Timing of exhumation of meta-ophiolite units in the Western Alps: New tectonic implications from 40Ar/39Ar white mica ages from Piedmont Zone (Susa Valley) (original) (raw)
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Pre-Alpine contrasting tectono-metamorphic evolutions within the Southern Steep Belt, Central Alps
Lithos, 2018
In the Southern Steep Belt, Italian Central Alps, relicts of the pre-Alpine continental crust are preserved. Between Valtellina and Val Camonica, a poly-metamorphic rock association occurs, which belongs to the Austroalpine units and includes two classically subdivided units: the Languard-Campo nappe (LCN) and the Tonale Series (TS). The outcropping rocks are low to medium grade muscovite, biotite and minor staurolite-bearing gneisses and micaschists, which include interlayered garnet- and biotite-bearing am- phibolites, marbles, quartzites and pegmatites, as well as sillimanite-bearing gneisses and micaschists. Permian intrusives (granitoids, diorites and mi- nor gabbros) emplaced in the metamorphic rocks. We performed a detailed structural, petrological and geochronological analysis focusing on the two main lithotypes, namely, staurolite-bearing micaschists and sillimanite-bearing paragneisses, to reconstruct the Variscan and Permian-Triassic history of this crustal section. The reconstruction of the tectono-metamorphic evolution allows for the distinction between two different tectono-metamorphic units during the early pre-Alpine evolution (D1) and predates the Permian intrusives, which comprise rocks from both TS and LCN. In the staurolite-bearing micaschists, D1 developed under amphibolite facies conditions (P=0.7- 1.1 GPa, T=580-660◦C), while in the sillimanite-bearing paragneisses formed under granulite facies conditions (P=0.6-1.0 GPa, T>780◦C). The two tectono- metamorphic units coupled together during the second pre-Alpine stage (D2) under granulite-amphibolite facies conditions at a lower pressure (P=0.4-0.6 GPa, T=620-750◦C) forming a single tectono-metamorphic unit (Languard-Tonale Tectono-Metamorphic Unit), which comprised the previously distin- guished LCN and TS. Geochronological analyses on zircon rims indicate ages ranging between 250 and 275 Ma for D2, contemporaneous with the emplacement of Permian intrusives. This event developed under a high thermal state, which is compatible with an extensional tectonic setting that occurred during the exhumation of the Languard-Tonale Tectono-Metamorphic Unit. The extensional regime is interpreted as being responsible for the thinning of the Adriatic continental lithosphere during the Permian, which may be related to an early rifting phase of Pangea.
Swiss Journal of Geosciences
In the Western Alps, different shear zones acting at different depths have been investigated for explaining multistage exhumation of (U)HP units, and several exhumation models have been proposed for explaining present-day stacking of different tectonometamorphic units. This study aims to reconstruct the tectonic evolution of the Susa Shear Zone (SSZ), a polyphasic first-order shear zone, outcropping in the Susa Valley. The SSZ consists of a thick mylonitic zone, along which units characterized by different Alpine metamorphic P–T peaks are coupled. In the study area, the footwall of the SSZ mostly consists of oceanic units (i.e., Internal Piedmont Zone), which record eclogitic conditions, whereas the hanging wall consists of oceanic units (i.e., External Piedmont Zone), which record blueschist-facies conditions. These tectonic units were deformed during subduction- and exhumation-related Alpine history, throughout four main regional deformation phases (from D1 to D4), and were couple...
