The geology and petrogenesis of the Capitan Pluton, New Mexico
Michael S. Allen and Virginia T. McLemore
The elongate Capitan pluton of the Lincoln County porphyry belt (LCPB) is the largest exposed Tertiary intrusion in New Mexico (280 km2 ). Study of 1340 m of vertical exposure within the Capitan pluton reveals it to be a zoned, hypabyssal, alkali feldspar granite with discordant and concordant intrusive relationships with Paleozoic country rock. The roof zone (west end) of the pluton consists of a high-silica (75.0%), micromiarolitic, granophyric aplite, enriched in Ta, Nb, Th, Y and HREE, and depleted in Fe, Mg, Ca, Al, Na + K, Ti, P, Zr, Sr, Ba, Co, Ni and LREE relative to a lower silica (71.2%), amphibole, biotite, subsolvus, fine-grained, granite-porphyry core (east end). Data are consistent with fractionation of apatite, titanite, zircon and feldspar to produce the observed zonation. Micromiarolitic samples display severe depletion of Ca, Fe, LREE, U and Th consistent with mobilization of these elements into an aqueous phase, which resulted in formation of the Capitan iron skam deposit, and thorium- and REE-bearing quartz veins. Fractionation in a high-level chamber to produce the zoning of the Capitan granite is problematic, due to its rapid crystallization. A model of flow differentiation is favored. The igneous centers of the LCPB were controlled by two major structures: the north-south Pedemal arch and the east-west Capitan lineament. These crustal flaws allowed leakage of magmas at different times, depending on fracture orientation and changes in tectonic stress regime. Igneous activity along the Pedemal trend consists of pre-rift (38.2-36.5 Ma) alkalic complexes, and early-rift (30-26.5 Ma) syenites and granites. Along the Capitan lineament, bimodal mafic alkalic and granite intrusions such as the Capitan pluton were emplaced during early rifting from 28 to 26.5 Ma. Contemporaneity of mafic alkalic and granitic magmatism along the Capitan lineament suggests that an areally extensive, early rift, mantlemelting event caused melting of the lower crust, generating granitic magmas. Isotope systematics (Sr, Nd, 0) and REE patterns are consistent with derivation of the Capitan pluton from a metaigneous lower crustal source.
- Allen, Michael S.; McLemore, Virginia T., 1991, The geology and petrogenesis of the Capitan Pluton, New Mexico, in: Geology of the Sierra Blanca, Sacramento and Capitan Ranges, New Mexico, Barker, James M.; Kues, Barry S.; Austin, George S.; Lucas, Spencer, G., New Mexico Geological Society, Guidebook, 42nd Field Conference, pp. 115-127.