KEYNOTE SPEAKER: DECIPHERING MULTIPLE METAMORPHIC EVENTS BY LASER ABLATION SPLIT-STREAM (LASS) PETROCHRONOLOGY OF MONAZITE AND XENOTIME IN THE CLEARWATER COMPLEX, NORTHERN IDAHO (Invited Presentation)

The Clearwater metamorphic core complex in northern Idaho records a polymetamorphic history, with both Proterozoic and Cretaceous-Eocene tectonism. Each of these events occurred at similar amphibolite facies conditions, thus interpreting P-T histories is hampered by uncertainty in the age of porphyroblast growth relative to each metamorphic event. This study presents in situ LASS U-Th/Pb geochronology and REE geochemistry of monazite and xenotime from pelitic rocks across the complex. Samples investigated include Grt-bearing Al-Mg schists in the internal zone footwall of the core complex, which record Eocene mica cooling ages, as well as Grt + Ky/Sil ± St schists in the external zone, which record Cretaceous mica cooling ages.

Matrix monazite in three Al-Mg schists from the internal zone of the complex yielded ²⁰⁶Pb/²³⁸U weighted mean dates of 45.9±0.5 Ma (n = 19), 47.7±0.2 Ma (n = 30), and 47.9±0.3 Ma (n = 37). Xenotime in one sample yielded a ²⁰⁶Pb/²³⁸U weighted mean of 48.9±0.4 Ma (n = 15). Monazite was not observed as inclusions in garnet in these samples. In contrast, a sample from the Eocene Jug Rock shear zone, the E-bounding detachment fault of the internal zone yielded a spread in ²⁰⁶Pb/²³⁸U dates of 67-47 Ma. Two monazite inclusions in garnet in this sample yielded ²⁰⁷Pb/²⁰⁶Pb dates of 1.43 Ga and 1.33 Ga, which is consistent with published 1.46 Ga Lu-Hf garnet dates from close proximity.

Monazite in samples from the external zone did not yield any Eocene dates and instead records a protracted history of Cretaceous crustal thickening as well as multiple Proterozoic events. Monazite and xenotime included in staurolite near the Collins Creek fault outside the eastern boundary of the complex record ²⁰⁷Pb/²⁰⁶Pb dates ranging from 1413-1188 Ma with a cluster around 1.34 Ga. One monazite inclusion in garnet yielded a date of 1.53 Ga. In two other samples near the Collins Creek fault, monazite and xenotime systematics are complicated and record mixing of Proterozoic (c. 1.5, 1.3, and 1.1 Ga) and Cretaceous (81-67 Ma) domains within single grains. Taken together, these results are consistent with previous interpretations of early garnet and staurolite growth in the Mesoproterozoic, protracted Cretaceous crustal thickening in the external zone, and strong 46-49 Ma overprinting in the internal zone during core complex unroofing.