Magmatic garnet in the Cordilleran-type Galiléia granitoids of the Araçuaí belt, Brazil : evidence for crystallization in the lower crust.
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2017
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Abstract
Magmatic garnet, together with epidote, is a rare mineral association in cordilleran-I-type granitoids and of special
petrogenetic significance. The metaluminous to slightly peraluminous (ASI = 0.97–1.07) Galiléia batholith
(Brazil) is a large (ca. 30,000 km2), Neoproterozoic (ca. 632–570 Ma) weakly foliated calc-alkaline granitoid
body, characterized by the widespread occurrence of garnet (grossular 25–43 mol%) and epidote (pistacite
9.3–22.7 mol%). Field, petrographic and mineral chemical evidence indicates 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 granitoids and other garnet-free cordilleran-type granitoids
worldwide. This evidence strongly suggests that the origin of the uncommon garnet+epidote parageneses is related
to the conditions of magma crystallization, such as pressure, temperature and water content. Comparison
between the mineral assemblages andmineral compositions fromthis 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 grossular-rich garnet, epidote and white
mica is consistentwith magma crystallization at pressures greater than 0.8 GPa (above 25 km depth) and at temperatures
below 700 °C, i.e. near the water saturated solidus. Furthermore, resorption textures around garnet
(plagioclase ± quartz coronas) and epidote suggest that these minerals have been partially consumed prior to
complete crystallization. These findings demonstrate that at 630 Ma the crust underneath the Araçuaí Orogen
was already at least 25–30 km thick and relatively cool. However, this contrasts with the marked high heat
flow registered from the neighbour Carlos Chagas Batholith located 50 km to the east. In fact such granitoids record
granulite-facies metamorphism at the same pressure and time (ca. 570 Ma) of Galiléia granitoids crystallization.
Thus, a more suitable geodynamic scenario is required in order to explain these two contrasting thermal
regimes within the same orogen. Eventually, field, petrographic and mineral chemical analogies with similar
garnet-bearing granitoids located in the fore-arc settings of the British Columbia subduction zone, possibly
imply that the Galiléia granitoids represent “rare” garnet- and epidote-bearing metaluminous Cordilleran-Itype
granites which can only form in a fore-arc setting.
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Keywords
Grossular-rich magmatic garnet, Magmatic epidote, High pressure crystallization, Cordilleran-type Galiléia granitoids
Citation
NARDUZZI, F. et al. Magmatic garnet in the Cordilleran-type Galiléia granitoids of the Araçuaí belt, Brazil: evidence for crystallization in the lower crust. LITHOS, v. 282-283, p. 82-97, 2017. Disponível em: <http://www.sciencedirect.com/science/article/pii/S002449371730083X>. Acesso em: 25 ago. 2017.