PRESSURE-TEMPERATURE EVOLUTION OF METAPELITES WITHIN THE ANACONDA METAMORPHIC CORE COMPLEX,SOUTHWESTERN MONTANA
This study shows the complete metamorphic evolution of metapelitic footwall rocks in
the Anaconda metamorphic core complex, located in southwestern Montana. Pseudosections were constructed using the computer program THERMOCALC (v. 3.26) in conjunction with the internally consistent dataset of Holland and Powell (1998, version 5.5, updated Nov. 2003) in the model system NCKFMASH (Na2O-CaO-K2O-FeO-MgO-Al2O3-
SiO2-H2O). Microprobe data reveals that garnet is very almandine rich and contains very little manganese, thus not requiring MnO as a component in the model system. X-ray mapping of garnet within metapelites indicate only one growth phase.
Petrographic analysis in conjunction with pseudosections reveal peak metamorphism
occurred at 3.3-5.3 kbars and 590-665°C with an assemblage of biotite + garnet + sillimanite + quartz + plagioclase + muscovite. Chemical composition of garnet, biotite,
and plagioclase all verify peak metamorphic conditions on contoured pseudosections.
Evolution of footwall rocks occurred along a prograde, clockwise path that was largely
stable within sillimanite stability as evidenced by its abundance. These amphibolite facies
metapelites equilibrated at or near the solidus.
Advisor:Dr. Julie Baldwin
School:The University of Montana
School Location:USA - Montana
Source Type:Master's Thesis
Date of Publication:08/07/2008