Cryogenian U-Pb (Shrimp I) zircon ages of anorthosites from the upper sequences of Niquelândia and Barro Alto Complexes, central Brazil

  

Correia, C.T.¹; Girardi, V.A.V.¹; Basei, M.A.S.¹; Nutman, A.²

 1. Departamento de Mineralogia e Geotectônica, Instituto de Geociências da Universidade de São Paulo. Rua do Lago, 562, CEP 05508-900, São Paulo, SP, Brasil.
²2. Research School of Earth Science, Australian National University, Camberra, ACT 2601.

 

 

 Abstract

The Niquelândia complex comprises two main superposed sequences dipping westward: the lower (LS), at the eastern, and the upper (US), at the western part of the body. The complex is either interpreted as a single body, or as two distinct layered massifs, yielding, according to the last model, respective ages of 0.79 Ga and 1.25 Ga for LS and US. However, new SHRIMP U-Pb data from igneous zircons of anorthosites indicate a crystallization age of 833 ±21 Ma for the US sequence. SHRIMP U-Pb determinations carried out on zircons of anorthosites from the upper sequence of the Barro Alto complex indicated a crystallization age of 733±25 Ma, thus supporting Cryogenian Neoproteroic ages for the igneous crystallization of the upper units of both complexes.

Keywords: Niquelândia, Barro Alto, Anorthosites, U-Pb geochronology, Central Goiás

 

 

INTRODUCTION AND GEOLOGICAL OVERVIEW

The Niquelândia and Barro Alto complexes belong to a 350 km NNE-trending mafic-ultramafic belt in Goiás State, central Brazil, which also includes the Cana Brava complex. They are part of the Goiás Massif, a complex geotectonic unit (Pimentel et al., 2004). The Niquelândia stratiform complex (Girardi et al., 1986; Ferreira Filho et al., 1992) is constituted by two main superposed sequences dipping westward: the lower (LS), at the eastern, and the upper (US), at the western part of the complex. LS includes a basal gabbro zone (BGZ), a basal peridotite zone (BPZ), composed by hazburgites, dunites and rare pyroxenites, a layered ultramafic zone (LUZ), made up of interlayered pyroxenites and peridotites, and a layered gabbro zone (LGZ), formed mainly by melanocratic to leucocratic gabbros. The top of this layer, formerly called “hydrous zone”, due to the enrichment in biotite, hornblende, apatite, zircon, incompatible elements and LREE, is here called “LGZ top zone”, as proposed by Correia et al. (1996). Leucocratic gabbros, the main components of the lower zone, contain abundant xenoliths of country-rocks, including quartzites, schists, calc-silicate rocks and gneisses. US comprises an upper gabro-norite zone (UGAZ), consisting of gabbros, norites and anorthosites and an upper amphibolite zone (UA). Although the preservation of primary igneous structures and mineral assemblages are common, the effects of a high temperature re-equilibration formerly attributed to slow sub-solidus event (Girardi et al., 1986), and later ascribed to granulite-amphibolite facies metamorphism (Ferreira Filho et al., 1998) are important features. The age and the stratigraphy of the complex are controversial. According to Girardi et al. (1986), the Niquelândia body represents a single igneous complex. Following this model, Correia et al. (1996), using SHRIMP U-Pb data in the LGZ top zone and the Re-Os systematic in LUZ, obtained an Re-Os age of 2.0 Ga for the LS sequence, interpreted as the crystallization age of the entire complex. U-Pb ages of 780 Ma, 1000-1400 Ma, and 1600-1880 Ma from the concordia were interpreted as probable periods of tensional relaxing due to extensional rifting in the area. Ferreira Filho et al. (1994) analyzed zircons from two rocks (CF 03, a diorite from LS, and CF-04, a quartz-rich mylonitic rock within gabbros of US) by conventional U-Pb method. The result was a poorly constrained discordia, where the upper intercept (1.56 Ga) was interpreted as the crystallization age of the complex. The authors attributed an age of 794 Ma to the metamorphism. Ferreira Filho and Pimentel (2000) proposed that LS and US constitute two distinct layered complexes, yielding ages of 2.0 Ga (LS) and 1.35 Ga (US). However, the last age originated from poorly defined Sm-Nd isochrons as mentioned by Pimentel et al. (2004b). The interval 770-795 Ma was interpreted as the period of high-grade metamorphism of both units. Pimentel et al. (2004b) using SHRIMP U-Pb method in zircons from the same samples (CF-03 and CF-04) formerly studied by Ferreira Filho et al. (1994), and Sm-Nd method from Ni-376, a gabbronorite of LS, concluded that LS and US yield respective igneous crystallization ages of 0.79 and 1.25 Ga.

The aim of this paper is to discuss new SHRIMP U-Pb zircon data from anorthosites of the US of Niquelândia, in order to contribute to a better knowledge of the age of this unit in the context of the tectonic evolution of the complex. We also display new similar data of anorthosites from the upper portion of Barro Alto, in order to contribute to further correlations among them.

