Major and trace element and isotope chemistry

Table 2 Sm–Nd whole-rock data for 1865–1850 Ma igneous rocks in the Kimberley region a Rock unit Sample Sm ppm Nd ppm 147 Sm 144 Nd 143 Nd 2s T Ma o Nd i T DM 144 Nd 7.32 41.01 0.1079 0.511372 Mondooma Ganite 7 8759.8007 1862 − 3.5 2542 Lennard Granite 8759.8011 6.34 42.49 0.0903 0.511144 6 1864 − 3.7 2460 6.62 36.35 0.1102 8759.8009 0.511386 Lennard Granite 7 1862 − 3.7 2577 6.87 38.77 0.1070 0.511405 Kongorow Granite 6 8759.8010 1852 − 2.7 2474 Neville Granodiorite 108532 4.83 24.38 0.1198 0.511542 10 1860 − 3.0 2589 Paperbark Granite 113416 8.30 45.79 0.1096 0.511438 10 1854 − 2.7 2487 7.80 39.00 0.1229 0.511541 Whitewater Volcanics 15 92777 1855 − 3.8 2679 91085 6.90 Whitewater Volcanics 39.00 0.1083 0.511354 8 1855 − 4.0 2577 2.80 12.20 0.1373 0.511800 Wombarella Quartz 9 95313 1850 − 2.2 Gabbro Wombarella Quartz 92721 4.00 19.60 0.1247 0.511711 8 1850 − 1.0 Gabbro Toby Gabbro 8333.0052 4.69 21.97 0.1292 0.511754 14 1855 − 1.1 a Samples 8759.XXXX were analyzed at the Australian National University. Samples of the Whitewater Volcanics and Wombarella Quartz Gabbro were analyzed by Ian Fletcher at Curtin University of Technology. Samples 108532 and 113416 were analyzed at LaTrobe University. Sample 8333.0052 is from Sun and Hoatson in press. All data have been recalculated relative to La Jolla = 0.511860. T DM calculated assuming mantle depletion beginning at 4.56 Ga, and using 143 Nd 144 Nd = 0.513144 and 147 Sm 144 Nd = 0.2136 for present day depleted mantle. remainder of the analyses. Therefore, the cores are not significantly older than the rest of the zircon crystal. Nineteen of the 20 analyses, with the exception of analysis 33-1, form a single popu- lation x 2 = 1.08 that defines an igneous crystal- lization age of 1861 9 4 Ma Fig. 4d. Sample 8759.8010 Fig. 2 is from the Kon- gorow Granite, a thoroughly recrystallized gneis- sic granodiorite containing garnet. The zircons from this sample show concentric oscillatory zon- ing; some crystals contain unzoned cores. How- ever, analyses that overlap these cores e.g. 52.1 and 53.1, Fig. 5c give ages indistinguishable from analyses from zoned rims, indicating that the cores are not significantly older. All 17 analyses from this rock plot on or near the concordia curve, defining an igneous crystallization age of 1852 9 4 Ma x 2 = 1.33; Fig. 4d. This result is significantly younger different at the 1 proba- bility level using Student’s t-test than other dated granites in the Hooper Complex. However, the intrusion shares the same field relationships as the other dated granites, and is inseparable from ages for the high-level porphyries and Whitewater Volcanics. Collectively, the Whitewater Volcanics, and high-level porphyry intrusions and granites of the Paperbark supersuite represent a period of volu- minous felsic magmatism at 1865 – 1850 Ma. However, the data presented do not allow us to determine whether the volcanic rocks have the same age range as the granites, or if they repre- sent a more restricted episode of magmatism. All of the dated samples appear to have a paucity of significantly older inherited zircon crystals.

