An oxygen and hydrogen isotope study of a granodiorite pluton from the Southern California batholith

Abstract

The core-zone of the Domenigoni Valley granodiorite pluton (about 9 sq. mi., 30% of the area of the pluton) has a uniform whole-rock δO18 = +7.24 ± 0.11. This is interpreted to be the approximate δO18 of the original granodiorite magma; it is abnormally depleted in O18 relative to the major granodiorites and quartz monzonites of the Southern California batholith. Both the data and the field relations thus suggest that the pluton is more closely related to the major tonalites and gabbros of the batholith. This hornblende-biotite granodiorite body was forcibly intruded into metasedimentary rocks composed largely of muscovite-biotite phyllites and schists (δO18 = +18 to +20) and minor meta-arkose (δO18 = + 13 to +15). Strong oxygen isotopic interaction took place between the low-O18 granodiorite and the high-O18 metasedimentary rocks, with the border zone (outer 650 ft, 8% of the pluton) being enriched in O18 by about 1.3 per mil and the intermediate zone (approximately 60% of the pluton) by about 0.4 per mil. Much stronger isotopic interaction effects are observed in a small granodiorite plug and in several dikes that are offshoots from the main pluton. The hydrogen isotopic ratios of biotite are very uniform throughout the granodiorite (δD = −65 to −74) and are identical to that of biotite from the metasedimentary rocks (δD = −71). These isotope effects are not accompanied by any significant chemical and mineralogical effects and thus cannot be explained by assimilation of the country rocks. They must in large part be brought about by exchange between the granodiorite and the surrounding metasedimentary rocks, probably by means of upward and inward migration of high-O18 aqueous fluids through the marginal zones of the pluton. Locally, sub-solidus exchange occurred at temperatures as low as 550°–600°C, based on the quartz-biotite geothermometer. Good correlations exist between the isotopic data and petrographic evidence for recrystallization in the granodiorite.

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