#1EGU2020-12777 First Cenozoic ages from the Roraima's region landscape (Northern Brazil): insights from hematite and goethite (U-Th)/He dating Caroline Sanchez', Cécile Gautheron¹, Rosella Pinna-Jamme¹, Frédéric Haurine1, Jean-Yves Roig 2, and Renaud Coueffë 2 1Université Paris-Sud, Géosciences Paris-Sud (GEOPS), France ([email protected]) 2Bureau de Recherches Géologiques et Minières (BRGM) RECA Géosciences pour une Terre durable brgm General GEOSCIENCES EGU Geely 2020 GEOPS PARIS SUD CC 30 BY ND#2EGU2020-12777 DEM of the Guyana Shield (Roraima plateau extent in blue line) Altitude (m) N Jurassic 1000 500 0 -500 -1000 20 km Paleocene Guyana shield paleo-surfaces Altitude (m) -123-47 147 74 74 -98 98 114 114 137 137-180 180-274 274-351 351 - 487 487 648 648-919 1919-2872 Early Cretaceous S CC BY The relationship between age and altitude of preserved surfaces has largely been discussed since the mid XIXth century (Davis, 1889; King, 1953, Vasconcelos and Carmos, 2018) with a specific emphasis on craton shields under tropical climates. Indeed, is there a positive correlation between paleosurfaces altitude and their age? Scenarios have been proposed for the Guyana Shield, at the North of the Amazon, assigning an age for different paleosurfaces at specific altitudes. In this study we focused on the large extended Roraima plateau developed on Paleoproterozoic rocks which age is supposed to be Jurassic at around 1000 masl by dating the duricrusts on the surface by (U- Th)/He. (Age of the paleosurfaces in the Roraima Brazilian region after Schaefer et al., 1996) 2#3EGU2020-12777 Crotago A INGARICO NA DOCTORAIMA PP46av Ale UIR gara 66 d PP3rs p3ra 156 d Au RORAIMA AM PP Pav LIL PP3ru Northern Brazil Roraima study zone In this study, two sites were sampled, the first one named Sierra Lilas (LIL) where developed on ~1.9 Ga sandstone from the on Roraima group sandstone at 835 m on a remnant hill. From the second zone, Uiramuta (UIR), 3 samples were taken on 1.7 Ga intrusives between at 835 m and around 950 m high. CC ④ BY Uiramuta (N 71 Mo -10 km PP34T5 (S 424 O MARCOS TO PR341 sa Sierra UIR-04 PP34Tsa Now Mo Lilas 1400 Sunahu UIR-03 PP3130 1200 LIL-04 PP3-su 1000 J AP3:3pp 120 ab AI SÃO MARCO 800 100000 Cab 46 dt 600 Roraima group sandstone Surumu intrusive group Saracura intrusive suite 400 200 Uatuma Gp. 200 Arai Fm Upp Mb Suite - Avanavero Arai Fm-Upp Mb 0 Pedro Pintada intrusive group Topographic profile of the study site and altitudes. Cenozoic cover North Roraima geological map (Boa Vista : 1/200000, CPRM) Suapi Gp 3#4EGU2020-12777 LIL 04 F ii 5 cm Intensity 500 450 400 350 300 250 200 150 100 50- iii Methods Ides characterization and (U-Th)/He dating CC ④ BY UIR 04 B iii 5 cm L4Eiii1b сминим Before dating the samples the mineralogy has been characterized on every visible generation first at macro scale (apparent density, colour and morphology) and micro scale (SEM and XRD). Here, the samples present various petrologies mainly pisolithic (LIL) and iron massive banded iron types (UIR). The XRD analysis revealed that the LIL sample are hematite and UIR sample are mainly goethite. The (U-Th)/He dating method is based on the accumulation of 4He, in the mineral struct to radioactive decay from mainly U and Th elements. For this purpose we measure the daughter and the father element concentration to obtain the age. We replicate several aliquots on the same generation. 0 eerium 1750 buil 0 5 10 15 20 25 30 35 40 45 Angle 20 Characterization SEM + XRD He Extraction LASER-QUAD-MS + U-Th Measurements ICP-MS#5EGU2020-12777 LIL 04 A i 1b la 2.5 cm Kaolinite dating (EPR dating in process method, see Allard et al., 2018) UIR 04B i 16- 12- Several generations LIL 04 age compilation Early Miocene n = 47 8 Late Eocene 0- 12- 8 4- 5 cm 0. Jurassic 0 20 40 60 80 100 120 140 160 180 200 (U-Th)/He age (Ma) UIR 04 age compilation n = 46 Late Miocene UIR 03Ciii All the ages obtained on that simple form a Oligocene single distribution peak 5 cm UIR 03 age compilation n = 7 CC BY U3C 0 20 40 60 80 100 120 140 160 180 200 -2 20 40 60 80 100 120 140 160 180 200 (U-Th)/He age (Ma) (U-Th)/He age (Ma)#6EGU2020-12777 Roraima's plateau age(s) The ages obtained from the Roraima plateau are principally Cenozoic ranging from late Miocene to Eocene ( with the exception of one aliquot indicating a Jurassic age), younger than proposed in the literature. The ages are robust and consistent for each sample. However there is no positive correlation between the altitude and the (U-Th)/He ages of the sample. : Climatic implications have been demonstrated different climates can explain the diversity of iron oxide mineralogies. Indeed the dehydrated hematite form preferentially under warm climate and goethite hydrated mineral are related to cooler and humid climatic conditions (Tardy et al., 1991). The strict differences in terms of iron oxide mineralogy, age distribution difference and the U-Th concentrations strongly suggest different processes in forming the duricrusts as topographic and lithologic which are still under investigation. • • • . • Bibliography: CC ④ BY Pinna- Allard, T., Gautheron, C., Riffel, S. B., Balan, E., Soares, B. F., Jamme, R., ... & Do Nascimento, N. (2018). Combined dating of goethites and kaolinites from ferruginous duricrusts. Deciphering the Late Neogene erosion history of Central Amazonia. Chemical Geology, 479, 136-150. Davis, W. M. (1899). The geographical cycle. The Geographical Journal, 14(5), 481-504. King, L. C. (1953). Canons of landscape evolution. Geological Society of America Bulletin, 64(7), 721-752. Schaefer, C. E. G. R., & Dalrymple, J. (1995). Landscape evolution in Roraima, North Amazonia: planation, paleosols and paleoclimates. Zeitschrift für Geomorphologie, 39(1), 1-28. Tardy, Y., Kobilsek, B., & Paquet, H. (1991). Mineralogical composition and geographical distribution of African and Brazilian periatlantic laterites. The influence of continental drift and tropical paleoclimates during the past 150 million years and implications for India and Australia. Journal of African Earth Sciences (and the Middle East), 12(1-2), 283-295. Vasconcelos, P. M., & Carmo, I. D. O. (2018). Calibrating denudation chronology through 40Ar/39Ar weathering geochronology. Earth-Science Reviews, 179, 411-435. Acknowledgments Thanks to everyone who helped to this work ; Thierry Allard, Benoit Baptiste, Claire Boukhari, Ludovic Delbes, Alexis Derycke, Karina Marques, Maximilien Mathian, Bruno Tourlière and Philippe Sarda 6