GEOCHEMISTRY OF VOLCANIC AND VOLCANICLASTIC ROCKS OF THE TRIASSIC-JURASSIC SAN HIPOLITO FORMATION, BAJA CALIFORNIA SUR, MEXICO

The San Hipolito Formation is a 2400 m thick section of Mesozoic marine sedimentary rocks that crops out in the Vizcaino Peninsula in Baja California Sur. The formation consists of chert, pelagic limestone, breccia and volcaniclastic sandstone and crops out in an area that covers almost 20 Km2 in Punta San Hipolito. The basal chert member conformably overlies pillow lavas. Interpillow areas are filled with dolomitized limestone that contains hemipelagic pelecypods that range from Middle to Late Triassic age. Individual pillows are up to 1m in diameter. The pillow lavas at Punta San Hipolito are most likely the top of an ophiolitic sequence, the La Costa Ophiolite, which is part of the Vizcaino Peninsula Ophiolite. The last includes from base to the top serpentinized harzburgite dunite, gabbro, isotropic gabbro, plagiogranite stocks and dikes, sheeted dikes and pillow lavas.

Both major and trace element geochemical analysis were carried out, and results were interpreted using discrimination diagrams to determine the tectonic setting of lavas and volcanic sandstone. Samples have high potassium, and fall in the basaltic andesite field in the TAS diagram. Th Yb, Ta and Hf abundances suggest a primitive island arc to MORB affinity. Spider diagrams as well as chondrite normalized REE plots, are similar to those from basalts formed in primitive island arcs. These results support the theory of other authors, like Moore. He proposed that the Vizcaino Peninsula Ophiolite was formed in the suprasubduction zone, perhaps in the backarc region of an island arc.