Page 121 - Microtectonics

4.4 · Lattice-Preferred Orientation (LPO) 109
4.4.5 olivine is at the limit of crystalplastic behaviour, diffuse
LPO Patterns of Other Minerals girdles of principal axes may occur. At medium tempera-
ture (≈1000 °C), a girdle of [0kl] normal to L and a point
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As in quartz, LPO in other minerals strongly depends on maximum of [100] parallel to L may form. At high tem-
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active slip systems and on the geometry and symmetry perature (>1100 °C) a point maximum of [010] normal
of the flow pattern, resulting in a similarity of LPO geom- to S and [100] parallel to L develops; and at hypersolidus
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etry and strain geometry. As for quartz, slip systems and conditions (>1250 °C) a point maximum of [100] paral-
fabric elements tend to rotate towards the fabric attractor, lel to L and partial girdles of [010] and [001] are formed.
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resulting in a common subparallel orientation of compo- Fabrics with external asymmetry do occur and can be
nents of the LPO, and S and L in the rock. used to determine shear sense (Avé Lallemant and Carter
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Calcite c-axis LPO patterns show a similar influence 1970; Mercier 1985).
of flow symmetry on pattern geometry. At low tempera- In clinopyroxene three main types of LPO have been
ture (<300 °C) coaxial progressive flattening produces a described; (a) [100] normal to S and [001] parallel to L or
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maximum around the Z-axis of strain. In constriction a S (Mainprice and Nicolas 1989; Phillipot and van Roermund
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girdle through Y and Z develops and in coaxial plane 1992); (b) [010] normal to S and [001] parallel to L or S r
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strain a maximum around Z and a minor girdle through Y (Mainprice and Nicolas 1989); (c) [010] normal to L and
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(Wenk et al. 1986a; Shelley 1993). In simple shear flow, a [001] parallel to L (Helmstead et al. 1972; van Roermund
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c-axis girdle develops as for quartz, but dipping in the 1983, 1992). (a) is mainly found in peridotite massifs with
opposite direction (in the direction of the shortening ISA). pyroxenite layers and may either be formed at low tempera-
This is due to the fact that in calcite at low temperature, e- ture and high strain rate or, if this is unlikely, by post-tec-
twinning is largely responsible for development of the LPO tonic crystal growth (van Roermund 1992). (b) and (c) form
fabric (Schmid 1982; Behrmann 1983; Schmid et al. 1987; by crystalplastic deformation by multiple slip in medium
Wenk et al. 1987; Rutter et al. 1994; Lafrance et al. 1994; to high temperature eclogites by activity of dislocations
Rutter 1995; Khazanehdari et al. 1998; Casey et al. 1998; with predominantly [001] and <110> Burgers vectors (van
Pieri et al. 2001). Roermund 1983, 1992; Buatier et al. 1991). Their difference
Experimental and computer models indicate that at seems to reflect different strain types, constriction for
high temperature in plane strain, split maxima may de- Type (b) and flattening for Type (c) (Helmstead et al. 1972;
velop around the shortening axis (Wagner et al. 1982; van Roermund 1992). Asymmetric fabrics have not been
Wenk et al. 1986a, 1987; Schmid et al. 1987; Takeshita et al. reported for clinopyroxene in the literature.
1987; de Bresser 1991). Natural examples of such a high In orthopyroxene deformed at high-grade metamor-
temperature LPO pattern in calcite were reported by de phic conditions, LPO of [100] has been reported at a high
Bresser (1989), Burlini et al. (1998), Kurz et al. (2000), Leiss angle to S and of [001] gently inclined with respect to L r
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and Molli (2003) and Barnhoorn et al. (2004). (Dornbush et al. 1994). This preferred orientation is as-
In plagioclase, (010) trends to parallelism with S and sociated with the dominance of (100)[001] as a slip sys-
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[001] with L at medium to high-grade metamorphic con- tem. The LPO reported by Dornbush et al. (1994) is slightly
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ditions (Olsen and Kohlstedt 1985; Montardi and Mainprice asymmetric and can be used to determine sense of shear.
1987; Ji and Mainprice 1988; Shaosheng and Mainprice 1988; Little is known about LPO in amphiboles, but the avail-
Mainprice and Nicolas 1989; Dornbush et al. 1994; Egydio- able data indicate a strong similarity with clinopyroxene.
Silva and Mainprice 1999). At very high grade, however, [001] is commonly parallel to L (Gapais and Brun 1981;
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[100] trends to parallelism with L (Kruhl 1987b; Dorn- Rousell 1981; Shelley 1994) and either (100) (Mainprice
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bush et al. 1994). In shear zones, the pole to (010) tends to and Nicolas 1989) or (110) (Gapais and Brun 1981) paral-
show an external asymmetry with respect to a planar and lel to S . Preferred orientations at medium to high grade
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linear shape preferred orientation, which can be used to may be due to crystalplastic deformation, but LPO in
determine shear sense as in the case of quartz c-axes. amphiboles at low-grade conditions may be due to rigid
Olivine has a complex behaviour that is strongly de- body rotation, dissolution-precipitation or oriented
pendent on temperature (Mainprice and Nicolas 1989). growth (Ildefonse et al. 1990; Shelley 1993, 1994; Imon et al.
At relatively ‘low’ temperatures (700–1 000 °C) corre- 2004). Hornblende fabrics reflect the strain symmetry
sponding to lower crustal or upper mantle levels where (Gapais and Brun 1981).