The Great Dividing Ranges and Stonehenge

Granites occur throughout much of the north coast and New England region. I use the term granite here loosely, in reality the rocks I’m referring to have a range of compositions and ages. The things they have in common are their relatively high quartz content and they are igneous intrusive (plutonic) rocks. They have cooled slowly and therefore have allowed large crystals to form – giving them that typical granite appearance. I’ve covered a few granites in previous blog posts but in this post I’ll cover one New England “granite” called the Wards Mistake Monzogranite. I’ll continue to cover others in future posts.

Stratigraphically the Wards Mistake Monzogranite is part of the Wards Mistake Suite which in turn is part of the Uralla Supersuite. The Wards Mistake Monzogranite outcrops in a relatively extensive area between Glen Innes and Guyra. In places it straddles the Great Dividing Range but mainly occurs just on the eastern side within the upper reaches of many Clarence River tributaries. The unit was formed around 250million years ago, during the Lower Triassic to Lopingian (early Permian period).

The Wards Mistake Monzogranite consists of monzonite (a rock containing moderate quartz with equal parts potassium and sodium-calcium feldspar) with some granodiorite (abundant quartz and calcium-sodium feldspar). It has a typical equigranular black and white speckled appearance which is common of the Uralla Supersuite. It is like the other Uralla Supersuite granites as it is derived from the melting of other igneous rocks - I-Type Granites (Bryant et al 2003). However, it does contain some xenoliths (inclusions of other rock) which are sedimentary. It is possible that when the Wards Mistake Monzogranite was emplaced into the crust it incorporated bits of the surrounding sedimentary rock. This may have affected the chemistry of the magma and may be one of the reasons why there is both monzonite and granodiorite in the unit.

Typical tor outcrops of the Wards Mistake Monzogranite near Glen Innes

Many New England granites contain mineral deposits. Being an I-Type granite usually means a good chance of mineral deposit formation. However, the Wards Mistake Monzonite contains very sparse mineralisation with only a few small areas where there is some alteration zones that have more concentrated ore minerals. These include wolframite (tungsten), molybdenite (molybdenum) and cassiterite (tin) (Brown 1997). Other surrounding granites such as the Kingsgate Granite and Red Range Leucogranite have abundant mineralisation that was historically mined and is still under active mineral exploration permits.

A lovely feature of most New England granites is the interaction with the climate. This produces wonderful looking granite tors. This is a result of onion skin weathering (frost wedging) where water penetrates into the rock and freezes during the cold winters. This repeated action causes large flakes of rock to peel off. Some of these Tors are given their own names. In the Stonehenge area on the New England Highway you can stop and walk among these Tors and see the Balancing Rock which looks like it will topple over at any moment.

The landscape around Stonehenge between Guyra and Glen Innes is my favourite landscape in Australia. The high country agriculture, the cold weather and the geological conditions that form the rolling hills and special tors make it a special place. The picture above is of a portion of the Wards Mistake Monzogranite and partly shows the landscape I’m talking about. The accessibility of the granite is certainly worth a quick stop if you are travelling on the New England highway.

References/bibliography:

*Barnes, R.G , Willis, I.L. 1989. Preliminary geological plan of the 1:250 000 Grafton-Maclean sheet area - SH 56-6, SH 56-7. New South Wales Geological Survey Report

*Brown, R.E. 1997. Mineral deposits of the Glen Innes 1:100 000 map sheet area. Geological Survey of New South Wales. Quarterly Notes 103 p1-19

*Bryant, C.J. , Chappell, B.W. , Blevin, P.L. 2003. Granites of the southern New England orogeny. In Blevin, P. et al (eds) Magmas to Mineralisation: the Ishihara Symposium Geoscience Australia. Record 14 - extended abstracts.

A special volcano on the edge of the Northern Rivers

I have previously mentioned several volcanoes that have existed during the Cenozoic period in and around the Northern Rivers region of the New England. But, it is worth noting that there was once a period of significant volcanism earlier in the Cenozoic which defines the landscape of the Great Dividing Range south of Glen Innes, near the villages of Glencoe (with its excellent pub: The Red Lion Inn) and Ben Lomond. This area is the headwaters of many wild rivers found flowing down the rugged New England escarpment that are tributaries of the Clarence River. On the other side of the divide eventually join the Darling and then Murray River. The Maybole volcano was apparently centred at the modern day and generally unheard of locality, Maybole. It erupted lavas over a large area in every direction including large areas to the west, east and south east.

Maybole lies just on or just outside of the headwaters of the Northern Rivers but none-the-less is worth mentioning because of the extent of volcanic rock that appears to have originated from it. The rocks that have come from the Maybole Volcano are mostly basalt type rocks which were once referred to as the Eastern division of the Central Volcanic Province (Coenraads & Ollier 1992), now referred to as the Maybole Volcanics but still part of the Central Volcanic Province according to Vickery et al (2007). The Maybole Volcanics are comprised of alkali olivine basalt to slightly less silica undersaturated basalt and andesite and reworked volcanic material (epiclastic and volcaniclastic sedimentary rocks) and was erupted around 36-39 million years ago.

Coenraads & Ollier (1992) identified that Maybole was a significant volcano by determining the thickness of basalt that occurred in the region and noticing that at Maybole the thickness was significant at several hundred metres. There are also apparently some dykes and vents that are present. Additionally, they had a close look at drainage patterns and realised that they radiated like the spokes on a bicycle, a classical indication of volcanic geomorphology.

Since Coenraads & Ollier (1992), Vickery et al (2007) has undertaken a major review of the Central Volcanic Province and delineated several constituents of the province. The most significant along this part of the Great Divide is now known as the Maybole Volcanics, obviously directly associated with the Maybole volcano. The age of the Central Volcanic Province including the Maybole Volcanics shows that these rocks are too old to be associated with the Eastern Australian hotspot which formed many of the other major volcanic centres in the region (such as the Focal Peak, Tweed and Ebor Volcanoes). Some time after the end of volcanism from the Maybole Volcano  other volcanoes between about 14-24Ma erupted their lavas over the top of the Maybole Volcanic suite rocks.

Interestingly, it appears that the Maybole Volcanics had affected exactly where the Great Divide was situated because the nature of the existing range was such that the lavas filled the valleys creating thick volcanic piles while the existing hills were only covered with thin layers. This meant redirection of streams and when the rock was eroded the more erodible hills were turned into valleys and the valleys became hills caped with basalt. This is termed an inverted topography. But more about this in another post.

Interestingly, Coenraads & Ollier (1992) have observed that the the great divide has moved over time with some of the old basalt filled valleys showing that they used to flow to the west but with the streams now flowing to the east. It actually appears that the Northern Rivers region is getting bigger!

Red Lion Inn (from Flickr)

PS. Like lots of geologists I like pubs with a good atmosphere and The Red Lion Inn at Glencoe is just such a beautiful place. It is an exceptional location to stop for a meal, especially during the middle of winter while snow is coming down. Alternatively, during autumn while the trees turn bright yellow and red, or during spring while the new leaves are coming out, or even summer! i.e. I recommend it!

References/bibliography:

*Coenraads, R. R., Ollier, C.D. 1992. Tectonics and Landforms of the New England Region in 1992 Field Conference - New England District. Geological Society of Australia Queensland Division.
*Vickery, N. M., Dawson, M.W., Sivell, W.J., Malloch, K.R., Dunlap, W.J. 2007. Cainozoic igneous rocks in the Bingara to Inverell area, northeastern New South Wales. Geological Survey of New South Wales Quarterly Notes v123.