Cooking the rocks at Emerald Beach

I have always been interested in the little things in life. The things that don’t get the attention that everything else seems to get. This even applies to rocks and rock outcrops. It applies to a little headland that I visited on a trip to Coffs Harbour earlier this year. The headland has no name but lies on the northern side of Emerald Beach and the village of the same name. It is made from a granite-like rock of a poorly understood suite of intrusions in north eastern NSW.

Boulder on Emerald Beach. Note the xenolith at the bottom

The rock is formally called the Emerald Beach Monzogranite. It is the eastern most granite on the Australian continent is also one of the youngest rocks in the New England area. The Emerald Beach Monzogranite has been dated at 228.5Ma and part of an informally super suite of granites called the Coastal Supersuite (Chisholm et al 2014). Originally the unit was formerly defined as the Emerald Beach Adamellite (Korsch 1978) but has been renamed to reflect the most up-to-date nomenclature. However, the name Monzonite (and hence Adamellite) is misleading. The composition of the rock is consistent with the definition of Granodiorite (Plagioclase Feldspar abundance greater than that of Potassium Feldspar (Korsch 1971, Chisholm et al 2014). No reference to Monzogranite (or Adamellite) have been made and the samples I’ve seen were plagioclase feldspar dominant so the present classification appears erroneous. Maybe the name Emerald Beach Granodiorite might be more correct.

The dating of the Emerald Beach Monzogranite was only conducted in the last couple of years. It is an example of using multiple techniques together to get an answer. The mineral Zircon is formed in magma chambers of granite and granite-like composition. This is a very stable mineral. Zircon locks up uranium in small amounts and this uranium undergoes radioactive decay to lead. By measuring the proportions of uranium to lead it is possible to determine how long ago the zircon had formed. By this method Chisholm et al 2014 narrowed the age down to about 228.5 million years old. This is the Upper Triassic era which was the time of the best known dinosaurs.


Xenoliths of country rock are present in the rock (you can see an example in the picture above). These darker coloured xenoliths are inclusions of country rock which has been caught up in the magma chamber and have not quite been completely melted into the rest of the liquid rock. In the case of the Emerald Beach Monzonite the xenoliths are slightly elongated and display a preferred orientation. This orientation is probably caused by following the direction of intrusion of the molten rock (Korsch 1971).

The intrusion of the magma heated up the surrounding rock into which it had been emplaced. This heating up forms what is termed a contact metamorphic aureole (a metamorphic zone of effect). The Emerald Creek Monzonite had heated the muds in the surrounding deep sea Coramba Bed rocks to such an extent that new minerals were formed including very small but abundant crystals of biotite mica. Biotite mica forms at approximately 500 degrees Celsius (but varies by pressure) and disintegrates when hotter than about 800 degrees. Therefore the temperature of the molten rock was probably at least this. This type of contact metamorphic rock is referred to as hornfels.

It is an interesting example how little aspects again can tell a lot about how rock forms. Preferred orientation of xenolith inclusions and the formation of biotite in the surrounding rock show both the direction that the magma was moving and its temperature at the time. Have a look if you are in the area and see if you can spot some of the xenoliths. Those that are really in the know can say that the Emerald Creek Monzonite seems to have been incorrectly named.

References/Bibliography:

*Chisholm, E.I., Blevin, P.L. and Simpson, C.J. 2014. New SHRIMP U–Pb zircon ages from the New England Orogen, New South Wales: July 2012–June 2014. Record 2014/52. Geoscience Australia
*Korsch, R.J. 1971. Palaeozoic Sedimentology and Igneous Geology of the Woolgoolga District, North Coast, New South Wales. Journal and Proceedings of the Royal Society of New South Wales. Vol. 104.
*Korsch, R.J. 1978. Stratigraphic and Igneous Units in the Rockvale-Coffs Harbour Region, Northern New South Wales. Journal and Proceedings of the Royal Society of New South Wales. Vol. 111.

How wonderfully marbleous!

There are some rock types that are very common around the country and around the world that just don’t seem to rate much of a mention in the Northern Rivers. One very common rock is limestone formed from corals in a shallow sea, just like the Great Barrier Reef. Limestone is made almost entirely of the mineral calcite. Some parts of the world have vast terrains dominated by limestone called karst landscapes and it is quite distinctive. Limestone terrains sometimes form amazing subterranean cave systems as the stone is dissolved by rainwater infiltration into the formation. These karst terrains include north-west Mexico and other parts of North America, a giant band through northern England and a wide area of South Australia along the Great Australian Bight. However, it is a landscape absent from the Northern Rivers.

