Application of geochronology/geochemistry of zircon in understanding the construction of the Peninsular Range Batholith

Abstract:

Trace-element signatures of zircon are becoming increasingly used as a tool to infer the petrologic history of the rock from which that zircon crystallized. In this study, we sampled 11 igneous rocks from west to east across the northern Peninsular Ranges Batholith (PRB) to test how well REE patterns in zircon reflect those of the whole rock in arc magmas (granodiorite–tonalite). Previous studies of the PRB show two transitions from west to east, with respect to their REE patterns; transition 1) a decrease in HREE, from the west to the central PRB, and 2) an increase in LREE from the central to the eastern PRB. Whole rock samples in this study, analyzed by XRF and ICPMS, reproduce this pattern and thus provide a variety of REE signatures with which to test whether zircon can be used as a proxy for whole-rock data.

Zircon from the 11 samples was analyzed by LASS (Laser Ablation Split Stream) ICP-MS, to measure both the age of the zircons and their trace-element compositions. In general, as expected, ages young from west (ca. 104 Ma) to east (ca. 90 Ma). Patterns of HREE in zircon correlate well with those of the whole rock, whereas the LREE correlation is weak. The distribution coefficient for HREE between zircon and whole rock, however, decreases with increasing HREE. Possibilities for this negative relation include: 1) minor changes in whole-rock chemistry, as samples become slightly more felsic from west to east, 2) changes in crystallization temperature from west to east, and 3) sampling bias of HREE-poor zircon rims with HREE fractionated in zircon cores.

Other trace element data measured in zircon and whole rock include P, Ti, Y, Nb, Hf, Ta, Th, and U and provide interesting results. Nb and Ta show a moderate correlation between zircon and whole rock, but the Nb/Ta ratio does not. Also, though U correlates well between whole rock and zircon and Th/U correlates well between zircon and age, the Th/U ratio of the zircon is negatively correlated with the whole rock; thus, in this case, Th/U changes in zircon are not a good indication of Th/U trends overall. Finally, Ti in zircon correlates strongly with location and whole-rock HREE concentration, indicating that either temperatures and/or Ti activity were higher in the west, early in the construction of the batholith.