Schematic profile from the European to the Adriatic plate (NW–SE) representing a model for the pre-orogenic situation in the area of the Central and Western Alps. This model is based on a series of radiometric data (mainly SHRIMP) for both protolith and metamorphic ages (see also text). Gray domains Continental crust; 

Context in source publication

Context 1

... Cathodoluminescence-controlled radiometric dating (U–Pb SHRIMP) was carried out on zircon domains from metabasic rocks of the Chiavenna unit, a major mafic/ultramafic-bearing unit in the Central Alps. Co-magmatic zircon domains from amphibolites near Chiavenna and Prata areas yielded weighted mean 206 Pb/ 238 U ages at 93.0±2.0 and 93.9±1.8 Ma, respectively, interpreted as the age of crystallization of the magmatic protoliths. These ages fit well with the time of late spreading in the Valais Ocean, as suggested by previous paleogeographic reconstructions. Inherited zircon grains and/or core domains (Permo-Triassic, Carboniferous, Proterozoic) are abundant, indicating proximity of the Chiavenna unit to thinned continental crust. This is in line with the origin of this unit from subcontinental mantle sources, as suggested previously on petrological and structural grounds. Metamorphic zircon domains from one amphibolite near Chiavenna yielded a weighted mean 206 Pb/ 238 U age at 37.1±0.9 Ma, identical to the 38.5±0.9 Ma SHRIMP age of an amphibolitized eclogite of the Antrona ophiolites (Valais domain, Western Alps). Precise metamorphic ages were difficult to obtain from the composite (poly)metamorphic rim domains of the Prata amphibolite. This is attributed to the location of the Prata area close to the granulite-facies Gruf unit (metamorphosed at ca. 33 Ma) and to the 24–25 Ma old Novate granite, where metamorphic/fluid events probably caused multi- ple resetting to various degrees. The ca. 93 Ma old magmatism, identified for the first time in the Chiavenna unit, is the youngest basic oceanic magmatism reported in the Alps. The 37.1±0.9 Ma old metamorphism in the Chiavenna unit, attributed to the Valais domain, confirms the model suggesting stepwise younging of metamorphic ages from the south (Adriatic plate) to the north (European plate). It is older than metamorphism in the European margin (ca. 35–31 Ma) lying to the north of the Valais domain and younger than that in the Piemont–Ligurian Ocean (ca. 44–45 Ma) lying to the south of the Valais domain. Multidisciplinary studies of ophiolitic, mafic/ultramafic rock associations in the Alps suggest the former existence of at least two oceans, originally located between Europe and Africa (Adria): (1) the Piemont–Ligurian Ocean, representing the Mesozoic Tethys, which opened from the Middle Jurassic onward (e.g., De Wever and Baumgartner 1995) and (2) the Valais trough, located NW of the Brianc ̧ onnais peninsula, probably opening from the Late Jurassic—Early Cretaceous onward (e.g., Frisch 1979; Tru ̈ mpy 1980; Stampfli et al. 1998). Sub- duction/collision of these ocean basins and intervening microcontinent(s) in response to convergence between Europe and Africa (Adria) resulted in the formation of the Alpine chain, characterized by a high complexity of nappe structures (e.g., Tru ̈ mpy 1975; Platt 1986; Le Pichon et al. 1988; Schmid et al. 1996a; Escher et al. 1997; Stampfli et al. 1998; Froitzheim 2001). Unraveling the pre-orogenic paleogeography and the complex Alpine tectono-metamorphic processes is a complicated and challenging task. Due to this complexity, a series of different models have been suggested for the evolution- ary history of the Alps over the many years of geological research (see e.g., review articles by Escher et al. 1997 or Froitzheim 2001 and references cited therein). Figure 1 shows one of these models with a schematic cross section of the paleogeographical situation in the Western and Central Alps, before the onset of convergence between Africa and Europe. The model is mainly based on a series of geochronological data (see summary by Gebauer 1999). The paleogeographic position, time of formation, and southward subduction of the Piemont– Ligurian Ocean are generally agreed on in the different models. In contrast, there are controversial views and missing information regarding the Valais Ocean, i.e., its paleogeographic extent, size, age, and duration of spreading, as well