40AR/39AR MUSCOVITE THERMOCHRONOLOGY OF PETACA DISTRICT PEGMATITES, NORTHERN, NM

⁴⁰Ar/³⁹Ar muscovite dates from pegmatites of the Petaca District in the Tusas Mts, NM reveal cooling ages between 1390 and 1334 Ma. The Petaca District is located along the eastern boundary of Rio Arriba County in northern NM. A once heavily mined area due to the abundance of muscovite; the Petaca district contains more than 200 pegmatites that may also contain important concentrations of REE minerals. We hypothesize that the coarse-grained muscovite will have ⁴⁰Ar* closure temperatures in excess of 400°C, and depending upon crustal emplacement depth, they might record the intrusion age of the pegmatites. We test this hypothesis by comparing ages of coarse-grained muscovite to ages of finer grained muscovite within metamorphic host rocks. The pegmatite muscovites from 10 samples have overall flat age spectra with integrated ages between 1390 and 1334 Ma and broadly group at 1390, 1370, 1360 and 1335 Ma. The finer grained host rock muscovites from two samples have integrated dates of 1346 and 1332 Ma. Because the basement muscovites are younger than the pegmatite muscovites, we cannot readily conclude that the pegmatite muscovites record intrusion ages. End-member interpretations range from all pegmatites being emplaced at 1390 Ma or older with all ages equaling cooling ages versus each age recording an intrusion over a ~60 Ma duration. Similar muscovite grain size with variable apparent age would argue that ⁴⁰Ar* closure temperature is not a simple function of physical grain size. We expect to employ in situ ⁴⁰Ar/³⁹Ar dating and electron microprobe monazite U+Th/Pb dating to better establish intrusion age. Regionally, the muscovite data contribute to the thermochronology database of New Mexico and ca. 1.4-1.35 Ga dates fit well with existing Tusas Mt. data. Expansion of pegmatite dating to more southern and cooler 1.4 Ga basement should identify where muscovite thermochronology can well approximate intrusion ages.