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MAGMATIC LAYERING (UNIDIRECTIONAL SOLIDIFICATION TEXTURES
AND Y-ENRICHED GARNET TRAIN TEXTURES) IN APLITE – PEGMATITES OF
THE CADOMIAN BRNO BATHOLITH, CZECH REPUBLIC
Hönig Sven1§
, Leichmann Jaromír1
, Novák Milan1
1
Department of Geological Sciences, Masaryk University, Kotlářská 2, 611 37Brno, Czech
Republic §
honig@mail.muni.cz corresponding author
Key words: Unidirectional Solidification Texture (UST), A-type granite, magmatic layering, aplite-pegmatite,
yttrium, garnet, Train texture.
INTRODUCTION
Rarely occurred magmatic structure - Unidirectional solidification textures (UST) - are known
as a specific type of the magmatic layering. This non-genetic term describes the
morphological features of layers with crystals oriented perpendicular to, or with high angle to,
the strike/plan of a layer. Typical morphological features of UST are comb-like or crenulate
shape of the crystals oriented perpendicularly to the layer. These crystals are present in layers
and multilayered sequences, with textures indicative of mineral growth in one direction from
a solid substrate.
The term UST was used for the first time by Shannon et al. (1982) for textures observed in
subvolcanic granitic bodies at the Henderson Mine, Empire, Colorado; however, older
Russian texts also described identical textural features in the Transbaykal region. Similar
textures were recently found in many strongly peraluminous, volatile-rich (B, F and H2O)
aplite–pegmatites as well (e.g., London 1992; Breiter et al. 2005).
Compared to these highly-fractionated, micas-bearing and peraluminous aplite-pegmatites the
UST were newly found in the felsic, A-type, metaluminous, garnet-bearing aplite-pegmatites
of the Brno Batholith, SE Czech Republic (Hönig et al. 2010), studied in this paper.
GEOLOGICAL SETTING
The Brno Batholith is Cadomian magmatic complex formed mainly by amphibole–biotite and
biotite granitoids, pegmatites or other more evolved leucocratic granitic rocks are evolved
very sporadically, in additional. Three distinct internal subunits were recognized in the Brno
Batholith, each separated by sharp tectonic boundaries: Central Metabasite Zone, Eastern
Granitoid (I-type) Complex and Western Granitoid Complex, inside which three distinctive
granitic suites were observed. Two of them are represented by evolved rocks of an active
continental margin with affinities to S- and/or I-type granites. The third suite (Hlína suitestudied
in this paper), is built by felsic, layered, garnet-bearing, aplite–pegmatite intrusions.
Now is classified as A-type granite (Leichmann et al. 1999).
Dykes and lens-shaped bodies of the layered Hlína aplite–pegmatites and associated aplitic
granites cut both other S-and I- type granodiorites of the Western Granitoid Complex. The
examined aplite–pegmatite bodies are ~2 to 50 m thick and up to 200 m long with a general
NW–SE strike and dip 40–80° to the northeast or the southwest. Sharp contacts of the aplite–
pegmatite dykes and host granodiorites are only scarcely exposed and commonly strongly
weathered.
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TEXTURES AND STRUCTURES OF THE HLÍNA LAYERED SUITE
Studied dykes are characterized by alternating of generally fine-grained layered garnetbearing
aplite units containing parallel comb-like coarse-grained UST units (Fig.1). Major
minerals of UST include K-feldspar, plagioclase (An 15-10) and quartz, accessory minerals
are extremely rare. The tops of elongated crystals are always growing into the fine-grained
aplite unit with sharp boundary. For the bottom/opposite sides are typical transitional
boundaries. Contacts of both (aplite and UST) units are typically sharp although gradual
transitions were also observed. Thickness of the individual zones varies from several cm to
cca 1–2 m for aplites and cca 10 cm for UST zones. The layered character of aplitic rocks is
pointed up by parallel stripes of Y-enriched garnet grains. This train textures are composed by
numerous of small (< 0.5 mm) individual garnet grains always arranged into long lines (up to
several meters long), where the individual garnet crystals are in direct contact (Fig.2 a,c,d).
Stripes are always parallel to the plane of the aplite unit. Less abundant are similar associated
strips of magnetite.
Cathodoluminiscence study of fine-grained aplite unit and coarse-grained UST exhibit several
internal structures inside both K-feldspar and plagioclase. Zoned feldspar from aplite unit
(Fig.2 e) are very common: K-feldspar, lacking perhithes, are marked by variable intensity of
blue color with well developed hourglass sector zoning in their middle parts as well as light
blue dissolved sector zones on corroded rims. Subhedral grains of plagioclase (light green
color) show multiple oscillatory zones. Feldspars represented in the coarse-grained UST unit
have zoned texture too. The comb-like shaped crystals of K-feldspar show light-blue sector
luminescence zoning with sharply jagged rims in their centre; no zoning were observed in
their dark-blue outer parts.
A different modal composition as well as textural dissimilarity between alternated UST and
aplite units is remarkable. The interlayered aplite units seems to have features of very rapid
cooling such as hourglass zoning, abundant symplectites-granophyres presented between
feldspars and quartz. Unlike UST that show effects of probably slightly slower crystallization
because oscillatory zoning of comb-like K-feldspar observed by CL.
