MAFIC HYALOCLASTITES AND PILLOW BASALTS OF THE BISBEE BASIN, SE AZ: RECORD OF LATE JURASSIC TECTONICS IN THE BORDER RIFT

A sequence of Upper Jurassic sea floor basalts exposed in the Chiricahua mountains of southeastern Arizona demonstrates that continental rift processes were active in Late Jurassic time. Detailed stratigraphic analysis documents three pulses of basaltic volcanism, each of which produced a vent-proximal complex of pillow basalts and lava flows surrounded by an apron of pillow breccias and hyaloclastites on the vent flank. At the distal edges of the volcanic complexes, hyaloclastites are intercalated with shales that contain Lower Kimmeridgian ammonites, securing the Late Jurassic age of the volcanism. The basalts are overlain by classic Bisbee basin stratigraphy, which represents filling of a continental rift basin during marine transgression and the end of thermal subsidence.

Pillows and lavas of the suite are presumably mafic (SiO₂=50.0-55.6%; MgO=3.6-7.3%) but have interacted with sea water during eruption, resulting in high volatile content (4.4-8.3%) and alkalis (2.8-6.0% Na₂O; 1.5-6.3% K₂O). Weakly correlated major element compositions suggest that seafloor alteration has changed the concentration of many, if not most, elements in these rocks. However, the immobile elements Ni, Cr, Zr, Nb, Y, and TiO₂ form coherent trends. For instance, there is no correlation between MgO and Ni or Cr, although Ni (36-108 ppm) and Cr (68-140 ppm) are positively correlated. Zr, TiO₂, Y and Nb (Zr: 125-229 ppm; TiO₂: 1.3-2.0%; Nb: 10-30 ppm; Y: 24-33 ppm) are positively correlated with each other. Each of the three complexes contains a range of compositions, suggesting that the vents were fed by magma chambers that were frequently replenished with fresh magma. Fractional crystallization models predict extensive fractionation to produce the range of trace element compositions. Thus, the stratigraphy, lithology, and geochemistry of this basalt complex all support the hypothesis that the lavas were erupted in a submarine rift environment from magma chambers that behaved much like mid-ocean ridge chambers. The Chiricahua pillow basalts comprise one of several examples of Late Jurassic mafic magmatism in the Border rift of northern Mexico and the southwestern US.