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Lewis S D Behrmann J H Musgrave R J and Cande S C Eds 1995 Proceedings of the Ocean Drilling Program Scientific Results Vol 141 28 COMPARISON OF THE ALTERATION AND PETROLOGY OF THE TAITAO RIDGE TO THE TAITAO OPHIOLITE1 Viktor Kurnosov 2 Randy Forsythe 3 Nancy Lindsley Griffin 4 Boris Zolotarev 2 Georgy Kashinzev 5 Vadim Eroshchev Shak 2 Andrey Artamonov 2 and Oleg Chudaev6 ABSTRACT 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

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