Chemical characteristics of zircon from A-type granites and comparison to zircon of S-type granites

Abstract

The trace element content in zircons from A-type granites and rhyolites was investigated by using back-scattered electron images and electron microprobe analyses. The studied Proterozoic (Wiborg batholith, Finland and Pará, Amazonas and Goiás states, Brazil) and Variscan (Krušné Hory/Erzgebirge, Czech Republic and Germany) plutons cover a wide range of rocks, from large rapakivi-textured geochemically primitive plutons to small intrusions of F-, Li-, Sn-, Nb-, Ta-, and U-enriched rare-metal granites. While zircon is one of the first crystallized minerals in less fractionated metaluminous and peraluminous granites, it is a late-crystallized phase in peralkaline granites and in evolved granites that may crystallize during the whole process of magma solidification. The early crystals are included in mica, quartz, and feldspar; the late grains are included in fluorite or cryolite or are interstitial. The zircon in hornblende–biotite and biotite granites from the non-mineralized plutons is poor in minor and trace elements; the zircon in moderately fractionated granite varieties is slightly enriched in Hf, Th, U, Y, and HREEs; whereas the zircon in highly fractionated ore-bearing granites may be strongly enriched in Hf (up to 10wt.% HfO2), Th (up to 10wt.% ThO2), U (up to 10wt.% UO2), Y (up to 12wt.% Y2O3), Sc (up to 3wt.% Sc2O3), Nb (up to 5wt.% Nb2O5), Ta (up to 1wt.% Ta2O5), W (up to 3wt.% WO3), F (up to 2.5wt.%F), P (up to 11wt.% P2O5), and As (up to 1wt.% As2O5). Metamictized zircons may also be enriched in Bi, Ca, Fe, and Al. The increase in the Hf content coupled with the decrease in the Zr/Hf value in zircon is one of the most reliable indicators of granitic magma evolution. In the zircon of A-type granites, the Zr/Hf value decreases from 41–67 (porphyritic granite) to 16–19 (equigranular granite) in the Kymi stock, Finland, and from 49–52 (biotite granite) to 18–36 (leucogranite) in the Pedra Branca pluton, Brazil. In the in situ strongly fractionated Cínovec cupola (Erzgebirge), the Zr/Hf value decreases from 33–51 in the protolithionite granite at a depth of 1255m to 7.5–25 in the zinnwaldite granite at a depth of 40m. At the scale of individual crystals, the Zr/Hf value decreases from 86 to 68 from the cores to the rims of the zircons from the Teplice rhyolite and from 64 to 33 in the zircons from the biotite granite at Krupka, Erzgebirge. The contents of Hf and U in zircon are dependent mainly on the degree of granite fractionation and the nature and volume of the volatile phases and are independent of the A- or S-character of the parental melt. The zircon Zr/Hf ratios 55 and 25 are proposed to approximately distinguish common, moderately evolved and highly evolved granites. Zircons from the moderately and highly evolved granites of A- and S-type can be discriminated on the basis of their HREE content and the U/Th ratios. Nb, Ta, and W are present in zircon from the highly evolved granites from all studied areas, while high As, Bi, and Sc contents are typical only for the Erzgebirge.