LCT (Lithium, Cesium, Tantalum) and NYF (Niobium, Yttrium, Fluorine) pegmatites in the Central Alps. Proxies of exhumation history of the alpine nappe stack in the lepontine dome

The study concerns pegmatites ourcropping in the Central Alps where geochemical, structural, petrological and mineralogical analysis were performedLo studio riguarda le pegmatiti delle Alpi centrali caratterizzate da un punto di vista geologico, petrologico, geochimico e mineralogic

Tertiary LCT and NYF pegmatites of the central Alps

A large field (about 100 km in E-W length and 15 km in N-S thickness) of Oligocene pegmatites extends in the central Alps from the Bergell pluton (to the east) to the Ossola Valley (to the west) within the Alpine nappes north of the Periadriatic Lineament. The pegmatite field geographically overlaps (i) the highest temperature domain of the Lepontine Barrovian metamorphic dome, and (ii) the zone of Alpine migmatization. Most pegmatites have a simple mineralogy consisting of K-feldspar, quartz and muscovite, but a minor amount (< 5%) includes Sn-Nb-Ta-Y-REE-U oxides, Y-REE phosphates, Mn-Fe-phosphates, Ti-Zr-silicates, Be-Y-REE-silicates, garnet (almandine-spessartine), and schorl-dravite-fluorelbaite tourmaline. Major and trace elements geochemistry of pegmatite bulk rock, rock-forming and accessory minerals al- lows the distinction of different pegmatite populations ranging from NYF (niobium, yttrium, fluorine) to LCT (lithium, cesium, tantalum) pegmatites, or mixed LCT-NYF ones. Actually, LCT pegmatites of the Central Alps did not reach a high degree of geochemical evolution. In the Codera Valley (on the western side of the Bergell pluton) LCT and NYF pegmatites are respectively hosted in tonalites and granodiorites; these pegmatites include the most evolved types which contain Mn-fluorelbaite, Mn-phosphates, pink-beryl and Cs-Rb-rich feldspar. From the structural point of view 2 main types of pegmatites can be distinguished: (i) pegmatites that were involved in ductile deformation, and (ii) pegmatite crosscutting the ductile structures of the SSB. Many peg- matites from Codera Valley belong to the first structural type: they were at emplaced at relatively high ambient temperatures (> 450 \u25e6 C) and locally show pervasive recrystallization of quartz. More to the east (Mesolcina and Bodengo Valleys) the main set of pegmatites crosscut the ductile deformation structures of the SSB, but the area also includes an earlier generation of boudinaged and folded dykes. The undeformed pegmatites from this area may contain large miarolitic pockets. There is no systematic difference in mineralogy and geochemistry between the 2 structural types of pegmatites. Structural data and the few existing radiometric ages suggest that pegmatites were emplaced over a time span between 29 and 25 Ma with the youngest dykes postdating the ductile deformations of the Alpine nappes. The present work presents a first comprehensive geochemical and mineralogical classification of the Oligocene pegmatite field of the central Alps. In order to constrain the timing of pegmatite formation monazite and xenotime have been sampled from the different generations of pegmatite

Age determination by \ub5-PIXE analysis of cheralite-(Ce) from emerald-bearing Pegmatites of Vigezzo Valley (Western Alps, Italy).

A green transparent homogenous crystal of cheralite-(Ce) was separated from an albitized pegmatitic dike outcropping at the summit of Mount Pizzo Marcio (Vigezzo valley, Western Alps, Verbania, Italy) and analysed with a proton-induced X-ray emission microprobe (\u3bc-PIXE) for total U-Th-Pb age determination. 9 spot analyses have been performed, obtaining ages in the range 30.4-34.3 Ma, with errors on single analysis of \ub11.6 Ma. These ages are not statistically different and do not show systematic distributions within the analysed crystal (i.e. either in the core or in the rim). Therefore an average age of 32.7 Ma was calculated, with a propagated error of \ub1 3.2 Ma, representing the youngest total U-Th-Pb age ever obtained on a mineral of the monazite group. Such an age indicates that during the Alpine event two potential sources of pegmatitic magmas producing the Vigezzo valley pegmatitic field can be taken into consideration. A first potential magmatic source is represented by the granodioritic-tonalitic Masino-Bregaglia pluton, aged from 32.9 to 28 Ma., associated with the peraluminous two micas granitic stock of S. Fedelino 25 Ma old (MOTICSKA, 1970, BERGER et al., 1996, HANSMANN, 1996). A second potential source could be related to the Barrovian metamorphism responsible for the development of the so called Lepontine Gneiss Dome (WINTER, 2001), that affected the Central Western Alps during Oligocene. Such metamorphism was concomitant with dextral strike-slip movements along the Insubric Line. Subsequently, the main thermal updoming producing migmatites, which started about 32-30 Ma ago with the intrusion of the Masino-Bregaglia pluton, migrated towards west reaching the metamorphic peak in the Simplon Alpine region, about 20 Ma ago (ENGI et al., 1995, BOSQUET et al., 1997 )

What is the actual structure of samarskite-(Y)? A TEM investigation of metamict samarskite from the garnet codera dike pegmatite (Central Italian Alps)

We investigated, by scanning and transmission electron microscopy (SEM, TEM), wavelength-and energy-dispersive spectroscopy (WDS, EDS), and electron diffraction tomography (EDT), several (Y-REE-U-Th)-(Nb-Ta-Ti) oxides from the Garnet Codera dike pegmatite (Central Italian Alps). These oxides have compositions in the samarskite-(Y) field and yield an amorphous response from the single-crystal X-ray diffractometer. Backscattered electron images reveal that the samples are zoned with major substitutions involving (U+Th) with respect to (Y+REE). At the TEM scale, the samples show a continuous range of variability both in terms of composition and in radiation damage, and the amount of radiation damage is directly correlated with the U-content. Areas with high U-content and highly damaged show crystalline, randomly oriented nanoparticles that are interpreted as decomposition products of the metamictization process. On the other hand, areas with lower U-content and radiation dose contained within 0.7 × 1016α-event/mg, although severely damaged, still preserve single-crystal appearance. Such areas, noticeably consisting of relicts of the original samarskite structure, were deeply investigated by electron diffraction techniques. Surprisingly, the retrieved crystal structure of untreated samarskite is consistent with aeschynite and not with ixiolite (or columbite), as believed so far after X-ray diffraction experiments on annealed samples. In particular, the resolved structure is a niobioaeschynite-(Y), with Pnma space group, cell parameters a = 10.804(1), b = 7.680(1), c = 5.103(1) Å, and composition (Y0.53Fe0.22Ca0.10U0.09Mn0.07)Σ=1(Nb1.07Ti0.47Fe0.34Ta0.07W0.06)Σ=2O6. If this finding can be confirmed and extended to the other members of the group [namely samarskite-(Yb), calciosamarskite, and ishikawaite], then the samarskite mineral group should be considered no longer as an independent mineral group but as part of the aeschynite group of minerals. It is finally suggested that the rare crystalline sub-micrometric ixiolite domains, occasionally spotted in the sample by TEM, or the nanoparticles detected in highly metamict areas interpreted as decomposition product of the metamictization process, which may have in fact the ixiolite structure, act as seeds during annealing, leading to the detection of ixiolite peaks in the X-ray powder diffractograms