Abstract

Bald Rock batholith, California, is one of a chain of small plutons that lies 20 miles west of the main Sierra Nevada pluton and parallels the N.15°W. structural trend of the northern Sierra. The country rocks are mainly late Paleozoic (?) basic volcanic rocks cut by small masses of basic plutonic rocks prior to late Jurassic time and folded and dynamothermally metamorphosed in the Nevadan orogeny. Foliation in the pre-batholith terrain was vertical, and its mineral assemblages were, with local anomalies, typical of greenschist facies. After regional metamorphism, gabbroic, dioritic, and granitic rocks were emplaced south and east of the site of Bald Rock batholith. The basic rocks of this sequence are in part magmatic, but the granitic bodies show gradational borders and internal structures that indicate static metasomatic growth, perhaps related to magmatic cores.

Bald Rock batholith was a forceful intrusion, as is shown by the abrupt concordant swing of bedding around it. However, local crosscutting relations show that about a fourth of its area at the exposed level was gained by other means. Large concentric outliers and a hull of injection migmatite suggest stoping. Inclusions are too scarce to account for the missing wall rocks, but gradational zoning of the batholith from a trondhjemite core through granodiorite to a heterogeneous tonalite rim suggests that basic stoped rock contaminated an originally trondhjemitic magma. This is supported by comparing a computed chemical composition of the granodiorite and tonalite with chemical analyses of two trondhjemites. The benign nature and pattern of flow structures shows that the different granitic rocks could not have been intruded separately. In contrast to the wall rock, plagioclase of the granitic rocks shows pronounced oscillatory zoning, indicating thorough reworking of the stoped rock. The reactions required for the changes were presumably exothermic, but the broad mixing shown here must have required additional heat.

Contact-metamorphic rocks of epidote amphibolite to possible pyroxene hornfels facies form an aureole that had an area nearly as great as the original kinetic intrusion. Squeezing and vertical stretching of these rocks is indicated by pebble and cyrstal lineation, most pronounced near the contact. Post-kinetic replacement veins of tonalite occur sporadically throughout the inner part of the aureole. The rise in anorthite content of plagioclase in metabasaltic rocks is gradual toward the batholith and sharp near the tonalite veins. The obvious fracture control of these high-temperature metamorphic structures indicates that heat addition to the aureole was largely through fluid transfer rather than simple thermal diffusion.

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