Granites of the intracontinental termination of a magmatic arc : an example from the Ediacaran Araçuaí orogen, southeastern Brazil.
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2016
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Abstract
The Araçuaí orogen of southeastern Brazil together with the West Congo belt of central West Africa form the
Araçuaí–West Congo orogen generated during closure of a terminal segment of the Neoproterozoic Adamastor
Ocean. Corresponding to an embayment in the São Francisco–Congo Craton, this portion of the Adamastor was
only partially floored by oceanic crust. The convergence of its margins led to the development of the Rio Doce
magmatic arc between 630 Ma and 580 Ma. The Rio Doce magmatic arc terminates in the northern portion of
the Araçuaí orogen. Granitic plutons exposed in the northern extremity of the arc provide a rare opportunity
to studymagmatismat arc terminations, and to understand the interplay between calc-alkalinemagma production
and crustal recycling. The plutons forming the terminus of the arc consist of granodiorites, tonalites and
monzogranites similar to a magnesian, slightly peraluminous, calcic- (68%) to calc-alkaline (24%), with minor
alkali-calcic (8%) facies, medium- to high-K magmatic series. Although marked by negative Nb–Ta, Sr and Ti
anomalies, typically associatedwith subduction-relatedmagmas, the combined Sr, Nd and Hf isotopic data characterize
a crustal signature related to anatexis of metamorphosed igneous and sedimentary rocks, rather than
fractional crystallization of mantle-derived magmas. Zircon U–Pb ages characterizes two groups of granitoids.
The older group, crystallized between 630 and 590 Ma, experienced a migmatization event at ca. 585 Ma. The
younger granitoids, emplaced between 570 and 590 Ma, do not show any evidence for migmatization. Most of
the investigated samples show good correlation with the experimental compositional field of amphibolite
dehydration-melting, with some samples plotting into the field of greywacke dehydration-melting. The studied
rocks are not typical I-type or S-type granites, being particularly similar to transitional I/S-type granitoids described
in the Ordovician Famatinian arc (NW Argentina). We suggest a hybrid model involving dehydrationmelting
of meta-igneous (amphibolites) and metasedimentary (greywackes) rocks for magma production in
the northern termination of the Rio Doce arc. The real contribution of each end-member is, however, a challenging
work still to be done.
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Keywords
Granites, Magmatic arc termination, Ediacaran Rio Doce arc
Citation
GONÇALVES, L. E. da S. et al. Granites of the intracontinental termination of a magmatic arc: an example from the Ediacaran Araçuaí orogen, southeastern Brazil. Gondwana Research, v. 36, p. 439-458, 2016. Disponível em: <http://www.sciencedirect.com/science/article/pii/S1342937X15001951>. Acesso em: 25 ago. 2017.