Paper No. 33-6
Presentation Time: 9:55 AM
CHANGES IN ARCHITECTURE, GEOCHEMISTRY, AND CONSTRUCTION TIMESCALES OF INTRUSIVE COMPLEXES IN THE UPPER ~35 KM OF A TRANSCRUSTAL MAGMATIC SYSTEM: THE CRETACEOUS CASCADES ARC, WASHINGTON, USA
The Cascades Core (Washington, USA) continental-arc crustal section allows investigating the depth-dependent architecture and temporal-chemical evolution of a magma plumbing system. We studied three intrusive complexes emplaced during a ~10-Myr magmatic flare-up event at ~5 to 40 km depth during mid-Cretaceous crustal shortening. The volume of magmatism increases and becomes less focused with depth. Each complex consists of multiple intrusive units exposing tonalite-granodiorite and subordinate gabbro to granite. Pluton-wide magma emplacement rates are similar at each crustal level, but rates for individual units increased at all depths ~5 Myr after flare-up initiation. During the ~2-Myr flare-up peak, the largest magma volumes were emplaced (~1´10-3 km3/yr), implying a thermally matured crust, facilitating rapid magma ascent. A vertical compositional stratification is evident by decreasing whole rock SiO2 and K2O and increasing Al2O3 and CaO contents with emplacement depth. Equilibrium major element melt compositions calculated from early-crystallizing amphiboles in dioritic to tonalitic samples (58–69 wt. % SiO2) parallel these trends but show more evolved compositions compared to whole rock data. Calculated equilibrium melt compositions and amphibole and pyroxene Fe-Mg partitioning relationships indicate crystal accumulation and melt loss at all crustal levels. With decreasing emplacement depth, intrusive units record the arrival of increasingly more homogenous and evolved equilibrium melts, supporting a transcrustal distillation process: 65–80 wt. % SiO2 in the deep mid-crust (~35 km) to ~75–80 wt. % SiO2 in the upper crust (~5 km). Whole rock Nd-Sr isotopes shifted to more juvenile values during the flare-up, independent from trends observed on a batholith-wide scale. These reflect a decrease in assimilation/partial melting in a local lower-crustal MASH zone or an increasingly more depleted mantle source.