Major intra-arc crustal loading and its tectonic implications, North Cascades crystalline core, Washington and British Columbia

Widespread high-pressure ([gt] 7 kbar) metamorphism in the Coast Plutonic Complex and its southeast extension, the Cascades crystalline core, is commonly attributed to thrust loading during mid-Cretaceous superterrane collision. Important information on the timing of crustal loading is provided by widely distributed mid-Cretaceous (96--88 Ma) plutons that intrude terrane boundaries within the Cascades core. Hornblende barometry from these plutons and the presence of early andalusite in wall rocks suggests that most of the plutons were intruded at relatively shallow levels ([lt] 4 kbar). Major crustal loading after pluton emplacement is indicated in the wall rocks by the replacement of andalusite by kyanite and garnet zoning patterns that record a pressure increase of generally between 3.5 kbar, but with little increase in temperature. Furthermore, the wall rocks to several of the mid-Cretaceous plutons were also intruded by 65--76 Ma, magmatic epidote-bearing plutons that yield pressures (from hornblende barometry) of 6--9 kbar. These relations suggest that loading occurred before or during emplacement of the 65--76 Ma suite. The evidence for major crustal loading after emplacement of terrane-stitching plutons is not consistent with proposed collisional models for high-pressure metamorphism in the Cascades core. The authors suggest that the core records either major intra-arcmore » thrusting or magmatic loading, a history which is probably shared by other parts of the Coast Plutonic Complex. Such loading may also be an important process in the roots of other arcs, although it has not been widely reported.« less