Petrology and Rb-Sr age of Precambrian rhyolitic dikes, Llano County, Texas

Abstract

Rhyolite porphyry (llanite) and iron-rich melarhyolite dikes intrude Precambrian metamorphic rocks in north-central and south-eastern Llano County, respectively. These dikes are chemically and mineralogically similar, except that rhyolite porphyry is richer in quartz and alkali feldspar phenocrysts, whereas melarhyolite contains a greater abundance of biotite and magnetite. There are corresponding enrichments of SiO 2 in rhyolite porphyry and of FeO + Fe 2 O 3 in melarhyolite. The two rock types may have originated as two fractions of a common parent magma enriched in salic and mafic phases, respectively, to produce rhyolite porphyry and melarhyolite. This proposed relation is compatible with major-element chemistry and Rb-Sr systematics. Isotopic analyses of five melarhyolite whole rocks, two rhyolite porphyry whole rocks, groundmass and feldspar phenocrysts from rhyolite porphyry, and a nearby diabase dike yield an isochron age of 1,106 ± 6 m.y. (λ Rb = 1.39 × 10 −11 yr −1 and initial 87 Sr/ 86 Sr of 0.7028 ± 0.0002. Other data, including Zartman9s (1964) analyses of the microcline fraction of alkali feldspar phenocrysts from rhyolite porphyry, indicate subsequent Rb-Sr redistributions. These may be related to devitrification of melarhyolite groundmass and to unmixing of alkali feldspar phenocrysts in rhyolite porphyry.