Outcrop of limestone north west of Tabulam

Having said that vast areas of limestone don’t exist in the region it is worth noting that they do exist in small areas here and there within the older rocks of the New England Orogen. The reason for this is interesting. The New England Orogeny was a period of mountain building during periods of plate collision which included a period of subduction of an oceanic plate under the Australian continental landmass during the Silurian period. The material on the surface of the oceanic plate was often accreted, that is scraped off and squashed onto the Australian continent. Seamounts are old islands in the middle of the sea. Such as, those around modern day Hawaii or Fiji. The seamounts were accreted onto the continental mass where they created little pockets of limestone in midst of the jumbled, squashed mass of deep seafloor sediments.

This means that if you find limestone in the New England area you are actually finding the preserved remnants of a little tropical island reef or lagoon. An especially nice thought, when you find some limestone on a cold frosty New England winter morning. One relatively accessible place to see some limestone is an old quarry on the Pretty Gully Road just north-west of the town of Tabulam which sits on the Bruxner Highway crossing of the Clarence River. The stratigraphic unit that the limestone of the area is part is the Emu Creek Formation which also includes areas of interesting fossils (more about that in yet another post). However, the quarry is interesting for more reasons than just as an occurrence of limestone.

Following the period of subduction and accretion a period occurred where intrusions of molten magma pushed their way into the accretionary sedimentary rocks. It occurred a couple of times including during the Late Permian to Early Triassic and created one part of what is referred to as the New England Batholith. The batholith is an array of granitic rocks that stretches through the whole New England Tablelands. The intrusions of the Late

fresh face of limestone - note the sparkles from the calcite crystals

Permian to Early Triassic included the emplacement of the Bruxner Monzogranite, a type of granite pluton (more about this specific rock in a future post). This pluton heated up and metamorphosed the rocks around it and one of which was that body of limestone near Tabulam. Contact metamorphism of limestone creates the rock called marble and this has happened at Tabulam. Although, the quality of marble is questionable because of the amount of impurities.

Other things happened to the limestone during metamorphism too. The transfer of fluids into and out of the cooling magma created chemical reactions which concentrated elements such as iron. This process develops what is called a skarn, a body of altered limestone with sometimes economic amounts of minerals. The minerals in a skarn can be diverse and very, very valuable but the minerals are based on the chemistry of the granite pluton. In the case of the chemistry of the Bruxner Monzogranite, there was not much of value except lots of iron which formed abundant amounts of the minerals magnetite and haematite. This has been considered for mining in the past but the small size and low grade means it is not a viable iron mine.

There are other small limestone deposits all around the New England and all of them are interesting for one reason or another. Some north of Inverell have lovely caves, others near Tamworth are mined for lime on a large scale. While others, just have interesting little features that illustrate what happened during the formation of our region.

References/bibliography:

*Bryant, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cordilleran Tonalitic I-type Intrusions in Eastern Australia. Journal of Petrology. v38.

*Lishmund, S.R., Dawood, A.D. & Langley, W.V. 1986. The Limestone Deposits of New South Wales. 2nd Ed. Geological Survey of New South Wales

In the hills of Valla and Nambucca Heads

The Valla Adamellite now termed the Valla Monzogranite to reflect modern naming conventions is an interesting small to medium sized intrusion about 10km north east of Nambucca Heads. It is one of the suites of coastal granites which are mostly I-Types (melted igneous material), this means that the coastal granites show abundances of ore minerals within the granite or in the surrounding metamorphosed country rocks. A monzogranite is a granite with roughly equal proportions of (alkali-feldspar (potassium and sodium rich) and plagioclase feldspar (calcium rich)). The monzogranite is thought to have formed during the Triassic period.

The metamorphic aureole for the Valla Monzogranite is actually quite interesting as it shows a classic zonation of metamorphism (high grade at the contact grading to low grade further away) and also excellent examples of mineral zonation associated with metasomatism (hot-water or fluid alteration of rock). The Valla Monzogranite has been shown to be associated with gold, silver, arsenic and molybdenum mineralisation (as well as others). The rock that the monzogranite has been intruded into is called the Nambucca Beds which are part of the Nambucca Block. The Nambucca Beds are Permian to Carboniferous in age and are mainly comprised of the regionally metamorphic rock type called phyllite which was originally deposited on the sea floor. The Nambucca Block was accreted onto the Australian continent in the New England Orogen and this caused the regional metamorphism of the beds.

