Phonolites and peralkaline rhyolites from a single magma source in the mantle : A new look at some Black Hills rocks
- Illinois State Univ., Normal, IL (United States). Dept. of Geography-Geology
A re-evaluation of existing data from the Deer Mountain-Terry Peak-Sugarloaf Mountain area of the Black Hills, plus some new data, suggests the real possibility that both silica-undersaturated and silica-oversaturated alkaline-peralkaline rocks evolved from the same mantle-derived parent magma. Mineralogically, aegirine rhyolites, phonolites, a minette and the mantle are linked by an association of Mg-rich olivine-phlogopite structures, zenocrystic phlogopite and diopside-cored pyroxene phenocrysts. Trends of silica vs. major elements, trace elements (V,Sc,Ni) and MgO/FeOt are continuous and preclude being fortuitous. Peralkalinity also increases with silica in a well-defined trend. Increasing ferric oxide to total iron oxide indicates increasing oxygen fugacity with silica saturation. A mantle origin for the phonolites is supported by Sr-isotope data of Beintema (1986) and Beintema and Montgomery (1986). Higher Sr-isotope ratios for the aegirine rhyolites, suggesting a lower crustal origin, actually may result from magmatic processes, as shown by others for ocean island basalt-phonolite-comendite associations. Early fractionation of mafic phases drives trends away from the Ne-minimum on the residua diagram, indicating that magma evolution took place above residua temperatures, thus avoiding the thermal divide. Later fractionation of alkali feldspars accounts for variation in the aegirine rhyolites. Rising alkalies and oxygen explain variations in peralkalinity and ferric iron content. Pressure-dependent immiscibility possibly may be the cause of a silica gap in rock types, as rocks with low quartz contents are not found, except as phaneritic inclusions. A model is suggested in which either a fractionating minette or trachyte magma could yield the series of rocks under study.
- OSTI ID:
- 5011595
- Report Number(s):
- CONF-9303210-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:3; Conference: 27. annual Geological Society of America (GSA) North-Central Section meeting, Rolla, MO (United States), 29-30 Mar 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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