Rift Volcanics or Rhyolitic Sills? The Jurassic-Cretaceous Rhyolitic Rocks of the Whetstone Mountains, Southeastern Arizona

Previously mapped as both intrusive and extrusive, the rhyolitic bodies found within the Bisbee Group rift sedimentary rocks of the Whetstone Mountains were thought to represent the silicic end member of bimodal volcanism associated with extension of the Late Jurassic Mexican Borderland rift. However, preliminary investigations indicate that all the rhyolitic bodies are shallow intrusive sills and dikes that have intruded the Bisbee Group sediments parallel/sub-parallel to bedding. The age of the intrusions can be constrained by the age of the Bisbee Group rocks (Late Jurassic) and cross cutting relationships with a 74 Ma pluton, meaning the rhyolitic bodies may be tectonically related to either rifting or to early Laramide deformation.

The following lines of evidence suggest that these rhyolitic bodies are related to the extension of the Mexican Borderland Rift rather than Laramide deformation: 1.) The rhyolites found in the Whetstones are consistent with a high silica end member of bimodal volcanism associated with rifting, versus the intermediate composition magmas associated with Laramide deformation. 2.) Local deformation within the Whetstone Mountains has folded both Bisbee Group sedimentary rocks and the rhyolitic sills. One such fold has been truncated by the 74 Ma pluton restricting the age of the compressional structures to early Laramide and providing further evidence that the sills where emplaced and cooled long before folding occurred. 3.) The low abundance of hydrous minerals (<2% biotite), no vesiculation, no magma-water interaction at intrusion contacts, and general lack of volcanism indicate that these were dry silicic rift related magmas. Columnar jointing, chilled margins, and aphanitic porphyritic textures point to emplacement of the magma at shallow depths. It is thought that the sedimentary rocks were dry at the time of magma emplacement due to lack of magma-water interaction. However, it is permissible that the magmas were emplaced only a few million years after sedimentary deposition suggesting that the magmas are Latest Jurassic to Early Cretaceous in age. If so, these rhyolites would extend the duration of rift magmatism within the Mexican Borderland Rift.