ZIRCON TRACE ELEMENT AND ISOTOPIC RECORDS OF SIERRA NEVADAN ARC MAGMATISM DURING AN ARCHETYPAL MAGMATIC FLARE-UP
Trace element and Hf and O isotopes from zircon provide complementary information that can together be used to probe the sources and assembly of the intrusions. Key trace element indicators (Ti-in-zircon model temperatures, REE patterns and ratios) show important time-dependent shifts that repeat along the arc, and thus reflect important arc-scale shifts in the environment of magma formation and also in subsequent higher-level storage conditions. The shifts record a pronounced change from overall hotter conditions of magma formation and initial storage (zircon undersaturation) near the "apex" of the flare-up to markedly cooler, more water-rich conditions of magma formation and storage (at or near zircon saturation) as arc-magmatism waned.
Hf and O isotopes in zircon show important variations in crustal and mantle sources that are generally harder to discern from whole rock isotopic trends. Although there is a modest shift to more crustal isotopic values (lower εHf and higher δ18O) in the suites, there are equally important arc-scale and intrusion-scale spatial variations that must reflect lithosphere-scale variations and shifts in magma source and/or plumbing.
The zircon data also can be used to evaluate/refine recent models for arc-scale differentiation that have been proposed for Cordilleran arcs.