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GEOLOGY AND GEOCHEMISTRY OF THE PROTEROZOIC REDROCK
GRANITE AND ANORTHOSITE XENOLITHS IN THE NORTHERN BURRO
MOUNTAINS, GRANT COUNTY, NEW MEXICO, USA
VIRGINIA T. MCLEMORE, NELIA DUNBAR, PAULA J. KOSUNEN,
O. TAPANI RÄMÖ, MATT HEIZLER AND ILMARI HAAPALA
McLEMORE, VIRGINIA T., DUNBAR, NELIA, KOSUNEN, PAULA J.,
RÄMÖ, O. TAPANI, HEIZLER, MATT, and HAAPALA, ILMARI 2002.
Geologyand geochemistry of the Redrock Granite and anorthosite xenoliths (Proterozoic)
in the northern Burro Mountains, Grant County, New Mexico, USA.
Bulletin of the Geological Society of Finland 74, Parts 1–2, 7–52.
Mineral ages from the A-type granites and anorthosite xenoliths in the Redrock
area in the northwestern Burro Mountains in southwestern New Mexico
cluster around ~1220–1225 Ma and provide yet another example of bimodal
igneous activity during this time period in the southwestern United States. The
metaluminous to peraluminous, marginally alkaline to subalkaline Redrock Granite
exhibits the textural, mineralogical, and geochemical features of A-type granite
that was emplaced at a relatively high crustal level. Field relationships, whole
rock and mineral geochemical and isotopic trends suggest that the four phases
of the Redrock Granite are genetically related, with the miarolitic biotite/alkali
feldspar granite being the youngest phase. Spatial relationships and geochemical
data suggest that the anorthosite xenoliths were coeval with the Redrock
Granite, which is consistent with the anorthosite being derived from the upper
mantle, possibly due to deep mantle upwellings, and the Redrock Granite from
the lower crust. The process involved melting in the upper mantle, emplacement
of anorthosite in the crust resulting in partial crustal melting and thinning,
and, finally, intrusion of shallow silicic plutons, the Redrock Granite. The Redrock
Granite and anorthosite were presumably derived from sources characterized
by subtle, long-term LREE depletion, with Nd (at 1220 Ma) values on
the order of +1.5 to +2.
Key words: granites, A-type granites, anorthosite, xenoliths, geochemistry, chemical
composition, electron probe data, isotopes, magmatism, Proterozoic, Burro
Mountains, New Mexico, United States
Virginia T. McLemore, Nelia Dunbar, and Matt Heizler: New Mexico Bureau
of Geology and Mineral Resources, New Mexico Institute of Mining and Technology,
Socorro, NM, 87801, USA
E-mail: ginger@gis.nmt.edu
Paula J. Kosunen, O. Tapani Rämö, and Ilmari Haapala: Department of Geology,
P.O. Box 64, FIN-00014 University of Helsinki, Finland