LADDER DIKES IN THE CATHEDRAL PEAK GRANODIORITE: MIGRATING FINGERED DIKES?

Ladder dikes in the Cathedral Peak granodiorite (Kcp), Tuolumne Intrusive Suite, California, appear to reflect outward injection of magma from a central source via tubular conduits. Ladder dikes are roughly tabular bodies, commonly 30-100 cm thick, composed of nested concave-up mafic-felsic modal layers. They are fairly common in felsic plutons but are rarely abundant. Weinberg et al. (2001, GSA Bulletin) interpreted (1) ladder dikes to record channelized magma flow parallel to the layers; (2) modal layers to be crystal concentrations at the base of the flow; and (3) the layers to young upward. We concur except that the modal layers are unlikely to reflect sedimentation because they commonly are monomineralic (e.g., titanite) and include a range of grain sizes. We can envision no sedimentary process that sorts only by composition, and suggest instead that the modal layers record chemical reactions between intruding magma and its walls. A Kcp ladder dike commonly terminates laterally/upward in a subcircular mass of leucocratic rock, 50-100 cm across, that fills the highest trough and is rimmed by titanite ± mafic minerals. We interpret this to indicate that a ladder dike reflects intrusion of a narrow magma finger that migrated upward with time, perhaps owing to buoyancy and thermal erosion, through a mushy host. Ladder dike orientations in the Kcp in the Tuolumne River and Steelhead Lake areas indicate that the dikes generally strike at a high angle to the external contact of the pluton and that trough axes commonly plunge gently to moderately outward toward that contact. These geometrical attributes are difficult to reconcile with a convective origin (cf. Weinberg et al., 2001) which appears to predict steep upward flow from the center of the pluton. This is consistent with the fact that the small length scale of ladder dikes and the high viscosity of felsic magma make Rayleigh-Taylor instability unlikely to be significant. The geometry is more consistent with outward injection of gently inclined magma fingers from a central source. The present consistent outward inclination of troughs may reflect doming of the entire intrusive complex as a result of continued vertical stacking of intrusive increments. We thus interpret ladder dikes to record an intrusive mechanism by which a pluton may grow incrementally.