Page 73 - Microtectonics

3.13 · Deformation of Polymineralic Rocks 61
rection of the b-axis has more than twice the length of 3.13 3.13
that in pyroxenes. Theoretically, due to the increased Burg- Deformation of Polymineralic Rocks
ers vector length, amphiboles should therefore be stronger
in ductile deformation than clinopyroxenes. In practice, 3.13.1
the opposite is commonly observed. Introduction
Presently available evidence on deformation of horn-
blende suggests that below 650–700 °C, amphiboles Since most rocks are composed of more than one min-
mostly deform by brittle deformation and dissolution- eral, it is interesting to see how individual minerals be-
precipitation, and aggregates of fine-grained hornblende have in a polymineralic rock. Minerals do not always
probably form by fracturing rather than dynamic recrys- show the same dependence in behaviour on temperature
tallisation (Allison and LaTour 1977; Brodie and Rutter and strain rate as in monomineralic aggregates, and may
1985; Nyman et al. 1992; Stünitz 1993; Lafrance and even behave in an entirely different way. The behaviour
Vernon 1993; Babaie and LaTour 1994; Berger and Stü- of polymineralic rocks is remarkably complex (Jordan
nitz 1996; Wintsch and Yi 2002; Imon et al. 2002, 2004). 1987, 1988; Handy 1989, 1992; Bons 1993; Handy et al.
Dissolution of hornblende is probably balanced by 1999; Stünitz and Tullis 2001). The concept of a stress-
deposition of amphibole of a different composition supporting network is important; if ‘hard’ and ‘soft’ min-
(Imon et al. 2004) or of other phases such as epidote, erals coexist, the strength of an aggregate does not in-
albite and biotite elsewhere in the rock (Berger and Stü- crease linearly with the amount of the hard mineral
nitz 1996). Core-and-mantle structures on hornblende present. If few hard grains are present, the strength of
formed below 650–700 °C may also be due to fracturing the aggregate is similar to that of a monomineralic ag-
(Nyman et al. 1992), but where recrystallisation is in- gregate of the soft mineral; the hard minerals may rotate
volved (Cumbest et al. 1989), it is probably driven by a in the flow of the soft material, and may form core-and-
difference in chemical composition rather than strain mantle structures if they recrystallise on the outside. The
energy (Fitz Gerald and Stünitz 1993; Stünitz 1993). The strength of the aggregate increases suddenly when the
main reason for this dominant brittle behaviour seems grains of the hard mineral are so common and large that
to be the excellent cleavage on {110} planes. At low tem- they touch and start to support the imposed differential
perature and/or high strain rate, amphiboles also deform stress. Obviously, the original shape of the grains is also
by deformation twinning on (101) or (100) (Buck 1970; important here. When the hard mineral is dominant, the
Rooney et al. 1975; Morrison-Smith 1976; Dollinger and strength of the aggregate will approach that of the pure
Blacic 1975; Biermann 1981; Hacker and Christie 1990) hard mineral, but at higher strain the pockets of the soft
and slip on (100)[001]. As in micas, slip on (100)[001] mineral may interconnect and form shear zones that
can lead to development of kinks. weaken the aggregate (Jordan 1987). The contrast in rhe-
At high temperature, above 700 °C and in dry rocks ology between two minerals may change and even re-
hornblende can apparently deform by crystalplastic de- verse with changing external conditions. Below, we dis-
formation, and shows strain energy driven dynamic re- cuss the behaviour of quartz-feldspar aggregates as an
crystallisation (Boullier and Gueguen 1998a; Kruse and example of a polymineralic rock.
Stünitz 1999; Fig. 3.24). At high temperature and/or low
strain rate, several slip systems have been documented, 3.13.2
mainly (hk0)[001] and (100)[001] but also {110}1/2<110> Quartz-Feldspar Aggregates
and (010)[100] (Rooney et al. 1975; Dollinger and Blacic
1975; Biermann and van Roermund 1983; Olsen and The study of deformed quartzofeldspathic rocks such as
Kohlstedt 1984; Montardi and Mainprice 1987; Cumbest granites shows an interesting dependence of structure on
et al. 1989; Reynard et al. 1989; Skrotsky 1992; Kruse and metamorphic grade (Vernon and Flood 1987; Tullis et al.
Stünitz 1999). Subgrains are elongated parallel to the 1990, 2000). At very low-grade conditions feldspar and
c-axis and subgrain boundaries consist of simple arrays quartz deform both by brittle fracturing (Fig. 3.42). Micro-
of [001], [100] or <110> dislocations and are parallel to structural observations suggest that feldspar is actually
{110}, (100) or (010) (Biermann and van Roermund 1983; weaker than quartz at these conditions (Chester and Lo-
Reynard et al. 1989). gan 1987; Evans 1988). This is probably due to the fact
A characteristic structure in hornblende schists is that that feldspar grains have cleavage planes that reduce their
of ‘garben’ (German for stack), bundles of elongate horn- strength. As a result, aggregates of elongate cataclased feld-
blende crystals that are oriented in fan-like arrangements spar and quartz develop (Fig. 3.42) where part of the feld-
usually parallel to the foliation plane. Such ‘garben’ may spar (especially K-feldspar) is transformed to kaolinite and
develop by growth of subgrains in the direction of the sericite. A cataclastic foliation of fragmented grain clus-
c-axis in previously deformed hornblende crystals (Bier- ters with fractures and preferred orientation of sheet sili-
mann 1979). cates commonly develops (Evans 1988).