TRACE ELEMENTS IN CPX AND HBL REVEAL MAGMA BATCHES AND CRYSTAL ACCUMULATION IN THE IRONSIDE MOUNTAIN PLUTON, KLAMATH MOUNTAINS, CA

The thought that many plutons represent accumulation of minerals is becoming widely accepted. Researchers are moving away from assuming bulk-rocks are melt compositions and towards utilizing single mineral compositions as tools to calculate melt compositions and the extent of accumulation in plutons. The Ironside Mountain pluton's (IMP) 3-pyroxene mineral assemblage, its evolution to potassic compositions, and its H₂O-poor and reducing parental magmas make it distinct from other plutons that intruded the Klamath Mountain terrane post- Middle Jurassic contraction. The IMP ranges in composition from gabbro to quartz monzonite. Previous work on the IMP used bulk-rock data to conclude it differentiated by fractional crystallization as a relatively closed system. In this study, trace element concentrations in augite (cpx) and hornblende (hbl) were measured by in-situ laser ablation inductively coupled plasma mass spectrometry to test the closed-system fractionation hypothesis and the extent of crystal accumulation.

Mineral-bulk rock ^Fe/MgKd values range between 0.88 and 1.30 and between 0.530 and 0.76 in hbl and cpx, respectively. High ^Fe/MgKd values suggest the minerals were in equilibrium with melts lower in Mg than the bulk rocks (i.e. the bulk rocks accumulated Mg-rich phases). Melt calculations of the rare earth elements (REE) result in melt compositions with large negative Eu* anomalies. The lack of Eu* anomalies in the bulk rocks suggests that they represent accumulations of plagioclase grains with positive Eu* anomalies. Based on trace element abundances and REE patterns, there are at least three distinct units in the IMP: a) a quartz diorite unit that is the most evolved, has the deepest negative Eu* anomaly, and is enriched in V and depleted in Ni and Co, b) a unit in the NW that is characterized by REE depletion and enrichment in Ni and V, and c) the bulk of the IMP which is enriched in REEs and follows its own fractionation trend.

Through detailed analyses of minerals, it was possible to test and negate previous theories of petrogenesis, to identify potential magma batches, and to better assess magmatic processes in the IMP.