Mesozoic thermal history and timing of structural events for the Yukon-Tanana Upland, east-central Alaska: ⁴⁰Ar/³⁹Ar data from metamorphic and plutonic rocks

Abstract

We present new 40Ar/39Ar ages for hornblende, muscovite, and biotite from metamorphic and plutonic rocks from the Yukon–Tanana Upland, Alaska. Integration of our data with published 40Ar/39Ar, kinematic, and metamorphic pressure (P) and temperature (T) data confirms and refines the complex interaction of metamorphism and tectonism proposed for the region. The oldest metamorphic episode(s) postdates Middle Permian magmatism and predates the intrusion of Late Triassic (215–212 Ma) granitoids into the Fortymile River assemblage (Taylor Mountain assemblage of previous papers). In the eastern Eagle quadrangle, rapid and widespread Early Jurassic cooling is indicated by ~188–186 Ma 40Ar/39Ar plateau ages for hornblende from plutons that intrude the Fortymile River assemblage, and for metamorphic minerals from the Fortymile River assemblage and the structurally underlying Nasina assemblage. We interpret these Early Jurassic ages to represent cooling resulting from northwest-directed contraction that emplaced the Fortymile River assemblage onto the Nasina assemblage to the north as well as the Lake George assemblage to the south. This cooling was the final stage of a continuum of subduction-related contraction that produced crustal thickening, intermediate- to high-P metamorphism within both the Fortymile River assemblage and the structurally underlying Lake George assemblage, and Late Triassic and Early Jurassic plutonism in the Fortymile River and Nasina assemblages. Although a few metamorphic samples from the Lake George assemblage yield Jurassic 40Ar/39Ar cooling ages, most yield Early Cretaceous 40Ar/39Ar ages: hornblende ~135–115 Ma, and muscovite and biotite ~110–108 Ma. We interpret the Early Cretaceous metamorphic cooling, in most areas, to have resulted from regional extension and exhumation of the lower plate, previously tectonically thickened during Early Jurassic and older convergence.

Re-Os systematics and age of pyrite associated with stratiform Zn-Pb mineralization in the Howards Pass district, Yukon and Northwest Territories, Canada

“…An alternative explanation is that the laminated sulphides formed concurrently with abundant, secondary, coarse-grained sulphide minerals that occur along cleavage planes and in thin fractures throughout the deposits. The northeast dip of the cleavage planes and their alignment with northwest-trending regional folds (Gordey and Anderson 1993;Martel 2015) suggest that the coarse-grained sulphides formed during tectonism, the onset of which regionally occurred in the Jurassic to Cretaceous (Dusel-Bacon et al 2002;Mair et al 2006). Furthermore, paleomagnetic dating of coarse-grained sulphide veins in the Howards Pass district indicates that they formed during the Middle Jurassic, leading to the further suggestion that the fine-grained sulphides may also have formed at this time (Kawasaki and Symons 2012).…”

Deformation history of the northwestern Selwyn Basin, Yukon, Canada: Implications for orogen evolution and mid-Cretaceous magmatism

“…Upper amphibolite facies gneisses are interpreted to represent the lower plate, or underthrust, rocks of the continental margin. The 40 Ar/ 39 Ar data of Dusel-Bacon et al (2002) suggest that the extension event in the Yukon-Tanana terrane uplands took place between 125 and 105 Ma, with most of the 40 Ar/ 39 Ar cooling ages from the lower-plate gneisses concentrated in the more restricted range of 115-105 Ma. In addition, Day et al (2002) investigated zircon and monazite from lower-plate gneisses in the Yukon-Tanana terrane uplands with a sensitive high-resolution ion microprobe (SHRIMP).…”

“…The Yukon-Tanana terrane is a geographically extensive entity, with notable differences in the tectonic history between the Yukon-Tanana terrane of southern Yukon and the Yukon-Tanana terrane uplands of east-central Alaska (Mihalynuk et al, 1999;Dusel-Bacon et al, 2002). Although Mesozoic collisional tectonism has affected the entire terrane, much of the Yukon-Tanana terrane in southern Yukon preserves a Devonian to Triassic history (Erdmer et al, 1998), whereas the Paleozoic history is strongly overprinted by Jurassic-Cretaceous collisional tectonism in the Yukon-Tanana terrane uplands.…”

“…The Jurassic to Cretaceous tectonic development of the orogen is summarized in schematic cross sections in Figure 12. Many workers cite strong evidence, based on structural relationships and 40 Ar/ 39 Ar cooling ages of high-grade metamorphic rocks, for major Early to mid-Cretaceous extension in the Yukon-Tanana terrane uplands of east-central Alaska (Hansen, 1990;Miller and Hudson, 1991;Pavlis et al, 1993;Hansen and Dusel-Bacon, 1998;Dusel-Bacon et al, 2002). The juxtaposition of upper amphibolite rocks against lower greenschist facies rocks by lowangle extensional detachment faults in eastcentral Alaska suggests the elimination of as much as 10 km of the crustal section (Pavlis et al, 1993).…”

“…Gabrielse and Yorath, 1991). Numerous studies have addressed the nature and timing of collision-related deformation in the Yukon-Tanana terrane (Tempelman-Kluit, 1979;Hansen, 1990;Mortensen, 1992;Mihalynuk et al, 1994;Erdmer et al, 1998;Dusel-Bacon et al, 2002). Equivalent studies in adjacent Selwyn Basin are largely restricted to map reports and memoirs (Gordey and Anderson, 1993;Murphy, 1997).…”