Jurassic ophiolites within the Valais domain of the Western and Central Alps: geochronological evidence for re-rifting of oceanic crust

Abstract

Metabasic rocks from different parts of the Antrona ophiolites, Western Alps, as well as from the Misox zone, Central Alps, were dated using ion microprobe (SHRIMP) U-Pb analyses of zircon, in association with cathodoluminescence (CL) imaging. HP metamorphism must have affected at least the major part of the Antrona ophiolites, although HP relics are rarely preserved, probably due to the Lepontine metamorphic overprint. HP metamorphism has affected also the area of the Misox zone. The origin of the Antrona ophiolites is arguable. They were interpreted as part of both the Piemont-Ligurian (PL) and the Valais ocean, the two main oceans in the area of the Alps before Alpine convergence. SHRIMP-analyses of co-magmatic zircon domains from the Antrona ophiolites (Guggilihorn, Passo del Mottone and Quarata areas) yielded identical (within uncertainty) weighted mean 206 Pb/ 238 U ages of 155.2±1.6 Ma, 158±17 Ma (or 163.1±2.4 Ma: one analysis; 1r error) and 155.6±2.1 Ma, respectively, interpreted as the time of crystallization of the magmatic protoliths. These Late Jurassic ages fit well to the time span considered for the formation of Piemont-Ligurian oceanic crust. The metagabbro of the Misox zone (Hinterrhein area), for which a Valaisan origin is generally accepted, gave also a Late Jurassic, PL protolith age of 161.0±3.9 Ma. The metamorphic zircon domains from the amphibolitized eclogite of Mottone yielded an age of 38.5±0.7 Ma, interpreted as the time of HP metamorphism. This age is in good agreement with the time of metamorphism reported from previous zircon SHRIMP-data for eclogites and amphibolites of other parts in the Valais domain. In order to bring in line the PL protolith ages with the Valaisan metamorphic ages, we suggest a scenario involving emplacement of part of the PL oceanic crust to the north of the newly formed Brianc¸onnais peninsula, inside the Valais geotectonic domain. This paleotectonic configuration was probably established when younger Valaisan oceanic crust formed by spreading and re-rifting, partly within PL oceanic crust.

New stratigraphic data from the Lower Penninic between the Adula nappe and the Gotthard massif and consequences for the tectonics and the paleogeography of the Central Alps

“…For the Misox zone a Jurassic age is testified by U-Pb dating on zircon (~161 Ma, Liati et al, 2005), for the Valser slices it is supported by facies analysis (Steinmann, 1994). It is noteworthy that part of the magmatic activity is recorded in tuffites and other volcanoclastic manifestations (Kupferschmid, 1977).…”

“…Above the Adula nappe and below the Middle Penninic (and Briançonnais) Tambo nappe, there is the Misox zone (Gansser, 1937), characterised by thick sequences of mostly Cretaceous calcschists (the so-called Bündnerschiefer) with few basement slices and intercalated Jurassic MORB-type metamafics and rare serpentinites (Dürr et al, 1993;Gansser, 1937;Kupferschmid, 1977;Liati et al, 2005;Nabholz, 1945;Steinmann, 1994;Steinmann and Stille, 1999). The Misox zone consists of two nappes and three imbricated zones, respectively and from top to bottom these are: the Tomül and the Grava nappes (with their basal mélanges), the Aul unit, the Upper Valser and the Lower Valser slices ("Valser Schuppen") (Wyss and Isler, 2007).…”

“…Dürr et al, 1993;Steinmann and Stille, 1999) proposed the presence of oceanic crust in the internal North Penninic. Where radiochronologically dated, the supposed oceanic rocks, are of late Middle-early Upper Jurassic time (~161 Ma, Liati et al, 2005). Concerning the Valais Trough, the birth of an early Cretaceous ocean has been proposed (Frisch, 1979;Stampfli, 1993) but has not yet been proven convincingly by the dating of Cretaceous oceanic crust.…”

Identification of repeated Alpine (ultra) high-pressure metamorphic events by U–Pb SHRIMP geochronology and REE geochemistry of zircon: the Rhodope zone of Northern Greece

“…It was more and more confirmed since, also in other (U)HP orogenic belts worldwide, that such long durations are unrealistic for single (U)HP metamorphic cycles (e.g., Gebauer 1996;Amato et al 1999;Rubatto and Hermann 2001 and references therein). Thus, for the Central and Western Alps, a very different model involving multiple subductions of different terranes at different Alpine times was suggested on the basis of SHRIMP-data, combined with a detailed structural, geological and petrological information (see e.g., summary by Gebauer 1999;Froitzheim 2001;Liati et al 2005b and references therein). Four distinct Alpine cycles are envisaged so far for the Western and Central Alps involving subduction/collision of different oceanic and continental fragments at ca.…”

Jurassic syn-rift and Cretaceous syn-orogenic, coarse-grained deposits related to opening and closure of the Vahic (South Penninic) Ocean in the Western Carpathians – an overview

“…Open ing of both the South and North Penninic oce anic branches was as cribed to the eastwards prop a ga tion of the Cen tral At lan tic and the North At lantic and Bay of Biscay rift sys tems, re spec tively (Frisch, 1981;Lemoine and Trümpy, 1987;Stampfli, 1994Stampfli, , 1996. The Cre taceous North Penninic rift sys tem (Valais Ocean) partly re-rifted the Ju ras sic South Penninic Ocean (Liati et al, 2005).…”