Heavy-mineral analysis and clinopyroxene geochemistry applied to provenance analysis of lithic sandstones from the Azuero-Sona Complex (NW Panama)

The volcaniclastic sandstones of the Middle Eocene to Lower Miocene Tonosi Formation were deposited within the forearc realm of an island are system w hich today is part of the southern Central American landbridge. Both heavy- mineral analyses and geochemical analyses of clinopyroxenes clearly reveal differences in the composition between sediments of the Lower and Upper Ton osi Formation, indicating changes within the tectonic setting as well as is land are evolution during time of deposition. Based on characteristic heavy -mineral suites three lithogroups can be determined and assigned to the Low er and Upper Tonosi Formation, respectively. Provenance analyses indicate t hat the sandstones of the Tonosi Formation were generally derived from an i sland are setting. The abundance of a mafic heavy-mineral suite relative to heavy-minerals derived from metamorphic and sialic intrusive rocks indicat es a convergent plate boundary setting for provenance of the Lower Tonosi F ormation. This result is supported by the chemistry of clinopyroxenes withi n the heavy-mineral suite, which indicates evolved basaltic magma types sou rced the detrital grains. Within the Hb-Ol + Id + Cpx(brown)-Opx + Cpx(gree n)-diapram most samples of the Upper Tonosi Formation plot in the field for divergent plate boundaries. The geochemical analyses also show that clinop yroxenes of the Upper Tonosi Formation additionally exhibit affinities to w ithin-plate alkaline magma types. As an integration of our results we prese nt a provisional two-stage model for the tectono-sedimentary evolution of t he western Panamanian forearc area during the deposition of the Tonosi Form ation. (C) 1999 Elsevier Science B.V. All rights reserved.