PENNSYLVANIAN-TRIASSIC MAGMATIC FLUX IN NORTHERN MEXICO AS INDICATED BY DETRITAL ZIRCON DATA FROM TRIASSIC-LOWER CRETACEOUS STRATA

The Southwestern Laurentian Borderland (SLaB) model for late Paleozoic deformation of the Laurentian margin envisions movement of the Caborca block from southern California to its present postion in Sonora by the beginning of middle Permian time. Recent U-Pb data from two plutons in western Sonora and clasts in Triassic strata have been invoked to suggest onset of the Cordilleran magmatic arc by early Permian time (~284 Ma), and by extension, the requirement that Caborca arrived in Sonora before the Permian. We compiled detrital zircon data from samples of Jurassic and Early Cretaceous age across northern Mexico from Sonora to Nuevo León to determine if we could discern a record of Pennsylvanian through Triassic magmatic flux from the sedimentary record. The underlying rationale is that Triassic volcanic rocks were likely to have been the first units eroded from Jurassic rift shoulders during Pangea breakup, followed by Triassic plutons, if those were locally present adjacent to the Jurassic basins. The Jurassic strata should therefore record the chronology of zircon production, and thus arc magmatism, in northern Mexico.

Early Permian (299-272 Ma) magmatism is indicated by zircon age modes in Jurassic strata from northern Chihuahua (Sierra Samalayuca and Plomosas uplift), but elsewhere evidence for early Permian zircon production is sparse. At most localities, a pulse of magmatism is recorded near the beginning of the late Permian (~260 Ma) and continuing through the Middle Triassic, to ~235 Ma. In northern Chihuahua and Sonora, there is evidence for a Late Triassic magmatic pulse that extended from ~225-211 Ma. Triassic magmatism in Sonora apparently continued unabated into the Jurassic.

The detrital zircon record does not demonstrate clear-cut coeval pulses of magmatism across northern Mexico. This may result from vagaries in the detrital record or from actual geographically diachronous onset of the Cordilleran arc. Our data set does not indicate significant onset of early Permian magmatism in Sonora, and thus does not preclude arrival of the Caborca block in Sonora in early Permian time as predicted by the SLaB model.