Territory Stories

Woodgreen, Northern Territory : explanatory notes



Woodgreen, Northern Territory : explanatory notes

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Northern Territory Geological Survey


E-Publications; E-Books; PublicationNT; Australia 1:100 000 Geological Map Series




Australia 1:100 000 Geological Map Wood 5458; Australia 1:250 000 Geological MapAlcoota SF5310; Australia 1:250 000 Geological MapAlcoota SF5310

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1:100 000


Made available via the Publications (Legal Deposit) Act 2004 (NT); Available from GEMIS - Geoscience Exploration and Mining Information System




Geology; Georgina Basin; Arunta Region

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Northern Territory Government; Northern Territory Government

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1st ed.


Australia 1:100 000 Geological Map Series

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Attribution International 4.0 (CC BY 4.0)

Copyright owner

Northern Territory Government



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https://geoscience.nt.gov.au/gemis/ntgsjspui/handle/1/81885 [GEMIS]

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that the Delny Metamorphics increase in metamorphic grade to the west. The Delny Metamorphics are intruded by the strongly foliated Crooked Hole Granite (eg, at 435064mE 511961mN) and both units share a common fabric. Although it is assumed that the Woodgreen Granite Complex intrudes the Delny Metamorphics, all exposed contacts appear to be tectonic and are strongly sheared or marked by a ridge of quartz breccia fault rock. Small bodies of foliated granitic rock intrude the Delny Metamorphics at several locations (eg 429800mE 51500mN, 434900mE 515400mN) and pegmatite intrusions are widespread. The Delny Metamorphics are typically nonmagnetic, the exception being a moderately magnetic feature concordant with the local foliation, centred on 434600mE 514300mN. Its radiometric response is low, with the weak signal that is present being dominated by K. Preliminary SHRIMP UPb zircon geochronological data (J ClaoueLong, Geocience Australia) suggests that the Delny Metamorphics has a maximum deposition age of about 1825 Ma. undivided metamorphic rocks (LPu ) Several small isolated outcrops of metamorphic rock belonging to the Arunta Region have not been assigned to any formal units. Near the northern boundary of Woodgreen, a strongly lineated biotitemuscovite gneiss is intruded by abundant pegmatite at 424500mE 564300mN. This outcrop is associated with a moderate local magnetic anomaly and slight K and U anomalism. Dark green, well layered and foliated amphibolite outcrops at 4444mE 541681mN and is intruded by granite and pegmatite. The mineral assemblage in the rock comprises actinolite, epidote, clinopyroxene, titanite, plagioclase and quartz, and may represent a metasedimentary calcsilicate rock. It locally exhibits weak, isolated U anomalies that contrast with the more Kdominated signature of the surrounding granitic material. North of Mount Harper, poorly exposed muscovite schist and minor amphibolite outcrop around 398800mE 545500mN and are overlain by the Grant Bluff Formation. Several small outcrops of metasedimentary rocks are associated with the Woodgreen Granite Complex. As the outcrops are isolated, it is uncertain if these represent margins to the granite or are large rafts within it. Such outcrops include a dark green hornfels at 42800mE 529100mN and epidoterich rocks around 429800mE 53000mN. Utopia Quartzite (LPut) Several small outcrops of deformed, massive to tabular clean quartzite, metasandstone and ironstone in northcentral Woodgreen have been tentatively assigned to the Utopia Quartzite. These have distinctly low magnetic and radiometric responses. In two localities (428621mE 554960mN, 431655mE 552292mN), it is clear that these rocks are unconformably overlain by the Boko Formation and Oorabra Arkose, respectively, and were deformed prior to the deposition of Georgina Basin sediments. The relationship between outcrops of Utopia Quartzite and nearby granites is not known, but it is inferred that the granites intrude the quartzite. The Utopia Quartzite is defined in adjacent Utopia, where it overlies the Ledan Schist, with both units extending eastward into HUCKITTA (Shaw etal 199). The Ledan Schist has also been recognised in southern BARROW CREEK (Haines etal 1991) and tentatively in adjacent Woolla (Shaw etal 199). SHRIMP UPb dating of detrital zircons from the Ledan Schist in Utopia gives a maximum deposition age of ca 180Ma (Maidment etal 2005), suggesting that it was deposited between the Yambah Event and Strangways Orogeny. In aLCOOTA, quartzite of the Mendip Metamorphics has a maximum deposition age of 160 Ma, was intruded by pegmatite at 130 4 Ma and has metamorphic zircon rims with an age of 123 9 Ma (Maidment etal 2005). These units are considered to be part of the same sedimentary package as the Utopia Quartzite (Ledan package of Scrimgeour 2003), in which case, the Utopia Quartzite was also deposited in the interval 165130 Ma. Crooked Hole Granite (LPgc) The Crooked Hole Granite is restricted to the southeastern corner of Woodgreen and is also present in the border regions of the adjoining 1:100 000 sheet areas. The type locality is situated in the low hills surrounding 435000mE 512000mN (Shaw etal 199). A typical sample from 440453mE 515968mN is a biotite granodiorite with abundant plagioclase (largely retrogressed to sericite) and accessory titanite and zircon. The unit intrudes the Delny Metamorphics and has a strong foliation defined by biotite. The contact with the Delny Metamorphics is well exposed at 435064mE 511961mN, near the type locality, where the Crooked Hole Granite can be seen as a distinctly tabular silllike body within the gneiss, which at this locality, dips gently west parallel to lithological layering and foliation (Figure6). Pegmatites are common within and around the margins of the granite and tend to intrude parallel to foliation. The Crooked Hole Granite is reported to be similar to the Copia Granite, which outcrops 20 km eastsoutheast of the southeastern corner of Woodgreen (Shaw and Warren 195). These granites are syn or pretectonic and are likely to predate less deformed granites of the nearby Woodgreen Granite Complex. Although the Crooked Hole Granite has not been dated, it is likely to be contemporaneous with other deformed granites that intruded at 18010 Ma in HUCKITTA (Zhao and Bennett 1995). The Crooked Hole Granite has a relatively low radiometric response in comparison to other granites in Woodgreen, and is non to moderately magnetic (susceptibilities up to 822 x 105 SI were recorded in the field). Portions of the intrusion (such as around 43500mE 513200mN) have apparently been subject to magnetitedestructive alteration. Woodgreen Granite Complex (LPgw) The Woodgreen Granite Complex comprises a diversity of granitic and minor migmatitic rocks that outcrop over a wide area in central and southern Woodgreen. Shaw and Warren (195) nominated a type area around 433000mE 519000mN, southwest of Dixon Bore, but in view of the lithological variety across the complex, a number of additional reference areas for mappable subunits are