Geochronology of high-pressure-low-temperature Franciscan metabasites: A new approach using the U-Pb system

Publisher Website
Google Scholar
Abstract
The U-Pb isochron method is a promising new approach to the geochronology of high-pressure-low-temperature metabasites. In samples with favorable U/Pb ratios, metamorphic minerals such as sphene, apatite, lawsonite, glaucophane, garnet, and hornblende partition U and Pb in such a way as to provide a range of U/Pb ratios suitable for isochron dating. In a manner analogous to Rb-Sr isochron dating, these U-Pb isochrons provide not only ages, but also information on the initial isotopic composition of Pb at the time of metamorphism, a significant petrogenetic tracer. Sphene is the key mineral for dating. Its relatively high-U/Pb ratio results in the evolution of moderately radiogenic Pb, and it is highly resistant to resetting. Some metabasites have U/Pb ratios that are extremely low, perhaps owing to severe U depletion at some stage of metamorphism. These samples are not useful for dating, but still provide valuable data on the initial isotopic composition of Pb. Analysis of a Type III metabasalt (blueschist) from the Taliaferro complex near Leech Lake Mountain yields a U-Pb isochron age of 162 ± 3 Ma., slightly older than the widely quoted 150–155 Ma. K-Ar ages for high-grade Franciscan tectonic blocks. Two garnet amphibolite blocks from the Catalina Schist terrane yield identical ages, with an isochron for both samples giving an age of 112.5 ± 1.1 Ma. These samples, plus three more Franciscan metabasites, have isotopic compositions of initial Pb (206Pb/204Pb = 18.40–18.85; 207Pb/204Pb = 15.55–15.66) that plot distinctly above the field for modern MORB; instead they plot in the fields for some island arcs and granodiorites from the Sierra Nevada.

This publication has 0 references indexed in Scilit: