Dredged volcanic rocks from the southern oceans: theEltanincollection

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Abstract
Here we synthesise available data and present 22 new chemical analyses of dredged volcanic rocks, recovered by the USNS Eltanin, from a wide variety of morphotectonic settings in the southern oceans. Generally, these samples have undergone the variable affects and styles of postmagmatic alteration. The products of alkali volcanism were recovered from scattered localities in the back-arc environment of the Scotia Sea. Volcanic rocks from the Pacific-Antarctic and Chile Rises mainly consist of quartz and olivine normative ocean-floor basalts similar in composition to those from the East Pacific Rise and Galapagos Spreading Centre. To the north of the Eltanin Fracture Zone, a wider compositional diversity seems to occur, including FeTi-basalts and trachyandesite, than that observed from within and to the south of this fracture zone and on the Chile Rise. Reported values for the ratios K/Rb Rb/Sr La/Yb and Sr87/Sr86 for fresh material, gen'era"lly hint at a derivation of these basalts from a typically depleted mid-ocean ridge basalt source region. Both geophysical and petrological interpretations point to a possible analogy between segments of the PacificAntarctic Rise and youthful migrating-propagating accretion axes, in areas of intense axial primordial gas emission and hydrothermal activity, such as the East Pacific Rise between 4° and 17°S and the Galapagos Spreading Centre. From the Ross Sea, phonolites, hawaiites, nepheline-mugearite, and alkali basalt were recovered. With the exception of the alkali basalt from the Scott Seamounts, these samples show chemical affinities to both the Erebus and Hallett volcanic provinces. The phonolites from Scott Island and platform are similar in composition to those of Cape Crozier (Ross Island), all of which probably have evolved from a common magma type by fractional crystallisation of a basanite or alkali basalt of similar composition to those of the Balleny Islands. Though altered, basalts from the Broken Ridge and Hjort Trench share some compositional similarities and might be termed ferrobasalts. The data support the inference that the Broken Ridge is at least m part volcanic in nature, while a relationship between the Hjort Trench and the West Pacific island-arc system remains inconclusive. The rough topography between the southern end of Ninetyeast Ridge and the Indian-Antarctic Spreading. Centre may represent intraplate volcanic activity.

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