Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

More than a dozen mafic–ultramafic layered intrusions were emplaced across >100 000 km2 in the Musgrave region of central Australia at c. 1075 Ma as part of the c. 1090–1040 Ma Giles Event. The intrusions crystallized from tholeiitic magma of variable composition (<7–12.5 wt% MgO) emplaced at mid- to upper crustal levels (at pressures of up to ~6.5 kbar). As a result, individual intrusions show distinctive compositions. Intrusions such as Wingellina Hills, Pirntirri Mulari, The Wart, Ewarara, Kalka, Claude Hills, and Gosse Pile contain significant ultramafic portions of wehrlite, harzburgite, and websterite. Others, including Hinckley Range, Michael Hills, and Murray Range, are of predominantly olivine-gabbronoritic composition. Mafic intrusions containing substantial troctolitic portions include Morgan Range, Cavenagh, Bell Rock, Blackstone, and Jameson–Finlayson. The latter three are tectonically dismembered portions of an originally contiguous body, here named the Mantamaru intrusion that had a strike length of >170 km and a width of at least 20 km, making it one of the world’s largest layered intrusions. The Giles Event intrusions were emplaced into the Musgrave Province, a complex Proterozoic terrane at the intersection between the West Australian, North Australian, and South Australian Cratons. The region underwent several episodes of orogeny and rifting over a time span of >200 Ma. The oldest event that clearly affected the entire province was the 1220–1150 Ma Musgrave Orogeny. It arose either in an intracratonic setting or as a distal back-arc and featured early, rapid, and substantial lithospheric thinning. These events allowed convecting mantle to be channelled upward along the contacts with surrounding craton keels to the newly exposed base of the Musgrave lithosphere. The result was large-degree mantle melting and subsequent ponding of basalt at (and intrusion into) the base of the lithosphere, lower-crustal melting, voluminous granite magmatism, and widespread mid-crustal ultra-high-temperature (UHT) metamorphism. The ductile (UHT) nature of the lower crust, and the development of substantial crystal-rich magma storage chambers — or melt, assimilation, storage, and homogenization (MASH) zones — prevented ascent of basic magmas into the upper crust. This resulted in the predominantly felsic character of magmatism during the Musgrave Orogeny. The c. 100 Ma (1220–1120 Ma) duration of UHT mid-crustal conditions suggests that re-establishment of lithospheric mantle was significantly retarded. Magmatism largely ceased between c. 1150 and 1090 Ma, possibly because the lower crust became too refractory, or because a buoyant lithospheric mantle began to form. Therefore, the MASH zone may have solidified; however, mid-crustal temperatures remained anomalously high, as suggested by the continued growth of migmatite-related zircon for more than another c. 80 Ma. Renewed mantle melting from c. 1090 Ma onwards led to the magmatism-dominated Giles Event (c. 1090 to 1040 Ma), comprising voluminous basic and felsic volcanic and intrusive rocks grouped into the Warakurna Supersuite. One particularly notable component of the Giles Event was the Warakurna Large Igneous Province, represented by doleritic intrusions that outcrop across ~1.5 million km2 of central and western Australia (Wingate et al., 2004). The source to the Giles basic magmas was largely asthenospheric, reflected by their relatively minor crustal component (low large ion lithophile elements [LILE], ƐNd up to +2), and low Pt/Pd ratios. The long-lasting magmatism and UHT metamorphism in the Musgrave Province suggests that magmatism was plate driven rather than plume driven. In many regards, the Giles Event can be viewed as an extension of the anomalous thermal regime established during the Musgrave Orogeny. Although initial extension and rifting, emplacement of the layered G1 Giles intrusions, and then significant uplift all happened between 1078 and 1075 Ma, mantle-derived magmatism lasted for >50 Ma and is unrelated to a deep mantle plume. Periods of deformation (both extension and compression) during both the Musgrave Orogeny and the Giles Event may be related to far-field compressive influences that allowed the formation of thick sill complexes, ultimately resulting in some of the world’s largest layered intrusions. A comparison of current models of ore formation with the geology generated by the Giles Event indicates that the region has potential prospectivity for the following types of mineral occurrences: - platinum group element (PGE) reefs in the ultramafic–mafic transition zones of layered intrusions, and in magnetite layers in the differentiated portions of the intrusions. Potential PGE reefs are more likely in the early (G1) intrusions, whose parental magmas failed to interact with abundant juvenile sulfur of relatively late-stage felsic volcanic rocks - Cu–Ni sulfide deposits within magma feeder conduits of late basaltic pulses that could assimilate sulfur-rich felsic volcanic rocks - vanadium in the lowermost magnetite layers within the most fractionated intrusions - apatite in the unexposed uppermost magnetite layers of the fractionated intrusions - ilmenite as granular disseminated grains in magnetite layers within the upper portions of the intrusions - iron, particularly in tectonically thickened magnetite layers or magnetite pipes of the upper portions of intrusions - gold and copper in the roof rocks and contact aureoles of the large intrusions, and in associated granites and felsic volcanic rocks - lateritic nickel in weathered portions of the olivine-rich ultramafic portions of intrusions.