Magmatic Inclusions in Rhyolites of the Spor Mountain Formation, Western Utah: Limitations on Compositional Inferences From Inclusions in Granitic Rocks

Inclusions of quenched mafic magma occur in a 21 Ma rhyolite lava and precursory non-welded tuff that form the Spor Mountain Formation in west-central Utah. The mafic inclusions are not lithic inclusions; no comparable volcanic unit was present at the surface when the Spor Mountain Formation erupted. Mineral and bulk compositions preclude liquid immiscibility. The mafic inclusions show clear morphologic and textural effects of magma mingling shortly before eruption of the rhyolite. Globular inclusions from both units are vesicular, phenocryst-poor, plagioclase-sanidine-clinopyroxeneorthopyroxene-magnetite-ilmenite latites and trachytes with quench temperatures of about 1000°C. Although overlapping in SiO2, TiO2, Zr, and Hf concentrations, inclusions from the underlying tuff lack negative Eu anomalies and are enriched in P2O5, K2O, A12O3, Sr, Pb, Cr, and Ni whereas those hosted in the overlying lava have small negative Eu anomalies and two-fold enrichments in Fe2O3, MnO, HREE, Y, Ta, Th, Rb, and Cs. The most reasonable explanation for the differences between the two sets of inclusions lies in selective chemical exchange between the rhyolite lava and the mafic inclusions after eruption. Limited mechanical mixing occurred after the inclusions solidified and became chemically modified. The textures of the inclusions in the lavas and the elements selectively mobilized in the inclusions imply that vapor-phase transport occurred in this low-pressure volcanic environment. If such substantial variations in inclusion compositions can arise during what must have been a short period of time before chemical reactions were halted by rapid cooling, it seems unlikely that the compositions of mafic inclusions formed by magma mingling in slowly cooled granites preserve their original compositions, mineralogies, or information about their ultimate sources. Using the compositions of such chemically modified inclusions as end-members for mixing calculations may lead to erroneous results regarding the significance of magma mixing in plutonic rocks.