SYNCHRONOUS SILL EMPLACEMENT, EXTENSIONAL DEFORMATION, AND GRANULITE FACIES METAMORPHISM WITHIN THE COAST PLUTONIC COMPLEX, BRITISH COLUMBIA

Andronicos, C. L., Chardon, D. H., Hollister, L. S.

Key words: granulite, extension, magmatism, Cordillera, arc

The Eocene Kasiks sill complex (KSC), located in the core of theCoast Plutonic Complex near Prince Rupert, B.C., provides a case study ofthe interactions of deformation, metamorphism, and plutonism. The KSC,dominated by a 5 km thick diorite sill, dips moderately north and isconcordant with a regional foliation in the core of a tens of km scale dome.Kinematic indicators within country rocks and orthogneiss within the KSC areconsistent with top-to-the northwest normal shearing during sillemplacement. The eastern side of KSC is defined by an east-directedextensional detachment that formed during decreasing temperature conditionsand omits structural section. Deformation around the KSC is localized to thesills, with older steeply dipping foliations preserved within a 1000-m ofthe KSC. Orthopyroxene (opx) occurs within the roof of the KSC, and iswidely distributed below it. The distribution of opx to regions in closeproximity to the KSC implies that heating to the granulite facies was due toheat advected by the sill magmas.

Relict garnets from earlier metamorphism in rocks adjacent to the sillcomplex have a variety of metamorphic corona textures. These include thedevelopment of halos of plagioclase + orthoamphibole +/- opx on garnet inamphibolite and orthogneiss, and rims of cordierite on garnet in paragneiss.Corona textures in orthogneiss and amphibolite define asymmetric fish thatindicate top-to-the north-northwest normal shearing. The degree ofdevelopment of the amphibole corona textures is related to the intensity ofdeformation in a general way, with the most intensely deformed horizonsshowing nearly complete replacement of garnet by the corona assemblages. Incontrast cordierite halos statically overprint garnet porphyroclasts thatwere formed during earlier transpressive deformation. The features of theKSC and its adjacent metamorphic rocks indicate the deformation, sillemplacement and metamorphism were synchronous and show that intrusion of thesill magma localized the extensional shearing and granulite faciesmetamorphism.

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