|
Silvio Seno, University of Pavia (Italy)
Giorgio Dallagiovanna, University of Pavia (Italy)
Laura Gaggero, University of Genova (Italy)
Massimo Tiepolo, CNR (Italy)
Matteo Maino, University of Pavia (Italy)
|
|
New in situ U-Pb ages by Laser Ablation (LA) ICP MS on zircon were performed on post-Variscan volcanites of the Briançonnais domain in the Ligurian Alps. These rocks belong to continental volcano-sedimentary sequences presently structured in Alpine tectonic units issued from the Paleo-European continent.
In the inner Briançonnais the Variscan basement is unconformably covered by early rhyolitic ignimbrites ("C. Lisetto Metarhyolites"), followed by coarse deposits ("Ollano Fm"). On the basement, or at the top of the Ollano Fm., fine sediments ("Murialdo Fm. and Viola Schists") are interbedded with andesitic lavas and pyroclastites ("Eze Fm."). The main magmatic episode is represented by calc-alkaline, rhyodacitic ignimbrites (A, B and C-Lithozones of the "Melogno Porphyroids"), largely outcropping also in the outer and intermediate Briançonnais sectors. The Late Palaeozoic volcanic activity ends with rhyolites of the D-Lithozone with high modal K-feldspar and alkaline-potassic character. An erosional surface separates the volcano-sedimentary complex from the Permo-Mesozoic covers.
Zircons were separated from the early rhyolite (2 samples), the main rhyodacite (1) and the high K rhyolites (3). The U-Pb analysis on the magmatic textural domain yielded ages of about 284 Ma for the C. Lisetto Metarhyolites, 273 Ma for C Lithozone and 258 for D Lithozone, assumed as the crystallization ages. The low-grade Variscan amphibolite-greenschist facies metamorphism in the basement, referable to the final exhumation, is set at 310-300 Ma. As the early rhyolites can be dated to 285.6±1.3 Ma, the average exhumation rate is c. 1 mm/a. Its mechanism is debatable as these velocities account both for a purely isostatic and for a tectonically-assisted kinematics. Therefore, following the collision, the exhumation was also driven by a general extensional regime causing decompressional melting of the lithospheric mantle and the volcanic activity. Later, in the time span between 284 (C. Lisetto Metarhyolites) and 273 (C-Lithozone) Ma, a spread volcanic activity took place. The upwelling and the emplacement of magma was favoured by transtensional faults, both tapping magmatic reservoirs at depth and offering an easy path of ascent. In this regard, the Ligurian Alps underwent a generalized lithospheric thinning associated with a large-scale mechanism of crustal wrenching as other sectors of the Variscan belt. The volcanic activity ended at c. 258 Ma (D Lithozone), when the change from calc-alkaline to alkaline activity suddenly occurred.
Thus, the crustal-scale extension and magmatism in the Ligurian Alps represent the transition from post-orogenic to anorogenic setting. At that moment were also originated the structural features that will be inherited and reactivated during the subsequent rifting of Pangea, and that will control the Mesozoic evolution of the European passive margin of the Jurassic Neotethys.
|
