THE ASSOCIATION BASANITIC NEPHELINITE - FELDSPAR IJOLITE - NEPHELINE MONZOSYENITE AT HOUSE MOUNTAIN VOLCANO, NORTH-CENTRAL ARIZONA

JAMES H. WITTKE
Bilby Research Center, Northern Arizona University, Flagstaff, Arizona 86001-6013, U.S.A.

RICHARD F. HOLM
Department of Geology, Northern Arizona University, Flagstaff, Arizona 86001-4099, U.S.A.

Abstract

Dikes of basanitic nephelinite at the predominantly basaltic House Mountain volcano (Arizona) host ocelli, dikes, pods and sheets of feldspar ijolite and nepheline monzosyenite that may be explained by differentiation according to the series: basanitic nephelinite feldspar ijolite nepheline monzosyenite. Titanaugite, nepheline, feldspar and olivine occur in all stages of this sequence, which shows systematic Fe enrichment in mineral chemistry and bulk composition. The nephelinite (mg = 76) contains Fo85-86 olivine with a thin Fo65-77 rim, and Wo48-52Fs8-14En36-44 titanaugite. The ijolite (mg in the range 51-61) contains titanaugite with a Wo48-53Fs14-18En30-35 core and a Wo48-52Fs20-24En28-31 rim. The monzosyenite (mg in the range 46-50) has Fo18-29 olivine, and titanaugite zoned from Wo48-50Fs15-27En29-34 to Wo48Fs21-42En10-31, with a green-brown rim of Wo48Fs37-45En7-15. Along the proposed sequence, LILE generally show a 2-3× enrichment, and compatible elements show a strong depletion. A petrogenetic model for the association, based on field relationships, geochemistry, and petrography, includes buoyant rise of a heterogeneous body of ijolitic + monzosyenitic magma hrough a large dike of molten nephelinite to shallow depths (<0.5 km) in the volcano. Turbulence or shear along the boundary may have stirred small globules (ocelli) of felsic magma into the nephelinite. The large body of ijolite - monzosyenite spread as a sheet when it encountered the chilled top of the dike, and fractionated to form cm- to m-scale banding and layering. Large crystals, intergrowth and skeletal textures, and miarolitic cavities with zeolite resulted from rapid crystallization of undercooled, fluid-rich felsic magma. Post-intrusion differentiation of the host nephelinitic magma produced wispy patches and schlieren of felsic liquid that were locally filter-pressed into small dikes of nepheline monzosyenite.