1,721,005 research outputs found
Monazite dissolution-reprecipitation in medium-grade metasedimentary rocks from the northern Apennines, Italy
No full textFluid-induced dissolution of parent phases and precipitation of products is a fundamental process in metamorphism
(Putnis & Austrheim, 2010). Here we present the dissolution-reprecipitation behaviour of monazite, one of the main
rare-earth element (REE)-bearing accessory minerals useful for obtaining geochronological and geochemical
information from a wide range of Ca-poor crustal rocks. The reactions involving monazite were studied in a micaschist
recovered from the Pontremoli 1 well (Tuscany) and the implications of fluid-controlled monazite breakdown on age
dating were discussed (Lo Pò et al., 2016). The micaschist belongs to the Variscan basement of the Northern Apennines.
In this rock monazite, either disseminated in the matrix or included in white mica and chlorite partially replacing garnet,
is surrounded by coronitic microstructures consisting of concentric shells of apatite + Th-silicate, allanite, epidote.
Studies of the element partitioning between garnet and accessory minerals and the garnet inclusion mineralogy suggest
that monazite was partially dissolved and replaced by apatite, allanite and epidote during garnet breakdown to chlorite
and muscovite. This stage was associated with the retrograde and decompressional evolution of the micaschist at fluidpresent
conditions. Through thermodynamic modelling, we determined the P-T conditions of the monazite partial
dissolution and replacement process at 510 ± 35°C, during a nearly isothermal decompression path from 8 kbar to 2-3
kbar. Fluid-induced alteration of monazite also resulted in a partial resetting of the monazite ages, which were
determined to be between 294 and 19 Ma. The maximum extent of the alteration process occurred in monazite located
within retrogressed garnet rims. In this microstructural site, Pb in the Variscan monazite was lost.
Lo Pò, D., Braga, R., Massonne, H.-J., Molli, G., Montanini, A., Theye, T. (2016): Fluid-induced breakdown of monazite in mediumgrade
metasedimentary rocks of the Pontremoli basement (Northern Apennines, Italy). J. Metamorph. Geol., 34, 63-84.
Putnis, A. & Austrheim, H. (2010): Fluid-induced processes: metasomatism and metamorphism. Geofluids, 10, 254-269
Monazite dissolution-reprecipitation in medium-grade metasedimentary rocks from the Northern Apennines, Italy.
No full textFluid-induced dissolution of parent phases and precipitation of products is a fundamental process in metamorphism
(Putnis & Austrheim, 2010). Here we present the dissolution-reprecipitation behaviour of monazite, one of the main
rare-earth element (REE)-bearing accessory minerals useful for obtaining geochronological and geochemical
information from a wide range of Ca-poor crustal rocks. The reactions involving monazite were studied in a micaschist
recovered from the Pontremoli 1 well (Tuscany) and the implications of fluid-controlled monazite breakdown on age
dating were discussed (Lo Pò et al., 2016). The micaschist belongs to the Variscan basement of the Northern Apennines.
In this rock monazite, either disseminated in the matrix or included in white mica and chlorite partially replacing garnet,
is surrounded by coronitic microstructures consisting of concentric shells of apatite + Th-silicate, allanite, epidote.
Studies of the element partitioning between garnet and accessory minerals and the garnet inclusion mineralogy suggest
that monazite was partially dissolved and replaced by apatite, allanite and epidote during garnet breakdown to chlorite
and muscovite. This stage was associated with the retrograde and decompressional evolution of the micaschist at fluid-
present conditions. Through thermodynamic modelling, we determined the P-T conditions of the monazite partial
dissolution and replacement process at 510 ± 35°C, during a nearly isothermal decompression path from 8 kbar to 2-3
kbar. Fluid-induced alteration of monazite also resulted in a partial resetting of the monazite ages, which were
determined to be between 294 and 19 Ma. The maximum extent of the alteration process occurred in monazite located
within retrogressed garnet rims. In this microstructural site, Pb in the Variscan monazite was lost.
Lo Pò, D., Braga, R., Massonne, H.-J., Molli, G., Montanini, A., Theye, T. (2016): Fluid-induced breakdown of monazite in medium-
grade metasedimentary rocks of the Pontremoli basement (Northern Apennines, Italy). J. Metamorph. Geol., 34, 63-84.
