MONAZITE THERMOCHRONOMETRY IN KYANITE + SILLIMANITE ± ANDALUSITE BEARING QUARTZITES AND SCHISTS FROM THE PICURIS RANGE, NORTHERN NEW MEXICO

Monazite-xenotime (Mnz-Xno) thermometry and microprobe U-Th-Pb chemical age dating in aluminous quartzites and schists from the northern Picuris range permit quantitative temperature-time estimates in rocks that lack minerals needed for Fe-Mg exchange thermometry. Mnz and Xno were analyzed in two quartzite samples (Qtz-Ky-Sil-Ms-Ctd-Hem) and two samples of schist (Ms-Ky-Sil-And-Ctd-Hem). Rim temperatures vary systematically with calibration and mineral assemblage; however, age determinations are consistent within error for all samples.

Mnz and Xno from the quartzites occur within both the matrix, and more rarely as inclusions in kyanite and sillimanite. Chemical zoning patterns are similar in all grains, showing 2-3 distinct compositional domains. Mnz cores have relatively low U, intermediate Y, and high Th; rims show relatively high U, high Y, and low Th. Assuming Mnz rims are in equilibrium with Xno, rim temperature estimates from an empirical calibration (Pyle, unpublished) are 442 °C (±12) and 453 °C (±14). Temperature estimates from the experimental calibration of Gratz & Heinrich (G&H, 1997) are higher at 464 °C (±24) and 485 °C (±28). Corresponding rim age determinations are 1408 Ma (±15; 4 grains, 9 analyses) and 1406 (±16; 5 grains, 15 analyses).

Within the schists, Mnz and Xno are found in both the matrix and as inclusions within large andalusite porphyroblasts. Chemical zoning patterns are similar across most grains and show 3-4 distinct compositional domains. Mnz cores show relatively low U, intermediate Y, and high Th; rims show relatively high Y, high U, and low Th. Assuming Mnz rims are in equilibrium with Xno, rim temperature estimates from Pyle (unpublished data) are 482 °C (±31) and 481 °C (±11). Temperature estimates from G & H (1997) are significantly higher at 538 °C (±54) and 542 °C (±19). Corresponding age determinations are 1395 Ma (±16; 5 grains, 17 analyses) and 1407 Ma (±16; 6 grains; 27 analyses).

Independent estimates of peak metamorphic temperatures in adjacent quartzites and pelites are 520-530 °C (Holdaway & Goodge 1990), consistent with temperatures from the schists (G&H, 1997 calibration). The lower temperatures derived from other sample-calibration combinations may reflect pre-peak temperatures, consistent with monazite inclusions within kyanite, sillimanite, and andalusite.