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Bimodal magmatism as a consequence of the post-collisional readjustment of the thickened Variscan continental lithosphere (Aiguilles Rouges-Mont Blanc Massifs, Western Alps)

Published online by Cambridge University Press:  03 November 2011

François Bussy ,
Jean Hernandez  and
Jürgen Von Raumer
François Bussy
Affiliation:
François Bussy, Earth Sciences Department, Royal Ontario Museum, Toronto, Canada; and IMP-BFSH2, UNIL, CH-1015 Lausanne,Switzerland e-mail: [email protected]
Jean Hernandez
Affiliation:
Jean Hernandez, IMP-BFSH2, UNIL, CH-1015 Lausanne,Switzerland e-mail: [email protected]
Jürgen Von Raumer
Affiliation:
Jürgen Von Raumer, Institut de Minéralogie, Pérolles Université, CH-1700 Fribourg,Switzerland e-mail: [email protected]
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Abstract

High Precision U-Pb zircon and monazite dating in the Aiguilles Rouges–Mont Blanc area allowed discrimination of three short-lived bimodal magmatic pulses: the early 332 Ma Mg–K Pormenaz monzonite and associated 331 Ma peraluminous Montées Pélissier monzogranite; the 307 Ma cordierite-bearing peraluminous Vallorcine and Fully intrusions; and the 303 Fe-K Mont Blanc syenogranite. All intruded syntectonically along major-scale transcurrent faults at a time when the substratum was experiencing tectonic exhumation, active erosion recorded in detrital basins and isothermal decompression melting dated at 327-320 Ma. Mantle activity and magma mixing are evidenced in all plutons by coeval mafic enclaves, stocks and synplutonic dykes. Both crustal and mantle sources evolve through time, pointing to an increasingly warm continental crust and juvenile asthenospheric mantle sources. This overall tectono-magmatic evolution is interpreted in a scenario of post-collisional restoration to normal size of a thickened continental lithosphere. The latter re-equilibrates through delamination and/or erosion of its mantle root and tectonic exhumation/erosion in an overall extensional regime. Extension is related to either gravitational collapse or back-arc extension of a distant subduction zone.

Keywords

Acid-basicexhumationextensiongranitemigmatitessyntectonic intrusionU-Pb datingzircon
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 91 , Issue 1-2: Fourth Hutton Symposium: The Origin of Granites and Related Rocks , 2000 , pp. 221 - 233
Copyright
Copyright © Royal Society of Edinburgh 2000

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