New Mexico Geological Society Annual Spring Meeting — Abstracts

The brick-red, silica-depleted, microcline microperthite syenites of the Red Hills at the Caballo Mountains occur within the Precambrian granites in the form ofelongated bodies. The syenites are studied with the aim of determining the petrogenesis with the aid offield studies, petrography, and geochemistry. Field studies include sample collection across a zone of metasomatism and from individual syenite bodies (some containing granite xenoliths). The syenites are being analyzed for major elements and trace elements.

Field observations, petrographic analysis and literature survey indicate that the petrogenesis ofthe syenites can be tested by two models. In Model 1 the syenites are magmatic with accompanying metasomatism and formed in a failed rift during Cambrian-Ordovician time (Ervin, S. D., and McMillan, N. 1., 1997, Ann. G. S. A. Meeting, Abs. with Prog. p. 231; McLemore, 1986, 37^th N.M.G.S. Field Conf. Guidebook: 151-159). Model 2 suggests that the syenites are late-stage hydrothermal alteration products ofthe granite accompanied by quartz dissolution and replacement (metasomatism) by secondary k-feldspars. Unfortunately, the syenites are devoid of datable minerals and all the rocks are significantly altered, precluding age determinations.

Modal analysis across two zones of metasomatism (29 samples) indicates an abrupt change in composition across the syenite/granite contact accompanied by a decrease in quartz from 40-0 % and increase in microcline from 1-94%. Petrographic studies of the Red Hills syenite have shown that the alteration of the syenites can be distinguished from that ofthe granites, based on the changes in primary mineralogy and the type of secondary vug filling (Cathelineau, M., 1986, Journal ofPetrology, v. 27: 4, p. 945-965). Thus, preliminary data support a magmatic origin for the syenites.