Contact Metamorphic Reactions and Processes in the Mt. Tallac Roof Remnant, Sierra Nevada, California

Contact metamorphic facies in a large roof remnant within thecomposite Sierra Nevada batholith vary from actinolite hornfelsto pyroxene-hornfels facies. Hornblende-hornfels facies rockssurround most plutons. The pyroxene-hornfels facies, definedby the appearance of hypersthene in mafic rocks, occurs onlywhere two aureoles overlap. The rate of reaction of biotite plus quartz to hypersthene plusK feldspar plus water in the pyroxene-hornfels facies was controlledby the rate of escape of water. The reactants are still presentin the rocks; hypersthene was not stable where water pressurewas as high as load pressure but was able to form locally aswater vapor escaped and water pressure fell below load pressure. The reaction of epidote to andradite took place in the high-gradepart of the hornblende-hornfels facies and in the pyroxene-hornfelsfacies rocks. Hydrothermal experiments show that this reactiontakes place in the range 575 to 600 C at 2,000 bars. The highestcrystallization temperature within the hypersthene-bearing rocksis estimated to have been 750 C. The recrystallization temperatureof the clinopyroxene-bearing mafic hornfelses in the hornblende-hornfelsfacies is estimated to have been 625 to 650C, and that ofthe hornblende-bearing mafic hornfelses to have been 525 to550C. Interpretation of the fabric of the hypersthene rocks suggeststhat most of the heat was transferred to the walls by conductionand radiation. The main effect of hot magmatic vapor was toproduce a retrograde zone at the end of the recrystallizationperiod. The duration of the high-temperature phase of the metamorphismis estimated to be of the order of 10⁵ years.