Winterer, E.L., Sager, W.W., Firth, J.V., and Sinton, J.M. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 14315. PETROLOGY AND GEOCHEMISTRY OF IGNEOUS ROCKS FROM ALLISONAND RESOLUTION GUYOTS, SITES 865 AND 8661P.E. Baker,2 P.R. Castillo,3 and E. Condliffe2ABSTRACTAt Site 866 (Resolution Guyot), the volcanic basement (> 1620 mbsf and < 128 Ma) consists of a series of subaerial lava flowsseparated by scoriaceous, rubbly, and clay (iron-rich smectite) intervals. The lavas may be divided into three main groups onpetrographic and geochemical grounds. Because of alteration, the geochemical evidence depends mainly on relatively immobileelements, such as Ti, Zr, Nb, and the rare earth elements. The lowest group (>1727 mbsf) is picritic and alkalic (high Nb/Ti andNb/Zr). The middle group (1673-1727 mbsf), with megacrysts and phenocrysts of Plagioclase, olivine, and clinopyroxene, is moremarkedly alkalic (e.g., steep mid-ocean ridge basalt-normalized light rare earth element-enriched patterns). The upper group(above 1673 mbsf) includes basalts rich in Plagioclase megacrysts overlain by picrites, and is more tholeiitic (lower Nb/Ti, Zr/Ti,and flatter rare earth element patterns). At Site 865 (Allison Guyot), altered basaltic sills are intrusive into Albian clayey dolomiticlimestones between 830 and 870 mbsf. Several lines of evidence indicate that the sediment was unconsolidated at the time ofinvasion by the basalt (<lll Ma). Salitic clinopyroxenes, abundances and ratios of the less mobile incompatible elements, andpronounced light rare earth element-enrichment all point to a decidedly more alkalic affinity than was seen at Site 866. Resolutionand Allison guyots probably originated within the region of intense hotspot volcanism referred to as the South Pacific isotopic andthermal anomaly or SOPITA. The two guyots probably followed a similar tectonic pathway and may have passed over more thanone hotspot. Geochemical evidence (e.g., Nb/Zr and Zr/Ti) suggests that they have more in common with the Society-Austral(Tubuai) Islands than with islands to the east (e.g., Marquesas, Easter) or the west (Cook, Samoa): this is also consistent withlineaments derived by backtracking.INTRODUCTIONA vast area of the South Pacific Ocean, about 3000 km across, wasthe site of intense mid-plate volcanism during the Early Cretaceous.The location of this activity probably corresponds with the present-daySouth Pacific Superswell (McNutt and Fischer, 1987), which includesthe Society, Cook, and Austral archipelagos. It also belongs toSOPITA, the area of the so-called South Pacific isotopic and thermalanomaly (Staudigel et al., 1991). The lavas of these islands also formpart of the isotopically distinct Southern Hemisphere belt referred to asthe Dupal Anomaly (Hart, 1984). In the western part of the Mid-PacificMountains (MPM), a series of broad plateaus is surmounted by flat-topped seamounts. The age of the oceanic crust beneath the MPM isestimated to be about 119 to 130 Ma near Allison Guyot and may be asold as 154 Ma farther west, around Resolution Guyot (Sager, Winterer,Firth, et al., 1993). The guyots are capped by shallow-water limestonesof Barremian-Albian age (124-98 Ma). The Cretaceous seamounts ofthe MPM probably formed over the South Pacific Superswell and weretranslated northwestward on zig-zag pathways, determined by changesin plate motion, to their present location (Fig. 1). From the hotspotlineaments calculated by Duncan and Clague (1985), the seamounts ofSites 865 and 866 lie close to the Easter Island track, but would havepassed near the Marquesas and Society hotspots during their transit. Aperiod of rejuvenation and uplift may have accompanied their passageover these other hotspots. The ages calculated by Duncan and Clague(1985) are consistent with the location of the guyots over the Marque-sas or Society hotspots at 100 to 120 Ma.Site 866 is discussed first, as it represents a substantial sectionthrough volcanic basement and was the only instance where basementwas reached during Leg 143. At Site 865, on Allison Guyot, igneous1 Winterer, E.L., Sager, W.W., Firth, J.V., and Sinton, J.M. (Eds.), 1995. Proc. ODP,Sci. Results, 143: College Station, TX (Ocean Drilling Program).2 Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, United King-dom.Geological Research Division, Scripps Institution of Oceanography, University ofCalifornia, San Diego, La Jolla, CA 92093-0220, U.S.A.rocks are confined to a group of basaltic sills intruded into the sedi-ments. Some comparisons are made with a few samples from Site869, on a sediment apron extending southward from WodejebatoGuyot and Pikinni Atoll. The detailed petrology and geochemistry ofthe volcaniclastics from Site 869 are treated separately (see Janney etal., this volume).ANALYTICAL METHODSX-ray fluorescence (XRF) analyses for major and trace elements(excluding rare earth elements) were conducted at the Department ofEarth Sciences, Leeds University, using a wavelength-dispersive auto-mated Philips PW 1400 spectrometer. Major elements were deter-mined on fused glass beads and trace elements on pressed powder pel-lets. Accuracy and precision for major elements are estimated at betterthan 3% for Si, Ti, Fe, Ca, and K and 7% for Mg, Na, Al, Mn, and P:for trace elements above 10 ppm they are estimated at better than10%. Rare earth element (REE) determinations were performed on aVG Instruments PlasmaQuad 11+ inductively coupled plasma massspectrometer (ICPMS) at the Scripps Institution of Oceanography.Multiplier voltage was set at 5 kV and nebulizing gas flow rate at 0.2L/m. 115In was used as an internal standard, and calibration wasconducted using standard solutions of 5, 10, 50 and 100 ppb REE.Accuracy and precision of the analyses were monitored using the rockstandards AGV-1 and BCR-1. Rock powders (0.014 g) were dis-solved in clean Teflon vessels using l mL 2:1 mixture of concen-trated HF and HNO3 and then heated overnight over a hot plate at lowtemperature. The resulting solution was evaporated to dryness, resus-pended in a small amount of concentrated HNO3, and evaporated todryness, and finally diluted to a factor of 1000 in a 1 % HNO3 solutioncontaining 100 ppb 115In. Accuracy of the analyses, based on repeatedmeasurements of BCR-1 and AGV-1 standards, is better than 5%,especially for the light elements.Microprobe analyses were performed using a CAMECA SX-50instrument fitted with three wavelength dispersive