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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 4. (April)

First Page: 758

Last Page: 758

Title: Icelandite and Aenigmatite-Bearing Pantellerite from McDermitt Caldera Complex, Nevada-Oregon, and Their Petrogenetic Significance: ABSTRACT

Author(s): John W. Drexler, Andy B. Wallace, Donald C. Noble

Article Type: Meeting abstract


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 fO2 and fH2O. 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.

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