OXYGEN ISOTOPE RATIOS OF CENTRAL AMERICAN CENOZOIC IGNIMBRITES

The Central American Cenozoic ignimbrites represent one of the largest known terrestrial provinces of silicic magmas. Evidence from tephra layers in ODP Leg 165 Caribbean deep-sea cores indicates that the ignimbrites were largely erupted during two well-defined episodes or flare-ups, one in the Miocene (Coyol group) and the other in late Eocene (Matagalpa group). This Cenozoic volcanism is markedly bimodal, with the huge volume of silicic magmas dominating the basaltic ones and intermediate compositions relatively minor. We obtained oxygen isotope compositions of forty whole-rock samples of volcanic rocks from Nicaragua and Honduras. Unaltered samples of ignimbrites were obtained by collecting glassy or obsidian clasts from the ignimbrite matrix, or more commonly by sampling dense, unaltered and glassy (10 to 200 cm thick) vitrophyres at the base of ignimbrite flows. Oxygen was extracted from these rocks using the BrF₅ method. During this period, NBS-28 yielded an average of 9.71±0.1 ‰ (w.r.t. V-SMOW). Analyses of Honduran silicic rocks range from d¹⁸O of +13.5 to 16.4+0.2 ‰ and oxygen isotopic ratios in Nicaraguan silicic vitrophyres are similar, in the range 12.2 to 17.7 ‰. In contrast, d¹⁸O of Honduran Cenozoic basalts ranges from 7.3 to 11.9 ‰ and Nicaraguan basalts from 5.8 to 10.1 ‰. Thus there is no overlap in the range of silicic and basaltic rocks from these regions. Cenozoic Honduran andesites have a range of d¹⁸O from 7.6 to 11.8 ‰. The uniformly high oxygen isotope ratios in Central American vitrophyres are unlikely to be due to post-eruption hydration or alteration of these glassy rocks. Alternatively, we consider that these high ratios may be a reflection of a crustal source in the generation of these silicic magmas.