MESOZOIC INVASION OF CRUST BY MORB-SOURCE ASTHENOSPHERIC MAGMAS, US CORDILLERAN INTERIOR

Mafic and ultramafic xenoliths entrained in lavas of the Cima volcanic field have Nd and Sr isotopic ratios indicative of a source similar t o that of mid-ocean ridge basalt (MORE). Nd and Sr internal isochrons demonstrate a Late Cretaceous intrusion age. These results, combined w ith evidence for emplacement in the lower crust and upper mantle, indi cate invasion of the lower crust by asthenospheric magmas in the Late Cretaceous. Constituting the first prima facie evidence for depleted-m antle magmatism in the Basin and Range province prior to late Cenozoic volcanism, these results lend key support to models suggesting crusta l heating by ascent of asthenosphere in the Mesozoic Cordilleran inter ior.