LADDER DIKES HOSTED IN METAVOLCANIC WALL ROCK AND “THE VORTEX”: MAGMATIC STRUCTURES FROM THE CENTRAL SIERRA NEVADA, CALIFORNIA

Meter-scale ladder dikes (LDs) of 88 Ma Cathedral Peak (CP) granodiorite of the Tuolumne Intrusive Suite (TIS) are exposed on glaciated outcrops in Yosemite National Park and adjacent Hoover Wilderness. Reid et al. (1993) described LDs within the CP pluton, located on open slabs next to the Tuolumne River. We observed 12 LDs at the eastern margin of the TIS, near Steelhead Lake (SL), hosted in Jurassic andesitic metavolcanic rocks. At SL, the pluton/wall rock contact strikes roughly N-S and is parallel to wall rock foliation. Foliation in the metavolcanics developed during regional deformation, prior to TIS intrusion, and dips steeply E to sub-vertical. LDs lie within 10 m of the pluton/wall rock contact and are oriented sub-perpendicular to the contact and foliation. Direction of axial migration of the LDs is E, away from the pluton/wall rock contact – arcuate ‘ladders' are convex to the contact. Post-crystallization deformation of LDs and the CP pluton overprints original magmatic fabrics. LDs are deformed to varying degrees and show evidence of shear with cm-scale offset as well as granulation of megacrysts. Wall rock foliation is slightly deflected at LD margins. Contacts between LDs and wall rock are irregular and wavy but sharp, suggesting that melt was not extracted into the host rock. Interaction of material at the LD/wall rock contact is limited to fragmentation and enveloping of the wall rock, however fragments of wall rock (xenoliths) are rare within LDs. The occurrence of LDs in wall rock supports the ideas of Weinberg et al. (2001), that LDs are cross-sections of migrating magma conduits (possibly feeder conduits for diapirs or plumes) that record a dynamic flow history. LDs are capable of piercing through and making space in wall rock during their formation. The occurrence also suggests that a surrounding crystal-rich-mush host is not necessary for LD formation.

“The Vortex” is an enigmatic and complex magmatic structure that is hosted in CP granodiorite, west of Tuolumne Meadows. The structure is ~1m x 2m in size and is composed of a central, megacryst-free felsic core, surrounded by several different graded, cross-cutting units of mafic schlieren. Internal contacts between different units are extremely sharp. The development and formation of “The Vortex” is not well understood.