1,721,248 research outputs found
Coronitic microstructures in patchy eclogitised continental crust : the Lago della Vecchia pre-Alpine metagranite (Sesia-Lanzo Zone, Western Italian Alps)
No full textThe Lago della Vecchia pre-Alpine coronitic metagranite (Sesia-Lanzo Zone, Western Italian Alps, Valle del Cervo) preserves igneous textures and mineralogy despite the complete eclogitization recorded by surrounding deformed metagranites and country rocks. The coronitic cores are metre to few metres undeformed metagranites preserving igneous cm-sized K-feldspar and mm-sized biotite. Microscale analysis shows that igneous association is partly preserved: biotite, white mica and K-feldspar are only partly replaced by Alpine assemblages, while the original plagioclase is pseudomorphically overgrown by aggregates of albite + phengitic mica + Fe-epidote ± garnet. Metamorphic reactions are also localized at the interfaces biotite – plagioclase and white mica – plagioclase. A continuous garnet corona with a distinctive partitioning of Ca2+ and Fe2+ occurs between biotite and plagioclase, suggesting diffusive mass transfer during a prograde burial path. Microdomains have been described and analysed in terms of their geometry, mineral composition and chemistry patterns using optical microscopy, X-rays compositional maps, chemical analyses and image analysis in order to detail the microstructures and to reconstruct the relative chronology of the reactions within each microdomain. The results allow demonstration of the chemical exchanges that occurred between adjacent igneous microdomains: Biotite microdomain at contact with plagioclase – two coronas are observed: 1) BtII + Phn-WmII + Alm-rich GrtI; 2) Phn-WmII + Grs-rich Grt coronas; White mica microdomain with plagioclase: a single Phn-WmII + Grt + Ep + Ab corona develops; K-feldspar microdomain with plagioclase: a single Ab + Phn-WmII ± Ep ± Grt corona develops; Plagioclase core microdomain: Ab + Phn-WmII (fine-grained) + Ep ± Grt;
microdomain: Ab + Phn-WmII (fine-grained) + Ep ± Grt; K-feldspar core microdomain: large igneous grains are partly to completely replaced by Ab from the plagioclase-K-feldspar boundaries or along fractures.
The absence of deformation seems to inhibit the complete development of eclogite-facies metamorphic parageneses, allowing the preservation of microstructures, mineral phases and chemistry. This information is commonly hidden by the widespread development of high pressure - low temperature eclogite-facies assemblages within surrounded deformed volumes
Kilometre-scale strain partitioning of the Mont Morion Permian metaintrusives during Alpine subduction (Dent-Blanche Unit, Valpelline - Western Italian Alps)
No full textLa presente carta mostra l’evoluzione strutturale e metamorfica dei litotipi dal Permiano ad oggi. Le rocce pre-Permiane preservate come incassanti all’interno degli intrusivi Permiani. Durante la riequilibrazione Alpina tali rocce sono state trasformate in metaintrusivi da coronitici a milonitici associati a gneiss a mica bianca e glaucofane. Sulla carta sono riportate le tracce delle foliazioni e dei piani assiali, distinte sulla base della loro cronologia relativa e mineralogia caratteristica. Tali informazioni raccolte in una singola carta permettono di ricostruire i diversi stadi evolutivi di tale porzione crostale, dall’estensione pre-Alpina, seguita dalla fase intrusiva Permiana, fino alla subduzione e relativa collisione Alpina.The structural and metamorphic evolution of Mont Morion Complex lithotypes, from pre-Permian to present, is presented. We identified pre-Permian rocks (e.g. amphibolite, biotite-bearing gneiss and acid granulite) preserved as roof pendants (i.e. xenoliths) within Permian intrusives. Permian intrusives and xenoliths are re-equilibrated during the Alpine time and coronitic to mylonitic metaintrusives are produced, due to meter to kilometer-scale fabric gradients, with associated white mica-, glaucophane-bearing gneiss. The tectono-metamorphic map shows the traces of the superimposed foliations and the fold axial planes. The foliations are distinguished on the basis of their relative chronology and mineralogical support. Using these structural data, reported on a single map, the successive stages of this portion of the Adria plate can be reconstruct: (i) a pre-Alpine extension, recorded by granulite- to amphibolite-facies xenolits; (ii) the Permian magmatic phase (e.g. Mont Morion, Mont Collon and Matterhorn intrusives); (iii) the Alpine subduction and subsequent collision
Prograde lawsonite during the flow of continental crust in the Alpine subduction : Strain vs. metamorphism partitioning, a field-analysis approach to infer tectonometamorphic evolutions (Sesia-Lanzo Zone, Western Italian Alps)
