Geological Survey of Western Australia 1998–99 Annual Review The nature of c. 2.0 Ga crust along the southern margin of the Gascoyne Complex by S. Sheppard1, S. A. Occhipinti, I. M. Tyler, and D. R. Nelson Remapping of the ROBINSON RANGE* Abstract and GLENBURGH 1:250 000 map sheets in 1997 and 1998, as part of the The southern part of the Gascoyne Complex consists of foliated and Southern Gascoyne Complex Project, gneissic granites of the Dalgaringa Supersuite, as well as pelitic and combined with SHRIMP U–Pb zircon calc-silicate gneisses of the Camel Hills Metamorphics, the protoliths geochronology (Nelson, 1998, 1999), of which were deposited between c. 2025 and c. 1960 Ma. The confirms that rocks of the Yilgarn Dalgaringa Supersuite mainly consists of 2005–1975 Ma foliated and Craton are not present north of the gneissic tonalite, granodiorite, and monzogranite. SHRIMP U–Pb Errabiddy Shear Zone in the mapped dating has not yet found any trace of Archaean rocks of the Yilgarn area (Fig. 1). Instead, the crust along Craton in the southern Gascoyne Complex. The complex may have the southern margin of the Gascoyne formed as a convergent continental margin above a northwesterly Complex mainly, or entirely, formed dipping subduction zone before it was accreted to the Yilgarn between 2005 and 1975 Ma. Craton at c. 1960 Ma during the Glenburgh Orogeny. Furthermore, this crust was deformed and metamorphosed at KEYWORDS: Proterozoic, structural terranes, granite, medium to high grade up to 150 geochronology, Gascoyne Complex, Dalgaringa million years before collision of the Supersuite, Camel Hills Metamorphics, Glenburgh Yilgarn and Pilbara Cratons between Orogeny 1840 and 1800 Ma (Tyler et al., 1998; Occhipinti et al., 1998). The rocks in the southern part of the Gascoyne Complex are divided into three main stratigraphic units. Introduction the Yilgarn Craton. Myers (1990) These are 2005–1975 Ma granitic interpreted the southern part of gneiss and granite of the Dalgaringa Granites and high-grade meta- the Gascoyne Complex as para- Supersuite, Palaeoproterozoic morphic rocks of the Gascoyne autochthonous Yilgarn Craton medium- to high-grade metasedi- Complex are an important part of interleaved with Proterozoic rocks mentary rocks of the Camel Hills the Capricorn Orogen, a major (his Zone B). He suggested that Metamorphics, and 1830–1800 Ma tectonic zone formed during the Errabiddy Shear Zone (Fig. 1) granite and pegmatite dykes and collision of the Archaean Yilgarn marks the boundary between para- plugs. In addition, two episodes of and Pilbara Cratons in the Palaeo- autochthonous Yilgarn Craton to deformation and medium- to high- proterozoic (Fig. 1). However, the north, and unreworked Yilgarn grade metamorphism that occurred tectonic models for the Gascoyne Craton to the south. The granitic between 1985 and 1945 Ma are Complex have been hampered by gneisses in the southern part of the collectively referred to here as the a lack of reliable geochronological Gascoyne Complex were interpreted Glenburgh Orogeny. The 1830– data. The timing of granite as Archaean because they are 1800 Ma granites are related to the intrusion, deformation, and similar in appearance to gneisses Capricorn Orogeny (Occhipinti et al., metamorphism are poorly in the Narryer Terrane of the 1998) and not discussed here. constrained. Yilgarn Craton, and because In the following sections, sample they have Sm–Nd chondritic numbers refer to samples analysed model ages of 2700–2500 Ma Williams (1986) suggested that the by Nelson (1999) unless otherwise (Fletcher et al., 1983). However, southern part of the Gascoyne referenced. The raw data and reconnaissance SHRIMP U–Pb Complex consisted mainly of concordia plots for these samples are reworked Archaean gneisses of dating by Nutman and Kinny (1994) failed to identify reworked Archaean crust in the southern Gascoyne * Capitalized names refer to standard 1 [email protected] Complex. map sheets. 56 Technical papers PILBARA CRATON GLENBURGH LANDOR GASCOYNE C A COMPLEX P R I C O R N O R O G Errabiddy E N ? YARLARWEELOR Shear Zone GNEISS COMPLEX NARRYER TERRANE YILGARN CRATON Meekatharra 300 km ? ? Dalgety 142925 Downs 142926 142927 142930 Gascoyne Junction 142933 Glenburgh 142932 e Zon 142928 142909 142923 142929 142910 142908 YALBRA r ERONG Mullewa ea Sh iddy Erong rrab E 142911 10 km 142912 SS66 25.10.99 Archaean granitic Alluvium, colluvium gneiss, granite Mount James Formation Undivided granitic gneiss Moorarie Supersuite Granite (1830–1780 Ma) Fault Wooramel Granite (c. 1960 Ma) Geological boundary suite Nardoo Granite (c. 1975 Ma) GSWA dating site Dalgaringa Supersuite Foliated and gneissic granite (2005–1985 Ma) Road Camel Hills Petter Calc-silicate Homestead Metamorphics Quartpot Pelite Figure 1. Simplified geology of the southern margin of the Gascoyne Complex on the GLENBURGH 1:250 000 map sheet. The names of the 1:100 000 map sheets are also shown contained in the same publication. granites dated at 2005–1975 Ma. 2005–1985 Ma foliated and gneissic Locations of the dated