Abstract

Metasedimentary rocks of the Tioga Pass roof remnant in the east-central Sierra Nevada, show systematic mineralogical changes with distance from the plutonic contacts. Rocks indicative of the albite epidote hornfels facies occur in the interior of the remnant, whereas those of the hornblende hornfels facies occur near plutonic contacts. Assuming Pfluid = Ptotal = 1 to 2 kb, calculated and experimentally determined phase equilibria suggest that calcareous rocks adjacent to the eastern and western contacts were metamorphosed in the presence of a water-rich vapor phase and at temperatures between 550° and 670°C. Basic metavolcanic rocks show no systematic mineralogical changes with distance from the plutonic contacts.

Contact metamorphism in the Standard area in the west-central foothills of the Sierra Nevada produced a narrow zone of sillimanite schists adjacent to the plutonic contact. The sporadic occurrence of K-feldspar in the sillimanite schists is interpreted to be the result of variations in the PH2O/Ptotal ratio of the rocks during metamorphism. Garnet occurs in some samples of sillimanite schist and is probably stabilized because of the combined effects of relatively high bulk-rock oxidation and high Mn component. Marble inclusions have been metasomatically altered by water-rich fluids supplied by the surrounding schist.

Studies of contact metamorphism in these and in other parts of the Sierra Nevada reveal the following: (1) Calcareous assemblages indicative of water-rich vapor phases are common. Large differences existed in the PCO2/PH2O ratio of nearby calcareous rocks in two areas, supporting the hypothesis that, for the most part, the fluid phase composition of calc-silicate units in Sierran aureoles was controlled by the original rock composition. Assuming Pfluid = Ptotal = 1 to 2 kb, calc-silicate assemblages, including grossularite and clinozoisite, suggest temperatures of 550° to 650°C in parts of many aureoles. Several areas show regular mineralogical changes in calc-silicate assemblages with distance from the plutonic contacts. (2) In several localities, pelitic rocks contain the assemblage sillimanite (or andalusite) + K-feldspar + quartz (± muscovite) close to plutonic contacts, whereas the assemblage Al2SiO5 + muscovite + quartz (no K-feldspar) occurs at greater distances from the contact. The coexistence of andalusite and sillimanite (fibrolite or prismatic, or both) has been described from several localities; this observation suggests a limited usefulness of these poly-morphs as PT indicators of Sierran contact aureoles. (3) Basic metavolcanic rocks in many areas have disequilibrium assemblages characterized by relict volcanic plagioclase. The failure of these rocks to recrystallize may indicate that they were closed to externally derived water during metamorphism. In several areas, the outer limit of contact metamorphism is defined by the transition from actinolite to hornblende toward the plutons. Epidote is common in hornblende-bearing basic assemblages, suggesting it may be stable in basic rocks of the hornblende hornfels facies. (4) Magnesian assemblages in contact metamorphosed serpentinites may have developed under the condition PH2O < Ptotal.

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