Seventeen Million Years of Episodic Volcanism Recorded Within the Geologist Seamounts: Implications for Tectonic Drivers of Intraplate Volcanism

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AbstractUpwelling and decompression of mantle plumes is the primary mechanism for large volumes of intraplate volcanism; however, many seamounts do not correlate spatially, temporally, or geochemically with plumes. One region of enigmatic volcanism in the ocean basins that is not clearly attributable to plume‐derived magmatism is the Geologist Seamounts and the wider South Hawaiian Seamount Province (∼19°N, 157°W). Here we present new bathymetric maps as well as 40Ar/39Ar age determinations and major and trace element geochemistry for six remote‐operated vehicle recovered igneous rock samples (NOAA‐OER EX1504L3) and two dredged samples (KK840824‐02) from the Geologist Seamounts. The new ages indicate that volcanism was active from 90 to 87 Ma and 74 to 73 Ma, inferring that in conjunction with previous ages of ∼84 Ma, seamount emplacement initiated near the paleo Pacific‐Farallon spreading ridge and volcanism spanned at least ∼17 m.y. Geochemical analyses indicate that Geologist Seamount lava flows are highly alkalic and represent low‐degree partial mantle melts primarily formed from a mixture of melting within the garnet and spinel stability field. The ages and morphology inferred that the seamounts were likely not related to an extinct plume. Instead, we build upon previous models that local microblock formation corresponded to regional lithospheric extension. We propose that the microblock was bounded by the Molokai and short‐lived Kana Keoki fracture zones. Regional deformation and corresponding volcanism among the Geologist Seamounts associated with the microblock potentially occurred in pulses contemporaneous to independently constrained changes in Pacific Plate motion—indicating that major changes in plate vectors can generate intraplate volcanism.