Petrologic studies of primitive alumina-rich basaltic magmas; an experimental, geochemical, and tectonic investigation
University of Oregon, Eugene
135 p., Illus.
Alaska Resources Library & Information Services: QE462 B3 D73 1991
My experimental and geochemical studies of compositionally similar basalts from disparate tectonic settings elucidate a range of upper mantle processes. Experiments on ID16, an Aleutian high-magnesia basalt, produced derivative liquids that resemble many published high-alumina basalt compositions, including some of the most Al$\sb2$O$\sb3$-rich. ID16 is multiply saturated with a plagioclase/spinel lherzolite assemblage immediately beneath the 12 kbar liquidus, suggesting that ID16 last equilibrated with the upper mantle at 1325$\sp\circ$C and depths of 35-40 km prior to eruption. This temperature is much hotter than steady-state thermal models imply, suggesting that ID16 ascended through a preheated conduit, deflecting isotherms upward $\sim$50 km. Experimental results also suggest that derivative melts of ID16 undergo a complex reaction relationship with olivine at 10 kbar and 1200$\sp\circ$C-1150$\sp\circ$C. These liquids resemble comparatively evolved high-alumina basalts, and it is likely that small amounts of H$\sb2$O are required to bring olivine onto the liquidus of more mafic high-alumina basalts. Involvement of H$\sb2$O in the development of ID16 itself would tend to increase the pressure of multiple saturation. However, ID16 closely resembles melts produced in dry peridotite melting experiments, so the amount of H$\sb2$O present when ID16 formed was probably minimal. The experiments also indicate that Plio-Pleistocene basalts from the Brothers Fault Zone in the northernmost Basin and Range, many of which resemble ID16, were derived from a shallow mantle source and underwent only minor shallow-level fractionation, probably promoted by progressive extension along the Brothers Fault Zone. This origin sharply contrasts with that generally proposed for mid-Miocene Columbia River and Steens Mountain basalts, which apparently stagnated at shallow depth and differentiated to basaltic andesite compositions. Bulk compositions of nearby silicic rocks, with physical and thermal considerations, suggest that they were products of crustal anatexis driven by the injection of mafic magmas. These petrologic features suggest that the arrival of the mantle plume presently beneath Yellowstone produced or strongly influenced most late Cenozoic magmatism in the Oregon northern Basin and Range. This proposition accounts for many magmatic and physiographic features of the northern Basin and Range in the Pacific Northwest.
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