Unravelling the genesis of young continental-arc shoshonites in the Talamanca Cordillera, Costa Rica

Six to five Myr. old continental-arc shoshonites and other high-K igneous rocks crop out among the summits of the Talamanca Cordillera, Central America. These rocks fill a previously assumed volcanic gap in eastern Costa Rica, showing that the Late Miocene-Pliocene magmatic arc was almost continuous along the current Talamanca Cordillera. Their trace element compositions (e.g., Sr ~ 1700 ppm at Rb ~ 120 ppm concentrations) together with fractional crystallization modeling, and isotopic signatures suggest a genesis related to subduction processes rather than lower crustal melting. Here we present a comprehensive geochemical study that integrates Monte Carlo simulations of trace element geochemistry and isotopic evidence in order to explain the origin of these highly enriched igneous rocks. The trace element budget of the Talamanca arc rocks requires the input of an incompatible element-enriched component, similar in composition to the Cocos Island Province (Pacific Ocean). We interpret the presence of this component to be the result of slab melting of subducted seamounts. Our model suggests that in southern Central American, contribution of OIB-related seamounts is a critical step for the generation of continental crust-like rock compositions.