Geologic insights from zircon inheritance [electronic resource]
Keywords: Geological Sciences.

Numerous studies illustrate that the mineral zircon is the preeminent material resource for understanding fundamental geologic processes that occur in continental crust. Zircon can grow and exist in a broad range of igneous and metamorphic environments within the Earth's crust, and incorporates trace concentrations of elements (U, Th, Hf, REE) that allow age and chemical fingerprinting of these "environmental" influences. This dissertation explores the tectonic and petrologic ramifications of zircon inheritance in the rock cycle, focusing primarily on insights from xenocrystic zircons found in igneous rocks. Pre-magmatic (xenocrystic) domains in zircons can convey otherwise inaccessible details regarding the composition and history of deep--seated magma source regions from which the xenocrysts were derived. The pre--Cenozoic kinematic and tectonic history of the Arctic Alaska Chukotka terrane (AAC) is explored in Chapters 1 and 2. Constraining the pre-opening paleogeography of the Canadian and Alaskan margins of the Canada Basin is a first-order objective in resolving the plate tectonic evolution of the Amerasia Basin of the Arctic Ocean. Paleogeographic models of Northern Alaska are tested using detrital zircon U-Pb geochronology of 12 samples from Middle Mississippian to Early-Middle Jurassic strata (Ellesmerian and lower Beaufortian megasequences) obtained from wells and outcrop along Alaska's North Slope. Arctic Alaska received sediments recycled from the Devonian foreland clastic wedge and underlying Franklinian Basin strata along the Canadian margin-- indicating northern Alaska was part of the foreland fold and thrust belt of the Franklinian mobile belt prior to the opening of the Canada Basin. During the Mesozoic, the AAC was involved in crustal shortening, followed by magmatism and extension, with localized high--grade metamorphism and partial melting, all of which obscured its pre--orogenic geological relationships. New zircon geochronology and isotope geochemistry results from Wrangel Island and western Chukotka basement rocks establish and strengthen intra-- and inter--terrane lithologic and tectonic correlations of the AAC. Temporal patterns of zircon inheritance and O--Hf isotopes are consistent with Cryogenian--Ediacaran AAC magmatism in a peripheral/external orogenic setting (i.e., fringing arc on rifted continental margin crust). In Chapters 3 and 4 fundamental questions about lithology, age, structure, and thermal evolution of the deep crust of the retro--arc hinterland of the North American Cordilleran orogen are addressed through systematic investigation of zircons from Cretaceous and Paleogene granitic rocks in the Snake Range, Kern Mountains, and Deep Creek Range in the east-central Great Basin. Regional metamorphism in Neoproterozoic strata succeeded by (and facilitating?) local underthrusting of low δ18O Paleoproterozoic cratonal basement at depth best explains the observed patterns of inheritance associated with retroarc magmatism during Late Cretaceous time. Ubiquitous mid--Cretaceous to Paleogene zircon overgrowths on xenocrysts from 40 Ma muscovite porphyry dikes indicate that fertile conditions for zircon growth existed throughout the time span of the Sevier and Laramide orogenies, persisting until 45 Ma. "Closed--system" like conditions allowed the overgrowths to preserve similar δ18O compositions to their cores. After 40 Ma in the study area, increased mantle heat flow related to foundering of the Farallon oceanic lithosphere exceeded conditions for pre--magmatic zircon preservation, making xenocrysts a unique archive of the deep crust history here.