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

The geology of the Proterozoic units in the Burro Mountains include metamorphic rocks (Bullard Peak, Ash Creek Group, >1633, 1550-1570 Ma) that were intruded by granitic and mafic rocks. Chemically distinct granitic rocks include (1) gneissic granite/granodiorite (older than 1440-1450 Ma), (2) Burro Mountain granite (~1440-1460 Ma), (3) Jack Creek Rapakivi Granite (~1465 Ma), (4) Redrock Granite (~1220 Ma), (5) rhyodacite-dacite porphyry dikes, (6) fine-grained alkali-feldspar and biotite granite dikes, and (7) pegmatite dikes (youngest). Mapping and chemical analyses of gneissic granite/granodiorite mapped as Burro Mountain granite, indicates that the mapped Burro Mountain granite is composed of multiple intrusions that can range in age from ~1100 to older than 1460 Ma and records growth of Laurentia. The mafic rocks include (1) older gabbro/diabase/diorite intrusions, possibly related to formation of the juvenile Mazatzal crust (~1633 Ma), (2) synplutonic lamprophyre (minette) dikes and numerous enclaves within the ~1465 Ma Jack Creek Rapakivi Granite, (3) ~50 anorthosite xenoliths (5-270 m long, 30 m wide) within the Redrock Granite, and (4) younger gabbro/diabase/diorite intrusions, mostly as dikes (<1200 Ma). The emplacement of these rocks was probably associated with repeated extensional/transtensional events along the southern margin of Laurentia and, given this monotonous overall tectonic setting, they should exhibit similar features in tectonomagmatic discrimination diagrams. However, they do not. In terms of aluminum saturation, the Jack Creek rapakivi granite is peraluminous, the gneissic granite/granodiorite and the Redrock granite straddle the metaluminous-peraluminous boundary. In the TiO2 vs. Zr diagram, the three suites fall into distinct fields: Jack Creek is lowest in Zr, gneissic granite/granodiorite is highest in TiO2, and Redrock is highest in Zr. In the Whalen diagrams, the Jack Creek rapakivi granite falls on the syn-COLG vs. VAG boundary, Redrock granite is in the WPG field, and the gneissic granite/granodiorite is intermediate between the two. In terms of Ga/Al, the Redrock granite is clearly A-type, whereas the Jack Creek rapakivi granite and the gneissic granite/granodiorite are transitional between the A-type and I- and S-type granites.