Tertiary Pegmatite Dikes of the Central Alps

Guastoni, Alessandro; Pennacchioni, Giorgio; Pozzi, Giacomo; Fioretti, A. M.; Walter, Jens M.

doi:10.3749/canmin.52.2.191

Cited by 20 publications

(20 citation statements)

References 36 publications

(52 reference statements)

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“…A virtually identical compositional trend was found in tourmaline from beryl-columbitephosphate pegmatites of the Sebago Pegmatite Group, Maine, USA (Wise & Brown 2010) which, however, sometimes contain abundant Li micas (and locally also pollucite and spodumene). Similar beryl-columbite phosphate pegmatites with Mn-rich elbaite have also been observed in the central Alps (Guastoni et al 2014, Vignola et al 2014.…”

Section: Mineral Fractionation Patternsmentioning

confidence: 87%

Mineralogy and Geochemistry of Pegmatites on Mount Begbie, British Columbia

Dixon¹,

Cempírek²,

Groat³

2014

Can Mineral

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The pegmatite group on the northern slope of Mount Begbie, near Revelstoke, B.C., comprises more than 50 relatively small pegmatite bodies. The pegmatites have diverse ranges of mineralogy; most of them are barren with micas and tourmaline as the only accessory minerals, but 12 dikes are more fractionated and correspond to beryl-columbite, beryl-columbite-phosphate, and lepidolite subtypes. Detailed study of tourmaline (dravite, schorl, fluor-elbaite, Mn-rich elbaite), cordierite-sekaninaite, garnet, rare element-bearing silicate minerals (beryl, chrysoberyl, bertrandite, euclase, trilithionite, Li-muscovite, petalite, pollucite), rare element-bearing oxide minerals (columbite-tantalite, bismutotantalite, Nb-rutile, cassiterite, hübnerite, qitianlingite), phosphate minerals (triplite, lithiophilite, Mn-rich apatite, xenotime, monazite), and zircon provides insight into the mineralogy and geochemistry of the individual dikes. Fractionation within the pegmatite group increases from the southeast to the northwest, suggesting that the source pluton may be located to the southeast of the study area at depth, likely related to the Ladybird granite suite. Elevated Be and Li contents in sekaninaite, the occurrence of Mn-rich elbaite in a beryl-columbite-phosphate pegmatite, and the relatively common presence of hübnerite and qitianlingite in the Mount Begbie pegmatite group are unusual compared to rare element pegmatite fields worldwide.

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Section: Mineral Fractionation Patternsmentioning

confidence: 87%

Mineralogy and Geochemistry of Pegmatites on Mount Begbie, British Columbia

Dixon¹,

Cempírek²,

Groat³

2014

Can Mineral

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“…Both the range in ages and crosscutting relationships indicate that seismicity on these faults was repetitive, as was fracturing in the Neves area. At the regional scale of the Alps, widespread Oligocene dikes are commonly oriented at a high angle to the long-term NNW-SSE shortening direction (Guastoni et al, 2014;Mancktelow et al, 2001). This dike orientation suggests a regional transient switch in stress axes during magmatic emplacement, similar to that producing the E-W set of discrete fractures in the Neves area.…”

Section: Discussionmentioning

confidence: 99%

Intermittent fracturing in the middle continental crust as evidence for transient switching of principal stress axes associated with the subduction zone earthquake cycle

Mancktelow

Pennacchioni

2020

Geology

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In the Neves area, eastern Alps, fractures that localized shear zones in middle continental crust above the Alpine megathrust are commonly oriented at a high angle to the inferred long-term shortening direction. Fractures show a segmentation geometry and, locally, a discernible offset, indicating movement opposite to the sense of subsequent ductile shear and implying a switch of principal stress axes σ1 and σ3 during fracturing. We propose that this repeated switch, demonstrated by overprinting relationships and different degrees of fracture reactivation, was due to sporadic co-seismic to early post-seismic rebound in the upper plate of the Alpine continental collision system. Fracturing occurred intermittently in the weak midcrustal rocks due to seismic stress release at high transient strain rates and pore-fluid pressures. Widespread transient fracturing in the hanging wall of the Alpine megathrust regionally controls the orientation of ductile shear zones in the middle crust, as well as the emplacement of magmatic dikes.

