The Jurassic Gran Cañon Formation (Cedros Island, Baja California, Mexico) constitutes an unusually well preserved and exposed example of ancient backarc-basin fill. Petrofacies analysis conducted on tuffaceous sandstone and tuff samples from this formation complement and reinforce prior lithofacies interpretations, but with some modification. When temporal and spatial trends in petrographic data (detrital modes) are analyzed and compared to models based on data collected from Deep Sea Drilling Project and Ocean Drilling Program cores, the trends indicate a second, heretofore unrecognized, phase of backarc rifting. Basalt lavas interstratified with dacitic pyroclastic rocks of the primary volcanic lithofacies, previously interpreted to record the eruption of differentiated magmas at the climax of growth of the Gran Cañon island arc, are now as a result of this study considered to be the product of arc extension and rifting.

Our method of modal analysis uniquely combines the quantification of textural attributes of pyroclastic and epiclastic debris that reflect eruption style and magma composition, as well as the effects of reworking and mixing in marine settings. This study demonstrates that detailed petrographic analysis is useful in the interpretation of ancient volcaniclastic deposits suspected of having formed in backarc-basin settings.

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