A Comprehensive Geochronology Study of the Rio Grande Rise: Evidence for
Prolonged On and Off-ridge Volcanism
Abstract
Expedition NBP1808 on the R/V Nathan B. Palmer completed 32 dredges
between October and December, 2018 from locations across the Rio Grande
Rise (RGR)—a largely unstudied oceanic plateau on the South American
plate—and several seamounts located between RGR and the Mid-Atlantic
Ridge (MAR). Eighteen samples from 10 dredge locations on RGR were dated
to better understand the geochronological history of this large igneous
province and to provide clues to its relationship with the Walvis Ridge
and Tristan-Gough hotspot(s) on the conjugate African plate.
40Ar/39Ar results from plagioclase
separates (and one biotite) show a prolonged emplacement history
throughout RGR ranging from ~84 to 48 Ma. Ages in
general decrease towards the MAR in accord with plate motions showing
that RGR as a whole was emplaced over at least several Ma and not as a
single pulse like some other oceanic plateaus. Using the recently
published tectonic reconstruction of Sager et al., most volcanism in the
NW and NE sectors on RGR was emplaced off-axis while that in the SE
sector was erupted on-axis. This suggests that the plume source for RGR
changed from more intraplate to more ridge-centered as the system
evolved through time. There is evidence of a possible reversed age
progression in the NE RGR which could provide evidence for micro-plate
activity that has been suggested in this region, though more ages are
needed to confirm this trend. Geochemistry studies are ongoing and will
be used in the future to better understand the eruptive processes.
Additional age analyses are also ongoing and will focus on the other
dredge locations throughout RGR as well as the seamounts to complete the
geochronological picture of the emplacement of RGR.