Serpentinite–Sediment Associations: Provenance Controlled by Competing Extensional–Contractional Tectonic Processes During the Evolution of the Northern Apennines (Eastern Elba Island, Tuscany)

Orogenic wedges juxtapose tectonic units that originated far from each other, and tracing these back to their origin is often difficult. We have studied two contrasting serpentinite-sediment associations of the Alpine-Apennine orogenic wedge of eastern Elba Island with the help of a detrital zircon study of the sediments and a geochemical comparison of the relic phases of their associated serpentinites. We demonstrate that these very likely originated in different branches of the Ligurian Ocean and in contrasting tectonic settings, one during opening of Alpine Tethys and the other during Apenninic contraction-exhumation. First, the Early Cretaceous Palombini shales are associated with abyssal ocean floor serpentinite-ophicalcites of a Ligurian ophiolite (LO) that originated in the western branch of the Ligurian Ocean during ultraslow spreading. They have an Adria/African zircon provenance, indicating proximity to Adria rather than Corsica-Europe and the associated serpentinites are highly depleted and relatively little deformed. The second sediment-serpentinite association has a tectonised serpentinite band in contact with highly deformed, Miocene blueschist facies metasediments. Detrital zircons of these metasediments (Acquadolce (AD) and Pseudomacigno) record major Eocene-Oligocene U-Pb zircon age peaks, with an igneous provenance in the western and central Alps respectively. An age peak at ca. 38 Ma links the Pseudomacigno sediments to calc-alkaline volcanic rocks of the central Adamello massif, whilst an Oligocene age peak at ca. 32 Ma indicates western Alpine sources for the AD Unit. The associated massive, highly tectonised AD serpentinite represents most likely a mantle sliver of subcontinental lithospheric mantle, which together with Oligocene blueschist facies rocks underwent synorogenic Apenninic tectonic extrusion during W-directed subduction-rollback of the eastern branch of the Ligurian Ocean.