THE GEOCHEMICAL CORRELATION BETWEEN THE MEDINA MIGMATITES BELONGING TO THE PARAÍBA DO SUL COMPLEX AND LEUCOGRANITES SUITES OF THE MINAS GERAIS STATE, BRAZIL

 

 

Mello, F.M.¹ & Bilal, E.²

 

^1. Departamento de Geociências, IA. Universidade Federal Rural do Rio de Janeiro, BR 465, Km 7,

23851-970, Seropédica - RJ, Brasil. fermamll@ufrrj.br

^2. CNRS-UMR 6425, Département GENERIC, Ecole Nationale Supérieure des Mines de Saint-Etienne, 158,

Cours Fauriel, 42023 Cedex 2, Saint-Etienne, France. bilal@emse.fr

 

ABSTRACT

 

The Medina migmatites belong to Paraíba do Sul Complex and outcrop in all the eastern part of Mantiqueira Structural Province. They are potential sources of genesis the leucogranites suite. The geochemical data of the Medina migmatites show a peraluminous composition and high part of immature sandstones. The peak anatexis of migmatites calculated with garnet geotherrnobarometry indicates a high temperature (800°C and 4.5 kbar). The leucogranitic melting involved at low temperature (550-700°C and 4-5 kbar) explained a geochemical trend of leucogranites suites. The distribution and behaviour of the many traces elements show a strong implication of the Paraíba do Sul metasediments source rocks in genesis of leucogranites suites.

 

Keywords: Leucogranite, migmatites, metasediment, Mantiqueira, perphosphorous

 

 

GEOLOGICAL SETTING

The Rio Doce region is located in the north-central part of the Mantiqueira Structural Province, east of São Francisco Craton (Fig. 1). This province is represented by Neoproterozoic mobile belts surrounding the São Francisco cratonic block and is associated to the Brazilian Orogeny (600 - 450 Ma). These mobile belts reworked the early-Proterozoic basement (high and low-grade metamorphic rocks of Piedade, Paraíba do Sul and Pocrane Complexes; Juiz de Fora, late-Proterozoic supracrustal sequences (Rio Doce group) and enabled the intrusion of granitoid plutons and pegmatites. Several rare metals and gem mineral rich pegmatite are in the São Tomé foliation plane.

The regional evolution was linked to the Governador-North Guaçui and Vitoria shear zones. Two main deformation phases (D1 and D2) pre- and post-dating the pluton emplacement was developed under amphibolite facies conditions. The first deformation (D1) was responsible for penetrative foliation (solid state) N 10º - 30º W middle to high angle and mineral lineation of the host rocks and the granitoids. It affected pre-tectonic granites and controlled magmatic foliation of the sin-tectonic granitoids.

This foliation, the associated oblique lineations and kinematics studies suggest that subvertical shear zones were important during the emplacement of these granitoids. The second one was characterized by the cleavage crenulation, boudinage and normal faults and it was associated to extensive phase late- and post-tectonic granitoids. The recent geochronology study demonstrates existence of two tectono-metamorphic events in this region dated at 590-565 Ma and 535-520 Ma (Pedrosa-Soares et al., 1999; Bilal et al., 2000 a, b; Nalini et al., 2000; Bilal et al., 2001).

 

Leucogranites suites

We study many little leucogranites bodies associated to the pegmatites located in south of Teofilo Otoni city (Mello & Bilal, 2004). They are controlled by a previous main compressive deformation phase Dl. Ten individual zircon crystals within leucogranites are aged 579 ± 5 Ma.  The very Sr-enriched and Nd-depleted initial ratios (0.782 ≤ ⁸⁷Sr/⁸⁶Sr_(i) ≤ 0.823 and -8.2 ≤ eNd₍₆₀₀₎ ≤ -7.4) must be related to an important role of a crustal source. The syn-tectonic magmatic series are related to crustal melting produced by decompression and thermal relaxation (550 – 700 °C and 4 - 5 kbar).

These perphosphorous leucogranites display porphyritic textures and are characterized by the presence of apatite phenocrystal (2 cm) and P-rich feldspars. The plagioclase in leucogranites varies from An₁₂ to An₀ composition and P₂O₅ values vary between 1.0 - 2.5 wt.%. Apatite in the leucogranites shows two groups based on chemistry and occurrence: (i) the phenocrystal apatite enriched in Mn, disseminated within the leucogranites, and (ii) the small apatite included in plagioclase are Mn-depleted.

 

 

Figure 1. Geological sketch map of south estern (Compilation data this work and Pinto, 1997;

Bilal et al., 2000 a, b; Nalini et al., 2000 ; Bilal et al., 2001).

 

Medina Migmatites

The Medina migmatites belong to the Paraíba do Sul Complex and outcrop in the eastern part of Mantiqueira belt. The Sm-Nd isotopic data for the Paraíba do Sul paragneiss indicate T_DM model ages between 1.61 and 1.74 Ga, which are interpreted as an upper limit for the sources of the original sediments which deposition occur during the Meso- or Neoproterozoic (Celino 1999).

