MAGMATIC DEVELOPMENT OF THE COAST MOUNTAINS BATHOLITH AND CONSTRAINTS ON THE TECTONIC CONFIGURATION OF THE NORTHERN CORDILLERA IN JURASSIC TO LATE CRETACEOUS TIME

The Coast Mountains batholith (CMB) of British Columbia, Canada, is one of the largest exposed continental arcs on Earth. It was emplaced into and across the Insular and Intermontane superterranes, making the CMB important not only to understanding crustal growth in arc settings, but also to the tectonic and paleogeographic history of the northern Cordillera. The CMB comprises roughly parallel Jurassic to mid-Cretaceous arcs that are built into the Insular terrane to the west and the Intermontane terrane to the east. Magmatism in the eastern arc segment is stationary and continuous, whereas magmatic patterns in the western segment are variable and in places marked by a magmatic gap at 140 – 120 Ma. From ca. 100 - 65 Ma, magmatism migrates progressively inboard, stitching across the two terranes. This suggests that by at least ~100 Ma, the Insular and Intermontane terranes are sutured together. Landward migration also indicates that the two superterranes and the now-amalgamated CMB are positioned above an east-dipping subduction zone. Isotopic data (Sr-Nd-Hf-O) for the CMB reveal a composite arc that is dominated by mantle-derived melting, though excursions toward negative zircon Lu-Hf values suggest variable contamination of melts by crustal sources. In general, eHf(t) values in CMB plutons record greater northward incorporation of more isotopically-evolved components in magmas. In the central CMB, where country rock affinities are better known, the pre-100 Ma arc emplaced into rocks of the Insular terrane has distinctly lower eHf(t) values that pull down below CHUR in the early Cretaceous. These zircon Hf results suggest involvement of a Precambrian component in pluton melt sources, which is inconsistent with archipelago models of the western CMB developing in a fringing oceanic arc position at that time. This interpretation is further supported by zircon trace element analysis, which indicates melts that are more fractionated, involve a greater crustal contribution, and were likely generated in a thicker crustal column than what is typical in most intra-oceanic arcs. The presence of geochronologically and geochemically-distinct western CMB detritus in the Chugach accretionary complex requires the Insular arc to be west-facing, though west-directed subduction along its inboard margin cannot be ruled out.