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| | | | Geological Survey of Western Australia |
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| | | Glenburgh Orogeny D2g/M2g (CGD2) | SP Johnson, S Sheppard, and SA Occhipinti | | | | | Event type | deformation: compressional | Parent event | | Child events | | Tectonic units affected | | Tectonic setting | orogen: collisional orogen | Metamorphic facies | | amphibolite: undivided | amphibolite: sillimanite - K-feldspar |
| Metamorphic/tectonic features | diatexitic; gneissose; foliated; schistose |
| | | Summary | Deformation and metamorphism associated with a second event of the Glenburgh Orogeny is referred to as D2g (and associated metamorphic assemblages as M2g), and it was pervasively imparted on all rocks from the Errabiddy Shear Zone through to the Mooloo Zone in the southern part of the Gascoyne Province. The earliest structure preserved in pelitic to semi-pelitic lithologies of the Moogie Metamorphics and Camels Hills Metamorphics is a subhorizontal, mm- to cm-scale gneissic banding that reflects discontinuous leucocratic melt segregations formed during migmatization. In the more psammitic and quartzitic lithologies, this D2g fabric is represented as an intense, bedding-parallel composite foliation. In the quartzofeldspathic meta-igneous lithologies, especially those of the Halfway Gneiss, D2g is heterogeneously developed as mm- to cm-scale gneissic banding defined by alternating leucocratic and mesocratic layers with abundant, discontinuous 1–25 cm-thick pegmatites. The banding is commonly complexly folded, showing both type 2 and type 3 interference folds, most of which show no consistency in orientation. Nearly all the fabrics and folds are, or were, subhorizontal, indicating the predominance of large horizontal shortening components during D2g deformation and metamorphism. Although poorly constrained due to the intense alteration of peak metamorphic mineral assemblages during subsequent metamorphic events, M2g metamorphism is interpreted to have peaked between 5–9 kbar and >650°C. The M2g metamorphism has been precisely dated by SHRIMP U–Pb geochronology of metamorphic zircon and monazite within the medium- to high-grade pelitic and semi-pelitic lithologies of both the Camel Hills Metamorphics and Moogie Metamorphics at between c. 1965 and c. 1950 Ma. | | | Distribution | Deformation and metamorphism associated with the second event of the Glenburgh Orogeny is referred to as D2g (and associated metamorphic assemblages as M2g). The D2g event is characterized by gneissic fabrics, folds, and metamorphic assemblages that are prevalent throughout the Paradise and Mooloo Zones of the Glenburgh Terrane, and the Errabiddy Shear Zone. | | | Description | In the Paradise Zone, D2g is characterized mainly by mesoscopic, tight to isoclinal, upright folds that are accompanied by a penetrative axial planar foliation that is especially well developed in the c. 1975 Ma Nardoo Granite (Occhipinti and Sheppard, 2001). Rare F2g fold hinges in the older parts (i.e. 2005–1985 Ma) of the Dalgaringa Supersuite and in the pelitic diatexite (Johnson et al., 2010, 2011) indicate that the D2g fabric is developed subparallel to both the gneissic banding and D1g foliation.
In the Mooloo Zone, heterogeneous D2g fabrics and folds are developed pervasively throughout the Moogie Metamorphics and Halfway Gneiss. The earliest structure preserved in the Moogie Metamorphics is subhorizontal mm- to cm-scale gneissic banding that reflects discontinuous leucosomes formed during migmatization. In the unmelted psammitic lithologies, this D2g fabric is represented by an intense bedding-parallel composite foliation. Although the medium- to high-grade M2g metamorphic diatexite assemblages were completely retrogressed to lower grade assemblages during the Capricorn Orogeny, many localities still show particularly well-preserved diatexite textures (Johnson et al., 2010, 2011). At one locality in particular, on the southwestern slopes of Mount Dalgety on DAURIE CREEK (SPJDAU000005), the retrogressed pelitic diatexite (sample GSWA 184161) contains euhedral 5–10 cm porphyroblasts of garnet (pseudomorphed by chloritoid), which sit within 1–10 cm thick biotite-rich restitic layers, that are wrapped by discontinuous leucosomes of quartz–feldspar–sillimanite (now sericite). The relict garnet porphyroblasts contain coarse quartz and biotite inclusion trails that are continuous with the external D2g fabric, but display minor dextral shear rotation; presumably the rotation was imparted during locally prolonged D2g deformation. Within the more psammitic parts of the Moogie Metamorphics, the local preservation of bedding (graded bedding and compositional layering) alongside diatexite textures in the pelites, demonstrate that melting, migmatization, and porphyroblast growth during D2g was a function of composition, rather than variations in metamorphic grade.
