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

Contact metamorphic facies in a large roof remnant within the composite Sierra Nevada batholith vary from actinolite hornfels to pyroxene-hornfels facies. Hornblende-hornfels facies rocks surround most plutons. The pyroxene-hornfels facies, defined by the appearance of hypersthene in mafic rocks, occurs only where two aureoles overlap. The rate of reaction of biotite plus quartz to hypersthene plus K feldspar plus water in the pyroxene-hornfels facies was controlled by the rate of escape of water. The reactants are still present in the rocks; hypersthene was not stable where water pressure was as high as load pressure but was able to form locally as water vapor escaped and water pressure fell below load pressure. The reaction of epidote to andradite took place in the high-grade part of the hornblende-hornfels facies and in the pyroxene-hornfels facies rocks. Hydrothermal experiments show that this reaction takes place in the range 575° to 600° C at 2,000 bars. The highest crystallization temperature within the hypersthene-bearing rocks is estimated to have been 750° C. The recrystallization temperature of the clinopyroxene-bearing mafic hornfelses in the hornblende-hornfels facies is estimated to have been 625° to 650°C, and that of the hornblende-bearing mafic hornfelses to have been 525° to 550°C. Interpretation of the fabric of the hypersthene rocks suggests that most of the heat was transferred to the walls by conduction and radiation. The main effect of hot magmatic vapor was to produce a retrograde zone at the end of the recrystallization period. The duration of the high-temperature phase of the metamorphism is estimated to be of the order of 105 years.