Document Type
Thesis
Date of Degree Completion
Spring 2020
Degree Name
Master of Science (MS)
Department
Geological Sciences
Committee Chair
Chris Mattinson
Second Committee Member
Wendy Bohrson
Third Committee Member
Jeff Lee
Abstract
The Dulan ultrahigh-pressure (UHP) terrane in western China hosts rocks that record continental collision, however; the timing of peak P-T conditions in UHP eclogites and HP granulites, and the tectonic significance of their spatial relationship within the Dulan UHP terrane are not well understood. This study, (1) evaluates rare earth element (REE) equilibrium between garnet and zircon, to assess coeval growth; (2) links timing (from zircon) to the P-T history (from garnet) to constrain the P-T-t path of the Dulan UHP terrane and interpret implications for zircon growth; (3) refines P-T conditions for HP granulite; and (4) evaluates the tectonic implications of these new results. Garnet-zircon REE partitioning reveals that garnet and zircon formed in equilibrium at or near peak P-T in four Dulan UHP eclogite samples, but not in two HP granulite samples. UHP eclogites experienced peak P-T of up to 35 kbar and 625-675˚C, corresponding to subduction to depths >100 km. Peak P increased over time, corresponding to a rate of ~3 mm/a from 436-423 Ma, suggesting that the buoyancy of iv the terrane may have slowed subduction prior to exhumation. Zircon growth occurred on parts of the P-T path where hydrous mineral breakdown is predicted to release fluid. In contrast, in HP granulite, zircon and garnet did not form in equilibrium. REE and inclusion data indicate that zircon grew from melt during early exhumation at 418 Ma, after the growth of garnet but before significant plagioclase growth. A new thermodynamic model for Dulan HP granulite constrains peak conditions of 21-23 kbar and 830-900˚C, suggesting burial depths >80 km. These data suggest that the HP granulite originated from the subducting slab, not the upper plate as previously proposed, as it is unlikely the upper plate was >80 km thick. Combining results from both the HP granulite and UHP eclogite suggests that these two rock types record processes near the top of the subducting slab where isotherms are predicted to be strongly compressed during exhumation, juxtaposing higher-T and lower-T rocks.
Recommended Citation
Weisberg, Wesley, "Applying Garnet and Zircon REE Linkages to Determine Timing of P-T Conditions and Tectonic Implications for UHP Eclogites and HP Granulites, Dulan UHP Terrane, China" (2020). All Master's Theses. 1376.
https://digitalcommons.cwu.edu/etd/1376