ARE-RIFT TRANSITION VOLCANISM IN THE PUERTECITOS VOLCANIC PROVINCE, NORTHEASTERN BAJA-CALIFORNIA, MEXICO

The Neogene Puertecitos Volcanic Province of northeastern Baja Califor nia records a transition from are-related volcanic activity to rift vo lcanism associated with opening of the Gulf of California. The eastern Puertecitos Volcanic Province is divided into three volcanic sequence s based on mapping, petrology, and Ar-40/Ar-39 geochronology. The lowe st sequence comprises early to middle Miocene (20-16 Ma) arc-related a ndesitic lava flows, volcanic necks, and proximal pyroclastic and epic lastic deposits up to 400 m in thickness, with minor basaltic lava flo ws. Following the initiation of crustal extension in the region (11-6 Ma), synrift volcanism produced two rhyolitic sequences that discordan tly overlie the are-related rocks. The older synrift sequence (6.4-5.8 Ma) is composed of rhyolite domes and a series of pyroclastic hows up to 300 m thick The upper sequence (3.2-2.7 Ma) consists of ash-flow t uffs and pumicelapilli pyroclastic flows, collectively up to 200 m thi ck Minor andesite eruptions followed each episode of silicic synrift v olcanism. Synvolcanic faults produced topographic relief that controll ed deposition of the pyroclastic flows and caused gentler dips upsecti on. Rhyolite domes are aligned parallel to the predominant north-north west to north-northeast fault pattern. All three volcanic sequences ar e calc-alkaline. However, the synrift andesite is characterized by low er K2O, lower incompatible element concentrations, and less fractionat ion of light rare earth elements than the are-related basalt and andes ite. This suggests that the primary melts were more primitive for synr ift andesite than for the are-related rocks.