Structure of the Pennine Zone (Central Alps): A New Working Hypothesis

Milnes, A. G.

doi:10.1130/0016-7606(1974)85<1727:sotpzc>2.0.co;2

Cited by 99 publications

(46 citation statements)

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“…1a). Pegmatites are abundant in the southern steepened "roots" of the Alpine nappes (Southern Steep Belt, SSB: Studer 1851, Milnes 1974, adjacent to the Periadriatic Fault, and progressively decrease in number towards the north, where the reported by Wenk (1970) and Burri et al (2005) outlined by dashed and dotted lines, respectively. The map also shows the two major Tertiary batholiths of Adamello (Ada) and Bergell (Br), in grey, and the smaller and younger Novate stockwork intrusion, in black.…”

Section: Introductionmentioning

confidence: 70%

Tertiary Pegmatite Dikes of the Central Alps

Guastoni¹,

Pennacchioni²,

Pozzi³

et al. 2014

Can Mineral

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Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbSTRACTThe largest field of Alpine Oligocene pegmatite dikes is in the Central Alps within the Southern Steep Belt (SSB) of the Alpine nappes; it extends for about 100 km in an E-W direction and 15 km in a N-S direction north of the Periadriatic Fault, from the Bergell pluton (to the east) to the Ossola valley (to the west). The pegmatite field geographically overlaps (1) the highest temperature domain of the Lepontine Barrovian metamorphic dome and (2) the zone of Alpine migmatization. We have studied pegmatites in two areas: (1) the Codera area on the western border of the Bergell pluton and (2) the Bodengo area between the Mera and the Mesolcina valleys. Most pegmatites show a simple mineral assemblage consisting of K-feldspar, quartz, and muscovite ± biotite, and only a minor percentage of the dikes (< 5%) contains Sn-Nb-Ta-Y-REE-U oxide, Y-REE phosphate, Mn-Fe-phosphate, Ti-Zr-silicate, Be-Y-REE-U-silicate and oxide minerals (beryl, chrysoberyl, bertrandite, bavenite, and milarite), garnet (almandine-spessartine), tourmaline (schorl to rare elbaite), bismuthinite, magnetite, and rarely dumortierite and helvite. The mineral assemblages, geological context, and chemical compositions allow the distinction between LCT (lithium, cesium, tantalum) and mixed LCT-NYF (niobium, yttrium, fluorine) pegmatites (with only one exception of an NYF dike in the Bodengo area). The LCT pegmatites of the Central Alps did not reach a high degree of geochemical evolution. The most fractionated pegmatites are found in the Codera area and contain Mn-rich elbaite, triplite, pink-beryl, and Cs-Rb-rich feldspar. In the Bodengo area pegmatites locally contain miarolitic cavities and the most evolved pegmatites correspond to the berylcolumbite-phosphate type. From a structural point of view two main types of pegmatites can be distinguished: (1) pegmatites that were involved in ductile deformation and (2) pegmatites that postdated the main ductile deformation of the SSB. Many pegmatites of the Codera valley belong to the first structural type: they were emplaced at relatively high ambient temperature (ca. 500 °C) and locally show a pervasive recrystallization of quartz and a mylonitic structure. The Codera dikes trend about 70° and are steeply dipping. In the Bodengo area the main set of pegmatites (trending approximately N-S to NNE-SSW) crosscuts the ductile deformation structures of the SSB, but the area also includes an earlier generation of boudinaged and folded pegmatite dikes. The undeformed ...

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Section: Introductionmentioning

confidence: 70%

Tertiary Pegmatite Dikes of the Central Alps

Guastoni¹,

Pennacchioni²,

Pozzi³

et al. 2014

Can Mineral

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“…This nappe complex belonged, like the adjacent Leventina, Maggia, Adula, and Cima Lunga units, to the Penninc Ocean prior to its subduction. This ocean, separating the European and African continental margin, was subdivided into two main units, which are (1) the South Penninic Ocean and Middle Penninic continental shelf, together also referred to as upper Penninic units, and (2) the North Penninic Ocean, the lower Penninc units (Milnes 1974;Frisch 1979;Frisch and Meschede 2005;Maxelon and Mancktelow 2005). Southward-directed subduction commenced in the Cretaceous and culminated in continental collision approximately 35 million years (Ma) ago (Frisch 1979;Frisch and Meschede 2005).…”

Section: Geological Settingmentioning

confidence: 99%

Formation of kyanite–quartz veins of the Alpe Sponda, Central Alps, Switzerland: implications for Al transport during regional metamorphism

