Magmatic tubes, pipes, troughs, diapirs, and plumes: Late-stage convective instabilities resulting in compositional diversity and permeable networks in crystal-rich magmas of the Tuolumne batholith, Sierra Nevada, California

Abstract

A complex array of widespread, but domainally developed, structures is preserved in the Tuolumne batholith, including stationary and migrating tubes, pipes, troughs, diapers, and plume heads. These structures, all formed by local magma flow through crystal-mush host magmas, are often associated with the formation of schlieren rich in accessory and mafic minerals, and are associated with filter pressing and accumulations of crystals with diverse magma histories and ages. Together they represent a network in which channelized flow occurred in an existing chamber of crystal-rich magmas, resulting in local compositional and structural diversity. These structures also are useful structural tools for evaluating the internal evolution of magma chambers. For example, the consistently steep tube and pipe axes indicate that neither the pluton nor features in the pluton were tilted during growth, thus excluding models in which subhorizontal layers tilted to form the existing steep contacts. Although the overall direction of younging established by geochronologic studies is toward the batholith center, local younging directions determined from troughs cutoffs indicate that outward growth occurred in many zones. The highly variable movement directions of local diapirs and plumes require interactions between buoyancy forces and other gradients. The existence and characteristics of these structures have several other implications. Interpretations derived about crystal residence times in chambers and about crystal mixing during eruptions need to be treated with caution, since mixed crystal populations existed well prior to eruptions, and in the case of the Tuolumne batholith involved crystals with ages ranging over ~10 m.y. A likely solution is that crystals in subvolcanic chambers become armored (rimmed) by other crystals or exist in crystal clusters that, in spite of changing environmental conditions, prevent rapid chemical communication with the surrounding melts. These structures also challenge many aspects of the incremental chamber growth model resulting in sheeted bodies championed by Glazner, Bartley, Coleman, and colleagues for the Tuolumne batholith. The thousands of preserved internal structures provide clear evidence against late annealing and removal of internal contacts, and are difficult to reconcile with either vertical sheeted or subhorizontal laccolith models; however, they are permissive of early pulsing leading to one or more large magma chambers.