Late Cretaceous adakitic magmatism in east-central Sonora, Mexico, and its relation to Cu-Zn-Ni-Co skarns

The Late Cretaceous-early Tertiary (Laramide) orogeny (90-40 Ma) in the northwestern part of Mexico produced an important calc-alkaline magmatism, accompanied by several associated porphyry copper and skarn deposits. In the Sierra Santo Niño, Sonora, the Bacanora batholith intrusive complex, composed oftonalite-granodiorite and biotite-hornblende granites intrudes both upper Paleozoic platform sedimentary rocks and Upper Cretaceous volcanic rocks, and is intruded by the San Lucas porphyry (quartz monzonite). The Bacanora batholith and the San Lucas porphyry yield crystallization ages between ~91 Ma and ~89 Ma, and represent the oldest Late Cretaceous intrusive event yet reported for east-central Sonora. Cu-Zn-Ni-Co skarn deposits were generated by this magmatism, and have not been previously described in Sonora. The Bacanora batholith and the San Lucas porphyry show average compositions of 65.1-65.3wt.% SiO2 and 15.7wt.% Al2O3; the main geochemical differences among both unitsare in 4.3wt.% FejO3, 1.9wt.% MgO, 3.8wt.% CaO and 3.3wt.% K2O for the Bacanora batholith versus 2.8wt.% Fe2O3, 1.7wt.% MgO, 2.1wt.% CaO and 5.1wt.% K2O for the San Lucas porphyry. Both units show a LREE enrichment as well as HREE depletion. These compositions match with that of adakitic magmas produced by partial melting ofsubduced oceanic slab, leaving garnet and amphibole in the restite.