Woodgreen, Northern Territory : explanatory notes
Northern Territory Geological Survey
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Geology; Georgina Basin; Arunta Region
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Australia 1:100 000 Geological Map Series
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Northern Territory Government
10 be transitional between migmatitic metasediment and Stype granite, and has a highly magnetic response (up to 5000 x 105 SI units). Its radiometric response is markedly less than that of LPgw1, primarily indicating lower K concentration. LPgw3 This subunit is mineralogically similar to LPgw1 but is distinctively finer grained and mostly equigranular. Although it locally displays layering, it lacks the distinctive flowaligned phenocrysts of LPgw1. Garnet phenocrysts are of similar size to those in LPgw1, but have a clotted appearance with strongly embayed margins. The reference area is around 432053mE 7533884mN. Subunit LPgw3 is also distinct from LPgw1 in that it has a very variable magnetic intensity and its radiometric signature is also inconsistent. Areas of very high magnetic intensity, in nonoutcropping areas shown as LPgw3 on the simplified geology map inset, may reflect the presence of migmatite (LPgw2) and/or metasedimentary rocks of the Anira Metamorphics. LPgw4 This subunit occupies a central position in the Woodgreen Granite Complex, south of the Mount Ida Shear Zone. It does not include outcrops immediately south of the shear zone, which are finer grained and generally strongly weathered and sheared. LPgw4 comprises coarsegrained porphyritic granite similar to LPgw1, but generally lacking garnet. Subordinate mediumgrained equigranular granite is also present. Minor garnet has been identified locally and, in some places, the granite contains spherical clots of biotite, which may represent retrogressed former garnet. The reference area is around 431400mE 7523000mN. Subunit LPgw4 has the lowest magnetic response of all subunits of the Woodgreen Granite Complex, being almost nonmagnetic. Th is a larger component of its radiometric signature than in other granitic units in Woodgreen, manifested in K/Th ratios in the range 0.020.03 (most granites on Woodgreen typically have K/Th >0.03). LPgw5 This subunit is the most southerly and is separated from LPgw4 by a major fault or shear zone. It comprises two contrasting phases that are intercalated on a scale that does not allow distinction on the mapsheet (Figure10). The dominant phase comprises distinctive porphyritic granite with a dark biotiterich groundmass (Figure11). Thin sections show significant dynamic deformation and evidence of recrystallisation, including significant quantities of secondary epidote in the groundmass. The subordinate phase comprises mediumgrained leucocratic granite. Isolated outcrops of nonporphyritic biotite granite near the western limit of exposure (eg at 428690mE 7521369mN) have also been included in LPgw5. The reference area is around 433000mE 7519000mN, which is also the type locality of the entire Woodgreen Granite Complex, as nominated by Shaw and Warren (1975). The unit has a relatively uniform, moderate to high magnetic response, with measured field susceptibilities of up to 2700 x 105 SI units associated with the porphyritic phase. Mollie Granite Complex (LPgm) Based on aeromagnetic interpretation, the scattered outcrops of granitoid rocks that lie in a northwesttrending belt from the Utopia tantalite prospects to the small outcrop at 426700mE 7556900mE are likely to comprise a single body and are distinguished on the map as Mollie Granite Complex (new name; see Appendix1). However, a wide lithological range is present, even in a single outcrop, and it is uncertain whether all phases are comagmatic or whether they comprise two or more unrelated intrusions. Interpretation is complicated by the fact that most outcrops are deeply weathered, although small fresh dome or tor outcrops, generally with little relief, have been found in several areas (eg 432459mE 7552773mN, 443999mE 7545626mN, 442936mE 7453632mN, 444309mE 7541775mN). One of the best exposures is around 432459mE 7552773mN, eastnortheast of New Bore, which is nominated as a reference area for this complex. At this locality, an early, dark biotiterich granodiorite with abundant dark enclaves is intruded by leucocratic microgranite and leucocratic porphyritic granite, with a distinctive flow foliation of tabular Kfeldspar phenocrysts. The later phases are intimately interlayered and appear to be comagmatic. Similar variants can be tentatively identified Figure10. Woodgreen Granite Complex (LPgw5). Typical exposure showing coarse, dark porphyritic phase interlayered with finer, leucocratic nonporphyritic phase. 432901mE 7518722mN, 6.5 km WNW of Ugnada Bore.
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