GEOMETRY AND EMPLACEMENT MECHANISM OF LADDER DIKES IN THE CATHEDRAL PEAK GRANODIORITE, YOSEMITE NATIONAL PARK

Ladder dikes (LDs)—crudely tabular arrays of nested, concave-up mafic-felsic layers, commonly 0.3-1 m thick—are sparse but relatively common in granitic plutons. Several recent papers interpret LDs to record subvertical plumes produced by small-scale convection in a magma chamber. Field observations from the Cathedral Peak Granodiorite (Kcp) clearly favor the migrating-pipe model of Weinberg et al. (2001); many LDs in Kcp terminate in a rod-shaped mass of leucocratic rock that fills the highest trough and records the final position of a pipe-like magma conduit. However, the geometry of LDs in Kcp is inconsistent with vertical plumes. At several widely dispersed locations in the Kcp, we measured orientations of >160 LDs and of the trough-shaped layers that record conduit orientation. The LDs generally strike at a high angle to the external contact of the pluton; troughs invariably plunge outward at an average angle of ~45°; and nesting of troughs uniformly indicates upward migration of the conduit. This pattern is difficult to reconcile with a subvertical original orientation and suggests that the conduits originally plunged away from the center of the pluton.

Wall-rock fragments are very sparse in Kcp but are common in LDs, indicating that the xenoliths were carried in by magma flow in the LD conduit. The xenoliths typically are ~2 -10 cm across and very fine-grained. Bulk-rock chemical analyses yield intermediate SiO₂ concentrations consistent with an andesitic protolith, but broad scatter of other major oxides suggests strong bulk-chemical modification.

Combined with chemical evidence that LD layering reflects liquid immiscibility (Glazner and Bartley, this volume), we suggest that LDs reflect conduits through which late-stage immiscible liquids migrated outward from the center of the growing pluton. The pipe-like shape of the conduits may reflect fingering of outward-propagating intrusive sheets during incremental growth of the pluton (Bartley et al., 2009). The descending trajectory of the conduits might result from negative buoyancy of the dense Fe-rich liquid component, which accumulated preferentially at the bottom of the conduit to form dark layers, while upward migration of the conduit may have been caused by the buoyancy of the felsic component.