New Mexico Geological Society Annual Spring Meeting — Abstracts

⁴⁰Ar/³⁹Ar muscovite dates from mainly pegmatites from the Tusas, Sandia and Manzano Mts provide cooling ages between about 1450 and 1330 Ma and in general record diachronous cooling across the region.Samples from the Petaca District pegmatites in the Tusas Mts have comparatively young muscovite ages between 1406 and 1328 Ma whereas data from the Sandia and Manzano Mts are generally older than 1400 Ma and have steps in their age spectra that approach igneous intrusion ages between 1440 and 1460 Ma. Our initial goal of directly dating the Petaca District pegmatites using muscovite argon data was not successful due to high (>500°C) ambient country rock temperatures related to 1.47 to 1.40 Ga metamorphism.The data do however constrain cooling of the basement to about 375°C between 1.4 and 1.35 Ga. The pegmatite muscovites from 36 samples have overall flat age spectra with integrated ages between 1406 and 1328 Ma and broadly group at 1400, 1380, 1360 and 1335 Ma. The finer grained host rock muscovites from six samples have integrated dates ranging 1363 to 1334 Ma. Preliminary electron microprobe monazite U+Th/Pb ages from the Globe and Meadow pegmatite yield ages at ~1430 Ma, confirming that argon muscovite ages are not pegmatite emplacement ages. Muscovite argon ages in the Sandia and Manzano Mts can approximate intrusion ages based on concordance with published U/Pb zircon ages from the Sandia and Priest plutons. The near concordance between argon and U/Pb ages indicates a higher level of crustal exposure in central New Mexico compared to northern New Mexico.State wide compilation of argon data show a strong occurrence of muscovite ages at ~1450-1470 Ma which compares well with the onset of “1.4 Ga” metamorphism recorded by published Lu/Hf garnet ages from the Tusas Mts.

Efforts to quantitatively extract thermal histories from the muscovites using laboratory derived diffusion coefficients based on the fraction of ³⁹Ar released seems to have mixed success. Despite long step-heating experiments to maximize the likelihood of muscovite stability during heating, coarse-grained samples appear to catastrophically dehydroxilate resulting is perhaps inaccurate diffusion coefficients. In contrast, finer grained samples appear to yield sensible age spectra and diffusion coefficients. A sample from the Bosque Peak area yields an age gradient from about 800 to 1500 Ma and thermal history inversion indicates nearly isothermal conditions at ~325°C between 1500 and 1400 Ma or cooling below at least 200°C at 1500 Ma followed by reheating to ~350°C at ~1400 Ma.This technique holds promise to provide the first quantitative thermal history measurements for the time period between 1.6 and 1.4 Ga.

In situ laser ablation ⁴⁰Ar/³⁹Ar ages of muscovites yield variable apparent ages and generally reveal younger ages near crystal edges and old uniform ages for crystal interiors. This spatial relationship indicates that the physical crystal size is defining the argon diffusion dimension.