Selection of Rock Specimens for the Interim Gallery Display – Namban Granite
By Peter Williams, NRG Steering Committee
The NRG Steering Committee is planning to demonstrate the educational and tourism attributes of the Rock Garden by the establishment of an Interim Gallery Display (IGD). This would be a proof of concept arrangement of selected rocks in the south east corner of the NRG site. The Interim Gallery Display will have rock exhibits grouped into Themes, which unify and contextualise the rock exhibits in time and space, laid out in a logical order along a meandering, closed-loop pathway. The centre of the feature will contain a display explaining the exhibit to visitors. The NRG Steering Committee is currently preparing a comprehensive Works Application for submission to the National Capital Authority and is building up a targeted database of specific specimens which can be procured in the near future to populate the IGD themes.
One rock from Western Australia which seems to be a good choice for the Interim Gallery Display is the Namban Granite, which is described in this article. This building stone is well known as a decorative material with spectacular visual impact, notably in the foyer of the Western Australian Parliament. It is also an excellent example of a rare granite type.
The Namban locality (see location map at https://www.whereis.com/wa/namban-6512) is situated about 10 km south of the town of Watheroo in the Central Wheatbelt of Western Australia, in the Shire of Moora.
The geology of the area is dominated by granite batholiths and smaller intrusions (plutons), one of which, the Namban Granite, is named after the locality of Namban. The pluton extends north from Namban and through Watheroo, and comprises dominantly coarse to very coarse-grained rocks with monzogranite, monzonite and quartz syenite compositions. These rock names refer to different types of granite according to the proportion of potassium feldspar (orthoclase) to sodium and calcium-rich feldspars (plagioclase), and on the total proportion of quartz in the rock to other constituents. A monzogranite has about equal proportions of orthoclase and plagioclase and more than 20% quartz; a monzonite is similar but with less than 5% quartz; whereas a quartz syenite has 5%-20% quartz and more than 90% of the feldspar is a potassium feldspar.
Six quarries have been opened up in the Namban Granite, and all have been described in the Geological Survey of Western Australia Mineral Resources Bulletin 24 on Dimension Stones of the Southern, Central Western, and Northern Regions by JM Fetherston. The location of these quarries is shown on the location map. The name of the quarries comes from the visual character of the rocks; Mulroy Green is the name of a dimension stone from the Donegal granite in Ireland, which may be the origin of the name. Valmere Green is the colour of green dinner-ware prized in the 1950’s.
The age of the Namban intrusion is Neoarchean. Three radiometric age determinations for Verde Lope quarry samples yielded ages of 2638 Ma, 2643 Ma and 2646 Ma, recorded in the Geoscience Australia geochronology database. Interestingly, these ages are similar to syenite and other late Archean small plutons that occur over vast areas of the Yilgarn Craton in southwest Western Australia. These rocks were intruded by dolerite dykes which form prominent ridges in the landscape of the region, and are overlain by Proterozoic Moora Group sedimentary rocks, mainly quartzite and siltstone.
Rock types like those of the Namban intrusion are unusually low in quartz and high in potassium compared to most granites intrusions. They form distinctive patterns in geophysical data, with high potassium and commonly (but not always) a strong magnetic signature. The Namban intrusion can be mapped readily in areas that are underlain by the granite, but where no outcrop can be seen, from the radiometric signature of overlying soils. This is derived from the elevated potassium in the large pink-coloured orthoclase feldspar crystals in the rocks.
This rock is suitable for the Interim Gallery Display because of its geologic and economic significance, and because large blocks are readily accessible in an existing quarry. The Steering Committee welcomes other suggestions of rocks which could be acquired with relative ease and which might illustrate significant stages in the evolution of our continent, its life forms, landscapes, resources, and cultures and provide links to many of Australia’s iconic tourist attractions.