Fluorite and REE constraints on fluid–melt relations, crystallization time span and bulk DSr of evolved high-silica granites. Tabuleiro granites, Santa Catarina, Brazil
Introduction
Fluorine-rich high-silica topaz rhyolites and their corresponding A-type granites have fluorite as a characteristic accessory phase (Burt et al., 1982; Collins et al., 1982; Christiansen et al., 1983; Whalen et al., 1987). In this setting, fluorite is believed to precipitate during magmatic and/or magmatic–hydrothermal stages (Huspeni et al., 1984; Congdon and Nash, 1988; Johnston and Chappel, 1992; Price et al., 1992; Nash, 1993; Webster and Duffield, 1994). Despite the relatively abundant petrological data on fluorine-rich high-silica rhyolites and granites, the stability conditions and genesis of the accessory fluorite are poorly constrained.
The main question concerning the fluorite formation within evolved high-silica rhyolites and granites is whether it formed under supersolidus conditions from a silicate melt or from a magmatic–hydrothermal fluid, or under subsolidus conditions from a post-magmatic fluid. Experimental data on fluorite-bearing topaz rhyolite vitrophyre from the Spor Mountain, UT, USA, has shown that fluorite is stable under supersolidus conditions (Webster et al., 1987; Tsareva et al., 1992) and that, with low water contents, fluorite and biotite are late-stage phases (Webster et al., 1987).
Fluorite may be used to trace magmatic to magmatic–hydrothermal processes as REE and Sr substitute for Ca in fluorite. It is well known that fluorite may concentrate REE from the melt or fluid from which it crystallizes (Mineyev, 1969; Marchand et al., 1976; Tsareva et al., 1992). On the other hand, fluorite has very low Rb/Sr ratios which allows us to determine the ratios of the melt or fluid from which it precipitated.
The Tabuleiro high-silica granite, Santa Catarina, Brazil, is a favorable rock to study the geochemistry of accessory fluorite because medium-grained and relatively abundant purple fluorite is present. In this study, we use for the first time the Sr isotope and REE geochemistry of accessory fluorites and of their host granites to understand the formation of the accessory fluorite. Our new data allow us to set tighter constraints on fluid–melt relations, the crystallization time span and the bulk distribution coefficient DSr of evolved high-silica magmatic systems.
Section snippets
Geologic setting
The Early Paleozoic Santa Catarina Composite Massif is located along the northeastern extremity of the Pelotas Batholith in the Dom Feliciano Orogenic Belt of southern Brazil (Fragoso Cesar et al., 1986; Sallet et al., 1989) (Fig. 1a). Regional mapping has defined three plutonic associations in the massif, namely: the Valssungana, the Pedras Grandes and the Tabuleiro associations (Horbach and Marimon, 1982; Kirchner and Morgental, 1983).
In the southernmost sector of the massif, there are two
Analytical methods
Tabuleiro granite samples with visible purple fluorite and weighing 10 to 20 kg were collected from road cuts. Granite analyses were performed at the University of Lausanne, Switzerland, by X-ray fluorescence (XRF) for major and trace elements and by inductively coupled plasma-atomic emission spectrometry (ICP-AES) for REE. F was determined by specific ion electrode at the C.R.P.G, Nancy, France. Li was analyzed by atomic absorption spectrometry (AAS) and Sn by inductively coupled plasma-mass
Petrography and geochemistry of the Tabuleiro granites
The Tabuleiro granites are chemically and mineralogically very similar to topaz rhyolites (Burt et al., 1982; Christiansen et al., 1983) and some A-type granites (Collins et al., 1982; Whalen et al., 1987). Sodic plagioclase, strongly perthitic alkali feldspar and quartz form an equigranular texture, locally porphyritic. Interstitial biotite almost completely transformed to chlorite and purple fluorite are late-stage phases. Fluorite occurs mainly as purple interstitial grains and occasionally
REE and other lithophile elements
Other than REE, trace elements Be, Nb, Ta, U, Th and Zr are present in fluorite in significant amounts, between 75 and 1100 ppm (Table 3). This element association, probably accounted for by minute mineral inclusions in fluorite, is typically enriched in F-rich high-silica magmas forming topaz rhyolithes and A-type granites (Christiansen et al., 1983; Congdon and Nash, 1988; Webster and Duffield, 1991; Charoy and Raimbault, 1994).
Chief minerals carrying REE in granites are phosphates,
Fluid–melt relations
Fluorine-rich high-silica melts forming topaz rhyolites and their A-type granitic equivalents are considered to be relatively dry. The low water contents of these melts are related to their source rocks (Burt et al., 1982; Collins et al., 1982; Christiansen et al., 1983; Clemens et al., 1986; Creaser et al., 1991). Since water solubility is strongly enhanced by increasing fluorine contents in haplogranitic melts (Holtz et al., 1993), it is suggested that fluorine-rich high-silica melts are
Crystallization time span and bulk DSr constrains
The magmatic origin of the recorded ratios in fluorite is the only mechanism to explain all the observed chemical correlations. Formulations by Cavazzini (1994)indicate that to reach the high values recorded by the granitic fluorites by means of fractional crystallization, it is required that the Tabuleiro granites represent very low fractions of residual magmas evolved during long crystallization time spans. Using the following equation from Cavazzini (1994):
Conclusions
and REE ratios associated with textural features of the Tabuleiro granitic fluorite, indicate that the fluorites crystallized from late stage melts at near solidus temperatures. REE patterns and Eu/Eu*, Rb/Sr and ratios of fluorite are directly inherited from melts with evolving differentiation degrees.
High and variable Tabuleiro granitic fluorite ratios are explained by decay of during fractional crystallization of high-Rb/Sr silicic systems with long
Acknowledgements
We would like to thank Jim Webster and an anonymous reviewer for their useful reviews of an early draft. Marcelle Falcheri and Fabio Caponi (University of Geneva, Switzerland) are thanked for their collaboration during labwork. RS thank Benjamin Carvalho and Clovis Bevilacqua (Mineração Santa Catarina, Grupo Votorantin) and Rui Philipp (Federal University of Rio Grande do Sul, Brazil) for their collaboration during field work. Germano Melo Jr. is thanked for improving the final text. This study
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