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The Role and Significance of Pegmatites in the Central Alps. Proxies of the Exhumation History of the Alpine Nappe Stack in the Lepontine Dome

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The Role and Significance of Pegmatites in the Central Alps. Proxies of the Exhumation History of the Alpine Nappe Stack in the Lepontine Dome Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010 THE ROLE AND SIGNIFICANCE OF PEGMATITES IN THE CENTRAL ALPS. PROXIES OF THE EXHUMATION HISTORY OF THE ALPINE NAPPE STACK IN THE LEPONTINE DOME Ph.D. candidate: ALESSANDRO GUASTONI Tutors: Prof. GILBERTO ARTIOLI, Prof. GIORGIO PENNACCHIONI Cycle:XXIV Abstract The pegmatitic field is located between Domodossola town (Western-Central Alps) and the Masino-Bregaglia Oligocene intrusion (Central Alps) ). Most of the pegmatitic dikes are hosted into the Southern Steep Belt (SSB), an alpine migmatitic unit characterized by high temperature greenschist and amphibolitic facies related to Barrovian metamorphism. Swiss authors aged some aplite-pegmatites between 29-26 m.y. Most of pegmatitic dikes (>90%) are composed by K-feldspar and subordinate quartz+ muscovite. A small population of more “geochemically evolved” population of pegmatites respectively belong to the LCT (lithium, cesium, tantalum) and NYF (niobium, yttrium, fluorine) families. Studies will focuse: to isotopes to define age of crystallizations and rate of coolings of deformed and miarolitic pegmatites; to geochemistry and relate the genesis of Alpine pegmatites to migmatites terrains of the Alpine Barrovian-type metamorphic belt or to granitic-tonalitic liquid melts of Masino-Bregaglia intrusion. Introduction Most of the pegmatitic dikes are hosted by the Southern Steep Belt (SSB), an alpine migmatitic unit, which includes several Lepontine nappes like the Antigorio, the Monte Rosa, the Camughera-Moncucco- Isorno-Orselina and the Adula. Alpine metamorphism in Central Alps is poliphasic and characterized by high temperature greenschist and amphibolitic facies related to Barrovian metamorphism during Oligocene-Miocene age (Burg & Gerya, 2005; Burri et al., 2005; Maxelon & Mancktelow, 2005). The changing from greenschists to amphibolites metamorphic grade in the Central Alps involves a main changing with depth in the fragile/ductile rheology of crustal rocks thrusted during the formation of the Lepontine Alps. Along SSB ductile deformations of pegmatites in amphibolitic facies are overprinted by fragile deformations in greenschists. Pegmatittes hosted by ultramafic rocks are deeply fenitized (albitized) and those in contact with the Masino-Bregaglia pluton are marked by synmagmatic deformations. Field studies based on textural, and mineralogical observations and structural relationships of pegmatitic dikes with the hosting rocks which outcrop in the Central Alps allow them to be assigned to different families and classes as reported in the classification of Ćerný & Ercit (2005). Most of pegmatitic dikes (90-95%) can be defined as “barren” being composed by K-feldspar and subordinate quartz + muscovite. A minor percentage (5%) can be assigned to a population of “geochemically evolved” pegmatites which contain accessory mineral phases, respectively belonging to the LCT (lithium, cesium, tantalum) and NYF (niobium, yttrium, fluorine) families as defined in the Cerny-Ercit classification . An indicative classification of the pegmatites population in the Western-Central Alps is the following: LCT pegmatites: Rare elements-Li class Li-beryl-columbite (Codera valley, Vigezzo valley, Bellinzona area) with beryl, chrysoberyl, columbite-Mn, muscovite, biotite, schorl, almandine-spessartine garnet