NEO Geochemistry

Geochemical analyses of rocks in the New England orogen have been important in revealing the temporal evolution of magmatic sources and thus potential plate interactions through time. In particular, isotopic dating and whole-rock chemistries of both plutonic and volcanic rocks of a variety of ages have provided a basis for several models to explain a transition from convergence to extension, and then a return to increased convergence, from Devonian through early Triassic time. This trend is apparent in granite geochemistry, which suggests a transition from sedimentary to igneous (or intracontinental) sources through the history of the orogen. Younger volcanic rocks show greater variety than older volcanics. These granite and volcanic rock trends imply a more complicated source region for these igneous rocks through time that may involve an increased contribution from the mantle in younger rocks.

 

Granite geochemistry

Granites outcrop sporadically throughout the spatial extent of the New England Orogen, though they are most widely exposed in the southern portion of the New England orogen (south of the Clarence-Moreton basin) (Figure 3.1). The granites are typically lumped into two age groups, one late Carboniferous to early Permian and the second late Permian to early Triassic. The older granites are typically S-type, having been derived through melting of a sedimentary source, and are fairly consistent in composition. More specifically, S-type granites are distinct in their lack of hornblende, high K/Na ratio, and high Al and low Ca contents (Caprarelli and Leitch, 1998). Strontium (Sr) and Neodymium (Nd) characteristics of sedimentary rocks of an appropriate age (middle to late Paleozoic) provide support for the interpretation that these granites were formed through melting of sediments (Hensel et al., 1985). Well-documented S-type granites include the Hillgrove and Bundarra suites (part of the New England batholith) in the southern portion of the New England orogen and the Urannah suite (part of the Connors arc), Claddagh and Gallangowan granodiorites (part of the north D'Aguilar block) in the north. The younger granites are I-type; they formed through melting of an igneous or intracrustal source, and they show considerable variability in composition. I-type granites exposed in the southern New England orogen include the Nundle, Moonbi, Uralla, and Clarence River suites.

 

Figure 3.1. Maps showing the distribution of igneous rocks in the southern (left) and northern (right) New England Orogen. Click on either image to enlarge (from Geology of the Mineral Deposits of Australia and Papua New Guinea)

 

Neodymium (Nd) isotope studies provide support that the change in granite geochemistry from S-type to I-type in the late Paleozoic reflects an addition of more mantle-derived magmatic source material through time. In particular, epsilon Nd values are fairly large, positive numbers for the younger I-type granites but tend to be small negative numbers for the S-type granites (Figure 3.2). However, the epsilon Nd values for the I-type granites show considerable variability, with values from some suites significantly larger than others. For example, the Nundle suite granites have some of the highest epsilon Nd values and are characterized by the presence of more gabbro and granodiorite than other I-type granite suites, whereas rocks of the Moonbi suite yield epsilon Nd values closer to those of the S-type granites. This variation provides support for a complex magmatic source region during the latest Carboniferous and early Permian.

     

Figure 3.2. These plots show strontium (Sr) and neodymium (Nd) isotope variations for rocks of differing ages and from different localities. The diagram on the left is a plot of epsilon Nd (time corrected to 250 Ma) for the Urannah suite, an S-type granite in the northern New England Orogen, and dikes associated with this suite. The diagram on the right shows representative ranges of epsilon Nd (time corrected to 280 Ma) for southern NEO volcanic rocks of Permian and Triassic age, as well as slightly older granitic plutons (blue field). These plots imply that magma sources have varied through time. In particular, the plot on the left shows how epsilon Nd values have become more positive for mafic dikes, which are younger than felsic dikes or Urannah granites. In addition, the plot on the right shows a trend of considerably high epsilon Nd values that decreases again by latest Permian. (data from Allen, 2000; Hensel et al., 1985; Caprarelli and Leitch, 1998).

