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| | | | Geological Survey of Western Australia |
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| | | Split Rock Magmatic Event (PCSME) | AH Hickman | | | | | Event type | magmatic: intrusive | Parent event | | Child events | | Tectonic units affected | | Tectonic setting | igneous: plume-related | Metamorphic facies | | Metamorphic/tectonic features | –– |
| | | Summary | The Split Rock Magmatic Event resulted in the intrusion of the Split Rock Supersuite across the eastern half of the northern Pilbara Craton. The Split Rock Supersuite comprises multiple intrusions of highly fractionated, Sn–Ta–Li-bearing, post-orogenic monzogranites that were emplaced in a broad, northwest-trending linear belt across the Kurrana and East Pilbara Terranes, and into the northeastern Mallina Basin. Dating of the Split Rock Supersuite has been limited to intrusions in the southeastern part of the East Pilbara Terrane and the Kurrana Terrane, where ages of 2851–2831 Ma have been recorded.
Geochemical features of the supersuite, and whole-rock Sm–Nd isotope data, indicate magma derivation from partial melting of Paleoarchean granitic crust. The distribution of the post-orogenic intrusions, in a northwest-trending zone, suggests that intrusion migrated along this trend. One possible explanation is drift of the east Pilbara Craton southeast to northwest across a hot spot. Geochronology on the more northwesterly intrusions is required to test this possibility and establish the true age range of the supersuite.
| | | Distribution | Unlike Paleoarchean granitic supersuites of the Pilbara Craton, which show no linear zones of intrusion, the Split Rock Supersuite was emplaced in a broad, northwesterly trending belt across the Kurrana and East Pilbara Terranes and into the northeastern part of the Mallina Basin. Another difference is while the intrusion of Paleoarchean granites was confined to the cores of granite–greenstone domes, intrusions of the Split Rock Supersuite were emplaced into both granites and greenstones (Hickman, 1975). | | | Description | The Split Rock Supersuite comprises multiple intrusions of highly fractionated, Sn–Ta–Li-bearing, post-orogenic monzogranites that were emplaced in a broad, northwesterly trending belt across the northeastern part of the Pilbara Craton (Blockley, 1980; Hickman, 1980, 1983; Sweetapple, 2000; Sweetapple and Collins, 2002; Van Kranendonk et al., 2006, 2007). Monzogranites of the supersuite are silica-rich (>73% SiO2), with high contents of large ion lithophile elements (LILE) and moderate to large negative Eu anomalies (Champion and Smithies, 2001a). The rocks are depleted in Sr and undepleted in Y. Moderate to high Rb, Rb/Sr, Rb/Ba, Ca/Sr and K2O/Na2O, and low K/Rb ratios are consistent with crystal fractionation. Fluorite is a common accessory mineral in most of the dated intrusions. Mineralization associated with the individual intrusions varies depending on if they intrude granites (Sn and Ta) or greenstones (W, Mo, Cu, F and Li) (Hickman, 1975).
Eoarchean or early Paleoarchean Sm–Nd two-stage depleted mantle model ages (TDM2) in the Split Rock Supersuite of the southeastern part of the east Pilbara are consistent with magma derivation from partial melting of much older granitic crust (Blockley, 1980; Davy and Lewis, 1986; Bickle et al., 1989; Champion and Smithies, 2001a,b, 2001; Smithies et al., 2003; Gardiner et al., 2017, 2018). However, Mesoarchean or late Paleoarchean Nd TDM2 ages in the western part of the east Pilbara (Carlindi and Yule Domes) indicate more juvenile sources.
U–Pb zircon dating of the Split Rock Supersuite has been limited to intrusions in the southeastern part of the east Pilbara, where dates of 2851–2831 Ma have been recorded (reviewed by Hickman, 2021). Kinny (2000) used dating of tantalite in pegmatite to suggest that some granitic intrusions in the Carlindi and Yule Domes were emplaced at c. 2880 Ma, but more data on the geochemistry of the intrusions related to these pegmatite units is required to confirm an assignment to the supersuite. The northwestern intrusions are locally weakly foliated and elsewhere non-foliated, suggesting the possibility of more than one intrusive event.
