K/Ar age dating of the ophiolitic Nicoya Complex of the Osa Peninsula, southern Costa Rica

Abstract

K/Ar ages were determined on a suite of 24 intrusive and extrusive basaltic rocks from the Nicoya Complex as part of a research program investigating the geology and placer gold mineralization of the Osa Peninsula. These isotopic ages indicate submarine volcanism on a regional scale over a 34 Ma period, from late Santonian through to late middle Eocene time (78-44 Ma). A statistical study of all available ages from the Osa-Golfito sector suggests that most of the igneous rocks formed (crystallized) during one of three volcano-plutonic events—or periods when submarine volcanism was especially active. The oldest event dates back to the Santonian (78±2Ma); a second well-defined event took place during the Paleocene, peaking at around 60.2±7.6Ma; and the third event occurred during the Eocene at around 44.1±4.4 Ma. There is excellent correspondence between these isotopic results and previous micropaleontologic dating of cherts and limestones that occur interstratified with the basaltic lavas. This paleontologic evidence provides a record of virtually continuous sedimentation from the Campanian through to middle Eocene time. In the extreme southeastern tip of the Osa Peninsula, marine sedimentary rocks containing microfossils of late Oligocene to early Miocene age occur interbedded with, and intruded by, basaltic igneous rocks. These deposits include an appreciable clastic component and, as such, are atypical of the sedimentary rocks elsewhere in the peninsula, which generally lack a terrigenous component. This local younger volcano-sedimentary sequence is considered to have developed around an isolated volcanic island.