Age and Geochemistry of Basement and Alkalic Rocks of Malaita and Santa Isabel, Solomon Islands, Southern Margin of Ontong Java Plateau

Abstract

Geochemical and 40 Ar—39 Ar studies of the Malaita Older Series and 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 Malaita Older Series and Sigana lavas fall within limited ranges [(87Sr/86Sr)T= 0.70369–0.70423, ENd(T)= + 3.7 to +6.0, and 206Pb/204Pb = 18.25–18.64] virtually indistinguishable from those found in the three OJP basement drill sites as far as 1600 km away, indicating a uniform hotspot-like mantle source with a slight ‘Dupal’ signature for the world's largest oceanic plateau. Three chemical types of basalts are recognized, two of which are equivalent to two of the three types drilled on the plateau, and one with no counterpart, as yet, on the plateau; the chemical data indicate slightly different, but all high, degrees of melting and slight variation in source composition. All but one of the 40Ar-39Ar plateau ages determined for Malaita 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, suggesting that two short-lived, volumetrically important plateau-building episodes took place ∼30 m.y. apart. Aside from OJP lavas, three isotopically distinct suites of alkalic rocks are present. The Sigana 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 displays a distinct ’HIMU‘ -type isotopic signature [206Pb/204Pb ≈20.20, (87Sr/86Sr) T ±0.7032, εNd(T) ≈±4.4], possibly representing small-degree melts of a minor, less refractory component in the OJP mantle source region. The Younger Series in southern Malaita has an 40Ar-39Ar age of 44 Ma and isotopic ratios [εNd(T)=-0.5 to +1.0, (87Sr/86Sr)T =0.70404–0.70433, 206Pb/204Pb = 18.57–18.92] partly overlapping those of the ‘PHEM’ end-member postulated for Samoa, and those 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 ∼44 Ma. The North Malaita Alkalic Suite in northernmost Malaita is probably of similar age, but has isotopic ratios [(87Sr/86Sr) T ≈0.7037, εNd(T) ≈+4.5, 206pb/204pb ≈18.8) resembling those of some OJP basement lavas; it may result from a small amount of melting of aged plateau lithosphere during the OJP's passage over these hotspots. Juxtaposed against OJP crust in Santa Isabel is an ∼62–46-Ma ophiolitic (sensu lato) assemblage. Isotopic and chemical data reveal Pacific-MORB-like, backarc-basin-like, and arc-like signatures for these rocks, and suggest that most formed in an arc—backarc setting before the Late Tertiary collision of the OJP against the old North Solomon Trench. The situation in Santa Isabel appears to provide a modern-day analog for some Precambrian greenstone belts.