APATITE COMPOSITIONS AS TRACERS OF IGNEOUS PROCESSES IN THE WOOLEY CREEK BATHOLITH, KLAMATH MOUNTAINS, CALIFORNIA

Apatite incorporates a variety of trace elements (including Sr, Y, and REE) and volatile elements (F and Cl) whose compositions are related to magma processes.

Apatite separates from the compositionally-zoned, calc-alkaline Wooley Creek batholith (WCb) were analyzed for major and trace elements (EMP and LA-ICPMS, respectively). Samples represent the three compositional and temporal zones (the dioritic-tonalitic lower zone, the tonalitic-granitic upper zone, and the transitional central zone) and a mafic selvage.

Apatite fluorine concentrations are 1.95-2.69 wt% (lower zone), 0.85-3.59 wt% (upper zone), 2.61-3.58 wt% (central zone), and 1.60-3.50 wt% (mafic selvage). Apatite chlorine concentrations are 0.22-0.94 wt% (lower zone), 0.02-0.86 wt% (upper zone), 0.05-0.19 wt% (central zone), and 0.11-1.57 wt% (mafic selvage). There are distinct F-Cl groups between samples. Apatite from upper zone samples 2308 and 8009 displays two distinct F-Cl groups in each sample.

Chondrite-normalized apatite REE patterns display negative slopes and negative Eu anomalies. Lower zone samples are distinct from each other in REE abundance and depth of Eu anomalies, as are some upper zone samples. Apatite from upper zone sample 2308 has two REE patterns, one with greater REE abundances and deeper Eu anomaly than the other. Sr-Y trends for lower zone samples are steeply positive slopes. Upper and central zone samples display shallow positive to constant Sr-Y slopes, except for one population from sample 2308 that overlaps with the steeper lower zone slopes. Mafic selvage apatite has positive Sr-Y slopes slightly shallower than the lower zone samples.

Apatite compositions are consistent with the model of Coint et al. (2013) in which lower zone magmas were emplaced as discrete batches and that the upper zone is relatively homogeneous because of convective mixing. This model can also explain the distinct Sr-Y populations in 2308, in which apatite with positive Sr-Y slopes is antecrystic from lower zone magma (or magma of similar origin) that was involved in convective mixing as the upper zone formed.