The Halfway Gneiss at most localities contains a mm- to cm-scale gneissic banding defined by alternating leucocratic (quartz–feldspar) and mesocratic (biotite–quartz–feldspar(–hornblende)) layers with abundant discontinuous 1–25 cm-thick pegmatites. On northern DAURIE CREEK, the gneissic fabric in the Halfway Gneiss contains abundant leucocratic material dated at c. 2006 Ma (Occhipinti et al., 2001), providing a maximum age for its development. On the northern part of GLENBURGH and DAURIE CREEK, the Halfway Gneiss is intercalated with pelitic diatexites and psammitic schists of the Moogie Metamorphics, but itself contains only a simple, openly folded gneissic fabric. This suggests that the Halfway Gneiss did not see both of the Glenburgh events and that the gneissic fabric must be contemporaneous with the medium- to high-grade D2g fabrics within the Moogie schists. Farther north on southern YINNETHARRA, these gneissic fabrics are commonly complexly folded showing both type 2 and type 3 interference folds (Ramsey and Huber, 1987), most of which show no consistency in orientation, or the development of any new fabrics. The age of this refolding event(s) is unclear but may either be related to locally prolonged D2g deformation, or to subsequent Paleoproterozoic to Neoproterozoic reworking events (Johnson et al., 2010). The continuity of the medium- to high-grade S2g fabrics in the Moogie Metamorphics with those in the Halfway Gneiss suggest that M2g in the Halfway Gneiss must have occurred at similar metamorphic grade to that in the Moogie Metamorphics.
Metamorphism (M2g) appears to have accompanied D2g deformation throughout the southern part of the Gascoyne Province (Occhipinti and Sheppard, 2001), but was not of uniform grade. In the Paradise Zone, the grade of M2g metamorphism is difficult to establish as there appears to be no significant retrogression or overprinting of the higher grade granulite facies M1g assemblages, most likely due to the anhydrous nature of these assemblages (Occhipinti et al., 2004). In the amphibolite grade gneisses, garnet appears to be replaced with clots of fine-grained biotite; in the calc-silicate rocks, pargasite and diopside are rimmed or replaced along fractures with tremolite (Occhipinti et al., 2004), whereas the gneissic fabric in the Nardoo Granite is defined by biotite–quartz–oligoclase to andesine–epidote (Occhipinti and Sheppard, 2001). Collectively, these features suggest that M2g metamorphism in the Paradise Zone peaked at lower amphibolite facies.
All lithologies in the Errabiddy Shear Zone are in faulted contact with each other and contain a single, pervasive fabric associated with D2g deformation and metamorphism. The Archean granitic rocks, such as the Warrigal Gneiss (Sheppard and Occhipinti, 2000), contain a gneissic fabric defined by alternating mesocratic and leucocratic layers, whereas rocks of the Camel Hills Metamorphics contain either a strong migmatitic or gneissic fabric depending on their lithological composition and whether or not they were melted. This uniformly consistent S2g fabric was originally flat lying and is parallel to lithological contacts, suggesting that it developed during tectonic imbrication (Occhipinti et al., 2004). In the pelitic to semi-pelitic rocks of the Quartpot Pelite, intensive migmatization produced alternating mm- to cm-scale mesocratic and leucocratic layers consisting of sillimanite–biotite and quartz–plagioclase–K-feldspar, respectively. The growth of abundant garnet over this composite S2g foliation–layering suggests that garnet grew synchronously with, to slightly post, D2g deformation (Occhipinti et al., 2004).
In the Errabiddy Shear Zone and Mooloo Zone, M2g was responsible for the melting of pelitic and semi-pelitic lithologies of the Camel Hills Metamorphics and Moogie Metamorphics, indicating that M2g was of significantly higher grade than that in the Paradise Zone. However, due to wholesale retrogression of the M2g metamorphic assemblages during the subsequent 1820–1770 Ma Capricorn Orogeny, it is difficult to precisely determine the peak conditions attained during D2g. Irrespective of this, textural pseudomorphs and the fragmentary preservation of peak metamorphic porphyroblasts within pelitic and semipelitic diatexite lithologies suggest that both garnet and sillimanite porphyroblasts were abundant phases in these rocks as part of an equilibrium assemblage (Occhipinti and Sheppard, 2001; Johnson et al., 2010, 2011). In typical pelitic compositions, garnet and sillimanite are stable over a wide range of pressures (5–9 kbar) and temperatures, although the presence of in situ melts within these lithologies suggests that temperatures exceeded the wet solidus (i.e. during partial melting at >650°C; e.g. Rigby, 2009). The lack of kyanite anywhere within the Gascoyne Province suggests that M2g metamorphism may have proceeded at moderately steep geothermal gradients. In the southwestern part of the Errabiddy Shear Zone, the pelitic and semipelitic lithologies of the Quartpot Pelite lack any features indicative of melting and migmatization. They contain an assemblage of quartz–muscovite–biotite with minor garnet and plagioclase, consistent with metamorphism only to mid-amphibolite facies (Sheppard and Occhipinti, 2000), suggesting that there was a decrease in the metamorphic grade of M2g from northeast to southwest across the Errabiddy Shear Zone (Occhipiniti et al., 2004). | | | | | | Geochronology | | | Glenburgh Orogeny D2g/M2g | Maximum age | Minimum age | Age (Ma) | 1966 ± 7 | 1947 ± 11 | Age | Paleoproterozoic | Paleoproterozoic | Age data type | Isotopic | | References | | |
| During D2g, the growth of new zircon (mainly as rims around older inherited zircon grains) and monazite was ubiquitous during high-grade metamorphism and migmatization of pelitic and semi-pelitic lithologies of the Camel Hills Metamorphics and Moogie Metamorphics. This period of metamorphic zircon and monazite growth occurred throughout the southern Gascoyne Province, from the Errabiddy Shear Zone to the Mooloo Zone. Nine samples yielded precise U–Pb ages constraining D2g to between c. 1965 and c. 1950 Ma (Johnson et al., 2010, 2011).
