Geology and Geochemistry of the Western Panamá Canal Basin Volcanic Arc Rocks

Panamá arc activity has been divided into three episodes, the Late Cretaceous to Eocene episode, the Miocene episode, and the Pliocene to recent adakite suite. Rocks from the oldest episode are dominantly hornblende bearing, contain a large negative Nb-Ta anomaly, and exhibit enrichment of large-ion lithophile elements (LILEs). These characteristics are all indicators that the Late Cretaceous-Eocene episode rocks formed from a hydrous mantle wedge derived subduction zone magma. Miocene episode rocks are commonly calc-alkaline throughout Panamá. In contrast, Miocene episode rocks from the Panamá Canal (PC) are strongly tholeiitic, lack hydrous minerals, have a decreased Nb-Ta subduction zone signal, and exhibit LILE depletion relative to older PC volcanism as well as the older Panamá arc episodes. New major element, trace element, and radiogenic isotope data has been analyzed for rocks from the Western Canal Basin (WCB) between El Valle volcano and the PC. The rocks contain both calc-alkaline and tholeiitic signatures, and range from basalt to dacite in composition. In terms of trace elements, the rocks exhibit enriched heavy rare earth element concentrations that are among the highest in all of Panamá. The Ba/Yb values for WCB samples are between the depleted PC samples and fluid rich El Valle samples. The WCB Ta/Yb values are similar to PC samples, with both exhibiting a lesser subduction zone signal than the Cretaceous-Eocene arc. On a V versus Ti tectonic discrimination diagram both PC and WCB samples plot in the MORB/BAB/CFB field. Pb and Sr isotope ratios for the Panamá Canal Basin rocks are similar to the Cordilleran arc, whereas Hf and Nd isotope ratios are the highest measured in all of Panamá and are consistent with the contribution of young asthenospheric mantle. Overall, trace element, isotopic and geophysical data indicate that the western Canal basin volcanic rocks formed due to the influx of young juvenile mantle in an extensional tectonic system. One explanation is the existence of an underlying tear in the subducted slab due to the ongoing collision of Panamá and South America. This is consistent with the fracturing of the Isthmus as proposed by Farris et al. (2011), and a tear in the underlying slab would allow for the influx of young mantle material. To place constraints on the crustal structure of the Canal basin, a series of gravity measurements were conducted. Bouguer gravity models exhibit a series of westward deepening half-grabens filled with low-density volcano-sedimentary material. Such horst and graben systems are interpreted to have formed due to extension in both east-west and north-south directions. Sub-orthogonal normal fault systems reflect the extensional nature of the Panamá Canal Basin. The models also indicate that 3 to 5 km thick volcano-sedimentary basins dominate the shallow subsurface, indicating that observed lavas are likely low in volume. Finally, the gravity models show the existence of a continuous sedimentary basin across the isthmus, which allows for the existence of a narrow strait connecting the Caribbean and Pacific during this time as proposed by Collins et al. (1996).