ORIGIN OF ZIRCONS IN METASEDIMENTARY ROCKS OF THE SOUTHERN COAST MOUNTAINS BATHOLITH

The tectonic history of the Coast Mountains Batholith (SCMB) in British Columbia is important to further understand continental growth by accreted terranes. This study focuses on the origin of zircon grains extracted from high-grade metasedimentary rocks preserved as pendants within the SCMB. Most of the grains in these samples yield ages older than 170 Ma, and help evaluate potential connections with the Wrangellia, Alexander, Yukon-Tanana, and Stikine terranes. These samples also yield numerous Jurassic to Late Cretaceous ages, which is problematic because the rocks are known to have been intruded by plutons beginning ~170 Ma and may have been metamorphosed during intrusion. The purpose of this project is to determine whether these post 170 Ma ages result from igneous material (e.g., small sills or veins) that was unintentionally included in the samples, or record the growth of metamorphic zircon. This question has been addressed by (1) acquiring detailed CL images of the young zircon crystals, and (2) constructing detailed maps of their U-Pb age, U concentration, and U/Th. Each zone of the crystal was targeted and between 5 and 33 analyses were conducted on each grain. This age mapping technique was applied to 16 zircon crystals from 5 samples, one from a leucocratic sill and four metasedimentary. Many of the zircons from the metasedimentary samples, and all of the grains from the leucocratic sills, display oscillatory zonation and yield U/Th values of less than 10. Given that zircons from adjacent plutons are of similar age and have similar characteristics, many of the post 170 Ma ages in our samples are interpreted to result from unintended incorporation of igneous material in our samples, despite very careful examination of the rocks during sample collection. Most of the <170 Ma ages, however, are from zircon rims or entire zircon grains that display convolute zoning and yield high U/Th values, which are not found in zircons from the leucocratic sills or adjacent plutons. These ages are interpreted to record metamorphism of the metasedimentary rocks during emplacement of the adjacent plutons.