History and geochemical evolution of igneous rocks forming the Panama land bridge since Late Cretaceous

Abstract

Major and trace elements, Sr-, Nd-, Pb-, and O-isotopes and 40Ar/39Ar dating of igneous rocks from the Cordillera de Panama and the Soná and Azuero Peninsulas define the magmatic evolution over the last 95 Ma in western Panama. An initial phase of intraplate magmatism derived from a Galápagos-type plume source and forms the Caribbean Large Igneous Province basement at 95-69 Ma. Younger accreted terranes with enriched trace element patterns were amalgamated between 71 and 21 Ma. A distinct magmatic suite in the Soná and Azuero Peninsulas shows trace element patterns suggesting the initiation of subduction at 71-68 Ma (Soná Azuero arc). Arc magmatism continues in the cordillera with a first pulse between 66 and 61 Ma in the Chagres and Bayano area and a second pulse from 50 to 42 Ma (Chagres Bayano arc). A third phase of younger arc magmatism started after a significant magmatic gap of about 20 Ma from 19-7 Ma along the whole Cordillera de Panama (Cordilleran arc). The youngest magmatic phase consists of isolated volcanic centers of adakitic composition in the Cordillera de Panama that developed over the last 2 Ma (Adakite suite). Initiation of arc magmatism at 71 Ma coincides with the cessation of Galápagos plateau formation, which insicates a causal link. The transition from intraplate to arc magmatism occurred relatively fast (3 Ma) and introduced a new enriched mantle source. The transition between early Chagres Bayano arc and younger Cordilleran arc (41 to 19 Ma) involves a change to more homogeneous intermediate mantle wedge compositions through mixing and homogenisation of sub-arc magma sources through time and/or the replacement of the mantle wedge by a homogeneous, relatively undepleted asthenospheric mantle. The break-up of the Farallon plate at this time (~ 25 Ma) may have triggered these changes. Adakite volcanism started after a magmatic gap, enabled by the formation of a slab window. The validity of these results is supported by alteration studies. Even though strong petrographic evidence for alteration and new growth of minerals is found, the compositional and isotopic systematics are changed relatively little by hydrothermal alteration at variable temperatures and diverse water rock ratios. Therefore source composition determinations are possible as long as oxygen isotopes are used as tracers even if the samples are affected by alteration.