Abstract
The pre‐Tertiary rocks of the Florida Mountains, southern New Mexico, have been previously mapped as Precambrian (Griswold, 1961) or Mesozoic (Corbitt and Woodward, 1973). Previously determined K‐Ar and Rb‐Sr mineral dates by R. E. Denison (Brookins, 1974a) range from 418 to 555 MYBP. Early attempts at using the Rb‐Sr whole rock method on weathered, surface samples (Brookins, 1974a) indicated a mix of Precambrian samples with samples which yielded apparent Paleozoic dates; these latter samples were considered unreliable because of their weathered nature. The presence of quartz syenite and syenite in addition to quartz monzonite‐granodiorite in the Floridas was confirmed however. Recently, drilling has been carried out in the Floridas and very fresh samples have been made available for study. These samples yield Precambrian dates for the quartz monzonite‐granodiorite suite, but the quartz syenites and syenites yield Rb‐Sr whole rock isochrons of 395 ± 27 MYBP and 371 ± 19 MYBP respectively. These isochrons, 8‐to‐10 data each, indicate plutonic activity in the mid‐Paleozoic in southern New Mexico. A survey of the literature and independent geochronologic work (Loring and Armstrong, 1980) suggest that Paleozoic plutonism in New Mexico may be more widespread than previously thought.
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