Wednesday, 8 February 2012

Lots of heat but low pressure between Ebor and Guyra

In the headwaters of the Aberfoyle and Guy Fawkes Rivers, tributaries of the Clarence River lies a rare zone (for Australia) of rocks that have experienced high temperatures but surprisingly low pressure. This is another one of those “we don’t have a clear answer” posts, in particular what has actually caused the metamorphism of the rock in this area, but research just published in January (Craven et al. 2012) has shed a lot of light on the matter.

Originally it was thought that the metamorphism at Wongwibinda (The Wongwibinda Metamorphic Complex) was directly associated with the emplacement of the Granites since the most intensely metamorphosed rocks are close to the Permian aged Abroi Granodiorite and other Permian granites with a decrease in intensity of metamorphism further away from these intrusions (Wilkinson 1969). The depth of metamorphism was never considered very deep because the minerals that are present in the metamorphic rocks are not formed where intense compression is found. However, it has since been observed that contacts with some of the granites shows no or little metamorphic effects, notably along the contact with the southern part of the Abroi Granodiorite. Additionally, the Abroi Granodiorite itself displays some metamorphic textures making the picture relatively unclear.

An old geological map of the area (NSW Geological Survey)-
Note that the Glen Bluff fault should not define the edge of the schist (gradational)
(Phag - Abroi, Plr - Ramspeck, Pl - Girrakool and Dyambarin, Tb - Basalt)
Like many parts of the New England, the geology can be quite complex with many aspects and relationships not fully understood and this holds for the Wongwibinda area which is a area of metamorphism, abundant faulting, granite intrusions, deep sea sedimentary and volcaniclasitic rocks and basalt lavas. Describing the generalgeology may be easiest from east to west. Before coming to the properties of Abroi and Springfield is the Palaeozoic aged Dyambarin Beds which is neatly faulted off to the east. The eastern side of the Wongwibinda Fault lies rock of the Abroi Grandiorite (a type of granite), sometimes referred to as the Abroi Gneiss (in this case metamorphosed granite which appears to have been affected by the Wongwibinda Fault). Then just to the west of this we enter what is significantly altered Girrakool Beds and this is were it gets even more interesting.

The eastern most part of the Girrakool beds has been significantly affected by heat maybe up to about 700 degrees Celsius, but has experienced very low pressures and this has created an unusual texture called migmatite. Migmatite is a type of rock were the parent (in this case sandstone and mudstone sedimentary rocks of the Girrakool Beds) has been heated so much that it just starts to become liquid like, here it also shows Ptygmatic folding. The liquid usually accumulates or is formed in some individual layers creating essentially layers of molten rock between sediments. Sometimes the hot liquid rock follows cracks in the rocks creating little dyke like structures too.

Moving further to the west we enter a zone of schist (called the Rampsbeck Schist), which is a medium grade metamorphic rock that has had some of the crystals in the rock reform into layers, this schist extends further west showing less and less metamorphic effects until it is indistinguishable from the rest of the Girrakool Beds. There are also some areas of quartzite and amphibolite (other metamorphic rocks) in the schist zone. I’ve also seen a pegmatite dyke a bit further to the west, which I have no idea where it fits into the picture.

Over the top of all this are remnants of comparatively recent Basalt referred to as Tertiary (or Mid-Cenozoic aged) Alkali Basalt. Given its location this basalt is probably Doughboy Basalt, part of the Cenozoic aged (~40Ma) Doughboy Volcanic Province.

But what caused the formation of the metamorphism? Many mechanisms have been proposed by different authors such as Wilkinson (1969), Danis et al (2010) and Craven et al (2012) and other authors. Craven (2012) has carried much work, including dating to try and gain an understanding:
  • Was it the Wongwibinda fault? No – otherwise the rock would pressure related textures. 
  • Was it the intrusion of the Abroi Granodiorite (or other granites in the area)? No – otherwise we’d see metamorphism around all the Abroi Granodiorite that we don’t see, and the age of the Abroi Granodiorite is older than the date of metamorphism. 
  • Was it an intrusion that we can’t see because it fairly deep underground? No – gravity surveys have been conducted and these don’t show any deep granites other than the Abroi Granodiorite in the area of maximum metamorphism.
  • Was it the eruption of the Cenozoic Basalts? No, the age of metamorphism vastly predates the Cenozoic period.

Wow, so what options are left? Craven et al (2012) have come up with a theory: following the tectonic events that formed the granites in the area there was a period of stretching of the earth, this thinned out the crust and allowed for heat to be more easily transferred from the mantle. All of the other options are more common elsewhere in Australia and around the world, but each option has been refuted by different evidence until the only reasonable explanation left at this stage is the extension of the crust to allow convective heat transfer from the mantle to very shallow levels during the Permian.

I seem to have lost my old photographs of the migmatite but I have found a good website on Finnish migmatites that has some great pictures. It can be linked to from here.

A follow up post on the Wongwibinda migmatites can be found here.


*Danis, C.R., Daczko, N.R., Lackie, M.A. and Craven, S.J. 2010. Retrograde metamorphism of the Wongwibinda Complex, New England Fold Belt and the implications of 2.5D subsurface geophysical structure for the metamorphic history. Australian Journal of Earth Sciences V57.
*Craven, S.J. Daczko, N.R. and Halpin, J.A., 2012. Thermal gradient and timing of high-T-low-P metamorphism in the Wongwibinda Metamorphic Complex, southern New England Orogen, Australia. Journal of Metamorphic Geology V30.
*Wilkinson, J.F.G. 1969 The New England Batholith - introduction. IN Packham G.H.(ed) - The geology of New South Wales. Geological Society of Australia. Journal V16.

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