CHARACTERISTICS OF VERTICALLY SHEETED ARC PLUTONS

The importance of vertical and horizontal sheets in the construction of plutons has been increasingly recognized. We have examined numerous deep- to shallow crustal, sheeted plutons in the Cascades, Sierra Nevada, and Donegal that share many features. They are typically steep sided and vertically extensive. In map view, these bodies are markedly elongate (aspect ratios reaching > 10:1) and have relatively straight contacts. Thin (commonly < 5 m wide), relatively mafic, steeply dipping sheets characterize the margins. Mingling between marginal sheets implies injection close in time. The marginal sheets are intruded inward by more felsic and thicker sheets, which in turn are intruded by larger, relatively felsic, and more irregularly shaped magmatic pulses. Variable amounts of host rock inclusions represent a continuum from septum, to rafts, and to widely dispersed xenoliths. Sheets separated by rafts and septum may show distinct or subtle modal and/or textural difference. Geochronological data (e.g. Matzel, 2004) indicate that construction of the sheeted plutons ranges from < a few myr. to > 10 myr. These plutons display a range of relationships to host rock structures. Some bodies are discordant to regional structures except in narrow structural aureoles, whereas others are concordant. None of these intrusions were obviously controlled by fault zones. We have also mapped relatively weakly elongate plutons, such as the Tuolumne Intrusive Suite (TIS) of the Sierra Nevada batholith, that have been interpreted by others to have been constructed incrementally by a multitude of sheets (e.g., Glazner et al., 2004). Geochronology and field relations indicate that these bodies formed from multiple magmatic pulses, but they lack many features typical of “classic” sheeted plutons. For example, although the TIS is sheeted in a narrow marginal zone, inward from this zone host rocks only occur as sparse isolated xenoliths rather than rafts or septum, steep external and internal contacts are typically irregular in geometry, large domains of homogeneous granodiorite are present, and there is evidence for fractionation of larger pulses. These features are probably most compatible with chamber construction by early sheets and subsequent much more voluminous (tens of cubic km?), irregularly shaped batches of magma.