A Rock of Gibraltar Range National Park - Part 2

Dandahra Creek Leucogranite

This post is a follow-on from an earlier post which can be read here.

The Dandahra Creek Leucogranite is mainly composed of granite which is depleted in dark (mafic) minerals. The crystals are of very similar size and medium to coarse grained. The crystals are mainly quartz with feldspars and occasional biotite mica. The term Leuco- simply refers to the light colour and lack of mafic minerals. There are also small amounts of other minerals that are disseminated through the rock these include the mineral zircon which is used for dating.

The dating of the Dandahra Creek Leucogranite 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. In the past in some cases the whole zircon crystal have been used to determine the ratio. However, this method has some complications.

Not all of the zircon crystals in rocks show the same age. In the case of the Dandahra Creek Leucogranite some seemingly having much older ages. These crystals are actually inherited from the parent rock. The stability of the zircons means that they have not fully melted in the magma chamber. Often a good way to determine if a zircon is older than the magma chamber is to look at the shape and determine whether there has been any melting of the edges of the crystal. However, sometimes it is very hard to tell because the zircon often builds itself up again with an old core and a new crystal face.

To overcome the problem of age zoning in zircon crystals an alternative method was developed measure the ratio of lead and uranium. A high accuracy ion beam is aimed at the different portions of crystal. The ion beam vaporises the elements in that tiny area. The vapour is then measured for the abundance of each element and then the ratio of elements can be calculated. This is called the Sensitive High Resolution Ion Micro Probe or SHRIMP.

SHRIMP was a method developed right here in Australia. It is regarded as one of the most reliable ways to analyse microscopic crystals to determine when and how they formed. The need for the special machine came from dating the Rocks that make up the oldest parts of Western Australia which are the oldest in the world. It has no become a recognised tool around the world (Ireland et al 2008). There are 20 SHRIMP analysers around the world with four built in the last couple of years in Japan, China and Poland. Like Wi-Fi, the Hills-Hoist and Pavlova it is another example of Australian scientific ingenuity.

The age of the intrusion given for the Dandahra Creek Leucogranite using the SHRIMP method is 237.6 Ma (plus or minus 1.8Ma). This makes it the youngest example of the Stanthorpe Suite of Granites (Chisholm et al 2014) and nearly the youngest in the whole Standthorpe Supersuite (Thanks for the correction rockdoc!).

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

*Clarke, Peter J. & Myerscough, Peter J. 2006. Introduction to the Biology and Ecology of Gibraltar Range National Park and Adjacent areas: Patterns, Processes and Prospects. Proceedings of the Linnean Society of New South Wales

*New South Wales National Parks and Wildlife Service 2005. Gibraltar Range Group of National Parks (Incorporating Barool, Capoompeta, Gibraltar Range, Nymboida and Washpool National Parks and Nymboida and Washpool State Conservation Areas) Plan of Management. February 2005. ISBN 0 7313 6861 4

*Ireland, T.R., Clement, S., Compston, W., Foster, J. J., Holden, P., Jenkins, B., Lanc, P., Schram, N. & Williams, I. S. (2008), "Development of SHRIMP", Australian Journal of Earth Sciences V55 p937–954

The right age for Mount Warning

In previous posts on the Tweed Volcano, especially those relating to the Mount Warning Central Complex I indicated that there were some strange anomalies to do with the dating of these intrusions. Graham (1990), in his natural history summary of the Tweed region illustrated how confusing the dates recorded for the rocks that made up the Mount Warning Central Complex could be.

Mount Warning Central Complex from the southern rim of eroded shield

Wellman and McDougall (1974) summarised existing and provided new evidence for the date of the Mount Warning Central Complex and the surrounding Lamington Volcanics. Wellman and McDougall (1974) and earlier researchers used a very good technique of dating called potassium-argon dating (K-Ar dating). This is a radiometric dating method based on measurement of the radioactive decay of an isotope of potassium (40K) into argon (40Ar). Note, that the numbers in front of the chemical symbol for each element refers to the number of neutrons in the atoms nucleus. The decay rate of 40K to 40Ar is known accurately because the time it takes for half of the 40K to turn into 40 Ar is about 1.25billion years (the half-life). Therefore, the ratio of the two can be used to determine just how old the rock is.

The accuracy of using the K-Ar dating method is very good, but has some provisos. The most important being that the rock sample must be very 'fresh'. There must be no weathering, alteration or metamorphism of the sample. Because potassium is more reactive than argon and it can be removed or added respectively during weathering and alteration. Additionally, the K-Ar dating 'clock' can be reset during any recrystallisation during metamorphism.

K-Ar dating by Wellman and McDougall (1974) and earlier authors showed that the intrusive complex at Mount Warning was emplaced between ~23.7Ma and 23.0Ma and the surrounding lavas erupted from ~22.3Ma to ~20.5Ma. This doesn't make a lot of sense because an intrusion of magma needs to intrude into something else (otherwise it is not an intrusion!). In the case of shield volcanoes this intrudes the earlier lava that was erupted before. The K-Ar dating shows this is apparently not the case.

What is going on? No one could suggest any reasonable ideas. Cotter (1998) suggested a possibility there may have been a large volume of pre-existing Palaeozoic and/or Mesozoic sedimentary rocks that have now been eroded away. However, Cotter (1998) did date a sample of basalt lava from the Terania Creek area at ~23.9Ma (using K-Ar). This suggested maybe the dating by Wellman and McDougall (1974) and earlier authors might have either missed later lavas or maybe there was something else wrong.

Cohen (2007) spent a lot of time resampling the K-Ar dated volcanic rocks of eastern Australia. This time instead of using K-Ar he used another technique called 40Ar-39Ar dating. This is similar to K-Ar dating in concept. It instead measures the abundance of two isotopes of argon and is much less affected by any effects of weathering and alteration (though not metamorphism). What did he find? He found some of the K-Ar dates were wrong.

Cohen (2007) found the actual date of the lavas was within the range ~24.3 to ~23.6 million years, about 2 million years older than first thought. Though the 40Ar-39Ar date of the Mount Warning Central Complex was quite close at ~23.1Ma it fell within the range of the K-Ar dating (23.7-23.0Ma). This reverses the idea the intrusion of the Mount Warning Central Complex was before the lavas. So, now we know that the final intrusions of the Mount Warning Central Complex does indeed fit the model for shield volcanoes. That is, the intrusions were likely to have been emplaced into already erupted volcanic rock. They were also erupted and emplaced over a period much quicker than first thought. The new dating shows volcanism possibly lasting 1 million years instead of the 3 million previously suggested.

References/bibliography

*Cohen, B.E. 2007. High-resolution 40Ar/39Ar Geochronology of Intraplate Volcanism in Eastern Australia. PhD Thesis, University of Queensland.
*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Graham, B.W. 1990. A Natural History - Tweed Gold Coast Region. Tweed River High School Library.
*Wellman, P. & McDougall, I. 1974. Potassium-argon Dates on the Cainozoic Volcanic Rocks of New South Wales. Journal of the Geological Society of Australia v21.