U-pb and K-Ar geochronometry of the coast plutonic complex, 53°N to 54° N, British Columbia, and implications for the insular-intermontane superterrane boundary

This study presents 48 new U-Pb and 35 new K-Ar dates for magmatic rocks of the Coast Plutonic Complex (CPC) between 53°N and 54°N. The eastern flank of the CPC is underlain by the Gamsby Complex, an early Late Jurassic (160-155 Ma) magmatic, metamorphic and ductile compressional belt superimposed on Early (197-190 Ma) and Middle (178 Ma) Jurassic magmatic rocks of the Intermontane superterrane. The Gamsby Complex was uplifted and cooled in latest Jurassic-Early Cretaceous time (145-132 Ma). Small Early Cretaceous intrusions (132-120 Ma) in the Gamsby Complex are clearly post-kinematic, and no evidence was found for a major middle Cretaceous orogenic episode in the eastern flank of the CPC. The Central Gneiss Complex in the core of the CPC contains large Late Cretaceous (80 Ma) and Eocene plutons that are commonly sill-like. The Central Gneiss Complex gives Eocene K-Ar dates, and is proposed to be an Eocene (51 Ma) metamorphic core complex, which underlies the eastern flank of the CPC beneath a gently to moderately dipping ductile extenslonal shear zone; the tectonic boundary with the western flank of the CPC is gently to steeply dipping. Biotites from positions structurally low in the Gamsby Complex give K-Ar dates (51-50 Ma) which appear to reflect resetting above a rapidly rising hot Central Gneiss Complex. The Eocene extenslonal boundary between the Central Gneiss Complex and the Gamsby Complex was disrupted by steep brittle faults of the Sandifer Lake fault zone, which localize Oligocene (33 Ma) lamprophyre dyke swarms. The west flank of the CPC contains three major tectonic belts, from west to east: 1) The Banks Island belt, composed of large, 160-155 Ma pre-, syn-, and post-kinematic plutons intrusive Into strata of the Insular superterrane; this belt cooled in latest Late Jurassic time (148-143 Ma). 2) The McCauley Island belt, containing Early Cretaceous (131-123 Ma) plutons which cooled in middle Cretaceous time (109-97 Ma). 3) The Ecstall belt, representing a major middle Cretaceous (110-94 Ma) magmatic pulse; this belt cooled by Late Cretaceous time (67 Ma). The new geochronometric results are integrated into a regional tectonic model in which all Jurassic-Eocene components of the CPC and adjacent belts are related to changing patterns of east-dipping subduction beneath a single allochthonous Alexander-Wrangellia-Stikinia (AWS) megaterrane, which was emplaced against the western margin of North America in Middle Jurassic time, closing off the Cache Creek-Bridge River ocean. Following accretion of the AWS megaterrane an early Late Jurassic magmatic arc and associated structures were built mainly along the new outboard margin of North America, but they also overprinted the Cache Creek suture and eastern terranes, including rocks of North American affinity. Early in its history the Washington to Alaska central part of the Late Jurassic arc was rifted (and perhaps displaced longitudinally as well) causing the formation of intra-arc flysch basins and in effect breaking up the AWS megaterrane into two fragments corresponding to the Insular and Intermontane superterranes. These were subsequently overprinted by magmatic belts that define the present CPC. A major middle Cretaceous magmatic pulsewas accompanied by widespread crustal shortening across the CPC, and Eocene magmatism was accompanied by crustal extension, which may have continued into late Tertiary time. Successive episodes are presumably related to changes in relative plate motions and velocities between the North American continent and Pacific oceanic lithosphere, within a primary setting of long-lived, obliquely east-dipping subduction beneath a single terrane. This intra-terrane model for the evolution of the CPC differs from the superterrane collision model of Monger et al. (1982), and is compatible with the scenario suggested by Brew and Ford (1983), who first suggested that the CPC may be an intra-terrane feature. There are no sutures in the CPC, and the CPC is not the result of a collision between discrete superterranes.

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