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The few geological and geophysical studies of the Lyra Basin at the western margin of the Ontong Java Plateau (OJP; Pacific Ocean) revealed that it is underlain by thicker than normal oceanic crust. The unusually thick oceanic crust is attributed to the emplacement of massive lava flows from the OJP. Dredging was conducted to sample the inferred OJP crust on the Lyra Basin but instead recovered younger extrusives that may have covered the older plateau lavas in the area. The Lyra Basin extrusives are alkalic basalts with (87Sr/86Sr)t = 0.704513–0.705105, (143Nd/144Nd)t= 0.512709–0.512749, εNd(t) = +3.0 to +3.8, (206Pb/204Pb)t = 18.488–18.722, (207Pb/204Pb)t= 15.558–15.577, and (208Pb/204Pb)t = 38.467–38.680 that are distinct from those of the OJP tholeiites. They have age-corrected (187Os/188Os)t = 0.1263–0.1838 that overlap with the range of values determined for the Kroenke-type and Kwaimbaita-type OJP basalts, but their (176Hf/177Hf)t = 0.28295–0.28299 and εHf(t) = +7.9 to +9.3 values are lower. These isotopic compositions do not match those of any Polynesian ocean island volcanics. Instead, the Lyra Basin basalts have geochemical affinity and isotopic compositions that overlap with those of some alkalic suite and alnöites in the island of Malaita, Solomon Islands. Although not directly related to the main plateau volcanism at 120 Ma, the geochemical data and modeling suggest that the origin of the Lyra Basin alkalic rocks may be genetically linked to the mantle preserved in the OJP thick lithospheric root, with magmatic contribution from the Rarotongan hotspot.

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