Sumatorios y medias aritméticas

The age distributions of detrital zircon populations in metasediments of the Etheridge Group have previously been reported by Black & McCulloch (1990), Black &

Withnall (1993), and Black et al. (2005). The inheritance patterns in samples 184,

255, and 264 presented here provide a useful comparison to these published detrital zircon populations, as well as to the inheritance patterns observed in the metasediments of other Palaeoproterozoic basins in eastern Australia. These broad similarities are discussed in more detail below.

Leucogneiss from the Einasleigh Metamorphics north of Gum Flats analysed by Black

et al. (2005) contained inherited zircons that these authors suggested were most likely

derived from a number of sedimentary protoliths. They defined a maximum age limit of ca. 1700 Ma for the leucogneiss and its equivalents, based on the presence of substantial detrital populations at ca. 1700 Ma in some samples of leucogneiss. Four samples from their study contained inherited zircon populations that have some similarities to the inheritance patterns of the Daniel Creek Formation samples (Fig 4.17). Three leucogneiss samples contained no ages younger than 1700 Ma, and their detrital patterns were thought to reflect the contribution of many different source terranes. The remaining sample contained a significant concentration of younger ages which they related to at least three episodes of crystallisation and recrystallization between 1570 and 1500 Ma. The most obvious similarities between the two lithologies are the presence of a major ca. 2600 Ma component and ca. 1800-2000 Ma component, coupled with the relative lack of ages between 2000 and 2400 Ma in three samples (Fig 4.17). The similarity in the inheritance patterns between the Daniel Creek Formation samples of this study and the Einasleigh Metamorphics leucogneiss

of Black et al. (2005) indicates that both lithologies have shared a similar provenance.

Inherited zircons were also reported in samples of Proterozoic granite from the Georgetown Inlier by Black & McCulloch (1990) and Black & Withnall (1993). As with the metasedimentary samples, the inherited zircons present a similar age distribution. Demonstrating this relationship is a sample of the Lighthouse Granite analysed by Black & Withnall (1993), which contains a number of inherited grains forming a similar inheritance pattern to that of sample 264, a sample of Dead Horse Metabasalt amphibolite (Fig. 4.18). The authors suggested that their inheritance

patterns reflected the input of material from numerous sources into the source magma of the Lighthouse Granite. This hypothesis may also account for the pattern observed in sample 264, indicating the passage of mafic magmas though similar sedimentary rocks.

Figure 4.17: (a) Cumulative probability diagrams for four leucogneiss samples from the Einasleigh Metamorphics (after Black et al., 2005). Also shown for comparison in (b) are detrital

zircon age data for Daniel Creek Formation samples 184 and 255 (this study).

An alternative hypothesis that may account for the detrital populations between 2500 and 2000 Ma is that the inherited grains were sourced from now unexposed late- Archaean/early-Proterozoic basement rocks underlying the Etheridge Group, similar to that proposed for inherited populations in the Mt Isa Eastern Succession (Giles & Nutman, 2003). This would account for the regional extent of the ca. 2500-2000 Ma ages that have been obtained from the U-Pb zircon studies of numerous lithologies

across the region (e.g. Black & McCulloch, 1990; Black & Withnall, 1993; Black et

al., 2005; and this study).

Lighthouse Granite Black & Withnall (1993) n = 27 1400 1800 2200 2600 3000 Age (Ma) Re la ti v e P ro b a b ilit y a Sample 264 Dead Horse Metabasalt n = 31 1400 1800 2200 2600 3000 Age (Ma) Rel at iv e Pr o b a b ilit y b

Figure 4.18: Comparison of cumulative probability diagrams for (a) a sample of the Lighthouse Granite (Black & Withnall, 1993) and (b) sample 264 from the Dead Horse Metabasalt (this

study).

The inheritance patterns of the Etheridge Group metasediments share a number of traits common to rocks of both the Broken Hill Group and Mt Isa Eastern Succession, including: (i) a significant late-Archaean to earliest Palaeoproterozoic component (ca. 3100-2500 Ma); (ii) an absent or relatively insignificant early Palaeoproterozoic component (ca. 2400-2100 Ma); and (iii) a significant mid-Palaeoproterozoic component (ca. 2100-1800 Ma). In the Broken Hill Group of the Willyama Inlier

(Willis et al., 1983) the spectrum of inheritance patterns listed above has been

recognised in numerous samples of metasedimentary and gneissic rocks, including lithologies associated with and hosting the Pb-Zn-Ag orebody at Broken Hill (Fig.

