Late Jurassic Igneous Rocks in South-Arizona and North-Central Sonora: Magmatic Record of Crustal Extension

Through most of Middle Jurassic time, the region of south-central Arizona and north-central Sonora was part of a continental magmatic arc. In Late Jurassic time, this magmatic arc gradually gave way to a transtensional regime characterized by strike-slip faulting, pull-apart basins, and basinal sedimentation. Late Jurassic rifting was accompanied by magmatism, typically small in volume relative to the preceding arc phase, and with moderately alkaline compositions. These igneous rocks, the Ko Vaya Suite, make up much or most of the Comobabi, Artesa, Quijotoa, and Brownell Mountains and Sierra del Cobre. The bimodal Ko Vaya Suite constitutes volcanic to shallow plutonic complexes comprising chiefly trachyandesite and volcanic wacke; monzodiorite; rhyolite porphyry; distinctive, compositionally and texturally heterogeneous, quartz-poor granite (Ko Vaya Granite); and A-type perthite granite and quartz syenite; aphyric intrusive rhyolite; and locally abundant hematite veins. Alteration, particularly potassic alteration, is widespread. A minority of minimally altered samples of granite and quartz syenite have the composition of trachyte and alkali rhyolite. The Late Jurassic Ko Vaya Suite is nonconformably overlain by the latest Jurassic Sand Wells Formation, approximately correlative with the Glance Conglomerate, the basal unit of the Bisbee Group. Ko Vaya Suite rocks and closely associated Middle Jurassic arc rocks have been affected by Cretaceous contraction and following episodes of Tertiary extension. Despite the resulting stratigraphic and structural obfuscations we infer that the Ko Vaya Suite is a manifestation of regional transtension that culminated with emplacement of the well-known Independence dike swarm of eastern California.