Swiss Bulletin of …, 2003
The frontal Penninic units (FPU) of the Western Alps underwent a complex tectono-metamorphic evolution during Alpine orogeny that can be divided into three major stages: a first one pre-to syn-collisional (D1 and D2), a second one (D3) and third one (D4) post collisional. This study aims to integrate detailed structural mapping with metamorphism (Kübler index and K-white mica b-cell dimension analysis) and thermochronology (fission track analysis). Metamorphic conditions in the FPU range from high anchizonal to epizonal conditions. The highest metamorphic temperatures (~300 °C) were deduced for the Houiller unit and the Cheval Noir unit. Structural analysis and fission track ages indicate that the temperature peak in the Houiller unit was reached during D1, i.e. in the Eocene. The metamorphic peak in the Cheval Noir unit was reached during D2 due to overthrusting of the more internal Subbriançonnais and Houiller units. Kübler indices and fission track data show a vertical gradient within the Cheval Noir unit with the higher temperatures recorded at the base of the stratigraphic series. This metamorphic gradient was successively folded during D3 in the Oligocene. The K-white mica b-cell dimension values do not show major differences within the FPU units. They present a clear bimodal distribution interpreted to be due to the superposition of the effects of the two stages of deformation. The Kwhite mica b-cell dimension values from the FPU are higher than those recorded in the Dauphinois domain indicating that the FPU were subjected to higher pressures compared to the Dauphinois domain. Metamorphism in the Dauphinois domain, which is affected by one major phase of deformation (i.e. D3) only, is related to the overthrusting of the FPU along the syn-D3 Roselend thrust. According to our tectonic reconstruction the first two phases of deformation (D1 and D2) developed during subduction of the Valaisan units, within the accretionary wedge. Our data indicate that metamorphism affecting the FPU south of Moûtiers is mainly related to the first stage (D1 and D2) when the FPU formed the frontal and upper part of the Alpine accretionary wedge. The second stage (D3), characterized by thrusting and folding, developed during the Oligocene in a post-collisional scenario and led first to low T-low P metamorphic overprint and finally to a partial exhumation of the FPU already in the Late Oligocene.
Mineralogy and Petrology, 2006
In the Eastern Alps, different tectonic units contain eclogites of Carboniferous, Cretaceous (eo-Alpine), and Cenozoic age: (i) In the structurally lower Penninic unit, which is exposed in the Tauern window, eclogites formed at peak P-T conditions of 2.4 GPa and 630 C as a result of subduction of the former European continental margin at the base of an imbricate stack of Mesozoic oceanic rocks below the Austroalpine domain. Amphibole and phengite 40 Ar-39 Ar and Rb-Sr multimineral isochron ages suggest an Eocene-Early Oligocene (<45-31 Ma) age for the stage of deepest subduction and rapid exhumation=cooling. (ii) Variscan MORB-type eclogites are present in the central Ö tztal basement (2.7 GPa=730 C; mean Sm-Nd age: 347 AE 9 Ma), the eastern Silvretta (ca. 350 Ma), and the Ulten high-grade crystalline (336 AE 4 Ma). These domains pertain to the N Apulian (Austroalpine) crust. (iii) In the southern Austroalpine units, eclogites and high-P metapelites are known from the Texel complex (1.3 GPa=520-600 C), the Schober area (1.8 GPa= 690 C), and the Saualpe-Koralpe-Pohorje domain (2.2-2.7 GPa= 750 C). Their tectonic position and near-identical Cretaceous ages suggest a common subduction-exhumation history. This ''eo-Alpine high-pressure belt'' (EHB) resulted from burial of the strongly thinned, distal passive Neotethyan margin of the Meliata back-arc basin and, further west, pre-Alpine Austroalpine crust along an intracontinental subduction zone during the Late Mesozoic convergence of Apulia and Europe. Peak metamorphism and initial decompression=exhumation is dated by the Sm-Nd, Lu-Hf, U-Pb and Rb-Sr systems as close to 90 Ma, with mean exhumation rates in the range of 5-10 km=Ma for the time between 90 and 85 Ma B.P.
