IGNEOUS PETROGENESIS AND TECTONIC SETTING OF PLUTONIC AND VOLCANIC ROCKS OF THE CORDILLERA DE TALAMANCA, COSTA RICA—PANAMA, CENTRAL AMERICAN ARC

摘要

The Cordillera de Talamanca represents the largest and highest mountain belt in Costa Rica and Panama. This study presents a petrologic description and petrogenetic interpretation of the plutonic and volcanic rocks of the Talamancas. The Late Miocene Talamanca Intrusive Suite (TIS) represents one of the youngest and most extensively exposed plutonic suites associated with orogenesis in the world. Rapid uplift associated with subduction of the Cocos Ridge (approx 5 Ma) is primarily responsible for exposing this plutonic core to the Chorotega tectonic block. In addition, a combination of Pleistocene glacial and recent rain forest conditions have provided an efficient erosional mechanism for unroofing the Cordillera de Talamanca and generating an eclectic mix of geomorphological features. On the basis of petrographic, geochemical, and geochronological information, four major lithologic groupings are made on igneous rocks of the Cordillera de Talamanca, which include mid-Oligocene tholeiitic gabbros, mid-Oligocene El Baru plutonic rocks, Late Miocene calc-alkaline TIS, and a Plio-Pleistocene volcanic group. The mid-Oligocene intrusives are interpreted to represent an early stage of island arc igneous activity following the initial ophiolite obduction event(s) recorded by the basement Nicoya Complex. The next major igneous episode is represented by the TIS (17-6 Ma), which is the focus of this study. A integrated study of major and trace element and isotopic (Nd and Sr) modeling indicates that the TIS originated by partial melting of a metasomatized (LIL enriched) mantle source. Magmatic differentiation of the TIS was primarily a function of fractional crystallization of a plagioclase— nornblende-biotite-magnetite assemblage generating the gabbro to granite compositional variation. Estimated intensive parameters of the TIS crystallization suggest epizonal (<1 kb) intrusion at temperatures of 805° to 860℃. Coevaf(12-9 Ma) calc-alkaline dacites have been found in the Talamancas and may represent volcanism cogenetic with the TIS, suggesting that the TIS intruded into its own volcanic ejecta. Talamanca volcanic rocks are primarily represented by Plio-Pleistocene andesites that display geochemical characteristics consistent with derivation from a subducted oceanic crustal source, such as hot, young Cocos Ridge material.