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| | | | Geological Survey of Western Australia |
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| | | Blackstone Sub-basin (PTBNB) | HM Howard, RH Smithies, and R Quentin de Gromard | | | | | Type | Sub-basin | Lithology | sedimentary and volcanic rocks | Parent unit | | Child units | No child units | Constituent lithostratigraphic units | | Affected by events | | Tectonic setting | | basin: intracratonic basin |
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| | | Summary | The Blackstone Sub-basin forms part of the greater Bentley Basin and the rocks of the sub-basin are folded in the Blackstone Syncline south of the Blackstone Range. The base of the sequence comprises units of the Kunmarnara Group (MacDougall Formation and Mummawarrawarra Basalt). These are overlain by volcanic rocks of the Tollu Group (Smoke Hill Volcanics and Hogarth Formation), which constitutes the dominant component within the sub-basin. The age of this part of the Bentley Supergroup ranges from c. 1085 to 1067 Ma. | | | Distribution | The preserved extent of the Blackstone Sub-basin is folded by the east-trending Blackstone Syncline in the northern part of BLACKSTONE, between Bell Rock Range in the east and Cavenagh Range in the west. The sub-basin lies entirely in the Mamutjarra Zone and is bounded to the north by the Blackstone Range and to the south by granitic and gneissic basement of the Musgrave Province. It likely extends eastwards under cover onto the western edge of BELL ROCK. | | | Description | Volcanic rocks in the Blackstone Sub-basin directly overlie the layered, mafic Blackstone intrusion (P_-WKG-o). Depositional layering in these volcanic rocks parallels igneous layering in the intrusion, and there is no evidence for a faulted contact between the intrusion and the volcanic rocks. Alcurra Dolerite dykes cut the layering within the layered mafic Cavenagh intrusion (P_-WKG-o) to the south and are interpreted as feeders to the geochemically identical lavas of the Hogarth Formation. These relationships indicate a significant time gap between deposition of the Kunmarnara Group (which pre-dates, and was intruded by, P_-WKG layered mafic intrusions) and eruption of the Smoke Hill Volcanics. During that time gap, the layered mafic–ultramafic intrusions were emplaced within the Mummawarrawarra Basalt (Kunmarnara Group), and the whole package was uplifted, eroded, and possibly folded.
The younger part of the Blackstone Sub-basin sequence is represented by the Tollu Group, subdivided into the Smoke Hill Volcanics and overlying Hogarth Formation (Daniels, 1974; Smithies et al., 2009). The Smoke Hill Volcanics is dominated by porphyritic rhyolite and dacite, occurring as lavas, sills and cryptodomes. Volcaniclastic deposits are locally common, and include breccia-sandstone and laminated sandstones to siltstones. Some volcaniclastic units are interpreted as rheoignimbrites. Intrusive and extrusive rocks are locally flow banded, amygdaloidal or spherulitic; lavas also show hyaloclastite breccias, indicating subaqueous deposition.
Rocks of the Tollu Group are locally folded about an east-trending axis that parallels the axis of the Blackstone syncline, and Coleman (2009, 2010b) established that mylonite zones that are either syn- or post-folding had developed by 1071 ± 8 Ma. This date most likely represents an effective minimum age of deposition of the Smoke Hill Volcanics, which likely evolved within a narrow interval between c. 1075 and 1070 Ma.
The overlying Hogarth Formation comprises andesitic to trachyitic lava flows. The dominant unit consists of variolitic andesitic to trachyitic lava, but there are also acicular- and comb-textured andesitic to trachytic lavas. Volcaniclastic rocks, including pebble breccia, laminated volcanic siltstones, and subordinate sandstones are interbedded with lavas near the base of the formation, and porphyritic, aphanitic rhyolite and lesser dacite lavas are common at the top of the formation.
A sequence of supracrustal rocks with a similar lithological range to the Kunmarnara and Tollu Groups forms the Tjauwata Group, which straddles the Western Australian – Northern Territory border to the north, and lies unconformably beneath the basal sedimentary rocks of the Amadeus Basin. Close et al. (2003) noted some stratigraphic, geochemical, and geochronological similarities between the Tjauwata Group and the combined Kunmarnara and Tollu Groups (i.e. the Tollu Group as previously defined by Close et al., 2003). | | | | | | Geochronology | | | Blackstone Sub-basin | Maximum age | Minimum age | Age (Ma) | 1085 | 1067 | Age | Mesoproterozoic | Mesoproterozoic |
| The oldest rocks of the Blackstone Sub-basin are the MacDougall Formation and the Mummawarrawarra Basalt of the Kunmarnara Group. The maximum depositional age of the MacDougall Formation is constrained by the youngest detrital zircon age component of c. 1179 Ma (Quentin de Gromard et al., 2016). A more realistic maximum age is the c. 1085 Ma regional maximum age of magmatism associated with the Giles Event (Edgoose et al., 2004; Evins et al., 2010), which must be nearly coeval with the onset of the Ngaanyatjarra Rift and deposition of the Kunmarnara Group. The Mummawarrawarra Basalt is considered to be the lateral equivalent of the c. 1085 Ma Mount Harris Basalt, and is, therefore, assumed to have the same age of c. 1085 Ma.
