Crystallization of hydrous mantle magmas at the base of extending continental crust: evidence from the Mafic Complex of the Ivrea-Verbano Zone (Italian Alps)

The composition of the lowermost continental crust is still enigmatic, mostly due to its typical inaccessibility. The archetypal Ivrea-Verbano Zone from Italian Alps exposes a nearly complete sequence of lower continental crust. The Ivrea-Verbano Zone encases the Permian Mafic Complex, an 8 km thick gabbro-dioritic batholith, crystallized during the post-collisional transtensional phase of the Variscan orogeny. The lower section of the Mafic Complex (hereafter referred to as Lower Mafic Complex) is remarkably mainly composed of amphibole gabbros. Evidence for crystallization of hydrous mantle magmas within or at the base of the extending continental crust was rarely reported. The Lower Mafic Complex from the Ivrea-Verbano Zone enables to shed light on the magmatic processes associated with deep-seated emplacement of hydrous mantle magmas at the base of the extending continental crust. The present contribution wishes to ascertain the nature of the H2O-rich magmas forming the Lower Mafic Complex, and the processes driving their chemical differentiation. The whole dataset also allowed us to provide evidence for involvement of primary mantle magmas with different compositions in the building of the whole Mafic Complex. In a broader perspective, the present study aims at acquiring new insights into the accretion of the lower continental crust by magmatic underplating in extensional continental settings.

This submission contains a pdf text file with the analytical methodologies used; a pdf file containing supplementary figures; supplementary tables; and references. However, a brief description of the analytical methodologies used is provided within the manuscript.