Bruxner Monzogranite on the Bruxner Highway

In a previous post, I discussed the metamorphism of limestone at an area north-west of Tabulam. I thought I’d take the opportunity to discuss the intrusion itself  that caused the metamorphism (a rock unit called the Bruxner Monzogranite). Also briefly, put it in the context of the formation of the broader New England Batholith.

Typical Bruxner Monzogranite monzogranite

The Bruxner Monzogranite is a geological unit that is composed of a series of ‘granite’ plutons (intrusions of molten magma). These occur in a hourglass shape between Drake and Tabulam. The biggest areas occurring north and south of the Bruxner Highway and the central thin part of the ‘hourglass’ occurring where the Bruxner Highway crosses it.

The Bruxner Monzogranite is part of the Clarence River Super Suite of granites which is an I-type granite (Bryant et al 1997). I-type granites are derived from melted igneous rock. It contains two different varieties of ‘granite’ (Thomson 1976). One variety is the rock type monzogranite which contains roughly equal amounts of the two main feldspar groups (plagioclase feldspar and alkali feldspar). It also includes quartz, amphibole and biotite mica.

The slightly less common variety is the granodiorite which contains more alkali feldspar than plagioclase. Therefore, it is richer in the elements sodium and potassium . It is worth noting that the granodiorite is often more altered and is more quartz rich. The easiest way to distinguish between the two Bruxner Monzogranite varieties in the field is their colour: The granodiorite usually has a pink colour and the monzogranite grey. The relationship between the two varieties of granite is not very clear to me. The following questions immediately spring into my mind:

• Does one granite intrude the other? 
• Were they both molten when they were emplaced? Or was one crystallised first? 
• Was it fluids from the crystallising monzogranite that caused the alteration of the granodiorite?

Maybe, they are questions that someone knows about but has not published their work on, or maybe they are just one of the many geological questions unanswered.

Bryant et al (1997) gives the potassium-argon age of the Bruxner Monzogranite as 250Ma. This places it in the Triassic Era, the same age as the other nearby Clarence River Supersuite. Such as, the Jenny Lind Granite which occurs a few kilometres north of the Buxner Monzogranite. The Clarence River Supersuite ‘granites’ are a similar age to many other granites which occur throughout the New England. This was certainly a busy time for intrusions. Indeed, these granites probably represent the magma source for an eroded volcanic arc system. It was caused by a large west dipping subduction zone that was active during this time (Scheibner & Basden 1998).

The Bruxner Monzogranite was intruded into Emu Creek Formation which is Carboniferous to Permian aged (Bottomer 1986). It is comprised of mudstones, greywacke, siltstones, shale, sandstones, conglomerate and limestone. As mentioned in my earlier post on limestone in the area, metamorphism of these rocks has in places been quite pervasive with a distinct metamorphic aureole. This has created some interesting rocks and altered zones such as marble and iron rich skarn.

The Bruxner Monzogranite is overlain in some areas by sediments of the Clarence-Moreton Basin. In particular, the Woogaroo Subgroup of the Bundamba Group, mainly the Laytons Range Conglomerate. Weathered exposures of the Laytons Range Conglomerate can be seen in road cuttings on the Paddys Flat Road.

The Bruxner Monzogranite was once called the Bruxner Adamellite (the term adamellite is no longer recognised). It is named after the Bruxner Highway which passes right through the unit.  Adjacent to the Bruxner Highway, approximately 2-3km west of Plumbago Creek, is one of the best places to see the outcrops of both the monzogranite and granodiorite. A good place to see the monzogranite is along the ridges along Sugarbag Road which is in the northern part of the unit, off Paddys Flat Road.


References/bibliography:

*Bottomer, L.R. (1986), Epithermal silver‐gold mineralization in the Drake area, northeastern New South Wales, Australian Journal of Earth Sciences. V33.
*Bryant, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cirdilleran Tonalitic I-type Intrusions in Eastern Australia. Journal of Petrology. V38.
Scheibner, E. & Basden, H. 1998 Geology of New South Wales – Synthesis. Volume 2 – Geological Evolution. Geological Survey of New South Wales, Memoir Geology 13.
*Thomson, J. 1976 Geology of the Drake 1:100 000 sheet, 9340. Geological Survey of New South Wales 1v.