Structural Evolution and Metasomatism of Subducted Metaophiolites in the Northwestern Alps
Tectonics, 2019
A subduction complex of the northwestern Alps consists of serpentinites, eclogitic metagabbros, flysch‐like metasediments, meta‐ophicarbonates, and gneissic slices. Unlike other subduction complexes, it contains unusual hybridized rocks described here for the first time in the northwestern Alps. They are preserved as patches interstitial in the metagabbro and as layers within metagabbros and serpentinites. The hybridized rocks are made of high modal zoisite/clinozoisite + white mica pseudomorphs of lawsonite, garnet, and amphibole associated with an Alpine eclogite‐facies fabric. While these eclogitic metagabbros are chemically comparable to oceanic oxide gabbros from the ultraslow Southwest Indian Ridge, the layers are extremely enriched in Al2O3 and CaO and depleted in TiO2, MgO, and SiO2 relative to metagabbros. Patches have a geochemical signature that is intermediate between that of layers and metagabbros. Trace element compositions of hybridized rocks suggest a contribution fr...
Tectonics, 1998
We explore the timing of deformation and exhumation of the Siviez-Mischabel Nappe (western Swiss Alps), which has been considered a classic example of a midcrustal crystalline nappe since the studies of Argand [1916]. This study presents 4øAr/39Ar ages obtained on both synkinematic white mica from Permo-Triassic cover sediments and more complex white mica populations from basement gneisses of the Siviez-Mischabel and middle Pennine Nappes. Primary foliation developed in cover units by nucleation, growth, and rigid rotation of mica grains during noncoaxial Alpine deformation. Although some samples show a crenulation of this primary foliation, mica growth appears to have occurred only during the development of primary foliation, the main phase of greenschist facies deformation related to imbrication of the Siviez-Mischabel Nappe and other middle Pennine Nappes. Good agreement exists between independent estimates of the timing of deformation and reported 4øAr/39Ar white mica ages from cover units of the central and southern Siviez-Mischabel Nappe. In cover units from the central and southern Siviez-Mischabel regions of the study area, 4øAr/39Ar ages appear to date synkinematic white mica growth. Results suggest that the Siviez-Mischabel •Nappe was emplaced and developed foliation during a 5 m.y. period from 41 to 36 Ma. In cover units from the eastern Siviez-Mischabel, however, 4øAr/39Ar white mica ages appear to date postkinematic thermal events. These thermal events may be related to Oligocene magmatic activity in the lower Pennine Nappes or to Miocene development of the Simplon fault zone. Variations in the relation between Alpine age and grain size for cover samples from the central, eastern, and southern Siviez-Mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamorphism. When considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western Swiss Alps,
High-pressure metamorphism U-Th-Pb geochronology Allanite Monazite Florencite Rare earth elements We present a petrological and geochronological study of "silvery micaschists" that crop out in the northern side of the Gran Paradiso Massif, Western Alps, with the aim of constraining P-T conditions and age of the Alpine high-pressure metamorphism. The studied "silvery micaschists", which are the products of metasomatic alteration of granitoids along ductile shear zones, consist of Mg-chlorite, talc, chloritoid, minor phengite, and accessory minerals. Microstructural relationships indicate the following prograde sequence in the growth of U-Th bearing accessory minerals: florencite → monazite → allanite. Thermobarometric calculations indicate that the Mg-chlorite + talc + chloritoid + allanite peak assemblage was stable at P = 1.9-2.7 GPa and T = 515-600°C, while monazite formed earlier at pressures over 2.0 GPa. SHRIMP dating of allanite yielded 33.7 ± 1.6 Ma, interpreted as the age of the high-pressure metamorphic peak. Prograde monazite yielded an age of 37.4 ± 0.9 Ma, implying a minimum duration of ∼ 2-4 Ma for the Alpine subduction event. Combining our ages with previous constraints, it can be concluded that the initial exhumation of the Gran Paradiso Massif occurred at a fast rate higher than 2 cm/year, and slowed down to ∼ 1 cm/year in the final stages. In a regional context, the new geochronological data align the subduction of the Gran Paradiso Massif with the other Internal Crystalline Massifs of the Western Alps. This supports a subduction model marked by alternating compressional events, related to the accretion of continental terranes, and extensional events, related to the episodic retreat of subduction zone hinges.
