Contrasting quartz and feldspar viscosities in natural
strain and temperature gradients: preliminary study
Prokop ZÁVADA, Karel SCHULMANN, Ondrej LEXA, Jiří KONOPÁSEK
Institute of Petrology and Structural Geology, Charles University, Albertov 6, 128 43, Prague
The rheological behaviour of polyphase rock systems such as conglomerates of granitoids
depends on the bulk rock viscosity, the phase viscosity contrast, weak and strong phase
fraction, and the initial clast shape. The viscosity contrasts were determined for pebbles of
various compositions in natural conglomerates (Gay, 1969, Freeman and Lisle, 1987, Lisle
and Savage, 1983). However, these rocks often exhibit non-uniform distribution of clast sizes,
highly variable compositions, internal anisotropies of pebbles and varied ratios of clast matrix
proportions. In addition, the rheological properties of pebbles are not known from
experimental studies.
We studied coarse-grained granitoids of similar original modal compositions (quartz
(19%), K-feldspar (49%) and plagioclase (28%)), and relatively constant original grain size
deformed at various temperatures. The samples were collected from two adjacent units
forming a nappe pile at the western border of the Saxothuringian domain, Czech Republic.
The upper unit consists of granulites, migmatized granulites, and coarse-grained orthogneisses
(Bt, Grt ± sillimanite) deformed to variable degree at high temperatures close to the granulite
facies conditions. The lower unit is composed of high pressure micaschists (Konopásek,
2001), mafic eclogitic bodies and sheets of medium-grade Ms-Bt orthogneisses. We compared
the internal strain and deformational microstructures of both high-grade and medium-grade
orthogneisses to depict the possible rheological behaviour of felsic rocks under various
thermal conditions. The estimated viscosity ratios of individual phases and bulk rock are
compared with their rheological properties determined experimentaly.
The shapes of K-feldspars and quartz were digitized from XZ and YZ sections, their
topology was traced in GIS Arc View environment and the shapes were analysed using the
PolyLX Matlab™ toolbox. The particle shapes were approximated by ellipses and the shape
ratios were calculated using the method by Harvey & Fergusson (1981). To estimate the bulk
strain, the Fry’s method (Fry, 1979) was applied. The quartz of high-grade orthogneiss
exhibits lower strain intensities than K-feldspar and plagioclase. The strain symmetry of all
phases is in apparent flattening field, except for one sample in which the weak feldspars are
strongly constricted. The medium-grade orthogneiss shows strong K-feldspar, weaker quartz
and the weakest plagioclase. For the low bulk strain intensities, the strain symmetry of all
phases is close to plane strain, but with increasing strain the shapes become oblate. The bulk
strain in strongly deformed samples is difficult to establish using Fry’s method, due to the
contribution of viscous deformation of strong phases and their passive displacement in weak
matrix. Therefore, the viscosity contrasts between strong and weak phases were assessed
using the Gay’s (1969) equation yielding values around μq/μfel = 0.5 – 0.6 for the high-grade
orthogneiss, around 1 for the low strained medium-grade orthogneiss, and 2 – 3 for the highly
sheared medium-grade rocks. This contrasting quartz and feldspar shape behaviour is
correlated with their microstructural development. The medium-grade orthogneiss exhibits
extremely fine-grained plagicolase (50) recrystallized even under very low strain intensities,
fractured K-feldspar, which shows some recrystallization at grain margins and recrystalized
quartz exhibiting a uniform grain size (0.5-1.2mmrarely 4mm). Weak grain boundary
migration and rather straight grain boundaries are characteristic of all deformational stages.
The high-grade orthogneiss consists of recrystallized aggregates of well-equilibrated
plagicolase mosaic (0.6-0.8mm) and recrystallized K-feldspar (1.75mm) coated with narrow
plagioclase-quartz films. Quartz grains with highly migrated grain boundaries (1.75-2.5mm in
size) form globular aggregates.
The microstructure is further quantified using the GIS Arc-View environment and the
PolyLX Matlab™ toolbox to provide precise information about grain size, grain contacts and
modal compositions of individual layers. In addition, the mineral CPO (crystallographic
preferred orientation) is studied in order to reveal possible deformation mechanisms. The
preliminary results show that the relative rheologies of feldspars and quartz in natural rocks
are not consistent with experimental results. Their mechanical behaviour is probably highly
sensitive to strain intensity as well as to temperature and recrystallization grain size.
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