GRANITIC COMPLEXES - The Field Description of Igneous Rocks - page 168

Table 7.2 Mineralogical and textural effects of contact metamorphism of
common rock-types (for details of minerals and textures not defined here, refer
to Fry, this topic series).

Pure limestone No mineralogical changes; increase in degree of

recrystallisation as contact is approached, leading to formation
of completely recrystallised marble . The thickness of the zone
of contact metamorphism and recrystallisation varies from a
few centimetres adjacent to minor intrusions, to hundreds of
metres adjacent to larger plutonic intrusions

Impure limestone
(inc. dolomite)

Complex sequence of mineralogical changes; the following
mineralogical assemblages may develop as the contact is
approached: (i) calcite ± dolomite ± clay minerals; (ii) calcite
± dolomite ± olivine (Mg-rich olivine, forsterite) ± amphibole
(tremolite); (iii) calcite ± serpentine ± chlorite ± tremolite;
(iv) wollastonite (Capyroxene) ± garnet (Ca-Mg-Fe rich garnet,
grossularite and andradite) ± pyroxene (Ca-Mg rich pyroxene
diopside). These changes are accompanied by recrystallisation
to marble. The thickness of the zones corresponds to those
described for pure limestone above. Typically, for a
metamorphic aureole several hundred meters wide, the zones
with diopside and wollastonite would be only a few metres
thick

Shale Complex sequence of mineralogical changes; the following

mineralogical changes may develop as the contact is
approached: (i) development of 'spots'; (ii) large crystals (or
porphyroblasts) of andalusite (chiastolite) (Figure 7.25); and
(iii) cordierite-andalusite-biotite. These changes are
accompanied by progressive recrystallisation to hornfels-
textured rocks (these are usually banded and may show
small-scale deformation as a result of partial melting). These
zones may all be developed within 1 - 2 km of the igneous
contact (cf. Figure 7.24)

Sandstones No mineralogical changes; recrystallisation limited to within a

few metres of the contact, and does not usually exceed a few
tens of metres even around large intrusions

Basic volcanic
rocks

Complex sequence of mineralogical changes; the following
mineralogical assemblages may develop as the contact is
approached: (i) epidote-chlorite-amphibole; (ii) plagioclase-
hornblende; and (iii) plagioclase-pyroxene. These changes are
accompanied by progressive recrystallisation to form basic
hornfels-textured rocks (as above) within 1 - 2 km of the
igneous contact