A Neoproterozoic hyper-extended margin associated with Rodinia's demise and Gondwana's build-up : the Araguaia Belt, central Brazil.
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2019
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The Araguaia Belt encloses a poorly constrained Pan-African (Brasiliano Cycle) continental suture marked by a
series of (~750 Ma) ophiolitic units which, when properly characterized, could provide important informations
on its geological history, closely linked with the Rodinia demise and further western Gondwana amalgamation.
We present new bulk-rock and mineral major and trace element compositions for these ultramafic and mafic
units. They mainly consist in fully serpentinized harzburgite, scarce dunite lenses and chromite pods, tectonically
overlain by basaltic pillow lavas. Low Al2O3/SiO2 ratios (0.01 to 0.06), rather highMgO concentrations (42.28 to
45.29 wt%) and spinels' Cr# and Mg# ratios comprised between 0.36 and 0.51 and 0.59 and 0.72, respectively,
indicate a depleted oceanic-like protolith. MORB-peridotite interactions are evidenced both by pyroxenite, olivine
gabbro and diabase occurrences in the serpentinites and by high TiO2 (up to 0.42 wt%) contents in spinels
from some Serra do Quatipuru serpentinites. These observations support that the Araguaia Belt ophiolitic bodies
are the remnants of the upper mantle section of a MOR or subcontinental lithosphere. The serpentinites wholerock
REE content can be modeled as resulting from a dry partial melting involving 14 to 24% of melt extraction,
coupled with refertilization by fertile melts, generated deeper in the mantle. Such an oceanic-like setting is also
supported by the N-MORB signature of Serra do Tapa and Morro do Agostinho pillow lavas basalts. All together,
these results tend to infirm the supra-subduction zone (SSZ) setting previously proposed for these ophiolitic
units. Important LILE, B and Li enrichments in the serpentinites likely result from a metasomatic event involving
sediments-derived fluids that occurred during the obduction of the units on the Amazonian Craton. Our results
combined with (1) the apparent scarcity of igneous crustal rocks, (2) the proximal nature of the
metasedimentary rocks hosting the ophiolitic units, and (3) the occurrences of Amazonian Craton fragments
eastward of the ophiolitic bodies, allow us to propose that the Araguaia Belt comprises a fossil ocean-continent
transition (OCT) accreted on the eastern border of the Amazonian Craton.
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Ophiolite, Serpentinites
Citação
HODEL, F. et al. A Neoproterozoic hyper-extended margin associated with Rodinia's demise and Gondwana's build-up : the Araguaia Belt, central Brazil. Gondwana Research, v. 66, p. 43-62, 2019. Disponível em: <https://www.sciencedirect.com/science/article/pii/S1342937X18302545>. Acesso em: 08 fev. 2019.