Geology and geochemistry of Proterozoic granitic and mafic rocks in the Redrock area, northern Burro Mountains, Grant County, New Mexico-A progress report
Virginia T. McLemore, O. Tapani Ramo, Paula J. Kosunen, Matt Heizler, Ilmari Haapala, and Christopher McKee


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). The Redrock granite consists of four textural and compositionally distinct phases (biotite-hornblende, hornblende, K-feldspar, and miarolitic biotite granites). Approximately 50 small anorthosite xenoliths (1216.9 ± 5.3, 1228.5 ± 4.5 Ma, 40Ar/39Ar; 1326 ± 830 Ma, Sm/Nd isochron) are scattered in a northeast-trending zone throughout the miarolitic biotite granite. K-rich minette (1420.5 ± 4.5 Ma, 40Ar/39Ar; 1135 ± 71 Ma, Sm/Nd isochron) occurs as coeval isolated enclaves, swams of enclaves, and synplutonie dikes within the Jack Creek rapakivi granite.  Diabase/gabbro/diorite stocks and dikes intruded the metamorphic and granitic rocks. The age relationships of the granites and minettes remain unclear and will hopefully be resolved by additional U/Pb dating of zircons. The number of 40Ar/39Ar age dates clustering about 1200 Ma strongly suggests that a heating event or uplift occurred at that time. Geochemical and isotopic data indicate that the Redrock granite, anorthosites, Jack Creek rapakivi granite, Burro Mountain granite, and metamorphic rocks were derived from different protoliths. The anorthosites and diabase probably tapped a moderately depleted mantle source. The Jack Creek rapakivi granite was presumably derived from a source characterized by subtle, long-term LREE enrichment, whereas that of the Redrock granite pluton was slightly depleted. Both were clearly more juvenile (Nd model ages ~1600 Ma) in nature than the metamorphic rocks of the region (Nd model ages ~1900 Ma) and may include a significant Middle Proterozoic mafic lower crust/subbcontinental lithospheric mantle component. The metamorphic rocks of the region register an older signature than what has been considered typical of the Nd province 3 of the westem United States.


  1. McLemore, Virginia T.; Ramo, O. Tapani; Kosunen, Paula J.; Heizler, Matt; Haapala, Ilmari; McKee, Christopher, 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: Southwest passage. A trip through the Phanerozoic, Lawton, Timothy F.; McMillan, Nancy J.; McLemore, Virginia T., New Mexico Geological Society, Guidebook, 51st Field Conference, pp. 117-126.

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