A study conducted by Allen (2000) on a series of granites known as the Urannah suite, located in the northern portion of the New England Orogen in central Queensland, provides significant information on changes in the nature of NEO magmatism through time. This particular suite of granites has been cut by two groups of subvertical dikes, one generally mafic and the other generally felsic. The mafic dikes always cross-cut the felsic dikes and are thus considered to be younger. A plot of their geochemical compositions on a TiO2 vs. FeO*/MgO diagram shows that the felsic dikes and Urannah suite granites fall largely within the "arc-front" field, whereas the mafic dikes fall within the "back arc" field (Figure 3.3). Most of the basaltic dikes have a calc-alkaline, or intra-plate geochemical signature and have epsilon Nd values of +4.19 to +7.07. In contrast, felsic dikes have epsilon Nd values of around -1.11 to -2.19. Allen (2000) considers the chemistries of the dikes to reflect a change in magmatic source between their respective times of emplacement that is probably related to the transition from convergence to extension in the NEO region during the early Permian, around ~280-300 Ma (see tectonic models).

Figure 3.3. This plot shows titanium (Ti), iron (Fe), and magnesium (Mg) oxide compositions of Urannah suite granites and younger felsic (black squares) and mafic (black diamonds) dikes in the same area. Mafic dikes always crosscut felsic dikes, such that the sharp division between areas where mafic and felsic dikes plot seems to imply a dramatic shift in magma source between times of their emplacement. This particular diagram is based on Andean magma chemistry and separates the Fe0*/Mg0 vs. Ti02 diagram into forearc and backarc fields. The felsic dikes plot in the forearc, or "arc front" field, whereas the mafic dikes plot in the "back arc" field. This change apparently reflects a shift to extensional tectonism in the New England Orogen at some time between felsic and mafic dike emplacement (from Allen, 2000)

Another geochemical study of granites was conducted by Bryant et al. (1997) on the Clarence River Supersuite, located in the southern portion of the New England orogen. Both S-type and I-type granites outcrop in this suite with the age relationship mentioned above, though there exists one granite body of I-type with anomalously old ages. This body, known as the Kaloe granodiorite, has been dated at around 293 Ma by Ar-Ar analysis of hornblende, in great contrast to the ~250-260 Ma ages of most I-type granites. In addition, these granites are located in the vicinity of a large-scale oroclinal fold and show a set of fractures associated with a phase of deformation that is not evident in other I-type granites. This evidence implies the possibility that the Kaloe granodiorite is somehow related to megafolding, though the authors of this study prefer early stage, presumably localized, intra-arc rifting as an explanation for the presence of the Kaloe body.

 

Volcanic rock geochemistry

Volcanic rocks are present in the New England orogen with ages that span virtually the entire late Paleozoic to early Triassic age range (Figure 1.2). The chemistry of these rocks shows a transition from calc-alkaline during the Devonian to late Carboniferous to a more variable chemistry from late Carboniferous to early Permian time, ranging from tholeiitic to calc-alkaline (Figure 3.4). From late Permian through Triassic time, volcanic rocks are more consistently calc-alkaline.

Figure 3.4. Plot showing N-MORB normalized compositions of Early Permian volcanics from the southern New England Orogen. Samples plotted are intended to be representative of the wide variation of chemical compositions of volcanics of this age, most likely due to a complex tectonic setting during this time. (from Caprarelli and Leitch, 1998)

Arc magmatism characterizes volcanic rocks from at least Silurian through late Carboniferous. The earliest of these arc rocks have an island arc chemistry and are believed to have been erupted above and formed within oceanic crust east of the eastern margin of Australia (Gondwana). In the mid-Devonian, this arc apparently collided with the Australian continent, and subsequent subduction beneath Australia resulted in the formation of a continental margin arc. This arc magmatism continued through late Carboniferous and is inferred from the presence of calc-alkaline lava flows, tuffs, and volcaniclastic sediments. A hiatus in arc magmatism, lasting about 60 Ma and of uncertain cause, occurred between late Carboniferous and late Permian, and this period is associated with emplacement of S-type granites and coeval volcanics of highly variable composition, from basalts to rhyolites and with geochemical signatures ranging from MORB to calc-alkaline to alkaline. Arc magmatism resumed in the late Permian, continued through the early Triassic, and is characterized by emplacement of I-type granites and some associated, largely silicic volcanics. This later period of arc magmatism appears to have a more easterly location than the previous Devonian through late Carboniferous arc. The mid-Triassic was characterized by yet another episode of extension with a cease in arc volcanism, gradually transitioning to a passive margin in the Jurassic.

 

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