New isotopic constraints on age and magma genesis of an embryonic oceanic crust: The Chenaillet Ophiolite in the Western Alps

Abstract

The Chenaillet Ophiolite is one of the best-preserved remnants of the Piemont-Liguria oceanic basin, a branch of the Central Atlantic that opened during the separation of Adria/Africa from Europe. Despite numerous studies of structure, petrology, geochemistry and isotope geochronology, the timing and genesis of various magmatic rocks within the Chenaillet Ophiolite are still controversial. We provide in this study integrated in situ analyses of zircon U–Pb age and O–Hf isotopes for the troctolite and albitite within the Chenaillet Ophiolite. Our new results indicate that the troctolite and albitite crystallized synchronously at ~165Ma. Zircons from the troctolite have homogeneous Hf and O isotopic compositions, with εHf(T)=+13.5±1.0 (2SD) and δ18O=5.4±0.4‰ (2SD), indicating crystallization from magmas that were derived from a depleted, MORB-like mantle. The albitite zircons give consistent εHf(T) values (+13.0 to +13.5) within errors with those of troctolite zircons, but variable δ18O values. The altered zircon domains have relatively low δ18O values of 4.7±0.6‰ (2SD) due to subsolidus hydrothermal alteration, whilst the least-altered zircon domains give δ18Ozir values of 5.1±0.4‰ (2SD), indistinguishable within errors with the troctolite zircons and the igneous zircons from the Mid-Atlantic and Southwest Indian Ridges gabbros, norites, and plagiogranites of modern oceanic crust. In situ zircon O–Hf isotopic data suggest that the troctolites and albitites are most likely cogenetic, with the albitites being formed by extreme fractional crystallization from the basaltic magma.Based on our new age results and compilation of the reliable literature U–Pb age data, the ophiolitic gabbros from Eastern, Central and, Western Alps, Liguria and Corsica crystallized nearly synchronously at 158–166Ma, suggesting a short life span of <11m.y. for the formation of the Piemont-Liguria oceanic domain, rather than a ~30m.y. life span as previously thought. The Chenaillet ophiolite is likely a remnant of embryonic oceanic crust, rather than a piece of “mature” oceanic crust. Provided the spreading velocities of <3cm/yr full rate for magma-poor MOR sequences, the maximum width to the Piemont-Liguria oceanic floor would have been in the order of 300km.