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“…The tertiary, oligocenic, Alpine pegmatite field [16,17] is located in the Central Alps, within the SSB (Southern Steep Belt) of the Alpine nappes. It extends for~100 km in an E-W direction and~15 km in a N-S direction north of the Periadriatic Fault, from the Bergell massif to the east, and the Ossola valley to the west (Figure 1).…”

Section: Geological Sketchmentioning

confidence: 99%

“…The hanging wall of the pegmatite in contact with the orthogneiss shows a lobate structure. The border zone, in the upper portion (upper end) of the dike, hosts albite, aeschynite-(Y), polycrase-(Y), xenotime-(Y) allanite-(Ce), yttrian-fluorapatite, zircon, and secondary cavities formed by the dissolution of yttrian-fluorite lined with gadolinite-(Y) and titanite [17]. The coarse-grained core zone is composed of euhedral pinkish to whitish perthite K-feldspar, brownish to colorless large masses of quartz and dark brown-black platy crystals of siderophyllite-annite, which host centimetric black granular masses of samarskite-(Y).…”

Section: Description Of Pegmatite Dikesmentioning

confidence: 99%

“…The Arvogno pegmatite dikes ( Figure 1) are unique within the tertiary alpine pegmatite field [16,17] showing marked geochemical and mineralogical NYF features. These pegmatites contain aeschynite-(Y), polycrase-(Y), and samarskite-(Y) associated with further typical NYF minerals such as Y-rich fluorite, Y-rich spessartine, allanite-group minerals, gadolinite-(Y), and xenotime-(Y).…”

Section: Introductionmentioning

confidence: 99%

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Non-Metamict Aeschynite-(Y), Polycrase-(Y), and Samarskite-(Y) in NYF Pegmatites from Arvogno, Vigezzo Valley (Central Alps, Italy)

et al. 2019

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At Arvogno, Vigezzo valley in the Central Alps, Italy, pegmatite dikes are unique in the scenario of a tertiary alpine pegmatite field because they show marked geochemical and mineralogical niobium–yttrium–fluorine features. These pegmatites contain AB2O6 aeschynite group minerals and ABX2O8 euxenite group minerals as typical accessory minerals including aeschynite-(Y), polycrase-(Y), and samarskite-(Y). They are associated with additional typical minerals such as fluorite, Y-dominant silicates, and xenotime-(Y). The Y–Nb–Ti–Ta AB2O6 and ABX2O8 oxides at the Arvogno pegmatites did not exhibit any textural and compositional features of oxidation or weathering. They are characterized by low self-radiation-induced structural damage, leading to the acquisition of unit-cell data for aeschynite-(Y), polycrase-(Y), and samarskite-(Y) by single-crystal X-ray diffraction. Aeschynite-(Y) and polycrase-(Y) crystals allowed for both to provide space groups whereas samarskite-(Y) was the first crystal from pegmatites for which cell-data were obtained at room temperature but did not allow for the accurate determination of the space group. According to the chemical compositions defined by Ti-dominant content at the B-site, the cell parameters, respectively, corresponded to polycrase-(Y), aeschynite-(Y), and the monoclinic cell of samarskite-(Y). Emplacement of Alpine pegmatites can be related to the progressive regional metamorphic rejuvenation from east to west in the Central Alps, considering the progressive cooling of the thermal Lepontine Barrovian metamorphic dome. Previous studies considered magmatic pulses that led to emplace the pegmatite field in the Central Alps. As an example, the pegmatites that intruded the Bergell massif were aged at 28–25 millions of years or younger, around 20–22 m.y.

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Tertiary Pegmatite Dikes of the Central Alps

Cited by 20 publications

References 36 publications

Mineralogy and Geochemistry of Pegmatites on Mount Begbie, British Columbia

Mineralogy and Geochemistry of Pegmatites on Mount Begbie, British Columbia

Intermittent fracturing in the middle continental crust as evidence for transient switching of principal stress axes associated with the subduction zone earthquake cycle

Non-Metamict Aeschynite-(Y), Polycrase-(Y), and Samarskite-(Y) in NYF Pegmatites from Arvogno, Vigezzo Valley (Central Alps, Italy)

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