MAGMATIC THROUGH MYLONITIC FABRIC EVOLUTION IN ARCS DURING COEVAL PLUTONISM, VOLCANISM AND REGIONAL DEFORMATION: EXAMPLES FROM NORTH AMERICAN CORDILLERAN BATHOLITHS

The construction and deformation of batholiths has long been known to be episodic, characterized by rapid and voluminous emplacement of lavas/magmas onto and into the crust often coupled with local and regional deformation. Magmatic and solid state fabrics developed within arc plutons can be used to constrain regional deformation within arcs and help understand orogenic evolution. 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 and solid state fabrics is to examine the crystallographic preferred orientation (CPO) of minerals like quartz. Mapping, microstructural and image analysis and electron back-scatter diffraction (EBSD) are used to quantify the orientation, shape, intensity and kinematics of both magmatic and shear zone fabrics in plutons from the Sierra Nevada batholith, California and the Coast Plutonic Complex (CPC), British Columbia. Along with geochronology and barometry, these data suggest that punctuated episodes of magma emplacement and associated volcanism coupled with localized vertical displacement occur within evolving regional strain fields within these arcs.

The Jackass Lakes pluton (JLP) in the Sierra Nevada and plutons from the Bella Coola region of the CPC are excellent examples of how punctuated construction and deformation of volcano-plutonic systems develop. The JLP exhibits a complex history of incremental emplacement and within the pluton there are several large volcanic pendants and innumerable xenoliths linked to the Ritter Range pendant located east of the pluton. Ages from metavolcanic pendants within the JLP overlap directly with U-Pb ages from the pluton indicating that coeval magmas intruded their own volcanic cover ca. 98 Ma. Geochronologic, barometric and structural data show that volcanism, plutonism and deformation in this region of the Sierran arc were occurring coevally. Bella Coola plutons are early Jurassic through Eocene in age that contain magmatic fabrics and shear zones recording regional deformation. A complete progression from magmatic through mylonitic fabrics allow for investigation of SPO and CPO evolution in arcs. Magmatic fabric and shear zone data record changing strain patterns in the arc through time.