Beitter

Wagner

Markl

2008

Contrib Mineral Petrol

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In this study, we have investigated the formation of quartz-kyanite veins of the Alpe Sponda, Central Alps, Switzerland. We have integrated field observations, fluid inclusion and stable isotope data and combined this with numerical geochemical modeling to constrain the chemical processes of aluminum transport and deposition. The estimated P-T conditions of the quartz-kyanite veins, based on conventional geothermometry (garnet-biotite, white mica solvus and quartz-kyanite oxygen isotope thermometry) and fluid inclusion data, are 550 ± 30°C at 5.0 ± 0.5 kbar. Geochemical modeling involved construction of aqueous species predominance diagrams, calculation of kyanite and quartz solubility, and reactionpath simulations. The results of the modeling demonstrate that (1) for the given chemical composition of the veinforming fluids mixed Al-Si aqueous species are dominant in transporting Al, and that (2) fluid cooling along a small temperature gradient coupled with a pH decrease is able to explain the precipitation of the quartz-kyanite assemblages in the proportions that are observed in the Alpe Sponda veins. We conclude that sufficient amounts of Al can be transported in typical medium-to high-grade regional metamorphic fluids and that immobile behavior of Al is not very likely in advection-dominanted fluid-rock systems in the upper and middle crust.

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“…In region II, the foliation strike has an E-W orientation and swings into a WSW-ENE orientation in region III (Figure 2b), where the foliation dips steeply to the SSE (∼60°toward 155°, Figure 2c) and forms part of the Southern Steep Belt [Milnes, 1974]. The stretching lineation plunges SW in region II and ENE in region III (Figure 2c), with a broad transition where the lineation is close to horizontal.…”

Section: Se Prolongation Of the Simplon Fault Zonementioning

confidence: 99%

“…The grade of this regional metamorphism increases systematically toward the internal regions, from middle greenschist facies in the NW (Rhone Valley) to middle amphibolite facies in the SE (Ossola Valley) [Bearth, 1958;Niggli and Niggli, 1965;Trommsdorff, 1972;Wenk and Wenk, 1984]. In the Upper Pennine zone (SW of the SL [Milnes, 1974]), maximum metamorphic conditions attain ∼620-700°C and 0.5 GPa in the deepest Camughera-Moncucco unit toward the south [Keller et al, 2005a]. In the Lower Pennine zone (i.e., NE of the SL), the Alpine metamorphism in the Mesozoic cover has been studied in detail by Frank [1983].…”

Section: Metamorphic Historymentioning

confidence: 99%

“…The immediate footwall of the SFZ is characterized by a transition from a broad ductile mylonitic zone (the "Simplon shear zone") to a discrete brittle detachment with identical kinematics. The mylonitic zone grades over a distance of 1-2 km into less strongly overprinted units of the Lower Pennine zone [Milnes, 1974]. The brittle detachment or "Simplon Line" (SL) [Mancktelow, 1985;Mancel and Merle, 1987;Steck, 1987] consists of a narrow zone (<10 m) of cataclasites overprinting the mylonites.…”

Section: Simplon Fault Zonementioning

confidence: 99%

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Geochronological evidence for continuous exhumation through the ductile‐brittle transition along a crustal‐scale low‐angle normal fault: Simplon Fault Zone, central Alps

et al. 2010

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[1] Major low-angle normal faults juxtapose different structural levels of the crust that record both brittle and ductile deformation. Field relationships alone cannot establish whether these different responses to deformation represent (1) parts of a single process of exhumation along the detachment or (2) two separate events, with the later, more discrete brittle detachment exhuming a fossil ductile shear zone from depth. These two general models are critically assessed for the lowangle normal Simplon Fault Zone (SFZ) in the central Alps. The SFZ shows a spatial transition from a broad ductile mylonitic shear zone to a discrete brittle detachment with identical kinematics. The age of the ductile shear zone and ductile-brittle transition is controversial. We present a detailed geochronological study based on fission track, 40 Ar/ 39 Ar, and Rb/Sr microsampling dating, coupled with structural, petrological, and chemical analyses that provides tight constraints on SFZ timing. Discontinuous mineral cooling ages over a broad range of temperatures across the fault zone argue for fault activity between 20 and 3 Ma. On the basis of synkinematic white mica in low-temperature shear zones and necks of foliation boudinage, the brittle-ductile transition in the footwall could be dated at ∼14.5-10 Ma. Overall, the data presented here are consistent with a continuous transition from ductile shearing to a more localized zone of brittle deformation within the same geological framework, over a period of ∼15 Ma. The SFZ is therefore an example of a telescoped crustal section within a single major lowangle fault, involving a continuous period of exhumation rather than a two-stage structure.

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Structure of the Pennine Zone (Central Alps): A New Working Hypothesis

Cited by 99 publications

References 0 publications

Tertiary Pegmatite Dikes of the Central Alps

Tertiary Pegmatite Dikes of the Central Alps

Formation of kyanite–quartz veins of the Alpe Sponda, Central Alps, Switzerland: implications for Al transport during regional metamorphism

Geochronological evidence for continuous exhumation through the ductile‐brittle transition along a crustal‐scale low‐angle normal fault: Simplon Fault Zone, central Alps

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