PETROLOGIC EVIDENCE FOR POSSIBLE ULTRAHIGH PRESSURE METAMORPHISM IN THE CHUACUS COMPLEX OF NORTHCENTRAL GUATEMALA

High-P eclogitic banded gneisses, schists, and migmatites with a dominant sedimentary protolith forming the Chuacús complex in Guatemala contain relic textural and mineralogical features that suggest UHPM prior to high-T, nearly-isothermal decompression and partial melting. Radial cracking around quartz and other inclusions in garnet and kyanite could be interpreted as due to the coesite-quartz inversion. The preservation of assemblages such as Cc/Ar-Rt-Qz, Zo-Ky-Gt-Rt-Qz as well as the abundance of lamellar inclusions of rutile/ilmenite in garnet, phengite and zoisite, and high sodium content of some garnets (up to 1200 ppm), are additional observations consistent with UHPM. However, despite intensive search for relic coesite none has been found, possibly because uplift of the Chuacús rocks occurred under high-T and abundant fluids leading to the partial melting of the gneisses and causing the complete disappearence of coesite. Preliminar thermodynamic mineral calculations based on equilibrium assemblages such as garnet-amphibole-zoisite-albite-quartz-rutile-biotite, amphibole-plagioclase, and garnet-biotite, yield a range of pressures between 20 and 30 kbar and temperatures between 700 and 800 °C for the decompressional event. Rapid uplift is inferred from lack of compositional zoning in garnet, coronas of titanite on rutile, of biotite on phengite, and of Ep-Qz on garnet, as well as symplectites of amphibole-Qz and amphibole-Pl-Cpx after omphacite. Partial melting of some rocks occurred at high T and P producing an Ab-rich neosome in equilibrium with the high-P assemblage quartz-garnet-rutile-amphibole-epidote/zoisite-phengite-clinopyroxene. Although K-Ar dating of micas and hornblende yield ages between 72-48 Ma, local and regional stratigraphic and structural data better support a pre-Mesozoic age for the multi-stage HP-UHP metamorphism and polyphase folding that affected the Chuacús complex. Its fault-bounded contacts and the extremely high P-T conditions discovered for these rocks open new lines of thinking about the tectonic evolution for the entire geologic history of the Maya-Chortís-Oaxaquia microcontinental blocks forming the structural backbone of Mesoamerica.