~1.4 GA PARTIAL MELTING AND REGIONAL DEFORMATION IN PROTEROZOIC MIGMATITES OF THE RINCON RANGE, NORTH-CENTRAL NEW MEXICO: APPLICATION OF MONAZITE U-TH-PB CHEMICAL DATING

Preliminary U-Th-Pb chemical age determinations from 16 matrix monazites and 5 monazites included within garnet in pelitic migmatites (Grt-Bt-Sil-Pl-Ms-Qtz-Ilm-St-Ap-Chl [incl. in grt]-Xt-Mnz) constrain the timing of partial melting and regional deformation to be ~1.4 Ga. The migmatites are spatially related to the ~1.68 Ga granitic Guadalupita pluton. Previous workers suggested that partial melting and regional deformation coincided with pluton emplacement and was overprinted by a second, lower grade metamorphic event and reactivation of existing deformational fabrics at ~1.4 Ga.

Peak P-T estimates from petrogenic grids are 675-850°C and 4-8 kbars. Three deformational fabrics are recognized within these samples. The dominant subhorizontal foliation, S2, is folded by F3 folds to form axial planar S3. Leucosomes, biotite, and sillimanite are aligned in S2. Sigmoidal inclusion trails (Si) within garnet are discontinuous with the matrix foliation and reflect earlier deformation. Garnet x-ray maps show core to rim increases in Fe/(Fe+Mg) from .91 to .95 and XGrs from .03 to .06, while XSps decreases from .09 to .03. Garnets are extensively resorbed. Garnet shows a sharp decrease in inclusion density, coincident with a minor break in Ca zoning near the rim. Garnet textures and chemical zoning suggest (1) syn-deformational garnet growth prior to melting, (2) dissolution during staurolite growth, followed by (3) resumed growth synchronous with melting (few inclusions), and then (4) resorption during cooling and possibly minor decompression.

Matrix monazite age distributions range from ~800-1560 Ma with a mean of 1360 ± 67 Ma. Monazite inclusions within garnet yield age distributions between ~1160 and ~1480 Ma with a mean of 1413 ± 26 Ma. X-ray maps of U, Th, Pb, and Y show some distinct core or overgrowth zoning patterns. No measurable age differences were observed across these chemical domains. Matrix monazites in leucosomes and bt-sil selvages are aligned in S2, consistent with monazite growth previous to or synchronous with S2 formation and partial melting. The occurrence of Si-aligned monazite inclusions within garnet and matrix monazite aligned in S2 suggests regional S2 deformation occurred near 1.4 Ga.