CONTRASTING STRUCTURAL SETTING FOR 1.4 GA GRANITES IN THE SOUTHERN SANGRE DE CRISTO MOUNTAINS, NEW MEXICO: EVIDENCE FOR PARTITIONED INTRACRATONIC DEFORMATION

Field and U-Pb zircon geochronology studies reveal structural-plutonic relationships surrounding Mesoproterozoic granites that intruded Paleoproterozoic gneiss of the Gallinas Canyon, southern Sangre de Cristo Mountains, NM, and within the Yavapai Mazatzal transition zone. The Cañon del Agua granite (CAG) intruded heterogeneous gneiss of the Lower Gallinas Canyon on the east side of the Hermit Peak thrust fault, a Laramide-age Front Range fault. The gneiss displays a strong ENE-trending sub-vertical axial planar foliation. The CAG is a strongly foliated k-feldspar megacrystic granite. Parallelism between the granite and country rock fabric and abundant granite pegmatite sills in the contact zone suggest syntectonic emplacement. Zircon analysis of two CAG samples revealed complex zonation. One sample from the megacrystic pluton interior contained sub- to euhedral, generally prismatic and elongate, 100-250 µm long zircons. CL images show concentric internal growth zonation that is truncated by a ~40 µm brightly luminescent unzoned rim. Ten core analyses yielded a mean 207Pb/206Pb age of 1696±9.5 Ma and concordia intercept of 1705±78 Ma. Twelve rim analyses yielded a mean 207Pb/206Pb age of 1454.0±7.5 Ma and concordia intercept of 1452±15 Ma. A second sample, collected from a pervasively foliated relatively higher-strain domain near the pluton contact, yielded subhedral, generally prismatic and elongate, 50-150 µm long zircons which show concentric internal growth zonation truncated by a narrower ~20 µm luminescent unzoned rim. Cores gave a mean 207Pb/206Pb age of 1706.1±36.9 Ma and concordia intercept of 1706±7.7 Ma. Most overgrowths were too narrow to analyze, but three targets yielded 1450 Ma ages. For both samples, we interpret older ages to represent inherited igneous cores and the younger ages to represent the pluton crystallization and emplacement age. These results contrast with previous work on the nonfoliated subhedral equigranular 1448±12 Ma Evergreen Valley granite, which unambiguously cross-cuts consistently NNW-oriented heterogeneous gneiss above the Hermit Peak thrust in the Upper Gallinas Canyon. The presence of deformed and undeformed granite of the same age in proximity supports models for partitioned intracratonic deformation associated with Mesoproterozoic magmatism.