TH131 – Influence of reaction kinetics on rock microstructure, texture and micro-chemistry… – poster806
Unidirectional solidification texture (UST) and
garnet layering textures from the Y-enriched
garnet-bearing aplite-pegmatites, western part of
the Cadomian Brno Batholith, Czech Republic
Hönig, S., Leichmann, J.*
& Novák, M.
Dept. of Geological Sciences, Masaryk University, Brno, Czech
Republic (*
leichman@sci.muni.cz)
Felsic 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° into ESE or
WSW, cut granodiorites to granites at the SW part of the Brno
Batholith, Brunovistulicum. Aplite-pegmatite bodies are
characterized by alternation of two main textural units: (i) a
fine-grained aplite unit and (ii) a coarse-grained pegmatite with
comb-like UST unit both developed in zones with the thickness
varying from several cm to ~ 1-2 m for the aplite unit, and ~ 10
cm for the UST unit. In this paper, we use the term “UST”
(unidirectional solidification texture) which describes rock layer
composed of crystals oriented perpendicularly to the plain of
layering.
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 concentrations of CaO (0.94 – 1.11 wt. %),
and low to very low concentrations of Fe2O3 (0.62 – 0.93 wt.%),
MgO (0.02 – 0.03 wt. %), and TiO2 (≤ 0.03 wt. %). Low K/Rb
(212–241), high K/Ba ratios (1034 – 2303) and deep Eu
anomalies indicate a high degree of fractionation. Both textural
units consist of perthitic microcline, plagioclase An15-8 and
quartz. The total amount of accessory minerals is typically very
low, commonly < ~ 1 vol.% in the aplite unit, 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 - 0.06 apfu, 0.53 wt.% Yb2O3 and 0.20 wt.%
Er2O3) with oscillatory and sector zoning. Other minerals
commonly 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 characterized by (i)
metaluminous to slightly peraluminous A-type (NYF) affinity
indicated by elevated concentration of Y, REE (especially
HREE), Zr, U, Th, (ii) specific accessory minerals including Yrich
garnet, Nb>>Ta, HREE>>LREE, (iii) almost entire absence
of micas and other minerals with volatiles (B, F, P and also
H2O) are remarkable. They differ from all other granitic rocks
with UST textures described to date typically characterized by
high activity of volatiles and peraluminous signature. Magmatic
layering of garnet is explained by the formation of a boundary
chemical front bordered margin of the coarse-grained UST unit.
Modelling of nonlinear inter-granular diffusion
under local equilibrium with a mineral: an
example in the system muscovite – quartz – H2O
Nishiyama, T.
Dept. of Earth & Environmental Sciences, Kumamoto
University, Kunamoto, Japan (tadao@sci.kumamoto-u.ac.jp)
Diffusion through an inter-granular fluid plays an important role
in texture formation of the metamorphic rocks such as reaction
zones and metamorphic layering. The nature of the diffusion is,
however, not well understood. This paper will discuss the
situation such that a fluid species diffuses down its chemical
potential gradient under the constraint of local equilibrium with
a co-existing mineral. The constraint will cause a coupling of
diffusive fluxes, which is described by nonlinear diffusion
equations. The aim of this paper is to evaluate the effect of the
coupling which possibly contributes to the texture formation in
metamorphic rocks.
Suppose a metamorphic rock in the system muscovite –
quartz – H2O, which consists of a muscovite – rich layer and a
quartz – rich layer. At an initial pressure - temperature
condition, the system is in an equilibrium, namely, muscovite,
quartz and H2O (an inter-granular fluid) are in equilibrium with
each other in both layers. The system will destabilize according
to a pressure – temperature change. If temperature is raised, the
solubility of minerals increases. The muscovite – rich layer will
reach the muscovite saturation condition before arriving at the
new quartz – muscovite equilibrium condition, because the
amount of dissolved species is proportional to the total surface
area of the mineral and hence the concentration of dissolved Al
species is higher than that at the new quartz – muscovite
equilibrium condition. Similarly, the quartz – rich layer reaches
the quartz saturation condition before arriving at the new quartz
– muscovite equilibrium condition. Then spontaneous diffusion
will take place between the two layers. The diffusion is assumed
to occur under local equilibrium with muscovite. The local
equilibrium condition with quartz is not introduced, because it
does not drive diffusion of Si species. We will discuss only
diffusion of Al and Si species, assuming that diffusion of K is
much faster than that. The model nonlinear diffusion equations
become coupled PDEs. By Boltzmann transformation, the
coupled PDEs are transformed into coupled ODEs. Perturbation
expansion decomposes the coupled ODEs into uncoupled
ODEs. Finally the uncoupled ODEs are solved numerically with
MATLAB. Essential features of the nonlinear diffusion can be
summarized as the following. (1) The coupling of diffusive
fluxes causes advection in Al concentration profiles. (2) The
case of smaller DSiSi/DAlAl shows larger advection effects in Al.
(3) The coupling causes suppression in Si concentration
profiles. (4) The case of smaller DSiSi/DAlAl shows uphill
diffusion of Si in a muscovite – rich layer. This study is a first
step to clarify the effect of nonlinear diffusion on the
development of texture in metamorphic rocks.
Fig. 1: Comparison of Si profiles between normal and nonlinear
diffusion. Nonlinear diffusion shows uphill diffusion near the boundary.