WHOLE-ROCK CHEMISTRY
All aplite–pegmatites of the Hlína suite have a felsic character documented by high contents
of SiO2 (74.6–75.7 wt. %), K2O (4.61–4.94 wt. %), Na2O (3.82–4.21 wt. %), moderate
concentrations of CaO (0.94–1.11 wt. %), and low to very low concentrations of Fe2O3
T
(0.62–0.93 wt. %), MgO (0.02–0.03 wt. %) and TiO2 (≤ 0.03 wt. %). Slightly elevated
concentrations of Rb (170–182 ppm) and MnO (0.12–0.32 wt. %) are positively correlated
with Y and HREE concentrations. Low K/Rb (212–241) and high K/Ba ratios (1034–2303)
with deep negative Eu anomalies indicate high degree of fractionation.
Typical is depletion in LREE (cca 10× enrichment over chondritic abundances) relative to
HREE (70–90× enrichment relative to chondrite); noticeable is also low contents of volatilies
(B,F, P and H2O).
The ASI of the Hlina suite is moderte (alumina saturation ~ 1.01); affinity to calc-alkaline
series (calc-alkali index = 15.1-16.2) is also typical. After tectonic diagrams of Pearce et al.
(1984) Hlína aplite-pegmatites plot to the WPG (Within-Plate Granites); Moreover, high
content of Y (64-116 ppm), Nb (12-28 ppm) and Ga classify these rocks into the fields of
post-collisional, post-orogenic and anorogenic A-type granites (Eby 1992).
CONCLUSSION
Based on discussed above, the crystallization of the magma was probably several time
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interrupted (alternation of UST and interlayered aplite units); moreover, the process of nonequilibrium
crystallization is characterized by the layered /striped structure of the magmatic
Y-rich garnet (spessartine-almandine solution) also with evolved internal oscillatory and
sector zoning. Mechanism for formation garnet-rich and garnet-poor layers is considered
oscillatory crystallization-related process at the evolving chemical boundary front (process
suggested by Webber et al. 1999) formed next to the partly solidified surface of the UST unit.
All these features are strikingly different from other known UST-bearing intrusions
worldwide, which are typically associated with strongly peraluminous, volatile-rich (B, F, P
and H2O) intrusions of distinct mineralogy.
ACKNOWLEDGEMENTS
This work was supported by the research projects GAČR IAA300130801 and
MSM0021622412.
REFERENCES
Breiter K, Müller A, Leichmann J, Gabašová A (2005) Textural and chemical evolution of a
fractionated granitic system: the Podlesí stock, Czech Republic. Lithos 80: 323–345
Eby GN (1992) The A–type granitoids: a review of their occurrence and chemical
characteristics and speculations on their petrogenesis. Lithos 26: 115–134
Hönig S, Leichmann J, Novák M (2010) Unidirectional solidification textures and garnet
layering in Y-enriched garnet-bearing aplite–pegmatites in the Cadomian Brno Batholith,
Czech Republic. J of Geosciences 55: 81-97
Leichmann J, Novák M, Sulovský P (1999) Peraluminous whole-rock chemistry versus
peralkaline mineralogy of highly fractionated, garnet-bearing granites from the Brno
Batholith. Ber Deutsch Miner Ges 1: 144
London D (1992) The application of experimental petrology to the genesis and crystallization
of granitic pegmatites. Canad Mineral 30: 499–540
Pearce JA, Harris NBW, Tindle AG (1984) Trace element discrimination diagrams for the
tectonic interpretation of the granitic rocks. J Petrol 25: 956–983
Shanon on JR, Walker BM, Carten RB, Geraghty EP (1982) Unidirectional solidification
textures and their significance in determining relative ages of intrusions at the Henderson
Mine, Colorado. Geology 10: 293–297
Webber KL, Simons WB, Falster AU, Foord EE (1999) Cooling rates and crystallization
dynamics of shallow level pegmatite–aplite dikes, San Diego County, California. Amer Miner
84: 708–717
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FIGURE CAPTIONS
Fig.1
Idealized vertical section of the Hlína layered aplite-pegmatite. Arrow shows direction of
crystallization. Individual layers and features are not to scale. Thickness of interlayered aplite
typically vary from 0.1 to 0.5 m, UST unites are usually 0.2 – 10 cm thick.
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Fig.2
Upper row images – macrotextures of the layered Hlína suite; a – Aplite unit is typical by
parallel garnet strips and associated magnetite. b – UST unit characterized by comb-like
shaped Kfs, Qtz and Plg. The tops of elongated crystals growing into the fine-grained aplite
unit with sharp boundary. Opposite sides of the UST have a transitional boundaries.
Lower row images – structures of garnets (c,d) and feldspars (e); c – BSE image of the train
texture, garnets from the strip. d – garnets from strip in BSE, typical is a complex inner fabric
(sector zones in the centre, oscillatory zones in the rims). e – CL image of the blue colored
hourglass and sector zoning K-feldspar.