CUADRO 4 FICHA METODOLÓGICA

The upper Irindina Gneiss was sampled ~4 km ESE of Lizzie Dam (Fig. 4.9C; GR 506827 7424908), where it consists of a quartz, biotite, plagioclase, garnet and sillimanite schist with a strong layer-parallel foliation. A slightly more quartz-rich and coarser-grained layer ~30 cm thick was sampled from within the pelite in order to obtain zircon of a slightly larger grain size for analysis.

Zircon from the semipelite is relatively large, typically around 150-200 µm in size, but ranging up to 500 µm long. Many grains are mantled by thin overgrowths, typically ~10 µm thick, which have a moderate to low intensity in CL images (Fig. 4.10). Seventy- four analyses of cores and six analyses of overgrowths are listed in Table 4.4 and plotted in Fig. 4.11. A large proportion of grains have ages between ~1.0 and ~1.3 Ga, with peaks at ~1.02, 1.11 and 1.23 Ga. There are also a range of younger ages, with a secondary peak at ~0.86 Ga and a spread between ~0.5 and ~0.75 Ga. The Grenvillean and higher ages are consistent with sources in the Musgrave and Arunta Inliers, however zircon with ages of ~0.86 Ga and 0.5-0.75 Ga has no obvious central Australian source. Ages between 0.5-0.7 Ga are common in Cambro-Ordovician sediments in southeastern Australia (Williams & Chappell, 1998; Williams et al., 2002), indicating a connection between the central and southeast Australian depositional systems (Chapter 3). The two youngest grains (8 and 31) were analysed multiple times

Chapter 4 HRG geochronology

to obtain better precision, yielding ages of 517 ± 13 and 504 ± 21 Ma respectively (2,

including the propagated error on the standard analyses). The schist has a similar detrital age spectrum to that of Irindina Gneiss near Mallee Bore (Buick et al., 2001a), which has a range of Grenvillean and Arunta ages, a group between ~0.5 and ~0.75 Ga and a youngest grain with an apparent age of ~511 Ma. The Irindina Gneiss was thus probably deposited in the Early to Middle Cambrian, at a similar time to carbonate-dominated deposition in the Amadeus and Georgina basins.

Most of the overgrowths were too thin to be analysed, however six analyses were able to be made, yielding a range of ages. One relatively thick, rounded overgrowth with high U (>~1500 ppm) and Th/U of 0.12 was analysed twice, yielding an average age of ~630 Ma (analyses 27.1, 27.2). An irregular overgrowth with a U content of 735 ppm and Th/U of 0.03 yielded an age of ~1050 Ma (analysis 41.2). Three euhedral overgrowths with moderate U content (280-386 ppm) and low Th/U (0.01-0.02) have equal 206Pb/238U within error, yielding a pooled age of 478 ± 15 Ma. These younger overgrowths have a similar appearance in CL imagery to other thin overgrowths in the sample and are interpreted to date metamorphism of the Larapinta Event. The older,

Figure 4.10. CL image of zircon from semipelite of the upper Irindina Gneiss, showing grains with some of the thicker overgrowths.

Chapter 4 HRG geochronology

more rounded overgrowths are interpreted to be remnant abraded overgrowths on detrital grains.

4.3.3 Harts Range Meta-Igneous Complex (sample 2001080226)

The Harts Range Meta-Igneous Complex (HRMIC) consists of numerous layer-parallel metabasite and minor anorthosite units within the Irindina Gneiss. Some of these have thicknesses in excess of 1 000 m and can be traced for over 100 km in the Harts Range area. The presence of detrital zircon in amphibolites from the HRMIC and the common intimate interlayering of metasediments and metabasite implies that at least some of the units are volcanic or volcaniclastic, rather than sills.

Two units from the Harts Range Meta-Igneous Complex were sampled with the aim of finding magmatic zircon that would precisely date the age of emplacement: 1) a thick, relatively homogeneous amphibolite unit near Rockhole Dam in the southeastern Harts Range and 2) an anorthosite layer from the ruby mine area west of the Entia Dome. However, no zircon could be found in either of these units, and so a relatively quartz- rich layer was sampled from within metabasite in the Mount Ruby area of the southern

Figure 4.11. Tera-Wasserburg concordia plot and probability density histogram of zircon U-Pb data from semipelite of the upper Irindina Gneiss ESE of Lizzie Bore (sample 2001080003).

Chapter 4 HRG geochronology

Harts Range in the hope that it contained a significant clastic sedimentary component with detrital zircon.

Quartz-rich amphibolite was sampled from near the southern margin of the large metabasite body (GR 532766 7435899), consisting of a medium to coarse grained quartz-hornblende-plagioclase gneiss with a weak compositional layering. Zircon from the amphibolite ranges up to ~200 µm in size and in CL images is seen to have a range of brightness, zoning patterns and shape, typical of a detrital population (Fig. 4.12). These grains are typically modified by weakly zoned, moderately bright overgrowths.

Sixty analyses of cores and 14 of zircon overgrowths are listed in Table 4.5 and plotted in Fig. 4.13. There is a relatively large proportion of Grenville ages, with peaks at ~1.15 and ~1.02 Ga, and a smaller group of zircons with ages between 1.51-1.78 Ga. Secondary peaks occur at ~0.85 Ga, ~1.59 and ~1.8 Ga, with a scattering of ages from ~0.7 Ga to ~0.56 Ga. The relative proportions of age clusters suggest that most sediment was derived from the Musgrave Inlier, with a minor input from the Arunta Inlier. There are no obvious central Australian sources for the ~865 Ma peak, which is similar to that Figure 4.12. CL image of zircon from quartz-rich unit in metabasite near Mount Ruby, showing cores with a range of luminescence and zoning patterns mantled by moderate intensity overgrowths.

Chapter 4 HRG geochronology

found in the upper Irindina Gneiss. The 0.56-0.7 Ga group is interpreted to be derived from the same distal areas to the southeast as similarly-aged zircon in the upper Irindina Gneiss.

The detrital zircon age signature of the HRMIC near Mount Ruby is similar to that of the HRMIC from Mallee Bore (Buick et al., 2001a) and the upper Irindina Gneiss, with which it is interlayered. The similarity between the detrital zircon ages of these units over a distance of ~50 km gives confidence that the measured age signature is representative of the detrital zircon population, with the youngest detrital ages of ~0.56 Ga indicating that the volcaniclastic was deposited during the Early Cambrian. An early Palaeozoic age for this unit is consistent with Nd data for the HRMIC collected by Sivell & McCulloch (1991), who found initial 143Nd/144Nd ratios higher than that reported for any other Proterozoic igneous rock. The inferred Palaeoproterozoic age for the HRMIC led these workers to propose the presence of an anomalous depleted mantle reservoir, however if a Cambrian age is used the Nd values they obtained from the

metabasites (+6.9 to +8.2) are compatible with models for the isotopic evolution of the depleted mantle (e.g. Galer et al., 1989).

Figure 4.13. Tera-Wasserburg concordia plot and probability density histogram of zircon U-Pb data from hornblende-rich quartzite in the HRMIC near Mount Ruby (sample 2001080226).

Chapter 4 HRG geochronology

The zircon overgrowths have low to moderate U contents (~40-415 ppm) and generally low Th/U, (0.02-0.46). Two analyses (15.2, 18.2) have significantly lower 206Pb/238U than the rest and are interpreted to have undergone lead loss. The remaining 12 analyses

have equal 206Pb/238U within analytical error and yield a weighted mean age of 462.2 ±

5.4 Ma (MSWD = 0.58).