O´Connor, J. M., Hoyer, P. A., Haase, K. M., Regelous, M., Homrighausen, Stephan, Geldmacher, Jörg and Geissler, W. (2020) Is the Rio Grande Rise a Large Igneous Province or a Microcontinent?. [Talk] In: Goldschmidt 2020 Virtual Conference. , 21.06.-26.06.2020, - . DOI 10.46427/gold2020.1962.

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Abstract

The Rio Grande Rise (RGR) isone of the most prominent bathymetric features in the South Atlantic Ocean.It has long been assumed that this massive oceanic plateau is a late-Cretaceous Large Igenous Province related to the Tristan-Gough mantle plume. But recent studies suggest that it might be a fragment of detached continental marginthathas been preserved asa'microcontinent'embedded in plume-influenced oceanic crust[1, 2].Here we present the first geochemical results from a combined geophysical and petrological cruise(MSM-82)carried out in spring 2019. We measured major and traceelement concentrations of 47whole rock samplesdredged from the flanks of a long rift valleythat cuts throughthe eastern (E) and western (W) parts of the RGR. Our preliminary data showthat theERGR differs in compositionfrom the WRGR. The latter rangesfrom trachybasaltto trachyandesite, while the former mainly consists of alkalinebasalt.Geophysical results show that the WRGR is underlain by thicker crust so the more evolved compositionsmight be due to longer residence times for melts in the associated magma chambers. Light rare earth elementenrichment[(La/Sm)N: 1.5 to 6.5]in all samples indicatesan enrichedmagmasource, consistent with the involvment of a mantle plume during the formation of the RGR. However, variable ratios ofimmobileincompatible trace elements (e.g. Nb/Zr) imply aheterogenoussource region.All samples have high Nb/Th (6.4 –14.3) similar to oceanic basalts, indicatingthat they have not undergone significant contamination bya continental component with low Nb/Th.Thus, our initial findingsare consistent withaplume-related magmatic origin for the RGRratherthana microcontinent.

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