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
The composition of felsic igneous rocks is largely a function of the source composition and
mineralogy, but granites from the same tectonic setting commonly share some broad similarities.
Sheppard et al. 1997a noted that the rocks stud- ied here are more silicic and potassic than
consistent with a lack of field or textural evidence for magma mixing in the volcanic rocks and
porphyries. An exception however, may be some of the tonalite- and granodiorite-dominated intru-
sions that contain widespread mafic clots. The mineralogy and textures of these clots are similar
to microgranitoid enclaves described by Vernon 1991 and attributed to magma mixing. These
intrusions may have formed in part by mixing i.e. large-scale homogenization of mafic and felsic
magmas.
The compositional gap between 55 and 61wt SiO
2
in the Paperbark supersuite is incompatible with crystal fractionation of mafic magmas to
produce the granites. Moreover, the dominance of silicic compositions Fig. 6 suggests that not all
the silicic granites are the products of crystal
Fig. 10. Harker variation diagrams for some of the individual porphyry intrusions.
Phanerozoic batholiths developed at convergent continental margins, such as the Peninsular
Ranges batholith of California or the Coastal batholith of Peru. In addition, the 1865 – 1850 Ma
Kimberley granites have Ba- and Sr-depleted, and Y-undepleted mantle-normalized patterns, which
contrast with the Sr-undepleted and Y-depleted patterns of most Cordilleran granites Wyborn et
al., 1992. Most of the Cordilleran batholiths also display a range in initial o
Nd
values, including some with positive initial o
Nd
values, indicating a juvenile component in the granites e.g. Pankhurst
et al., 1988. The compositions of the granites in the Hooper
and Lamboo Complexes are broadly similar to high-K post-collisional granites from the pre-
Cordilleran of South America Mpodozis and Kay, 1992; Rapela et al., 1992, the southern Alps
in Italy Rottura et al., 1998, Dabieshan in east- central China Ma et al., 1998, and the Badjal
intrusive suite in far east Russia Grigoriev and Pshenichny, 1998. These post-collisional intru-
sions range from gabbro to syenogranite, but are generally dominated by monzogranite and gran-
odiorite. In common with the Paperbark super- suite the rocks are high-K calc-alkaline to
shoshonitic in composition Fig. 12. Mantle-normalized patterns for granites of the
Paperbark supersuite are similar to granites from the Badjal intrusive suite in Russia and Group III
granites from Dabieshan Fig. 13. These rocks have comparable abundances for most elements,
and are all characterized by negative Ba and Sr anomalies, and are undepleted in Y relative to
Fig. 11. o
Nd
values for granite and gabbro of the Paperbark supersuite, and for the Whitewater Volcanics at 1850 Ma. The
o
Nd
values for depleted mantle and late Archaean crust at 1850 Ma are from Patchett and Arndt 1986.
Fig. 12. Comparison of granites of the Paperbark supersuite with Phanerozoic high-K post-collisional granites. Field for Paperbark supersuite includes \ 95 of analyses. Data for Dabieshan granites from Ma et al. 1998; Badjal intrusive suite from Grigoriev and
Pshenichny 1998; Triassic granites of Patagonia from Rapela et al. 1992; Triassic granites of Chile from Mpodozis and Kay 1992. Fields in a from Peccerillo and Taylor 1976.
Na. These characteristics are consistent with par- tial melting in the stability field of plagioclase.
The sample from the Group II granites of Da- bieshan is characterised by small positive ‘spikes’
in Ba and Sr, and a strong depletion in Y. This pattern may be produced by extensive fractiona-
tion of amphibole without plagioclase as sug- gested by Ma et al. 1998, or equilibration of
the melt with amphibole or garnet either in the residue, or en route to the level of emplacement.
Nevertheless, the felsic igneous rocks of the Kim- berley differ from Phanerozoic post-collisional
granites in some respects. For example, the Kim- berley rocks have lower Na
2
O and thus higher K
2
ONa
2
O ratios, and higher Rb and Y con- tents than most post-collisional granites Fig.
12. In addition, the Kimberley rocks show a positive correlation between Y and SiO
2
, whereas post-collisional granites generally show a weak
negative correlation Fig. 12. The behaviour of Y in the Kimberley granites may reflect the ab-
sence of hornblende in these rocks. Other chemi- cal differences may be related to the particular
source composition for the Kimberley granites.
The Phanerozoic high-K post-collisional gran- ites generally have moderately or strongly negative
initial o
Nd
values indicating that they contain a significant component of evolved continental crust.
Nevertheless, their isotopic compositions preclude wholesale melting of country rocks or known
basement rocks e.g. Rapela et al., 1992; Rottura et al., 1998. Basement to the felsic igneous rocks
of the Kimberley is not exposed, but it is clear that they were not derived by melting of late Archaean
crust similar to that exposed as basement elsewhere in northern Australia Fig. 11.
7. Tectonic setting
