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 increase with rising Th concentrations. A-type granites have the highest Th/U ratios and high Th concentrations, followed by I-type granites. Th/U ratios in S- to I-type granites are lower than in A-type and I-type granites, but higher than in S-type granites. The variation of Th/U ratios in all three types of granite cannot be explained by fractional crystallization of monazite, zircon and other Th and U bearing minerals alone, but are mainly due to source heterogeneities and uranium mobilization processes.
Highlights
Granitoid rocks dominate the upper and middle continental crust ([1,2] and references therein)
Our results show that it is possible in a first approach to use the Th/U ratios in order to distinguish between different granite types
The A-type Malani granites and the I-type, I-to S-type and S-type Variscan granites from the Fichtelgebirge, Oberpfalz and Bavarian Forest show a large variation in Th and U concentrations, with the A-type granites showing a higher variation in Th than in U, whereas S-type granites vary the most in U
Summary
Granitoid rocks dominate the upper and middle continental crust ([1,2] and references therein). A reliable tracer that is not altered by subsequent processes after the melting process would provide information about the source rock of the granite and the petrogenetic processes leading to magma generation. A very common method found in the literature is the use of tectonic discrimination diagrams in which trace element concentrations and their ratios and Rb/Sr and Sm/Nd isotope ratios give information about the tectonic setting. These diagrams are controversially discussed, because at the time of their development, the complexity of tectonic evolution was still unknown [15]
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