DEFROSTING A PLUTON, JURASSIC ASHLAND PLUTON, KLAMATH MOUNTAINS, OR AND CA

The composite Ashland pluton is tilted to the east, exposing several km of structural relief. The pluton consists of three main units. The lower tonalitic unit grades eastward to quartz monzodiorite and granodiorite (qmd unit--the focus of this study), which is host to swarms of mafic enclaves. The qmd unit is intruded and overlain by the granitic unit, which consists of biotite ± hornblende (Hbl) granite with local, dense enclave swarms. The qmd unit is enriched in SiO₂, K₂O, and P₂O₅, compared to the tonalite unit; its enclaves generally display low SiO₂ and have high P₂O₅ and K₂O values. Equant apatite is the main accessory mineral in the tonalite unit, whereas the qmd and granitic units contain accessory titanite (Ttn), apatite, and allanite.

Petrographic features in the qmd unit such as glomerocrysts, broken crystals in which zoning is truncated, resorption of Hbl, Plag, and Ttn, and pseudomorphed, melt-filled fractures, all indicate defrosting and magma movement. Apatite occurs mainly as fine inclusions in other phases and as clusters of fine crystals (± scant zircon and Fe-Ti oxides) in quartz and K-feldspar.

Magmatic Hbl in the tonalite unit is similar to Hbl in coeval Klamath plutons. Compared to qmd-unit Hbl, it is richer in Ba and P and poorer in total REE, Y, Nb, Ta, and Mn. Hbl in the qmd unit displays striking enrichments in REE, Nb and Ta, enrichments weakly correlated with Ti or T. Ttn grains in the qmd unit vary from euhedra to ragged anhedral. Many display normal trace element zoning (e.g., core-to-rim decreases in Nb) but some display high-Nb rims.

We explain the petrographic and mineral zoning features in the qmd unit as the result of multiple defrosting events related to mafic recharge, as exemplified by abundant mafic enclave swarms. Resorption of Ttn led to redistribution of REE, Nb, and Ta into Hbl, and locally into Ttn rims. Remelting of phases with apatite inclusions freed and partly resorbed the small apatite grains, which then collected in low-T melt pools. At least some melts lost during defrosting rose into the granitic unit, consequently imparting cumulate compositions on the residual qmd mush. We conclude that the effects of defrosting may be preserved in pluton textures, mineral zoning patterns, and bulk compositions.