GEOLOGY AND GEOCHEMISTRY OF REDROCK ANORTHOSITE AND GRANITE, NORTHERN BURRO MOUNTAINS, GRANT COUNTY, NEW MEXICO

Virginia McLemore, New Mexico Bureau of Mines and Mineral Resources, February, 2001, updated August 2011


A complex Proterozoic terrain in the northern Burro Mountains consists of metamorphic rocks (Bullard Peak and Ash Creek Series, 1550-1570 Ma, U/Pb) that are intruded by granitic and mafic rocks. The granitic rocks include 1) Burro Mountain granite (oldest, ~1445 Ma, U/Pb), 2) gneissic granite/granodiorite (~1380 Ma, K/Ar), 3) Jack Creek rapakivi granite (1220.9±3.5, 1198±4.7 Ma, 40Ar/39Ar), 4) Redrock granite (1210±5, 1204.8±12 Ma, 40Ar/39Ar; 1328±58 Ma Sm/Nd isochron), 5) fine-grained alkali-feldspar and biotite granite dikes, 6) rhyodacite/dacite porphyry dikes, and 7) pegmatite dikes (youngest).
 

Rapakivi texture in Jack Creek rapakivi granite. Rapakivi is Finish for rotten or crumbly rock and describes the tendency of the rapakivi granite to easily weather, found throughout the eastern portion of the game refuge in the northern Burro Mountains. The rapakivi texture refers to the mantling of K-feldspar phenocrysts (pink) by plagioclase (white). Most rapakivi granites are middle to early Proterozoic, although rapakivi textures have been described from some Phanerozoic and Archean granites. The petrogenesis of rapakivi granites is controversial and questions remain as to the source of the rapakivi magma, the genetic relationships between the granite and penecontemporaneous mafic rocks, the origin of the rapakivi texture, and the tectonic setting of these rocks.
 

 

Photo showing comingling textures along the edges of a minette pillow in Jack Creek rapakivi granite in Jack Creek, northern Burro Mountains. K-rich minette as isolated enclaves, swarms of enclaves, and synplutonic dikes range in size from a few centimeters to several meters in diameter. The minette is dark gray, fine to medium grained, and porphyritic with dark mica and, presumably, pyroxene phenocrysts that have been altered into fine-grained amphibole; the main constituents in the groundmass are K-feldspar, dark mica, and amphibole . The enclaves are distinctly finer grained then the enclosing granite. The crenulate to cuspate margins of the synplutonic dikes and many of the enclaves show evidence for intensive mingling with the rapakivi granite host; quartz and K-feldspar megacrysts have been mechanically incorporated into the minette and the quartz xenocrysts are locally mantled by biotite or hornblende. Adjacent to the minette, feldspars within the rapakivi granite are locally surrounded by biotite and amphibole from the minette. Other minette pillows have sharp to diffuse contacts with the host granite. Locally, coarse-grained pegmatitic phases of the granite form a halo surrounding the minette pillows.
 
 

Minette pillows in rapakivi granite.



This study began in 1987, when a preliminary investigation of the Proterozoic rocks was undertaken in order to assess their economic potential (McLemore and McKee, 1988). This early investigation also indicated that previous mapping by Hewitt (1959) and Hedlund (1980a, b) did not adequately differentiate the diverse Proterozoic rocks. Subsequent mapping by Finnell (1987) grouped the Proterozoic rocks together as one unit. Additional studies by the authors continued in 1988, 1996, and 1998-2000 and are ongoing (McLemore et al., 1999b; Kosunen et al., 1999; Rämö et al., 1999). Current studies by the authors include detailed mapping and sampling of the Proterozoic rocks in order to explain the petrogenesis of the anorthosites and spatially related granites and minettes, and to assess their economic potential. Approximately 77 km2 are being mapped at a scale of 1:12,000 and transferred to a map scale of 1:24,000 using standard U.S. Geological Survey (USGS) 7½-minute topographic quadrangle maps (Redrock, Brushy Mountain, Mangas Springs, Bullard Peak).

