Intermittent Proterozoic plutonic magmatism and Neoproterozoic cooling history in the Caballo Mountains, Sierra County, New Mexico: Preliminary results
Virginia T. McLemore, O. Tapani Ramo, Matthew T. Heizler, and Aku P. Heinonen


New mapping, geochemistry, and biotite 40Ar/39Ar thermochronology of granitic and metamorphic rocks in the Caballo Mountains indicate a period of ~1480 Ma plutonism followed by cooling to ~300°C by 1411±3 Ma. Isotope and geochemistry data allow correlation with other Proterozoic terrains in southern New Mexico. At least five Proterozoic granitic plutons are found in the Caballo Mountains; the Caballo granite, Longbottom Canyon granite/granodiorite pluton, Northern Caballo granite, Palomas Gap granite, and granite dikes that intruded ~1680 Ma supracrustal amphibolite and felsic gneiss and granite gneiss. The Longbottom Canyon pluton and Caballo granite yield indistinguishable U/Pb zircon derived intrusion ages of 1486±16 and 1487±24 Ma (Amato and Becker, 2012, this guidebook). Nd isotope data support ~1480 Ma magmatism in the region and suggest that the Palomas Gap granite also may be a ~1480 Ma intrusion. No direct dating is available for the granite dikes, however a complex 40Ar/39Ar biotite age spectrum is permissive of one dike being part of the ~1480 Ma intrusive suite. Small, alkaline igneous dikes and near-vertical pipes intruded the Proterozoic granitic rocks in several areas of the Caballo Mountains and are unconfomrably overlain by the Cambrian-Ordovician Bliss Formation (north and south of Palomas Gap, Longbottom Canyon, Apache Gap area, and the Red Hills area of the southern Caballo Mountains). Alkaline magmatism occurs in the Caballo Mountains and may be associated with regional Cambrian-Ordovician magmatic episodes, but no direct radiometric dating has yet identified intrusions of this age in the Caballo Mountains. These alkaline intrusives, which have been interpreted to be Cambrian-Ordovician in age based upon field relationships and regional correlations may indeed be older as suggested. Additional 40Ar/39Ar thermochronology on K-feldspar was conducted. K-feldspar age spectra are variable with 3 samples yielding age gradients between about 1100 to 800 Ma that support cooling from ~275°C to 175°C related to Grenville contraction and associated exhumation. Ages as young as ~70 Ma in the K-feldspar age spectra are consistent with Phanerozoic burial followed by late Tertiary rift-related exhumation. A variety of mineral deposits are found in the igneous rocks in the Caballo Mountains, and most are small, low grade, and uneconomic. Only the gold veins in Proterozoic rocks, placer gold deposits, and REE-U-Th veins in possible Cambrian-Ordovician alkaline igneous rocks and associated metasomatic episyenites have future mineral resource potential.


  1. McLemore, Virginia T.; Ramo, O. Tapani; Heizler, Matthew T.; Heinonen, Aku P., 2012, Intermittent Proterozoic plutonic magmatism and Neoproterozoic cooling history in the Caballo Mountains, Sierra County, New Mexico: Preliminary results, in: Geology of the Warm Springs region, Lucas, Spencer G.; McLemore, Virginia T.; Lueth, Virgil W.; Spielmann, Justin A.; Krainer, Karl, New Mexico Geological Society, Guidebook, 63rd Field Conference, pp. 235-248.

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