"SEDIMENTARY" STRUCTURES IN A LAYERED GRANODIORITE: AN EXAMPLE OF MAGMA MULTIPHASE DYNAMICS FROM THE TUOLUMNE INTRUSIVE SUITE, SIERRA NEVADA, CALIFORNIA

The Tuolumne Intrusive Suite (TIS) is a type example of a concentrically zoned, composite, intrusive body. Mapping, geochronology and limited chemical analyses have led to the suggestion that magma was episodically assembled into the TIS. However, the physical processes that contribute to the shared characteristics in the units of the TIS are not obvious, due to the special rheological requirements for the preservation of fluid processes. Hence the sparse sedimentary-type features exposed in layered outcrops of the TIS provide a unique opportunity to assess fluid dynamic processes.

Outcrops of centimeter scale, rhythmic, modally graded layers have been mapped near the eastern margin of the TIS, locally cross-cutting all members of the suite. This layering involves all mineral phases, demonstrating ‘fluidity’ up to moderate degrees of crystallinity. The layering is different than other seemingly dynamic features such as ladder-dikes and schieren, or K-spar rich magma “worms.” Some layers of granodiorite demonstrate features similar to cross bedding in sedimentary rocks. Other layers display slumping or truncation of bedding similar to soft-sediment deformation. In some cases a sharp, sub-planar, upper and/or lower contact of the layers is visible. In all cases dikes containing K-spar megacrysts cross cut the layers.

Field relations suggest that the layers occupy "slot-like" openings in the Kuna Crest and Half Dome granodiorites. This is similar to the geometry invoked by Clarke and Clarke (1998) for layered outcrops of granodiorite in the South Mountain Batholith of Nova Scotia. However, the TIS layers appear to originate by dynamic deposition rather than in-situ crystal growth. The layers capture the process of Cathedral Peak magma being drawn into near horizontal planar conduits. As the process is repeated, rhythmic layers develop.