The Nambucca Beds are intruded by the Monzogranite. The Beds are extensive and
extend far into the rugged Nambucca Hinterland. This photo is west of Bowraville.

The Valla Monzogranite seems to be a Climax Molybdenum Deposit named after the Climax Mine in North America. This means that when the Monzogranite was cooling the upper portion of the pluton became residually enriched with fluids, metals and silica. These fluids cause alteration of the upper portion of the pluton forming what is called greisen and also are injected into the surrounding rock through veins and sometimes aggressively through breccia pipes. One of the first minerals to form in these veins is silica, quartz with metal sulphide such as molybdenite (molybdenum ore) and wolframite (tungsten ore). Further away from the intrusion the degree of alteration becomes less grading through potassic through to argillic which are defined alteration zones based on the changes in the rock forming minerals. As the degree of alteration becomes less so the types of metal ores change with increasing amounts or arsenic, gold and silver. Further out in the alteration zone minerals such as galena form (lead ore) and finally stibnite (antimony ore). These ore deposits seem to be fairly common in the New England area with Glen Eden being the most studied (Somarin 2001, Somarin & Ashley 2004) and have in some areas been extensively explored such as Kingsgate east of Glen Innes.

Some attempts of mining have occurred in the Valla Monzogranite in the past, the most significant being the Valla Gold mine which was located just to the north of Valla Beach. The mine was abandoned with very little rehabilitation and therefore has become an environmental problem for the local creek. However, rehabilitation efforts have recently been undertaken, though these will need another post to discuss in more detail.

References/bibliography:

*Somarin, A.K. 2011. Petrography, Geochemistry, and Petrogenesis of Late-Stage Granites: An Example from the Glen Eden Area, New South Wales, Australia. Earth and Environmental Sciences.
*Somarin, A.K. & Ashley, P.M. 2004. Hydrothermal Alteration and Mineralisation of the Glen Eden Mo-W-Sn deposit: A Leucogranite related hydrothermal system, southern New England Orogen, NSW, Australia. Mineralium Deposita.

The Dummy you'll find north of Armidale

One of the imposing landscape features on the north side of Armidale is the 1400m high Mount Duval. Some of my secondary education was in Armidale and I remember that the logo of my school actually had Mount Duval in it. Mount Duval is part of granite-like pluton called the Mount Duval Monzogranite. It was previously called the Mount Duval Adamellite; however the term Adamellite is no longer formally recognised. The intrusion actually extends in a crescent shape further to the west and includes Little Mount Duval which is roughly were the watershed for the Great Dividing Range sits, draining to the east all the way to the Macleay River. The monzogranite is considered to be middle Permian in age and intrudes several different complex rock units, one of these is a relatively small unit called the Dummy Creek Conglomerate.

Dummy Creek Conglomerate in the Sunnside area
metamorphosed by the Highlands Igneous Complex

The Dummy Creek Conglomerate is situated to the north of Mount Duval and extends to the east to the area of Puddledock, the northern side is intruded by the Highlands Igneous Complex. The Dummy Creek Conglomerate is comprised mainly of conglomerate but not exclusively. Lithic sandstone is a major component and it is actually what is in these sandstones that allow us to determine when the unit was formed, but more of that later. The abundance of conglomerate as well as sandstone and rarity of fine grained sediments like mudstones shows us that the sediments, gravels, etc that made up the Dummy Creek Conglomerate have not travelled far from their source. The clasts in the conglomerate show that the source rock was the underlying Carboniferous aged Sandon Beds (part of the Texas-Woolomin Block).

Korsch (1982) concludes that the original Sandon Beds was domed and uplifted by the intrusion of granite bodies of the New England Batholith such as the Mount Duval Monzogranite and the Highlands Igneous Complex. The hills formed from the deformation of the Sandon Beds began shedding rock, eroding and the sediments were deposited a short distance from these new hills. The intrusions continued to intrude shortly after the sediments were deposited which according to Holland (2001) created a complex system of overlapping zones of contact metamorphism. The intrusions were therefore emplaced in a very shallow crustal situation and volcanism was abundant and the Dummy Creek Conglomerate was quickly covered and preserved by a volcanic unit that is called the Annalee Pyroclastics which includes lavas, pyroclastic deposits and the like. It is worth noting that other models of formation by various other authors were summarized by Holland (2001) for instance some authors suggest that rock fabric studies may show a source only from the south.

