David M. Haag¹ and Nancy J. McMillan¹

Extension in the southern Rio Grande rift began 32-29 Ma ago, followed by the eruption of the Uvas Volcanic Field (28-27 Ma). Rifting accelerated at approximately 10 Ma after a mid-Miocene volcanic lull. Major, trace element and isotopic variations in lavas erupted between 19-3 Ma reflect changes in mantle source regions and magma evolution processes associated with rift development.

The oldest post-Uvas Volcanic Field lavas found in southern New Mexico are 18.3 Ma old trachyandesites (SiO₂ = 56-57 wt. %, £_Nd = -2.2 to -1.7, ⁸⁷Sr/⁸⁶Sr = 0.706106-0.706191). These lavas are compositionally and isotopically similar to the Uvas Volcanic Field lavas but have distinctly higher Ba (1671-1872 ppm), Sr (1053-1090 ppm), Al₂O₃ (17.1-17.7 wt. %), K₂O (2.7-2.9 wt. %) and lower Mg#.

By 13 Ma, intermediate volcanic rocks are absent. A tholeiitic basalt, at Hatchita, erupted at 11.8 Ma has a isotopic composition indicative of a lithospheric mantle source (£_Nd = +1.3, ⁸⁷Sr/⁸⁶Sr = 0.704764). This was erupted as a single event with no derivative magmas, indicating that lithospheric mantle could produce small volumes of magma while being cool enough to prevent assimilation.

By 10 Ma, volcanism accelerated, producing both tholeiitic and alkali basalts from a depleted mantle (asthenospheric) source (£_Nd = +6.5 to +7.4, ⁸⁷Sr/⁸⁶Sr = 0.702974-0.703963). Variations in major and trace element composition are attributed to degree of partial melting, crustal contamination, trace element composition of asthenospheric mantle, and, for basalts younger than 9.8 Ma, thermal transformation of lithospheric mantle to asthenospheric mantle (Perry, et al., 1987).

1991 New Mexico Geological Society Annual Spring Meeting April 5, 1991, Macey Center Online ISSN: 2834-5800

Pages: 14