The Pre-Permian basement rocks preserved within the Alpine fold belt record a number of magmatic and metamorphic events. In contrast to sedimentary lithologies, which may have originated at various places and been amalgamated tectonically, magmatic rocks reflect the geodynamic development of substantial segments of the crust. Key geochemical parameters and high-precision age determinations may reveal similarities between certain magmatic bodies and allow us to correlate them in terms of geodynamic scaling of magmatic cycles.
The studied area in the Central Swiss Alps consists of two adjacent but distinct tectonic units: the Ultrahelvetic Gotthard unit and the Lucomagno unit, a frontal part of the lower Penninic nappe complex. Pre-Permian ortho- and paragneisses, amphibolites, quartzites and schists of different chemical and mineralogical composition form a major part of these units. Structurally, the Pre-Permian paragneisses, amphibolites and quartzites occur as extensive stripes and lenses, some of them originating from tectonic wedges in the Caledonian orthogneiss of the Gotthard, and as zones among the mixed gneisses of the Lucomagno unit. Amphibolite horizons and their boudins vary in thickness from less than one meter to some hundreds of meters, and show concordance with and sometimes finger like projections into the country rocks.
Principally two types of amphibolite can be distinguished in the Gotthard and Lucomagno units: (Type I) dark-green, homogeneous amphibolites without banding, and (Type II) spotted to banded (green-white) amphibolites. Type I amphibolites, generally occurring within paragneisses, were traditionally interpreted as metasedimentary marls. In contrast, petrographical and field characteristics favour a magmatic origin for type II amphibolites. Our new major and trace element data of the amphibolites, however, demonstrate they are both metabasalts. Type I amphibolites show features which are typical for tholeiitic continental basalts: enrichment of light REE, generally with no Eu anomaly, higher total REE, Sr, and Nb concentrations compared to MOR basalts and type II amphibolites. The latter are more depleted in incompatible elements, show constant REE enrichment with slightly positive Eu anomalies, and are interpreted as andesitic metabasalts. The Rb/Sr data for both amphibolite groups suggest complete resetting of the Rb/Sr isotopic system at around 255 Ma, which is in good agreement with previously established ages of regional metasomatic alteration and aplite formation. The more resistant Sm/Nd isotopic system, indicates a wide range of Pre-Caledonian model TDM ages for all amphibolites. Additionally, various zircon populations within the Gotthard and Lucomagno basement gneisses point to a Pre-Caledonian multistage magmatic and tectonic evolution. A remarkable feature is the strong geochemical affinity of amphibolites from the above tectonic units, which were juxtaposed during Variscan and Alpine compressive tectonics.
The completion of a geodynamic evolutionary model for the Gotthard and Lucomagno Pre-Permian basement especially requires high-resolution single-zircon U/Pb chronometry. This permits to recognize deposition ages of the volcano-sedimentary units within the Pre-Permian basement. On a larger scale, the Late Proterozoic and Palaeozoic geodynamic history of the Central Alpine domain can be described as a complex succession of extensional and compressive regimes within a wide mobile zone, situated between the northern margin of the Laurasian/Gondwanan craton and the Eurasian/Northern Atlantic continents (e.g. Pin, 1991): the Late Proterozoic episode of crustal growth in Europe, further rifting, continental break-up and ocean opening in the Cambrian-Ordovician, the Variscan continent-continent collision in the Early Carboniferous leading to large-scale nappe tectonics, crustal melting, and emplacement of numerous granitoid intrusives. Many of the Pre-Permian rocks, especially paragneisses, were thereby subjected to anatectic melting. The age of the amphibolites, as the most resistant remnants of previous volcanic pulses accompanying the above dramatic changes of geodynamic regimes, may therefore help to construct an evolutionary time-scale.
Preliminary morphological studies on accessory amphibolite zircons indicate a predominant magmatic zircon population, nevertheless a majority of the euhedral grains contains a visible inherited zircon component suggesting crustal contamination from older basement rocks. In addition, some zircon grains display rounded surfaces and may represent incorporated detrital components. First single-zircon U-Pb age results, performed on the magmatic population, are interpreted as age of volcanic event(s) and will be presented and discussed.
Pin, C., In The West African Orogens and Circum-Atlantic Correlatives, (Dallmeyer, R. D. & Lécorché, J. P., Eds.) 295-306 (Springer-Verlag, 1991).