Geological Society, London, Special Publications, 2008
This paper highlights the use of synkinematic white mica, biotite and phlogopite for the dating of deformation in ductile shear zones within crystalline rocks under low-grade metamorphic conditions. The Mont Blanc shear zones range from 1 mm to 50 m in width and have localized intense fluid flow, resulting in substantial differences in mineralogy and whole-rock geochemistry. On the basis of their synkinematic alteration assemblages and geographic distribution within the Mont Blanc Massif, three main metamorphic zones are distinguished within the network of shear zones. These are: (i) epidote±white mica-bearing assemblages; (ii) chlorite–phlogopite-bearing assemblages; and (iii) white mica±biotite±calcite±actinolite±epidote- bearing assemblages. 40Ar/39Ar age spectra of biotite and phlogopite are complex, and reflect significant variations in chemical composition. In biotite, this is partly due to inheritance from precursor Variscan magmatic biotite. In contrast, new white mica grew ...
Radiometric ages of Alpine metamorphic rocks in the western and central Alps
The Island Arc, 1993
The chronological characteristics of Alpine metamorphic rocks are described and Alpine metamorphic events are reinterpreted on the basis of chronological data for the western and central Alps from 1960 to 1992. Metamorphic rocks of the Lepontine, Gran San Bernardo, Piemonte, Internal Crystalline Massifs and Sesia-Lanzo mostly date Alpine metamorphic events, but some (along with granitoids and gneisses from the Helvetic and Southern Alps) result from the Variscan, Caledonian or older events and thus predate the Alpine events. Radiometric age data from the Lepontine area show systematic age relations: U-Pb monazite (23-29 Ma), Rb-Sr muscovite (15-40 Ma) and biotite (15-30 Ma), K-Ar biotite (10-30 Ma), muscovite (15-25 Ma) and hornblende (25-35 Ma), and FT zircon (10-20 Ma) and apatite (5-15 Ma), which can be explained by the different closure temperatures of the isotopic systems. A 121 Ma U-Pb zircon age for a coesite-bearing whiteschist (metaquartzite) from the Dora-Maira represents the peak of ultra-high pressure metamorphism. Coesite-free eclogites and blueschists related to ultra-high pressure rocks in the Penninic crystalline massifs yield an 40Ar-39Ar plateau age of about 100 Ma for phengites, interpreted as the cooling age. From about 50 Ma, eclogites and glaucophane schists have also been reported from the Piemonte ophiolites and calcschists, suggesting the existence of a second high PIT metamorphic event. Alpine rocks therefore record three major metamorphic events: (i) ultra-high and related high P/T metamorphism in the early Cretaceous, which is well preserved in continental material such as the Sesia-Lanzo and the Penninic Internal Crystalline Massifs; (ii) a second high P/T metamorphic event in the Eocene, which is recognized in the ophiolites and calcschists of the Mesozoic Tethys; and (iii) medium PIT metamorphism, in which both types of high P/T metamorphic rocks were variably reset by Oligocene thermal events. Due to the mixture of minerals formed in the three metamorphic events, there is a possibility that almost all geochronological data reported from the Alpine metamorphic belt show mixed ages. Early Cretaceous subduction of a Tethyan mid-ocean ridge and Eocene continental collision triggered off the exhumation of the high pressure rocks.
Rendiconti online Società Geologica Italiana, 2023
New structural data have been collected in the high-pressure meta-ophiolites exposed in the Lanzo Valleys (i.e., the Lanzo Valleys Ophiolite, LVO) and tectonically overlying the continental crust of the Dora-Maira and Gran Paradiso massifs in the Western Alps. Detailed geological mapping and structural analysis carried out in the southwestern sector of the LVO (i.e., the Upper Viù Valley) allow to distinguish four ductile deformation phases, corresponding to the subduction-related D1 phase, the early exhumation-related D2-phase and the late exhumation-related D3 and D4 phases. The geometry of structures and kinematics have been reconstructed, and their regional tectonic meaning is discussed in the frame of the Alpine tectonic evolution of the Internal Piedmont Zone (i.e., the remnants of the Alpine Tethyan oceanic lithosphere) in the Western Alps.