Abstract

The Cub River Diabase is a dike that cuts tuffs and limestones of the Pliocene Cache Valley Formation (Salt Lake Group) in northeastern Cache Valley, Idaho. The diabase exhibits intergranular, porphyritic and, to a lesser extent, xenomorphic- to hypidiomorphic-granular textures with aphanitic (vitrophyric) chilled margins. Contact metamorphism of limestone has resulted in crystallization of diopside, wollastonite, vesuvianite and calcic garnets. These textures and contact relations suggest a hypabyssal environment. Material of a more felsic composition occurs as monzonite outcrops west of the main trend of the intrusion and as segregation veins (3 to 15 centimeters wide) primarily within the diabase.

Plagioclase of the main intrusion averages An₅₉, and often displays strong marginal zoning. Augite is the most abundant pyroxene pigeonite and hypersthene occur in smaller quantities. The percentage of mesostasis is variable, consisting of micrographic and microperthitic intergrowths or, less commonly, felsitic groundmasses, all heavily stippled with iron oxides. The monzonite outcrops contain pyroxene and/or amphibole along with An₅₁ plagioclase. The discontinuous segregation veins of fine-grained felsic rock contain plagioclase of An₄₇, with pyroxene or amphibole present only in subordinate amounts.

The main intrusion is basic to intermediate, with SiO₂ content ranging from approximately 50 to 60 percent and averaging 54.45 percent. The monzonite outcrops and segregation veins have average SiO₂ percentages of 63.84 and 68.67, respectively.

The main diabase intrusion is interpreted as a differentiation series from a tholeiitic basalt parent magma. The intermediate derivative magmas (monzonite and segregation veins) are considered to be more evolved differentiates of this melt.