U-Pb ages of igneous xenoliths in a salt diapir, La Popa basin: Implications for salt age in onshore Mexico salt basins

Crystallization ages of igneous xenoliths entrained in a salt diapir in La Popa basin, northeastern Mexico, indicate the timing of magmatism in onshore Mexico salt basins and elucidate igneous rock-salt relations. Three equigranular phaneritic mafic to intermediate xenoliths have U-Pb zircon ages from 158 to 154 Ma (Oxfordian-Kimmeridgian). Phaneritic textures, hydrothermal alteration, zircon zonation, and previously published 40Ar/39Ar cooling ages from a nearby diapir, which are younger than Upper Jurassic strata overlying the salt, combine to suggest that these samples were intruded into salt and exhumed during diapirism. A porphyritic mafic rock with a U-Pb zircon age of 150 Ma (Tithonian) has a crystallization age coeval with marine strata overlying the salt. It is interpreted as a shallow intrusion into salt emplaced after the onset of salt diapirism. Magmatism thus took place from 158 to 150 Ma. The Oxfordian-Kimmeridgian ages of the three older xenoliths overlap with published biostratigraphic ages in carbonate strata intercalated with evaporites of the Minas Viejas Formation in the Sierra Madre Oriental south of Monterrey, Mexico. The entire time interval represented by the xenolith ages is coeval with mafic flows and pillow basalts in Border rift basins in northern Mexico and the southwestern United States. Comparison of the new ages with 40Ar/39Ar ages of igneous xenoliths from coastal Louisiana diapirs implies that rift magmatism began later in northeastern Mexico than in the Gulf of Mexico. The magmatic time interval indicated by the diapiric xenoliths is younger than accepted ages for Jurassic arc magmatism in northern and central Mexico and supports the inference that the Late Jurassic magmatism was not a result of subduction. The U-Pb ages and biostratigraphy are consistent in indicating a late Oxfordian-early Kimmeridgian salt age in northeastern Mexico, younger than the Louann Salt of the Gulf of Mexico. The contrasting salt ages indicate incursion of water into La Popa basin and adjacent onshore salt basins of northeastern Mexico from a fully marine Gulf of Mexico.