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| | | | Geological Survey of Western Australia |
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| | | Glenburgh Orogeny D1g/M1g (CGD1) | SP Johnson, S Sheppard, and SA Occhipinti | | | | | Event type | magmatic: intrusive | Parent event | | Child events | | Tectonic units affected | | Tectonic setting | orogen: continental arc | Metamorphic facies | | amphibolite: undivided | granulite: cordierite - K-feldspar |
| Metamorphic/tectonic features | diatexitic; gneissose; foliated; schistose |
| | | Summary | The oldest tectonic fabric that can be attributed to the Glenburgh Orogeny is referred to as D1g (and the corresponding mineral assemblage, M1g). This predominantly gneissic fabric is recognized only within the Paradise Zone of the Gascoyne Province and is developed in meta-igneous rocks of the Dalgaringa Supersuite, and in strips of pelitic diatexite and mafic granulite that are intercalated with the gneissic granites. The Dalgaringa Supersuite metagranites have geochemical and isotopic compositions that indicate formation in an Andean-type arc. Associated M1g assemblages record metamorphism mainly in the middle amphibolite facies, but pelitic diatexites and mafic granulites in the central part of the Paradise Zone indicate peak conditions, at least locally, in the granulite facies. Metamorphism and deformation were contemporaneous with arc magmatism between 2005–1985 Ma and suggest that D1g was associated with the construction of the arc in the middle crust. | | | Distribution | The oldest tectonic fabric that can be attributed to the Glenburgh Orogeny, D1g (and the corresponding mineral assemblage, M1g), is recognized only within the Paradise Zone of the Gascoyne Province. The predominantly gneissic fabric is developed within 2005–1985 Ma-aged metatonalites, metagranodiorites, and metamonzogranites of the Dalgaringa Supersuite (Occhipinti and Sheppard, 2001; Occhipinti et al., 2004), and in discontinuous strips of mafic granulite and pelitic diatexite that are intercalated with these gneisses. | | | Description | In the Paradise Zone the D1g fabrics are represented predominantly by a gneissic banding and foliation. Folding, including tight to isoclinal, moderately plunging folds, is only locally developed.
Occhipinti and Sheppard (2001) recorded the presence of relict garnet porphyroblasts within many of the Dalgaringa Supersuite metagranites. Intercalated strips of amphibolite suggest that metamorphic conditions in D1g peaked at upper amphibolite facies throughout most of the zone. However, in the central part of the zone around Paradise Well, discontinuous lenses of mafic granulites and pelitic diatexites indicate that some parts of the Dalgaringa Supersuite underwent peak metamorphism at upper amphibolite to granulite facies; these metamafic rocks commonly contain a peak metamorphic assemblage of either hypersthene–clinopyroxene–plagioclase or plagioclase–clinopyroxene–hypersthene(–garnet) (Occhipinti and Sheppard, 2001).
The pelitic diatexites are described by Johnson et al., (2010, 2011) as coarse-grained granoblastic rocks with mm- to cm-scale discontinuous leucosomes parallel to the regional D1g gneissic fabrics. The diatexite leucosomes (GSWA 185942) contain an assemblage of cordierite–sillimanite–hercynitic-spinel–biotite–quartz–corundum. The cordierite forms 1–5 mm-diameter porphyroblasts, in which the core regions are intergrown with abundant fibrolite mats. The outer 0.25 – 1 mm part of the porphyroblasts are free of sillimanite, but are partially mantled by coarse-grained (0.5 – 2 mm diameter) euhedral sillimanite blades. The cores of these sillimanites host hercynitic spinel and corundum. Tabular biotite blades are intergrown within the cordierite–sillimanite coronas and appear to be in textural equilibrium with these phases; however, in the matrix, which comprises quartz–cordierite–biotite, the biotite blades are not in textural equilibrium and are mantled by fringes of fibrolite and cordierite. The mesosomes (GSWA 144823) contain an assemblage of garnet–sillimanite–gahnitic-spinel–plagioclase–biotite–quartz. Garnet is found within 10–20 mm-thick biotite-rich seams and forms 5–10 mm-diameter porphyroblasts that contain inclusions of both biotite and sillimanite. Outside of these seams, quartz and plagioclase are more prevalent. Biotite plates are mantled by fringes of fibrolite and cordierite, and have exsolved small pockets of ilmenite. Discontinuous sillimanite-rich seams up to 10–15 mm wide are composed of euhedral sillimanite blades with minor biotite–quartz–plagioclase. The sillimanite cores are replaced by anhedral gahnitic spinel.
