And I-Type Granites: Petrogenetic Evidence from Whole Rock Th/U Ratio Variations
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minerals Article Origin of S-, A- and I-Type Granites: Petrogenetic Evidence from Whole Rock Th/U Ratio Variations Anette Regelous 1,*, Lars Scharfenberg 2 and Helga De Wall 1 1 GeoZentrum Nordbayern, Friedrich-Alexander-Universität Erlangen-Nürnberg, Schlossgarten 5, 91054 Erlangen, Germany; [email protected] 2 Institut für Geologie, Universität Wien, Althanstr. 14, 1090 Wien, Austria; [email protected] * Correspondence: [email protected]; Tel.: +49-8526065 Abstract: The origin and evolution of granites remain a matter of debate and several approaches have been made to distinguish between different granite types. Overall, granite classification schemes based on element concentrations and ratios, tectonic settings or the source rocks (I-, A-, S-type) are widely used, but so far, no systematic large-scale study on Th/U ratio variations in granites based on their source or tectonic setting has been carried out, even though these elements show very similar behavior during melting and subsequent processes. We therefore present a compiled study, demonstrating an easy approach to differentiate between S-, A- and I-type granites using Th and U concentrations and ratios measured with a portable gamma ray spectrometer. Th and U concentrations from 472 measurements in S- and I-type granites from the Variscan West-Bohemian Massif, Germany, and 78 measurements from Neoproterozoic A-type Malani granites, India, are evaluated. Our compendium shows significant differences in the average Th/U ratios of A-, I- and S-type granites and thus gives information about the source rock and can be used as an easy classification scheme. Considering all data from the studied A-, I- and S-type granites, Th/U ratios Citation: Regelous, A.; Scharfenberg, L.; De Wall, H. Origin of S-, A- and increase with rising Th concentrations. A-type granites have the highest Th/U ratios and high Th I-Type Granites: Petrogenetic concentrations, followed by I-type granites. Th/U ratios in S- to I-type granites are lower than Evidence from Whole Rock Th/U in A-type and I-type granites, but higher than in S-type granites. The variation of Th/U ratios in Ratio Variations. Minerals 2021, 11, all three types of granite cannot be explained by fractional crystallization of monazite, zircon and 672. https://doi.org/10.3390/ other Th and U bearing minerals alone, but are mainly due to source heterogeneities and uranium min11070672 mobilization processes. Academic Editor: José Keywords: granites; uranium; thorium; continental crust; petrogenesis Francisco Molina Received: 27 May 2021 Accepted: 17 June 2021 1. Introduction Published: 24 June 2021 Granitoid rocks dominate the upper and middle continental crust ([1,2] and references Publisher’s Note: MDPI stays neutral therein). Despite the wide distribution and intense research, the origin of granitic rocks with regard to jurisdictional claims in is still a matter of debate ([3,4] and references therein). It is widely believed that many published maps and institutional affil- granites are a result of fractional crystallization of mantle-derived basaltic magmas, even iations. in active continental margin settings (e.g., [5,6]). Alternatively, granites are thought to be products of partial melting of middle-lower crustal protoliths, during orogenesis and crustal thickening or thinning [7–14]. The ongoing discussion shows that it is difficult to decipher the petrogenesis of granite formations. Obviously, there are several variables that control the magma composition, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. such as pressure, temperature, volatiles, source rocks and potentially tectonic setting. To This article is an open access article make things even more complicated, the magma composition is subsequently modified distributed under the terms and by assimilation, fractional crystallization and mixing processes. A reliable tracer that is conditions of the Creative Commons not altered by subsequent processes after the melting process would provide information Attribution (CC BY) license (https:// about the source rock of the granite and the petrogenetic processes leading to magma gen- creativecommons.org/licenses/by/ eration. A very common method found in the literature is the use of tectonic discrimination 4.0/). diagrams in which trace element concentrations and their ratios and Rb/Sr and Sm/Nd Minerals 2021, 11, 672. https://doi.org/10.3390/min11070672 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 672 2 of 19 isotope ratios give information about the tectonic setting. However, these diagrams are controversially discussed, because at the time of their development, the complexity of tectonic evolution was still unknown [15]. Crust formation can be a multi-stage process, thought to occur in at least two stages, because felsic melts are not in equilibrium with mafic and ultramafic rocks [16]. In a first stage, basaltic magmas are produced by partial melting of the mantle; in a second stage, either fractional