HYDROUS ARC MAGMATISM ASSOCIATED WITH ACCRETION, THE CENTRAL COAST MOUNTAINS OROGEN OF SOUTHEASTERN ALASKA AND NORTHERN COASTAL BRITISH COLUMBIA

100-50 Ma magmatic arc activity in the Coast Mountains is associated with subduction related to terrane accretion. Two arcs are identified in the study area, between Wrangell, Alaska and Prince Rupert, British Columbia. An older one, ~100 to 90 Ma, lies along the western side of the orogen; a younger one, 70-52 Ma, lies to the east across a major zone of ductile deformation, the Coast shear zone. Tonalite is the dominant rock type in both arcs with lesser amounts of gabbro and granodiorite. The younger arc also contains 52-50 Ma felsic plutons that formed during rapid uplift of the eastern arc. In the western arc crustal scale tilting due to post-55 Ma extension, documented using minerals in the aureoles of the plutons, has exposed a ~15 km thick section of crust (5 kbar on the west to 9 kbar on the east). The presence of pegmatitic hornblendite gabbro, of magmatic epidote in deep-seated and shallow tonalite plutons, and of plagioclase porphyritic bodies among the shallower intrusions, document magmas with high water contents. One large pluton, the 90 Ma Bell Island body, contains relict clinopyroxene, less igneous epidote and may be somewhat less hydrous than other plutons in the 90-100 Ma arc. Magmatic garnet occurs in the deeper eastern part of this pluton. The 75-60 Ma plutons east of the Coast shear zone also were less hydrous than those to the west as shown by olivine and pyroxene cumulates and the absence of magmatic epidote or plagioclase porphyritic plutons. Emplacement depths are estimated to be between 5 and 7 kbar; chemistry suggests greater crustal interaction than in the western arc plutons. Small low-K plutons in both arcs and, in the eastern arc, highly melted migmatite zones lacking K-feldspar adjacent to several of the larger bodies of tonalite, document crustal melting in the presence of water. These are interpreted as resulting from crustal melting in the presence of fluids derived from the more mafic magmas. The 52-50 Ma granitic plutons are chemically distinct from the older plutons and probably formed by dehydration melting during rapid uplift of the orogen. We suggest that the petrographic and chemical variations recorded in these two continent-margin arcs document interactions of hydrous mantle-derived subduction related magmas with overlying crust of arc and continental affinity.