Publication Type:

Book Chapter


Paleozoic evolution and metallogeny of pericratonic terranes at the ancient Pacific margin of North America, Canadian and Alaskan Cordillera, Geological Association of Canada, Toronto, ON, Canada, Volume 45, p.281-322 (2006)




Alaska, Alaska Range, alkalic composition, back-arc basins, basalts, basins, Canada, chemical composition, chemical ratios, continental crust, continental drift, continental margin, crust, Devonian, geochemistry, igneous rocks, island arcs, mafic composition, magmatism, mantle, mantle plumes, mid-ocean ridge basalts, neoproterozoic, North America, orogenic belts, Pacific Plate, paleogeography, Paleozoic, peralkalic composition, petrography, plate tectonics, Precambrian, proterozoic, recycling, rifting, Rodinia, subduction, tectonics, terranes, tholeiitic composition, Trace elements, United States, upper Precambrian, volcanic rocks, VOLCANISM, Western Canada, Yukon Territory, Yukon-Tanana Terrane


Devonian to Permian igneous rocks in the Yukon-Tanana terrane (YTT) record six cycles of arc, arc-rift, continental rift and back-arc basin magmatism, each set apart from the others by changes in the locus and/or character of igneous activity, as well as deformational episodes and unconformities. The first four cycles, from mid-Devonian to Late Mississippian, record largely bimodal arc magmatism above a west-facing (east-dipping) subduction zone, with or without accompanying back-arc basin magmatism and continental margin rifting. The fifth, Pennsylvanian-Early Permian cycle, involved more primitive, mafic to intermediate volcanism in a west-facing arc with a corresponding marginal back-arc basin to the east. The sixth, Late Permian cycle reflects subduction reversal, and continental-arc magmatism associated with an east-facing (west-dipping) subduction zone. Mafic rocks in all cycles of both arc and non-arc character were derived from variably enriched sources, with contributions from depleted mantle wedge or back-arc asthenosphere, and enriched lithospheric mantle, with or without a subducted slab component, Felsic rocks in arc, arc-rift and back-arc geodynamic settings were derived predominantly from melting and recycling of upper continental crustal material (UCC; La/Sm (sub UCN) nearly equal 1). Arc felsic rocks have calc-alkalic and tholeiitic signatures, whereas non-arc rocks are enriched in high field strength elements and rare earth elements (A-type or peralkaline signatures). Notably, throughout the late Paleozoic magmatic history of the YTT that spanned over 150 m.y., there are no systematic temporal variations in the composition of most mafic and felsic rocks. Igneous source regions and igneous processes were essentially unchanged throughout the tectonic history of the YTT. An important aspect of many mafic rocks from YTT intra-arc rifts and back-arc basins is their high Nb/Th (sub mn) and Nb/La (sub mn) >1 (mn = primitive mantle normalized), implying excess Nb relative to Th and La compared to primitive mantle ratios. Excess Nb in these rocks implies a recycled oceanic crustal component in their genesis, and is a common feature of plume-derived magmas and magmatic rocks from large igneous provinces. The recurrence of this recycled component over the extended magmatic history in YTT, and the common occurrence as low volume eruptions, argues against a direct plume or large-igneous province origin; however, it is possible that this signature reflects the reactivation of recycled plume or large igneous province components in the YTT lithospheric mantle that were originally derived via lithospheric fertilization by magmatism associated with Neoproterozoic breakup of Rodinia.


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