TABLE 1. Age relationships of the Proterozoic and younger rocks in the northern Burro Mountains.
Units
Volcanic-epithermal veins
Tertiary rhyolite and quartz monzonite dikes and plugs
Tertiary-Cretaceous andesite sills and dikes
Cretaceous Beartooth Quartzite
Proterozoic rocks
 Diabase/gabbro/diorite dikes
 Pegmatites dikes
 Serpentinite veins
 Fine-grained biotite and K-feldspar granite dikes (1206±15 Ma, 40Ar/39Ar)
 Rhyodacite-dacite porphyry dikes
 Redrock granite (1328±58 Ma Sm/Nd isochron)
    Miarolitic biotite granite
    K-feldspar granite
    Hornblende granite (1210±5, 1204.8±12 Ma, 40Ar/39Ar)
    Biotite-hornblende granite
 Jack Creek rapakivi granite (1220.9±3.5, 1198.0±4.7 Ma, 40Ar/39Ar)
 Minette (1420.5±4.5 Ma, 40Ar/39Ar; 1135±71 Ma Sm/Nd isochron)
 Anorthosite/leucogabbro (1216.9±5.3, 1228.5±4.5 Ma, 40Ar/39Ar; 1326±830 Ma Sm/Nd isochron)
 Diabase/diorite/gabbro (1500-1535 Ma, U/Pb, Stacey and Hedlund, 1983; but the dike intruded the 1445 Ma Burro Mountains granite)
 Gneissic granite/granodiorite (1380±45 Ma, K/Ar, Hedlund, 1980b) (Ygd of Hedlund)
 Burro Mountain granite (1169±11 Ma, 40Ar/39Ar; correlated to 1445±10 Ma U/Pb by Hedlund, 1980b) (Yg of Hedlund, 1980a, b)
 Bullard Peak and Ash Creek Metamorphic rocks (1550-1570 Ma, U/Pb, Stacey and Hedlund, 1983)
 Quartz-feldspathic gneiss (1202.3±2.4 Ma, 40Ar/39Ar) (granulite of Hedlund, 1980a)

TABLE. Summary of age determinations. * disturbed age spectra.

Sample Published ages (Ma) (Hedlund, 1980a, Stacey and Hedlund, 1983) 40Ar/39Ar (Ma) (mineral dated) Nd/Sm isochron Ma Rb/Sr isochron Ma
Gabbro/diabase/diorite dikes
NM118-98 *~1000 (horndblende)
Rhyodacite-dacite porphyry dikes
NM1G *~1200 (hornblende)
Fine-grained biotite granite dike (related to Redrock Granite)
NM84-98 1206±15 (biotite)
Redrock Granite 1328±58 (n=4) ~1250
NM19-98 *~1200 (hornblende)
NM93-98 1210±5 (hornblende)
1204.8±12 (biotite)
NM279-99 *~1200 (biotite)
Jack Creek rapakivi granite 1135±71 (n=5) ~1300
Red17 1198.0±4.7 (biotite)
NM17-96 1220.9±3.5 (biotite)
NM217-99 1208.6±1.9 (biotite)
NM230-99 1384±3 (biotite)
NM234-99 1335.4±0.7 (biotite)
Anorthosite 1326±830 (n=4)
Red7A 1228.5±4.5 (hornblende)
Red7B 1216.9±5.3 (hornblende)
Diabase dike 1500-1535 (U-Pb on zircon)
Metamorphic rocks 1550-1570 (U-Pb on zircon)
NM101-98 1202.3±2.4 (muscovite) ~1340

 

REFERENCES

McLemore, V. T., 1988, Geology and geochemistry of the Redrock anorthosites, Burro Mountains, southwestern New Mexico; Preliminary Observations; abstr. and poster presented at the Penrose Conference on Origin of massif anorthosites, Wyoming, Aug. 14-19, 1988. (unpublished).

McLemore, V. T., and McKee, C., 1988, A preliminary geochemical study of the Redrock anorthosite and granite, Burro Mountains, southwestern New Mexico: New Mexico Geological Society, Guidebook 39, pp. 83-88.

Kosunen, P. J., Rämö, O., T., McLemore, V. T., McKee, C., and Haapala, I., 1999, Lamprophyric (minette) magmatism and Proterozoic rapakivi granites: Preliminary observations from the Redrock area, Burro Mountains, New Mexico (abstr.): Geological Society of America, Abstracts with Programs, v. 31, p. A-261.

