U/PB ZIRCON AGES OF FELSIC VEINS IN THE SAWTOOTH METAMORPHIC COMPLEX, IDAHO, U.S.A: IMPLICATIONS FOR MAGMATISM AND PLAGIOCLASE CATHODOLUMINESCENCE RESPONSE AS A SOURCE PROXY

An assemblage of high-grade rocks in the Sawtooth Metamorphic Complex (SMC), Idaho, is crosscut by numerous felsic veins of unknown origin. The fine-to-coarse grained veins consist primarily of alkali feldspar, plagioclase, quartz, ± biotite, ± zircon, ± apatite, and ± opaques. Constraining the ages and sources of these veins can help elucidate the melting events, relative timing of metamorphism, and the regional tectonics. The melts that formed these veins could be related to several sources: (1) Sawtooth batholith (ST), c.a. 48 Ma; (2) Idaho batholith (IB), 98-67 Ma; (3) an anatectic melt (AM) of aluminous gneisses, >100 Ma; (4) a younger melting or hydrothermal event; or (5) a combination of sources. U/Pb ages (by LA-ICP-MS) of zircons extracted from 11 vein samples augment previously published ages for likely sources. New ages, ranging from 75 ± 1 Ma to 101 ± 1 Ma, are consistent with an origin as peripheral melts related to the IB age. In addition, representative plagioclase (pl) grains from the vein samples and potential source materials were characterized with optical cathodoluminescence (OM-CL) to determine the efficacy of using OM-CL as a proxy for sources and correlations.

Fourteen samples from potential vein sources (5 ST, 4 IB, and 5 AM) have distinctive plagioclase signatures. ST pl luminesces pink to dark brownish-green and orange to bright green; IB luminesces dark green and dark brownish-green; and pl in anatectic melts samples luminesce turquoise to green, greenish blue, and dark blue. Seven of 11 vein samples displayed dark green OM-CL plagioclase responses, similar to plagioclases from the IB. One sample showed brownish-green and orange responses similar to the ST and three samples have greenish blue, brown, and yellow responses unlike those observed in the potential source sample set. Thus, over 60% of the vein samples have OM-CL plagioclase responses similar to the age-equivalent rocks in the area as determined by U/Pb zircon geochronology. These data suggest that plagioclase OM-CL responses in some samples can provide a first order linkage to possible sources in the absence of geochronology. In addition, this study underscores the multiple episodes of melt formation and migration in the SMC over a ca. 25-million-time period.