Pré-Publication, Document De Travail Année : 2024

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

Résumé

We studied the age, history of emplacement, and source of the Mount Hillers intrusive center (MHIC, Henry Mountains, Colorado Plateau), with in-situ U-Pb and Lu-Hf isotopic analysis on the same zircon grains. Inherited cores in all analyzed zircon grains, allowing us to document the Proterozoic history of the Colorado Plateau lithosphere. In addition, K-Ar age dates on feldspars from microgranular matrix and 40Ar/39Ar age dating on hornblende phenocrysts to compare zircon crystallization and cooling ages. U-Pb data from zircon rims from all samples provide 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 MHIC emplacement. We propose that the 220 kyr covered by the analytical uncertainties is the minimum duration of emplacement. The maximum duration is, on the other hand, given by ± 2 SD of the sample population, i.e. 640 kyr. The distinct magmatic pulses identified in the field display similar ages within analytical uncertainties. With these durations, the averaged vertical displacement rate during the MHIC emplacement was around 0.3-0.9 cm/yr. The average magmatic flux was between 0.5 and 1.6.10-4 km3/yr at the scale of both the whole MHIC and individual satellite intrusions. The U-Pb ages of the zircon cores range between 1.05 Ga and 2.16 Ga, with a major frequency peak between 1.3 and 1.45 Ga and several minor peaks between 1.6 Ga and 1.75 Ga. The 1.8-1.7 Ga Yavapai and 1.7-1.6 Ga Mazatzal age signatures of are both 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 1.3-1.8 Ga depleted-mantle reservoir, and incorporating variable amounts of recycled continental crust with time from 1.5 Ga. Even if both sources were partly mixed, they still can be observed in the Oligocene rims crystallized during the emplacement of the MHIC. With this core-rim duality, the data tell two stories that might 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 these two stories are coupled: our data document a magmatic legacy spanning 1.5 billion years. Our work shows unequivocally 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 maximum duration or at least to discriminate temporally the different pulses in a pluton.

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insu-04798407 , version 1 (22-11-2024)

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  • HAL Id : insu-04798407 , version 1

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Jean-Louis Paquette, Michel de Saint Blanquat, Guillaume Delpech, Eric Horsman, Abdelmouhcine Gannoun, et al.. 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. 2024. ⟨insu-04798407⟩
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