Rates of production and origins of pegmatitic peraluminous granites over 50 million years in the Great Basin of the western United States



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The Ruby Mountain–East Humboldt Range metamorphic core complex is one of a series of cordilleran complexes that span the western half of North America. At least four generations of peraluminous granites have been identified in the Ruby Mountains, three of which are geochronologically well constrained. One unit, a pegmatitic granitic gneiss, has yielded a more complex U-Pb dataset that presents challenges for interpretation of its age and origin. A ~50 M.y. span of dates have been reported from this pegmatitic granitic gneiss unit suggesting that there has been a prolonged period of thermal activity as well as the production of multiple low temperature granitic melts within the same geographic region. Textural complexity in zircon from the pegmatitic granite reveals a wealth of geochemical information that has been used to interpret how this granitic system evolved through time. A significant factor in understanding complex igneous systems is the application of high precision geochronological techniques that enable the time scales of magma emplacement and crystallization to be measured. Accurate measurements of elements such as U, Th, Pb, Hf and REE provide a mechanism for interpreting conditions of zircon crystallization (Hoskin and Schaltegger, 2003) giving insight into the ongoing debate on the origins of granitic melts and magmas. The application of chemical abrasion techniques is shown in this study to improve the accuracy of in situ micro-analysis, eliminating some of the difficulties associated with zircon analysis and the interpretation of the data. U-Pb analysis for pegmatitic granites displaying discrete crystallization periods at 75-71 Ma and 40-36 Ma and observable migmatitic textures in the field, coupled with evidence for dissolution-reprecipitation, suggest that the coarse more leucocratic units from Lamoille Canyon represent in situ partial melts (leucosome) of the preexisting finer banded Cretaceous granite gneiss (paleosome).



Ruby Mountains, Zircon, Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), Magma, Granite