Metasomatized mantle xenoliths from Canastra-1 kimberlite, southern edge of the São Francisco Craton

GOMES BRAGA, LUISA; JALOWITZKI, TIAGO; PRES ROCHA, MARCELO; GERVASONI, FERNANDA; MAZZUCCHELLI MAURIZIO; GIOVANARDI, TOMMASO; NOVAIS-RODRIGUES, EDUARDO; DALLA COSTA, MARINA; FUCK REINHARDT ADOLFO; RUBIANO LORENZONI, GEORGINA; BERTOTTO GUSTAVO W.; VENTURA SANTOS ROBERTO

Lyon

Congreso; Goldschmidt 2021; 2020

European Association of Geochemistry and the Geochemical Society

The diamond-bearing Canastra-1 kimberlite represents a Cretaceous intrusion that belongs to the Alto Paranaíba Igneous Province (APIP), Brazil. This kimberlite hosts mantle xenoliths including garnet-bearing lherzolites, websterites (two samples are amphibole-bearing), clinopyroxenites and one eclogite. Magnesium number (Mg#) of lherzolitic olivine (88-91), orthopyroxene (90-92) and clinopyroxene (89-91) are typical of mantle origin. Minerals from the hydrated websterites contain lower Mg# (opx = 74-75; cpx = 78-79) than the anhydrous ones (opx = 90; cpx = 89), indicating a cumulate origin for these samples. The clinopyroxene Mg# of clinopyroxenites is low (86-87), except for one sample that contains high Mg# (90). The Mg# of eclogite omphacite is 87. Clinopyroxenes of the hydrated websterites show high CaO (>21 wt.%) and Al2O3 (>5 wt.%) contents coupled with low MgO (<14 wt.%), defining a trend in which CaO and Al2O3 contents decrease and MgO increase, characteristic of partial melting process. Pyrope endmember in garnets decreases from lherzolite (71-76) to anhydrous websterite (70-72), clinopyroxenite (62-69), eclogite (57) and hydrated websterite (42-46). The rare earth element (REE) patterns ofclinopyroxenes display variable enrichment in light-REE (LREE) related to heavy-REE (HREE) (Ce/YbN = 9.1-60.9), which indicate cryptic metasomatism. Garnets show normal REE pattern characterized by the domain of high HREE and gentle decrease of LREE (Ce/YbN <0.32). Representative temperature (T) and pressure (P) estimates demonstrate that lherzolites, anhydrous websterites and clinopyroxenites represent the deepest mantle column of Canastra-1 lithosphere (T = 1262-1338 °C; P =5.0-5.5 GPa), whereas hydrous websterites (T = 887-926 °C; P =2.7-3.3 GPa) and eclogite (T = 972 °C; P = 4.4 GPa) samples are shallower. Therefore, we propose that cryptic metasomatism recorded by the lithospheric mantle beneath Canastra-1 occurred due to the collision between the São Francisco Craton and the Paranapanema Block during the Neoproterozoic. The lowvelocityanomaly observed in the seismic tomographic data corroborates the high temperature estimates observed in the deeper xenoliths and it might be related to the lithosphericthinning and consequent asthenosphere upwelling in the Brasília Mobile Belt or could be associated to a Cretaceous fossil plume.