Abstract
Metamorphic soles are thin (<500 m-thick) sheets of high-pressure, high-temperature (HT) metabasalt, metachert, and metaclastic rocks structurally underlying an ophiolite. They are often interpreted to be formed at the initiation of subduction with the age determined through a mix of Ar-Ar hornblende, U-Pb zircon, and Lu-Hf garnet geochronology on mafic amphibolites. Only recently has U-Pb zircon dating been applied to metaclastic rocks within soles. The result has been either a high-temperature metamorphic age using the youngest concordant zircons or a detrital age spectrum, but not both. Here we describe LA-ICP-MS U-Pb geochronology of detrital zircons from a quartz-biotite-garnet schist from the amphibolite-facies metamorphic sole of the Devonian Feather River Belt of the northern Sierra Nevada exposed near Downieville, California, in which both detrital zircon ages and a metamorphic age can be determined. This sample was analyzed with analytical spots (N=207) chosen in grain cores guided by BSE images. Most of the analyzed zircons were discordant with only the youngest zircons (<400 Ma) being concordant. This youngest concordant group of zircons (N=34) yielded a concordia age of nearly equal 354 Ma which we interpret to be a HT metamorphic age causing either lead loss or growth of metamorphic zircon as the date lies at the lower concordia intercept of several linear arrays of discordant data. This date is consistent with previously published 344 Ma Ar-Ar and 345 Ma K-Ar ages associated with an early HT event on the unit. In previously published work, strongly (<-10% and >30%) discordant data were discarded to create a detrital zircon age spectrum. However, this approach is suspect as even zircons with relatively low discordance may have significantly different upper-intercept ages. To address this, we also use the concordant HT metamorphic age to calculate a discordia upper intercept for each discordant analysis to generate a corrected age spectrum. Preliminary results show a spread of mostly Precambrian ages that broadly match a North American provenance.