Petrography of volcaniclastic rocks in intra-arc volcano-bounded to fault-bounded basins of the Rosario segment of the Lower Cretaceous Alisitos oceanic arc, Baja California, Mexico

The Rosario segment of the Early Cretaceous Alisitos oceanic magmatic arc in Baja California displays a record ofarc-axis sedimentation and volcanismthat iswell preserved in outcrops within a southern volcano-bounded anda northern fault-bounded basin that flanked an intervening subaerial edifice. This record includes volcanic andvolcaniclastic rocks that range from felsic to mafic in composition. Volcaniclastic/tuffaceous sandstone samplesfromtwo previously published measured sections aremainly composed of volcanic clastswithmoderate plagioclasecontent. Locally quartz and/or potassium feldspar are present in trace to moderate amounts. The proportionsof volcanic lithic types exhibiting vitric, microlitic, lathwork, and felsitic textures are highly variable withno distinct stratigraphic trends, likely as a function of the mixed styles of eruption and magma compositionsthat produced pyroclasts, aswell as erosion-produced epiclastic debris. The volcaniclastic fill of the basins is consistentwith an oceanic arc setting, except for the relatively high felsitic volcanic lithic content, likely associatedwith subaerial, as opposed to the more common submarine felsic magmatism associated with arc extension inoceanic settings. There are no major differences in compositional modes of tuff and sandstone between thefault-bounded and volcano-bounded basin strata, even though they exhibit distinctly different volcaniclastic facies.This suggests that proximal arc-axis basins of varying types around a singlemajor subaerial edifice provide afaithful record of volcanic trends in the arc segment, regardless of variation in transport and depositionalprocesses.