Many of the granites and pegmatites of the Split Rock Supersuite were intruded as subhorizontal sheets into the older granites and greenstones of the craton. This is the situation in the Mount Edgar Dome, where the Moolyella Monzogranite includes a subhorizontal sheet of monzogranite underlain and overlain by banded orthogneiss of the Tambina Supersuite (Hickman, 2021). Likewise, field exposures of the Cooglegong Monzogranite in the Shaw Dome reveal this to be a subhorizontal, sheet-like intrusion (Van Kranendonk, 2003). At Wodgina, on the boundary between the Yule and Carlindi Domes, almost horizontal Sn–Ta–Li-bearing pegmatites cut across folded strata of the Soanesville Group (Van Kranendonk et al., 2006). Intrusion of horizontal sheets suggests reduced vertical pressures within the crust, possibly due to rapid erosion of the crust after the Mosquito Creek and North Pilbara Orogenies. | | | | | | Geochronology | | | Split Rock Magmatic Event | Maximum age | Minimum age | Age (Ma) | 2851 | 2831 | Age | Mesoarchean | Mesoarchean | Age data type | Inferred | | References | | |
| The oldest U–Pb zircon date on the supersuite is 2851 ± 2 Ma (GSWA 142879, Nelson, 1998) on the Spear Hill Monzogranite (Shaw Dome, TAMBOURAH). This date has been interpreted to indicate the maximum age of the supersuite, although until more intrusions are analysed and dated the maximum age of the supersuite will remain uncertain.
The youngest U–Pb zircon date on the supersuite is 2831 ± 12 Ma from the Moolyella Monzogranite in the Mount Edgar Dome (GSWA 169044, Nelson, 2004). This currently considered to provide the best estimate of the minimum age of the supersuite, because alternative geochronological constraints are geologically unrealistic, being limited to dates on unrelated stratigraphy in the unconformably overlying 2775–2629 Ma Fortescue Group. | | | Tectonic Setting | The Split Rock Supersuite was intruded into tectonically stable crust after orogenic episodes (North Pilbara and Mosquito Creek Orogenies) that cratonized the Pilbara Craton. The northwesterly trending zone of intrusion suggests either some form of structural control, e.g. a failed rift (Van Kranendonk et al., 2006) or a process, producing a succession of intrusions along this trend. One possible explanation is that intrusion of the supersuite took place as the Pilbara Craton crust drifted southeast to northwest across a hot spot (Hickman, 2016). In this scenario, the northwesterly intrusions are likely to have been emplaced before the dated intrusions in the southeast. | | | | References | Bickle, MJ, Bettenay, LF, Chapman, HJ, Groves, DI, McNaughton, NJ, Campbell, IH and de Laeter, JR 1989, The age and origin of younger granitic plutons of the Shaw batholith in the Archean Pilbara Block, Western Australia: Contributions to Mineralogy and Petrology, v. 101, p. 361–376. | Blockley, JG 1980, The tin deposits of Western Australia, with special reference to the associated granites: Geological Survey of Western Australia, Mineral Resources Bulletin 12, 184p. View Reference | Champion, DC and Smithies, RH 2001a, Archaean granites of the Yilgarn and Pilbara cratons, Western Australia, in Extended Abstracts — 4th International Archaean Symposium, Perth, 24–28 September 2001 edited by Cassidy, KF, Dunphy, JM and Van Kranendonk, MJ: AGSO (Geoscience Australia), Record 2001/37, p. 134–136. | Champion, DC and Smithies, RH 2001b, The geochemistry of the Yule Granitoid Complex, East Pilbara Granite-Greenstone Terrane: Evidence for early felsic crust, in Geological Survey of Western Australia annual review 1999–2000 edited by Day, L and Reddy, DP: Geological Survey of Western Australia, Perth, p. 42–48. View Reference | Davy, R and Lewis, JD 1986, The Mount Edgar Batholith, Pilbara