An exotic Cretaceous kimberlite linked to metasomatized lithospheric mantle beneath the southwestern margin of the São Francisco Craton, Brazil.
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2022
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We present major and trace element compositions of mineral concentrates comprising garnet xenocrysts,
ilmenite, phlogopite, spinel, zircon, and uncommon minerals (titanite, calzirtite, anatase, baddeleyite and
pyrochlore) of a newly discovered Late Cretaceous kimberlite (U-Pb zircon age 90.0 ± 1.3 Ma; 2r) named
Osvaldo França 1, located in the Alto Paranaíba Igneous Province (APIP), southeastern Brazil. Pyrope
grains are lherzolitic (Lherz-1, Lherz-2 and Lherz-3), harzburgitic (Harz-3) and wehrlitic (Wehr-2). The
pyrope xenocrysts cover a wide mantle column in the subcratonic lithosphere (66–143 km; 20–43 kbar)
at relatively low temperatures (811–875 C). The shallowest part of this mantle is represented by Lherz-1
pyropes (20–32 kbar), which have low-Cr (Cr2O3 = 1.74–6.89 wt.%) and fractionated middle to heavy rare
earth elements (MREE-HREE) pattern. The deepest samples are represented by Lherz-2, Lherz-3, Harz-3,
and Wehr-2 pyropes (36–43 kbar). They contain high-Cr contents (Cr2O3 = 7.36–11.19 wt.%) and are char-
acterized by sinusoidal (Lherz-2 and Wehr-2) and spoon-like (Lherz-3 and Harz-3) REE patterns.
According to their REE and trace elements, pyrope xenocrysts have enriched nature (e.g., Ce and Yb vs.
Cr2O3), indicating that the cratonic lithosphere has been affected by a silicate melt with subalkaline/
tholeiite composition due to their low Zr, Ti and Y concentrations. Besides minerals with typical kimber-
litic signatures, such as ilmenite and zircon, the exotic compositions of phlogopite and ulvöspinel suggest
an enriched component in the magma source. The formation of rare mineral phases with strong enrich-
ment of light-REE (LREE) and high field strength elements (HFSE) is attributed to the late-stage kimber-
litic melt. We propose a tectonic model where a thermal anomaly, represented by the low-velocity
seismic anomaly observed in P-wave seismic tomography images, supplied heat to activate the alkaline
magmatism from a metasomatized cratonic mantle source during the late-stages of Gondwana fragmen-
tation and consequent South Atlantic Ocean opening. The metasomatism recorded by mineral phases is a
product of long-lived recycling of subducted oceanic plates since the Neoproterozoic (Brasiliano Orogeny)
or even older collisional events, contributing to the exotic character of the Osvaldo França 1 kimberlite, as
well as to the cratonic lithospheric mantle.
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Osvaldo França 1 kimberlite, Alto Paranaíba Igneous Province, Metasomatized cratonic lithosphere, Zircon U-Pb geochronology
Citação
CARVALHO, L. D. V. de. et al. An exotic Cretaceous kimberlite linked to metasomatized lithospheric mantle beneath the southwestern margin of the São Francisco Craton, Brazil. Geoscience Frontiers ,v. 13, 2022. Disponível em: <https://www.sciencedirect.com/science/article/pii/S1674987121001456>. Acesso em: 29 abr. 2022.