A lot was happening in the Mount Duval-Tilbuster-Puddledock area during a relatively short period of geological time, indeed even during this time of change a substantial forest must have been growing in the area. The sandstone layers in the Dummy Creek Conglomerate preserve fairly common plant fossils. Most of the fossil remnants are fragments but there is enough to identify many plants with certainty. The most common fossil identified was the deciduous plant Gangopteris, a relative of the more commonly known Glossopteris, the main plant that formed the coal of the Sydney Basin. This plant existed abundantly in the middle of the Permian and so given that many of the rocks appeared to be forming at the same time these can be assumed to be close to this age too.

References/bibliography:

Holland, R. 2001. South western Margin and Contact Rocks of the Highlands Igneous Complex near Orana Falls, North of Armidale, NSW. Unpublished undergraduate research thesis, University of New England.
Korsch, R.J. 1982. The Dummy Creek Association: Rim Syncline Deposits. Journal and Proceedings of the Royal Society of New South Wales. V115.

Weirdly Wonderful Wongwibinda

I finally found them, photos of some of the strange metamorphic rock at Wongwibinda. I recently moved house and in the process I’ve lost many things but also found some things. Early this year I did a post on what were the broader conditions that lead to the geology of this area between Guyra and Ebor, namely thinning of the continental crust leading to increased heat flow and corresponding thermal metamorphism. I mentioned a rock type called migmatite and since I found my photos of the Wongwibinda migmatite, I thought I should go into a little more detail on this curious metamorphic feature.


Close angular folds in the Girakool Beds, Rockvale

The migmatites are strongly metamorphosed rocks of the Girrakool beds. The Girrakool beds are Carboniferous in age and were deposited in a marine environment. These beds were then accreted onto the edge of the Australian continent as part of the New England Orogen, much deformation occurred during this time. During or following this stage of tectonic forces that affected the New England region the Girrakool beds were subjected to a period of intense metamorphism. This affected one end of the beds more than the other. The western most part of the Girrakool beds in the Rockvale area remained relatively ‘uncooked’ but further to the east the effects of thermal metamorphism became greater creating schists known as the Ramspeck Schist and finally the zone of migmatites. The migmatites are faulted off by the Wongwibinda fault on the eastern side or are intruded by the Abroi Granodiorite which itself has been later metamorphosed into Gneiss.

Migmatite in the Aberfoyle-Wongwibinda area.
Note the ptygmatic folds and dyke on the left

The odd thing about the Wongwibinda migmatites generally is that they are actually three rocks in one: metamorphic sedimentary rocks becoming igneous at the same time. Usually rocks fit into the igneous and sedimentary categories neatly and then metamorphism can affect these rocks. In the case of migmatite the metamorphism is so great that the rock actually begins to melt, that is, it becomes an igneous rock with some of the sedimentary rock remaining unmelted. A characteristic of migmatite is ptygmatic folding, which is intense small scale folding with alternating light and dark bands. The dark bands are called the palaeosome which is the remains of the sedimentary rock and the lighter bands is insitu accumulation of melted rock called the Leucosome,. The leucosome is here comprised mainly of the minerals quartz, feldspar, mica and some garnet. Sometimes the leucosome can ‘break free’ from the ptygmatic folds and create dyke like structures. All of these features are visible in the picture opposite. 

What can be seen at Wongwibinda is essentially the formation of a granite, specifically a S-type (sedimentary derived), frozen in time. Craven et al 2012 demonstrated that this time was very close to the Carboniferous-Permian age boundary, probably just in the Permian, that is around 297 million years ago. There are some fancy geological features in the New England highlands and in my mind this is one of them. If you travel up that way and see some rocks by the side of the road be sure to stop and look closely, there are so many unusual things to find.

References/bibliography:
*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.

A magma chamber under Cabarita Beach

Again and again, I am amazed at how little we know about what is under our feet. It often takes an unexpected source of information to reveal some incredible knowledge of our region. The lastest information that has recently come to hand has been the preliminary geophysical survey results for the Grafton to Tenterfield survey. There are many results that may indicate some strange goings on, from some inconsistent features in the Mount Warning area (possibly indicating that the Tweed Shield Volcano might actually be a myth! More of this in a future post or two), to strange lineaments and responses showing hidden intrusions. This post is about just such a possible hidden intrusion in the Cabarita area.

