LATE JURASSIC MAFIC VOLCANISM OF THE BORDER RIFT, SOUTHWESTERN U.S. AND NORTHERN MEXICO

Associated with the opening of the Gulf of Mexico at its southeastern termination and the process of slab cutoff at its northwestern termination, the Late Jurassic Mexican Border rift consists of a series of horsts (Coahuila and Aldama platforms) and grabens (Sabinas basin, Chihuahua trough, and Mar Mexicano). Syn-rift sedimentary rocks in the grabens underlie the classic Bisbee basin section that represents filling of rift basins during slowing subsidence. We document four areas of mafic volcanism in the Border rift: (1) Blocks of greenschist facies metaigneous rocks exposed within an evaporite diapir in the Laramide La Popa foreland basin, NE Mexico, yield ⁴⁰Ar/³⁹Ar ages of 145 Ma (late Tithonian) that record the cooling of rift-related heat flow. (2) Mafic flows and related intrusions near Cucurpe, Sonora, Mexico, are intercalated with Oxfordian strata. (3) Andesitic lava flows are intercalated with the syn-rift Glance conglomerate in the Huachuca Mts. of SE AZ. Also, mafic pillow lavas, hyaloclastites, and subaerial lava flows in the Chiricahua Mts. of SE AZ are intercalated with shales containing Kimmeridgian ammonites. (4) Mafic lavas of the Broken Jug Fm. in the Little Hatchet Mts. of SW NM are intercalated with Upper Jurassic strata.

Geochemical data are complex due to differing degrees of metamorphism, sea floor and hydrothermal alteration, and weathering. However, consistent relationships have been observed in immobile incompatible trace element concentrations. The La Popa and Chiricahua suites represent asthenosphere-derived continental rift basalts with high [Nb/Zr]_N (1.4-7.1) and [Nb/Y]_N (1.4-15.6). The Cucurpe, Little Hatchet, and Glance suites, which have lower [Nb/Zr]_N (0.4-2.97) and [Nb/Y]_N (0.5-5.7) are interpreted as melts of lithospheric mantle during early rifting. We suggest that: 1) Late Jurassic sedimentary basins were formed during regional rifting; 2) the rift produced magmas from both lithospheric and asthenospheric mantle sources; 3) although the rift system has since been geographically reorganized by Cretaceous and Tertiary tectonic events, it was originally a contiguous feature similar in size to the Cenozoic Rio Grande rift.