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Early Proterozoic Magmatism and Geodynamics — Evidence of a Fundamental Change in the Earth’s Evolution
Published in O.A. Bogatikov, R.F. Fursenko, G.V. Lazareva, E.A. Miloradovskaya, A. Ya, R.E. Sorkina, Magmatism and Geodynamics Terrestrial Magmatism Throughout the Earth’s History, 2020
Magmatism here was less intense than in the Baltic Shield and was manifested mainly as intrusions. Dykes of titaniferous picrite, forming thin (0.25 to 25 m) bodies, occur in the Udokan Group and in the Archaean Olekma granite–greenstone terrane (Pukhtel and Zhuravlev, 1992). They are noted for higher TiO2 concentrations (on average 2 wt.%) and their high enrichment in the most incompatible elements (Zr and LREE). According to Pukhtel and Zhuravlev (1992), the nature of the variations in the petrogenetically important components (Al, Y, V and LREE) points to the leading role of garnet fractionation in the formation of the parental magmas of picrite. An isochron Sm–Nd age of 2202 ± 41 Ma gives the time of dyke emplacement. A positive value of εNd(T)= + 1.6, along with strong enrichment in the most incompatible elements, provides evidence of the supply of an enriched component, probably induced by mantle metasomatism, into a long-lived depleted mantle only a short time before partial melting.
Petrogenesis of the Kalka, Ewarara and Gosse Pile layered intrusions, Musgrave Province, South Australia, and implications for magmatic sulfide prospectivity
Published in Australian Journal of Earth Sciences, 2023
W. D. Maier, B. Wade, Sarah-Jane Barnes, R. Dutch
Ewarara (as well as Kalka) contains a number of mafic–ultramafic olivine-rich plug-like bodies, forming part of a group of at least 16 bodies intrusive into both granulites and Giles intrusions, as suggested by lack of deformation and recrystallisation as well as sharp contacts with the country rocks. The plugs are generally up to 100–1000 m across and may contain doleritic chilled margins. Several types have been distinguished. A peridotite group comprises dunites, lherzolites, harzburgites and peridotites with cumulus olivine (Fo87–90) and minor chrome spinel, with interstitial pyroxenes, plagioclase, hornblende and biotite. A picrite group contains 50–65% cumulus, semi-dendritic olivine (Fo82–85) enclosed by poikilitic pyroxene and plagioclase. In addition, there are olivine mesogabbros, olivine leucogabbros and leucogabbros.
Warrumbungle Volcano: facies architecture and evolution of a complex shield volcano
Published in Australian Journal of Earth Sciences, 2021
K. F. Bull, A. L. Troedson, S. Bodorkos, P. L. Blevin, M. C. Bruce, K. Waltenberg
Unlike the mafic-dominated oceanic hot-spot volcanic shields such as the Hawaiian Islands, the alkalic lavas and pyroclastic deposits that make up the Warrumbungle shield range from mafic to felsic. This compositional and eruptive complexity is instead more similar to island hotspot volcanoes such as the Canary Islands, and Samoa, where the early shield phase erupted basalt and picrite through trachyte, and are distinct at least in terms of trace elements from the ‘rejuvenated’ later stage products (Ancochea et al., 1990; Hoernle & Carracedo, 2009; Konter & Jackson, 2012). The coherent units in the WVC are also predominantly texturally trachytic—very few units have glassy, or formerly glassy groundmass material, implying a significant degree of pre-eruptive crystallinity.
Depositional age and correlation of the Oonah Formation: refining the timing of Neoproterozoic basin formation in Tasmania
Published in Australian Journal of Earth Sciences, 2018
J. A. Mulder, R. F. Berry, J. A. Halpin, S. Meffre, J. L. Everard
Neoproterozoic sequences in Tasmania comprise predominantly shallow marine dolomitic and siliciclastic strata and mafic volcano-sedimentary sequences (Figures 1 and 2). The Togari Group in far northwest Tasmania is the most completely preserved Neoproterozoic sequence in Tasmania. The Togari Group comprises a locally developed basal conglomerate and quartz sandstone unit (Forest Conglomerate), up to 125 m thick, which is conformably overlain by dolostone, black shale, chert and diamictite of the Black River Dolomite (Everard et al., 2007). A late Tonian age for the Black River Dolomite is supported by the presence of vase-shaped microfossils (Riedman, Porter, & Calver, 2018; Riedman, Porter, Halverson, Hurtgen, & Junium, 2014; Saito, Tiba, & Matsubara, 1998), which are interpreted to be a global ca 789–729 Ma biostratigraphic marker (Porter & Knoll, 2000; Riedman et al., 2018; Strauss, Rooney, Macdonald, Brandon, & Knoll, 2014). The Black River Dolomite contains a glacial diamictite (Julius River Member) correlated with the Sturtian Glaciation (Calver, 1998; Calver, Black, Everard, & Seymour, 2004), which is overlain by black shale with a 641 ± 5 whole-rock Re–Os age (Kendall, Creaser, Calver, Raub, & Evans, 2009). The Black River Dolomite is conformably overlain by shale, volcaniclastic siltstone, diamictite and dominantly mafic volcanic rocks of the Kanunnah Subgroup (Figure 2). The volcanic rocks within the Kanunnah Subgroup (Spinks Creek Volcanics) and correlatives elsewhere in Tasmania were emplaced at ca 580 Ma (Calver et al., 2004; Meffre et al., 2004) and include voluminous rift-tholeiites and lesser alkali basalt, picrite and felsic volcanic rocks (Brown, 1986; Everard et al., 2007; Holm et al., 2003; Meffre et al., 2004). The Kanunnah Subgroup is conformably overlain by shallow marine dolostone of the Ediacaran Smithton Dolomite (Calver, 1998) and the lower Cambrian Salmon River Siltstone (Everard et al., 2007). Further south near Corinna (Figure 1), a similar sequence has been termed the Ahrberg Group (Spry, 1962; Turner, Brown, McClenaghan, & Soetrisno, 1991). Like the Togari Group, with which it is correlated, the Ahrberg Group rests unconformably on the Rocky Cape Group (Figure 2). In contrast to the relatively thin Forest Conglomerate, however, the basal unit of the Ahrberg Group (Donaldson Formation) consists of several hundred metres of conglomerate, sandstone, micaceous siltstone, pelitic siltstone and minor chert, which are interpreted in part as a proximal marine turbidite fan deposit (Turner, 1993). The Donaldson Formation is conformably overlain by the Savage Dolomite—a correlate of the Black River Dolomite (Figure 2).