The Trinity peridotite is an enormous ultramafic massif situated in the eastern Klamath Mountains of northern California and composed of a diverse assemblage of ultramafic rocks including dunite, harzburgite, plagioclase lherzolite and clinopyroxene-rich lithologies. These rocks preserve an excellent record of a complex mantle history involving plastic deformation, partial melting, recrystallization and reaction with transient silicate melts in a large peridotite mass as it ascended through the upper mantle. Structural data are utilized to determine the relative ages of events that affected the peridotite, and petrochemical data are utilized to place constraints on the pressures and temperatures at which the events occurred. The Trinity peridotite is inferred to have ascended through the upper mantle from an initial depth of not less than 30 km, based on evidence for replacement of spinel by plagioclase. Ariegite bands and dikes formed and isoclinal folding occurred while the peridotite was in the spinel lherzolite stability field (P>10 kbar). The peridotite passed its solidus and partially melted at a pressure of ?10 kbar, producing a basaltic melt with tholeiitic affinities. Plagioclase lherzolite formed where partial melt was trapped and minerals reequilibrated during subsequent low pressure cooling of the peridotite. Clinopyroxene-rich dikes were emplaced after the peridotite had passed into the plagioclase lherzolite field (P<10 kbar). Plastic deformation, recorded by penetrative foliation and lineation, occurred continuously during the diapiric rise of the Trinity body. The emplacement of undeformed gabbro marks the arrival of the Trinity at the base of the crust. Large, tabular dunite bodies, clinopyroxene-rich dikes and ariegite dikes mark channels through which melts passed and reacted with the peridotite en route to the surface. The interiors of the dunite bodies and the dikes are interpreted to have formed as crystal cumulates from the transient melts. The margins of the dunite bodies and depleted zones around some clinopyroxene-rich dikes formed as a 'restite' by assimilation of pyroxene and plagioclase from the peridotite wall rocks by the transient melts. The melts are hypothesized to have formed by partial melting of peridotite at a great depth (e.g., 45--60 km) and to have penetrated the Trinity peridotite at shallower depths (<30 km) where they were not in equilibrium with pyroxene or plagioclase. Structural and stratigraphic data suggest that these events this occurred near an oceanic volcanic highland in a setting analogous to a modern island arc or back-arc basin.