Lewis, S.D., Behrmann, J.H., Musgrave, R.J., and Cande, S.C. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 14128. COMPARISON OF THE ALTERATION AND PETROLOGY OF THE TAITAO RIDGETO THE TAITAO OPHIOLITE1Viktor Kurnosov,2 Randy Forsythe,3 Nancy Lindsley-Griffin,4 Boris Zolotarev,2 Georgy Kashinzev,5Vadim Eroshchev-Shak,2 Andrey Artamonov,2 and Oleg Chudaev6ABSTRACTSamples from the Taitao Ridge collected at Site 862, Leg 141, consist of a bimodal suite of subalkalic basalts and dacites torhyolites. Effusives and sheeted dikes from the Taitao Ophiolite consist of basalts, andesites, and dacites to rhyolites. The ophioliticbasalts 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, basaltsof 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 mainsecondary mineral. In contrast, the degree of alteration in the effusives and sheeted-dike complex of the Taitao Ophiolite ismoderate 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 primarymelt that shared some features with mid-oceanic ridge tholeiite basalts (MORB) and with intraplate-rise alkaline basalts. Thesubalkaline basalts and silicic volcanic rocks comprising the Taitao Ridge and the Taitao Ophiolite may have formed synchro-nously with Pliocene-Pleistocene arrival of the Chile Rise and Taitao Fracture Zone at the continental slope and shelf. During thecomplicated interval of trench-transform-trench to trench-ridge-trench triple junction evolution, the ophiolite originally generatedfrom melts within the Taitao Fracture/Transform's collision zone was uplifted and faulted against latest Miocene and Pliocenenear-trench plutons and the pre-Late Jurassic metamorphic rocks of the Taitao Peninsula. The uppermost effusives of the ophiolitesequence may have been altered about as much as the Taitao Ridge basalts while in an oceanic setting, and then subjected toadditional alteration at the time of emplacement on the continental margin. Alternatively, the higher state of alteration seen in theonshore ophiolitic samples may merely represent differences in the original magmatic/hydrothermal setting, such as the degree ofsediment burial, the rates and durations of volcanism, or the age of the samples.INTRODUCTIONThe Taitao Ophiolite lies on the Taitao Peninsula of southernChile, and the Taitao Ridge is located just offshore from the TaitaoOphiolite, south of the northward-migrating junction between theChile Rise and the Chile Trench (Fig. 1). Kaeding et al. (1990)hypothesized that the Taitao Ophiolite originated either through mag-matic activity associated with Pliocene development of an incipientfore-arc rift during collision of the Chile Rise (just south of the TaitaoFracture Zone) or through structural offscraping of a recently formedfragment of oceanic crust at the subduction zone. Offshore geophysi-cal data discussed by Cande and Leslie (1986) and Cande et al. (1987)support the hypothesis that the Taitao Ridge may represent an off-shore extension of the Taitao Ophiolite and that both may have beenobducted together as young buoyant oceanic crust located within theTaitao Fracture Zone impinged on the continental margin in the latePliocene or early Pleistocene. Comparison of the age and geochemi-cal signature of the Taitao Ridge oceanic basement with those of theonshore ophiolite is necessary to determine the origin and kinematicevolution of this zone. The goal of this paper is to present mineralogicand geochemical data that further define the similarities and differ-ences in petrogeochemistry and alteration of the igneous rocks of theTaitao Ridge and the Taitao Ophiolite.Lewis, S.D., Behrmann, J.H., Musgrave, R.J., and Cande, S.C. (Eds.), 1995. Proc.ODP, Sci. Results, 141: College Station, TX (Ocean Drilling Program)." Geological Institute, Pyzhevsky Per. 7, Moscow 109017, Russia.Department of Geography and Earth Sciences, University of North Carolina, Char-lotte, NC 29223, U.S.A.4 Department of Geology, 214 Bessey Hall, University of Nebraska, Lincoln, NE68588-0340, U.S.A.5 Institute of Oceanology, Krasikov St. 23, Moscow 117218, Russia.6 Far East Geological Institute, Vladivostok 690022, Russia.Site 862: Taitao RidgeSite 862 of Leg 141 of the Ocean Drilling Program (ODP) is locatednear the crest of the Taitao Ridge (Fig. 1). Three holes were drilled atSite 862. Hole 862A penetrated 22.1 m below seafloor (mbsf), Hole862B to 42.9 mbsf, and Hole 862C to 102.1 mbsf. The total sedimentthickness at Site 862 is about 23 m. Hole 862A contained only twovolcanic clasts at the top of the core, but Holes 862B and 862Crecovered a series of igneous lithologies to sub-bottom depths of 42.9and 102.1 m, respectively. Shipboard geochemical data indicate thatthe Taitao Ridge volcanic rocks have silica contents typical of basalts,dacites, and rhyolites. The basalts are subalkalic to tholeiitic, withphenocrysts of olivine, clinopyroxene, and Plagioclase; MgO contentsare 8.0-8.6 wt%, and K2O values less than 0.5 wt%.The recovered materials probably represent highly fractured, in-situ pillowed flows (Behrmann, Lewis, Musgrave, et al., 1992). Allsamples have at least some glassy, devitrified, or cryptocrystallinegroundmass. The coarsest samples are represented by intersertal tosubophitic textures, and none of the basaltic rocks has true ophitictextures typical of the centers of thick flow units or dikes. Thus, a thinor pillowed flow model is generally consistent with estimates of unitthickness and the observed textures.Taitao OphioliteThe Taitao Ophiolite, exposed on the Taitao Peninsula 20 km eastof Site 862 (Fig. 1), was resampled by R. Forsythe in 1992 followingLeg 141 in order to supplement the existing suites for a comparativeanalysis of the alteration mineralogy. Dacites and rare rhyodacites andrhyolites have been reported from the Taitao Ophiolite, together withbasalts and basaltic andesites (Forsythe et al., 1986; Kaeding et al.,1990). Radiometric ages from the Taitao Ophiolite are latest Mioceneto late Pliocene