Sr and Pb isotope variations of igneous feldspars in the Do�a Ana mountains

Abstract

A bstrAct — Igneous rocks in the Doña Ana Mountains record a poorly studied portion of mid-Tertiary magmatism affecting southern New Mexico. Here, Sr and Pb isotopes of feldspars are used to evaluate the origins and potential magmatic relationships between compositional-ly variable igneous rocks. Overall, plagioclase, sanidine and orthoclase crystals from igneous rocks in the Doña Ana Mountains crystallized from magmas with very different Sr and Pb isotope characteristics. Plagioclase crystals from Early Eocene Palm Park Formation andesite and dacite have similar 87 Sr/ 86 Sr ratios but vary widely in 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb. These variations are consistent with the presence of variable magmatic sources that were not involved in Late-Eocene/Early Oligocene magmatism in the Doña Ana Mountains. Sanidine crystals from the Doña Ana Rhyolite and nearby Goat Mountain Rhyolite have similar 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb ratios but variable 87 Sr/ 86 Sr ratios, which precludes a simple petrogenetic relationship between them and which also confirms the presence of mul- tiple, highly-evolved magma compositions at ~36 Ma. Orthoclase crystals from syenite dikes that crosscut volcanic rocks in the Doña Ana Mountains, which were presumed to have been parental to the rhyolites, have different 206 Pb/ 204 Pb, and that undermine any direct magmatic connections between the syenite and rhyolites in the Doña Ana Mountains.