Rapid transition from shield to post-erosional volcanism at O’Higgins guyot, Juan Fernández Ridge, Pacific SE

Abstract:

Post-erosional volcanism is a common, usually low volume stage in the archetypical evolution of oceanic islands and seamounts. Magmas related to this transient rejuvenated stage are mostly alkaline with an enriched source different that the average mantle plume that fed the shield stage. Being scarce even in oceanic islands, only a few rock samples with geochemical features of post-erosional volcanism have been recovered from seamounts worldwide, although usually with a great uncertainty about the eruptive age. We report on the finding of lava flows with a fresh morphology as shown by high resolution bathymetry, just on top of the O’Higgins guyot, Juan Fernández Ridge in the Pacific South East, ca. 560 mbsl. Basanites recovered are quiet similar to other post-erosional lavas recognized along the Juan Fernández Ridge and were dated in ca. 8.1 Ma (⁴⁰Ar/³⁹Ar groundmass), slightly younger than the arguably youngest section of the partially eroded seamount (8.4-9.3 Ma), which carries the geochemical signature of the main shield stage. Such a rapid transition calls for a process explaining both the summit erosion of the shield volcano at the Miocene sea level, and the inception of the post-erosional volcanism with a contrasting signature pointing to a different source below the eastward moving (at ca. 8 cm/yr) Nazca Plate. Paleodepth inferred from the height of O’Higgins guyot (3,205 mbsl) is less than expected for normal seafloor (predicted from isostatic uplift of a reheated lithosphere) and thus a thermal anomaly of the underlying mantle is implied. Flexural loading due to a fast growing of the former shield volcano may have triggered decompression of this metasomatized and heterogeneous mantle source with rapid ascent assisted by the local structural conditions later achieved at erosional stage. This could be a general model and thus an explanation for the protracted shield to rejuvenated post-shield transition observed along the Juan Fernández Ridge.