Porphyroblast-Matrix Microstructures as a Tool for Understanding Orogenic Processes

JOHNSON, Scott E., School of Earth Sciences, Macquarie University, New South Wales, 2109, Australia (Scott.Johnson@mq.edu.au)

Porphyroblast-matrix microstructures can provide useful information on a wide range of geological problems, including: (1) metamorphic P-T-t paths; (2) rates of structural fabric development in relation to changes in metamorphic conditions; (3) relative timing between deformation, metamorphism and magmatism; (4) regional orientations of early foliations that have been largely obliterated in the surrounding matrix; (5) grain-scale structural and metamorphic processes; (6) mechanisms of folding and crenulation-cleavage development; (7) shear senses during foliation development; and (8) finite strains during foliation development.

All of these lines of investigation can become part of an overall P-T-D-t history, which is a primary goal of many modern microstructural studies. Complete microstructural evaluation of P-T-D-t histories requires specific evidence, including: (a) sequentially grown porphyroblasts that can be timed relative to surrounding foliations; (b) partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development; (c) a tectonic marker foliation allowing correlation of foliations from one location to another; and (d) single samples containing all of the foliations and all generations of porphyroblast growth within a single metamorphic zone. A basic requirement for this activity is continuity between porphyroblast inclusion trails and an external foliation. In the absence of continuity, relative timing between different porphyroblasts, and between porphyroblasts and foliations, can only be inferred. This presents a particularly difficult problem where grain coarsening of matrix minerals, or deformation post-dating porphyroblast growth, has destroyed continuity. In these circumstances there is a strong possibility of misinterpreting the timing of porphyroblast growths, and the effects on an inferred P-T-D-t history can be substantial.

Only rarely can all lines of evidence be acquired from a particular study area, and so a certain amount of inference is generally necessary. For this reason it is instructive to evaluate multiple P-T-D-t histories that are consistent with the available evidence. It is also instructive to consider how an inferred P-T-D-t path might change for various porphyroblast rotation-non rotation scenarios. Different scenarios will certainly affect the "D" part of the history, but what about the metamorphic path? Finally, new tools such as compositional maps of porphyroblasts and precise radiometric dating of single metamorphic crystals are important steps for the future, enabling more confident interpretation of porphyroblast-matrix microstructures, and providing a more complete picture of orogenic processes.