4. Major and trace element and isotope chemistry

The results presented in this section are based on 331 new whole-rock analyses of the Whitewa- ter Volcanics 47 analyses, high-level porphyry intrusions 53 analyses and coarse-grained gran- ites 210 analyses and coeval gabbros 21 analy- ses. The complete data set, along with sample collection and analytical techniques, and data for standards, is presented in Sheppard et al. 1997c. Representative analyses of the Whitewater Vol- canics, and high-level porphyry intrusions, granite and gabbro of the Paperbark supersuite are pre- sented in Table 1. Eleven samples six granites, two Whitewater Volcanics, and three gabbros were also analyzed for their Nd isotopic compo- sitions Table 2. 4 . 1 . Major and trace element chemistry The rocks range from about 46 – 80 wt SiO 2 , with a pronounced minimum at 58 wt SiO 2 Fig. 6. All of the analyses below 59 wt SiO 2 belong to intrusions of gabbro and hybrid rock, or mafic enclaves in granite plutons, with the exception of three granite samples Fig. 7. These three samples have been moderately to strongly overprinted by low- to medium-grade metamorphic assemblages, and their relationship to the remainder of the granites is unclear. The volcanic rocks and high-level porphyry intru- sions do not contain any mafic compositions. Among the felsic rocks, the volcanic rocks are overall slightly more mafic than the granites and high-level porphyries Fig. 6. The porphyries and granites have a peak at : 70 – 72 wt SiO 2 , whereas the volcanic rocks have a peak at : 68 wt. The volcanic rocks, porphyries and granites are I-types with high K 2 O, Rb, Y, Th, and K 2 ONa 2 O, and low Sr, SrY and KRb Fig. 7; Sheppard et al., 1997c. They also have high contents of large ion lithophile elements LILE such as Ba, Rb, K and Th, relative to the high field strength elements HFSE such as Zr, Y and Nb Sheppard et al., 1997c. The three groups of rocks share similar rare earth element REE abundances, although the granites show a wider range. A minority of granites and vol- canic rocks have positive Eu anomalies or very small negative Eu anomalies Fig. 8. These rocks contain cumulate feldspar, and are proba- bly cumulate rocks. Granites of the Paperbark supersuite are similar to Palaeoproterozoic gran- ites from elsewhere in northern Australia e.g. Wyborn, 1988. The gabbros and hybrid rocks of the Paper- bark supersuite range from about 46 to 55 wt SiO 2 Fig. 6. There are no trends apparent within the group, and for some elements e.g. Sr, Fig. 7, the gabbros and hybrid rocks do not plot on the same overall trend as the granites. The gabbros have REE contents that overlap with those of the granites, but chondrite-nor- malised patterns for the gabbros are less frac- tionated [LaYb] N = 5 than for the granites [LaYb] N = 10 – 30 Fig. 8b. The gabbros plot in the tholeiite field on an AFM diagram Shep- pard et al., 1995, and have incompatible trace element abundances similar to many continental tholeiites Sun and Hoatson, in press. The mafic enclaves within the coarse-grained granites have similar compositions to the intru- sions of gabbro and hybrid rock of the Paper- bark supersuite Fig. 9a, but with on average, higher K 2 O, Ba, Rb, Zr and Th contents. These relative abundances may be explained by con- tamination of the enclaves by the host granite magma. One of the enclaves has a large positive Eu anomaly Fig. 9b consistent with incorpora- tion of feldspar phenocrysts from the granite host. No analyses were obtained from the smaller mafic clots in some granodiorite and tonalite intrusions. There is compositional heterogeneity within the granites and the porphyries. For example, analyses from the Greenvale and Castlereagh Hill Porphyries do not plot on trends defined by the Bickleys Porphyry and the Mondooma Granite Fig. 10. Some of the porphyries e.g. Fig. 6. Histogram of whole-rock SiO 2 abundances for the Whitewater Volcanics, porphyries, and granites and gabbros of the Paperbark supersuite. The volcanic rocks are under-rep- resented owing to difficulty of access and the weathered nature of many of the rocks. Fig. 7. Harker diagrams for the Whitewater Volcanics, porphyries, and granites and gabbros of the Paperbark supersuite. Mondooma Granite have a large range in SiO 2 contents, whereas others, such as the Greenvale Porphyry, have a very restricted range. Trends for individual granite intrusions not shown show more scatter than for the porphyries, but it is still apparent that not all intrusions plot on the same trends. The presence of different chemical trends in the intrusive rocks indicates that the volcanic rocks, porphyries and granites are not comag- matic; that is, they do not represent a single magma lineage. 4 . 2 . Nd isotope compositions Neodymium isotope compositions for six sam- ples of coarse-grained granite, two samples of the Whitewater Volcanics, and three samples of bi- otite-bearing gabbro are shown in Table 2 and Fig. 11. The samples of granite and Whitewater Volcanics define a narrow range in initial o Nd from − 2.9 to − 4.2. The values at either end of this range are almost within error of each other, given an uncertainty of about 9 0.5 o Nd units. In con- trast, the three gabbroic rocks have initial o Nd values of − 1.1 to − 2.4. Depleted mantle model ages for the granites range from about 2500 – 2600 Ma or 2230 – 2320 Ma using the calculation of McCulloch, 1987, similar to most other Palaeoproterozoic granites of northern Australia. Fig. 11 shows that the granites must contain a large proportion of older crust, but they cannot be derived by wholesale melting of Archaean crust, which McCulloch 1987 also noted.

5. Petrogenesis