T249 Getting to the Root of It—Metamorphism, Tectonics, and Crustal Evolution (Posters)

Wednesday, 7 November 2012
9:00 AM - 6:00 PM
Hall B

PHASE EQUILIBRIA AND LASER ABLATION SPLIT-STREAM (LASS) PETROCHRONOLOGY OF METAPELITES IN THE PRIEST RIVER METAMORPHIC CORE COMPLEX, NORTHERN IDAHO

Paper number: T249-6
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Phase equilibria modeling and in situ LASS petrochronology of monazite and xenotime reveal the timing of metamorphism, anatexis, and exhumation in the Priest River metamorphic core complex (PRC), which extends from northern Idaho to southeastern BC. The Purcell Trench and Newport faults bounding the PRC footwall exhume the metapelitic Hauser Lake Gneiss (HLG), the metamorphic equivalent of the Mesoproterozoic Prichard Formation of the Belt Supergroup. This study provides new P-T-t constraints on the evolution of the HLG by integrating petrography, phase equilibria, and in situ geochronology and REE geochemistry of monazite and xenotime. Pseudosections based on the assemblage Grt + Ky/Sil + Bt + Pl + Or + Ilm + Qtz constrain peak P-T conditions of c. 8.3-9.4 kbar and 760-785°C. U/Th-Pb monazite and xenotime geochronologic and trace element geochemical data were collected simultaneously in thin section using the LASS method. 208Pb/232Th monazite and xenotime dates of 94-44 Ma for HLG samples across the PRC are consistent with results from previous studies; however, in situ analyses in this study provide a petrographic context for interpreting the significance of these dates.

The oldest monazite dates occur in the north as matrix grains in a sample near the Cretaceous Kaniksu batholith, and are synchronous with published zircon crystallization ages of ~94 Ma. Monazite and xenotime inclusions in porphyroblasts (Ky, Sil, Grt) in samples farther to the south range from 75 to 55 Ma. The earliest and latest porphyroblast growth is recorded in Ky-bearing and Sil-bearing assemblages, respectively. Monazite in leucosome of Ky-bearing assemblages indicates that partial melting occurred at 54-50 Ma, consistent with ages of Eocene plutons that intrude the HLG. Leucosome monazite in samples with greater melt retention have REE patterns consistent with growth during garnet breakdown. The youngest xenotime and monazite dates (48-44 Ma) occur in migmatites near the East Newport Fault. Together these data place constraints on the onset of partial melting and the extension and development of the PRC, and indicate that rocks were still at mid-crustal depths until 44 Ma in the vicinity of the East Newport Fault in the western footwall, suggesting a diachroneity in the timing of exhumation across the Purcell Trench and Newport faults.