Geochemistry, petrography, and zircon U–Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon–Tanana terrane

Abstract

We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Alaska: Zircon U-Pb geochronology, whole-rock geochemistry, and Pb isotopes

“…2). Movement along the northeast-trending faults has been attributed to the clockwise rotation of blocks resulting from the right-lateral movement along the Tintina and Denali faults (e.g., Page et al, 1995;O'Neill et al, 2010 Bacon et al, , 2013. The Fortymile River assemblage comprises amphibolite facies metasedimentary and metavolcanic rocks, marble, and granodioritic to tonalitic orthogneiss of Early Mississippian age.…”

“…Similarities in protoliths, arc geochemical signatures of metaigneous rocks, and conodont age ranges of rocks of the Chicken metamorphic complex and the Fortymile River assemblage suggest a shared origin for both units , the difference likely being one of metamorphic grade. Mylonitic tonalitic gneiss interlayered with mafic metavolcanic rocks of the Chicken metamorphic complex yielded a sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon protolith crystallization age of 332.6 ± 5.6 Ma (Dusel- Bacon et al, 2013).…”

“…Consists of argentiferous galena, sphalerite, chalcopyrite, and supergene minerals in vug fillings, boxworks, and quartz veins within marble of the Fortymile River assemblage (Werdon et al, 2004a; of amphibolite in the Fortymile River assemblage that is bounded by northeast-trending faults west of the Mount Veta ridge (Dusel- Bacon et al, 2013;Day et al, 2014). The metaharzburgite lens is too small to show at the scale of map, but is located due east of the Drumstick mineral prospect shown in Figure 3.…”

“…Location information for whole-rock samples is given in Supplemental File 5 5 . All samples were analyzed by wavelength-dispersive X-ray fluorescence and inductively coupled plasma-mass spectrometry at the GeoAnalytical Laboratory, Washington State University (see Dusel-Bacon et al, 2013, for a detailed description of analytical procedures).…”

“…Exhumation occurred during southeast-vergent (orogen parallel) extensional ductile deformation and metamorphism documented throughout much of east-central Alaska (Hansen and Dusel-Bacon, 1998). An apparent top-to-the-south-southeast fabric was observed in tonalitic mylonite gneiss in the Chicken metamorphic complex east of the Kechumstuk fault (Dusel- Bacon et al, 2013) and in a few other locations in the nearby Molly Creek area just west of the Mount Veta ridge (Hansen and Dusel-Bacon, 1998). Top-to-the-southeast-vergent deformational fabrics overprint the earlier top-to-the-northwest fabrics in our study area, and like elsewhere in the region, the younger fabrics are interpreted to be generally structurally shallower than the older fabrics and to have formed during regional extension (Hansen and Dusel-Bacon, 1998).…”