 

Results and Discussion

The analyses were performed on zircons from two anorthosites: one from Niquelândia (NQ-1552: 95% labradorite and minor amounts of green hornblende, epidote, scapolite and zircon; subhedral granoblastic texture; epidote locally euhedral in apparent equilibrium with plagioclase and hornblende; grain size between 0.5-3 mm) and the other from Barro Alto (BA-1541: 92% labradorite and minor amounts of hornblende, epidote and zircon; texture predominantly subhedral granoblastic, partially sheared; relicts of the original subophitic texture still preserved; grain size between 0.9-4 mm).

The radiometric analyses (Tables 1 and 2) were carried out at the Australian National University at Canberra, using SHRIMP I, according to the procedures presented by Compston et al. (1984), Williams (1998) and Stern (1998). The CZ3 standard was used for the isotopic corrections and calculation of element concentrations. Lead concentration corrections were based on the isotopic measurements and on the common Pb composition at the approximate time of rock formation (Cumming and Richards, 1975). The ages were calculated using the decay constants and the present value for the ²³⁸U/²³⁵U ratio recommended by Steiger and Jäger (1977). The isotope data were processed using the Isoplot Ex program (Ludwig, 1998).

The cathodoluminescence images (Fig. 1) for both samples display a pale-gray homogeneous zircon pattern. Only few grains show clearly thin overgrowths and oscillatory growth zonation.

 

Figure 1.  Selected cathodoluminescence images for zircons from NQ1552 and BA 1541 samples.

The results are plotted in Tera-Wasserburg diagrams (Fig. 2), which clearly shows that the zircon populations in both samples yield very different ages, thus indicating that the Barro Alto anorthosite (BA1541) is an younger intrusion.

Taking into account that all the different zircon types (homogeneous, with oscillatory growth and displaying thin overgrowths) were dated, the resulting ages should reflect the time of zircon crystallization followed by, if present, an almost simultaneous sub-solidus re-equilibration or metamorphism. The absence of inherited cores does not support previous interpretations, which suggest a Mesoproterozoic age for these rocks.

Pimentel et al (2004b) published SRHIMP data on zircons from samples CF–04 (a quartz rich mylonite from a shear zone cutting gabbros, at the base of US, close to the limit of the LS top zone) and CF–03 (a diorite from LS top zone); and Sm-Nd data from Ni-376 (a gabbronorite from LS top zone).

Zircons from both CF-03 and CF-04 display inherited cores bordered by overgrowth rims. Based on analysis of these rims and regions with oscillatory zoning from sample CF-03, the authors proposed a concordia age of 797±10 Ma, as representative of the igneous crystallization of the LS unit of Niquelândia. The Sm-Nd results from Ni-376 were not conclusive.

 

Table 1. SHRIMP I  zircon data from NQ1552 sample.

 

 

Table 2. SHRIMP I  zircon data from BA 1541 sample.

 

 

 
Figure 2. Tera-Wasserburg plots for NQ1552 andBA 1541 anorthosites.

Sample CF-04 displayed zircon cores varying from 0.76 to 1.25 Ga. Based on four ²⁰⁷Pb/²⁰⁶Pb discordant analyses the authors interpreted the upper intercept age of 1248 ± 23 Ma as the igneous crystallization of the US unit of Niquelândia.

These interpretations and also the crystallization age proposed by Correia et al. (1996) and Correia (2001) are questionable due to the abundant country-rock xenoliths, which contaminate the LS top zone. The crustal contamination caused by the assimilation of these rocks by the intrusive magma possibly accounts for the particular geochemical characteristics of gabbros and diorites of this zone (see previous section). Both samples CF-03, and mainly CF-04, seem to be close to the limit between LS and US (see fig. 3 in Pimentel et al, 2004b). Moreover CF-04 contains kyanite, which indicates contamination probably by metasediments. Therefore, it is not possible to distinguish between zircons from the intrusion and from the country-rocks. Thus, the age of 1248±23 Ma may have no relationship with the igneous crystallization of the Niquelândia complex. In addition, the 1.35 Ga Sm-Nd isochron from leucogabbros (Ferreira Filho and Pimentel, 2000) does not help to support the Mesoproterozoic age for US. It is poorly defined, as mentioned by Pimentel et al. (2004b), and also might be the result of mixing lines.

The morphological characteristics, the internal structure, homogeneity, and the predominant high Th/U ratios (see Table 1) of the NQ-1552 anorthosite zircons indicate that 833±21 Ma can be interpreted as the crystallization age of the US unit of the Niquelândia complex. We believe that the age of LS unit remains an open question, demanding further SHRIMP U-Pb and Re-Os analyses on rocks devoid of crustal contamination.

 

Acknowledgements

The authors acknowledge the Brazilian Agencies FAPESP (Proj. 2004/03032-6) and CNPq for financial support.

 

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