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The Ashland pluton is a calc-alkaline plutonic complex that intruded the western Paleozoic and Triassic belt of the Klamath Mountains in late Middle Jurassic time. The pluton comprises a series of compositionally distinct magma pulses. The oldest rocks are hornblende gabbro and two-pyroxene quartz gabbro with initial 87Sr/86Sr = 0˙7044, δ18O = 8˙7%, and REE patterns with chondrite normalized La/Lu = 7. These units were followed by a suite of tonalitic rocks (LaN/LuN = 7) and then by a suite of K2O- and P2O5 rocks of quartz monzodioritic affinity (LaN/LuN = 13–21; LaN/SmN = 2˙4–3˙) The quartz monzodioritic rocks were then intruded by biotite granodiorite and granite with lower REE abundances but more fractionated LREE(LaN/LuN = 13–19; LaN/SmN = 4˙3–6 and they, in turn, were host to dikes and bosses of hornblende diorite. The latest intrusive activity consisted of aplitic and granitic dikes. Combined phase equilibria and mineral composition data, indicate emplacement conditions of approximately Ptotal = 2˙3kb, PH2O between 1˙5 and 2˙2 kb, and fO2 between the nickel-nickel oxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO2 together with increasing δ18O and decreasing initial 87Sr/86Sr. The isotopic data are consistent with either (1) combined fractional crystallization of andesitic magma and concurrent assimilation of crustal material characterized by low Sr1 and high (δ18O or, more probably, (2) a series of partial melting events in which sources were successively less radiogenic but richer in 18O Each intrusive stage displays evidence for some degree of crystal accumulation and/or fractional crystallization but neither process adequately accounts for their compositional differences. Consequently, each stage appears to represent a distinct partial melting or assimilation event. The P2O5-rich nature of the quartz monzodiorite suite suggests accumulation of apatite. However, the suite contains abundant mafic microgranitoid enclaves and most apatite in the suite is acicular. These observations suggest that magma mixing affected the compositional variation of the quartz monzodiorite suite. Mass balance calculations are consistent with a simple mixing process in which P2O5-rich alkalic basalt magma (represented by the mafic microgranitoid enclaves) was combined with a crystal-poor felsic magma (represented by the tonalite suite), yielding a quartz monzodioritic magma that then underwent differentiation by crystal fractionation and accumulation.