ZIRCON XENOCRYST EVIDENCE OF MAGMATIC MOBILIZATION AND HOMOGENIZATION OF EAST CENTRAL NEVADA DEEP CRUST DURING THE EOCENE MAGMATIC SWEEP

A magmatic front migrating south across the Great Basin crossed east-central Nevada (ECN) ~40-35 Ma, producing 1000’s of km³ of predominantly metaluminous, intermediate composition volcanic rocks and associated plutons and dikes. This magmatic sweep has been attributed to post-Laramide progressive foundering of the subhorizontally subducting Farallon slab. In order to better understand the crustal evolution of the retroarc region of the North American Cordillera, we have carried out detailed age and chemical investigations of zircon in ECN magmas through time. Inheritance of pre-magmatic (xenocrystic) zircon within ECN granitic rocks is ubiquitous in Jurassic, Cretaceous and the earliest erupted magmas in the Eocene, but conspicuously minor to non-existent in the most volumetrically significant portion of Eocene magmas. Eocene magmatism reflects a renewed input of heat into the crust after a ≥30 m.y. hiatus and resulted in mixing, assimilation and homogenization of Proterozoic crust with large volumes of mantle-derived magmas, as previously inferred by Sr-Nd-Pb isotopes.

Pre-Eocene magmatism documented in exposed ECN crust is dated at ~160, 100, 90, 85, 75, and 70 Ma. Xenocrystic zircon populations in 40 Ma dikes indicate zircon growth in the deep crust at ~175-140 and ~110-65 Ma broadly correlating to observed ages of magmatism and associated metamorphism. Rare inheritance of large, euheral, oscillatory zoned ~61 and ~53 Ma zircons may reflect localized partial melting in the deep crust ongoing during the upper crust hiatus. Some xenocrystic zircons exhibit <~45 Ma metamorphic overgrowth zones that are geochemically distinct from and overgrown by magmatic rims. The preservation of xenocrystic zircon from the deep crust argues that pre-Eocene ECN magmatism and metamorphism lacked the thermal/chemical potential to fully reconstitute portions of the deep crust, whereas the main pulse of Eocene magmatism here at ~ 40-35 Ma thoroughly homogenized crustal source regions, as evidenced by isotopic data and the disappearance of inherited zircon in magmas < 40 Ma. In summary, detailed study of zircons inheritance offers new insights regarding the history of deep crust of the retroarc region before it was fundamentally altered by the magmatic sweep.