uraninite, tapiolite, zircon, microlite, U-silicates, REE-phosphates and secondary Be-silicates Li-beryl-phosphates (Bodengo valley) with beryl, muscovite, biotite, schorl, almandine-spessartine garnet, tapiolite-Mn, columbite-Mn, zircon, graftonite, ferrisicklerite, stanekite, vivianite and secondary Be-silicates 1 Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010 Li-complex elbaite-beryl-phosphates (Codera valley) with muscovite, biotite, Mn-rich elbaite, schorl, triplite, unclassified Mn-phosphates, microcline var. amazzonite, colourless to pinkish beryl, spessartine garnet Albitized Li-beryl-columbite (Alpe Rosso and Pizzo Marcio, Vigezzo valley) with beryl var. emerald, tremolite, phlogopite-talc, schorl-dravite, almandine-spessartine garnet, fersmite, vigezzite, tapiolite, wodginite, microlite-pyrochlore, secondary Be-silicates, zeolites, cheralite, titanite, zircon, vanadinite LCT pegmatites: Miarolitic-Li class Li-beryl-tantalite (Bellinzona area) with beryl var. acquamarine, schorl-elbaite, muscovite, almandine-spessartine, tantalite-columbite-Mn, wodginite, zircon, secondary Be-silicates NYF pegmatites: Rare elements class REE-euxenite group minerals-monazite (Codera valley) with biotite, muscovite, almandine-spessartine, euxenite-(Y), Sc-rich columbite, xenotime-(Y), monazite- (Ce), and rare beryl REE-allanite, euxenite group minerals, gadolinite-(Y) (Vigezzo valley) with albite var. clevelandite, muscovite, biotite, Y-rich fluorite, Y-spessartine garnet, allanite-(Y), aeschynite-(Y), gadolinite-(Y), monazite-(Ce), pyroclore group minerals NYF pegmatites: Miarolitic –REE (Bellinzona area) with biotite, almandine-spessartine garnet, monazite-(Ce), allanite-(Ce), zircon References BURG J.P. and GERYA T.V. 2005. The role of viscous heating in Barrovian metamorphism of collisional orogens: thermomechanical models and application to the Lepontine Dome in the Central Alps. J. Metamorphic Geol., 23, 75-95. BURRI T., BERGER A. and MARTIN E. 2005. Tertiary migmatites in the Central Alps. Schweiz. Mineral. Petrogr. Mitt., 85, 215- 232. ĆERNÝ P. and ERCIT S. 2005. The classification of granitic pegmatites revisited. Can. Mineral., 43, 2005-2026. MAXELON M. and MANCKTELOW N.S. 2005. Three-dimensional geometry and tectonostratigraphy of the Pennine zone, Central Alps, Switzerland and Northern Italy. Earth Science Rewievs 71, 171-227. 2 Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2009-2010 SUMMARY LAST YEAR’S ACTIVITY Courses: Scuola AIC-IUCr adsorption, absorption and crystal growth, Gargnano, Brescia International school on carbonatites, San Venanzo, Terni Communications: Guastoni, A., Artioli, G., Pennacchioni, G. - The role of pegmatites in the Central Alps. Proxies of the exhumation history of the Alpine nappe stack in the Lepontine Dome. 20° General Meeting, IMA, Budapest, UngheriaAS REFERENCES. YOUR NAME Posters: Guastoni A., Nestola F. - Sn-rich thortveitite intergrowth with xenotime-(Y): Y versus Sc fractionation in NYF miarolitic pegmatites at Baveno (Southern Alps, Italy). Poster session, 20° General Meeting, IMA, Budapest, Ungheria Pandolfo, F. , Guastoni, A., Nestola, F., Cámara, F. - Crystal-chemistry study of an outstanding sarcolite gemstone. Poster session, 20° General Meeting, IMA, Budapest ISI Publications: Guastoni A., Kondo D., Nestola F. – Bastnäsite-(Ce) and parisite-(Ce) from Mount Malosa. Gems & Gemology, Vol. 46, 42- 47. Guastoni A., Càmara F., Nestola F. – Arsenic-rich fergusonite-beta-(Y) from Mount Cervandone (Western Alps, Italy): crystal structure and genetic implications. American Mineralogist, Vol. 95, 487-494 Teaching activities: Didattica di terreno: Campo finale di valutazione, Bastia, Corsica 3 .
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