The alkaline rocks of Port Cygnet, Tasmania

217 Pages·2012·29.93 MB·English

THE ALKALINE ROCKS OF PORT CYGNET,
TASMANIA
By
RAMSAY J. FORD
Submitted in fulfilment of the requirements
for the degree of Doctor of Philosophy
University of Tasmania
HOBART
1983
APPENDIX 'VOLUME
-CONTENTS-
APPENDIX I SOME TYPICAL SYENITE ASSOCIATIONS
AND OCCURRENCES
APPENDIX 2 X-RAY FLUORESCENCE ANALYTICAL
DATA
APPENDIX 3 NORMATIVE DATA
APPENDIX 4 ELECTRON MICROPROBE DATA
APPENDIX 5 SPECIMEN LIST
APPENDIX 6 GEOLOGICAL AND LOCALITY MAPS
Al-i
APPENDIX I
SOME TYPICAL SYENITE ASSOCIATIONS AND OCCURRENCES
Syenites, and their related rock types show a diversity of
occurrences and associations in nature. In this section it is pro-
posed to summarize their characteristics, particularly with respect
to those which might have origins comparable with the Port Cygnet
alkaline rocks.
GNEISSOSE SYENITES AND SYENITES ASSOCIATED WITH METAMORPHIC ROCKS
Alkaline syenites can form in metamorphic terranes as granular,
foliated or gneissic rocks, which may in fact represent earlier
igneous rocks subsequently metamorphosed by later events.
Some of the more important metamorphic gneisses from the follow-
ing areas are generally representative of these rocks.
Haliburton-Bancroft Area of Canada
Gittins (1961), Appleyard (1967, 1969) have described these rocks
as being metasomatically recrystallized Grenville metasediments.
Transformations were caused by a nepheline syenite magma which accord-
ing to Appleyard was synchronous with the deformation and recrystalliz-
ation. Payne (1968) considers most of the rocks as intrusive masses
with foliation in the rocks being produced by flow,alignment of
minerals in the original intrusion. The syenites have higher soda
than potash (approximately 2:1). Trace element concentrations are
as follows:- Rb: 33-85 ppm, Sr: 23-88 ppm, Ba: 19-84 ppm, - Zr: 5.6-88 ppm,
which makes them unique when compared with other groups of alkaline
rocks.
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Gittins (1961) considered the rocks of the area to be regionally
metamorphosed metasomatic (nephelinized) rocks and also igneous rocks.
Gittins showed specifically a reciprocal relationship of (la 20 + K20)
with CaO for metasomatized igneous rocks, but no relationship for the
metasomatic nepheline rocks.
Syn-Orogenic Rocks of Eastern Ontario and Northern Norway
In discussin the occurrence of both gneissic and massively
textured alkaline rocks in linear belts within the Grenville tectonic
province of eastern Ontario and the Caledonian fold belt of west
Finmark, northern Norway, ,Appleyard (1974) cthne to the conclusion that
the magmas of both regions were miaskitic suggesting a common genetic
mode but with independent evolution of magmas, at a number of localities.
Appleyard thought that the Ontario rocks may have formed from anatexis
of supra-crustal metasediments including marble quartzo-feldspathic
gneisses and para-amphibolites with evaporites in the assemblage acting
as a fluxing agent.
Appleyard does not rule out a mantle origin but prefers the
former model. With regard to the west Finmark complex, because of
the high barium and strontium coupled with a relatively low rubidium
value, Appleyard prefers a mantle origin for these with the initial
process being anatexis within the mantle, followed by a complex process
of fractionation of a miaskitic magma.
Mbozi, Tanzania
This is a two feldspar gneiss with aegirine-augite, amphibole
and quartz or nepheline. It is associated with the Mbozi syenite-
gabbro complex and was considered by Brock (1968) to be formed by
volatiles.and solutions from the nepheline syenite metasomatizing
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quartzo-feldspathic metasediments of the Ubendian gneisses.
Analysis of marginal rocks:
Syenite from Mbozi Late Syenite Dyke
Si02 61.97 56.90
TiO2 0.90 0.27
A1203 18.06 20.41
Fe203 1.54 2.96
FeO 1.54 2.75
MnO 0.10 0.27
MgO 0.95 0.24
CaO 1.51 1.20
Na20 6.57 8.16
K20 6.22 6.20
P205 0.39 0.05
•
H20+ 0.23 0.36
H20- 0.11 0.06
100.09 99.83
Brock came to the conclusion that because of the spread of values
for the A.F.M. plot of the Mbozi syenites, they did not represent a
simple differentiation series.
The Kilonwa Syenite, Tanzania
This syenite is a gneissose syenite occurring on Waswankha Hill,
northeast of Chilonwa in central Tanzania and has been briefly described
by Kempe (1968). The hill north of this locality consists of sheared
syn-orogenic granite, amphibolite and hornblende-epidote-biotite
schists. Kempe considers the syenite to be of igneous origin and of
interest as a gneissose syenite occurring as a small isolated mass in
a highly metamorphic terrane, with the gneissose structure being the
result of shearing and cataclasis. According to Kempe there is
evidence of other small syenitic bodies throughout the region.
