New Mexico Geological Society Annual Spring Meeting — Abstracts

Late Cambrian plutonic rocks, dated at 503 ± 10 Ma U-Pb zircon date (Evans and Clemons, 1988, Am. Jour. Sci., v. 288, p.735-752), and at 514 ± 3 Ma, U-Pb zircon date (Matheneyet. al., 1988, NMGS Geo. Gdbk. 39th Fld. Cont., p. 99-106) located in the Florida Mountains south of Deming, New Mexico, present an intriguing problem in interpreting the tectonic history of southern New Mexico. Alkali-feldspar granites and syenites in the Florida Mountains have been mapped by Clemons (1982, 1984, 1985, NMBMMR maps 52, 56, 59) and Clemons and Brown ( 1983, NMBMMR map 58) as two distinct bodies, separated by the South Florida Mountains Fault (SFMF). The plutonic rocks are overlain by the Late Cambrian Bliss Sandstone, which was locally derived from the intrusions. During the Late Cambrian, southern New Mexico is thought to have been a passive continental margin, a tectonic setting commonly devoid of igneous activity. The presence of Cambrian age intrusions in the Florida Mountains and the rapid unroofing implied by the Bliss Sandstone suggests a more active tectonic setting. This study uses detailed petrographic and geochemical analyses to indicate depth of emplacement, model magmatic processes that occured during magma genesis, to estimate the amount of unroofing necessary to expose the pluton, to determine source regions for the intrusion, and identify the structural relationship between the igneous bodies north and south of the SFMF.

All Florida Mountain igneous rocks are hyper-solvus, one-feldspar plutons containing potassium feldspar with exsolved plagioclase, indicating that the intrusion crystallized at P_H2O<5 kb (<16.5 km). Low pressure textures, such as miarolitic cavities, are absent. Thus, the pluton was probably emplaced in the upper crust but not at extremely shallow depth. Exsolution laminae are broader and more well developed in rocks south of the SFMF; north of the SFMF, the rocks contain orthoclase, but south, orthoclase has partially inverted to microcline. This indicates that the alkali-feldspar granites in the south have cooled more slowly than the syenites and alkali-feldspar granites in the north. Mafic enclaves with chilled margins and potassium feldspar xenocrysts (1-3 cm) identical to phenocrysts in the alkali-feldspar granites are abundant south of the SFMF. These enclaves are interpreted as partially mixed mafic magmas injected in the lower part of the chamber. Cenozoic movement along the fault thrust the southern alkali-feldspar granites over the upper part of the pluton.

The presence of Late Cambrian magmatic activity and evidence for up to 16 km of erosion between 503 ± 10 Ma and deposition of the Late Cambrian-Early Ordovician Bliss Formation (ca. 505 Ma) suggests that this continental margin was atypical. Whole-rock major and trace element analyses are in progress to identify magma sources and mechanisms of melting, which should aid in the re-interpretation of Late Cambrian tectonics of the region.