crystallization or re-melting of the basalt produces the more evolved rocks of which the upper-middle continental crust is dominantly composed. The products may then undergo further differentiation by one or more cycles of re-melting [16–18]. Therefore, the upper-middle continental crust has been produced throughout Earth history by mantle extraction and is mostly composed of granitoid rocks [2,19–21]. The thickness of the granitic upper crust is estimated to range between 10–20 km [22–24]. In the field, granitoid rocks are normally classified after Le Maitre [25,26] based on their chemical variability and after Streckeisen [27] based on their relative modal proportions of quartz, alkali feldspar and plagioclase. The scheme from Shand [28,29] is based on the chemical composition, in which granites are either peraluminous, metaluminous, or peralkaline. Peraluminous rocks have an excess of Al2O3 over that required to crystallize feldspar and have an alumina-saturating phase; metaluminous rocks are deficient in alumina relative to the alkalis and lime, whereas peralkaline granites are deficient in Al2O3 relative to the alkalis. However, this scheme has the disadvantage that it gives not always a distinct petrogenetic link. The most common used geochemical classification scheme is that of Chappell and White [30,31], which subdivides granites into (a) I-type, product from an igneous or intracrustal source, (b) S-type, product of anatexis of metasedimentary or supracrustal protoliths, (c) A-type, which are alkaline, anhydrous, and/or anorogenic such as cratons or rift zones and (d) M-type mantle-like granites (e.g., [32–36]). Other types (H- and C-) have been also proposed but these classes are not widely used [37,38]. However, this classification of granitic rocks has been criticized [34,39] as it assumes a simple source for granitic rocks, which is probably oversimplifying. Nonetheless, to our understanding, this descriptive classification is still the best one, given that different types of granitic rocks are, in general, characteristic for different tectonic settings. U and Th concentrations as well as Th/U ratios are useful parameters in the charac- terization of granitic rocks (e.g., [23]). Granites have high concentrations of U and Th [40]. However, Th and U concentrations vary significantly within crustal rocks as a result of petrogenetic processes responsible for their formation [41]. U and Th are incompatible elements during melting of mantle rocks, as they have large ionic radii and high valence, which prevents them from being incorporated into the structure of the main rock-forming minerals and are enriched in felsic/crustal rocks [41,42]. Th and U occur mostly in ac- cessory minerals, such as zircon, apatite, monazite, titanite, xenotime and allanite within igneous and metamorphic rocks and thus show only a weak correlation with SiO2 and thus fractional crystallization trends (e.g., [43–45]]. However, in some I-type granites Th and U contents increase with differentiation, i.e., SiO2 contents (e.g., [46,47]). Similarly, Th decrease and U increase with differentiation in some S-type granites, especially in phosphorous-rich suites (e.g., [48]). Th and U are transported to the middle and upper crustal levels through several different processes associated with high-grade metamor- phism, metasomatism, partial melting, and fluid and melt migration [49]. During partial melting and crystal fractionation, Th and U are preferentially fractionated into the silicate melt and, over time, these elements have been enriched in the Earth’s continental crust. Geochemical models predict a global mean ratio for Th/U = 3.7–4.0 for the bulk Earth and primitive mantle [49]. Globally, the average Th/U ratio of the continental crust is estimated to be about 4 [40,50–52], with a significant variability reported for the continental crust [2,51,53–55]. Th and U behave very similar, because they are both tetravalent species in the mantle, and therefore, difficult to fractionate significantly by melting processes. In contrast, under more oxidized surface conditions, the elements show contrasting behavior, with Th remaining tetravalent and immobile during weathering, unlike highly water- Minerals 2021, 11, 672 3 of 19 soluble hexavalent U species [56]. Bea et al. [57] showed, in a study about metapelites in the Ivra-Verbano Zone, that U was compared to Th enhanced into the fluid phase that was released at the end of crystallization of leucosomes. On the other hand, studies from Variscan I- and S-type granites from central Spain show that Th/U ratios can also be affected by fractionation [47,48], as the Th concentrations are influenced by this process. So far, a systematic global study on Th and U concentrations and ratios in order to distinguish between different types of granites has not been carried out. This is therefore the first systematic Th and U study of S-, A-, and I-type granites on a larger scale, including 550 measurements. Here we present data from the western Bohemian Massif comprising I-type to S-type granites and NW India with vast exposure of A-type granites.