Abstract

The metamorphic rocks of the Santa Rosa Range of north-central Nevada represent a sequence of Upper Triassic (and Jurassic?) sediments that was 20,000 feet thick before it was strongly folded, metamorphosed, locally faulted, and intruded by granitic stocks. Each of the six formations is dominantly metapelitic, although quartzitic, calcareous, and dolomitic rocks are widespread. All the rocks were converted to metamorphic assemblages of quartz, albite, muscovite, chlorite, and carbonates before and during the regional folding. The nine stocks consist mainly of sodic granodiorite, but adamellite, trondhjemite, and tonalite are locally abundant. Early chilled apophyses show the magmas originally contained few if any crystals, and structural relationships prove the stocks are dominantly crosscutting (stoped?) bodies.

Forty-six chemical analyses of metapelites show that most of the phyllites have percentage compositions close to: SiO2, 61.3; TiO2, 1.0; A12O3, 19.3; Fe2O3, 1.2; FeO, 5.5; MnO, 0.07; MgO, 1.9; CaO, 0.6; Na2O, 1.5; K2O, 3.0; H2O, 3.9; P2O5, 0.15. Besides dehydration, the only metasomatism during progressive contact metamorphism was minor desilication and addition of Fe around one stock.

Although their compositions are similar, the rocks developed four combinations of critical minerals in the outer and intermediate parts of the aureoles, namely, cordierite-biotite, cordierite-andalusite-biotite, staurolite-andalusite-biotite, and andalusite-biotite. Textures and structures show that directed stress was equally effective in all these rocks. Around most of the stocks, contact metamorphism began with the reaction of chlorite and muscovite to biotite and andalusite; cordierite shortly joined these minerals. Near the stocks, muscovite and biotite were converted to K feldspar, cordierite, and iron ores, and sillimanite formed locally. Staurolitic rocks occur locally in the outer part of one of the cordieritic aureoles and form the major assemblage of another aureole, apparently having remained stable (or metastable) to the point where sillimanite formed, very near the stock. Andalusite-biotite rocks, typically mixed with the staurolitic rocks, formed also in the outer parts of the cordieritic aureoles, as well as by retrogressive reaction of cordierite to biotite deeper within these aureoles. Differences in original composition were considered as a possible reason for the four assemblages, and the cordierite-biotite rocks can be explained in this way; the others, however, cannot. The staurolitic assemblage apparently formed where magmatic fluids catalyzed the lower-temperature reactions, whereas the cordieritic rocks formed in drier aureoles where reactions could not go appreciably until higher temperatures were reached. Mineral assemblages of basaltic, calcareous, and impure dolomitic rocks show that the aureoles are almost entirely in amphibolite facies and perhaps partly in greenschist and pyroxene hornfels facies.

In the final stages of igneous crystallization, fluids from the stocks desilicated the hornfels locally and added one or both alkalies. Later, cooler fluids rehydrated most of the contact-metamorphic rocks and added considerable K2O to them.

This study emphasizes the influence of magmatic water on contact-metamorphic reactions; the relationships suggest that the rapid rate of heating by intrusive magma may be the most significant characteristic of contact, as compared to regional, metamorphism.

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