LATE PERMIAN (CA. 267–257 MA) MAGMATISM, DEFORMATION, AND METAMORPHISM AND LITHOTECTONIC ASSOCIATIONS OF THE LADUE RIVER UNIT IN EAST-CENTRAL ALASKA

The Ladue River unit (LRu), previously mapped across >750 km² in the Tanacross quadrangle of eastern Alaska, was originally correlated with the Permian Klondike assemblage of western Yukon, Canada. The Klondike assemblage, comprising metavolcanic rocks and co-magmatic plutons (now schist and orthogneiss, respectively), is a well-characterized regional lithotectonic unit within the allochthonous Yukon-Tanana terrane (YTT). Recent mapping in eastern Alaska indicates that discontinuous LRu exposures include quartz-mica schist, locally chromium-bearing chlorite schist, biotite schist, and amphibolite. U-Pb zircon dates of ca. 267 and 261 Ma from quartz-mica schist and quartz-eye schist are interpreted as crystallization ages of volcanic protoliths, supporting correlation of the easternmost LRu with the Klondike assemblage of western Yukon. Other LRu exposures yield ca. 366–352 Ma zircon ages and are interpreted to be part of older allochthonous assemblages, but not the Klondike assemblage. Most LRu exposures have Late Triassic to Jurassic biotite ⁴⁰Ar/³⁹Ar ages, and we interpret these rocks to represent a thin (up to a few hundred meters), gently dipping structural panel of allochthonous YTT assemblages. In the northern Tanacross quadrangle, a concordant, meter-thick layer of felsic gneiss interlayered with quartzite, mica schist, and amphibolite of the Fortymile River assemblage also yielded a ca. 262 Ma zircon crystallization age, suggesting that it is a meta-intrusive body that was deformed together with surrounding rocks at amphibolite facies after it was emplaced. Approximately 45 km to the north, an undeformed ca. 257 Ma granitic pluton 3.5 km northwest of Mt. Warbelow cuts across the main foliation in rocks of the Fortymile River assemblage. These observations and age constraints potentially bracket at least one episode of deformation and metamorphism between ca. 261 and 257 Ma, consistent with published constraints on the Late Permian Klondike orogeny in western Yukon. Subsequent deformation and metamorphism that occurred during Jurassic shortening and mid-Cretaceous extension overprinted and has almost completely obscured evidence of Permian tectonism in eastern Alaska.