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The Lyra Basin is believed to be a contiguous part of the Ontong Java Plateau (OJP), based on geophysical studies. Volcaniclastic rocks dredged at two sites in the Lyra Basin document another post-plateau episode of magmatism on the OJP; they are olivine-titanaugite-phyric alkali basalts with as much as ~30% modal phenocrysts. Lyra Basin basalts have compositions that vary from picritic (MgO ~22 wt%) to more evolved (MgO ~5 wt%) and have low SiO2 (41–46 wt%), high TiO2(2–4 wt%), and high Na2O + K2O (1–5 wt%) contents that are distinctly different from tholeiites that compose the main OJP. The 40Ar-39Ar weighted mean age of Lyra Basin basalts is 65.3 ± 1.1 Ma, determined using a single-grain laser fusion method of the ground-mass from the least altered alkali basalt and of biotite separates from differentiated samples. This age is interesting because it is much younger than the main stage of OJP formation (122 Ma) and no ca. 65 Ma alkaline basalts have been found previously near or on the OJP. Incompatible trace element modeling suggests that the volcanic rocks of the Lyra Basin may have been formed by a low degree of partial melting (~3%), predominantly at the garnet-lherzolite stability field from the same OJP mantle source preserved in its thick lithospheric root. However, major and trace elements and isotopic compositions can be better explained by magma mixing of Rarotongan alkali magma and magma derived from OJP-source mantle melting (12% partial melting at garnet stability field) in the ratio of 1:2. Although the trace element compositions of Lyra basalts can be reproduced by OJP-source mantle melting with or without contribution from the Rarotongan hotspot, the lower potassium content of the calculated Rarotongan hotspot-influenced melt is more compatible with that of an average composition of Lyra basalt. These results are consistent with previous reconstruction of the OJP path from 120 Ma to its present position, indicating that it may have passed over the Rarotongan hotspot at 65 Ma. In either case, the petrogenesis of Lyra Basin basalts highlights the role of the thick lithospheric root of the OJP in the late-stage development of the plateau. Additional evidence for episodic late-stage magmatic activity on the OJP helps to elucidate the magmatic evolution of the plateau and may provide insights into the origins of other large igneous provinces.

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