Construction of mid-crustal sheeted plutons: Examples from the North Cascades, Washington

Abstract

Large parts of many orogenic belts consist of highly elongate plutons constructed by the injection of multiple sheets of magma. We describe in detail two such intrusions, the deep (∼20–25 km), mid-crustal Cardinal Peak and Entiat plutons, and emphasize the dynamic nature of these magma chambers. These ca. 72–73 Ma plutons consist of heterogeneous mafic complexes in their margins and tips that give way inward to thicker, but still abundant, sheets of tonalite. Intrusive and petrologic relationships provide evidence for injection of early mafic (mantle?) and crustal melts, development of heated pathways utilized by larger batches of more felsic magma, and potentially a filtering mechanism in which conduits represented by the plutons fed larger, more homogenized magma bodies at shallower crustal levels. Well-developed, gently to moderately dipping magmatic and subsolidus foliations in these plutons are commonly discordant to sheet contacts, internal layering, and pluton contacts, but are continuous with host- rock foliation. They define small to map- scale magmatic folds, which have orientations and geometries similar to those of regional host-rock folds. These magmatic structures thus primarily record regional contractional tectonism during chamber construction. Host rocks also record complex processes during chamber evolution with narrow structural aureoles displaying remarkable lateral variability. Deflection of gently to moderately dipping host-rock markers toward parallelism with steep pluton contacts indicates vertical and largely downward ductile material transfer along sheet margins during emplacement. Several sharply discordant segments of pluton contacts imply late stoping. Regional ductile flow and folding also played a role, but regional faulting did not. Further evidence for chamber construction is provided by host- rock rafts, which are abundant along sheet contacts in the Cardinal Peak pluton and particularly between marginal sheets of the Entiat pluton, but are sparse elsewhere in the latter pluton. We interpret these relationships to indicate a temporal sequence in which magmatic sheets initially intruded host rock, then wedged aside this rock and coalesced, preserving pieces of the host along margins, and eventually formed large, relatively inclusion-free chambers as host rock was detached and transported vertically downward through the chamber.