Magmatic evolution of ignimbrites in the Bagaces Formation, Guanacaste Province, Costa Rica

Silicic volcanism (>65 wt.% SiO2) in the northern Costa Rican segment of the Central American volcanic arc was widespread from the Miocene through the Late Pleistocene. Some of the most silicic volcanic rocks in Costa Rica are found in the northern Guanacaste Province, and include ignimbrites of the Bagaces Formation. Bagaces ignimbrites are among the earliest exposed high-silica products in this part of the arc (<10 Ma) and are thus important to understanding its chemical, temporal and spatial evolution. While the northern part of the arc is built upon Paleozoic crust of the Chortis block, the arc in Costa Rica is built upon thick (∼40 km) oceanic plateau of the Chorotega block, located on the western extent of the Caribbean Large Igneous Province (CLIP).; Although detailed geochemical studies have been done on Pleistocene ignimbrites in central and northern Costa Rica, no such studies have been performed on Bagaces ignimbrites. This study describes and delineates ten (10) discrete ignimbrite units in the Bagaces Formation. Three of these units cannot be related to the other ignimbrites by simple fractional crystallization or partial melting of the same source rocks. Five of the units are clearly related to the same high-silica magma body. A robust statistical technique relatively new to igneous petrology, Polytopic Vector Analysis (PVA), was used to test mixing relationships among three of these units, corroborating petrographic and geochemical evidence of magma mixing. The geochemical connection of the five tuffs to the same magma batch supports the presence of long-lived system of high-silica magmas that was erupted periodically and punctuated by several episodes of mixing in a dynamic subvolcanic plumbing system.; A change in Sr/TiO2 values from Bagaces ignimbrites to ignimbrites associated with the younger Guachipelin caldera may indicate a change in the mantle or crustal source for the magmas; a similar pattern is observed in lavas, with a shift from low-Sr/TiO2 in the 22-11 Ma Sarapiqui arc to high-Sr/TiO2 values in more recent lavas. Diagnostic trace element ratios used to indicate the relative contributions from slab fluids to mantle melts (e.g. Ba/La) have similar ranges in pumice fragments from Bagaces ignimbrites as those for younger pyroclastic rocks in northern Costa Rica. The overall similarity of trends in these ratios between basaltic lavas and silicic volcanics along the length of arc is convincing evidence for a genetic relationship between previously emplaced calc-alkaline basalts from the mantle and silicic magmas (Vogel et al., 2006).; This study presents a similar model, whereby high-silica Bagaces magmas were produced by melt extraction from stalled, partially crystalline mantle melts or partial melting of crystallized magmas in warm lower crust of the CUP. Bagaces ignimbrites have trace element patterns and Na2O/K 2O values that resemble those of upper continental crust, although the crust in northern Costa Rica is modified oceanic plateau. Given the absence of evolved crust in Costa Rica, ignimbrites of the Bagaces formation may represent a step in the transformation of oceanic crust to juvenile continental crust.