Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil : clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon.
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2016
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Abstract
The Carlos Chagas batholith (CCB) is a very large (~14,000 km2) S-type granitic body formed during the
syn-collisional stage of the Araçuaí orogen (southeastern Brazil). Zircons extracted from the CCB record a wide
range of U–Pb ages (from 825 to 490 Ma), indicating a complex history of inheritance, magmatic crystallization
and partial melting during the evolution of the orogeny.Magmatic zircons (ca. 578–588Ma) aremarked by similar
Hf isotope compositions and REE patterns to those of inherited cores (ca. 825–600Ma), indicating that these
aspects of the chemical signature of the magmatic zircons have likely been inherited from the source. The U–Pb
ages and initial 176Hf/177Hf ratios from anatectic and metamorphic zircon domains are consistent with a twostage
metamorphic evolution marked by contrasting mechanisms of zircon growth and recrystallization during
the orogeny. Ti-in-zircon thermometry is consistent with the findings of previous metamorphic work and indicates
that the twometamorphic events in the batholith reached granulite facies conditions (N800 °C) producing
two generations of garnet via fluid-absent partial melting reactions. The oldest metamorphic episode (ca. 570–
550Ma) is recorded by development of thin anatectic overgrowths on older cores and by growth of newanatectic
zircon crystals. Both domains have higher initial 176Hf/177Hf values compared to relict cores and display REE
patterns typical of zircon that grewcontemporaneouslywith peritectic garnet through biotite-absent fluid partial
melting reactions. Hf isotopic and chemical evidences indicate that a second anatectic episode (ca. 535–500Ma)
is only recorded in parts from the CCB. In these rocks, the growth of new anatectic zircon and/or overgrowths is
marked by high initial 176Hf/177Hf values and also by formation of second generation of garnet, as indicated
by petrographic observations and REE patterns. In addition, some rocks contain zircon crystals formed by
solid-state recrystallization of pre-existing zircon, which exhibit similar Hf isotope composition to those of
inherited/magmatic core domains. The first anatectic event is interpreted as result of crustal thickening after
the intrusion of the batholith. This introduced the batholith to a depth in excess of 30 km and produced
widespread anatexis throughout the batholith. The second event was associated with asthenospheric upwelling
during extensional thinning and gravitational collapse of the orogen, this produced anatexis in parts fromthe CCB
that had been re-fertilized for anatexis by retrogression along shear zones following the first granulite facies
event.
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Keywords
Repeated partial melting, Crustal recycling
Citation
MELO, M. G. de et al. Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: clues from U–Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon. Geomorphology, Amsterdam, v. 1, p. 730–750, 2016. Disponível em: <http://www.sciencedirect.com/science/article/pii/S0169555X16301957>. Acesso em: 25 ago. 2017.