MANTLE INFLUENCES ON MIOCENE MAGMATISM IN CENTRAL PANAMA

South America collided with the Panama arc in the late Oligocene/early Miocene, fracturing the Panama block, inducing zones of extension and contraction, and ultimately resulting in the current tectonic configuration of Panama. During the Miocene, the Panama Canal Basin (PCB) lay within such a zone of extension. Evidence for extension throughout the basin comes not only in the form of structural and geophysical data but geochemical signatures of volcanic rocks consistent with shallow melt generation. Miocene lavas and pyroclastic rocks from the PCB have been analyzed for whole rock major and trace elements as well as radiogenic isotopes (Pb, Hf, Sr, Nd). PCB rocks have calc-alkaline arc and MORB-like signatures on tectonic discrimination diagrams.

The geochemical signatures of Panamanian arc rocks are influenced by mantle sources derived from the Galapagos hot spot. These sources include the Caribbean large igneous province (CLIP), Galapagos isotopic domains (north, central, south, east), the Galapagos spreading center (GSC), and seamount ridge tracks associated with the Galapagos domains (Cocos Ridge, Coiba Ridge, Malpelo Ridge). Trace element fractionation models require a GSC contribution to reach the level of enrichment observed in PCB volcanic rocks. Isotope mixing lines support a strong mantle influence on Miocene PCB rocks. The models result in a 50-70% contribution of GSC mantle compositions to the primary melts that produced Miocene magmatism in central Panama. This is possible through an influx of fresh mantle material, which may have resulted from slab tear, slab window, or slab detachment processes that ended orthogonal subduction in central Panama.