MAGMATIC THROUGH MYLONITIC FABRIC DEVELOPMENT IN THE BELLA COOLA AREA OF THE CENTRAL COAST PLUTONIC COMPLEX, BRITISH COLUMBIA, CANADA

Magmatic and solid state fabrics developed within plutons can be used to constrain deformation within arcs and help understand orogenic evolution. Magmatic fabrics (foliations and lineations), formed during the latest stages of crystallization, can record increments of regional strain. Magmatic fabrics in granitoids are commonly recognized as a shape preferred orientation (SPO) of minerals such as feldspars, biotite and amphibole. Another approach to characterize magmatic fabrics is to examine the crystallographic preferred orientation (CPO) of minerals like quartz, a common approach in shear zones.

The Bella Coola region is located in the central Coast Plutonic Complex, British Columbia and consists of early Jurassic through Eocene plutons that variably contain magmatic fabrics and regional shear zones. The Pootlass Shear Zone (PSZ), which is a series of northwest-southeast trending, moderately to steeply dipping shear zones initiated in the Early Cretaceous, exhibit weak subsolidous through ultramylonitic deformation. Thus a complete progression from magmatic through mylonitic fabrics exist in the Bella Coola region making it an ideal location to investigate SPO and CPO evolution in an arc.

Mapping, microstructural and image analysis and electron back-scatter diffraction (EBSD) were used to quantify the orientation, shape, intensity and kinematics of both magmatic and shear zone fabrics in Early Cretaceous Desire suite plutons near Bella Coola. Desire Suite plutons consistently show a NW-SE steeply dipping magmatic foliation a steeply to shallowly SE plunging lineation. Quartz CPO determined using EBSD in magmatic samples yield conflicting results with no CPO in some cases and well developed CPO indicating c-slip or possible preferential growth of quartz during crystallization. Desire suite plutons cut by the PSZ show variable fabric intensities from weak subsolidous fabrics to ultramylonites. Microstructures range from undulose extinction in quartz to bulging grain boundary migration. Quartz CPO is also variably developed but is dominated by prism <a> and rhomb <a> slip, with asymmetric girdles suggesting sinistral shear in the PSZ. Magmatic fabric and shear zone data are interpreted to reflect weakly sinistral transpression in the CPC during Jurassic through Early Cretaceous.