The Castle Crags pluton intrudes mafic and ultramafic rocks of the Trinity ophiolite complex in the Klamath Mountains of northern California. Contacts between the three concentrically arranged rock types which compose this 30 km2 epizonal stock are gradational. The outermost unit is a fine-grained granodiorite which has a chilled intrusive contact against the adjacent country rocks. The bulk of the pluton is a porphyritic sodic granodiorite which is characterized by euhedral potassium feldspar megacrysts. The core is occupied by a fine-grained alkalic trondhjemite. The pluton is intruded by a genetically related series of hypabyssal dikes, a series of porphyritic trondhjemitic dikes, and a suite of remarkably fresh spessartite lamprophyre dikes. Smooth variation in modes, major and trace elements, specific gravity, and porosity occurs from the margin to the core of the pluton.

The Castle Crags pluton represents a single intrusion with an original composition approximating that of the chilled granodiorite. After it was chilled, the remaining magma crystallized inward from its margins. The concentric chemical variation resulted from fractional crystallization of the alkali-rich parent magma toward a residuum with mild peralkaline tendencies and was aided by inward transfer of alkalis through a volatile phase. Abundant miarolitic cavities in the inner half of the pluton indicate that second boiling occurred when the magma was approximately two-thirds crystallized. The fine-grained textures of the trondhjemite resulted from pressure quenching that accompanied escape of volatiles from the pluton.

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