Genesis and evolution of a neoproterozoic magmatic arc : the cordilleran-type granitoids of the Araçuaí Belt, Brazil.
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2018
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The Araçuaí orogen (SE Brazil) is one of the largest (350,000 km2
) and long-lived (ca.
630 – 480 Ma) granitic province in the world. The peculiarity of this Orogen is its wide
variety of granitoids recording mid- to lower crustal P - T conditions that allow direct
investigation of petrological processes occurring in the deepest part of the continental
crust. This study investigates the field, textural, geochemical, geochronological and
isotopic evolution of the pre-collisional Galiléia Batholith outcropping in the central
part of the Araçuaí orogen. This metaluminous to slightly metaluminous (ASI = 0.97–
1.07) batholith, is a large is (ca. 15,000 km2
), Neoproterozoic (ca. 632–550 Ma) weakly
foliated, calc-alkaline, granitoid body characterized by the widespread occurrence of
grossular-rich magmatic garnet and magmatic epidote. This is a rare mineral association
in Cordilleran-I-type granitoids and of special petrogenetic significance. Field,
petrographic, and mineral chemistry evidence indicate that garnet, epidote, biotite as
well as white mica crystals (low-Si phengite), are magmatic. There is no difference in
bulk rock major and trace element composition between the Galiléia and other garnetfree cordilleran-type granitoids worldwide. Thus the uncommon garnet+epidote
parageneses are related to the conditions of magma crystallization, such as pressure,
temperature and water content. Comparison between the mineral assemblages and
mineral compositions from this study and those recorded in crystallization experiments
on metaluminous calc-alkaline magmas, as well as within garnet-bearing metaluminous
volcanic rocks and granitoids, indicates that the supersolidus coexistence of grossularrich garnet, epidote and white mica is consistent with magma crystallization at
pressures greater than 0.8 GPa (above 25 km depth). This indicates that the Galiléia
batholith was assembled in the lower crust during the accretionary/collisional stages of
the Neoproterozoic Brasiliano Orogeny. It is evident that this granitic body represents a
natural laboratory for studying the history of a lower crustal magma reservoir. Indeed
the lifetime of deep magma chambers and the duration of magmatic activity in them
remains a puzzle, contrary to young upper crustal magmatic systems. Despite being
homogeneous with respect to mineralogy/texture and major/trace elements, all samples
from the central part of the batholith record extreme variability in U-Pb magmatic ages
from ca. 630 to 555 Ma. Trace element geochemistry and Hf isotopes from the igneous
zircons – here interpreted as autocrysts (ca. 555 – 560 Ma) and antecrysts (> 560 Ma) –
are all consistent with an open-system crystallization, rather than a simple cooling
following fractional crystallization at the level of magma emplacement. We interpret
the age variability recorded by the Galiléia samples as the result of a long-lived,
uninterrupted injection of magmas of similar composition during assembly of the
batholith. Such continuous injections of magma took place in the lower crust, keeping
the system above its solidus through the 80 Ma of zircon crystallization. Unradiogenic
176Hf/177Hf and 143Nd/144Nd isotopic values of the Galileía samples indicate no direct
mixing with mantle-derived magmas during the assembly and growth of the Galiléia
batholith. This explains the scarcity of mafic products in the region. All of these
characteristics need a more suitable geodynamic scenario. Indeed mineral textural,
geochronological and isotopic similarities with other younger and older granitic plutons
constructed within accretionary / fore-arc settings, better explain the characteristics
showed by the Galiléia granitoids. Thus it is suggested that this giant batholith was
assembled in an accretionary prism during the Brasiliano Orogenic stages. Eventually,
it is likely that during the Brasiliano/Pan-African orogeny, accretionary prism, fore- and
back–arc setting were sites of voluminous silicic magmatism and commonplaces for the
stabilization of continental crust and its differentiation.
Descrição
Programa de Pós-Graduação em Evolução Crustal e Recursos Naturais. Departamento de Geologia, Escola de Minas, Universidade Federal de Ouro Preto.
Palavras-chave
Magmatismo, Orógeno Araçuaí - MG, Geocronologia, Batólito
Citação
NARDUZZI, Francesco. Genesis and evolution of a neoproterozoic magmatic arc : the cordilleran-type granitoids of the Araçuaí Belt, Brazil. 2018. 245 f. Tese (Doutorado em Evolução Crustal e Recursos Naturais) – Escola de Minas, Universidade Federal de Ouro Preto, Ouro Preto, 2018.