New geochronological data from mafic and felsic meta-intrusives of the Western Alpine Ophiolites: the missing magmatism of the Ligurian-Piedmont Ocean?

The Ligurian-Piedmont Ocean (LPO) is inferred as a relatively narrow oceanic basin in palaeogeographic restorations, but the actual amount of oceanic lithosphere generated and the timing of magmatic accretion are still debated. Magmatic ages obtained from LPO intrusive rocks predominantly range between 165 and 160 Ma, supporting the interpretation of the LPO as a magma-poor ocean. However, this relatively short timespan of magmatic accretion may also suggest that the orogen sampled older sectors of the oceanic lithosphere, while younger (and more oceanward?) sectors could have been deeply subducted without returning.

We therefore focus on studying a poorly-known stack of oceanic lithosphere (i.e., the Susa and Lanzo Valley Ophiolites; SLVO), which is exposed in the inner-central sector of the Western Alps and tectonically juxtaposed with the Gran Paradiso and Dora-Maira massifs. The SLVO were metamorphosed under eclogite-facies peak conditions and consist of large volumes of serpentinite hosting up to kilometer-sized metagabbro bodies, with Fe-Ti-rich differentiated masses and rare metaplagiogranite dykes. The metaophiolite sequence also includes widespread metabasaltic rocks and a metasedimentary cover consisting of minor quartzite and marble levels overlain by calcschist.

Two pairs of Fe-Ti metagabbro and metaplagiogranite s.l. sampled close to the Avigliana (lower Susa Valley) and Mondrone (middle Ala Valley) localities have been selected for zircon U-Pb dating. In each sample, the dated zircons yield magmatic ages falling within the uppermost Jurassic Period (~150 Ma). The common age, along with similar major and trace element compositions, suggests a cogenetic origin within differentiation trends for the two pairs of metagabbro-metaplagiogranite (De Togni et al., 2024). Consequently, the SLVO were sampled from a sector of the LPO characterized by magmatic activity at ~150 Ma, significantly younger than most of previously reported ages for the LPO magmatism. We argue that the SLVO represent the youngest oceanic lithosphere accreted in the Western Alps and they may provide new constraints on the structural architecture of the LPO.

 

De Togni, M., Balestro, G., Rubatto, D., Castelli, D., Gattiglio, M., & Festa, A. (2024). Late Jurassic magmatism in the Ligurian-Piedmont Ocean constrained by zircon ages of mafic and felsic meta-intrusives. Terra Nova, 00, 1–11. doi.org/10.1111/ter.12723