McLemore, V. T., Rämö, O. T., McKee, C., Heizler, M. , Haapala, I., and Kosunen, P. J., 1999, Origin of the Redrock anorthosite and rapakivi granite (Proterozoic), Redrock, New Mexico: Preliminary Observations (abstr.): Geological Society of America, South-central section meeting, Abstracts with programs, v. 31, no. 1, p. A-29.

Rämö, O., T., McLemore, V. T., Kosunen, P. J., Haapala, I., McKee, C., and Heizler, M., 1999, Nd Isotopic assessment of the Proterozoic granitoid and associated mafic rocks of the Redrock area, Burro Mountains, New Mexico (abstr.): Geological Society of America, Abstracts with Programs, v. 31, p. A-260.

*Haapala, I., McLemore, V. T., Rämö, O., T., and Kosunen, P. J., 2000, Epidote-chlorite alteration of a Mesoproterozoic miarolitic granite from the Burro Mountains, New Mexico (abstr.): 31st International Geologic Congress, Brazil.

McLemore, V. T., Rämö, O. T., and Kosunen, P. J., 2000, Supplemental road log 1, from Lordsburg to Redrock to Ash Creek in the northern Burro Mountains, New Mexico; in Lawton, T. F., McMillan, N. J., McLemore, V. T., Austin, G., and Barker, J. M. eds., Southwest Passage, A Trip through the Phanerozoic: New Mexico Geological Society Guidebook 51, pp. 45-50.

McLemore, V. T., Rämö, O. T., Kosunen, P. J., Haapala, I., Heizler, M., and McKee, C., 2000, Geology and geochemistry of Proterozoic granitic and mafic rocks in the Redrock area, northern Burro Mountains, Grant County, New Mexico; A Progress Report; in Lawton, T. F., McMillan, N. J., McLemore, V. T., Austin, G., and Barker, J. M. eds., Southwest Passage, A Trip through the Phanerozoic: New Mexico Geological Society Guidebook 51, pp. 117-126.

McLemore, V. T., Rämö, O. T., Kosunen, P. J., Heizler, M., Haapala, I., and McKee, C., 2000, Geology and geochemistry of Proterozoic granitic and mafic rocks in the Redrock area, northern Burro Mountains, Grant County, New Mexico; A Progress Report; in S. Lukkari and I. Haapala, Rapakivi granites and associated mineralization: IGCP Project 373, Field conference in southern Finland, p. 48.

McLemore, V. T., Ramö, O. T., Kosunen, P. J., Heizler, M., Haapala, I. and McKee, C., 2000, Geology and geochemistry of Proterozoic granitic and mafic rocks in the Redrock area, northern Burro Mountains, Grant County, New Mexico; A progress report (abstr.): New Mexico Geological Society, Spring Meeting, Proceedings Volume, New Mexico Geology, v. 22, p. 37-38.

McLemore, V. T., Heizler, M. T., Ramö, O. T., and Kosunen, P. J., 2000, Significance of the 1200 Ma Redrock Granite, Burro Mountains, New Mexico (abstr.): Geological Society of America, Abstracts with Programs, v. 32, no. 7, pp. 317-318.

*McLemore, V. T., Dunbar, N., Kosunen, P. J., Ramö, O. T., Heizler, M. T., and Haapala, I., 2001, Geology and geochemistry of the Redrock Granite and anorthosite xenoliths (Proterozoic) in the northern Burro Mountains, Grant County, New Mexico: Geological Society of Finland, Bulletin, http://geoinfo.nmt.edu/staff/mclemore/documents/McLemoreetal.pdf

McLemore, V. T., 2005, Mineral Resources of the Wild Horse Mesa Area, Northeastern Burro Mountains, Grant County, New Mexico: New Mexico Bureau of Geology and Mineral Resources, Open-file Report 486, 46 p. 1 map, http://geoinfo.nmt.edu/publications/openfile/details.cfml?Volume=486

Amato, J.M., Heizler, M.T., Boullion, A.O., Sanders, A.E., Toro, J., McLemore, V.T., and Andronicos, C.L., 2011, Syntectonic 1.46 Ga magamtism and rapid cooling of a gneiss dome in the southern Mazatzal Province: Burro Mountains, New Mexico: Geological Society of America Bulletin, v. 123, p. 1720-1744.