DIKE ASSEMBLY OF THE TUOLUMNE INTRUSIVE SUITE, YOSEMITE NATIONAL PARK, CALIFORNIA

Were plutons largely molten all at once or were they constructed over longer periods of time by incremental addition of small batches of magma? Incremental addition of magma during tectonic dilation has been criticized because thermal models show that rapid dilation rates (>10 mm/yr) are necessary to maintain a steady-state magma chamber; without such rapid expansion, the pluton would consist of a multitude of frozen dikes and should show textural evidence for such. Here we show that outer units of the Tuolumne Intrusive Suite (TIS) are sheeted and may have formed by dike amalgamation, consistent with incremental formation over several million years as suggested by ongoing geochronologic studies (Coleman et al., this meeting).

The TIS has long been viewed as a series of large batches of magma that were emplaced in rapid succession. Mapping the margin of the TIS near May Lake demonstrates that at least locally the outer units of the suite are made up of dikes. At May Lake the outermost unit of the TIS (tonalite of Glen Aulin) intrudes the May Lake metamorphic screen. The tonalite is choked with large (up to 250x20 m) inclusions of metamorphic rock whose foliation and lineation parallel that in the screen. We interpret these screens as in-place bodies of wall rock that are separated by dikes. The tonalite is modally sheeted parallel to the contact and grades over 10-50 m into the next inner and younger unit, the Half Dome Granodiorite. The granodiorite is modally sheeted and contains abundant mafic enclaves near the contact but grades inward to a more homogeneous rock. Similar relationships occur on the eastern side of the TIS. To the south, Calkins, Kistler, and Peck showed that the outermost unit of the TIS at Yosemite Valley invaded its wall rocks as a large dike swarm.

These relationships suggest that the TIS, an archetype of batholithic magmatism, began life as a dike swarm that evolved into a more texturally homogeneous body with time. This evolution is consistent with thermal models of dike emplacement which indicate that early dikes in a dilating shear zone should show evidence of chilling that may disappear as the system heats up and establishes a steady-state magma chamber. Given a 15 km width and ~5 m.y. of growth, this requires dilation rates of ~3 mm/yr, similar to ongoing dilation across the Coso magmatic system.