The assemblages documented above, along with mesoscale leucosomes, imply moderate- to high-temperature conditions that were conducive to incipient melting. The replacement of hercynitic spinel with corundum was interpreted by Schulters and Bohlen (1988) to indicate moderate to high pressure and temperature conditions in the range ~7–10 kbar and 800–1000°C. The alignment of the mesoscale melt fabrics with the regional S1g foliation suggest that this melting event was synchronous with D1g, and that migmatization may have been a response to the inclusion and burial of these pelitic strips within the mid-crustal parts of an active continental arc. The structural relationship between these granulite facies rocks and the surrounding amphibolite rocks is unclear, but it is suggested that the two were juxtaposed after the D1g/M1g event (Occhipinti et al., 2004). | | | | | | Geochronology | | | Glenburgh Orogeny D1g/M1g | Maximum age | Minimum age | Age (Ma) | 2002 ± 3 | 1987 ± 4 | Age | Paleoproterozoic | Paleoproterozoic | Age data type | Isotopic | | References | | |
| The age of the D1g event has been precisely dated at 1997 ± 10 Ma, using SHRIMP U–Pb geochronology of rims on zircons from melt veins that developed within a pelitic diatexite (GSWA 185942) during high-grade metamorphism (Wingate et al., 2010). However, the Dalgaringa Supersuite records a near-continual record of arc magmatism and accompanying deformation between 2005 and 1985 Ma (Occhipinti and Sheppard, 2001), suggesting that D1g was a much longer-lived event than the apparent single event recorded in the diatexite. The age of metamorphism recorded in the diatexite represents the time that the metasedimentary rocks underwent melting with production of a near-anhydrous mineral assemblage, which would have precluded the growth of any new metamorphic zircon during the subsequent prolonged M1g event. Therefore, the age of D1g is taken to be the age range of the oldest and youngest deformed rocks of the Dalgaringa Supersuite (i.e. 2005–1985 Ma). | | | Tectonic Setting | The Dalgaringa Supersuite represents a near-continuous record of continental arc magmatism (Sheppard et al., 2004) and accompanying deformation between 2005 Ma and 1985 Ma (Occhipinti and Sheppard, 2001). However, detrital zircons with near-continuous ages from c. 2000 Ma up to c. 2080 Ma, extracted from the Camel Hills Metamorphics, have Hf isotopic compositions similar to magamatic zircon from the 2005–1975 Ma Dalgaringa Supersuite (Johnson et al., 2011). These data suggest that arc magmatism may have been initiated at c. 2080 Ma in a currently unexposed part of the arc (termed the proto-Dalgaringa Arc). Combined with the moderate- to high-pressure nature of M1g metamorphism (Johnson et al., 2010, 2011), this suggests that D1g occurred as a result of construction of the arc in the middle crust. | | | | References | Johnson, SP, Sheppard, S, Rasmussen, B, Wingate, MTD, Kirkland, CL, Muhling, JR, Fletcher, IR and Belousova, E 2010, The Glenburgh Orogeny as a record of Paleoproterozoic continent-continent collision: Geological Survey of Western Australia, Record 2010/5, 54p. View Reference | Johnson, SP, Sheppard, S, Rasmussen, B, Wingate, MTD, Kirkland, CL, Muhling, JR, Fletcher, IR and Belousova, EA 2011, Two collisions, two sutures: punctuated pre-1950 Ma assembly of the West Australian Craton during the Ophthalmian and Glenburgh Orogenies: Precambrian Research, v. 189, no. 3–4, p. 239–262, doi:10.1016/j.precamres.2011.07.011. | Nelson, DR 1999a, 142925.1: biotite monzogranite, Challenger Well; Geochronology Record 312: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Nelson, DR 1999b, 142923.1: foliated biotite monzogranite, Glenburgh Homestead; Geochronology Record 343: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Occhipinti, SA and Sheppard, S 2001, Geology of the Glenburgh 1:100 000 sheet: Geological Survey of Western Australia, 1:100 000 Geological Series Explanatory Notes, 37p. View Reference | Occhipinti, SA, Sheppard, S, Passchier, C, Tyler, IM and Nelson, DR 2004, Palaeoproterozoic crustal accretion and collision in the southern Capricorn Orogen: The Glenburgh Orogeny: Precambrian Research, v. 128, p. 237–255. | Schulters, JC and Bohlen, SR 1988, The stability of hercynite and hercynite-gahnite spinels in corundum- or quartz-bearing assemblages: Journal of Petrology, v. 30, p. 1017–1031. | Sheppard, S, Occhipinti, SA and Tyler, IM 2004, A 2005-1970 Ma Andean-type batholith in the southern Gascoyne Complex, Western Australia: Precambrian Research, v. 128, p. 257–277. | Wingate, MTD, Kirkland, CL, Johnson, SP and Sheppard, S 2010, 185942.1: pelitic diatexite, Paradise Well; Geochronology Record 861: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference |
| | | | Recommended reference for this publication | Johnson, SP, Sheppard, S and Occhipinti, SA 2018, Glenburgh Orogeny D1g/M1g (CGD1): 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 June 2018. | | | | | 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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