as metamorphic age(s) and grade(s). It is generally accepted that, however, the Valais basin was a small ocean SSE of Europe, separated from the main Piemont–Ligurian Ocean by the Brianc ̧ onnais peninsula. Rifting/spreading processes at the site of future devel- opment of the Valais basin probably started in Late Jurassic times (e.g., Schmid et al. 1990; Stampfli et al. 1998 and references therein). The area studied within the framework of the present paper belongs to the Chiavenna unit, Central Alps (Figs. 2 and 3), classically considered to belong to the Valais Ocean, in the prolongation of the so-called Misox zone (e.g., Schmid et al. 1996b). This zone is undoubt- edly considered to contain rocks of the Valais basin (see below). Here we present geochronological data on metabasic rocks from two localities of the Chiavenna unit: (1) close to the town of Chiavenna, and (2) close to the village of Prata (Fig. 3). Our aim was to determine the age of the protolith formation of the amphibolites, as well as their age of metamorphism, because the knowl- edge of the absolute time of magmatism and metamorphism are very crucial for paleogeographic reconstructions. We applied the U–Pb ion microprobe dating (SHRIMP) technique to analyze magmatic and metamorphic zircon domains in sectioned zircon grains. In order to get information about the origin of zircons (e.g., magmatic, metamorphic) and to distinguish between the magmatic and metamorphic domains we used cathodoluminescence imaging of the zircon crystals selected for dating. The Chiavenna unit is situated along the southeastern Swiss–Italian border. It is one of the four Penninic units that form a nappe stack in the Central Alps. These units are, from top to bottom and with increasing metamorphic grade, the Suretta and Tambo nappes, the Chiavenna unit, and the Gruf unit (Fig. 3). The Chiavenna unit consists mainly of metaperidotites, amphibolites, metagabbros, and rare carbonate rocks. It lies tectonically above the Gruf unit to the south and below the Tambo nappe to the north (e.g., Wenk 1955; Froitzheim and Manatchal 1996; Schmid et al. 1996a). The Gruf unit consists of high-grade, upper amphibolite- to granulite-facies metamorphic rocks (migmatitic gneisses, metapelites, amphibolites, and calc-silicate marbles) and is thought to represent the southeastern continuation of the Adula nappe (see e.g., overview papers by Schmid et al. 1996a; Frey and Ferreiro-Ma ̈ hlmann 1999). This view was recently confirmed by geochronological data on felsic granulites from the Gruf unit (Liati and Gebauer 2002), which reveal a metamorphic age at 32.7±0.5 Ma, identical to ages for granulite-facies events of crustal and mantle rocks of the Adula–Cima Lunga nappe system (e.g., Alpe Arami or Cima di Gagnone; Gebauer 1994, 1996). The Tambo nappe, located between the overlying Adula nappe and the underlying Suretta nappe, consists mainly of paragneisses intruded by the Permian Truzzo granite (e.g., Gulson 1973; Marquer et al. 1998). It shows increasing metamorphic grade from middle greenschist- facies in the north to lower-middle amphibolite-facies in the south-southeast. Generally, the Chiavenna unit and, with less confidence, also the Bellinzona–Dascio zone, which is re- stricted to the Southern Steep Belt (a zone of steeply north-dipping foliated rocks bounded by the Insubric line at its southern margin; Milnes 1974) have been considered to represent the Valais (north Penninic) suture zone (e.g., Schmid et al. 1996b). The mafic–ultra- mafic rocks of the Chiavenna unit were interpreted to represent an inverse ophiolite sequence (Schmutz 1974). They have also been considered as an ‘‘incomplete’’ ophiolite sequence, where subcontinental mantle rocks instead of typical oceanic lithosphere are tectonically exposed on an ocean floor and covered by tholeiitic N-type MORBs (Talerico 2001). Similarly, Huber and Marquer (1998) consider the mafic/ultramafic rocks of the Chiavenna unit as subcontinental to oceanic in origin exposed along normal faults on a thinned continental margin. A progressive metamorphism under PT conditions ranging from 3–4 kbar, 520 ° C in the north to temperatures up to 700 ° C in the south is suggested by Schmutz (1974). Also, Talerico (2001) suggests a progressive metamorphic evolution at temperature conditions vary- ing between ca. 500 ° C in the northern ...