Two rhyolite samples from the Smoke Hill Volcanics, taken from around Mount Jane in the east of the Blackstone Syncline, have crystallization ages of 1071 ± 8 Ma (GSWA 191728, Coleman, 2010b) and 1073 ± 7 Ma (GSWA 191706, Coleman, 2010a). A sample of Smoke Hill Volcanics taken from the southwestern part of the Blackstone Syncline, approximately 10 km west of Naries Hill, gave a crystallization age of 1073 ± 8 Ma (GSWA 189561, Wingate et al., 2019). The overlying Hogarth Formation yielded a date of 1068 ± 7 Ma (GSWA 185518, Kirkland et al., 2013), consistent with the geochemically related c. 1067 Ma Alcurra Dolerite (Seat, 2008; Howard et al., 2009) that intruded the Smoke Hill Volcanics in the western part of the Blackstone Syncline.
In addition, a monzogranite pluton that intrudes the Smoke Hill Volcanics, and possibly the overlying Hogarth Formation, at Barnard Rocks in the central part of the Blackstone Syncline, has a crystallization age of 1065 ± 9 Ma (GSWA 189563, Kirkland et al., 2014). A vitric dacite sampled to the south of the Blackstone Range and north of Smoke Hill gave a date of 1026 ± 26 Ma (GSWA 187177, Kirkland et al., 2010), which was interpreted as the age of igneous crystallization of a subvolcanic sill in the upper part of the Smoke Hill Volcanics (Smithies et al., 2009). | | | | Volcanic rocks of the northern part of the Blackstone Sub-basin directly overlie the layered, mafic Blackstone intrusion. Depositional layering in these volcanic rocks parallels igneous layering in the intrusion, and there is no evidence for a faulted contact between the intrusion and the volcanic rocks. To the south, the sub-basin developed over the granitic and gneissic basement rocks of the Wankanki Supersuite and Wirku Metamorphics in the Mamutjarra Zone. | | | Tectonic setting | Rocks of the Blackstone Sub-basin, along with the Talbot Sub-basin and the Tjauwata Group, form part of the larger Bentley Supergroup, and were deposited into the Mesoproterozoic Ngaanyatjarra Rift in central Australia. This long-lived, but failed, intracontinental rift (Evins et al., 2010) is unusual in that the crust was thickened by ~15 km and overall extension was very limited (Aitken et al, 2013). The Ngaanyatjarra Rift is the main crustal expression of the Giles Event that was responsible for producing almost 50 Ma of continued magmatism in the west Musgrave region. The Giles Event does not therefore reflect a simple single event but rather a protracted and complex geodynamic setting. The event included the c. 1075 Ma Warakurna Large Igneous Province (LIP), the associated granitic rocks of which have volcanic equivalents in the Smoke Hill Volcanics, and emplacement of the regional Alcurra Dolerite dyke swarm and its volcanic equivalent in the Hogarth Formation (Blackstone Sub-basin).
The long duration of mantle-derived mafic and felsic magmatism included the development of a silicic LIP over a period of >30 Ma, formed by a series of large rhyolite eruptions, including some of supervolcano size, interleaved with regional tholeiitic basalt flows (Smithies et al., 2013). Smaller volume eruptions filled the Blackstone Sub-basin, compared with the more extensive volcanic succession that filled the Talbot Sub-basin; nevertheless, both are components of the Bentley Basin. | BookMark | | | | | Constituent lithostratigraphic units | | | Unit name | Unit code | Rank | GSWA status | | | Formation | Formal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Formation | Formal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Member | Informal | | | Group | Formal |
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| | | | | | References | Aitken, ARA, Smithies, RH, Dentith, MC, Joly, A, Evans, S and Howard, HM 2013, Magmatism-dominated intracontinental rifting in the Mesoproterozoic: The Ngaanyatjarra Rift, central Australia: Gondwana Research, v. 24, no. 3–4, p. 886–901, doi:10.1016/j.gr.2012.10.003. | Close, DF, Edgoose, CJ and Scrimgeour, IR 2003, Hull and Bloods Range, Northern Territory: Northern Territory Geological Survey, 1:100 000 Geological Map Series Explanatory Notes, 46p. | Coleman, P 2009, Intracontinental orogenesis in the heart of Australia: Structure, provenance and tectonic significance of the Bentley Supergroup, western Musgrave Block, Western Australia: Geological Survey of Western Australia, Record 2009/23, 50p. View Reference | Coleman, PM, Kirkland, CL, Wingate, MTD and Smithies, RH 2010, 191728.1: rhyolite, Mount Jane; Geochronology