Crustal-scale magmatic systems during intracontinental strike-slip tectonics: U, Pb and Hf isotopic constraints from Permian magmatic rocks of the Southern Alps |
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Authors: | Urs Schaltegger Peter Brack |
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Institution: | 1. Section des Sciences de la Terre, Université de Genève, rue des Mara?chers 13, 1205, Geneva, Switzerland 2. Department of Earth Sciences, ETH Zürich, Clausiusstrasse 25, 8092, Zurich, Switzerland
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Abstract: | The Southern Alps host volcano-sedimentary basins that formed during post-Variscan extension and strike-slip in the Early
Permian. We present U–Pb ages and initial Hf isotopic compositions of magmatic zircons from silicic tuffs and pyroclastic
flows within these basins, from caldera fillings and from shallow intrusions from a 250 km long E–W transect (Bozen–Lugano–Lago
Maggiore) and compare these with previously published data. Basin formation and magmatism are closely related to each other
and occurred during a short time span between 285 and 275 Ma. The silicic magmatism is coeval with mafic intrusions of the
Ivrea-Verbano Zone and within Austroalpine units. We conclude that deep magma generation, hybridisation and upper crustal
emplacement occurred contemporaneously along the entire transect of the Southern Alps. The heat advection in the lower crust
by injected mantle melts was sufficient to produce crustal partial melts in lower crustal levels. The resulting granitoid
melts intruded into the upper crust or rose to the surface forming large caldera complexes. The compilation of Sr and Nd isotopic
data of these rocks demonstrates that the mantle mixing endmember in the melts may not be geochemically enriched but has a
depleted composition, comparable to the Adriatic subcontinental mantle exhumed to form the Tethyan sea floor during Mesozoic
continental breakup and seafloor spreading. Magmatism and clastic sedimentation in the intracontinental basins was interrupted
at 275 Ma for some 10–15 million years, forming a Middle Permian unconformity. This unconformity may have originated during
large-scale strike-slip tectonics and erosion that was associated with crustal thinning, upwelling and partial melting of
mantle, and advection of melts and heat into the crust. The unconformity indeed corresponds in time to the transition from
a Pangea-B plate reconstruction for the Early Permian to the Late Permian Pangea-A plate assembly (Muttoni et al. in Earth
Planet Sci Lett 215:379–394, 2003). The magmatic activity would therefore indicate the onset of >2,000 km of strike-slip movement along a continental-scale
mega-shear, as their model suggests. |
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Keywords: | Southern Alps Permian basins U– Pb age determinations Zircon Hf isotopes Pangea reconstructions |
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