Page 249 - Microtectonics

240 8 · Primary Structures

Fig. 8.3. Oscillatory zoning in plagioclase, typical for magmatic crystallisation. Quartz diorite from Tijuca, Rio de Janeiro, Brazil. Width of
view 2 mm. CPL
Lack of a Deformation Fabric [001] direction parallel to the magmatic lineation, and
feldspars in gabbros and tonalites have [100] parallel to
This can be a strong indication for magmatic flow. How- the magmatic lineation (Benn and Allard 1989). In the
ever, it is not conclusive since static recrystallisation and case of olivine distinction can be made between mag-
grain growth may produce, especially in the granulite matic and high grade solid state deformation since in
facies, isotropic fabrics, usually hard to distinguish from the former [001] is parallel to the lineation and in the
magmatic fabrics. latter [100] (Benn and Allard 1989).

Distribution of Mineral Phases Angular Fragments in Flow Structures

In igneous fabrics the mineral phases are usually uni- Flow structures in some lavas may resemble mylonitic
formly dispersed whereas in recrystallised metamorphic foliation (Fig. 8.4). However, solid particles in lava may
rocks they are often concentrated in clusters. This is be- retain their angular shape during magmatic flow (Fig. 8.4),
cause in the latter case they may be derived from few large whereas porphyroclasts in mylonites are usually rounded
parent grains by recrystallisation (Blenkinsop 2000). (e.g. Figs. 5.22–5.25).

Crystallographic Preferred Orientation Elongated Shape of Enclaves

Since magmatic fabrics are produced by flow of euhedral Although this book is essentially concerned with micro-
phenocrysts these have, apart from a grain shape fabric, structures it is often advantageous to consider mesoscopic
also a pronounced lattice preferred orientation. Accord- evidence in conjunction with microstructures. For exam-
ing to Benn and Allard (1989) olivines, pyroxenes and ple the elongated shape of microgranitoid enclaves that
feldspars have their (010) crystal faces parallel to the do not show evidence of internal plastic deformation in-
magmatic foliation. Olivine and clinopyroxene have their dicates that they flowed as magma globules (Vernon 2000).