Abstract

Potassic feldspar-bearing augen granitoids are a fundamental component of the architecture of the Yukon-Tanana terrane and the ancient Pacific margin of the northern Cordillera. These augen granitoids form a belt that extends from Alaska to southeast Yukon Territory, vary in age, and provide probes of the crustal evolution and tectonic history of the Yukon-Tanana terrane and ancient Pacific margin of North America in the Paleozoic. We present results of an integrated field mapping, geochemical, Sm-Nd tracer isotopic, and U-Pb zircon geochronologic study of the augen granitoids in the Stewart River area in an attempt to understand their role in the crustal evolution and tectonic history of the Yukon-Tanana terrane and ancient Pacific margin of North America.

Augen granitoids of the Stewart River area are of three distinct ages: Late Devonian, early Mississippian, and Permian. U-Pb zircon geochronology of these augen granitoids has yielded ages of 362.1 ± 2.7 Ma (Stewart River augen granite), 347.5 ± 0.7 Ma (Mount Burnham augen granite), and 264.8 ± 3.7 Ma (Wounded Moose augen granite). All of the augen granitoids, regardless of age, have negative ϵNdt values (−2.0 to −15.3) and Proterozoic-Archean depleted-mantle model ages (TDM = 1.37–2.56 Ga). These geochemical and isotopic attributes, coupled with the presence of inherited zircon with Precambrian ages, suggestthatthesegranitoidsaretheproductof crustal melting and crust-mantle mixing during three different cycles of arc magmatism in the Paleozoic. Furthermore, these granitoids represent net crustal recycling along the ancient Pacific margin of North America in the Paleozoic. Importantly, however, there are minor secular variations in crustal recycling, and the younger Permian augen granitoids exhibit higher ϵNdt, Nb/Ta, V/Yb, and Sc/Yb, consistent with a greater juvenile component in their genesis. This juvenile component is probably due to assimilation of underplated mafic material derived from older early Mississippian Yukon-Tanana terrane arc magmatism and/or a greater mantle component due to enhanced infiltration of underplated mafic material into augen granitoid magma chambers through rheologically weak crust associated with Permian subduction.

The older Late Devonian and early Mississippian augen granitoid suites represent two pulses of Yukon-Tanana terrane arc magmatic activity that developed in response to east-dipping subduction along the western edge of the North America craton in the mid-Paleozoic. This east-dipping Yukon-Tanana terrane arc system continued to evolve throughout the Mississippian to Early Permian and was coincident with the development of the Slide Mountain backarc basin that formed between the Yukon-Tanana terrane arc system and the North American craton; this east-dipping arc-backarc system continued until ca. 275 Ma. After ca. 275 Ma, the east-dipping arc and backarc magmatism ceased and was replaced by ca. 270–269 Ma high-pressure metamorphism and the establishment of a new subduction zone that formed in response to the closure of the Slide Mountain backarc basin. The Permian augen granitoids from the Stewart River are the magmatic record of this new west-dipping subduction zone.

Although there are subtle variations, the petrogenetic and tectonic histories of the three suites of augen granitoids in the Stewart River area are remarkably similar and attest to the constancy of magmatic and tectonic processes that occurred along the ancient Pacific margin of North America in the Paleozoic.

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