Structure and Mode of Emplacement of Caribou Mountain Pluton, Klamath Mountains, California

Abstract

The Caribou Mountain trondhjemitic pluton is one of several Jurassic-Cretaceous granitic intrusions in the central metamorphic belt of the southern Klamath Mountains, California. This pluton, emplaced wholly within metacherts of the pre-Cretaceous Stuart Fork Formation, is elongated in a north-south direction and has an area of about 7 square miles. Both eastern and western contacts dip eastward at steep to moderate angles parallel to metamorphic foliation in the country rock. Foliation is markedly deflected around the intrusion except at the pluton9s southern end where foliation is crosscut. Contacts are sharp, and chilled zones are absent. Hornfelses (hornblende-hornfels facies) border the pluton in an aureole as wide as 1500 feet. The intrusion is characterized by a conspicuous primary igneous foliation defined principally by subparallel alignment of biotite flakes. Magmatic foliation and arcuate, fine-grained, trondhjemitic sheets parallel to foliation define a dome, the core of which is located asymmetrically in the northwestern corner of the intrusion. Mesozonal emplacement of this pluton and the adjacent Middle Fork and Canyon Creek plutons was primarily effected by pronounced forceful shouldering aside of country rock. Orientation of primary magmatic foliation in the Caribou Mountain intrusion indicates upward and outward expansion to the northwest of the dome during emplacement, resulting from addition of crystal-charged magma at depth and accompanied by attenuation of flow layers along the flanks of the intrusion. A well-developed system of primary cross joints and aplite-pegmatite dikes oriented radially about the core of the dome indicates that expansion of the pluton continued during late stages of consolidation. Rocks within the pluton are slightly more alkalic and silicic toward the core; this chemical variation is represented by a lithologic variation from calcic trondhjemite to trondhjemite. A trend toward trondhjemitic rocks is generally characteristic of Jurassic-Cretaceous plutons in the Klamath Mountains and Sierran Foothills and contrasts with the calc-alkaline variation of Late Cretaceous granitic rocks of the Sierra Nevada and Southern California batholiths. Petrological differences in Californian granitic rocks may be related to temporal controls rather than to the spatial controls suggested by Moore (1959).