26-8 Xenocrystic (XZ) and detrital (DZ) zircon age spectra as tracers of deep crustal processes

Session: Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism

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Abstract:

Xenocrystic zircon age spectra (XZ spectra) in mafic intrusions provide a window into the history of the Cordilleran deep crust, but are seldom investigated owing to the high solubility of zircon in mafic melts. We investigate XZ spectra from two shallow mafic sills from the easternmost portions of the Mesozoic-Cenozoic Cordilleran arc that intrude inner-shelf Ediacaran strata near the base of the Cordilleran passive margin wedge, which include the Johnnie Formation in the Hanaupah Canyon area in eastern California, and the Stirling Quartzite in the Caliente area, southern Nevada. Zircon preservation is accompanied by high bulk-rock [Zr] of 704 ppm in an early Jurassic lamprophyre in eastern California, and 170–214 ppm in a late Paleogene sub-alkaline diabase in southern Nevada. Similar to detrital zircon (DZ) spectra in the region, the XZ spectra record various Proterozoic magmatic events, and the youngest grains record Jurassic and Paleogene intrusion. However, the XZ spectra also contain 20-25% early Paleozoic grains that cluster between 450 and 400 Ma. Our results are highly counterintuitive given the deep source regions of the host magmas, and the amagmatic, thermal subsidence evident in the lower Paleozoic supracrustal record in the region.  Our results suggest that mafic intrusions can entrain zircons horizontally displaced >100 km from their sources (in this case, eastward). Using bulk-rock [Zr] as a screening tool, systematic exploration of the deep crust focused on XZ spectra of mafic intrusions may be a critical addition to DZ spectra.  The presence of lower Paleozoic zircons in the deep crust below the passive margin wedge support the hypothesis that significant intermediate to silicic magmatism occurred either below or west of the passive wedge during lower Paleozoic time, a history which is otherwise absent in the extant supracrustal record of the southwestern Cordillera

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