Unidirectional solidification textures and garnet layering in Y-enriched garnet-bearing aplite-pegmatites in the Cadomian Brno Batholith, Czech Republic

Abstract

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 Fe2O3T (0.62-0.93 wt. %), MgO (0.02-0.03 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 garnet is explained by formation of a boundary chemical layer bordering the surface of the coarse-grained UST unit.