GEOLOGY OF THE ELKHORN MOUNTAINS, NORTHEASTERN OREGON: PART 2. WILLOW LAKE INTRUSION

Abstract

The Willow Lake intrusion is funnel-shaped with contacts inclined inward at 60°-70° and has a sheetlike appendage to the southwest, inclined northwest at 50°. The pluton lies athwart the southern contact of the Bald Mountain batholith and was emplaced before the main batholithic phases. It contains normal metagabbroic rocks, rhythmically layered rocks similar to those described from the Stillwater and Skaergaard intrusions, and abnormal banded rocks in which the minerals are elongated and arranged with their longer axes at high angles to the banding. Similar abnormal banded rocks have been found in a number of localities in Oregon and California, invariably in steep-walled intrusions of small size, including dikes a foot or less in width. The Willow Lake type of rhythmical layering cannot have been formed by a process combining gravitative sedimentation of crystals with convection currents in the magma, such as accounts for Skaergaard-type layered rocks. The Willow Lake-type layered rocks probably formed by undercooling and crystallization in the oversaturated (metastable) region, combined with intermittent convective and turbulent currents, in a magma near a univariant condition. Banded rocks of this type may be expected in epizonal metamorphic regions in steep-walled mafic intrusions, emplaced late in the regional metamorphic cycle. Criteria for so-called gravity stratification are examined; the various features of density stratification may in part be due to processes other than those operating under the influence of gravity. The perpendicular feldspar rock of the Skaergaard intrusion and the harrisite structure of layered periodotites from the island of Rhum are cited as products of moderate undercooling of magma in a bivariate condition. Translation-gliding effects and a set of conjugate shears developed in Willow Lake banded rocks are attributed to stresses in the crystal mush resulting from magma movements past surfaces of crystallization. The role of diffusion in the interstitial melt is discussed, and the formation of mono-mineralic layers in the Willow Lake banded rocks is attributed to diffusion between interstitial melts of adjacent bands of contrasting mineralogy and reactions between the melts and the surrounding solid phases.