Putnis, A. & Austrheim, H. (2010): Fluid-induced processes: metasomatism and metamorphism. Geofluids, 10, 254-269
Styles and regimes of orogenic thickening in the Peloritani Mountains (Sicily, Italy): new constraints on the tectono-metamorphic evolution of the Apennine belt
No full textThe Peloritani Mountains constitute the Sicilian portion of the Calabria–Peloritani Arc (Italy), a tectono-metamorphic edifice recording the history of the subduction–exhumation cycle during Tertiary convergence between the African and European plates. Here, we describe the kinematic and the petrological characteristics of the major shear zones bounding the lowermost continental-derived metamorphic units cropping out in the eastern portion of the Peloritani Mountains. Both meso- and micro-scale shear sense criteria indicate a top-to-the-SSE tectonic transport, during a general evolution from ductile to brittle deformation conditions. Quantitative thermobarometry on texturally equilibrated phengite–chlorite pairs crystallized along the shear bands indicates pressure of 6–8 kbar at temperatures of 360–440 °C for the structurally highest units and 3–4 kbar at 380–440 °C for the lowest ones. This documents an overall inverse-type nappe arrangement within the tectonic edifice and a transition from an Alpine- (13–18 °C km−1) to a Barrovian-type (28–36 °C km−1) geothermal gradient during the progress of the Alpine orogenic metamorphism in the Peloritani Mountains. The integration of these results allows the Peloritani Mountains to be considered as a constituent element of the Apennine orogenic domain formed during the progressive space–time transition from oceanic to continental subduction at the active convergent margin
Feedback between fluid infiltration and rheology along a regional ductile-to-brittle shear zone: The East Tenda Shear Zone (Alpine Corsica)
No full textThe 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
P-T conditions of mylonitic shearing in the Granite Harbour intrusive complex of the Wilson Terrane, Deep Freeze Range, northern Victoria Land, Antarctica
No full textIntensive Fluid-Rock Interaction and Rheology of the East Tenda Shear Zone (Haute Corse, France)
No full textFeedback between fluid flow and rheology during the evolution of the East Tenda Shear Zone (Haute Corse, France).
No full textThe East Tenda Shear Zone (ETSZ) is the major Alpine tectonic boundary marking the overthrusting of the oceanicderived
Schistes Lustrés nappe onto the Hercynian crystalline basement of western Corsica. In this work we present
new structural and geochemical investigations along a transect ranging from the undeformed protolith (PR) to the
contact with the Schistes Lustrés. The results are used to construct a rheological model for the ETSZ.
Shear deformation within the ETSZ is heterogeneously distributed with high-strain domains (shear zones, SZ)
wrapping sigmoid shaped low-strain domains (massive lenses, ML). Locally, mica-rich mylonites occur (phyllonites).
The main foliation is concordant with that in the overlying Schistes Lustrés, strikes NW-SE, and is dominantly
shallow-dipping to NE. The ML mineralogy consists of an assemblage made of quartz, phengite and (relict)
feldspar (epidote, Fe-oxides, zircon and allanite as accessory phases). The SZ mineralogy is invariably dominated
by highly celadonitic (Si4+= 3.5-3.7 a.p.f.u.) phengite (40 ± 10 vol%) and modally abundant quartz (35 ± 5
vol%), albite (15 ± 5 vol%) epidote (<5 vol%) and microcline (10 ± 5 vol%). Locally, Na-amphibole (10-20
vol%) also occurs in the SZ assemblage to form thin (up to 1 m thick) dark mylonitic levels. Stretching lineations
strike WSW-ENE to E-W and consist of quartz-phengite-albite in ML and of Na-amphibole-quartz-albite-phengite
in SZ. Deformation is progressive and evolves from ductile-to-semibrittle conditions. Sense of shear is predominantly
top-to-the-SW and is locally reworked in the phyllonites with top-to-the-NE sense of shear.
Whole rock geochemistry suggests an increasing chemical alteration moving from the undeformed rocks to ML
and SZ. In particular, Ca++ is progressively leached while Na+ and K+ contents systematically increase as deformation
proceeds. Destabilization of Ca-bearing phases, such as plagioclase and epidote, and neoblastesis of
feldspars (albite and microcline) is consistently observed in the more evolved shear zones. These observations
indicate that progressive shear deformation was governed by intensive fluid-rock interaction characterized by increasingly
higher fluid/rock ratios.
