Woodgreen, Northern Territory : explanatory notes
Northern Territory Geological Survey
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Geology; Georgina Basin; Arunta Region
Northern Territory Government; Northern Territory Government
Australia 1:100 000 Geological Map Series
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Northern Territory Government
of the eastern Arunta, where highgrade metamorphic rocks have Neoproterozoic to Cambrian precursors (Buick etal 2001). All Arunta Region rocks in Woodgreen form part of the Aileron Province. The oldest known rocks in the Aileron Province are a widespread succession of pelites and psammites of the Lander package, deposited in the interval 18651820 Ma. The Lander package varies in sedimentary facies from shallow to deep marine, and is metamorphosed from greenschist to granulite facies. A younger, Ongeva package of siliciclastic and volcaniclastic metasedimentary rocks occurs in the eastern Arunta Region, and has depositional ages in the range 18101800 Ma (Scrimgeour 200). This latter package includes most of the Strangways Metamorphic Complex (Hussey etal 200), and the Bonya Schist and associated metasedimentary rocks in HUCKITTA (Scrimgeour etal 2001, ClaouLong and Hoatson 2005). The Aileron Province was affected by a number of tectonothermal events during the late Palaeoproterozoic to early Mesoproterozoic, the most important of which were the 18101800 Ma Stafford Event, the 17801770 Ma Yambah Event, the 1701700 Ma Strangways Orogeny and the 15901570 Ma Chewings Orogeny. These events were separated by periods of siliciclastic sedimentation, including the 1780 Ma Reynolds Range Group in the northern Arunta and the 17651740 Ma Ledan package in the eastern Arunta (Maidment etal 2005). The Georgina Basin is the largest of the intracratonic NeoproterozoicPalaeozoic basins on the North Australian Craton. The basin's present extent of 0 000 km2 is an erosional remnant of a series of interconnected basins, the early phases of which are included in the Centralian Superbasin (Walter etal 1995). A series of intraplate orogenic events, including the EdiacaranCambrian Petermann Orogeny and Palaeozoic Alice Springs Orogeny, led to exhumation of basement from beneath the superbasin, forming the series of isolated sedimentary basins (Ngalia, Georgina, Amadeus, Officer and Wiso basins and the Irindina Province) seen today. Regionally, the Georgina Basin has a depositional history ranging from approximately 800 Ma to Late Devonian (Walter etal 1995, Haines etal 2001), although only an incomplete succession is preserved in Woodgreen. In excess of 1.5 km of Neoproterozoic sedimentary rocks are preserved in downfaulted blocks and halfgrabens on the southern margin of the basin, and depocentres and synclines contain up to 2.2 km of Cambrian to Devonian section. Previous estimates of as much as 9 km total thickness, based on magnetic depth modelling, did not recognise lowmagneticintensity granites and, hence, have overestimated the thickness of the sedimentary basin. In Woodgreen, the maximum preserved thickness of the NeoproterozoicCambrian section is interpreted to be 1120 m (Dunster etal 2007). A highgrade metamorphic event (Larapinta Event) affected the Arunta Region and overlying sediments in the Harts Range region to the southeast of Woodgreen during the Ordovician (at 480460 Ma; Mawby etal 1999, Hand etal 1999). The Larapinta Event is inferred to have been of extensional nature, and was followed by a prolonged period of episodic convergent tectonic activity, the Alice Springs Orogeny, which peaked during the Devonian and Carboniferous (Haines etal2001). This event produced regionalscale retrogressive shear zones through the Arunta Region (Scrimgeour and Raith 2001) and was responsible for the uplift and removal of large areas of sedimentary cover, as well as moderate deformation in surviving basin regions. The Alice Springs Orogeny was also associated with widespread foreland deposition (Haines etal 2001), but such deposits are not preserved in Woodgreen. The Dulcie Syncline to the northeast preserves Devonian fluviatile sandstones that probably include detritus removed from the Woodgreen area. The region preserves little record of postPalaeozoic events, apart from local Cenozoic deposition, such as that of the TiTree Basin. Stratigraphy The stratigraphy of the Arunta Region, Georgina Basin and Cenozoic cover in Woodgreen are summarised in tables1, 2and3, respectively. PalaeoProterozoic arunta region Mapata Gneiss (LPnm) The Mapata Gneiss is defined to the east of Woodgreen in delny (Shaw etal 1979). Several outcrops near the southeastern corner of Woodgreen are tentatively assigned to the Mapata Gneiss, based on the regional mapping of Shaw and Warren (1975). These outcrops are dominated by biotitequartzfeldspar migmatite with ptygmatically folded leucosomes, together with lesser biotite schist and minor amphibolite. In Woodgreen, this unit is in sheared contact with the Delny Metamorphics. Shaw and Warren (1975) and Shaw etal (1979) assigned the Mapata Gneiss to the Strangways Metamorphic Complex and implied that the Delny Metamorphics should unconformably overlie the unit, although there is no definitive field or geochronological evidence to support this. The unit is essentially nonmagnetic, but is distinctly high in all radiometric channels. Anira Metamorphics (LPan) The Anira Metamorphics (new name; see appendix1) comprise several small outcrops of granulitefacies metasedimentary rocks in the area northwest of Anira Dam, the best exposures being at about 41985mE 750881mN. The rocks are dominantly pelitic and preserve well defined primary sedimentary layering (Figure3). Psammite layers represent 1020% of the succession and are typically 15 cm thick, with internal sedimentary layering. Metapelitic units have a high volume of melt and a quartzpoor cordierite, sillimanite and spinelrich melanosome. Leucosome is usually confined within pelitic layers. Local quartzpoor cordieritesillimanitespinelbiotite rocks are interpreted to be restitic following the extraction of melt (Figure4). Psammites contain biotite and garnet, and are generally melt free (Figure5), although they are locally intruded by melt that has mobilised from pelitic layers. Localised larger melt accumulations with tabular Kfeldspar may be equivalent to the Woodgreen Granite Complex. The Anira Metamorphics are intruded by several small bodies of granite, assigned to the Woodgreen Granite Complex, and rafts of similar metasedimentary lithology to the former unit are present
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