AGE AND GEOCHEMISTRY OF BASEMENT AND ALKALIC ROCKS OF MALAITA AND SANTA-ISABEL, SOLOMON-ISLANDS, SOUTHERN MARGIN OF ONTONG-JAVA PLATEAU

Geochemical and (40) Ar-(39) Ar studies of the Malaita Older Series an d Sigana Basalts, which form the basement of Malaita and the northern portion of Santa Isabel, confirm the existence of Ontong Java Plateau (OJP) crust on these islands. Sr, Nd, and Pb isotopic ratios of Malait a Older Series and Sigana lavas fall within limited ranges [(87) Sr/(8 6) Sr) (T) = 0.70369-0.70423, epsilon(Nd)(T) = + 3.7 to + 6.0, and (20 6) Pb/(204) Pb = 18.25-18.64] virtually indistinguishable from those f ound in the three OJP basement drill sites as far as 1600 km away, ind icating a uniform hotspot-like mantle source with a slight 'Dupal' sig nature for the world's largest oceanic plateau. Three chemical types o f basalts are recognized, two of which are equivalent to two of the th ree types drilled on the plateau, and one with no counterpart, as yet, on the plateau; the chemical data indicate slightly different, but al l high, degrees of melting and slight variation in source composition. All but one of the (40) Ar-(39) Ar plateau ages determined for Malait a Older Series and Sigana Basalt lavas are identical to those found at the distant drill sites: 121.3 +/- 0.9 Ma and 92.0 +/- 1.6 Ma, sugges ting that two short-lived, volumetrically important plateau-building e pisodes took place similar to 30 m.y. apart. Aside for OJP lavas, thre e isotopically distinct suites of alkalic rocks are present. The Sigan a Alkalic Suite in Santa Isabel has an (40) Ar-(39) Ar age of 91.7 +/- 0.4 Ma, the same as that of the younger OJP tholeiites, yet it displa ys a distinct 'HIMU'-type isotopic signature [(206) Pb/(204) Pb approx imate to 20.20, ((87) Sr/(86) Sr) T approximate to 0.7032, epsilon(Nd) (T) approximate to + 4.4], possibly representing small-degree melts of a minor, less refractory component in the OJP mantle source region. T he Younger Series in southern Malaita has an (40) Ar-(39) Ar age of 44 Ma and isotopic ratios [epsilon(Nd)(T) = - 0.5 to + 1.0, ((87) Sr/(86 ) Sr) (T) = 0.70404-0.70433, (206) Pb/(204) Pb = 18.57-18.92] partly o verlapping those of the 'PHEM' end-member postulated for Samoa, and th ose of present-day Rarotonga lavas; one or both of these hotspots may have caused alkalic volcanism on the plateau when it passed over them at similar to 44 Ma. The North Malaita Alkalic Suite in northernmost M alaita is probably of similar age, but has isotopic ratios [((87) Sr/( 86) Sr) (T) approximate to 0.7037, epsilon(Nd)(T) approximate to + 4.5 , (206) Pb/(204) Pb approximate to 18.8) resembling those of some OJP basement lavas; it may result from a small amount of melting of aged p lateau lithosphere during the OJP's passage over these hotspots. Juxta posed against OJP crust in Santa Isabel is an similar to 62-46-Ma ophi olitic (sensu lato) assemblage. Isotopic and chemical data reveal Paci fic-MORB-like, backarc setting before the Late Tertiary collision of t he OJP against the old North Solomon Trench. The situation in Santa Is abel appears to provide a modern-day analog for some Precambrian green stone belts.