The effect of chemical alteration of the host rock by fluids on the rheology of the ETSZ has been estimated taking
into account the modal composition and the fabric of the main lithotypes (PR, ML, SZ, phyllonites). Flow laws
are obtained using an averaging procedure based on weighted averages of single-phase rheology. These flow laws
are used to infer strain rates, construct deformation maps, and estimate the depth of the brittle-ductile transition
for each lithotype during progressive deformation. The combined effects of the feldspar-to-mica reaction and the
development of a strong planar fabric induce weakening and strain localization along the shear zones. Fluid channelling
along these shear zones enhances dominance of Na and K over Ca and, particularly, albite and microcline
neoblastesis. The latter, in turn, generates strain hardening. Among the possible consequences of such feedback
processes between strain localization and fluid-rock interaction are episodes of transient rheology.
The main result of our observations and rheological estimates is that reworking during top-to-the-E regional extension
occurred only in the uppermost part of the deforming crustal section and localized within the weaker
phyllonite levels
Pumpellyite-(Al), a new mineral from Bertrix, Belgian Ardennes
Full text linkpeer reviewedPumpellyite-(Al), ideally Ca-2(Al,Fe2+,Mg)Al-2(SiO4)(Si2O7)(OH,O)(2)center dot H2O, is a newly approved mineral species from Bertrix, Ardennes mountains, Belgium. It occurs as radiating fibrous aggregates reaching 5 mm in diameter, constituted by acicular crystals associated with calcite, K-feldspar and chlorite. Pumpellyite-(Al) is transparent to translucent and exhibits an emerald-green to white colour, sometimes with bluish tinges. The lustre is vitreous and the streak is colourless. The mineral is non-fluorescent, brittle, and shows a perfect {100} cleavage. The estimated Mobs hardness is 51/2, and the calculated density is 3.24 g/cm(3). Pumpellyite-(Al) is biaxial positive, alpha = 1.678(2), beta = 1.680(2), gamma = 1.691 (1) (lambda = 590 nm), colourless in thin section, 2V = 46 degrees, Y = b, no dispersion. Electron-microprobe analyses gave SiO2 37.52, Al2O3 25.63, MgO 1.99, FeO 4.97, MnO 0.11, CaO 23.21, BaO 0.01, Na2O 0.03, K2O 0.02, H2Ocalc. 6.71, total 100.20 wt. %. The resulting empirical formula, calculated on the basis of 8 cations, is (Ca1.99Na0.01)(Sigma 2.00)(Al0.42Fe0.332+Mg0.24Mn0.01)(Sigma 1.00)Al-2.00(SiO4)(Si2O7)(2.42) center dot 0.58H(2)O. The simplified formula is Ca2AlAl2(SiO4)(Si2O7)(OH)(3), which requires SiO2 38.16, Al2O3 32.38, CaO 23.74, H2O 5.72, Total 100.00 wt. %. The unit-cell parameters, refined from X-ray powder diffraction data, are: a = 8.818(2), b = 5.898(2), c = 19.126(6) angstrom, beta = 97.26(3)degrees, V = 986.7(4) angstrom(3), space group A2/m. The eight strongest lines in the powder pattern [d-values(in angstrom)(I)(hkl)] are: 4.371(65)(200), 3.787(80)(202), 3.040(70)(204), 2.912(95)(300), 2.895(100)(30 (2) over bar), 2.731(40)(20 (6) over bar), 2.630(35)(31 (1) over bar), 2.191(45)(40 (2) over bar). Pumpellyite-(Al) belongs to the pumpellyite group, and corresponds to the Al-rich compositions where the M1 and M2 sites contain Al as predominant cation. The crystal structure of pumpellyite-(Al) has been refined by the Rietveld method, based on an X-ray powder diffraction pattern, to R-Bragg = 7.09 %. The infrared spectrum is similar to those of minerals of the pumpellyite group. The mineral species and name were approved by the Commission on New Minerals and Mineral Names, IMA (no. 2005-016)
Sodic Pyroxene Bearing Phyllonites From the East Tenda Shear Zone: Constraining P-T Conditions and Timing of the Ligurian-Piemontese Ocean Overthrusting Onto the Variscan Corsica
No full textLinking the formation of coronitic microstructures around monazite to the growth of garnet in the Pontremoli well metapelites (Northern Apennines, Italy).
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