Deformation, crustal melting and magmatism in the crustal-scale East-Variscan Shear Zone (Aiguilles-Rouges and Mont-Blanc massifs, Western Alps)
Résumé
The Aiguilles-Rouges and Mont-Blanc massifs represent a segment of a crustal-scale transpressional shear zone named the East Variscan Shear Zone (EVSZ) along which two plutonic pulses occurred during Early and Late Carboniferous times. The aim of this study is to constrain the relationships between the dynamics of the crustal-scale shear zone, the mechanisms of pluton emplacement at different structural levels within the crust and the magma sources. A detailed structural analysis of the whole massif highlights the crustal-scale anastomosed network of the EVSZ. Microstructural observations and LA-ICPMS U–Th–Pb zircon ages from large plutons constrain the beginning of the transpression at ca. 340 Ma. From 340 to 305 Ma, the EVSZ broadened and formed a 25 km-wide dextral S–C–C’ anastomosed shear zone network with dilation zones acting as preferential pathways for melt migration and pluton growths. Moreover, LA-ICPMS U–Th–Pb zircon ages from small magmatic bodies (i.e. pegmatite, aplite and microgranite) indicate an Ordovician-age inheritance component. Field evidences and zircon inheritance indicate that Late Carboniferous granitic melts are mainly derived from water-fluxed melting of Ordovician orthogneiss with the input of mantle-source derived magmas. Over time, the growth of the dextral anastomosed network enhanced water transfer through the shear zones and water-fluxed melting to produce more anatectic melts.
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