CHANGES IN THE COMPOSITION OF MIOCENE MAGMATISM FROM EASTERN TO WESTERN PANAMA

New trace element and radiogenic isotope data has been analyzed for rocks from the Miocene Panama volcanic arc, including the Western Canal Basin (WCB) and Bocas del Toro (BdT), roughly representing the western and eastern reaches respectively of Miocene volcanism in Panama. The new data is used here to show changes in mantle source and subduction zone inputs across the arc and discuss their implications on subduction zone processes in Panama. BdT lavas are shoshonitic with strong enrichments in large ion lithophile elements (LILE) and moderate enrichments in other incompatible elements. The WCB magmas are sub-alkaline and vary from calc-alkaline to tholeiitic, and while the calc-alkaline varieties display strong LILE enrichments the tholeiitic lavas do not. WCB lavas have higher 𝜀Nd (>7.3) compared to the BdT lavas (<6), consistent with other measurements made of the Miocene arc lavas throughout Panama. Similarly, BdT lavas have more radiogenic Sr (0.7037 – 0.7041) and Pb (206Pb/204Pb = 18.91 – 19.35) isotopic compositions than those of the WCB lavas (<0.7037 and <19.05).

Most all Miocene volcanics have more radiogenic Pb isotopic (206Pb/204Pb>18.8) composition than Central American sediments, therefore sediments can be ruled out as the radiogenic Pb end member for these magmas. WCB lavas have isotopic compositions consistent with Caribbean Plate mantle (Caribbean Large Igneous Province, CLIP). The addition of a slab component (SC) to the CLIP mantle can explain the isotopic composition of the calc-alkaline WCB lavas, while the tholeiitic lavas are likely the resulting of melting CLIP mantle that has experienced relatively little SC addition in comparison. The BdT lavas have too low 𝜀Nd at radiogenic Pb compositions to be modeled reasonably by the addition of Central American SCs to a CLIP mantle. Instead of CLIP, a Galapagos mantle domain may be the radiogenic Pb end member in eastern Panama, as suggested by previous authors. The BdT compositional array can be explained by the addition of SCs to this flavor of Galapagos mantle. The continuing subduction of the Panama Fracture Zone as well as the Cocos and Coiba Ridges may have allowed for the Galapagos style mantle to enter the mantle wedge and for the latter to contribute to the hampering of the subduction of the Nazca Plate beneath western Panama.