Icelandite, apparently the first to be recognized in the western United States, is a petrographically important component of the volcanic suite of the middle Miocene McDermitt caldera complex. Median major-element composition of seven rocks that both pre-date and post-date the major ash-flow sheets exposed on the northern margin of the Long Ridge caldera have been determined. Very high Fe contents (9.1 to 10.2 wt % FeO) are associated with low to very low MgO (0.4 to 2.0 wt %); FeO/Mg ratios of from 5.4 to 25 are strongly "tholeiitic." Both alumina and total alkali contents are relatively low. With the exception of their significantly higher K₂O contents (3.1 to 4.7 wt %), the rocks are chemically similar to icelandites from hot-spot-related oceanic islands suc as Iceland and the Galapagos that are situated near spreading centers. A very thin unit of crystal-rich pentellerite welded tuff containing 1.5 vol % aenigmatite phenocrysts underlies the lower major ash-flow sheet exposed at the northern margin of the Long Ridge caldera. Analyses of progressively Fe-rich intermediate and silicic rocks given by Greene provide evidence for a coherent and continuous rock series from icelandite to peralkaline rhyolite. The high FeO/MgO ratios of the icelandites and the presence of aenigmatite in the tuff support a petrogenetic model for the intermediate and silicic rocks of the McDermitt complex involving extensive high-level differentiation of mantle (diapir?)-derived subalkaline mafic magma under conditions of low f_O2 and f_H2O. The K₂O and U (4 to 5 ppm) contents of the icelandites suggest that the parent magmas were moderately rich in these and other lithophile elements.

End_of_Article - Last_Page 758------------