Abstract

Tourmalines (dravite – schorl – uvite – povondraite) from intragranitic pegmatites of the Třebíč Pluton (porphyritic, amphibole–biotite melasyenite to quartz melasyenite – melagranite), in the Czech Republic, were examined using an electron microprobe, Mössbauer spectroscopy and LA–ICP–MS analyses. Pegmatites with the common assemblage Kfs + qtz + pl (An0–13) + Fe-rich phlogopite (0.49 < XMg < 0.70) and accessory allanite-(Ce), ilmenite, tourmaline and titanite, were divided into three types generally related to the NYF (REL–REE) pegmatites: allanite-type, euxenite-type (+ euxenite, aeschynite, pyrochlore, beryl, niobian rutile, zircon), and the most evolved euxenite-type pegmatite, Klučov I (with annite, XMg 0.09), cassiterite, herzenbergite, and abundant graphic intergrowths of quartz + feldspars and quartz + tourmaline). Black tourmaline occurs in several distinct morphological, paragenetic and textural types with very high variability in zoning; however, the morphological types of tourmaline do not differ chemically. They generally show two distinct trends: Fetot/(Fetot + Mg) = 0.243–0.882, 0.657–0.978; Altot = 5.039–6.464, 5.040–7.030 apfu; Ca ≤0.508, ≤0.174 apfu; Mn ≤0.079, ≤0.445, respectively. High concentrations of Ti ≤0.490, ≤0.361 apfu are typical, as well as low F, ≤0.288, 0.046–0.508 apfu, and low vacancy at the X site, 0.152, 0.348 pfu, respectively. Mössbauer spectroscopy revealed~21% of the Fe as Fe3+ for allanite-type, 17 to 26% for euxenite-type, and 22% for tourmaline from the pegmatite Klučov I, respectively, values that are high relative to ordinary granitic pegmatites. Our LA–ICP–MS study revealed highly variable concentrations of the individual trace elements within single grains and within the individual types of pegmatites. Explicit compositional evolution from less to highly evolved pegmatites was revealed only rarely, e.g., increase in Ni, Co, Zn, Ga, Sc and Ce from less to more evolved pegmatites. Two distinct sets of substitutions are dominant: R3+O R2+−1(OH) −1 and □R3+ Na−1R2+−1. In addition, there are minor substitutions, CaR2+ Na−1Al−1, Fe2+ Mg−1 and MnF Fe2+−1(OH) −1, the latter exclusively at Klučov I. They are similar to those from ordinary Li-poor to slightly Li-enriched granitic pegmatites. Tourmalines from allanite-type and euxenite-type pegmatites exhibit less convincing plots, with the dominant substitutions CaR2+3O2 Na−1R3+−3(OH)−2 and Fe2+ Mg−1, and minor Al Fe3+−1. High contents of Ti4+ imply the presence of the component XCa2+YR2+1.5YTi1.5ZR3+5ZMg2+ Si6O18 (BO3)3O3(OH). These substitutions are different from those normally encountered in granitic pegmatites, but show some features of non-pegmatitic Fe3+-enriched tourmalines. Textural relations suggest very late solidus to early subsolidus origin of the tourmaline. Discussions of a tourmaline’s stability in moderately aluminous granitic systems is shifted to the role of tourmaline composition along with the melt’s composition.

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