The Panamanian Isthmus presents an ideal opportunity to study arc magmatism, arc-hotspot interaction, and the effects of the South America-Panama collision. These effects include changing mantle wedge compositions and localized tectonic activities in western Panama. The Bocas del Toro sedimentary basin contains interbedded Miocene volcanic lava flows that range in age from 12 Ma to 8 Ma and sit behind the main body of the arc. The volcanic rocks of Bocas del Toro consist of trachy-basalt to trachy-andesite with SiO₂ content ranging from 45 wt. % to 64 wt. %. The MgO content is relatively low ranging from 0.35 wt. % to 3.43 wt. %, and with moderate depletion in FeOt (3.9 wt. %-8.0 wt. %) and CaO (2.8 wt. %-10 wt. %). However, K₂O content is extremely high (2.0 wt. %-5.2 wt. %), and these rocks are among the most alkaline in Panama. In terms of trace element geochemistry, the Bocas del Toro rocks exhibit a distinct, but decreased slab dehydration signature with a low Nb-Ta anomaly, enriched fluid-mobile LILEs and low Ti content. We have grouped Miocene and younger western Panama and eastern Costa Rica volcanic rocks into five groups: main arc tholeiite (~17-11 Ma) and calc-alkaline (~12-8 Ma), Bocas del Toro (~12-8 Ma), backarc alkaline (~8-2 Ma), and adakite ([less than]2 Ma) groups. In terms of trace element ratios, the Bocas del Toro rocks have relative low Ba/La, and have values that are higher, but approach the Cocos Ridge, that tracks of the Galapagos hot spot. The La/Yb and Th/Yb vs. Ta/Yb also show values that plot between the tholeiite and general calc-alkaline groups. Overall, trace element geochemistry indicates that an enriched OIB-like component mixed into the melts of the mantle wedge. With constraints from referenced Pb-Nd-Sr isotopes and the pattern of trace element geochemistry, the data range of the Bocas del Toro samples should be distributed close to the general calc-alkaline and backarc alkaline groups. According to the trace element geochemistry and isotope modeling, the percentage of enriched (OIB / Cocos track) geochemical component mixed into the melts of the mantle wedge, was quantified at 1-3%. Moreover, the pattern indicates that, as arc evolution continued, a progressively greater enriched geochemical components mixed into the wedge melts. Geochemical modeling allows estimates of the pressure, temperature etc., conditions under which magma formed. The MELTS software was used for major element modeling and show that the Bocas del Toro rocks underwent low-pressure (0.5-1.0 kbars) fractional crystallization, from 1200° C to 900° C with 50%-55% fractionation from a starting magma with ~11 wt. % MgO. In addition, trace element models and the ARC BASALT SIMULATOR 3.0 were used to simulate partial melting in the mantle wedge. These models indicate a component of enriched OIB-like mantle. The simulator also shows that the mantle wedge underwent 3.5%-6.0% of melting fraction under dry conditions at pressures of 1.8 Gpa to 1.9 Gpa (~60km) with temperatures of 1150-1350°C. In conclusion, we suggest that the geochemical variations of Bocas del Toro were caused by an influx of an OIB-like component into the mantle wedge by 12 Ma, and that possible effects from crustal extension reduced the overall subduction signature.
