APATITE COMPOSITIONS INDICATE FRACTIONAL CRYSTALLIZATION OF MAGMA BATCHES IN THE IRONSIDE MOUNTAIN BATHOLITH, KLAMATH MOUNTAINS, CALIFORNIA

Apatite contains trace elements (including Sr, Y, Mn, Mg, REE) and volatile elements (F, Cl) that are sensitive to magma processes.

Apatite separates from the Ironside Mountain batholith (IMb) were analyzed for major elements (EMP), trace elements (LA-ICPMS), and initial ⁸⁷Sr/⁸⁶Sr (LA-ICPMS). Samples analyzed were three from the gabbroic to quartz monzonitic Ironside Mountain pluton (IMp), one from the quartz diorite of Happy Camp Mountain (qdHCM), which underlies and is intruded by the IMp, and one from the olivine pyroxenitic to quartz dioritic Denny intrusive complex (Dic).

Apatite fluorine concentrations are 1.67-3.40 wt% (IMp), 2.95-3.46 wt% (qdHCM), and 2.89-3.25 wt% (Dic). Apatite chlorine concentrations are 0.17-1.40 wt% (IMp), 0.11-0.49 wt% (qdHCM), and 0.02-1.81 wt% (Dic). IMp samples have broad overlap in F concentrations, but each have distinct Cl concentrations with little to no overlap.

Apatite from the IMp has chondrite-normalized REE patterns with negative Eu anomalies and elevated LREE relative to HREE. Each IMp sample has a distinct REE concentration range with broad overlap between samples. Dic apatite has negative-sloped REE patterns, shallower negative Eu anomalies, and lower concentrations of REE than the IMp. A single analysis from qdHCM has lower REE abundance than all other samples.

Apatite REE concentrations positively correlate with one another and Y in IMp samples. IMp sample 224 has a more restricted range of REE and Y concentrations than other IMp samples. Individual IMp samples display distinct trends in Mn-Sr, Mn-Mg, REE-Sr, and REE-Y plots, and differ in concentrations of these elements from Dic and qdHCM samples.

Initial ⁸⁷Sr/⁸⁶Sr values of IMp apatite are 0.7036-0.7039 (uncertainties [unc]: 0.0004-0.0006). QdHCM values are 0.7043 and 0.7046 (unc: 0.0005-0.002). Dic values are 0.7031 and 0.7036 with (unc: 0.0005-0.0008).

Apatite compositions and initial ⁸⁷Sr/⁸⁶Sr of the IMp are consistent with evolution of magma primarily through fractional crystallization with minimal crustal contamination (Barnes et al., 2006) and batchwise assembly (Angulo and Barnes, 2020). Positive correlation of Y-REE, Mn-Mg, Sr-Mn (samples 224, 11A), and negative correlation of La-Sr (sample 224) indicate fractional crystallization of batches. Apatite compositions confirm the qdHCM and Dic as originating from distinct magmas from the IMp samples.