Smith (1999), curiously reported that within the Neranleigh-Fernvale Beds at Norries Head, Cabarita (located on the coast midway between Tweed Heads and Mullumbimby) there appeared to be evidence of thermal metamorphism in the rocks there, but no evidence of what caused the heating. Metamorphism is a characteristic of the Neranleigh-Fernvale Beds, but the style of metamorphism is pressure related due to the formation being accreted (squashed) onto the Australian continent during a period of subduction during the Palaeozoic period. Not much heat was generated in this formation and based on the minerals identified in the rocks it is possible to estimate the pressure and temperature when these rocks were squashed. The feature that Smith (1999) identified was biotite crystallisation (a variety of the mica mineral group). This mineral is indicative of heating of rocks to a medium to high grade but the lack of a preferred orientation of this platy shaped mineral shows us that the metamorphism postdates the accretion period. ie. the heating of the rock has occurred some time after the pressure, meaning at least two periods of metamorphism.

As discussed in a previous post, the New South Wales Geological Survey has been collecting geophysical data over the region. One measurement has been the intensity of magnetism (related to the iron content of rocks). Magnetic results can display what is happening under the earths surface, not just on top. It is known to show a characteristic feature where intrusions are known, either a strong negative or strong positive anomaly, depending on the rock type. The picture to the left shows the total magnetic intensity map (courtesy of the 2012 preliminary data package from the geological survey) for the area around Cabarita. I’m sure you can pick out the obvious red and blue anomaly. the pattern is consistent with intrusions, indeed exactly the same feature can be seen in the Mount Warning area (and others that I will discuss in future). As such, I suggest that this anomaly is actually good evidence of an intrusion hidden below the heat affected surface rocks. Smith (1999) thinks that the biotite grade metamorphism occurred during the Mesozoic period (well before the Cenozoic aged Lamington Volcanics) and that there was once a body of molten rock below the ground in this area.

I’m so pleased to be able to see the preliminary dataset, it is obvious that there are many features that can be better understood.


References/bibliography:

*Smith, J.V. 1999. Structure of the Beenleigh Block, northeastern New South Wales. New England Orogen: Regional Geology, Tectonics and Metallogenesis. Papers presented at a conference at the University of New England.
*Geological Survey of New South Wales. 2012. Grafton Tenterfield Airborne Geophysical Survey: Gridded and imagery data. Preliminary package from the Department of Trade and Investment: Resources and Energy.

Lismore GEMFEST 2012

This year I was able to visit the gem festival GEMFEST in Lismore. I had never been before and I must say that I was very impressed! The number of exhibitors was incredible. I was told it is the biggest gem and lapidary show in New South Wales... or was it Australia? I can believe either. It is a lapidary delight!... even for someone like me who, surprisingly, is not really into gems.

The weather was wonderful, the people friendly, the curry stall smelled great. It was nice to see activities for the kids and exhibitions on using various equipment. The variety of exhibitors was large too, from hippie jewels and alternative uses for crystals to fossils, gems and obscure minerals. My interest was on the less shiny things, on the fossils and unpolished and uncut minerals.

I was struck by the number of specimens, especially fossils, that come from Morocco (in north Africa). But, if you are aware of the most famous paleontological sites in the world this will not be a surprise. Morocco has wondrous numbers and variety of exquisitely preserved Permian aged sea life especially Trilobites. However, my favourite was actually not super expensive but something I still could not afford. It was both sides of some Miocene? aged mudstone that revealed a wonderful small fish and some leaves in the same layer. The vendor had identified the fish and plant but I cannot remember what they were. But, it was a perfect example of identifying the environment in which the fish lived: a still water environment probably a late or maybe an estuary. And the specimen just looked stunning.

Getting late on day one.
Just one of the Vendor avenues.

Since there was so much I could have bought if I had a spare $50 000, I thought the best thing to do was focus on one theme (I had less than $100). Although, I would loved to have obtain some of the wonderful fossils I instead bought from several different vendors, some uncut mineral specimens. The balance of these being minerals with a metamorphic theme such as Kyanite, Staurolite, Forsterite and Almandine Garnet plus some others. I will do a future blog post on Kyanite and how it and other minerals can be used to estimate the temperature and pressure of metamorphism.

I know that if I get the chance I will go next year and get some more metamorphic minerals and maybe if I save enough look at other rock forming and ore minerals and of course, fossils. Anyone that is as boring as me will be delighted... as will others who like prettier things!

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.

References/bibliography:

*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.