In situ U–Pb dating and trace element composition of apatite in the Chamrousse ophiolite (Western Alps)

The Chamrousse ophiolite is located in the external crystalline massifs of the Western Alps and has long been considered one of the best-preserved Cambro-Ordovician ophiolites of the Variscan belt. However, recent in situ U–Pb zircon dating indicates that the Chamrousse ultramafic-mafic complex consists of a Cambro-Ordovician continental basement unit (peak ages around 490Ma) and a Devono-Carboniferous ophiolitic sequence (350-360 Ma). In this study, we present new in situ U–Pb apatite ages and trace element compositions from the Chamrousse complex. In the Cambro-Ordovician unit, apatite ages (350 Ma), combined with rare earth element (REE) compositions, indicate that apatite crystallization is metamorphic. In contrast, the ages (350 Ma) and REE compositions of apatite in the Devono-Carboniferous mafic and ultramafic rocks suggest a magmatic origin. These results highlight how apatite can serve as an effective petrogenetic tool to constrain and distinguish the magmatic and tectono-metamorphic evolution of mafic and ultramafic units in the Variscan basement.