K-AR AND U-PB ZIRCON CHRONOLOGY OF LATE CRETACEOUS AND TERTIARY MAGMATISM IN CENTRAL CHIHUAHUA-STATE, MEXICO

Magmatism in central Chihuahua occurred from at least 68 to 27.5 Ma, a duration that can be separated into two periods by an order of magnit ude increase in the intensity of magmatism starting at 46 Ma. Activity prior to 46 Ma was volumetrically minor and possibly intermittent. Th e oldest rocks (68 Ma) are a localized, very thick, intermediate-compo sition sequence of lava flows, debris flows, and related sediments. Fr om 62 until at least 52 Ma, numerous small intrusives and felsic tuffs were emplaced locally. Between 46 and 27.5 Ma, the area was inundated with a quasi-steady supply of volcanic products. The post-46 Ma activ ity evolved through four distinct stages that produced two sequences o f felsic ash-flow tuffs separated by a sequence of massive coarsely po rphyritic lavas and tuffs of intermediate to felsic composition. These three calc-alkaline stages were gradually succeeded at about 31 Ma by mildly alkaline basaltic andesite lavas and rhyolitic tuffs with pera lkaline characteristics. All volcanic activity terminated abruptly at 27.5 Ma. The most significant events in the evolution of this magmatis m are its onset, its termination, and the profound increase in activit y beginning at 46 Ma. In its timing and subdued intensity, the pre-46 Ma magmatism fits a setting within the interior of the Laramide Cordil leran orogen. A convergent magmatic arc had advanced eastward to centr al Chihuahua by at least 68 Ma. The oldest rocks are tilted, but volca nic rocks 46 Ma and younger were not deformed. The abrupt increase in magmatism at 46 Ma may be a manifestation of diminished compressional stress related to a decrease in the rate of plate convergence along we stern North America. The regional stress field remained in compression until about 31 Ma, which corresponds approximately to the time of tra nsition from calc-alkaline to mildly alkaline mafic and peralkaline fe lsic volcanism. Magmatism ended at 27.5 Ma, prior to extensional fault ing in the area, but generally coincident with the end of plate conver gence along part of the continental margin of western Mexico.