DEEP CRUST HISTORY OF EAST-CENTRAL NEVADA USING ZIRCON XENOCRYST PETROCHRONOLOGY

Petrochronology of zircon xenocrysts in crustally contaminated granitic rocks provides a unique avenue to assess the geologic history of otherwise inaccessible parts of the crust. U-Pb age dating, combined with trace element analysis and isotopic tracer studies (O, Lu-Hf) of remarkably well preserved xenocrysts reveals fundamental insights about the much debated and poorly understood tectonic and thermal history of the deep crust of the Great Basin region of the western US. The diversity of zircon xenocrysts that contaminated Cordilleran Interior magmas during the Cretaceous to Paleogene Sevier and Laramide orogenies shows that magma source regions systematically varied and fundamentally changed as the orogen evolved- progressively involving more cratonal and anomalously low ¹⁸O crust. Coeval with a slightly retrograde Late Cretaceous thermal history in the Snake Range metamorphic core complex, xenocrysts (entrained by and preserved in 70 Ma and 40 Ma granitic rocks) indicate prolonged and perhaps intensifying partial melting of the crust at greater depths. The best explanation for this thermal dichotomy is that the Late Cretaceous source region for xenocrysts was cratonal basement that lay in a lower plate position to the miogeocline rocks exposed in the MCC- implying the xenocryst source reached peak metamorphic conditions subsequent to tectonic burial and as the miogeocline rocks were slightly cooling. The subsequent Paleogene southward magmatic sweep 30 m.y. later re-invigorated magmatism locally and mobilized more fragments of the underthrust basement in early magmas, but the waxing of this magmatic flare up eliminated the potential for xenocryst preservation in later magmas.