Timescales of pluton assembly and growth history of Colorado Plateau lithosphere: Geochronology (U-Pb, K-Ar, and Ar-Ar) and zircon Hf isotopic study of the Mount Hillers intrusive center (Henry Mountains, Utah, USA)

We studied the age, history of emplacement, and source of the Mount Hillers intrusive center (Henry Mountains, Colorado Plateau, Utah, USA) with in situ U-Pb and Lu-Hf isotopic analysis of the same zircon grains. Inherited cores in all of the zircon grains analyzed allowed us to document the Proterozoic history of the lithosphere of the Colorado Plateau. In addition, K-Ar age dating of feldspars from microgranular matrix and ⁴⁰Ar/³⁹Ar age dating of hornblende phenocrysts enabled us to compare zircon crystallization and cooling ages. U-Pb data from zircon rims from all of the samples provided consistent ages with a mean value of 25.97 ± 0.11 Ma. Our field mapping and structural data show that the sample set is representative of the entire history of the Mount Hillers intrusive center emplacement. We propose that the 220 k.y. covered by the analytical uncertainties is the minimum duration of emplacement. On the other hand, the maximum duration of 640 k.y. is given by ±2SD of the sample population. The distinct magmatic pulses identified in the field display similar ages within the analytical uncertainties. Given these durations, the averaged vertical displacement rate during emplacement of the Mount Hillers intrusive center was ∼0.3−0.9 cm/yr. The average magmatic flux was between 0.5 km³/yr and 1.6.10⁻⁴ km³/yr at the scale of both the entire Mount Hillers intrusive center and individual satellite intrusions. The U-Pb ages of the zircon cores range between 2.16 Ga and 1.05 Ga, with a major frequency peak between 1.45 Ga and 1.3 Ga, and several minor peaks between 1.75 Ga and 1.6 Ga. The 1.8−1.7 Ga Yavapai province and 1.7−1.6 Ga Mazatzal province age signatures are present beneath this part of the Colorado Plateau. Lu-Hf analyses of most of the zircon rims are consistent with those of the cores and show a lower crustal mafic source derived from a 1.8−1.3 Ga depleted-mantle reservoir, which incorporated variable amounts of recycled continental crust over time from 1.5 Ga. Even if both sources were partly mixed, they can still be observed in the Oligocene rims that crystallized during emplacement of the Mount Hillers intrusive center. With this core-rim duality, the data tell two stories that may seem totally disconnected: that of the Proterozoic lithosphere and that of the Oligocene magmatism of the Colorado Plateau. But the U-Pb and Lu-Hf data clearly show that these two stories are coupled. Our data document a magmatic legacy spanning 1.5 b.y. Our work unequivocally shows how important it is to combine robust geochronology based on careful sampling with relative chronology deduced from field-based structural mapping to correctly evaluate the emplacement duration and possibly to temporally discriminate between the different pulses in a pluton.