Abstract

Chemical compositions for 310 igneous rocks from the Cordillera de Panama and the Soná and Azuero peninsulas were supplemented by 40Ar/39Ar dating and Sr-, Nd-, Pb-, and O-isotope analysis to determine the magmatic evolution and oceanic plate interactions over the past 100 Ma in western Panama. An initial phase of intraplate magmatism, having geochemical characteristics of the Galápagos hotspot, formed the oceanic basement of the Caribbean large igneous province from 139 to 69 Ma. Younger accreted terranes with enriched trace element patterns (accreted ocean island basalt [OIB]) were amalgamated between 70 and 20 Ma. A second magmatic phase in the Azuero and Soná peninsulas has trace element patterns (Soná-Azuero arc) suggesting the initiation of subduction at 71–69 Ma. Arc magmatism continued in the Chagres basin region (Chagres-Bayano arc) from 68 to 40 Ma. A third phase formed discrete volcanic centers across the Cordillera de Panama (Cordilleran arc) from 19 to 5 Ma. The youngest phase consists of isolated volcanic centers of adakitic composition (Adakite suite) in the Cordillera de Panama that developed over the past 2 million years.

Initiation of arc magmatism at 71 Ma coincides with the cessation of Galápagos plateau formation, suggesting a causal link. The transition from intraplate to arc magmatism occurred relatively quickly and introduced a new enriched mantle source. The arc magmatism involved progressive transition to more homogeneous intermediate mantle wedge compositions through mixing and homogenization of subarc magma sources through time and/or the replacement of the mantle wedge by a homogeneous, relatively undeleted asthenospheric mantle. Adakite volcanism started after a magmatic gap, enabled by the formation of a slab window.

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