ORIGIN OF THE LOW δ18O SILICIC IGNIMBRITES ASSOCIATED WITH THE GUACHIPELÍN CALDERA, NW COSTA RICA

NW Costa Rica has a history of silicic volcanism that ranges from at least 8 to 0.66 my recorded in silicic ignimbrite deposits. The regional chemical trends of selected trace elements and oxygen isotopes in these deposits from Nicaragua to Costa Rica were discussed in a recent report (Vogel et al., 2006), and are similar to the trends in the basaltic lavas from the active volcanic front. Oxygen isotopes of phenocrysts from Nicaraguan and Costa Rican pyroclastic deposits show an increase in δ¹⁸O from northwest to southeast of 1.5‰, except for very low δ¹⁸O associated with the youngest ignimbrites (Guachipelín) in NW Costa Rica, which is the subject of this report. The oxygen isotopes of the coexisting phases in the silicic deposits were in equilibrium and therefore we use the ubiquitous magnetite as a proxy for the overall variation of oxygen isotopes of whole rock (magma). Values of δ¹⁸O (Mgt) are extremely low (to 1.3 ‰) in the Guachipelín deposits.

These ignimbrites were erupted from a nested caldera complex. The source of the older deposits is not known because the younger deposits covered earlier calderas and continuous outcrops are absent. The older ignimbrites may have erupted from the same caldera complex.

In the Guachipelín deposits, three end-member magma types can be identified based on geochemical and mineralogical data from the glassy pumice fragments. Each magma type has a unique chemistry and mineralogy. The mineralogy is characterized by distinct FeMg phases. The chemical variations among all of the Guachipelín pumice fragments are due to magma mixing. Based on hornblende compositions in some of the end members, these magmas equilibrated at about 1.8 kb. The mixed magmas are a result of mixing magmas from distinct sources all with low δ¹⁸O values. The only process that has been suggested for the production of low δ¹⁸O magmas is by melting or assimilation of rocks that have been altered by hot meteoric (or sea water). The low δ¹⁸O magmas may represent melting distinct sources that were igneous material associated with older caldera systems that have interacted with hydrothermal systems.