CONTRASTING PRECAMBRIAN TECTONIC HISTORIES IN NORTHERN NEW MEXICO AND SOUTHERN COLORADO

The Needle Mountains of Colorado contain compelling evidence for major deformation and metamorphism prior to 1.6 Ga. Amphibolite facies gneisses are poly-deformed and record two episodes of isoclinal folding that must have formed between 1774 and 1706 Ma based on cross-cutting relationships. These gneisses are overlain uncomformably by the Uncompahgre Group, with a maximum age of 1713 Ma, based on detrital zircon geochronology (Jones et al., 2005). The Eolus granite and Electra Lake gabbro crosscut and contact metamorphose older rocks and were intruded between1442 and 1435 Ma (Gonzalez and Van Schmus, 2007). Argon cooling ages are consistent with metamorphism prior to 1600 Ma as well as during intrusion of 1.4 Ga granites (Shaw et al., 2005). Thus, the rocks in the Needle Mountains record a complex, polyphase history. This rock record appears similar throughout much of Colorado and Arizona, leading most workers to extrapolate these events across the southwest, including to the aluminosilicate triple point metamorphic terrane of Northern New Mexico.

It has been known for more than a decade, based on Ar cooling ages, that the triple point terrane in New Mexico has experienced amphibolite facies metamorphism at 1.4 Ga (Grambling and Dallmeyer, 1993). Extrapolating results from Colorado and Arizona to New Mexico has lead to the proposal that the triple point mineral assemblages and associated deformation are the result of superposed tectonic events, with the Ar ages reset during 1.4 Ga intracontinental tectonism. Recently published detrital zircon ages (Jones et al., 2011) and newly obtained Lu/Hf garnet ages (Aronoff et al., this meeting), however, demonstrate sedimentation and prograde metamorphism between 1.49 and 1.4 Ga. These results strongly indicate that the triple point metamorphic terrane experienced a single progressive deformation and metamorphic event at ca. 1.4 Ga that reworked preexisting crust. Structures across the triple point terrane are contractional in nature suggesting deformation driven by collision or convergent margin process. Thus the present data set strongly indicates that Northern New Mexico was affected by a 1.4 Ga orogenic event distinct from the Mazatzal and Yavapai orogenies.