PALEOZOIC-EARLY MESOZOIC EVOLUTION OF THE ALEXANDER TERRANE: CURRENT PERSPECTIVES AND FUTURE OPPORTUNITIES

The global significance of the Alexander terrane has been a subject of debate in the scientific literature for decades. When and where did it originate? When and how was it transported into the paleo-Pacific realm? What stratigraphic, magmatic, or tectonic ties does it have with other Cordilleran terranes? To answer these questions and develop new hypotheses for Cordilleran evolution, we investigated the Alexander terrane in the Saint Elias Mountains of NW Canada and adjacent Alaska. Newly determined field relationships, zircon U-Pb ages, lithogeochemical compositions, and tectonic reconstructions support the following: (1) Cambrian-Ordovician rock units formed part of an arc-backarc system near the Scandinavian and Russian High Arctic regions of Baltica; (2) some Silurian-Devonian strata comprise now-displaced equivalents of the Old Red Sandstone and were deposited in the northern Caledonian realm following the Scandian collision between NE Laurentia and Baltica; (3) Devonian gabbros share a similar tectonic setting as those present in Wrangellia arc basement and establish the oldest potential tie between the two terranes; (4) Devonian-Mississippian arc-rifting along NW Laurentia opened a backarc oceanic basin that separated the Wrangellia arc from a passive margin developed on the Alexander terrane; (5) Pennsylvanian granitoids that tie together parts of the Alexander, Wrangellia, and Peninsular terranes were produced by a slab breakoff event following backarc ocean closure and the resultant arc–passive margin collision; and (6) Permian collision with the Admiralty Island microcontinent led to the final amalgamation of the Alexander-Wrangellia-Peninsular composite terrane. Future studies aim to test the origins of poorly understood Paleozoic-Mesozoic rock units in the Saint Elias Mountains, with key targets that include: (1) Permian siliciclastic rocks and their proposed stratigraphic ties with the Farewell terrane; (2) Triassic (225-230 Ma) bimodal rift-related lavas of Alexander terrane and their geodynamic linkages with coeval Wrangellia flood basalts; and (3) newly recognized, Triassic (217 Ma) felsic intrusive and volcanic rocks of Wrangellia.