Geochemistry of the Burro Mountains syenites and adjacent Proterozoic granite and gneiss and the relationship to a Cambrian-Ordovician alkalic magmatic event in New Mexico and southern Colorado
Virginia T. McLemore and Christopher McKee
More than 12 brick-red, lenticular to elongate bodies of syenite occur scattered throughout Proterozoic granite and quartzofeldspathic gneiss in the Burro Mountains, southwestern New Mexico. The syenites consist of microcline with subordinant quartz, muscovite, hematite, chlorite and plagioclase. A few syenites consist of subequal amounts of microcline and plagioclase. Accessory minerals include apatite, zircon, calcite, fluorite and magnetite. Pervasive hematitization has obscured the textures and mineralogy of most syenites. Compositions range from syenite to quartz syenite to alkali granite. Two distinct geochemical groups of syenites occur: high-K (9-15% K20, 0.3-1.5% Na2O) and high-Na (4-9% K20, 3-6% Na2O) syenites. Both groups are relatively high in Al203 (12-18%) and low in SiO2 (58-71%). The granites contain 69-78% SiO2 , 3 6% K20, 3-5% Na2O and 12-15% Al203 , whereas the gneisses contain 62-82% SiO2, 3-9% K20, 0.4-3% Na2O and 9-13% Al203. Outcrop relationships, textures, trace element distributions and high K2O/Na2O ratios suggest that the high-K syenites are a result of potassium metasomatism of either a high-Na syenite and/or adjacent granite and gneiss. The syenites may be part of a widespread alkalic magmatic event, characterized by carbonatite dikes, syenites and metasomatism, that affected New Mexico and southern Colorado during late Precambrian to Cambrian-Ordovician times. Radiometric dating of the Burro Mountains syenites is needed.
- McLemore, Virginia T.; McKee, Christopher, 1988, Geochemistry of the Burro Mountains syenites and adjacent Proterozoic granite and gneiss and the relationship to a Cambrian-Ordovician alkalic magmatic event in New Mexico and southern Colorado, in: Cretaceous and Laramide tectonic evolution of southwestern New Mexico, Mack, G. H.; Lawton, T. F.; Lucas, S. G., New Mexico Geological Society, Guidebook, 39th Field Conference, pp. 89-98.