Abstract

Compton made a detailed study of the Bald Rock batholith, California, and suggested that the tonalites and granodiorites of the rim of the batholith originated through contamination. He believed that trondhjemite magma, represented by rocks in the core of the batholith, assimilated meta-basaltic country rocks. This paper reports a detailed study of 22 rocks from the batholith, including all the main rock types. Values for Qu, Or, Ab, and An calculated from the modes show that the rocks fall along a trend slightly below the cotectic boundary between quartz and feldspars and directed toward this surface. Compositions calculated from the modes indicate that formation of the tonalites by contamination is possible but requires improbably high proportions of metabasalt xenoliths in the rim of the pluton. Habits of zircons separated from the rocks show a distribution pattern which emphasizes contrasts between trondhjemitic core and granodiorite-tonalite rim of the batholith but does not support the idea of formation of the mantle rocks by contamination. Dimensional zircon data indicate that zircons of the core are unimodal, whereas those of the mantle of the batholith are bimodal. Comparisons with the Bald Mountain batholith, Oregon, demonstrate many similarities, as well as some striking contrasts. Compositional trends of tonalite to granodiorite are different for the two plutons. Zircons are the same throughout the Bald Mountain batholith. The writers conclude that both plutons were emplaced by forceful injection. At Bald Mountain, Oregon, a uniform magma was injected, but at Bald Rock, California, a migma-magma was emplaced. The trondhjemitie core of the Bald Rock batholith was mainly liquid at emplacement, but the tonalite-granodiorite rim consisted of mobile, viscous solid phases with interstitial melt. The liquid line of descent at Bald Mountain, Oregon, was from tonalite to granodiorite and at Bald Rock, California, only from trondhjemite to leucotrondhjemite.

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