is commonly the dominant siliciclastic rock type, many times more abundant than pelite. Using calculated pseudosections in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system, the partial melting of metagreywacke is investigated using several natural protolith compositions that reflect the main observed compositional variations. At conditions appropriate for regional metamorphism at mid-crustal depths (6–8 kbar), high-T subsolidus assemblages are dominated by quartz, plagioclase and biotite with minor garnet, orthoamphibole, sillimanite, muscovite and/or K-feldspar (±Fe–T…
Migmatites in the Ivrea Zone (NW Italy): Constraints on partial melting and melt loss in metasedimentary rocks from Val Strona di Omegna
2013
Abstract The mid to lower crustal metamorphic field gradient through amphibolite and granulite facies rocks in the Ivrea Zone offers the potential to study partial melting and melt loss in the crust. Metapelitic rocks in Val Strona di Omegna show a progressive evolution in migmatite structures from metatexites with rare isolated leucosome veins in the amphibolite facies rocks to stromatic migmatites and diatexites in granulite facies rocks. Little field or petrographic evidence for melting can be seen on crossing the position of the modelled wet solidus, consistent with the small amounts of melt predicted to occur by H2O-saturated melting. The first field evidence for partial melting, in th…
Long-term deformation and fluid-enhanced mass transport in a Variscan peridotite shear zone in the Ivrea Zone, northern Italy: a microtextural, petro…
1995
The microtextural, petrological and geochemical study of a ductile shear zone in the phlogopite peridotite of Finero/Ivrea Zone (northern Italy) reveals the long-term deformation of this zone. The zone is divided into a protomylonitic and an ultramylonitic part. Both parts reflect different periods of deformation, although the orientation of the mineral lineations does not change. In the coarse-grained part (period 1) the deformation started under granulite facies conditions (about 775°C). Olivine, ortho- and clinopyroxene and phlogopite recrystallized dynamically. In the ultramylonitic part relics of the granulite facies event and evidence for a continuous or two-stage deformation history …
Eclogite xenoliths from Kimberley, South Africa — A case study of mantle metasomatism in eclogites
2009
Abstract Eclogite xenoliths from the Kimberley diamond mine dumps were studied for major, trace elements and oxygen isotopes. The suite consists of typical garnet and clinopyroxene rocks as well as kyanite-eclogites and olivine- or orthopyroxene-bearing samples. All samples have Al[6]/Al[4] ratios > 2 which indicate high equilibration pressures, i.e. in the eclogite stability field as opposed to pyroxenites. Most samples are modally and cryptically metasomatized and contain up to 10 vol.% phlogopite plus ilmenite and rutile. Trace element geochemical characteristics of phlogopite and Ti-oxides are similar to those described for metasomatized peridotite xenoliths from the Kaapvaal craton, an…
Roman quarries of the Iberian Peninsula: "Anasol" and "Anasol-type".
2007
The Iberian peninsula is rich in marble, both white and coloured, of excellent quality, used in the past (Pliny NH, XXXVI, II) and also at the present time as material for building and decoration. Many types of these marbles are similar to some, better known, Greek, Italian, Egyptian and Asian marbles. During the Roman Empire “ marmor carystium ” was extensively used. Taking into account the great similarity of this marble with the “ anasol ” and “ anasol ”-types mined in Spain and Portugal, this work presents a minero-petrographic, chemical and geochemical characterisation of these lithotypes, aiming at finding parameters to distinguish them from the better-known Greek and Italian “ cipoll…
Local disequilibrium of plagioclase in high-temperature shear zones of the Ivrea Zone, Italy
1995
Microstructural and chemical analysis of plagioclase in 20 superficially similar amphibolite facies ductile shear zones in metagabbors and amphibolites of the Ivrea Zone in Italy reveals significant differences in An and Ba contents. Plagioclase, which was deformed at P-T conditions lower than those of the wall rocks, occurs in the following four different microstructural situations with different chemical compositions: (i) relatively undeformed porphyroclasts, (ii) dynamically recrystallized grains and subgrains rimming the porphyroclasts, (iii) infill of microcracks cross-cutting the porphyroclasts and (iv) fine-grained recrystallized grains in the matrix of the shear zones. The differenc…
Contrasting metamorphic evolution of metasedimentary rocks from the Çine and Selimiye nappes in the Anatolide belt, western Turkey
2003
P-T conditions, mineral isograds, the relation of the latter to foliation planes and kinematic indicators are used to elucidate the tectonic nature and evolution of a shear zone in an orogen exhumed from mid- crustal depths in western Turkey. Furthermore, we discuss whether simple monometamorphic fabrics of rock units from different nappes result from one single orogeny or are related to different orogenies. Metasedimentary rocks from the Cine and Selimiye nappes at the southern rim of the Anatolide belt of western Turkey record different metamorphic evolutions. The Eocene Selimiye shear zone separates both nappes. Metasedimentary rocks from the Cine nappe underneath the Selimiye shear zone…