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