Abstract
Aseismic ridges are variably broad linear topographic features built by submarine volcanic activity that are thicker and more buoyant than the surrounding oceanic crust. Together with oceanic plateaus they constitute a non-trivial ∼30% of the oceanic floor. As such, aseismic ridge subduction significantly affects subduction dynamics as well as deformation of the overriding plate. Indeed, it is considered to exert major controls on volcanic activity, the generation of juvenile Earth's crust and the formation of porphyry Cu-Au deposits. Previous studies claimed that magma chemistry changes associated with aseismic ridge subduction are mainly due to source processes, such as melting of the subducting ridge, melting of the edges of the oceanic plate following slab tear, or subduction erosion of the continental margin with subsequent partial melting of the scraped and subducted continental material. Here we explore the role of the subducted aseismic Carnegie ridge on modulating frontal arc magma chemistry in the Ecuadorian-southernmost Colombian frontal arc. We show symmetric changes of several geochemical indices (Th, Nd, Sm, Nb, Th/La, Ba/Th) and evolutionary paths (Na2O-SiO2 correlations) in frontal volcanic arc rocks with respect to the equatorial latitude, which coincides with the projection of the crest of the subducting Carnegie ridge at the frontal arc. We explore the systematic along-arc changes of these geochemical indices through geochemical modelling using a Monte Carlo approach, and conclude that the trends are controlled to a significant extent by intracrustal processes. In particular we argue that magmatic systems associated with the central part of the Ecuadorian volcanic arc evolve through fractionation, partial melting of crustal rocks and mixing, at average greater depths than those in the northern and southern sides. We speculate about possible links between the systematic along-arc geochemical changes and the subduction of the Carnegie ridge. Our findings highlight a new facet of the controls that are exerted by the overriding plate on the geochemical variability of arc magmas.
Highlights
A variant of the subduction of “normal” oceanic crust, subduction of topographic asperities occurring upon the oceanic plate is not uncommon and may have profound impacts on major geological processes
Based on previous studies on Ecuadorian arc magmatism (Bourdon et al, 2003; Bryant et al, 2006; Chiaradia et al, 2009b; Samaniego et al, 2010; Schiano et al, 2010; Chiaradia et al, 2014a-‐b; Ancellin et al, 2017), this can occur by a combination of three potential processes: (i) mixing of primitive, mantle-‐derived magmas with high-‐SiO2 slab melt components; (ii) mixing of primitive, mantle-‐ derived magmas with SiO2-‐rich partial melts of the crust; (iii) AFC of mantle-‐ derived parental magmas assimilating crustal rocks
Because the Na2O-‐SiO2 slopes are reasonably the result of intracrustal processes, such correlations suggest that symmetric changes of Ba/Th and Th/La in Ecuadorian frontal arc volcanoes are controlled somewhat by magmatic evolution in hot zone-‐type crustal processes (AFC + mixing)
Summary
A variant of the subduction of “normal” oceanic crust, subduction of topographic asperities (seamounts, oceanic plateaus, aseismic ridges) occurring upon the oceanic plate is not uncommon (e.g., present day Andean margin) and may have profound impacts on major geological processes.
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