Cretaceous intraplate volcanism of Govorov Guyot and formation models of the Magellan seamounts, Pacific Ocean

Abstract

ABSTRACT The OIB-type volcanic rocks of Govorov Guyot in Magellan Seamounts (Pacific Ocean) include basalts, trachybasalts, basaltic trachyandesites, and trachyandesites dated by the 40Ar-39Ar plateau from 121 ± 2.8 to 98.5 ± 1.4 Ma. Some rocks bear amphibole-spinel wehrlite xenoliths and clinopyroxene-amphibole (Cpx-Amp) mantle vein fragments in xenoliths and xenocrysts, which are remnants of metasomatized oceanic lithosphere. The pressure and temperature crystallization for pargasitic amphibole were the highest (1170–1130°C and 2.5–1.6 GPa, 73–53 km depth) in wehrlite peridotite and the lowest (1.6–1.1 GPa, shallower than 52 km, and 1070–980°C) in Cpx-Amp metasomatic mantle veins. Ti-bearing Amp phenocrysts and microlites crystallized from the parent melts of basaltic rocks at 1060–910°C and 1.2–0.4 GPa (40–15 km). The ascending basaltic melts lost H2O from 8.6 ± 1.2 to 2.6 ± 0.4 wt% upon decompression. Amphibole mantle xenocrysts became replaced by Ti-magnetite- and rhönite-bearing mineral assemblages in intermediate magma chambers within oceanic crust (3–7 km). Pargasitic Amp in wehrlite and metasomatic mantle veins may have crystallized at different depths from hydrous (6–4.5 ± 0.7 wt% H2O) mafic melts that percolated through peridotite. The sources of heat and metasomatic agents (silicate melts and/or fluids enriched in volatiles) most likely had no relation to the Pacific Plate subduction but may be associated with the Southern Pacific Superplume in the South Pacific Thermal and Isotopic Anomaly region, or may be located at the Lithosphere-Asthenosphere Boundary (LAB) level in the plume absence. Repeated events of Cretaceous intraplate volcanism in the Magellan guyots can be explained in terms of plume (hotspot) activity or decompression partial melting of enriched or metasomatized asthenosphere and/or metasomatized oceanic lithosphere caused by Pacific Plate deformation and fracturing from the LAB level.