Page 247 - Microtectonics

238 8 · Primary Structures
8.1 8.1 van der Molen and Paterson 1979) and although it is usu-
Introduction ally around 30%, it may be as high as 50% (Vernon et al.
1988) or as low as 10–20% for gabbroic rocks (Nicolas
The objective of this chapter is to review various micro- et al. 1988). The transition from submagmatic to mag-
structures from igneous and sedimentary rocks that are matic flow probably roughly coincides with the critical
similar to structures in metamorphic rocks. Igne- melt fraction (e.g. Blenkinsop 2000, his Fig. 6.1), and cor-
ous rocks tend to crystallise gradually passing through responds to the transition from grain-supported flow to
transitional stages between the liquid and the solid state, suspension flow.
that is with various percentages of solid crystals within
a melt. If they experience deformation during this proc- 8.2.2
ess a number of specific microstructures may form in- Evidence for Magmatic Flow
dicative of these stages; these will be revised below. Sedi-
mentary rocks may contain a large variety of sedimen- Most of the criteria mentioned below are also discussed
tary structures (e.g. Collinson and Thompson 1982), by Vernon (2000) and Blenkinsop (2000).
some of which can be conveniently studied on the mi-
croscopic scale. It is not our objective to review these Grain Shape Preferred Orientation of Euhedral Crystals
structures here; only a few structures that may be mis-
taken for tectonically induced structures in fully lithified The best criterion to recognise magmatic flow is a
rocks will be discussed. grain shape preferred orientation of inequant euhedral
crystals that are not internally deformed (Figs. 8.1, B.5.3).
8.2 8.2 The preferred orientation may be an S, L or, more
Primary Structures in Rocks of Igneous Origin commonly, an SL fabric and is usually defined by
or in Migmatites feldspar and/or mica in felsic rocks and by feldspar,
pyroxene, amphibole or olivine in mafic rocks. Charac-
Many bodies of igneous rocks contain structures at- teristic is the lack of rounded corners and lack of de-
tributed to magmatic flow that are partially or com- formation in an isotropic matrix that can be very reduced.
pletely overprinted by “solid state” deformational fabrics, Some relatively weak undulose extinction in quartz
often related to regional deformation. To unravel the is not uncommon. One has to realise that the preserved
igneous, metamorphic and structural evolution of these fabric relates probably to the later stages of magma
rocks it is fundamental to analyse microtectonic evidence solidification, since earlier stages are easily destroyed
for magmatic, submagmatic and solid state flow. This by the flowing magma. Magmatic foliation does not
analysis is also essential for the understanding of magma always reflect magma flow planes (Yuan and Paterson
formation and migration in anatectic migmatites (e.g. 1993; Paterson and Vernon 1995; Paterson et al. 1998)
Sawyer 2001). but in several studies that interpretation explains best
We will first shortly discuss the concepts of magmatic, the field and laboratory data (e.g. Cruden et al. 1999;
submagmatic and solid state flow and then review the McNulty et al. 2000).
microstructural evidence for each of them.
Imbrication (“Tiling”) of Elongate Euhedral Crystals
8.2.1 that Are Not Internally Deformed
Magmatic and Submagmatic Flow
This structure also implies the presence of enough melt
Magmatic flow is defined as flow by displacement of melt, to enable the crystals to rotate without plastic deforma-
with consequent rigid-body rotation of crystals but with- tion (e.g. Blumenfeld 1983; Shelley 1985; Blumenfeld and
out sufficient interference between crystals to cause crys- Bouchez 1988; Mulchrone et al. 2005). Blenkinsop (2000)
tal plastic deformation (Paterson et al. 1989; Smith 2002). considers this structure as non-diagnostic since it may
Submagmatic flow can be defined as deformation involv- also be formed in mylonites (compare Sect. 5.6.17), but
ing flow of melt and crystals, assisted by crystal plastic if the crystals are euhedral and the matrix isotropic it
deformation. seems a reliable microstructure.
Experiments have indicated that the strength of a rock
decreases by about an order of magnitude when the melt Ophitic Fabric, Oscillatory Zoning and Growth Twins
fraction increases from zero to only a few percent. In-
creasing the melt fraction to a value around 30% the vis- These microstructures, especially the first two (Figs. 8.2,
cosity of the rock system with melt (magma) decreases 8.3), are characteristic for magmatic rocks. Twinning is
again abruptly by several orders of magnitude. This value more difficult to interpret since many types of twins may
is known as the critical melt fraction (CMF; Arzi 1978; also form in metamorphic rocks.