Rb-Sr and 40Ar/39Ar Mineral Ages of Granitoid Intrusives in the Mabujina Unit, Central Cuba: Thermal Exhumation History of the Escambray Massif

We present new and critical isotopic data for late- to postkinematic granitoid intrusions and their epidote-amphibolite-facies country rocks (Mabujina unit) in central Cuba. The Mabujina unit tectonically overlies a nappe sequence of high-pressure (HP) metamorphic rocks in the Escambray Massif. These granitoid rocks have a variable granodioritic to granitic composition, are generally rich in large ion lithophile elements, and resemble normal midocean ridge basalts in their high field strength element concentrations. On the basis of low initial 87Sr/86Sr ratios, it is inferred that Sr is very likely to be mantle derived. Primary coarse-grained muscovite from pegmatitic intrusions yields Rb-Sr crystallization ages of ∼86-88 Ma (Rio Jicaya region) and ∼80-82 Ma (Yayabo region). Laser 40Ar/39Ar ages for coarse- and fine-grained white mica (72-74 Ma) and Rb-Sr ages for biotite (73 Ma) outline the cooling history of these granitoids and their country rock. Dating of pegmatites within the Mabujina unit places minimum age limits on the epidote-amphibolite-facies metamorphism, on the end of the magmatic activity and initial collision of the Cretaceous island arc, and on the HP metamorphism and juxtaposition of tectonometamorphic units in the Escambray Massif. The combination of the new thermochronological data with constraints from the geological record indicates thermal relaxation during the initial stages of the arc-continent collision.