CE/YB AND TH/U FINGERPRINTING OF ZIRCONS: A WAY TO DISTINGUISH MESOZOIC MAGMATISM IN CONTINENTAL MARGIN ARCS FROM CONTEMPORANEOUS MAGMATISM IN THE ADJOINING CRATON FOR THE SW USA

Abstract

Common Pb isotopic studies in the 1980s-90s established that the Proterozoic of the Mojave crustal providence is characterized by a high average Th/U and that the Mojave Mesozoic intrusions had sources with long term high Th/U. Trace element (TE) analyses of zircon from the Mojave Mesozoic igneous rocks document that many of these zircons also have high Th/U relative to zircons from Mesozoic continental margin batholithic rocks (avr. Th/U=0.4). This difference in Th/U has been used to identify Mesozoic detrital zircons that have probable sources in the Mojave province. However the range of Th/U in Mojave zircons (0.4-1.6+) overlaps with that seen in the marginal batholiths (0.2-1.0) so that only the higher values from the Mojave are distinctive. It is therefore desirable to find additional zircon trace element signatures that can identify zircons from the Mojave (or other cratonic areas) vs the marginal batholiths. Inspection of zircon TE data from MORB, oceanic arcs, continental margin arcs and continental magmatism shows a progressive increase in average Ce/Yb. This increase in Ce/Yb can be correlated to the increasing slope of the REE for the rocks from these same magmatic environments. This observational evidence indicates that Ce/Yb in zircons is dominated by melt REE concentrations and not melt redox in spite of Ce concentration in zircons being controlled by Ce+4, not the more common Ce+3. For the SW USA Ce/Yb in zircons from the marginal arcs mostly ranges from 0.01-0.08 (individual sample averages, Late Cretaceous Sierra differs ranging 0.1-0.2) and from the Mojave and eastern Great Basin from 0.02 to > 0.2 with most values being > 0.08. The contrast in Ce/Yb is particularly sharp for Jurassic zircons with a value of 0.08 separating almost all marginal from cratonic data. For the Cretaceous a combination of age and Ce/Yb provides very good separation with the high Ce/Yb of the Sierra Late Cretaceous being older than the Late Cretaceous of the Mojave. The combination of zircon Th/U, Ce/Yb and age (additional zircon TE such as U/Yb also helps) therefore provides a strong guide to general provenance for the Mesozoic of the SW USA.