SIGNIFICANCE OF A CRUSTAL-SCALE EXTENSIONAL STRUCTURE IN THE CORDILLERA OF WEST-CENTRAL YUKON

In the McQuesten area of west-central Yukon, Canada, strongly deformed mid-Paleozoic greenschist to amphibolite facies rocks of the Yukon-Tanana terrane are juxtaposed, along the Willow Lake fault, against relatively undeformed and unmetamorphosed Late Devonian to Early Mississippian plutonic rocks belonging to the polyphase Reid Lakes Complex and its overlying cover of coeval volcanic and volcaniclastic rocks. The fault is not exposed, but can be relatively well delineated by truncations in aeromagnetic data as well as by an outcrop of protomylonite. This abrupt truncation of lithologies with significantly differing regional tectonic strain and metamorphic grade suggests that the Willow Lake fault may be a crustal scale discontinuity and is probably a normal fault. Primary U-Pb zircon crystallization ages in conjunction with ⁴⁰Ar/³⁹Ar muscovite, biotite, and hornblende cooling ages sampled along a transect perpendicular to the fault constrain the tectonothermal history of the region and help bracket the timing of movement on the fault. Preliminary Devono-Mississippian U-Pb crystallization ages of zircon confirm age correlation of key foliated and undeformed geologic units on either side of the fault. Lithologic similarity and preliminary c. 345 Ma SHRIMP protolith ages from the protomylonite suggest it is a strained component of the c. 360-345 Ma Reid Lakes Complex, and constrain movement to be younger than 345 Ma. The strong regional foliation that is truncated on the south side of the fault is well developed in granitoid bodies that yielded Permian ages, suggesting the fault must be Permian or younger. Initial Devono-Mississippian ⁴⁰Ar/³⁹Ar cooling ages of biotite-hornblende pairs in the undeformed Reid Lakes Complex coupled with similar U-Pb zircon ages confirm this plutonic suite cooled relatively quickly between c. 360-340 Ma, and was locally affected by a Jurassic thermal event that is strongly evidenced by regional ⁴⁰Ar/³⁹Ar cooling ages in neighboring areas to the northwest and southeast.