Tectonic and climate history influence the geochemistry of large-volume silicic magmas: New δ18O data from the Central Andes with comparison to N America and Kamchatka

Abstract

New δ18O data from magmatic quartz, plagioclase and zircon crystals in Neogene large-volume, rhyodacitic ignimbrites from the Central Andean Ignimbrite Province reveal uniformly high-δ18O values (δ18O(Qtz) from +8.1 to +9.6‰ — 43 analyses from 15 ignimbrites; δ18O(Plag) from +7.4 to +8.3‰ — 10 analyses from 6 ignimbrites; δ18O(Zrc) from +6.7 to +7.8‰ — 5 analyses from 4 ignimbrites). These data, combined with crustal radiogenic isotopic signatures of Sr, Nd and Pb, imply progressive contamination of basaltic magmas with up to 50vol.% upper crust in these large volume silicic systems. The narrow range of δ18O values also demonstrate that surprising homogeneity was achieved through space (100's km) and time (~10Ma to recent) in these large-volume magmas, via residence in their parental middle to upper crustal bodies. Low-δ18O values of many large volume (>10km3) silicic magmas in North America and Kamchatka, discussed here for comparison, reflect the influence of meteoric-hydrothermal events and glaciations in lowering these δ18O values via the assimilation of hydrothermally-altered crustal material. Conversely, there is a scarcity of a low-δ18O signature in the Central Andes and subduction-related or influenced systems in North America, such as the Oligocene Great Basin of Nevada and Utah, the Southern Rocky Mountain Volcanic Field of Colorado, and the SW Nevada volcanic field system. In these regions, the generally heavy-δ18O magmatic signature is interpreted as a reflection of how a broadly compressional regime, high elevation, aridity and evaporation rates limit availability and infiltration of large amounts of surface meteoric water and hydrothermal alteration of the shallow crust. This leads us to speculate that the δ18O values of large volume silicic magmas in these areas record a paleoelevation and paleoclimate signal. If this is the case, δ18O values of ignimbrites can potentially be used to track the effects of a meteoric-hydrothermal derived δ18O signature from upper crustal rocks that are subsequently assimilated to produce these magma types.