ASHLAND PLUTON (KLAMATH MOUNTAIN PROVINCE) REVISITED: THE VIEW FROM MINERAL COMPOSITIONS AND ZONING

The late Middle Jurassic Ashland pluton is a composite body that straddles the OR–CA border. The pluton and overlying Cretaceous and Tertiary sedimentary rocks are tilted gently to the east. Three magmatic units comprise the pluton. The tonalitic unit is structurally deepest, it crops out parallel to the western contact. The tonalite unit grades eastward to a heterogeneous unit of quartz monzodiorite and granodiorite (qmd unit), which hosts swarms of mafic enclaves. The qmd unit is intruded by the granitic unit: bodies of biotite ± hornblende granite that are locally host to enclave swarms. The tonalite unit has lower SiO₂, K₂O, and P₂O₅ contents than the qmd unit and the granitic unit contains high SiO₂ and K₂O but the lowest P₂O₅ values. Mafic enclaves are generally low in SiO₂ but have high P₂O₅ and K₂O values, similar to the qmd unit.

Trace element zoning in magmatic hornblende (Hbl) also distinguishes the three units. Hbl in the tonalite unit is richer in Ba and P and poorer in total REE, Y, Nb, Ta, and Mn than Hbl from the qmd unit. In addition, tonalite-unit Hbl is essentially identical to Hbl from coeval, broadly tonalitic plutons in the Klamath province, with most trace element values decreasing with decreasing Ti and temperature (T). In contrast, qmd-unit Hbl displays compositional arrays that are nearly orthogonal to those of tonalite-unit Hbl and are weakly correlated with Ti or T.

Trace element abundances and chemographic calculations indicate that (1) Hbl in all units crystallized from zircon-bearing rhyodacitic to rhyolitic melts, (2) in the qmd unit Ba and P were depleted prior to Hbl crystallization. Apatite in the qmd unit occurs mainly as clusters of fine inclusions in all primary phases, which suggests either thermal, or more probably compositional undercooling. The latter explanation is consistent with magma mixing during and after emplacement, and is consistent with the abundant swarms of enclaves (which are enriched in P₂O₅, REE, and Y) and the poor correlation of trace element variation with temperature in qmd-unit Hbl. Zircon saturation calculations [ref 1] indicate that rocks from the tonalite unit represent at least 40% crystal accumulation, qmd rocks represent 0–70% accumulation, and the granitic unit mainly reflects melt compositions.

[1] Barnes et al., Jour. Petrology, doi: 10.1093/petrology/egaa008.