THE COMONDÚ ARC-GULF OF CALIFORNIA RIFT TRANSITION: GEOCHEMISTRY

New data from both the Comondú arc and the early phases of the Gulf of California rift show that tectonic setting has a significant impact on magma composition. The Comondú arc is an extensional continental arc that was active between 20 and 11 Ma in the region where the Gulf of California would eventually open; continental rifting initiated at 11 Ma, when subduction ended and the Pacific-North American plate boundary jumped inboard, to occupy the thinned and thermally-weakened Comondú arc axis.

Our new whole rock geochemical data (from Bahía Concepción, Santa Rosalia and La Trinidad) show that arc and rift volcanics are dramatically different from one another. For example, while nearly all rocks (both arc and rift) are calc-alkaline in the AFM diagram, rift-related samples are shifted to higher FeOt and extend to greater alkali contents. Trace elements are also distinctive. Most arc-related samples have Sr/Y>40 and Y<20, while most rift-related volcanics have Sr/Y<10 and Y>30; some arc samples from Santa Rosalia and forearc samples from Concepcion are transitional with respect to Sr/Y but are still distinct from rift samples in having lower FeOt in the AFM projection. These contrasts are consistent with the arc-related samples being differentiated at greater depths. At La Trinidad, forearc samples are also distinct from arc samples in having higher FeOt and lower CaO/Al₂O₃ at a given MgO, which indicates distinct P-T conditions for the forearc magma plumbing system.

Also intriguing are volcanic rocks referred to as “bajaites” (Sr/Y>100, Sr>1500 ppm). Elsewhere in Baja, these have been attributed to post-subduction opening of a slab window; our new data yield Pliocene examples of these at La Trinidad. But at La Trinidad, some volcanics of clear bajaite-affinity erupted within the arc and forearc. This range in Sr/Y might indicate variations in depths of magma storage. In any case, it is clear that not all bajaites indicate slab window opening.

New isotopic and mineral composition data are needed to test these hypotheses and so better understand how tectonics processes affect volcanic rock geochemistry.