Paleozoic tectonic evolution of the Arctic: geochronologic constraints provided by the Alexander, Arctic Alaska and Pearya terranes

The middle Paleozoic tectonic history of the Laurentian Arctic margin is contentious. Terranes that have been interpreted to have Baltican and Siberian affinities are thought to have been transferred outboard of the Arctic margin of Laurentian into the Panthalassa Ocean. The timing and mechanism(s) of this translation are poorly understood. Refining models requires better constraints, which are provided by studying the Paleozoic geology of terranes thought to be displaced during this time period: 1) Alexander terrane, 2) Pearya terrane, and 3) the Arctic Alaska terrane.\nThe Alexander terrane is divided into the Craig and Admiralty subterranes. The timing of the juxtaposition of the two subterranes has been the subject of recent debate. Devonian sedimentary rocks in the Craig and Admiralty subterranes have nearly identical detrital zircon signatures suggesting that the two subterranes have been linked since the Devonian. Stratigraphic differences between the subterranes are explained by interpreting the Admiralty subterrane as a deep water basin adjacent to the Craig subterrane. The Pennsylvanian to Permian strata of the Craig and Admiralty subterrane have detrital zircon that, while different from each other, are consistent with derivation from Wrangellia. This supports links between the Craig and Admiralty subterranes, and reinforces the idea that Wrangellia was built on Alexander basement.\nThe Pearya shear zone is a large scale sinistral structure that could be involved in the displacement of outboard terranes; however, the timing of displacement on the Pearya shear zone is not well constrained. Titanite aligned parallel to the fabric of the Pearya shear zone yielded middle Paleozoic ages (ca. 380 Ma). Two stages of monazite growth are described based on age. The oldest monazite formed around 980 Ma, consistent with zircon crystallization ages of the protolith. An Upper Ordovician age (ca. 460) is reported for the second phase of monazite growth. The monazite and titanite ages suggest that displacement accommodated by the Pearya shear zone was episodic. The Upper Ordovician tectonic event is interpreted to represent the approach of the Pearya terrane to the Franklinian margin, while the titanite ages are thought to date continued sinistral displacement in the middle Paleozoic post-accretion that may be related to strike-slip migration of outboard terranes.\nThe tectonic setting of the Arctic Alaska terrane in the Middle to Late Devonian is poorly constrained. Geochronology, geochemistry and field mapping of igneous rocks from the North Slope subterrane provide new clues into the tectonic history. The intrusions yielded 370-362 Ma zircon U-Pb ages that are younger than plutons that intrude the Hammond and Coldfoot subterranes of the Arctic Alaska terrane. Whole rock geochemistry of the northern Yukon intrusions suggest that they formed in an arc setting. Field mapping suggests that all intrusions are found to the northeast of the Porcupine shear zone. Coeval intrusions with similar geochemistry are located on Northern Axel Heiberg and Ellesmere Islands. Correlation between these intrusions is inconsistent with the widely held rift setting for the Arctic Alaska terrane intrusions and suggest that the overlying Endicott group was deposit in a foreland basin rather than a rift flank.

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