JURASSIC BACK-ARC IGNEOUS PROVINCE OF THE NORTHERN GREAT BASIN: EVIDENCE FOR SLAB BREAK-OFF FOLLOWING ARC COLLISION

A unique feature of the western U.S. Cordillera is a Jurassic back-arc igneous province (JBAIP) in the northern Great Basin that extends far east of the Jurassic arc, as far as western Utah. The province is mostly characterized by plutons, although volcanic rocks are locally preserved. We obtained high-resolution U-Pb zircon ages and geochemical data from most of these igneous rocks. Ages primarily fall in a restricted time interval, from 165-157 Ma, and are notably younger from west to east. Basement rocks vary from Triassic back-arc basin and shelf, to Paleozoic rocks of the Golconda and Roberts Mountains allochthons and Cordilleran miogeocline. Compositions are mostly granodiorite to quartz monzonite, except in the east where monzogranite and 2-mica granite are locally common. Geochemical analyses indicate the back-arc plutons are distinct from arc igneous rocks to the west. They are more potassic, trending to shoshonitic, with mostly adakitic compositions. Spidergram and trace element discrimination plots document affinity with slab failure and/or syn-collisional granites, rather than arc granites. Sr and Nd ratios plot close to bulk earth, indicating the JBAIP magma source was dominantly subcontinental lithospheric mantle, with limited contribution from older continental crust. This is consistent with our zircon analyses which generally show only scarce evidence for inherited zircon components. Based on these data, tectonomagmatic origin of the JBAIP cannot be attributed to arc magmatism. More consistent with the data is a model wherein JBAIP magmatism was associated with a slab-breakoff or slab tear event, following arc-arc collision to the west. This model is consistent with regional relations in arc rocks to the west, specifically evidence for major regional deformation that preceded back-arc magmatism, and reconstructions supporting arc-arc collision as the driving force for this deformation. We infer that collision resulted in slab-breakoff or tear, and allowed magmas derived from sublithospheric mantle to penetrate into the crust over a restricted time interval, with magmas younging to the east as the slab progressively descended into the mantle.