Definition card for: Galiwinku Dolerite

State(s): NT,
Status of unit: Formal
Usage: Defined
Stratno: 77145

Reserved by: on:
Approved: Y
Approved by: Normington, V. on: 11-FEB-16

Name source: After the township of Galiwinku located in the Arnhem Shelf (see, Record ID NT G527) GDA94 Zone 53L 561500mE 8670281mN (12 1' 41" S, 135 33' 54"E)

Unit history: First informally named by Goldberg 2010 and chemically defined by Hollis and Glass 2012

Geomorphic expression: Subsurface radial dyke swarm.

Type section locality: Howship 1:100 000 geological mapsheet, Cameco Australia Pty Ltd drill hole PLD0001 (available for viewing at the Northern Territory Geological Survey, Core Library, Darwin). Grid Reference: GDA94 Zone 53L 335346m E 8591678m N (12.7351S, 133.483E) Hollis and Glass 2010).

Description at type locality: Intrusive into Mamadawerre Sandstone of the Kombolgie Subgroup, McArthur Basin in southeast Howship 1:100 00 geological mapsheet.

Extent: Dykes intrude McArthur Basin (onshore) and extend radially to the north offshore into the sedimentary rocks beneath the Arafura Sea.

General description: Subsurface extent of northeast and northwest-trending radial dykes which intrude the Paleo-Mesoproterozoic recognised from magnetic response. Extensive under cover in Milingimbi, Arnhem Bay Gove, Wessel Islands Truant Island and Junction Bay 1:250 000 geological maphseets.

Thickness range: Unknown.

Lithology: Quartz normative and olivine normative dolerite.

Depositional environment: Genesis: Intrusive dyke swarm, possibly related to rifting (Goldberg 2010).

Relationships and boundaries: Intrusive into the Paleo- to Mesoproteroic McArthur Basin. Chilled margin contact to Kombolgie Subgroup onshore in southeast Howship 1:100 000 geological mapsheet.

Identifying features: Distinctive alkaline "hawaiite" geochemistry, strong magnetic response for dyke swarm under cover.

Structure and Metamorphism: Not metamorphosed.

Age reasons: The Galiwinku Dolerite has a SIMS U-Pb baddeleyite age of 207Pb/206Pb age of 1329 +/- 55 Ma (95% confidence, n = 8, MSWD = 1.3; Whelan et al in press).

Correlations: Geohemically correlated Maningkorrirr and Mudginberri phonolites in Milingimbi and Alligator River 1:250 000, respectively. The latter has a Rb-Sr age of 1316 +/- 40 Ma (Page et al 1980). The Galiwinku Dolerite has similar geochemical signature to outcropping dolerite in Wellington Range 1:100 000 map geological, Cobourg Peninsula 1:250 000 geological mapsheet (Hollis and Glass, 2012) and dolerite intruding the Arnhem Province in Gove 1:250 000, but further investigation is required to confirm these relationships. Furthermore, alkali gabbro intruding the Tomkinson Province is geochemically similar to the Galiwinku Dolerite and yielded a SHRIMP U-Pb baddeleyite age of 1295 +/- 14 Ma (Melville 2010).

Alteration and Mineralisation: Some chloritisation of feldspars, unmineralised.

Geophysical Expression: Strong magnetic response.

Geochemistry: Chemical similarities to Oceanic Island Basalt (OIB) with potassic hawaiite geochemical signature, characterised by elevated High Field Strength Element (HFSE) abundances (Hollis and Glass 2012).

Defn author: LM Glass and JA Hollis 11-FEB-2016

References: Goldberg AS, 2010. Dyke swarms as indictors of major extensional events in the 1.9-1.2 Ga Columbia supercontinent. Journal of Geodynamics, 50, 176-190. **Hollis JA and Glass LM, 2012. Howship-Oenpelli, Northern Territory. 1:100 000 geological map series explanatory notes, 5572, 5573. Northern Territory Geological Survey, Darwin. **Melville PM, 2010. Geophysics and drilling collaboration final report for drilling program, Lake Woods Project, EL23687, EL24520, EL25631, EL27317, EL27318. Northern Territory Geological Survey, Open File Report, CR2010-0226. **Page RW, Compston W and Needham RS, 1980. Geochronology and evolution of the late-Archaean basement and Proterozoic rocks in the Alligator Rivers uranium field, Northern Territory, Australia: in Ferguson J and Goleby AB (editors) Uranium in the Pine Creek Geosyncline: proceedings of the International Uranium Symposium on the Pine Creek Geosyncline. International Atomic Energy Agency, Vienna, 3968. **Whelan JA, Glass LM, Hollis JA, Bleeker W, Chamberlin K and Soderlund U and Ernst RE, in press. Summary of results. TIMS and SIMS U-Pb baddeleyite dating of the Oenpelli and Galiwinku dolerites. Northern Territory Geological Survey, Record