1,721,329 research outputs found
Age determination by μ-PIXE analysis of cheralite-(ce) from emerald-bearing pegmatites of Vigezzo valley (Western Alps, Italy).
No full textPhosphates in archaeological finds: implications for environmental conditions of burial.
No full textThe occurrence of coarse-grained vivianite and mitridatite aggregates in a potsherd, a grand ring and a timber imprint from the Second Iron Age site of Adria (Rovigo, northeastern Italy) suggest contrasting environmental conditions of burial. In particular, bone fragments were replaced by vivianite at relatively low pH and Eh, due to the presence of deteriorating organic matter, together with slag and iron flakes. Subsequent interactions with Ca-rich groundwater characterized by higher pH and Eh determined the growth of mitridatite after vivianite. Although phosphates crystallized after burial, the examined samples were not involved in pervasive chemical contamination
Studio archeometrico di materiali ceramici. Sviluppo e applicazioni delle metodologie dâ€TMindagine mineralogiche e petrografiche allo studio di reperti archeologici
No full textCaratteri del metamorfismo ercinico nella fillade sudalpina ad ovest di Bressanone.
No full textThe aim of this paper is to contribute to the solution of the numerous problems concerning the South-Alpine crystalline basement of the Eastern Alps. An area located west of Bressanone (Brixen) (Fig. 1) was considered and the petrographic and microtextural features of its metapelites analyzed. The main rock-types are quartz-phyllites and phyllitic schists, within which metarhyolites ("porphyroids"), quartzites and metabasites are interlayered.
The distinction between the upper ("M. Cavallino Formation", "M. Cane Phyllites") and the lower ("Bressanone Phyllites") phyllitic complex is only possible according to mesostructural criteria: the metapelites making up the two complexes do display identical micro- and mesoscopic features. In particular, the almandine-biotite phyllites, which display the most complicated tectono-metamorphic history, have identical mineral assemblages in the two rock complexes, identical chemistry of the main mineral phases (Tables 1-4) and identical microtextural relationships between crystallization and deformation.
The tectono-metamorphic history of the considered metapelites was reconstructed. It consists of five crystallization stages, including two thermal climaxes which correspond to the higher-temperature part of the greenschists facies. The mineral compatibilities of each crystallization stage were also ascertained (Figs. 8-11).
Geothermometric estimations were deduced from the prevailing mineral assemblages (Fig. 13). Geobarometric estimations were based on the b value of the potassic white micas: the mean b value (8.774A) indicates low-pressure conditions of about 3 Kb. Considering the pattern of the b isopleths in the P-T field, an approximate value of 40°C/Km was assigned to the metamorphic thermal gradient of the Hercynian event
Further data concerning the pressure character of the Hercynian metamorphism in the West Carpathians (Czechoslovakia).
No full textNew data on the pressure character of the Hercynian metamorphism in the Czechoslovak West Carpadlians arc presented and discussed. Geobarometric estimations arc based on the bo values of muscovites from phyllites.
Three sample populations are considered, corresponding to a total of 172 new ho values. One sample population (phyllites from the Brusnik Anticline) gives a mean b0 value of 8.994 A (s = 0.005; n = 60). The other two sample populations (Stos Formation from the Gemericum; Predna Hola Complex from the
Veporicum) supply significantly lower b0 values, due to
the high Al-content of the rocks, shown by the systematic
occurrence of paragonite.
These new analytical data confirm that, in the West
Carpathians as wdl as in the central part of the Alpine-Mediterranean Belt, the Hercynian metamorphism
recorded in the phyllites, is of low-pressure (metamorphic
thermal gradient about 40°C/km), with only one
exception (Poiana Rusca, South Carpathians, Romania)
I frammenti delle matrici di fusione da Fondo Paviani (Verona) - Scavi Università degli Studi di Padova. Caratterizzazione tipologica e petrografica.
No full textAge determination by μ-PIXE analysis of cheralite-(Ce) from emerald-bearing Pegmatites of Vigezzo Valley (Western Alps, Italy).
No full textA 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 (μ-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 ±1.6 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 ± 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 )
Subduction related processes in the Borka Nappe (Inner Western Carpathians): a geochemical and petrological approach
No full textIn the Inner Western Carphatians, the rocks of the Bôrka Nappe record clear effects of subduction-related Alpine petrogenetic processes. They are metabasalts and associated metapelites.
Notwithstanding the mobility of some chemicals (alkalis and light elements) during metamorphism, the geochemistry of metabasalts show features which suggest either an oceanic or a back-arc scenario for their emplacement. Specifically, trace and RE elements indicate that these rocks have a sub-alkaline to moderately alkaline affinity, and their chemical features are consistent with those of OFB and BABB. Most of the normalised REE patterns show a slight LREE enrichment, a relatively high La/Sm ratio and a clear depletion of Lu, suggesting an E-MORB affinity for these rocks. However, some samples better fit a typical N-MORB chemistry.
As regards the metamorphic evolution of these metabasalts, Gln - Chl - Ep - Ab - Ttn +/- Phn and Na-Px - Gln - Chl - Ep - Ttn +/- Phn mineral assemblages indicate that these rocks underwent a blueschist facies metamorphism. This event was followed by a stage of isothermal decompression, recorded by the overprinted Act - Chl - Ep - Ab - Ttn mineral assemblage. These mineral phases nucleated at the rims of former Gln crystals, in veinlets and in small, irregularly shaped, coarser-grained domains within the matrix.
The associated metapelites are characterised by the Cld - Chl - Ab - Phn +/- Pg and Gln - Grt - Ab - Phn +/- Pg mineral assemblages, referable to the earlier HP metamorphic phase. Gln was destabilised during a later metamorphic stage and locally replaced by Chl - Qtz +/- Ab. The Grt - Chl - Ms - Bt - Ab and Cld(II) - Chl - Ms - Ab mineral assemblages therefore crystallised.
Both whole rock geochemistry and polyphase metamorphic evolution of metabasites and associated metapelites are consistent with a scenario involving subduction of an oceanic crust of a narrow ocean, followed by a rapid exhumation along an isothermal path
The gneisses of Monte Filau (Capo Spartivento, SW Sardinia): Petrographic and chemical features.
No full textThe Monte Filau orthogneisses show peculiar metamorphic features. They are probably "Caledonian" granites of anatectic-crustal origin which, before being intruded by the late Variscan granites, underwent two different metamorphic events. The older event caused their alteration into gneisses, with crystallization of andalusite, sillimanite and garnet, and the appearance of pockets of melts, in sites of suitable composition (T>650°C and P 3.5-4.8 Kb).The younger event had mainly cataclastic-mylonitic effects, with recrystallization in the Qtz-Ab-Ep-Chl subfacies. Local contact effects by the late Variscan granites concluded the metamorphic history of the M. Filau orthogneisses
- …
