Arc-rift transition volcanism in the Puertecitos Volcanic Province, northeastern Baja California, Mexico
The Neogene Puertecitos Volcanic Province of northeastern Baja California records a transition from arc-related volcanic activity to rift volcanism associated with opening of the Gulf of California. The eastern Puertecitos Volcanic Province is divided into three volcanic sequences based on mapping, petrology, and ^(40)Ar/^(39)Ar geochronology. The lowest sequence comprises early to middle Miocene (20–16 Ma) arc-related andesitic lava flows, volcanic necks, and proximal pyroclastic and epiclastic deposits up to 400 m in thickness, with minor basaltic lava flows. Following the initiation of crustal extension in the region (11–6 Ma), synrift volcanism produced two rhyolitic sequences that discordantly overlie the arc-related rocks. The older synrift sequence (6.4–5.8 Ma) is composed of rhyolite domes and a series of pyroclastic flows up to 300 m thick. The upper sequence (3.2–2.7 Ma) consists of ash-flow tuffs and pumice-lapilli pyroclastic flows, collectively up to 200 m thick. Minor andesite eruptions followed each episode of silicic synrift volcanism. Synvolcanic faults produced topographic relief that controlled deposition of the pyroclastic flows and caused gentler dips upsection. Rhyolite domes are aligned parallel to the predominant north-northwest to north-northeast fault pattern. 
 
All three volcanic sequences are calc-alkaline. However, the synrift andesite is characterized by lower K_(2)O, lower incompatible element concentrations, and less fractionation of light rare earth elements than the arc-related basalt and andesite. This suggests that the primary melts were more primitive for synrift andesite than for the arc-related rocks.