AN ALTERNATIVE MODEL FOR LARAMIDE MAGMATISM IN THE SW US: IMPORTANCE OF LATE JURASSIC CONTINENTAL RIFTING

The cause of Laramide magmatism in Arizona, New Mexico, Trans-Pecos Texas, and northern Mexico is an enigma because the magmatic zone extended further inboard and to younger ages than Mesozoic arc magmatism elsewhere in the Cordillera. Early models for Laramide magmatism relate an eastward sweep of magmatism to progressive shallowing of the Farallon plate. Advances in geochronology and petrologic models of subduction zones now preclude shallow subduction as the cause for Laramide magmas. For instance, at 80 Ma, intrusion in the Sierra Nevada batholith waned while Laramide magmatism began in Arizona and New Mexico, suggesting that the locus of magmatism shifted from one area to the other rather than sweeping systematically eastward. Investigation of shallow-angle subduction worldwide demonstrates that magmatism ceases as the angle of subduction decreases; modern shallow-angle subduction zones are nearly devoid of active volcanoes.

We propose that Laramide magmatism in Arizona, New Mexico, Trans-Pecos Texas, and northern Mexico was not caused by subduction processes, although the igneous rocks have arc-like geochemical characteristics. Instead, we suggest that Laramide magmatism was controlled by the mantle structure inherited from the preceding tectonic event. During Late Jurassic time, southwestern North America experienced extension, resulting in the Border continental rift. The Border rift is defined by thick accumulations of fault-bounded alluvial and oceanic strata intercalated with rhyolitic ash-flow tuffs and asthenosphere-derived basalts. Border rift basalts have been documented in Kimmeridgian/Tithonian marine and terrigenous strata in the Chiricahua Mountains of SE AZ, in Upper Jurassic strata in the Little Hatchet Mountains of SW NM, and as allochothonous blocks in diapiric Upper Jurassic salts in the La Popa basin of NE Mexico. The position of the Border rift coincides with the zone of Laramide magmatism. Thus, we interpret Laramide magmatism as the result of dehydration of the Farallon slab, and subsequent mantle melting, as the slab entered hot asthenospheric mantle emplaced at shallow depths during Border rift extension. Partial melts of hydrated mantle experienced crustal contamination during ascent, resulting in Laramide igneous rocks with arc-like geochemical signatures.