2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 208-28
Presentation Time: 3:45 PM


CHISOM, Charles1, CECIL, M. Robinson2, RUSMORE, Margaret E.1 and WOODSWORTH, Glenn J.3, (1)Geology, Occidental College, Los Angeles, CA 90041, (2)Department of Geological Sciences, California State University Northridge, 18111 Nordhoff St, Northridge, CA 91130-8266, (3)Geol Survey of Canada, 101-605 Robson St, Vancouver, BC V6B 5J3, chisom@oxy.edu

Although arc magmatic patterns have been closely studied in the central and northern Coast Mountains batholith (~ 52–54 °N; e.g. Gehrels et al., 2009), the geochronology and spatial distribution of plutons in the southern Coast Mountains batholith are not well known. In this study, we aim to constrain the magmatic history of the batholith at the latitude of Mt Waddington (~ 50 °N). New zircon U-Pb ages from nine plutons help define the eastward migration the magmatic front, delineate the locus of Late Cretaceous magmatism, constrain the timing of deformation in the core of the orogen, and contribute to estimates of magmatic flux from Late Jurassic through Early Tertiary time. Samples were collected from a ~100 km long NE-trending transect up Knight Inlet and past Mt. Waddington, ~ 75 km northwest of the area discussed by Rusmore et al., this meeting. Samples range in age from Late Jurassic to Eocene and vary in their composition and relation to major deformation events. Plutons in the westernmost part of the transect consist of slightly altered quartz diorite. East of these, we find epidote-bearing tonalites that appear similar to mid-Cretaceous plutons in the northern batholith. Samples from the core of the batholith include orthogneiss with a protomylonitc fabric, the sill-shaped Tiedemann intrusion from the Waddington massif, and large plutons that cut the Late Cretaceous eastern Waddington thrust belt and are early Tertiary in age. Cathodoluminescence imaging reveals distinctive zonation patterns and a wide variety of zircon morphologies. Samples from the central part of the transect, which record the greatest amount of deformation, have chemically and morphologically distinctive cores and thin, low-U rims, suggestive of zircon inheritance and possibly metamorphic overgrowths. Comparison of our new pluton ages to those from the northern segment of the Coast Mountains help define the magmatic history of the batholith, particularly the location and temporal evolution of the mid-Cretaceous arc. This information will contribute to our evaluation of what is primarily driving episodic magmatism in the Coast Mountains and potentially in cordilleran arcs globally.