Origin and Emplacement of the Academy Pluton, Fresno County, California

Abstract

The Academy Pluton is located in the western Sierra Nevada foothills northeast of Fresno, California (Fig. 1). It cuts the northern end of the Kings-Kaweah ophio-lite belt, which forms a 125-km-long northwest-trending zone of highly deformed mafic and ultramafic country rocks between the Kings and Tule Rivers of the foothills region (Saleeby, 1975, 1976a, 1976b). The Academy Pluton is one of a number of Jurassic and Early Cretaceous plutons which have intruded and metamorphosed the ophiolitic rocks; these crosscutting plutons form the western margin of the Sierra Nevada Batholith in the latitude of the Kings-Kaweah ophiolite belt. They range in composition from olivine-hornblende melagabbro to hornblende-biotite granodiorite, and many of then appear to have been preferentially emplaced into the structurally weakened zone provided by the disrupted ophiolite belt (Saleeby, 1975, 1976b). To the east, the main body of the Sierra Nevada Batholith is underlain primarily by more silicic middle to Late Cretaceous plutons which range in composition from granodiorite to granite. North of the Academy area, this compositional trend is found to exist all along the western Sierra Nevada foothills. The Guadalupe complex east of Merced (Best, 1963), the Pine Hill gabbro east of Sacramento (Springer, 1971), and the Bucks Lake Pluton (Hietanen, 1973) at the north end of the western metamorphic belt are representative of the more basic and older plutons comprising the western margin of the Sierra Nevada Batholith. The Academy Pluton is one of the most complexly zoned plutons in the western sierra Nevada. The earliest reference to the pluton was by Macdonald (1941), who described the pluton as being composed of hypersthene-bearing quartz diorite. His map (1941, P1.1) showed an indefinite contact between hypersthene-bearing quartz diorite and surrounding pyroxene-free quartz diorite and tonalite. The present investigation has established that pyroxene-free quartz diorite and tonalite lying peripherally to the morth and east of Macdonald's contact is part of the Academy Pluton but tonalite South of the pluton cuts across the Academy structure and thus represents a later magmatic esisode. In preliminary studies, the senior author of this paper noted that there were at least three, and possibly four, ring-like zones comprising the pluton and that the innermost zone was composed of more basic rock than the outermost zones. The difficulty in explaining the gradational contacts and the apparent reversed zoning in the pluton prompted this study.