Title

Modeling Metamorphic History of High Pressure Rocks from Western China

Presenter Information

Jesse Walters

Document Type

Oral Presentation

Location

SURC 140

Start Date

16-5-2013

End Date

16-5-2013

Abstract

During continental collision, rocks at the surface can subduct to great depth, metamorphose at extreme pressures, and return to the surface. This study uses computer modeling to construct the pressure-temperature (P-T) history of a gneiss from an extinct collision zone to test the extent of ultra high-pressure metamorphism (greater than or equal to 25 kbar at 750°C). Field evidence of crystallized melt and an alkali-feldspar reaction found in the sample suggest a history of melt loss during the early stages of metamorphism. A P-T diagram constructed from the pre melt-loss sample composition, predicts ~20-35 vol percent melt coexisted with the observed assemblage, ~80-85 percent of which was lost during peak metamorphism. Before melt-loss, the gneiss passed through the alkali-feldspar reaction (~15.5 kbar, 750°C) to the formation of the observed mineral group (~12 kbar, 850°C). Diagrams constructed for the post melt-loss composition predict the observed minerals to form at ~11-15 kbar and ~810-870°C, which is consistent with previous estimates for rocks hosted by the gneiss. Predicted mineral abundances match observed values, with 8-18 vol percent garnet and 0-10 vol percent biotite in the diagram, and 10-15 vol percent garnet and 10 vol percent biotite in the sample. Mineral compositions converge at ~12-12.5 kbar and ~660-700°C, which agree with previous P-T estimates for the gneiss and suggests diffusion altered mineral compositions. Modeling of the gneiss suggests a shared history of metamorphism with the hosted rocks, and that it did not reach the ultra high-pressure metamorphic conditions experienced by adjacent rocks to the east.

Faculty Mentor(s)

Chris Mattinson

Additional Mentoring Department

Geological Sciences

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May 16th, 3:00 PM May 16th, 3:20 PM

Modeling Metamorphic History of High Pressure Rocks from Western China

SURC 140

During continental collision, rocks at the surface can subduct to great depth, metamorphose at extreme pressures, and return to the surface. This study uses computer modeling to construct the pressure-temperature (P-T) history of a gneiss from an extinct collision zone to test the extent of ultra high-pressure metamorphism (greater than or equal to 25 kbar at 750°C). Field evidence of crystallized melt and an alkali-feldspar reaction found in the sample suggest a history of melt loss during the early stages of metamorphism. A P-T diagram constructed from the pre melt-loss sample composition, predicts ~20-35 vol percent melt coexisted with the observed assemblage, ~80-85 percent of which was lost during peak metamorphism. Before melt-loss, the gneiss passed through the alkali-feldspar reaction (~15.5 kbar, 750°C) to the formation of the observed mineral group (~12 kbar, 850°C). Diagrams constructed for the post melt-loss composition predict the observed minerals to form at ~11-15 kbar and ~810-870°C, which is consistent with previous estimates for rocks hosted by the gneiss. Predicted mineral abundances match observed values, with 8-18 vol percent garnet and 0-10 vol percent biotite in the diagram, and 10-15 vol percent garnet and 10 vol percent biotite in the sample. Mineral compositions converge at ~12-12.5 kbar and ~660-700°C, which agree with previous P-T estimates for the gneiss and suggests diffusion altered mineral compositions. Modeling of the gneiss suggests a shared history of metamorphism with the hosted rocks, and that it did not reach the ultra high-pressure metamorphic conditions experienced by adjacent rocks to the east.