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Metamorphic Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
In the outermost zone of the aureole, farthest from the contact, small but noticeable changes are produced: the shales are somewhat hardened and specks of opaque magnetite are formed. Somewhat nearer the contact incipient crystals of chiastolite and cordierite appear, as small dark spots; and with further progress towards the igneous contact andalusite and cordierite are better developed, as distinct spots, and small flakes of biotite have grown from the chlorite and sericite in the shale. This is the spotted-rock zone in Fig. 7.3. The recrystallization is continued in the zone nearest the contact, including the growth of muscovite flakes and quartz and larger magnetites; crystals of rutile (TiO2, tetragonal) may also be present, derived from titanium in the original sediment. The shale has now been completely recrystallized, and is called a hornfels, or to give it its full name, a biotite andalusite - cordierite-hornfels. It is hard, with a ‘horny’ fracture, and has a finely granular hornfels texture (Fig. 7.4). In some hornfelses, next to a high temperature contact, needle-shaped crystals of sillimanite may be formed from excess alumina, in addition to the other minerals. The ‘zones’ described above are not sharply separated but grade into one another; and it is assumed that there has been neither addition nor subtraction of material during the metamorphism.
The interplay of structure and metamorphism at Broken Hill, NSW, Australia
Published in Australian Journal of Earth Sciences, 2022
Complications arise in the interpretation of the distribution of peak metamorphic assemblages. First, the peak metamorphic grade distribution represents the product of two overlapping grade distributions, as both the D1 and D2 structural events occurred at high metamorphic grade. Second, Fitzherbert (2015) concluded that the andalusite and some cordierite were formed before the first deformation, mainly on the basis that much andalusite (and chiastolite) occurs as randomly oriented crystals. Earlier, Vernon et al. (1993) found that porphyroblasts of andalusite, microcline and biotite had grown before the first tectonic foliation.