Geology and Petrology or Part or Toodyay District, W.A. 123 
Under the microscope the constituents observed were : blue-green horn- 
blende (20 per cent), oligoclase (55 per cent), quartz (20 per cent) with 
diopside, sphene, magnetite, apatite, and microcline as accessories. The struc- 
ture is granoblastic with no tendency to parallel alignment of the constituents. 
Hornblende, usually idioblastic, occurring in small lenticular clots is, in 
places, altered to a paler greenish fibrous amphibole. Xenoblastic oligoclase 
(AbgyAuig), in slightly turbid grains averaging 1 mm. diameter, is the only 
felspar — it occurs in granoblastic aggregates with quartz and is frequently 
elongated slightly, parallel to the banding. The absence of microcline is un- 
usual, but is explained by the fact that the microcline in the granite gneiss 
has crystallised early, forming large phenocrysts, and that specimen 15397 
is the result of the reaction between the residuum and the basic xenoliths. 
Apatite, magnetite, and sphene aj-e all abundant accessories ; the sphene is 
usually idioblastic and carries central inclusions of magnetite (fig. 15B). 
The analysis of this rock, together with that of the microcline granite gneiss 
and coarse plagioclase amphibolite appears in Table 4. It is clearly inter- 
mediate between the other two t3q>es. The main discrepancy is in the alkalis, 
but this, as noted above, is due to the removal of the microcline phenocrysts 
from the magma prior to its reaction with the amphibolite. 
TABLE 4. 
Aymh/sis of Hornblende Granite Gneiss {Col. 2) compared with Coarse Plagioclase 
Amphibolite (1) and Microcline Granite Gneiss (2). 
SiOo 
TiO; 
AI2O3 
Fe,03 
FeO 
MgO 
CaO 
NagO 
K2O 
H3O+ 
H^O- 
MnO 
P2O5 
FeSa 
1. 
2. 
3. 
49-05 
64-16 
71-85 
1-07 
0-74 
0-25 
15-03 
15-12 
15-00 
3-16 
1-95 
0-55 
9-08 
2-90 
1-20 
6-96 
2-13 
0-42 
10-47 
6-63 
1-52 
1-70 
4-48 
3-67 
0-95 
0-20 
4-45 
1-43 
0-67 
0-54 
0-25 
0-04 
0-02 
0-09 
0-18 
trace 
0-08 
0-32 
0-11 
0-13 
0-15 
nd. 
Total 
99-45 99-67 99-58 
1. Coarse amphibolite xenolith (refer Table 1, No. 3), Toodyay. 
2. Hornblende granite gneiss (No. 15397), Toodyay. Analyst, R. T. Prider. 
3. Biotite-microcline granite gneiss (Table 2, No. 1). 
Much more work, both field and laboratory, is required before any definite 
statement can be made regarding the origin of these acid hornblende bearing 
gneisses. So far as examined, however, they appear to represent a hybridisa- 
tion of the intrusive granite gneiss. 
(g) Summary of the conclusions regarding the origin of the Lower Granite 
Gneiss and the associated xenoliths. 
The gneiss is essentially the same as the upper band but has not suffered 
such granulation — it is essentially a porphyritic granite, with flow orientation 
of the microcline phenocrysts. A less abundant biotite-oligoclase gneiss is con- 
sidered to represent a more sodic phase of the main mass, which crystallised from 
residuum squeezed out from the earlier phenocrystal microcline. Late stage 
veins of garnet-aplite and pegmatite are developed from this magma. 
