TESTING MODELS OF MAZATZAL (1650 MA) AND PICURIS AGE (1400 MA) METAMORPHISM IN NEAR–GRANULITE FACIES METAMORPHIC ROCKS OF THE NORTHERN TAOS RANGE, NEW MEXICO

Cathodoluminescence (CL) imaging of zircon in metamorphic rocks with differing bulk compositions from Cedro Canyon reveals metamorphic zircon grains and metamorphic overgrowths on preexisting zircon. Preliminary LA-ICP-MS 207Pb/206Pb ages from metamorphic zircon range from 1490 Ma to 1400 Ma, and metamorphic monazite yield ages between 1400 Ma and 1370 Ma. These ages are consistent with upper amphibolite facies metamorphism (~700 °C, 7 kbar) and deformation associated with the ca. 1450–1360 Ma Picuris Orogeny. No evidence of an earlier high temperature metamorphic event associated with the 1650 Ma Mazatzal Orogeny metamorphism was observed.

The structurally highest sample, a gneissic granite (CC15-06), lies within the hanging wall of a northwest-dipping, top-to-southeast thrust fault, and yields a crystallization age of 1714 Ma from concentrically zoned zircon cores with U/Th < 10. Metamorphic overgrowths with U/Th > 10 range from ~1410 Ma to ~1260 Ma. This new crystallization age is significantly older than previously reported ages of 1643 Ma and 1678 Ma for orthogneiss in the area. Undeformed pegmatite (CC15-02) crosscuts the gneissic granite and yields an upper intercept age of 1209

Sil-bearing felsic gneiss, amphibolite and ky-sil bearing quartzite from the footwall of the thrust fault were also sampled. Zircon cores from sil-bearing felsic gneiss (CC15-07) yield a crystallization age of ~1706 Ma with metamorphic overgrowths of 1422 Ma. Monazite from two samples of sil-nodule bearing felsic gneiss yield similar ages of 1381 Ma and 1371 Ma. Amphibolite (CC15-05) bears metamorphic zircon with relatively bright CL cores and dark CL rims that all show U/Th >> 10. Preliminary ages from the bright cores range from 1490 Ma to 1450 Ma and overgrowths yield an age of 1402 Ma. Quartzite (CC15-01) yields a unimodal detrital zircon age distribution (n=285) with a peak age of 1695 Ma. Metamorphic overgrowths, U/Th > 10, yield an age of ~1404 Ma. Monazite from the quartzite yield an age of 1400 Ma. These new metamorphic ages are consistent with previously reported U/Pb TIMMs ages of 1380 Ma on titanite, and 1420 Ma for zircon from an amphibolite.