MATERIAL TRANSFER PROCESSES IN PLUTONS DURING EMPLACEMENT; EVIDENCE FROM CORDILLERAN PLUTONS FOR LARGE-SCALE INTERNAL RECYCLING

Studies of pluton emplacement have focused on material transfer processes in external host rock, but much less is known about transfer processes within plutonic systems during intrusion of younger magmatic pulses, such as in zoned intrusive suites. We address this problem by describing internal relations in two Cordilleran plutons, emplaced at very different structural levels. Previous studies and our work on the western margin, eastern margin, and segments of internal contacts of the shallow (< 10 km) Tuolumne Intrusive Suite (Sierra Nevada) indicate that this classic zoned intrusive system (Bateman & Chappell 1979) displays a range of internal contacts. The Cathedral Peak Granodiorite intrudes in sharp to gradational contact the enclosing Half Dome Granodiorite, which in turn intrudes the outermost Kuna Crest phase along a sharp contact. This zonation is interrupted where the Cathedral Peak cuts across the outer phases (without deflecting their contacts) to the N and NE margin of the suite. Most of the suite displays only magmatic foliation, but high-T subsolidus, reverse shear zones are found within 200 m of the western and eastern host rock contacts in the Kuna Crest phase. These moderately to steeply dipping zones record dominantly pluton-side-up shear in the W margin (and less certainly in E margin), and imply modest steep subsolidus ductile flow resulting from regional shortening and/or emplacement of the younger phases. The cross-cutting Cathedral Peak contact, local blocks of older phases in younger ones, and overall weak solid-state strain, however, suggest that vertical material transfer by magma flow and breaking off and sinking of earlier solidified material removed large amounts of the earlier phases within the magma chamber. In the much deeper (> 25 km), 91-92 Ma Tenpeak pluton of the Cascades crystalline core early internally sheeted phases, which are commonly separated by m-scale rafts of metamorphic host rock, are intruded out by several much larger, more elliptical-shaped phases of tonalite. This truncation without deflection of the sheets again implies vertical removal of older phases in accord with numerous host rock xenoliths in the pluton. The vertical transfer of large volumes of early phases during intrusion of younger magmatic pulses in these and other Cordilleran plutons we have studied implies that major internal recycling occurs in plutons over a wide range of crustal levels.