Publication details
Unidirectional solidification textures and garnet layering inY enriched garnet bearing aplite pegmatites in the CadomianBrno Batholith, Czech Republic
| Authors | |
|---|---|
| Year of publication | 2010 |
| Type | Article in Periodical |
| Magazine / Source | Journal of Geosciences |
| MU Faculty or unit | |
| Citation | |
| Field | Geology and mineralogy |
| Keywords | Unidirectional Solidification Texture; magmatic layering; garnet; yttrium; aplite pegmatite; Brno Batholith |
| Description | Layered, garnet bearing aplite pegmatite dykes of the Hlína granitic suite, ~2 to 50 m thick and up to ~200 m long with general NW SE orientation and dip 40 80 to NE or SW, cut granodiorites to granites at the SW part of the Brno Batholith, Brunovistulicum, Czech Republic. Aplite pegmatite bodies are characterized by alternation of two main textural units: (i) fine grained aplite unit with garnet stripes and (ii) coarse grained pegmatite unit with oriented Qtz, Plg and Kfs comb like crystals known as Unidirectional Solidification Textures (UST). Both units developed in zones with the thickness varying from several cm to ~1 2 m for aplite unit, and ~10 cm for the UST unit. All rock types are characterized by high contents of SiO2 (74.6 75.7 wt. %), K2O (4.61 4.94 wt. %), Na2O (3.82 4.21 wt. %), moderate concentration of CaO (0.94 1.11 wt. %), and low to very low concentrations of Fe2O3 T (0.62 0.93 wt. %), MgO (0.0 0.03 wt. %), and TiO2 ( 0.03 wt. %). Low K/Rb (212 241) and high K/Ba ratios (1034 2303) with deep Eu anomaly indicate high degree of fractionation. Both textural units consist of perthitic microcline, plagioclase An15 8 and quartz. Total amount of accessory minerals is typically very low, commonly < ~1 vol. % in aplite unit, and in the UST unit they are almost absent. Accessory minerals include relatively common Y rich garnet Sps42 38 Alm32 28 And15 7 Grs21 15 Prp2 1 (1.10 wt. % Y2O3, 0.53 wt. % Yb2O3 and 0.20 wt. % Er2O3) with oscillatory and sector zoning. Other minerals closely associated with garnet are extremely rare: magnetite, chloritized biotite, muscovite, Ta rich titanite I, Al, F rich titanite II, and ilmenite. Primary zircon, xenotime (Y), monazite (Nd), fersmite, ferrocolumbite, REE,Y rich pyrochlore are strongly altered. Geochemical and mineralogical features of the Hlína aplite pegmatites are characterized by (i) subaluminous chemistry with Nb >> Ta and HREE >> LREE, A type (NYF) affinity, indicated by elevated concentrations of Y, REE (especially HREE), Zr, U, Th, Nb, and Ta, (ii) occurrence of specific accessory minerals including Y rich garnet, and (iii) a remarkable, almost complete absence of micas and other minerals with volatiles (B, F, P and also H2O). The studied suite differs from all other granitic rocks with UST described to date, which are typically characterized by volatile bearing minerals and peraluminous signatures. Magmatic layering involving g |
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