TAG 166 - Geological Society of Australia

A spectacular hornblende–diorite orbicule about 150 mm wide. Imagecourtesy Mike Fetherston.Bevan (2004) proposed that the orbicules were formed bycrystallisation from a fluid-rich, supercooled dioritic magma. Variationsin orbicule type and layer structure reflect a very dynamic history ofcrystallisation influenced by changes in magma composition and degreeof cooling. The presence of broken fragments and variations in zonemineralogy indicate that proto-orbicules were very mobile in the magmachamber, probably propelled by convection currents within fluid-richmargins. Orbicules were eventually subject to gravity settling to formthe deposit visible today. It is evident that, at the time of settling,orbicules were still in a plastic state as they are often deformed ormoulded together. Some orbicules appear to have been dragged alongby the current while others show evidence of mass movement aftersettling. Further recent information on the nature and origin of theorbicular granite is given in Bevan and Bevan (2009).Because of its visually spectacular orbicular structure, relative rarityand ability to take a high polish, the orbicular granite has been quarriedat different times, supplying blocks and slabs mainly to artisans andmonumental masons. Prime examples of completed works include highqualityornamental works such as contemporary sculptures, interiordecorative panels and tables. Floating orbicular granite spheresmeasuring up to one metre in diameter are also popular.MIKE FETHERSTONIndustrial minerals specialistGeological Survey of Western AustraliaNational Rock Garden updateTwo recent developments have given the National Rock Garden (NRG) amajor boost. First, the Australian National University (ANU) has agreedto become an Education Partner of the NRG. Second, the NRG has beenmade part of the Centenary of Canberra celebrations.At a time when university budgets are stretched to the limit, the ANUhas generously agreed to provide $100 000 in funding for the NRG. A draftMemorandum of Understanding has been drawn up and, once it is approvedand signed, ANU will become an Education Partner with the NRG.Dean of the ANU College of Physical and Mathematical Sciences (andGSA member), Professor Andrew Roberts, summed it up pretty well.“In Australia, we owe much of our national prosperity to our geologicalresources. The NRG is a great way to communicate to the general publicabout Australia’s geological heritage. As one of the world’s great EarthScience research establishments, it makes sense for ANU to partner withthe Geological Society of Australia in the NRG.”On 13 October 2013, in Earth Science Week and during Centenaryof Canberra celebrations, an inauguration ceremony will be held at thesite of the NRG on the western side of Lake Burley Griffin. Not to beconfused with an opening ceremony, which is still some years away, theNRG inauguration ceremony will mark the arrival of the first big rockson-site — eight to be exact, if things go according to plan.[L–R] Andrew Roberts and Brad Pillans at the NRG tourist sign — a sign ofthings to come! Image courtesy Brad Pillans.R E F E R E N C E SBevan, J, 2004, Archaean orbicular granitoids from Boogardie, near Mt Magnet, inWestern Australia: Dynamic Earth: Past, Present and Future: Geological Society ofAustralia, Abstracts 73, 17th Australian Geological Convention, Hobart, p. 252.Bevan, JC and Bevan, AWR, 2009, Nature and origin of the orbicular granodioritefrom Boogardie Station, Western Australia: an ornamental stone of monumentalproportions: The Australian Gemmologist (2009), v. 23, p. 373–432.Fetherston, JM, 2010, Dimension stone in Western Australia, Volume 2, Dimensionstones of the southern, central western, and northern regions: Geological Survey ofWestern Australia, Mineral Resources Bulletin 24, p. 35–43.The Canberra Foundation Stone, laid in 1913, and relocated to the lawns infront of New Parliament House in 1988. Image courtesy Brad Pillans.TAG March 2013| 15