Record 917: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Coleman, PM, Kirkland, CL, Wingate, MTD and Smithies, RH 2010, 191706.1: mylonitic rhyolite, Mount Maria; Geochronology Record 915: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Daniels, JL 1974, The geology of the Blackstone region, Western Australia: Geological Survey of Western Australia, Bulletin 123, 257p. View Reference | Edgoose, CJ, Scrimgeour, IR and Close, DF 2004, Geology of the Musgrave Block, Northern Territory: Northern Territory Geological Survey, Report 15, 46p. | Evins, PM, Smithies, RH, Howard, HM, Kirkland, CL, Wingate, MTD and Bodorkos, S 2010, Devil in the detail: the 1150–1000 Ma magmatic and structural evolution of the Ngaanyatjarra Rift, west Musgrave Province, central Australia: Precambrian Research, v. 183, p. 572–588. | Howard, HM, Smithies, RH, Kirkland, CL, Evins, PM and Wingate, MTD 2009, Age and geochemistry of the Alcurra Suite in the western Musgrave Province and implications for orthomagmatic Ni–Cu–PGE mineralization during the Giles Event: Geological Survey of Western Australia, Record 2009/16, 16p. View Reference | Kirkland, CL, Wingate, MTD, Howard, HM, Quentin de Gromard, R and Smithies, RH 2014, 189563.1: hornblende–biotite syenogranite, Barnard Rocks; Geochronology Record 1196: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Kirkland, CL, Wingate, MTD, Howard, HM and Smithies, RH 2013, 185518.1: rhyolite, Barnard Rocks; Geochronology Record 1128: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Kirkland, CL, Wingate, MTD and Smithies, RH 2010, 187177.1: metadacite, Hogarth Wells Rockhole; Geochronology Record 847: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Quentin de Gromard, R, Wingate, MTD, Kirkland, CL, Howard, HM and Smithies, RH 2016, Geology and U–Pb geochronology of the Warlawurru Supersuite and MacDougall Formation in the Mitika and Wanarn areas, west Musgrave Province: Geological Survey of Western Australia, Record 2016/4, 29p. View Reference | Seat, Z 2008, Geology, petrology, mineral and whole-rock chemistry, stable and radiogenic isotope systematics and Ni-Cu-PGE mineralisation of the Nebo-Babel intrusion, west Musgrave, Western Australia: The University of Western Australia, Perth, Western Australia, PhD thesis (unpublished). | Smithies, RH, Howard, HM, Kirkland, CL, Werner, M, Medlin, CC, Wingate, MTD and Cliff, JB 2013, Geochemical evolution of rhyolites of the Talbot Sub-basin and associated felsic units of the Warakurna Supersuite: Geological Survey of Western Australia, Report 118, 74p. | Smithies, RH, Howard, HM, Maier, WD and Evins, PM 2009, Blackstone, WA Sheet 4545: Geological Survey of Western Australia, 1:100 000 Geological Series. View Reference |
| | | | Recommended reference for this publication | Howard, HM, Smithies, RH and Quentin de Gromard, R 2019, Blackstone Sub-basin (PTBNB): Geological Survey of Western Australia, WA Geology Online, Explanatory Notes extract, viewed 05 August 2025. <www.dmp.wa.gov.au/ens> |
| | | This page was last modified on 14 June 2019. | | | | | Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Locations mentioned in the text are referenced using Map Grid Australia (MGA) coordinates, Zones 49 to 52. All locations are quoted to at least the nearest 100 m. Capitalized names in text refer to standard 1:100 000 map sheets, unless otherwise indicated. WAROX is GSWA’s field observation and sample database. WAROX site IDs have the format ‘ABCXXXnnnnnnSS’, where ABC = geologist username, XXX = project or map code, nnnnnn = 6 digit site number, and SS = optional alphabetic suffix (maximum 2 characters). All isotopic dates are based on U–Pb analysis of zircon and quoted with 95% uncertainties, unless stated otherwise. U–Pb measurements of GSWA samples were conducted using a sensitive high-resolution ion microprobe (SHRIMP) in the John de Laeter Centre at Curtin University, Perth, Western Australia. Digital data related to WA Geology Online, including geochronology and digital geology, are available online at the Department’s Data and Software Centre and may be viewed in map context at GeoVIEW.WA. | | | Further details of geological publications and maps produced by the Geological Survey of Western Australia are available from: Information Centre Department of Mines, Industry Regulation and Safety 100 Plain Street EAST PERTH, WA 6004 Telephone: +61 8 9222 3459 Facsimile: +61 8 9222 3444 |
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