A User Guide to Migmatites in Australia - 2013

Hydrogeology, Geology, & Environmental Issues

Pawley, and three co-authors, reported in 2013 that migmatites are complex, high-grade rocks that have formed by partial melting. They have been recognized across most of geological time, develop in various tectonic settings, and can affect a wide range of protoliths. Consequently migmatites are commonly encountered in the field where their complexity can confuse and intimidate the geologist. These rocks are particularly common in basement areas of South Australia and are therefore likely to be encountered by geologists working in this state.

The aim of this report is to provide an understanding of the terminology applied to migmatitic rocks, the processes that form them, and their usefulness. To achieve this, the report is divided into three main sections. The first section is conceptual and provides the background that is necessary to understand and describe these rocks. This will include how they form, the parts of a migmatite, and the different types of migmatitic rocks and what they mean. The second part will provide some practical information including what information can be extracted from a migmatite, dating of these rocks, their relationship to mineralisation, and some ideas on how to deal with migmatitic rocks in the field. The final section includes a collection of case studies from South Australia which will give the reader some idea of what information can be extracted from migmatitic rocks.

 A migmatite is also a type of rock found in medium- and high-grade metamorphic areas that can be heterogeneous at the microscopic to macroscopic scale and consists of two, or more, petrographically different parts, which are petrogenetically related to each other, and to their protolith, through partial melting or segregation of the melt from the solid fraction (Sawyer 2008a). The partially melted part of a migmatite will often, but not always, contain pale-colored rocks that are quartzo-feldspathic or feldspathic in composition, and dark-colored rocks that are enriched in ferromagnesian minerals. These two parts essentially represent the liquid and solid products of the melt reaction respectively, but this will be explained in the next section. In some cases however, the partially melted part may have changed mineralogy, microstructure and grain size without segregating into separate light or dark parts. This can make recognition in the field difficult and would most likely require petrographic analysis (the petrographic study of migmatites will not be covered in this report book but readers are directed to Sawyer 2008a). 

The first part of this review is largely based on the Atlas of Migmatites by E.W. Sawyer (2008a), which is an excellent, amply illustrated account of the description and origin of migmatitic rocks. This book also has a very comprehensive background section with a bibliography of the more detailed literature and is highly recommended.