Formation of a Granite Bodies in Depleted Granulite Terranes: the Wuluma Granite, Central Australia
Abstract
The Wuluma Granite (ca.17 km2) is hosted by Palaeoproterozoic, granulite facies metasedimentary and metaigneous rocks. It is believed to have formed by in situ partial melting of quartzo-feldspathic gneisses at 1728±3 Ma due to the influx of an externally derived aqueous fluid after the granulite facies metamorphism. We have reinvestigated the Wuluma Granite and find that most contacts between the granite and the host granulites are intrusive, not gradational. Granite occurs as thin (<1m) subconcordant sheets and dykes in country rocks that contain fresh orthopyroxene and cordierite without much replacement by hydrous minerals. Screens of country rock are common within the granite, and many contain metapelitic rocks that have leucosome and melanosome structures similar those found in the country rocks. Although some of the migmatite structures in the screens still contain garnet, cordierite and orthopyroxene, in most these minerals are replaced by biotite. Biotite is the only ferromagnesian mineral in the thinnest screens of country rock. All the screens contain subconcordant sheets and dykes of granite; typically a narrow selvedge is developed between the intrusive granite and the rocks of the screen; selvedges are either rich in biotite or in quartz depending on the host rock type. Schlieren are common throughout the granite and represent the last vestiges of the country rocks in the granite; there is much morphological and mineralogical variation among the schlieren. The Wuluma granite consists of innumerable thin (less than a metre) subparallel sheets and cross-cutting dykes, that are distinguished by variations in grain size, microstructure and the proportion of minerals present. The earliest phase to be porphyritic and rich in biotite, whereas the last is leucocratic, coarse grained and locally forms dykes up to 20m wide. The centre of the granite contains large (1 cm) crystals of garnet and, more rarely, cordierite. However, in many places these have been replaced by biotite (or chlorite), although, small crystals of garnet within the matrix persist locally. Virtually all of the granite contains a magmatic foliation, and this together with the presence of small dykes of granite in shears and in fold hinges indicates that the granite body formed during regional deformation (the local D3 event). Thus, the Wuluma granite did not form by in situ partial melting. Rather, it formed at a site where small increments of anatectic melt extracted from the surrounding granulite terrane during regional deformation were able to accumulate.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSM.V31C..02L
- Keywords:
-
- 3690 Field relationships (1090;
- 8486);
- 8035 Pluton emplacement;
- 9330 Australia