BROKEN JUG BASALTS: JURASSIC CONTINENTAL RIFT MAGMATISM IN SOUTHERN NEW MEXICO

Recent work in SW NM demonstrates that basaltic volcanism accompanied extension in Late Jurassic time in the southwest US. The Upper Jurassic Broken Jug basalts, exposed in the Little Hatchet Mountains, are subaerial lava flows intercalated with sandstones and conglomerates composed of quartzose and limestone clasts. This suggests that the lava flows erupted into a subsiding rift basin concurrently with erosion of flanking horsts. The geochemistry of these basalts is the key to understanding mantle dynamics during continental rifting. Although the Broken Jug basalts have been thermally metamorphosed during intrusion of Laramide arc magmas, relic igneous textures exhibit pl±ol±pyx phenocrysts in a subophitic groundmass. The mobile elements Rb, Sr, K, Na, Ca, and Mg provide unreliable information on magmatic history. However, ratios of the immobile trace elements Zr, Ti, P, Y, and Nb are constant for the suite (Zr/Nb=7.1 - 12.1 with one sample at 19.3; TiO₂/P₂O₅=3.9-7.4; Y/TiO₂=11.6-16.8), suggesting that the lavas are related by either fractional crystallization or by different degrees of partial melting. Fractional crystallization is precluded as the main process because of the large degree of fractionation required to model the range in element concentrations. In contrast, the lavas can be modeled by different degrees of mantle partial melting. Jurassic lavas are also exposed in the SE Arizona in the Chiricahua and Huachuca mountain ranges (Lawton and McMillan, 1999, Geology, v. 27, p. 779-782). Older lavas, intercalated with Glance Conglomerate, are interpreted as partial melts of lithospheric mantle; younger pillow lavas of the Crystal Cave and Onion Saddle Formations are interpreted as melts of upwelling asthenosphere. Comparison of the Broken Jug lavas to Arizona Jurassic lavas (Glance: Zr/Nb=10.9-14.2, TiO₂/P₂O₅=1.7-2.2, Y/TiO₂=22.3-30.3; Crystal Cave/Onion Saddle: Zr/Nb=5.2-10.3, TiO₂/P₂O₅=2.2-6.1, Y/TiO₂=13.7-22.2) suggests that the Broken Jug lavas were also derived from upwelling asthenosphere during continental rifting.