Abstract
The Urucum suite (582 ± 2 Ma, zircon U-Pb age), situated in the Mid-Rio Doce Valley, eastern part of Minas Gerais State, is characterized by elongated, NW-SE and N-S trending granitic massifs associated with the Conselho Peña-Resplendor high-angle shear zone. It corresponds to a syn to late collisional magmatism that presents dominant solid-state foliation. Four facies are distinguished within the Urucum suite: (i) a porphyritic (Urucum); (ii) a medium- to coarse nequigranular (Palmital); (iii) a tourmaline-bearing; and (iv) a pegmatitic facies. These facies are peraluminous, with alumina saturation index varying from 0.98 to 1.38. SiO2 contents vary from 70.7 to 73.7 wt%, with K2O values ranging from 3.5 to 5.7 wt%, Na2O from 1.9 to 4.4 wt%, MgO from 0.6 to 1.2 wt%, and CaO from 0.3 to 0.9%. Harker-type diagrams show rather continuous trends from the less-evolved Urucum facies to the more evolved tourmaline-bearing and pegmatitic facies. The behavior of several major oxides and trace elements (Fe2O3, MgO, MnO, CaO, TiO2, Al2O3, K2O, Rb and Ba) reflects the role played by fractionation of ferromagnesian minerals, feldspars and accessory minerals. Initial Sr87/Sr86 ratios vary from 0.711 to 0.716, with εNd (580 Ma) values between -7.4 to -8.2, and Sm-Nd TDM model ages ranging from 2290 to 1840 Ma.
Highlights
Several papers discuss the generation of significant amounts of granites by crustal melting in the late periods of crustal thickening and the role of erosion upon the decompression melting and its effect on increasing partial melting (England & Thompson 1984; Hollister 1993; Inger 1994)
1986), and 3. substitution in mica (Monier & Robert 1986). These results indicate crystallization temperatures in the range of 700 – 750oC for the Urucum and Palmital facies, from 650 – 700oC for the tourmaline-bearing facies, and about 600 – 650oC for the pegmatitic facies (Nalini 1997)
The different facies define an evolution in the following sequence: Urucum suite granites, Palmital granite, tourmaline-bearing granite and pegmatitic granites
Summary
Several papers discuss the generation of significant amounts of granites by crustal melting in the late periods of crustal thickening (collision) and the role of erosion upon the decompression melting and its effect on increasing partial melting (England & Thompson 1984; Hollister 1993; Inger 1994). The metamorphic assemblage of these rocks (staurolite and almandine) suggests metamorphism of lower amphibolite-facies, that occurred under the following conditions: P- about 4-5 kbar, and T ranging from 500 to 600oC (Nalini 1997) In this sense, the emplacement of the Urucum Suite is considered syn-tectonic with respect to D1- phase, and its generation is related to a regional metamorphic event that affected the above schists and that took place during crustal thickening (Nalini 1997). An increase in MnO/(MnO+Fe2Ot) values is observed from the Urucum suite granites to the pegmatites (Fig. 3D) This explains the importance of garnet crystallization in the more evolved (pegmatitic and tourmaline-bearing) facies. The values for elements such as Ba, Th and Ta approach those for oceanic-ridge granites, whereas Ce, Hf, Zr, Sm, Y and Yb show values around 0.1 times the normalization values (Fig.6)
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