### U-Pb Geochronology and Tectonic Implications of a Silurian Ash in the Farewell Terrane, Alaska

By: Dwight Bradley**Julie A. Dumoulin**Dan B. Bradley

Edited by: Julie A. Dumoulin**

                            https://doi.org/10.3133/pp1814F

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-                                 Larger Work: This publication is Chapter                                F of                                                                              Studies by the U.S. Geological Survey in Alaska, Volume 15                                                                    
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#### Abstract

First posted December 10, 2018

**For additional information, contact:**
Alaska Science Center staffU.S. Geological Survey4210 University Dr.Anchorage, AK 99508Alaska Mineral ResourcesAlaska Science Center 

The Farewell terrane is an exotic continental fragment in interior Alaska that during the early Paleozoic was the site of a passive margin. We report a 238U/206Pb zircon age of 432.9±3.0 Ma from a Farewell terrane ash in Mt. McKinley quadrangle, Alaska. This age overlaps with prominent detrital zircon age maxima reported from Silurian and Devonian strata from the Farewell, Arctic Alaska-Chukotka, White Mountains, Alexander, and Yreka terranes, and from parautochtonous Silurian and Devonian foreland-basin strata along the Laurentian margin in the Canadian Arctic and Alaska. These findings can be explained in terms of refinements to the extrusion model of Colpron and Nelson (2011). In the original model, the Farewell terrane was interpreted as having been extruded westward into the paleo-Pacific realm from an initial position along the Siberian margin of the Uralian seaway, that is, the early Paleozoic ocean between Siberia and Baltica. We suggest (1) that the Farewell terrane was deposited along a passive margin that faced into the Uralian seaway; (2) that the terrane more likely originated along the northern or eastern margin of Baltica (present directions), rather than Siberia; and (3) that the Silurian ash and Silurian detrital zircons were derived from a magmatic source along a convergent margin that overrode distal parts of the Farewell passive margin during the Late Ordovician and Silurian. The Farewell terrane was eventually dislodged from Baltica, began to travel with the extruding plate, and was conveyed toward the Pacific to its eventual resting place in Alaska.

#### Suggested Citation

 Bradley, D.C., Dumoulin, J.A., and Bradley, D.B., 2018, U-Pb geochronology and tectonic implications of a Silurian ash in the Farewell terrane, Alaska, *in* Dumoulin, J.A., ed., Studies by the U.S. Geological Survey in Alaska, vol. 15: U.S. Geological Survey Professional Paper 1814–F, 13 p., https://doi.org/10.3133/pp1814F. 

ISSN: 2330-7102 (online)

#### Study Area

#### Table of Contents

- Abstract
- Introduction
- Geologic Setting
- U-Pb Geochronology
- Discussion
- Acknowledgments
- References Cited

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