TAG 166 - Geological Society of Australia

NEWSIn the news this issuen NRG Rock of the Monthn National Rock Garden updaten International Geological Correlationn Programme (IGCP)n University honours geologistn Case studies soughtn Ediacaran Golden Spike localityn China, Australia and geoparksn Fossil problematica from Moretonn National ParkNational Rock Garden —Boogardie orbicular graniteGSA Rock of the Month — March 2013The Boogardie orbicular granite deposit is located 35 km west of MountMagnet in Western Australia. This unusual granitic rock, located onBoogardie Station, is one of a few orbicular granite localities knownworldwide. The orbicular granite is hosted by a pink, medium-grained,late Archean granitic rock, comprising myrmekitic biotite granodioritethat becomes tonalitic in places. Information from diamond drillholesindicates that orbicular granite bodies may have formed as saucershaped,sill-like structures within the host granodiorite–tonalite. Overthe area of the Boogardie quarry, the orbicular granite appears as anoval-shaped body, about 40 m wide and at least 55 m long. Maximumthickness at the centre is 11.4 m, tapering off in all directions torelatively small thicknesses at the outer rims.Within the sill-like structures, abundant granitic orbicules arecontained in a leucogranitic body of variable tonalitic composition. Theblack and white, concentrically banded orbicules are of spectacularappearance. Orbicules are mostly ellipsoidal but also include lessernumbers of near-spherical, irregular and broken shapes. Orbicules havean average length of about 140 mm along their longest axis by 75 mmin width. Orbicule spacing varies from orbicular masses in tangentialcontact with one another to a maximum separation of about 100 mm,although overall orbicule distribution appears roughly uniform throughoutthe rock, as shown in the photograph of the matrix cut by apegmatite vein below.Individual orbicules are generally separated from the enclosinggranitic matrix by a mid- to dark grey outer shell up to 10 mm thick, whichis made of mainly fine- to medium-grained hornblende, biotite and plagioclasefeldspar, with minor opaque oxide and titanite. Inside this protectiveshell, orbicules have a hornblende-diorite composition, largelycomposed of plagioclase feldspar and hornblende with lesser amounts ofbiotite, opaque oxide and titanite. Orbicules usually contain five to seven(or even more) major, well-defined concentric zones of variable width,structural complexity and mineralogy. Some major zones may be up to15 mm wide and commonly contain large, irregular, elongate, radiallyaligned hornblende grains interspersed with white plagioclase. Thesehornblende grains may extend across the entire width of the zone and arecommonly tapered internally to a point, as shown by the spectacular imageof the individual orbicule opposite (Fetherston, 2010).Orbicules enclosed in a granodiorite–tonalite matrix cut by a late-phase pegmatite vein. Image courtesy Mike Fetherston.14 |TAG March 2013