The most abundant variety of the Medina rocks are leucocratic and mesocratic migmatites. They occupy large bands oriented SE-NW in eastern part of Mantiqueira belt. The leucosome is mainly composed of quartz, plagioclase (An_20-31), cordierite, perthitic K-feldspar, biotite, rare sillimanite, and occasional garnet and muscovite. The melanosome comprises alternating granoblastic and restites bands defining the main foliation. It is composed of biotite, quartz, plagioclase (An_26-30), garnet, cordierite, rare sillimanite, and locally graphite. Apatite, zircon and the small monazite crystals are the main accessory minerals. Garnet geothermobarometry indicates that the anatexis peak occurred at 800 °C and 4.5 kbar. The majority of the zircon crystals from the Medina migmatites-gneisses give an age of 585 ± 7 Ma. They are contemporaneous to the Urucum and to the  perphosphorous leucogranites (Nalini et al. 1997; Bilal et al., 2000).

 

GEOCHEMISTRY

The Medina migmatites are highly peraluminous (1.07 < ASI < 1.38) in comparison with the porphyritic granites from the aplopegmatitic facies. The P₂O₅ contents (0.28 to 1.06 wt.%) decrease with SiO₂ increasing (72 - 75 wt.%). Very high concentrations of P₂O₅ in silicic peraluminous granites are characteristics of strong differentiation. In a same way, a decrease of major elements (CaO, Fe₂O₃, MgO, TiO₂) and of trace elements (Zn, V, Sc, Co, Cr, Ni, REE) is observed for the Medina Migmatites, Urucum granites suite to the aplopegmatites (Figs. 2, 3). The MgO/TiO₂ ratio is near 3, which may be compared to the typical granites of crustal origin.

 

 

 

Figure 2. (A) P₂O₅ versus SiO₂ diagram and (B) CaO versus SiO₂ diagram of per phosphorous leucogranites (P), Urucum suite (Ur) and Medina migmatites (Mig)  from Minas Gerais state.

 

The perphosphorous leucogranites are associated with the Urucum granite suite and migmatites suites in eastern of Minas Gerais state. The plot of P₂O₅ versus SiO₂ shows a regular variation of P₂O₅ in the granite decreases at SiO₂ (72 %) from this point onwards P₂O₅ increases.

Mineralogically, these perphosphorous leucogranites and pegmatites are characterized by the presence of rare phosphates (amblygonite, triphyllite, triplite, brazilianite). The trend is related to the later stages of magmatic differentiation. The P₂O₅ is concentrated in the late magmatic stage with Rb, Li and shows a negative correlation with CaO. The Ca activity may be influenced the apatite solubility.

The Medina migmatites have a peraluminous compositions and the presence of corundum normative indicates a metasedimentary source. However, the Medina migmatites trend (Fig. 2) seems to be compatible with the presence of restitic apatite. We observed in biotite restitic the small apatite and zircon inclusions. The phosphorus enrichment and discrete variation of the major and some trace elements content would be associated to restite unmixing during the segregation of anatectic melt, variable melting degree of plagioclase, and crystallization of microcline and micas.

Smoler (1987) and Häussinger (1990) used correlations between Al₂O₃, TiO₂ and Zr to discriminated shales and sandstones and identified lithostratigraphic units in metamorphosed belts. The Medina Migmatites are binary mixing between shales and sandstones, with high part immature sandstones (Fig. 3). The trend of leucogranites extending in opposite direction linked to early crystallisation zircon and ilmenite and the solubility of these phases is very low under hydrous conditions and low temperature, it’s a case for the leucogranites suite their temperature of crystallisation is between 550 and 700°C. The immature sandstones are potentially fertile sources for leucogranites (Clemens & Vielzeuf, 1987). Melting and restite segregation are important processes in genesis of peraluminous granites.

 

Figure 3. Distribution of the perphosphorous leucogranites (P), Urucum suite (Ur) and Medina migmatites (Mig) in the Al-Ti-Zr diagram and positions of the immature and mature sandstones, quartzite shales. The solids contours correspond to the field of peraluminous granites in word.

The REE patterns for the migmatites (Fig. 4) show two groups: one has the higher REE-contents, less fractionated pattern, flattening through the intermediate and heavy REE, and a conspicuous positive Eu anomaly. The second group is the most fractionated, has the lowest HREE and small Eu negative anomaly. The variable quantities of apatite, zircon and garnet are responsible for the higher intermediate and heavy REE contents. The different intensities of Eu anomaly are related to variations in the partial melting degree of the metasedimentary source rocks.

 

 

Figure 4. Distribution of the perphosphorous leucogranites (P), Urucum suite (Ur) and Medina migmatites (Mig) in the REE patterns diagram.

 

 

CONCLUSION

The geochemical data of the Medina Migmatites show a peraluminous composition and high part of immature sandstones. The Medina Migmatites are potential sources for the leucogranites suite. The peak anatexis of migmatites calculated with garnet geotherrnobarometry indicates a high temperature (800 °C) and the leucogranitic melting involved at low temperature (550 – 700 °C) explained a geochemical trend of leucogranites suites. The distribution and behaviour of the many traces elements show a strong implication of the Paraíba do Sul metasediments source rocks in the genesis of leucogranites suites.

 

REFERENCES

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