Date of Award

1988

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Gary R. Byerly

Abstract

Igneous rocks from southeastern Costa Rica and southwestern Panama, a Cenozoic magmatic arc, have been studied. Compositionally, the rocks span the range basalts through andesites to alkali basalts. The young andesites ($<$0.5 Ma) from Chiriqui, in Panama, and San Vito (1-2 Ma), in Costa Rica, have been studied in detail, and can be divided into low-K (high-Mg) and high-K andesites. Similar mineralogy is observed in both the suites, but the high-K andesites contain a higher proportion of hornblende. All the young andesites are characterized by high levels of large ion lithophile elements, such as Ba (550-1550 ppm) and Sr (1000-1900 ppm), and light rare earth elements, La (18-36 ppm). The high-Mg andesites have relatively lower levels of Ba and Sr, but high concentrations of compatible elements Cr (200-300 ppm) and Ni (80-130 ppm). The older rocks of the region are also characterized by high Ba and Sr. A suite of xenoliths, including hornblende gabbro, pyroxenite, and dunite, has been found on the western flank of Chiriqui. All of them have minor but conspicuous phlogopite high in Ba. The hornblende gabbros and the pyroxenite were cumulates from older andesite magmas, unrelated to the young andesites. The dunite came from the upper mantle. The high levels of Ba and Sr in many of these rocks are indicative of mantle metasomatism in the region. The origin of the young andesites can be explained by mixing of a mantle component which produces the Mid-Oceanic Ridge type of basalt and an incompatible element enriched fluid. Geochemistry and ages of the andesites and the older rocks suggest that metasomatism probably has taken place in the region since the Middle Miocene. The Panama Fracture Zone provided an uninterrupted pathway for the eruption of the young andesites. In contrast to most Central American orogenic basalts and andesites, these andesites were not affected by plagioclase fractionation; in addition, olivine fractionation did not occur from the high-Mg andesites.

Pages

330

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