GEOCHRONOLOGY OF PROTEROZOIC GRANITIC AND MAFIC ROCKS IN THE NORTHERN BURRO MOUNTAINS, GRANT COUNTY, NEW MEXICO

New U/Pb and Ar/Ar isotopic data confirm that the Burro Mountains in southwestern New Mexico comprise a complex Proterozoic terrain that documents three distinct mid-Proterozoic magmatic events (at 1633 Ma, 1460 Ma, and 1220 Ma) for southern Laurentia. The oldest rocks are metamorphic schists and gneisses (Bullard Peak and Ash Creek Group) that were intruded by mafic and granitic rocks. The mafic rocks include (1) an older period of gabbro/diabase/diorite intrusions (1633 Ma, U/Pb), (2) several synplutonic lamprophyre (minette) dikes and numerous enclaves (1465 Ma, U/Pb) within the Jack Creek Rapakivi Granite, (3) approximately 50 anorthosite xenoliths (5 m in diameter to 270 m long and 30 m wide) (1223-1230 Ma, U-Pb) within the Redrock Granite, and (4) a younger period of gabbro/diabase/diorite intrusions (<1200 Ma, field relationships). Chemically and petrologically distinct granitic rocks in the Burro Mountains include (1) Burro Mountain granite (~1440-1460 Ma, Ar/Ar), (2) gneissic granite/granodiorite (~1440-1450 Ma, Ar/Ar), (3) Jack Creek Rapakivi Granite (1465 Ma, U/Pb), (4) Redrock Granite (1220 Ma, U/Pb), (5) rhyodacite-dacite porphyry dikes, (6) fine-grained alkali-feldspar and biotite granite dikes (~1205-1220 Ma, Ar/Ar), and (7) pegmatite dikes (youngest). The granitic suites were presumably emplaced inboard of an extensional arc in response to mafic underplating associated with backarc extension. Nd isotopic data imply an overall juvenile craton margin and presence of a light-rare-earth-element-depleted mantle, and show that pre-1633 Ma metamorphic rocks of the central Mazatzal province had an unusually old provenance.