area, Western Australia: Geochemistry and petrography: Geological Survey of Western Australia, Report 17, 73p. View Reference | Gardiner, NJ, Hickman, AH, Kirkland, CL, Lu, Y, Johnson, T and Zhao, J 2017, Processes of crust formation in the early Earth imaged through Hf isotopes from the East Pilbara Terrane: Precambrian Research, v. 297, p. 56–76, doi:10.1016/j.precamres.2017.05.004. | Gardiner, NJ, Hickman, AH, Kirkland, CL, Lu, Y, Johnson, TE and Wingate, MTD 2018, New Hf isotope insights into the Paleoarchean magmatic evolution of the Mount Edgar Dome, Pilbara Craton: Implications for early Earth and crust formation processes: Geological Survey of Western Australia, Report 181, 41p. View Reference | Hickman, AH 1975, Precambrian structural geology of part of the Pilbara Region, in Annual report for the year 1974: Geological Survey of Western Australia, p. 68–73. View Reference | Hickman, AH 1980, Excursion guide — Archaean geology of the Pilbara Block: Second International Archaean Symposium, Perth 1980: Geological Society of Australia, W.A. Division, Perth, Western Australia, 55p. | Hickman, AH 1983, Geology of the Pilbara Block and its environs: Geological Survey of Western Australia, Bulletin 127, 268p. View Reference | Hickman, AH 2016, Northwest Pilbara Craton: A record of 450 million years in the growth of Archean continental crust: Geological Survey of Western Australia, Report 160, 104p. View Reference | Hickman, AH 2021, East Pilbara Craton: a record of one billion years in the growth of Archean continental crust: Geological Survey of Western Australia, Report 143, 187p. View Reference | Kinny, PD 2000, U–Pb dating of rare-metal (Sn–Ta–Li) mineralized pegmatites in Western Australia by SIMS analysis of tin and tantalum-bearing ore minerals, in Abstracts and Proceedings: Beyond 2000, New Frontiers in Isotope Geoscience, Lorne, Victoria, p. 113–116. | Nelson, DR 1998, 142879.1: biotite monzogranite, Cooglegong Creek; Geochronology Record 350: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Nelson, DR 2004, 169044.1: muscovite–biotite monzogranite, Ripon Hills Road – Yandicoogina Creek crossing; Geochronology Record 130: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Smithies, RH, Champion, DC and Cassidy, KF 2003, Formation of Earth's early Archaean continental crust: Precambrian Research, v. 127, p. 89–101. | Sweetapple, MT 2000, Characteristics of Sn–Ta–Be–Li industrial mineral deposits of the Archaean Pilbara Craton, Western Australia: AGSO, Canberra, Record 2000/044, 54p. | Sweetapple, MT and Collins, PLF 2002, Genetic framework for the classification and distribution of Archean rare metal pegmatites in the North Pilbara Craton, Western Australia: Economic Geology, v. 97, p. 873–895. | Van Kranendonk, MJ 2003, Geology of the Tambourah 1:100 000 sheet: Geological Survey of Western Australia, 1:100 000 Geological Series Explanatory Notes, 57p. View Reference | Van Kranendonk, MJ, Hickman, AH, Smithies, RH, Williams, IR, Bagas, L and Farrell, TR 2006, Revised lithostratigraphy of Archean supracrustal and intrusive rocks in the northern Pilbara Craton, Western Australia: Geological Survey of Western Australia, Record 2006/15, 57p. View Reference | Van Kranendonk, MJ, Smithies, RH, Hickman, AH and Champion, DC 2007, Review: Secular tectonic evolution of Archean continental crust: interplay between horizontal and vertical processes in the formation of the Pilbara Craton, Australia: Terra Nova, v. 19, no. 1, p. 1–38. |
| | | | Recommended reference for this publication | Hickman, AH 2024, Split Rock Magmatic Event (PCSME): 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 05 March 2024. | | | | | 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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