U-Pb analyses of 48 zircon and/or monazite fractions from ten intrusive rocks that range in composition from quartz diorite to leucogranite yield crystallization ages between 40 and 29 Ma. The new data indicate that Tertiary intrusive rocks constitute a widespread and significant magmatic component of the Ruby-East Humboldt metamorphic core complex. Some of the intrusive rocks are gneissic to mylonitic and have been penetratively deformed in a shear zone that can be traced for >100 km along the west flank of the core complex. These data, when coupled with previously published geochronologic data, indicate that an important episode of mylonitization occurred between ∼29 and 23 Ma (late Oligocene). Some field relations and geochronologic data, however, also suggest an earlier history of mylonitization in parts of the core complex. This earlier mylonitic history may have occurred in the Eocene or even late Mesozoic time, although more-detailed geochronologic data are needed to clearly define the exact time interval.

Sr, Nd, and Pb isotopic data from the dated intrusive units indicate that a relatively sharp, approximately east-west-striking crustal boundary separating predominantly Archean basement in the north from Proterozoic basement in the south transects the complex. We speculate that the crustal boundary represents the continuation of the Cheyenne belt into this part of northeastern Nevada. These new data cast doubt on the existence of the proposed 2.0-2.3 Ga crustal province in this part of the northern Great Basin. The isotopic data from the plutonic rocks coupled with published data from at least in part coeval volcanic rocks indicate that mantle-derived magmas were important in their genesis. Collectively, these data indicate that significant crustal growth, via the addition of mantle-derived magmas, occurred during large-magnitude extension in this part of the northern Great Basin.

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