Redistribution of Strontium and Rubidium Isotopes during Metamorphism, World Beater Complex, Panamint Range, California

Abstract

An earlier Precambrian gneiss dome in the Panamint Range of California and its mantle of later Precambrian and Paleozoic rocks have been metamorphosed in late Mesozoic time. Uranium-lead analyses of zircons indicate the primary age of the older gneiss to be about 1800 m.yrs and the age of a younger cross-cutting granite to be about 1300 to 1400 m.yrs. Potassium-argon ages on biotite give ages ranging from 103-130 m.yrs. Rubidium-strontium isotopic studies of all the constituent minerals and their associated total rocks yielded biotite-total rock isochrons indicating ages ranging from 64 to 156 m.yrs. All of the total rock ages deviate considerably from the apparent primary age indicated by the zircon ages. The intercept values of Sr^(87)/Sr^(86) for the isochrons from the various rocks range from 0.85 to 1.08. Nearly complete isotopic homogenization of strontium has occurred locally during this metamorphic episode for all mineral systems except apatite and muscovite. The "total rock systems" were open in some cases, including samples as large as 85 kg. There is obvious mineralogic and field evidence for metamorphism in the mantling sediments but no evidence for gross recrystallization and mobilization in either the mantling rocks or the underlying gneiss and granite. Even though original textures and structures, both sedimentary and igneous are preserved, the observed homogenization of strontium indicates that extensive migration of strontium occurred and affords a sensitive test of metamorphism. Conditions causing the redistribution of strontium in the gneiss did not lower the lead-uranium ratios of the zircons by more than 30%. A basic dike of later Precambrian age was found to contain radiogenic strontium due to partial equilibration with the neighboring gneiss. This rock gives an apparent age of 31.4 x 10^9 yrs and presents evidence for strontium transport over a distance of 5m.

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