DETRITAL ZIRCON POPULATIONS FROM PALEOPROTEROZOIC AND MESOPROTEROZOIC ROCKS IN ARIZONA AND NEW MEXICO: RAISING QUESTIONS ABOUT THE NATURE AND DEFINITION OF THE CA. 1.65-1.60 GA MAZATZAL AND CA. 1.45-1.36 GA PICURIS OROGENIES

Recognition of Mesoproterozoic age (ca. 1.6 Ga to 1.45 Ga) detritus in metasedimentary rocks formerly thought to be Paleoproterozoic in age (ca. 1.65 Ga to 1.7 Ga) in both Arizona and New Mexico naturally leads to questions about our understanding of the 1.65-1.60 Ga Mazatzal Orogeny and the more recently recognized 1.45-1.36 Ga Picuris Orogeny.

The Mazatzal Mountains of Arizona represent a classic field locality where the term "Mazatzal Revolution" was used to describe the last prominent contractional deformation in Precambrian rocks of central Arizona. However, Mesoproterozoic (ca. 1.54-1.57 Ga) detrital zircon at the top of the deformed section in both areas suggest that regional map-scale thrusting and folding is Mesoproterozoic in age. Similar data and observations occur in the Yankee Joe Group, Arizona and Trampas Group, New Mexico.

In central New Mexico, supracrustal rocks of the Manzano Group of the Manzano Mountains are exposed in a fold and thrust belt that extends for about 100 km across strike and were deposited between about 1.66 Ga and 1.60 Ga. Detrital zircon age populations show dramatic loss of locally sourced ca. 1.65 Ga detritus and an increase in older > 1.7 Ga detritus up-section suggesting loss of local topography and/or a maturing depositional system migrating headward into an older continental province. Regional deformation, map-scale folding and thrusting must postdate deposition of the ca. 1.60 Ga Blue Springs rhyolite at the top of the section. The juxtaposition of ca. 1.45-1.43 Ga amphibolite facies metamorphic rocks in the hanging wall of the Monte Largo thrust over Paleoproterozoic ca. 1.65-1.60 Ga greenschist facies rocks in the footwall is consistent with Mesoproterozoic regional deformation of the Picuris Orogeny. Although questions remain about the tectonic setting and evolution of the continental lithosphere associated with both of these orogenic events we propose that the regional map scale fold and thrust belts in the Mazatzal and Manzano mountains, formerly attributed to the Mazatzal Orogeny are instead a manifestation of the Mesoproterozoic Picuris Orogeny.