| Popis |
About 2600 spot analyses of mica (combined electron microprobe and laser ablation ICP-MS) from 120 samples from eight rare-metal granite (RMG) plutons of different geochemical affiliations were performed to characterize the contents of major (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, F), minor (Li, Rb, Zn), and trace elements (Sc, Nb, Sn, Cs, Ta, W). This allowed to constrain changes in mica chemistry during magma fractionation and to characterize the relation between the trace-element spectra in mica and geochemical specializations of parental granites. The studied samples covered metaluminous to peralkaline A-type granites of the Madeira pluton (Brazil), metaluminous to peraluminous A-type granites (Kimi, Finland; Orlovka, Russia; Cínovec, Czech Republic) and strongly peraluminous S-type granites (Panasqueira and Argemela, Portugal; Beauvoir, France; Nejdek, Czech Republic). Micas from non-mineralized S- and I-type granitoids from the Czech Republic were analyzed for comparison. For the monitored rare metals, the maximum contents are 1200 ppm Sn (Madeira), 350 ppm W (Beauvoir, Nejdek), 2300 ppm Nb (Madeira) and 200 ppm Ta (Orlovka, Cínovec and Nejdek); Sc reaches maximally 300 ppm at Orlovka. Micas from peraluminous RMG are generally rather enriched in W and Cs, while micas from subaluminous A-type granites are relatively enriched in Nb and Sc. Micas from the studied peralkaline rocks are rich in Nb and poor in Cs, Sc and W. With increasing fractionation of the parental magma, the contents of F, Li, and Rb in mica increase, while the contents of high-field-strength elements (HFSE) usually decrease. This can be attributed to the crystallization of late Li-mica after the HFSE accessories rather than to the effect of the mica crystal structure. Micas affected by high-temperature hydrothermal events are usually completely re-equilibrated. By contrast, low-temperature muscovitization connected with washing out of Li and an increase in Sn affected only rims of mica flakes, leaving crystal cores in their original composition. In a succession from early biotite granites to late Li-mica granites, the share of mica in bulk Sn, Nb and Ta contents in the rocks decreases (50–80 › 5–10%, 10–80 › 1–10% and 100 › <5%, respectively), and rutile, cassiterite and columbite/pyrochlore become the main hosts of rare metals.
|