GEOCHRONOLOGIC CONSTRAINTS ON THE TECTONIC EVOLUTION OF THE BOEHLS BUTTE-CLEARWATER CORE COMPLEX: EVIDENCE FROM 1.01 GA GARNETS

Preliminary Lu-Hf garnet-whole rock geochronologic dating has provided new evidence for 1.0 Ga metamorphism in northern Idaho. Garnets from amphibolite-grade aluminous schist, located along the eastern boundary of the Boehls Butte-Clearwater Core Complex (BBCCC), have yielded ages of 1.01 Ga and carry the implication that a much older metamorphic event is present and preserved in the Northwestern U.S. Cordillera. The BBCCC is a small terrain of intensely deformed poly-metamorphic schist, anorthosite, and amphibolite that lies along the northern border of the Idaho Batholith and is widely interpreted as predating the 1.45 Ga Belt Supergroup. The basement rocks of the BBCCC are exposed in a tectonic window and are separated from the surrounding metamorphic rocks of the Mesoproterozoic Belt Supergroup by ductile shear zones, which presumably were active during an episode of Eocene extension that has been documented in other core complexes in the northern Rockies. Recent Lu-Hf garnet-whole rock geochronology, initially aimed at constraining the timing of shearing of the BBCCC, has instead revealed evidence for a much more complex and long-lived structural history. These ages indicate that an older, major contractional event occurred and is preserved in the Northwest U.S. Cordillera. The metamorphic history of rocks from within the BBCCC has generally been interpreted as consisting of three metamorphic events. M2 and M3 events record peak pressure and temperature conditions during the Cretaceous (M2), and nearly isothermal decompression (M3) during Eocene extension. M1 is only sporadically preserved and the age and tectonic significance of this metamorphism has been limited to speculation. The Lu-Hf garnet ages may be evidence for the timing of the poorly constrained first metamorphic event (M1).