samples are The supersuite comprises two granite shown on Figure 1. episodes of magmatism, which are separated by a deformation and Foliated and gneissic granites high-grade regional metamorphic outcrop in a wide, easterly to east- 2005–1975 Ma Dalgaringa event. The two episodes consist of northeasterly trending belt on the Supersuite 2005–1985 Ma foliated to gneissic southern part of the GLENBURGH tonalite to monzogranite, and 1:100 000 map sheet (Fig. 1). The The Dalgaringa Supersuite consists c. 1975 Ma tonalite and granodiorite rocks range from strongly deformed of massive, foliated, and gneissic plutons. and completely recrystallized, 57 Geological Survey of Western Australia 1998–99 Annual Review foliated and gneissic granite in zones fine- to medium-grained, foliated several metres thick, and locally of high strain, to statically recrystall- biotite monzogranite and lesser contain up to 30% or more of the ized granites with intrusive relation- leucocratic tonalite to form a foliated and gneissic granites. Two ships preserved in areas of low composite gneissic granite unit. In of the sheets were dated. One of strain. areas of low strain, preserved these, a foliated monzogranite igneous relationships show that the (GSWA 142923), was re-sampled The most abundant, and oldest, rock monzogranite intruded the tonalite. from site NP 19 of A. P. Nutman type so far identified is a grey, fine- However, locally preserved net-vein (1996, pers. comm.) and gave a date to medium-grained, foliated to textures imply that there is little age of 1987 ± 4 Ma, within error of the gneissic tonalite. The rock commonly difference between the two rock c. 1990 Ma age reported by Nutman has a banded appearance owing to types. A sample of the biotite and Kinny (1994). Sample GSWA the presence of heterogeneously monzogranite (GSWA 142927) gave 142930 from a foliated biotite deformed pegmatite veins and dykes a date for igneous crystallization of pegmatite was dated at 1994 ± 2 Ma. (Fig. 2a). In low-strain zones the rock 1999 ± 5 Ma. This age is is a medium-grained, even-textured indistinguishable from that of biotite–quartz diorite or tonalite. tonalite sample GSWA 142926, Nardoo Granite Although this rock type resembles which has been net-veined by the Archaean mesocratic granitic gneiss monzogranite. The tonalite has also The 2005–1985 Ma foliated and in the Narryer Terrane (Occhipinti et been extensively intruded by sheets gneissic granites were intruded by a al., 1998; Sheppard and Swager, of medium- to coarse-grained biotite large pluton of biotite(–hornblende) 1999), two samples of tonalite and granodiorite and monzogranite, one tonalite and granodiorite. This quartz diorite (GSWA 142926 and of which (GSWA 142925) was dated intrusion, known as the Nardoo 142933) on GLENBURGH gave SHRIMP at 2002 ± 3 Ma. Granite, is equivalent to the gneissic U–Pb zircon dates for igneous biotite–hornblende granodiorite of crystallization of 2002 ± 2 Ma and All of the above rock types are the Dalgety Gneiss Dome of 1989 ± 3 Ma respectively. intruded by veins and sheets of Williams (1986). The Nardoo Granite medium- to coarse-grained biotite consists of weakly to strongly The tonalite is commonly inter- monzogranite or syenogranite, and foliated or locally gneissic tonalite layered tectonically with subordinate pegmatite. The sheets are up to and granodiorite (Fig. 2b). The a) b) SS70 16.8.99 SS71 16.8.99 Figure 2. a) Pegmatite-banded tonalite gneiss in creek pavement about 500 m east of sample site GSWA 142933; b) Foliated granodiorite of the Nardoo Granite with an inclusion of low-strain tonalite gneiss, about 6 km east-southeast of sample site GSWA 142932 58 Technical papers margins of the intrusion are commonly more strongly deformed than the core, and the contact with the surrounding gneissic tonalite to monzogranite is not sharp. The Palaeoproterozoic contact is marked by inclusions and 100 high-K granite rafts of the gneissic tonalite to monzogranite in the Nardoo Granite, and decreasing numbers of veins and sheets of Nardoo Granite 144819 intruded into the country rock. 144836 The Nardoo Granite consists of two imordial mantle mappable rock types: a medium- 10 grained, porphyritic biotite– k / pr hornblende tonalite, and a medium- grained, even-textured or weakly Roc porphyritic leucocratic biotite ‘Andean-type’granites granodiorite. Contacts between the two are generally sharp, but in places the two rock types grade into each other. In low-strain zones, irregular veins and dykes of Pb Nb Ce granodiorite have consistently Ba U Nd Zr Y intruded the tonalite. Nevertheless, Rb Th K La Sr P Ti Na the tonalite and granodiorite are SS67 18.10.99 the same age; a sample of the tonalite (GSWA 142932) gave a date of 1977 ± 4 Ma, while a sample of the Figure 3 Mantle-normalized plot comparing the Nardoo Granite (GSWA 144819 and 144836) with Andean-type granites from the Cretaceous Coastal Batholith of Peru (Atherton and granodiorite (GSWA 142928) was Sanderson, 1985) and the Eocene Coast Batholith of Alaska (Arth et al., 1988).