REE and Y Mineralogy of the Krudum Granite Body (Saxothuringian Zone)

Miloš René

doi:10.3390/min8070287

Abstract

The Krudum granite body comprises highly fractionated granitic rocks ranging from medium-F biotite granites to high-F, high-P2O5 Li-mica granites. This unique assemblage is an ideal site to continue recent efforts in petrology to characterize the role of zircon, monazite, and xenotime as hosts to rare earth elements (REEs). The granitic rocks of the Krudum body analyzed in this study were found to contain variable concentrations of monazite and zircon, while xenotime was only found in the high-F, high-P2O5 Li-mica granites and in the alkali-feldspar syenites of the Vysoký Kamen stock. Intermediate trends between cheralite and huttonite substitutions are characteristic for analyzed monazite grains from all magmatic suites. The highest concentration of cheralite was found in monazite from the alkali-feldspar syenites (up to 69.3 mol %). The proportion of YPO4 in analyzed xenotime grains ranges from 71 to 84 mol %. Xenotime grains are commonly enriched in heavy rare earth elements (HREEs; 9.3–19.5 wt % HREE2O3) and thorite-coffinite and cheralite exchange was observed. Some xenotime analyses return low totals, suggesting their hydration during post-magmatic alterations. Analyzed zircon from granite suites of the Krudum granite body contains moderate Hf concentrations (1.0–4.7 wt % HfO2; 0.010–0.047 apfu Hf). The highest concentrations of HfO2 were found in zircon from the high-F, high-P2O5 Li-mica granites (1.2–4.7 wt % HfO2). Analyzed zircon grains from the high-F, high-P2O5 Li-mica granites and alkali-feldspar syenites are enriched in P (up to 8.29 wt % P2O5; 0.24 apfu P), Al (0.02–2.0 wt % Al2O3; 0.00–0.08 apfu Al), Ca (up to 3.9 wt % CaO; 0.14 apfu Ca), Y (up to 5.5 wt % Y2O3; 0.10 apfu Y), and Sc (up to 1.17 wt % Sc2O3; 0.03 apfu Sc). Zircon grains from the high-F, high-P2O5 Li-mica granites were sometimes hydrated and fluorized. The concentrations of F in zircon from partly greisenised high-F, high-P2O5 Li-mica granites reached up to 1.2 wt % (0.26 apfu F).

Highlights

In the last ten years, several studies have emphasized the role of monazite, xenotime, and zircon as major hosts for rare earth elements (REE) and Y in granites [1,2,3,4,5,6,7,8,9]
The presented study concentrates on petrological and geochemical observations connected to the occurrence of monazite, xenotime, and zircon in compositionally different granitic rock types of the Krudum granite body
The alkali-feldspar syenite is described as feldspathite in some papers (e.g., [16,17]), but this name does not agree with the magmatic nature of this rock, which is in some places underlined by its striking magmatic layering (Figure 3)

Summary

Introduction

In the last ten years, several studies have emphasized the role of monazite, xenotime, and zircon as major hosts for rare earth elements (REE) and Y in granites [1,2,3,4,5,6,7,8,9]. Several factors controlling the composition of the above mentioned accessory minerals remain unclear. To expand on this knowledge, unique assemblages of variable fractionated granites in the Karlovy Vary pluton were selected for further analyses on these minerals. The presented study concentrates on petrological and geochemical observations connected to the occurrence of monazite, xenotime, and zircon in compositionally different granitic rock types of the Krudum granite body. This granite body is a subsidiary intrusion of the Karlovy Vary pluton in the Slavkovský Les Mts. Minerals 2018, 8, 287; doi:10.3390/min8070287 www.mdpi.com/journal/minerals

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Conclusion