MIOCENE ARC MAGMATISM IN WESTERN PANAMA AND ITS CONSTRAINTS ON MANTLE WEDGE AND TECTONIC CHANGE

The Panamanian Isthmus presents an ideal opportunity to study arc magmatism and the effects of the South America-Panama collision. These include changing mantle wedge compositions and localized tectonic activities. The Bocas del Toro sedimentary basin contains interbedded Miocene volcanic lava flows that ranges in age from 12 Ma to 8 Ma and sits behind the main body of the arc.

The volcanic rocks of Bocas del Toro consist of trachy-basalt to trachy-andesite with SiO₂ 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 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 Panama/Costa Rica volcanic rocks into five groups: main arc tholeiite (~17-11 Ma) and calc-alkaline (~12-8 Ma), backarc alkaline (~8-2 Ma) and Bocas del Toro (~12-8 Ma), and adakite (< 2 Ma) groups. In terms of trace element ratios, the Bocas del Toro rocks have low Ba/La, and have values that are higher, but approach the Cocos Ridge/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.

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 rocks underwent low-pressure (~1 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. They also show that the mantle wedge underwent 3.5%-6% of melting under dry conditions at pressures of 1.8 Gpa to 1.9 Gpa 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 that reduced the overall subduction signature.