Alpine and pre-Alpine magmatism in the root-zone of the western Central Alps

Romer, Rolf L.; Schärer, Urs; Steck, A.

doi:10.1007/s004100050147

Cited by 74 publications

(74 citation statements)

References 27 publications

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“…The intrusion age (from zircon U/Pb) of 29 to 26 Ma for synkinematic to postkinematic pegmatites within the interpreted prolongation of this major shear zone in the steep zone north of the Periadriatic Line provides a minimum age for this shearing event [Romer et al, 1996]. However, as noted above, the thermochronological jump and crosscutting relationship of the back folding in the footwall demonstrates Neogene activity on the brittle SL detachment.…”

Section: Previous Thermochronological and Geochronological Workmentioning

confidence: 76%

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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Section: Previous Thermochronological and Geochronological Workmentioning

confidence: 76%

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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“…These conditions are similar to those of the Codera area and similarly exceed the conditions of intrusion of the pegmatites. The pegmatites of the main set of the Bodengo area are mostly undeformed and crosscut the folds of the D3 phase, indicating that they postdated the main ductile deformation in the Romer et al (1996) dated post-tectonic aplite-pegmatites at 20.2 + 4.4/−7.8 Ma (zircon U-Pb ages). This dike does not show any discernible foliation or other evidence of internal deformation and crosscuts the mesoscopic foliation of the host gneiss.…”

Section: Pressure-temperature Conditions Of the Host Rocks During Pegmentioning

confidence: 99%

“…Therefore, in the SSB the leucocratic dikes (including pegmatite and aplite) are often associated with small granitoid bodies (meter to decameter in size) and in situ leucosome segregates of the host rock (Burri et al 2005). The interpretation of this structural assemblage is locally difficult due to the occurrence of pre-Alpine leucocratic dikes, migmatites, and granitoids (Hänny et al 1975, Romer et al 1996, Galli et al 2012. The age of Alpine migmatization and of emplacement of leucocratic dikes in the Central Alps apparently spans approximately the same age bracket as the Tertiary intrusions, whose endmembers are represented by the large Bergell intrusion (32-28 Ma, von Blanckenburg et al 1992, Berger et al 1996, Oberli et al 2004) and the younger and smaller Novate stockwork intrusion (24.0 ± 1.2 Ma, Liati et al 2000).…”

Section: Introductionmentioning

confidence: 99%

“…Gebauer (1996) dated the migmatization event of the SSB within the Cima Lunga nappe at 32.4 ± 1.1 Ma by SHRIMP U-Pb zircon dating; this age indicates that migmatization was coeval with the early phases of the Bergell intrusion. Similarly, in the western part of the pegmatite field, pegmatites intruding amphibolite-facies mylonites of the Simplon ductile shear zone have been dated from 29 to 25 Ma (with a discordant dike as young as 20 Ma) by the U-Pb method applied to monazite and xenotime (Romer et al 1996. It is noteworthy that Romer et al (1996) distinguished two generations of pegmatite-aplite from the same outcrop at Malesco: (1) the oldest, which intruded along the host-rock foliation and is slightly deformed, yielded an U-Pb age from monazite of 29.2 ± 0.2 Ma; (2) the youngest, which is undeformed and is discordant to the foliation, yielded an U-Pb age from xenotime of 25.5-26.5 Ma.…”

Section: Introductionmentioning

confidence: 99%

“…Similarly, in the western part of the pegmatite field, pegmatites intruding amphibolite-facies mylonites of the Simplon ductile shear zone have been dated from 29 to 25 Ma (with a discordant dike as young as 20 Ma) by the U-Pb method applied to monazite and xenotime (Romer et al 1996. It is noteworthy that Romer et al (1996) distinguished two generations of pegmatite-aplite from the same outcrop at Malesco: (1) the oldest, which intruded along the host-rock foliation and is slightly deformed, yielded an U-Pb age from monazite of 29.2 ± 0.2 Ma; (2) the youngest, which is undeformed and is discordant to the foliation, yielded an U-Pb age from xenotime of 25.5-26.5 Ma. This is consistent with the SHRIMP U-Pb zircon age of 25.1 ± 0.6 Ma obtained for a pegmatite cross cutting the Alpine foliation of the SSB more than 50 km to the east, just north of San Vittore (Gebauer 1996).…”

Section: Introductionmentioning

confidence: 99%

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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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Rock uplift and erosion rate history of the Bergell intrusion from the inversion of low temperature thermochronometric data

Fox

Reverman

Herman

et al. 2014

Geochem. Geophys. Geosyst.

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The Bergell intrusion (European Alps) was one of the first locations where thermochronometry was used to resolve changes in erosion rate, yet, relating these changes to variations in climate or in local tectonics remains challenging. One approach that enables changes in erosion rate to be related to changes in climate or rock uplift rate is to utilize landscape evolution models, as topographic response to these forcing parameters is unique. Furthermore, low temperature thermochronometric systems have the potential to resolve topography through time and thus topographic response. We present new (U-Th)/He data for samples collected across 2 km of relief from the Bergell. The ages range from $2 to $16 Ma and define an age elevation with an apparent exhumation rate of 0.1 km/Myr. In order to infer erosion rates, we use a thermokinematic model to solve the heat equation in the crust, track material points through time and predict thermochronometric data. Paleo-topography and erosion rate are parameterized using the stream power model. We find that rock uplift rates were 0.4 km/Myr from $25 to $20 Ma and subsequently decreased to 0.05 km/Myr. This results in a gradual decrease in erosion rate from rates of 0.4 to 0.1 km/Myr. A recent increase in rock uplift rate at $4 Ma to $0.6 km/Myr is required to explain the youngest ages and high topographic relief.

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Alpine and pre-Alpine magmatism in the root-zone of the western Central Alps

Cited by 74 publications

References 27 publications

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

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

Tertiary Pegmatite Dikes of the Central Alps

Rock uplift and erosion rate history of the Bergell intrusion from the inversion of low temperature thermochronometric data

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