STRUCTURE AND GEOCHEMISTRY OF A PANAMA ARC SECTION: JUXTAPOSITION VIA CONTRACTIONAL TECTONICS

The goal of this study is to define the structural and geochemical characteristics along the Caribbean coast of south Panama. In particular, this study focuses on an extensive exposure of pillow basalt and gabbroic rocks exposed near Puerto Obaldia. The two competing theories for origin of these rocks are that: 1) they are part of the Caribbean crust upon which the Panama arc was built, or 2) that the rocks are part of the arc itself. Geologic, mineralogic, and major and trace element data for this study were obtained from the Kuna Yala region of Panama. The rocks at the Puerto Obaldia location consist of two distinct parts, an upper section containing extensive pillow basalt and a lower section containing crystalline hornblende gabbroic rocks. Structural data suggests that the gabbro section has been folded and thrust over the pillow basalt section capped by layered volcano-sedimentary rocks. Major, trace element geochemistry data and U/Pb zircon geochronology have been utilized to determine the origin of the Puerto Obaldia section. In general, the rocks exhibit trace element geochemistry that includes elevated LILEs and a distinct negative Nb-Ta anomaly. Zircons separated from the gabbroic section yielded a concordant age of 58 Ma. The geochemical signature and the age strongly suggest that the combined gabbroic rocks and pillow basalts are derived from an early submarine portion of the Panama arc. The existence of the extensive pillow basalt upper section suggests that there may have been a connection between the Atlantic and Pacific at this time. In addition, REE trace element modeling was used to derive the gabbroic and pillow basalt from a common initial melt, with the gabbroic rocks forming a resitiic cumulate and the pillow basalt forming from the fractionated liquid. Finally, juxtaposition of the underlying hornblende gabbro and overlying pillow basalt via folding and thrusting must have occurred post-Eocene. In addition, the orientation of the observed contractional deformation is consistent with the modern GPS vector between the Panama block and the Caribbean plate, but better constraints on the age of contraction do not yet exist.