THE BAHIA TORTUGAS FAULT OPHIOLITIC MELANGE, BAJA CALIFORNIA SUR, MEXICO: A CORRELATIVE TO CALIFORNIA-OREGON OPHIOLITE-ARC-FOREARC ASSEMBLAGES?

South of the California Coast Ranges, the Vizcaino Peninsula, Baja California Sur provides the best view of Mesozoic forearc basement along the outboard edge of the southwest Cordillera. Relatively intact ophiolite complexes of Late Triassic and Middle Jurassic age are well-known from the Vizcaino region. A, lesser known, serpentinite matrix ophiolitic mélange is well-exposed within an ~10 km-long belt in the northern Vizcaino Peninsula where it is bounded by the northwest-trending Late Miocene-Early Pliocene Bahia Tortugas Fault. The mélange is interpreted as a serpentinite protrusion related to transpression along the fault. It could represent local basement to the adjacent Late Jurassic-Early Cretaceous Eugenia Formation, part of the Mesozoic forearc sequence. The talc-serpentinite matrix of the mélange has a weak subvertical foliation that wraps around the mélange blocks. Mélange blocks range in size up to approximately 100 x 250 m, average 5-15 m² and occur in poorly defined zones where specific block types can dominate. Mélange block types include serpentinized harzburgite-dunite, gabbro, comb-textured hornblendite/diorite, pyroxenite, amphibolite, plagiogranite, metabasalt and meta-limestone. Although, there is no internal stratigraphy, the basic elements of a classic ophiolite are present and local semi-coherent zones exist. The semi-coherent zones occur dominantly in the southeast third of the mélange, where plagiogranite is associated with comb-textured hornblendite/diorite. Zircon analysis from plagiogranite blocks, at two localities, yields U-Pb crystallization ages of approximately 188.7±1.5 and 195.0±5.3 Ma. The ages are tentatively interpreted as the crustal formation age of the ophiolite suite. The zircon ages provide the first evidence for early Jurassic ophiolite basement in the region. Whole rock major and trace element geochemistry of metabasalts reveal fractionated compositions (Mg# 54-38). The immobile trace element patterns are consistent with a transitional MORB to island arc tholeiitic affinity. Possible correlatives to the Bahia Tortugas Fault ophiolitic mélange occur in the western Sierran foothills and Klamath Mountains of California-Oregon.