Geology of the Bungalow pluton, Clearwater county, Idaho

The Bungalow granite pluton is one of several relatively unstudied Eocene age (37-52 Ma) plutons located within the Clearwater igneous province of northern Idaho, This province lies along the Kelly Forks fault, a major northwest-trending strike-slip structure that separates the Cretaceous age (60-90 Ma) Idaho batholith from Proterozoic (850-1300 Ma) metamorphic rocks. This study used geologic mapping, together with whole rock geochemical analysts and petrographic thin section observations, to document the internal chemical, petrographic and structural characteristics of the pluton and to reconstruct the crystallization behavior and mode of emplacement. The pluton forms a 225 km(2) by 1600 m exposure of granite with steep, unaltered and undeformed margins and a well defined negative gravity anomaly. The west half of the pluton has an upper surface of porphyritic granite. The large NW -trending Kelly Forks fault and 3 N-S striking normal faults cut the pluton, These new observations, interpreted in context of previous limited radiometric age dating and structural analysis of the Kelly Forks fault, suggest that the pluton is an isolated body passively emplaced along a strike-slip tectonic setting which was active during the Paleogene. This setting contrasts with the northeast-trending normal faults associated with Eocene igneous provinces adjacent to the southern part of the Idaho barholith. The pluton is composed of a suite of texturally diverse (fine-grained to porphyritic), yet remarkably homogeneous high-silica (75-77 wt % SiO2), metaluminous, biotite granite. The preferential occurrence of porphyritic textures and hydrous minerals near the edges of the pluton suggest that the pluton crystallized from the margins inward with no significant differentiation. In addition, the pluton was intruded by minor occurrences of rhyolite porphyry dikes, quartz syenite, lamprophyre dikes and has subsequently undergone limited. post magmatic hydrothermal alteration.