Geochronologic studies of the Florida Mountains, New Mexico
— Ronald K. Matheney, Muhammad Shafiqullah, Douglas G. Brookins, Paul E. Damon, and E. Timothy Wallin

Numerous geochronologic studies have been carried out on samples from the Florida Mountains, southern New Mexico. Early results indicated a range of dates for Precambrian to Mesozoic syenite-alkali granite core rocks, although questionable assumptions were involved in some of these studies. The core igneous rocks are in fault contact with sedimentary rocks in much of the Florida Mountains, but are unconformably overlain by sedimentary rocks elsewhere. These sedimentary rocks are Cambro-Ordovician and younger, placing a limit on the age of some of the igneous rocks.

K-Ar dates from the igneous core rocks range from 418 to 555 my, while a mineralized monzonite dike yielded an age of 61.5 my and an andesite dike 27.9 my. These last two ages may be associated with mineralization in the area. Rb-Sr dates range from 380 to 470 my and cluster around 400-420 my. These ages are defined by scattered linear arrays suggestive of open system conditions for the rocks, and this is supported by a wide range of apparent initial Sr87/Sr86 ratios for presumably coeval rocks. U-Pb dates on zircons from the Floridas yield a 514 ± 3 my concordia intercept from our studies, consistent with U-Pb dates reported by Evans and Clemons (1987) of 503 my.

Collectively, the geochronologic data argue for Paleozoic plutonism at about 500-515 my, with a resetting event that affected Rb-Sr systematics in the core syenite-alkali granite and overlying Bliss Formation, possibly at about 400 my. The nature of this resetting event is not well understood. The geochronologic data also support the extension of Paleozoic plutonism proposed by Loring and Armstrong (1980) into southern New Mexico, as well as possible extension of the Texas lineament into the area from the east, which may provide valuable information on Paleozoic plate boundary positions.