THIN WALL ROCK SCREENS AND INTRUSIVE PROCESSES IN THE SIERRA NEVADA BATHOLITH

Field study of thin screens of wall rock found between, and more rarely inside, Sierran granitoid plutons illuminates the origins of the screens and the processes that formed the adjacent plutons. Wall rocks in the screens invariably are ductilely deformed. The significance of that deformation and its timing relative to pluton emplacement vary markedly from one field example to another, but in no example we have yet studied did ductile deformation directly produce the elongate shape of a screen. The process by which the thin screens formed invariably was opening of closely spaced, subparallel fractures to admit magma. This conclusion implies that emplacement of the bounding pluton(s) invariably began with injection of dikes that split thin sheets of wall rock away from their neighbors. In at least two areas, the McDoogle pluton and the Glen Aulin and Half Dome plutons (Glazner et al., this volume), there is clear evidence that the plutons continued to grow incrementally by amalgamation of dikes. Wall-rock screens in these areas now are surrounded by intrusive rocks but never were engulfed by magma because only a small fraction of the pluton was magmatic at any one time; thus, finding inclusions of wall rock in a pluton need not record stoping. All other field examples we have studied permit incremental pluton growth by amalgamation of dikes or sills, but evidence is insufficient to require it. An unanticipated outcome of the field study was recognition that interpluton screens commonly had gentle initial dips and represent the floors of overlying plutons. Finding exposed pluton floors so accidentally suggests that they may be more common than is widely believed. Thus, the Split Mountain screen forms the floor of the Tinemaha granodiorite, and the Cedar Grove screen is the floor of the Lookout Peak granodiorite. Although mafic enclaves are more abundant at floors, both examples lack a significant volume of wall-rock blocks sitting on the floor that could represent blocks stoped from the pluton roof. This relation reinforces our interpretation that the sharp, fractured contacts of interpluton screens record intrusion of dikes rather than stoping of blocks from the roof or walls of a large magma body.