229-1 Petrogenesis of the Twilight Trondhjemite in the Needle Mountains, CO

Session: Crustal Petrology (Posters)

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Abstract:

The Needle Mountain Proterozoic Complex is made up of 1.8-1.4 Ga crystalline basement rocks that were uplifted through a series of orogenic events during the accretion of the southern margin of Laurentia. In addition to exposing some of the oldest rocks in the surrounding areas, the Needles are also of interest from a geochemical perspective. The emplacement of the high-Na (> 3 wt%) trondhjemite in the Twilight Gneiss formation against the high-K (> 4 wt%) Eolus Granite is separated by a period of only 400 Ma. The sodium-rich trondhjemites that make up the TTG suite are rarely produced after the end of the Archean, indicating a shift in the conditions of continental crust production after 2.5 Ga. Corresponding to this decline in trondhjemite production, global trends of crustal composition show a sharp decline in Na/K ratios of felsic continental crust across the Archean-Proterozoic boundary. The ~1.7 Ga sodic (>3 wt%) trondhjemite in the Twilight Gneiss formation is therefore of scientific interest because of its distinct composition and comparatively young age. Here, we present zircon U-Pb SIMS geochronology of 6 samples of the Twilight trondhjemite from the western Needles, with upper intercept ages spanning from 1759.8 +/- 4.3 - 1741.6 +/- 4.1 Ma. Discordia arrays show Pb loss in samples located in the northwestern Needles, as well as lower-intercept ages that appear to correspond with Tertiary magmatism in the San Juans. The age range of the trondhjemites from this work is ~10 Ma younger than zircon U-Pb TIMS upper-intercept ages of trondhjemites from a similar area from Gonzales and Van Schmus (2007). These ages will be supplemented by petrologic modelling based on bulk rock major and trace element geochemistry for further insights into the emplacement history of the Twilight trondhjemites. Understanding the emplacement history and the conditions that produced this Proterozoic trondhjemite will provide insight into the secular geochemical evolution of Earth’s continental crust.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9496

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