NEW U-PB AGES AND ZIRCON HF ISOTOPE DATA FROM THE SOUTHERN COAST MOUNTAIN BATHOLITH AND IMPLICATIONS FOR ALONG STRIKE VARIATION IN MAGMATISM

New geochronology and hafnium isotope work in the southern Coast Mountain batholith (CMB) are used to evaluate the timing, spatial distribution and petrogenesis of plutons making up part of the southern arc. Samples were collected from 20 plutons along an inlet that transects the arc about 51°N latitude. Plutons along this transect range from granitic to dioritic in composition and are variably deformed. Zircon U-Pb ages from this transect range from 175-90 Ma. Although plutons comprising the central batholith show a younging eastward trend in post-100 Ma plutons, no age patterns have been observed in the southern batholith. New U-Pb data reveal plutons with ages between 140 and 120 Ma in the southern batholith, a time period that has been characterized with low magmatic activity in the central batholith. Hafnium isotope signatures throughout the southern batholith are juvenile, with pluton averages ranging from +8 to +14. The most evolved εHf values are found in ca. 150 plutons, and generally become higher through time. The εHf values presented here overlap with, but are generally more primitive than, those documented in the central and northern batholith (+1 to +12). The observed differences in age populations between the central and southern parts of the batholith suggests that magmatism is being controlled by along-strike variation in the upper plate. Higher εHf values in the southern batholith indicates that magmas generated are dominantly mantle-derived melts, or are assimilating crust that is younger than that to the north.