EVIDENCE FOR REGIONAL ~1.4 GA METAMORPHISM IN THE RINCON RANGE, NORTH-CENTRAL NEW MEXICO: PRELIMINARY RESULTS FROM ELECTRON MICROPROBE CHEMICAL AGE DATING OF MONAZITE

Preliminary chemical age dates from metamorphic monazite in the Rincon Range, north-Central New Mexico are consistent with a single, regional metamorphic event at ~1400 Ma. Previous work in the region proposed that migmatites were associated with the emplacement of a ~1680 Ma granitic pluton, and subsequently overprinted by a second amphibolite-to-granulite facies regional metamorphism at ~1400 Ma. However, no monazites were observed with ages that correspond with the emplacement of the pluton, or the ~1650 Ma Mazatzal Orogeny, in general.

Monazite grains within a Grt-St-Bt schist and two Grt-Sil-Bt migmatites show simple to complex Y, U, Th and Pb zoning, and significant variation in ages within single grains and between multiple grains within a single sample. However, all samples show a maximum in the number of ages between 1350 and 1400 Ma. An array of younger monazite ages may reflect Pb loss, or the intermittent growth of monazite during slow cooling and dissolution of garnet, or both processes.

Eight matrix monazites were analyzed within the Grt-St-Bt schist (96-125) yielding ages between 1422 (±33) Ma and 1213 (±77) Ma. The majority of ages cluster near 1340 Ma. No monazite grains were observed within garnet or staurolite. Thermobarometry yields ~530 °C and 4-8 kbar, but is complicated by the dissolution of garnet via the reaction Grt + Chl=St + Bt, and variable plagioclase compositions adjacent to garnet. Peak metamorphic temperatures were likely closer to 570 °C based upon petrogenetic grid constraints.

Seven matrix monazites within a Grt-Bt-Sil migmatite (96-106) yielded ages between 1395 (±67) Ma and 706 (±18) Ma. The majority of ages cluster between 1350 and 1400 Ma. No monazite inclusions within garnet were observed. A second migmatite (96-117) sample contains Grt-St-Sil-Bt and shows monazite both within the matrix and as inclusions within garnet. Nine matrix grains yielded ages between 1466 (±66) Ma and 952 (±24) Ma with a majority of ages between 1350 and 1400 Ma. Five monazite grains included within garnet yield similar ages ranging from 1418 (±23) Ma to 1361 (±38 Ma). Thermobarometry is difficult to interpret in these samples due to considerable garnet dissolution and diffusion. Peak metamorphic conditions are estimated at 650-750 °C and 3-9 kbar based upon petrogenetic grid constraints.