SPATIAL AND TEMPORAL EVOLUTION OF MAGMATIC SYSTEMS IN A CONTINENTAL MARGIN ARC: THE CASCADES CORE, WASHINGTON

In the Cascades core magmatic systems ranging in age from ~96 Ma to 46 Ma formed during active contraction (~100 –75 Ma), transpression (~75-57 Ma), and transtension (~57-45 Ma) with the largest magmatic flux occurring at ~92 Ma and smaller peaks at 72 and 46 Ma. Both dispersed and focused magmatic systems formed with the latter evolving temporally from the former. Spatially and temporally complex compositional patterns (gabbros to granites) are equally developed in both systems although systems with mafic magmas evolve into focused systems. Significant magma fluxes occur in both systems, although larger diapiric batches are more common in the focused systems. Statistical studies show no correlation between faults and either type of system. However, sheeted bodies of all ages are prevalent in a NW-SE striking zone in the Chelan block raising the question of whether this region represents a long-lived, crustal-scale discontinuity. Examination of surrounding host rock indicates that both types of magmatic systems were constructed during tectonism with the largest magmatic flux occurring during contraction and crustal thickening and younger peaks occurring during transpression (and significant exhumation) and transtension, respectively. Material transfer in aureoles is dominated by magma wedging in dispersed systems and downward flow around focused systems. A comparison of two focused systems, the ~3-4 kbar, ~91-96 Ma Mt. Stuart batholith and the ~7-9 kbar, ~89-93 Ma Tenpeak batholith indicates that these systems formed over millions of years, that both mafic and felsic pulses continued to utilize these pathways, that the homogeneity of chambers may not increase at shallow crustal depths, and that younger magma pulses often displaced older batches by vertical mass transfer. Our observations suggest that once a magmatic "plumbing system" develops, it typically is utilized for millions of years, continues to tap a range of magma compositions, and is increasingly used for ascent of large magma batches. As these systems evolve, host rock and older parts of chambers are displaced by local processes largely resulting in vertical transport rather than by regional tectonism.