Document Type
Thesis
Date of Degree Completion
Spring 2013
Degree Name
Bachelor of Science
Department
Geological Sciences
Committee Chair
Dr. Chris Mattinson, Geology
Second Committee Member
Dr. Walter Szeliga, Geology
Third Committee Member
Dr. Audrey D. Huerta, Director Science Honors Research Program
Abstract
In order to better understand metamorphism during continent-continent collision, the pressure-temperature history of a garnet-k:yanite paragniess from the Dulan region of the N. Qaidam high-pressure (HP)/ ultrahigh-pressure (UHP) belt is modeled. The gneiss contains the assemblage Bt + Pl + Kfs + Grt + Ky + Qtz + Rt + Ap. An equilibrium assemblage diagram constructed for the sample's pelitic bulk composition predicts the observed assemblage to form at ~ 11-15 kbar and ~810-870 °C. The diagram predicts predicts ~2-8 vol% melt in the observed assemblage. Predicted mineral abundances closely match observed values, with 8-18 vol % garnet and 0-10 vol % biotite in the pseudosection, and 10-15 vol % garnet and 10 vol % biotite in the sample.
Compositional contours for garnet rims and plagioclase intersect at lower temperature than the above values predicted for the observed assemblage field. Plagioclase contours intersect at 12.5 kbar, ~ 700 °C, and garnet rim contours converge at ~ 12-12.3 kbar and ~660-670 °C. This is similar to previous pressure-temperature (P-T) estimates of 13-15 kbar and 730-775 °C for the gneiss using the Grt-Bt thermometer and GASP barometer. The average garnet size is 0.2 mm, and exhibits limited zoning with average core values of AlmssPrp17Grs23 and rim values of Alm62Prp19Grs18. The small size, limited zoning, and P-T estimates for the garnets suggests re-equilibration~ 190 °C down temperature, representing a segment of the P-T path from the observed assemblage to the isopleth intersections.
To model melt loss potential, a melt-reintegrated bulk composition is estimated by calculating the composition of the initial (low-temperature) melt, and adding it to the sample's bulk composition until the melt-in reaction is bounded by H2O saturated fields. The melt-reintegrated diagram predicts a maximum of ~20-35 vol % melt in coexistence with the observed assemblage, ~ 80-85 % of which is lost during peak metamorphism. Melt loss is supported by field evidence for melt escape and the lack of retrograde muscovite. The reintegrated diagram displays an alkali feldspar-in reaction, which is consistent with reaction textures in thin section that display alkali feldspar growing around plagioclase.
A P-T path is developed using equilibrium assemblage diagram analysis in conjunction with electron microprobe and textural data, combined with P-T data from previous studies. The prograde P-T path passes through the alkali feldspar-in reaction (~15.5 kbar, 750 °C) to peak P-T conditions of 14.5-17 kbar and 800-870 °C recorded by mafic granulite lenses hosted in the Grt-Ky gneiss. After peak metamorphism, the gneiss continued down pressure to the observed assemblage(~ 13.5 kbar, ~ 850 °C), during which melt was lost. Retrograde metamorphism then continued from the observed assemblage to where garnet compositions re-equilibrate at ~12-12.3 kbar and ~660-670 °C. A final retrograde P-T point is represented by mineral reactions at 8 kbar and 550 °C observed in the mafic granulites.
Recommended Citation
Walters, Jesse, "Modeling the Metamorphic History of Ultrahigh Pressure Rocks: Western China" (2013). Undergraduate Honors Theses. 115.
https://digitalcommons.cwu.edu/undergrad_hontheses/115
Comments
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