Age of the Altar Schist and Cretaceous Deformation in the Caborca – Altar Region of Northwestern Sonora

Zircon U-Pb ages of Mesozoic and Tertiary schist, volcaniclastic sequences, and granitic rocks in the Caborca area indicate Cretaceous sedimentation was followed by regional deformation in Late Cretaceous time. The greenschist facies Batamote complex (Altar schist of Damon et al., 1962) has yielded K-Ar and Rb-Sr ages that suggest a Cretaceous or Tertiary age (Damon et al., 1962; Hayama et al., 1984) and potential correlation with metamorphic rocks exposed in SE California. New detrital zircon U-Pb ages suggest correlation of the protolith of the Altar Schist with the El Chanate Group, and perhaps elements of the underlying Bisbee Group. Five El Chanate Group sandstones from Sierra El Chanate yielded zircons of Mesozoic to Archean age. The youngest detrital zircons define a depositional interval from about 105 to 74 Ma, consistent with the Aptian – Albian age of the underlying upper Bisbee Group and the 72 Ma age of the overlying El Charro volcanic complex. Meta-sandstone samples collected at the base of the Batamote complex and from the Altar schist near Altar contain detrital zircons as young as 74 Ma, documenting that the sandstone and conglomerate protolith of the Altar schist, at least in part, is also Late Cretaceous in age. The Batamote Complex and Altar schist were deformed during northeast-directed contraction prior to intrusion of the 74 Ma Pitiquito granodiorite. The age and style of sedimentation and deformation characterizing the Batamote complex, Altar Schist, and El Chanate Group suggest parallels with the McCoy Mountains Formation and related units in SE California and SW Arizona.