Recycling plus: A new recipe for the formation of Alpine-Himalayan orogenic mantle lithosphere

The origin of the lithospheric mantle beneath accretionary orogens is enigmatic; although severe compression of the buoyant crust occurs, the mantle lithosphere is generally thought to be removed and returned to the convecting mantle. We suggest that during the accretion of oceanic arcs and small continental blocks in the Mediterranean region, and more generally throughout the whole Alpine-Himalayan orogenic belt, the mantle lithosphere is newly created and composed of intimately mixed peridotite and crustal material from the forearc region. Potassium-rich volcanic rocks emplaced sometimes more than 30Ma after the formation of this lithosphere carry evidence for the presence of extremely depleted peridotite in their sources, but also for mica-bearing pyroxenites formed by reaction between subducted continental sediments and peridotite. Olivines crystallized from the magmas and mantle-xenocrysts derived from the enriched mantle, have elevated concentrations of Li that correlate positively with ⁸⁷Sr/⁸⁶Sr of the lavas, indicative of an origin from continental crust-derived sediments. If much of the continental crust is formed in accretionary orogens of this type, then extensive tracts of the continental lithosphere may contain mixtures of ultradepleted peridotite and recycled crustal material. In this case a portion of the subducted sediment is not returned to the convecting mantle, but becomes stored within the subcontinental lithospheric mantle.