No full textDetailed mapping of superposed fabrics and their mineral support allows for reconstruction of the tectonometamorphic evolution of the Ivozio Complex, within the inner portion of the Sesia-Lanzo Zone (Western Italian Alps). The resulting evolution is characterized by a multi-stage structural and metamorphic re-equilibration during Alpine subduction, starting from the pre-Alpine igneous association (Amp0 + Cpx0). The prograde associations begin with S1a marked by AmpI + ZoI which pre-date the growth of GrtI (S1b); successive increase in pressure stabilizes a second generation of Amp + Grt (S1c AmpII + ZoI + GrtII). The growth of prograde lawsonite and omphacite occur during S1d (OmpI + Lws + GrtII + AmpII) within lawsonite-bearing eclogites, while S1e is associated with the break-down of lawsonite, producing the association OmpI + Ky + ZoII + GrtII + AmpII (lws-bearing eclogites); S1d-e stages are associated with AmpII + ZoI + GrtII + OmpI in eclogites. The second generation of penetrative foliation (S2), describing the retrograde evolution, is divided into S2a (AmpII + GrtII + Pg + ZoII) and S2b (Chl + AmpIII + Pg + Ab). The comparison between the reconstructed evolution of the Ivozio Complex and P-T paths inferred in the Southern Sesia-Lanzo Zone suggests a non-uniqueness of the Sesia-Lanzo Zone continental crust, during the Alpine subduction
Tectono-metamorphic map of the Mont Morion Permian metaintrusives (Mont Morion - Mont Collon - Matterhorn Complex, Dent Blanche Unit), Valpelline - Western Italian Alps
Full text linkThe presented map displays the structural and metamorphic evolution of lithotypes from pre-Permian to present. We distinguish pre-Permian rocks (e.g., amphibolite, biotite-bearing gneiss and acid granulite) preserved as roof pendants (i.e., xenoliths) within Permian intrusives. Permian intrusives and hosted xenoliths are then re-equilibrated during Alpine evolution, producing coronitic to mylonitic metaintrus-ives, due to meter to kilometer-scale fabric gradients, and associated white mica-, glaucophane-bearing gneiss. The map also shows the traces of the superimposed foliations and the fold axial planes. The traces are distinguished on the basis of their relative chronology and mineralogical support. This information, reported on a single map, allows us to reconstruct the successive stages of this fragment belonging to the African plate continental crust, from the pre-Alpine extension, recorded by granulite- to amphibolite-facies xenolits, to the Permian intrusive phase (e.g., Mont Morion, Mont Collon and Matter-horn intrusives) lasting with the Alpine subduction-collision related evolution. The Mont Morion, part of the Mont Morion-Mont Collon-Matterhorn Complex of the Dent Blanche unit, may be interpreted as a multi-stadial Alpine km-scale shear zone, where Permian intrusive rocks are transformed into white mica chlorite-bearing or glaucophane-bearing gneisses along high-strain horizons (100 m-thick), while within low-strain cores (100- to 1000 m-thick), meta-intrusives preserve igneous features and xenoliths of am-phibolites, acid granulites and biotite-bearing gneisses. In this paper, an outcrop tectono-metamorphic map (1:10,000 scale) is presented, based upon fieldwork at 1:5,000 together with an interpretative map (1:15,000 scale), in which three dimensional relationships are described, and micro- to mesoscopic fabric types are shown, corresponding to finite strain states recorded by rocks
Meso and microstructural evolution of the Mont Morion metaintrusive complex (Dent-Blanche nappe, Austroalpine domain, Valpelline, Western Italian Alps)
No full textThe Mont Morion complex of the Dent Blanche nappe is interpreted as a multi-stadial Alpine km-scale shear zone. Permian intrusive rocks are transformed into white mica chlorite-bearing or glaucophane-bearing gneisses along high-strain horizons (100m-thick), while within low-strain cores (100- to 1000m-thick), metaintrusives preserve igneous features and xenoliths of amphibolites, acid granulites and biotite-bearing gneisses.
Ten superposed evolutionary stages (1 to 10) occurred: from pre-Permian (1 and 2, only in pre-intrusive host rocks) to Permian-Triassic (3 and 4) and Mesozoic-Cenozoic (5 to 10). Stages 1 and 2 developed under amphibolite to granulite-facies conditions. During stage 3, the Mont Morion complex was intruded (P-range = 0.25-0.70 GPa at T between 600-760°C). Stage 4 occurred under greenschist facies conditions. Stages 5 and 6 are associated with the development of tectonitic mylonitic km-scale domains under blueschist facies conditions. Stage 7 is represented by folding of S6 under greenschist-facies conditions. Metre-scale shear zones developed during stages 8, 9 and 10. The pre-Alpine part of the P-T-t-d path is interpreted as trace of post-collisional thinning Variscan continental crust, related to the upwelling of magmas (e.g. Mont Collon-Matterhorn gabbros; Arolla granites); the Alpine part of the P-T-t-d path is interpreted as record of continental crust subduction and exhumation, related to the Alpine oceanic-continental subduction and continental collision
- …
