Abstract

Two kinds of amphiboles—actinolite and aluminous Ca-amphibole—and two to three epidoteminerals coexist with chlorite and plagioclase in the meta-volcanic and meta-igneous rocks inthe contact aureoles of the Cretaceous plutons in the Feather River area, northern SierraNevada. Small prisms and rims of actinolite occur with green hornblende outside the l-mile-wide contact aureoles of the plutons. In the zone next to the plutons, blue-green hornblendeforms rims around large grains of actinolite and also forms small prisms that are included inplagioclase. Electron-microprobe analyses show that the alkali and Al content of the amphi-boles increases with an increase in the grade of metamorphism, the ratio of Na+K to tetra-hedral Al remaining close to 1:3. The ratio between two coupled substitutions, edenitic(NaAAliv for □ASiiv) and tschermakitic (Alvi Aliv for Mgvisiiv remains 3:7 and leads to thecomposition Na0.5Ca2Mg2.8Fe2+1.0(Al,Fe3+)1.2Si0.3Al1.7)O22(OH)2 at the highest grade of meta-morphism in the Feather River area and further to Na0.6Ca2R2+3.6R3+1.4Si6Al2O22(OH)2, the ex-treme member of this actinolite-hornblende series. The Mg/Fe ratio changes with the bulk com-position and with the degree of oxidation of the host rocks. Epidote and clinozoisite coexist inall zones as separate grains. In addition, many grains of clinozoisite include high index domains, lamellae, and stringers of epidote. In the highest grade zone next to the plutons, zoisite occurswith epidote, clinozoisite, and anorthitic plagioclase. Farther from the plutons, the plagioclase isalbite. There is no regular change in the distribution coefficient KD(Mg/Fe) for chlorite/amphi-bole although it is highest in the zone next to the plutons.

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