4.19; Page & Laing, 1992; Raetz et al., 2002; Page et al., 2005a). However, the

present in both the Broken Hill Group and Mt Isa Eastern Succession (Fig. 4.19 and 4.20 respectively), may also have important implications for the prospectivity of the Etheridge Group. In particular, that the Dead Horse Metabasalt-Daniel Creek Formation horizon within the Etheridge Group did not share the same provenance as the otherwise lithologically similar terranes further west.

Hores Gneiss n = 191 1400 1800 2200 2600 3000 Age (Ma) Re la ti v e P ro b a b ilit y

Figure 4.19: Compilation of U-Pb SHRIMP detrital zircon analyses for four samples of the Hores Gneiss in the Broken Hill Group (Page & Laing, 1992). The dominant peak at ca. 1680 Ma

represents the inferred crystallisation age of the host rock.

The metasediments of the Broken Hill Group were also found to contain a large

abundance of detrital zircons aged between ca. 1870 and 1780 Ma (Page et al.,

2005a). These zircons were thought to have been sourced from terranes adjacent to the Willyama Inlier that contained rocks formed during the Barramundi Orogeny (ca.

1870-1850 Ma; Scott et al., 2000; Page et al., 2005a), and between ca. 1820 and 1780

Ma (Page et al., 2005a). As with the metasediments of the Etheridge Group, the late-

Archaean component present in the metasediments of the Broken Hill Group has been attributed to incorporation of detrital zircons from unexposed basement material into

the metasediments (Page et al., 2005b).

The analysis of metasedimentary and meta-igneous rocks from the Mt Isa Eastern Succession by Page & Sun (1998) and Giles & Nutman (2003) revealed detrital zircon age populations consistent with the inheritance patterns described above (Fig 4.20). In both cases their analyses showed broad similarities across the length of the terrane, as well as correlations with the detrital populations in metasediments from the rest of the

Mt Isa Inlier (Scott et al., 2000). In particular, Giles & Nutman (2003) described two significant detrital populations occurring at ca. 2600-2300 Ma and ca. 1750-1720 Ma (Fig. 4.20). The older population of detrital ages overlaps with and may relate to the population thought to represent late-Archaean basement inheritance in both the Etheridge Group and Broken Hill Group. The younger detrital population is believed to have originated locally, as metasedimentary and magmatic rocks of ca. 1780-1720 Ma age are widely exposed in the Kalkadoon-Leichhardt Belt further west (Giles & Nutman, 2003). The absence of a detrital population at ca. 2000 Ma is significant, as this peak is present in the metasediments of the Etheridge Group. However, its absence could be accounted for if the source for this population was localised to the Etheridge Group, in a similar way to the localised sourcing of Barramundi-age material into the Willyama Inlier and Mt Isa Eastern Succession.

Sam ple : 9220 8004 Gandry Dam gne is s Soldie rs Cap Group n = 53 1400 1800 2200 2600 3000 Age (Ma) Re la ti v e P ro b a b ili ty c Sam ple : 9220 8031 m e tas e dim e nt Toole Cre e k Volcanics n = 14

1400 1800 2200 2600 3000

Age (Ma)

d

Sam ple : CAD051 M igm atitic gne is s Cannington n = 39 1400 1800 2200 2600 3000 R e la ti v e Pr o b a b il it y a Sam ple : DGC96.20 Lle w e llyn Cre e k Form ation Cannington n = 57

1400 1800 2200 2600 3000

b

Figure 4.20: Cumulative probability diagrams for selected metasedimentary samples from the Mt Isa Eastern Succession. Data sources: (a) and (b) Giles & Nutman (2003), (c) and (d) Page & Sun

(1998).

From the overview of inheritance patterns in the Palaeoproterozoic terranes of northeastern Australia outlined above, it is clear that a small but significant component that occurs throughout is the detrital population of late-Archaean age (between 3000 and 2500 Ma). This component has been inferred to represent the

addition of material from unexposed or eroded Archaean rocks that were basement to the Palaeoproterozoic terranes. A similar inheritance pattern was identified in the Belt

Supergroup of western USA and southern Canada by Blewett et al. (1998), and may

have important implications for reconstructions of the Rodinia supercontinent

involving north Queensland (e.g. Ross et al., 1992; Brookfield, 1993). The presence

of this Archaean component throughout the Proterozoic basins of northeastern Australia, including the Georgetown Inlier, indicates that these terranes may have shared a common Archaean basement, and would therefore all lie within the Proterozoic framework of the North Australian Craton.