Pozzi, Giacomo (2014) Tertiary pegmatite dikes in the Codera and Bodengo areas of the Central Alps. [Magistrali biennali]
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The tertiary pegmatite field of the Central Alps extends for about 100 km in an E–W direction from the Bergell pluton (to the east) to the Ossola valley (to the west). It intrudes the steepened roots of the Alpine nappes (Souther Steep Belt) north of the Periadriatic Fault and gradually fades within 15 km toward north. The pegmatite field geographically overlaps (1) the highest temperature domain of the Lepontine Barrovian metamorphic dome and (2) the zone of Alpine migmatization. In this thesis is attempted a first multisciplinary study on these pegmatites over a limited surface of the whole extension of the pegmatite field. Pegmatites were studied in two different areas: (1) the Codera area, located on the western border of the Bergell Pluton, and (2) the Bodengo area, located between the Mera and the Mesolcina valleys. Results show that Codera and Bodengo pegmatites differ under many aspects including structural, geochemical and radiometric data. The main set of pegmatite dikes of Codera area is steeply dipping and strikes WSW-ENE. Crosscutting relationships suggest the presence of at least two generations. All dikes were involved in ductile deformation and in some cases localize mylonitic shear zones. Codera pegmatites were emplaced at relatively high temperature of at least 500°C, which is constrained by the mineral assemblage along the mylonites and the strong CPO of recrystallized quartz (with c-axis maximum in the direction of the Y kinematic axis of the mylonite). The main set of pegmatites of Bodengo area trends approximately N-S to NNE-SSW and crosscuts the ductile deformation structures of the SSB. Most of these dikes are undeformed but some show ductile reactivation at the borders. Bodengo area also includes an earlier generation of boudinaged and folded pegmatite dikes oriented at a small angle to the host rock foliation. Miarolitic pockets are contained only in some undeformed dikes of Bodengo area. The mineralogical content of pegmatites of the two areas does not differ substantially. Most pegmatites have a simple mineral assemblage consisting of K-feldspar, quartz, and muscovite ± biotite, and only a minor percentage of the dikes contain rare accessories including almandine-spessartine garnet, pale blue beryl and schorl tourmaline. On the other hand chemical analyses on minerals, especially on garnets, allow distinguishing further the two selected areas. Codera garnets are systematically richer in spessartine than Bodengo ones and some Codera tourmalines show higher degrees of evolution (toward elbaite compositions) than Bodengo schorls. Radiometric data of monazite crystals yielded different 208Pb/232Th ages for Codera and Bodengo pegmatites, which do not overlap and are respectively older and younger than 24 Ma. We concluded that pegmatites of Codera area and Bodengo area belong to different intrusion events.
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