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
The gabbros of the Paperbark supersuite crys- tallized from tholeiitic magmas separate from the
felsic magmas that formed the coeval Whitewater Volcanics, porphyries and coarse-grained granites.
The presence of disseminated biotite and quartz in the gabbro intrusions, and the orthopyroxene-rich
nature of many of the rocks, suggests that they assimilated some granitic magma. In addition to
abundant field evidence of mingling and limited hybridization at the level of emplacement Blake
and Hoatson, 1993; Sheppard, 1996, there is evidence for limited mixing before intrusion. The
relatively uniform distribution of quartz and bi- otite in the gabbros indicates that some assimila-
tion
occurred before
emplacement. The
abundance of quartz and plagioclase xenocrysts in the hybrid rocks show no spatial relationship to
enclosing granite veins, suggesting that most of the xenocrysts were incorporated at depth Shep-
pard, 1996.
Although mixing was important in the genera- tion of the hybrid rocks, there is little evidence for
mixing in most of the granites. This conclusion is
Fig. 8. Chondrite-normalised rare earth element abundances for the Whitewater Volcanics, porphyry intrusions, granites, and gabbros and hybrid rocks. The bulk of the felsic rocks have large negative Eu anomalies. Samples of each rock type with positive
or small negative Eu anomalies are plotted individually. All gabbro samples are plotted individually. Normalizing values from Nakamura 1974.
Fig. 9. a Samples plutons of gabbro and hybrid rock, and mafic enclaves in granites normalized to average E-MORB of
Sun and McDonough 1989; b chondrite-normalized rare earth element plot for plutons of gabbro and hybrid rock, and
mafic enclaves in granites. Normalising values from Nakamura 1974.
fractionation from intermediate granites. Some silicic intrusions e.g. the Greenvale Porphyry do
not plot on the same trends as the intermediate intrusions Fig. 10, and the two therefore, cannot
be related by crystal fractionation. The presence of positive Eu anomalies in some of the granites
and volcanic rocks suggests that they are the cumulate products of fractional crystallization.
The presence of sharp internal contacts in many granite and porphyry intrusions indicates that
they were constructed from a number of magma batches. The magma batches may be of similar or
contrasting compositions. Collectively the data are consistent with variable amounts of crystal
fractionation superimposed on a range of primary intermediate to silicic magmas.
There are few constraints on the age of the source for the 1865 – 1850 Ma granites, porphyries
and felsic volcanic rocks in the Kimberley; no basement is exposed, and the felsic igneous rocks
appear to contain very few xenocrystic zircons. Experimental data indicate that potassic and sili-
cic I-type granites, such as those studied here, can be generated by melting of calc-alkaline igneous
rocks at moderate pressure : 8 – 10 kbar and at temperatures of 900°C or more Conrad et al.,
1988; Rutter and Wyllie, 1988; Singh and Johan- nes, 1996. The granites in the Kimberley were
accompanied by intrusion of numerous gabbro plutons, and it was probably the initial emplace-
ment of these mafic magmas into the lower crust that induced crustal melting e.g. Huppert and
Sparks, 1988. High temperatures of partial melt- ing may explain the paucity of inherited zircon in
felsic igneous rocks of the Kimberley. At tempera- tures greater than about 850°C, only the largest
zircons in the source are likely to survive melting Watson, 1996.
6. Comparison with Phanerozoic granites
