U-Pb zircon and geochemical evidence for bimodal mid-Paleozoic magmatism and syngenetic base-metal mineralization in the Yukon-Tanana terrane,Alaska

摘要

New SHRIMP (sensitive,high-resolution ion microprobe) U-Pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic Yukon-Tanana terrane in east-central Alaska help define the tectonic setting of mid-Paleozoic magmatism and syngenetic hydrother-nial Zn-Pb-Ag mineralization along the ancient Pacific margin of North America.We compare data from similar greenschist-facies sequences of bimodal volcanic and subvolca-nic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks,in the Wood River area of the Alaska Range and the Salcha River area of the Yukon-Tanana Upland,and from amphibolite-facies augen gneiss and mafic gneiss (amphibolite) in the Goodpaster River area of the upland.Allowing for analytical uncertainties,igneous crystallization age ranges of 376-353 Ma,378-346 Ma,and 374-358 Ma are indicated by 13 new SHRIMP U-Pb dates for the Wood River,Salcha River,and Goodpaster River areas,respectively.Bimodal magmatism is indicated by Late Devonian crystallization ages for both augen gneiss (371 ± 3 and 362 ± 4 Ma) and associated orthoamphibolite (369 ± 3 Ma) in the upland and by stratigraphic interleaving of mafic and felsic rocks in the Alaska Range.Metabasites in all three study areas have elevated HFSE (high field strength element) and REE (rare earth element) contents indicative of generation in a within-plate (extensional) tectonic setting.Within-plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the Bonnifield district in the Wood River area and for metarhyolite tuff interlayered with the carbonaceous Nasina assemblage,which hosts sedimentary exhalative sulfide occurrences in the Salcha River area.Most of the other felsic metaigneous samples from the Alaska Range and the Yukon-Tanana Upland have geochemical signatures that are similar to those of both average upper continental crust and continental-margin arc rocks generated in thick continental crust.Given the absence in our study areas of intermediate-composition magmatic products generally found in most arcs,and the presence of bimodal magmatism,the alkalic within-plate chemistry of the mafic rocks and some of the felsic rocks,and the widespread occurrence of interlayered carbonaceous sedimentary rocks indicative of deposition within a restricted marine basin or submerged continental margin,we consider it most likely that this prolonged Late Devonian to Early Mississippian magmatic episode resulted from attenuation of the ancient continental margin of western North America,rather than development of an arc,as proposed by many others.