Structure and Petrology of the Royal Stock, Flint Creek Range, Central-Western Montana

Abstract

The Royal stock, a late Cretaceous-early Tertiary epizonal pluton approximately 30 square miles in area, is in the Flint Creek Range of central-western Montana. The stock is composed of granodiorite, and is rather homogeneous in terms of overall mineral composition as well as the An content of the plagioclase. A metamorphic aureole with a maximum width of about 1 mile surrounds the stock. All of the structural and petrologic features are consistent with forceful intrusion of a magma. The regional north south trending structure in the country rock is strongly deflected in the vicinity of the stock. Although the regional structure swings concordantly around the stock, the contacts are discordant in detail cutting across the bedding in the country rock. Igneous contact breccia is present mainly along the northern border of the stock. Xenoliths are rare in the interior of the stock. A set of primary joints strikes N. 65° W. and dips 55° SW. in the stock and in places in the adjacent country rock. These joints are believed to be extension fractures produced by the upward push of magma in a prismatic-cylindrical body which plunges to the southwest.