Page 199 - Microtectonics

7

Porphyroblasts and Reaction Rims

7.1 I nformation contained in the structure in and around porphyroblasts and in re-
Introduction action rims is discussed in this chapter. Porphyroblasts often contain inclusion
patterns that reflect the structure of the matrix during their growth. Once formed,
7.2 a rigid porphyroblast protects these patterns from further modification, whereas
Porphyroblast the matrix may be subjected to later crenulation, recrystallisation, grain growth
Nucleation and Growth etc. For this reason comparison of porphyroblasts and their surrounding matrix
gives valuable information on the sequence of deformation phases and the rela-
7.3 tive time interval of porphyroblast growth with respect to these phases. A dis-
Inclusions tinction is made between pretectonic, intertectonic, syntectonic and post-tec-
tonic porphyroblasts. Using this technique a relative sequence of metamorphic
7.4 events and deformation phases can be established.
Classification of Special inclusion patterns, such as millipede structures, deflection folds and
Porphyroblast-Matrix oppositely concave microfolds are explained and discussed. Attention is also given
Relations to possible pitfalls in the interpretation of porphyroblast microstructures, such
as inclusion shapes and patterns controlled by crystallographic directions rather
7.5 then representing passive inclusions reflecting a pre-existing matrix foliation.
Millipede, The relative rotation or non-rotation of porphyroblasts with respect to their
Deflection-Fold and surrounding matrix or to geographic coordinates is still a hot topic in the litera-
Oppositely Concave ture and a revision is given of the current state of the art.
Microfold (OCM) A new topic refers to amalgamated porphyroblasts, a possible source of mis-
Microstructures interpretation that may produce in some cases apparently enigmatic inclusion
patterns.
7.6 The final part of this chapter deals with reaction rims, an important tool to
Problematic unravel the metamorphic history of the rock. Attention is given to coronas, moats,
Porphyroblast symplectites and pseudomorphs.
Microstructures

7.7
Crystallographically
Determined
Inclusion Patterns

7.8
Reaction Rims