Late Holocene hydrous mafic magmatism at the Paint Pot Crater and Callahanflows, Medicine Lake Volcano, N-California and the influence of H2O in thegeneration of silicic magmas

This paper characterizes late Holocene basalts and basaltic andesites at Me dicine Lake volcano that contain high pre-eruptive H2O contents inherited f rom a subduction related hydrous component in the mantle. The basaltic ande site of Paint Pot Crater and the compositionally zoned basaltic to andesiti c lavas of the Callahan flow erupted approximately 1000 C-14 years Before P resent (C-14 years B.P.). Petrologic, geochemical and isotopic evidence ind icates that this late Holocene mafic magmatism was characterized by H2O con tents of 3 to 6 wt% H2O and elevated abundances of large ion lithophile ele ments (LILE). These hydrous mafic inputs contrast with the preceding episod es of mafic magmatism (from 10,600 to similar to 3000 C-14 years B.P.) that was characterized by the eruption of primitive high alumina olivine tholei ite (HAOT) with low H2O (<0.2 wt%), lower LILE abundance and different isot opic characteristics. Thus, the mantle-derived inputs into the Medicine Lak e system have not always been low H2O, primitive HAOT, but have alternated between HAOT and hydrous subduction related, calc-alkaline basalt. This inf lux of hydrous mafic magma coincides temporally and spatially with rhyolite eruption at Glass Mountain and Little Glass Mountain. The rhyolites contai n quenched magmatic inclusions similar in character to the mafic lavas at C allahan and Paint Pot Crater. The influence of H2O on fractional crystalliz ation of hydrous mafic magma and melting of pre-existing granite crust bene ath the volcano combined to produce the rhyolite. Fractionation under hydro us conditions at upper crustal pressures leads to the early crystallization of Fe-Mg silicates and the suppression of plagioclase as an early crystall izing phase. In addition, H2O Powers the saturation temperature of Fe and M g silicates, and brings the temperature of oxide crystallization closer to the liquidus. These combined effects generate SiO2-enrichment that leads to rhyodacitic differentiated lavas. In contrast, low H2O HAOT magmas at Medi cine Lake differentiate to iron-rich basaltic liquids. When these Fe-enrich ed basalts mix with melted granitic crust, the result is an andesitic magma . Since mid-Holocene time, mafic volcanism has been dominated primarily by hydrous basaltic andesite and andesite at Medicine Lake Volcano. However, d uring the late Holocene, H2O-poor mafic magmas continued to be erupted alon g with hydrous mafic magmas, although in significantly smaller volumes.