Sample 264 (DHM – Robinhood)

Sample 264 was collected from an amphibolite-facies outcrop of a Dead Horse Metabasalt flow SW of Robinhood station (Fig. 4.1). The sample contained a number of coarse-grained felsic segregations that appear consistent with the fractionation of a felsic component prior to pervasive metamorphism and re-crystallisation. Zircons collected were all of igneous origin. Small unbroken grains are moderately uniform in size and elongation, averaging ~100 x 50 µm. A number of larger, broken grains were also found, and vary in dimension up to ~110 x 80 µm. All grains are euhedral and generally characterised by first-order pyramidal, or lesser prismatic faces. Terminations are either sharp on grain edges and broken surfaces, or are only very slightly rounded (smaller grains) or rounded (larger grains). Individual grains are

morphologically simple, consisting of only a primary igneous growth phase; however larger grains contain some form of compositional zoning. Although the metamorphic grade experienced by sample 264 is significantly higher compared to other mafic samples in this study, no significant metamorphic rims or zoning were detected on the analysed zircons.

Some 35 analyses were conducted on the separated zircons from sample 264 over two separate sessions. Three analyses were omitted due to a combination of significant Pb- loss and common-Pb. As with sample 172, the presence of significant common-Pb

contamination coupled with minor Pb-loss in several grains prevented the use of 204Pb

corrected 207Pb/206Pb ages, so 207Pb-corrected 206Pb/238U ages were instead used for

selected analyses (Table 4.1). Sample 264 is unique among the amphibolite samples analysed in this study, as it appears to contain a spectrum of isotopic ages that suggest a significant inherited component in the sample. There is a large spread in the isotopic age distribution (Fig. 4.8b), with the majority of analyses scattered between 1600 and 2600 Ma. Based on individual isotopic ages, the analyses falling between 1580 and 1710 Ma can be separated into a number of relatively distinct groups. However, only the most statistically acceptable groupings were used in order to determine weighted mean ages.

The youngest group (Group 1) contains 7 analyses that yield a weighted mean age of 1602 ± 17 Ma (MSWD = 0.74, probability of equivalence = 0.62). This age is within error of the crystallisation age determined for the younger phase of mafic intrusions represented by sample 172. However, the large isotopic errors for some Group 1 analyses and similarities in Th/U values to the Group 2 analyses may indicate that these zircons were partially reset by later metamorphism (Hoskin & Black, 2000). The next oldest group (Group 2) contains 15 analyses that yield a weighted mean age of 1663 ± 13 Ma (MSWD = 1.05, probability of equivalence = 0.40). As this group contains the majority of concordant analyses, the 1663 ± 13 Ma age represents the best estimate for the crystallisation age of this rock. Combining the Group 1 and 2 analyses did not return a statistically acceptable grouping.

The remaining analyses are separated into three groups based on isotopic ages. The two younger groups (Groups 3 and 4) contain 3 and 2 analyses respectively, with weighted mean ages of 1809 ± 50 Ma (MSWD = 0.52, probability of equivalence = 0.60) for Group 3, and 1905 ± 27 Ma (MSWD = 1.02, probability of equivalence =

0.31) for Group 4. The two groups most likely represent inherited sources, and may reflect the peperitic incorporation of Barramundi-aged material from adjacent metasediments into the magma. The oldest group (Group 5) contains 3 analyses that combine for a weighted mean age of 2531 ± 110 Ma (MSWD = 1.7, probability of equivalence = 0.18). This age may represent zircons inherited from a much older (pre- Barramundi?) source that were subsequently incorporated into the ca. 1660 Ma phase of magmatism. The significance of this age is discussed in more detail below. A further three analyses returned ages between 2200 and 2700 Ma, but do not form a statistically acceptable grouping.

d 1400 1800 2200 2600 0.0 0.4 0.8 1.2 1.6 2.0 Th/U Ag e ( M a ) Group 1 Group 2 Group 3 Group 4 Group 5 All Other Analyses

a 1500 1700 1900 2100 2300 2500 2700 0.07 0.09 0.11 0.13 0.15 0.17 0.19 0.21 1.4 1.8 2.2 2.6 3.0 3.4 3.8 4.2 238_U/206_Pb 207 Pb/ 206 Pb b 1300 1500 1700 1900 2100 2300 2500 2700 Age (Ma) Rel a ti ve Pr obabi li ty c 1480 1520 1560 1600 1640 1680 1720 1760 1800 Ag e ( M a ) Group 1 Mean = 1602 ± 17 MSWD = 0.74 probability = 0.62 Group 2 Mean = 1663 ± 13 MSWD = 1.05 probability = 0.40

Figure 4.8: (a) Tera-Wasserburg U-Pb concordia plot, (b) cumulative probability plot, (c) weighted average plot for Group 1 and 2 analyses (see text for explanation), and (d) Th/U versus

age diagram for analyses from sample 264. All error bars are 1σ.

The age groups outlined above are also distinguished by their Th/U characteristics (Fig. 4.8d). The younger groups (Groups 1, 2, and 3) have fairly distributed Th/U values (0.33-1.06, average = 0.71), whereas Group 4 analyses have Th/U values of 1.66 and 1.70 (average = 1.68). Th/U values for Group 5 also form a distinct group, ranging from 0.43 to 0.52 (average = 0.48). The difference in Th/U values, as well as the range in values for the younger groups, is partially a consequence of the younger

zircons possessing higher U and Th relative to Group 4 and 5 (averaging 770 ppm U and 541 ppm Th for Groups 1 and 2, compared to averages of 248 ppm U and 119 ppm Th for Group 5). Zircon morphology may also be linked to the distribution in isotopic ages in this sample, as analyses conducted on larger grains returned ages above ca. 1700 Ma.

It is possible that the spectrum of isotopic ages obtained for sample 264 may indicate that eruption of the metabasalt occurred sometime around the crystallisation of the Group 1 zircons (ca. 1600 Ma). However, the weighted mean age obtained for the Group 1 zircons is questionable, as in addition to the possibility of some isotopic re- setting occurring within the grains, the majority of analyses contained some common-

Pb. Due to these effects, the use of 207Pb-corrected 206Pb/238U ages rather than the

preferred 207Pb/206Pb ages was required. Consequently, I conclude that the Group 2

age of 1663 ± 13 Ma is the best estimate of the crystallisation age of this rock.