Isotopic evidence for a link between the Lyra Basin and Ontong Java Plateau

The few geological and geophysical studies of the Lyra Basin at the western margin of the Ontong Java Plateau (OJP; Pacifi c 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 fl ows 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, ε_Nd(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 ε_Hf(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 affi nity and isotopic compositions that overlap with those of some alkalic suite and alnoïtes 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.