In detail, precise ages from metamorphic zircon were obtained from two samples of Quartpot Pelite (GSWA 142905 and 142910) at 1952 ± 14 Ma and 1959 ± 6 Ma, one of Petter Calc-silicate (GSWA 142908) at 1944 ± 5 Ma, and three samples of the Mumba Psammite from the Moogie Metamorphics (GSWA 184161, 164369, and NP20; Kinny et al., 2004) at 1952 ± 4 Ma, 1959 ± 6 Ma, and 1934 ± 43 Ma. Three precise ages were obtained from metamorphic or recrystallized monazite from three samples of the Mumba Psammite (GSWA 164369, 164713, and 164333) at 1958 ± 6 Ma, 1966 ± 7 Ma, and 1947 ± 11 Ma (importantly, the age of metamorphic monazite and zircon within sample GSWA 164369 are statistically identical). From these data, two potential age outliers are present: the date of Kinny et al. (2004) and that from the Petter Calc-silicate. The result of Kinny et al. (2004) of 1934 ± 43 Ma was acquired from three analyses of three metamorphic rims; the resulting age is imprecise but within uncertainty of those obtained by the GSWA of 1965–1950 Ma. The metamorphic age obtained from the Petter Calc-silicate (GSWA 142908) of 1944 ± 5 (1σ) Ma was acquired from a single analysis of a single metamorphic zircon rim, and the resulting age is slightly younger than that obtained from the other GSWA samples (but still within 2 sigma uncertainty). Without additional analyses from this sample, it is also not possible to determine if this single analysis is an outlier in a slightly older age component. Therefore, the maximum and minimum ages for D2g metamorphism and deformation are taken to be the ages obtained from GSWA samples 164713 (1966 ± 7 Ma) and 164333 (1947 ± 11 Ma), for which there are sufficient analyses to define the age components. | | | Tectonic Setting | Metamorphism and deformation associated with the D2g event is present throughout the entire Glenburgh Terrane and is the first event common to all zones including those exotic to the Glenburgh Terrane (i.e. the Errabiddy Shear zone), suggesting that juxtaposition and interleaving of units took place at this time. Throughout the region, M2g peaked in the upper amphibolite facies with the widespread and extensive production of anatectic melts in pelitic to semi-pelitic rocks of the Camel Hills Metamorphics and Moogie Metamorphics. Although no relict ophiolitic slices appear to have been preserved, the interleaving of lithologies with different provenance along the Errabiddy Shear Zone, the pervasive nature of subhorizontal D2g deformation (i.e. thrusting), and moderate-pressure, moderate- to high-temperature metamorphism all suggest that D2g deformation and accompanying metamorphism were the result of collision between the Pilbara Craton – Glenburgh Terrane and the Yilgarn Craton along the Errabiddy Shear Zone. Collision was accompanied by intrusion of syntectonic granites of the Bertibubba Supersuite, which is also the first common magmatic component across the region, having intruded from the Yilgarn Craton through to the Paradise Zone in the Glenburgh Terrane. | | | | 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. | Kinny, PD, Nutman, AP and Occhipinti, SA 2004, Reconnaissance dating of events recorded in the southern part of the Capricorn Orogen: Precambrian Research, v. 128, p. 279–294. | 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, Myers, JS, Tyler, IM and Nelson, DR 2001, Archaean and Palaeoproterozoic geology of the Narryer Terrane (Yilgarn Craton) and the southern Gascoyne Complex (Capricorn Orogen), Western Australia - a field guide: Geological Survey of Western Australia, Record 2001/8, 70p. 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. | Ramsay, JG and Huber, MI 1987, The techniques of modern structural geology, Volume 2: Folds and fractures (1): Academic Press, London, UK, 392p. | Rigby, MJ 2009, Conflicting P-T paths within the Central Zone of the Limpopo Belt: A consequence of different thermobarometric methods?: Journal of African Earth Sciences, v. 54, p. 111–126. | Sheppard, S and Occhipinti, SA 2000, Geology of the Errabiddy and Landor 1:100 000 sheets: Geological Survey of Western Australia, 1:100 000 Geological Series Explanatory Notes, 37p. View Reference |
| | | | Recommended reference for this publication | Johnson, SP, Sheppard, S and Occhipinti, SA 2018, Glenburgh Orogeny D2g/M2g (CGD2): 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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