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Samples from the Taitao Ridge collected at Site 862, Leg 141, consist of a bimodal suite of subalkalic basalts and dacites to rhyolites. Effusives and sheeted dikes from the Taitao Ophiolite consist of basalts, andesites, and dacites to rhyolites. The ophiolitic basalts are more vesicular than basalts from the Taitao Ridge and are dominated by plagiophyric and pyroxene-plagiophyric textures. Basalts from the Taitao Ridge are dominated by plagioclase-clinopyroxene (±olivine) vitrophyric textures. Geochemically, basalts of both suites fall within the alkaline olivine series, but samples from the Taitao Ridge exhibit more tholeiitic characteristics. The degree of alteration of basalts from Site 862 on the Taitao Ridge is low (to rarely moderate) with smectite as the main secondary mineral. In contrast, the degree of alteration in the effusives and sheeted-dike complex of the Taitao Ophiolite is moderate to strong, resulting in the development of greenschist facies minerals. The basalts and silicic volcanic rocks in each suite are comagmatic and may have been derived from a subalkaline primary melt that shared some features with mid-oceanic ridge tholeiite basalts (MORB) and with intraplate-rise alkaline basalts. The subalkaline basalts and silicic volcanic rocks comprising the Taitao Ridge and the Taitao Ophiolite may have formed synchronously with Pliocene-Pleistocene arrival of the Chile Rise and Taitao Fracture Zone at the continental slope and shelf. During the complicated interval of trench-transform-trench to trench-ridge-trench triple junction evolution, the ophiolite originally generated from melts within the Taitao Fracture/Transform's collision zone was uplifted and faulted against latest Miocene and Pliocene near-trench plutons and the pre-Late Jurassic metamorphic rocks of the Taitao Peninsula. The uppermost effusives of the ophiolite sequence may have been altered about as much as the Taitao Ridge basalts while in an oceanic setting, and then subjected to additional alteration at the time of emplacement on the continental margin. Alternatively, the higher state of alteration seen in the onshore ophiolitic samples may merely represent differences in the original magmatic/hydrothermal setting, such as the degree of sediment burial, the rates and durations of volcanism, or the age of the samples.

Comparison of the Alteration and Petrology of the Taitao Ridge to the Taitao Ophiolite