The typical mineralogy is of large perthitic alkali feldspars in
a groundmass of granulated albite and perthite with bands of ferro-
magnesian and accessory minerals. The ferramagnesian minerals vary
with dark brown biotite always present. This may or may not be sub-
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ordinate to pale green aegirine and alkali amphibole. The rocks are
slightly oversaturated and soda always exceeds potash. The range of
analyses is represented by the following:
Si02 64.28 61.65
A1203 17.33 14.73
Fe203 2.63 4.56
FeO 1.16 3.68
MnO 0.03 0.20
MgO 0.22 0.70
CaO 0.03 1.87
Na20 7.36 6.69
K20 5.47 4.65
H20' 0.73 0.58
TiO2 0.38 0.52
P205 0.18 0.17
99.80 100.00
Kishangarh, India
At Kishangarh are banded amphibolite-nepheline two feldspar
gneisses. These are enclosed by metasediments and diorite-gneiss and
were formed by metasomatic transformation of gabbro. The process was
one primarily of addition of soda and alumina plus some potash.
Niyogi (1966) claims the process at Kis angarh was essentially one of
alkalinization akin to granitization. The alkali gneisses are highly
alumina oversaturated. The following is a typical analysis of alkali
gneiss from this locality.
Si02 54.52
TiO2 0.00
A1203 24.32
Fe203 6.62
FeO 0.00
CaO 1.71
MgO 0.43
Na20 10.62
K20 1.60
99.82
The alkaline syenite gneisses are characterized by variable trace
elements, however where a more albite-rich plagioclase and a potash
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mica phase are present the values for strontium and barium could
increase. Chemical trends are not clearly defined as for "more normal"
igneous rocks.
Shaki, Nigeria
Oyawoye (1967) has described the petrology of a potassic syenite
and its associated biotite pyroxenite, from Shaki in western Nigeria.
A syenite gneiss with oval outcrop, about 14 miles long is elongated
nearly parallel to the north-south structural trend of the surrounding
country rock which is a biotite-muscovite gneiss. The biotite
pyroxenite occurs as patches of varying sizes widely distributed
throughout the syenite. Trace elements are characteristic of both
magnesian ultrabasic rocks and granitic rocks. The trace element
concentrations are remarkably uniform in both rock types except for
zirconium and strontium which show a strong antipathetic correlation
with each other.
Some trace elements of interest are strontium (1000 ppm),
barium (2000-3000 ppm), lanthanum (200-300 ppm), nobium (0-20 ppm),
yttrium (20 ppm). Oyawoye believes both rocks were formed by two
metasomatic processes, one involving Ca+Fe0+Mg0 and the other K20 in
a process akin to fenitization. Pyroxenes are identical in both
rock types with some degree of development of amphibole.
Oyawoye states that chemically the syenite is between the biotite
pyroxenite and the surrounding biotite muscovite gneiss with the
chemical character being largely derived from the biotite pyroxenite.
The zoned distribution of pyroxene and amphibole in the syenite with
pyroxene in the core and amphibole on the outside is consistent with
the influence of chemical or temperature gradients. In the inter-
mediate zone where pyroxene and amphibole occur together the amphibole
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in many cases has crystallized first to be followed later by pyroxene.
This would indicate a rising temperature gradient such as would occur
where there is a flow of heat from a centre. The inference is that
the Shaki syenite had an origin resembling the process of contact
metasomatism and essentially formed by feldspathization of the basement
gneiss.
Analyses of Shaki syenite:
Amphibole-rich Pyroxene-rich
syenite syenite
SiO 60.80 59.10
TiO 0.80 0.80
Al 0 14.30 13.50
Fe 0 1.10 1.30
FeO 3.30 2.90
MnO 0.10 0.10
MgO 3.80 4.70
CaO 4.00 5.30
Na 0 2.50 1.90
KO 8.30 9.10
P0 0.40 0.40
H 04" 0.60 0.60
Total 100.00 99.70
Kunararam, India
The Kunararam series is a group of alkaline rocks from the
Khammam district, Andhra Pradesh, India. These have been described
by Subbarao (1971). Syenite and nepheline syenites have been
emplaced in highly folded metamorphosed igneous •and sedimentary rocks
of the eastern Ghats of south India. The syenites originated from
two independent magmas each of which had its own fractional crystalliz-
ation series. They were subsequently metamorphosed and also caused
metasomatic action on the country rocks. Soda is greater than
potash. The nepheline syenites are associated with pyroxene-, horn-
blende-, and biotite-nepheline-syenite variants. The syenites have
pyroxene-, amphibolite-, and biotite-syenite and syenodiorites.
A1-7
Granite gneisses and amphibolites also occur. The syenites occur as
irregular patches and often show gneissic banding. Biotite melteigite
patches in the nepheline syenite are interpreted as indicating that the
melteigite was a primary magma. Subbarao also suggests that some of
the nepheline-bearing syenodiorites and syenites have formed from the •
metasomatism of pre-existing rocks such as the garnetiferous biotite
gneiss.
Glen Dessary Area, Scotland
Described by Richardson (1968), and thought by him to be a pre-
or synmetamorphic intrusion with Caledonian age metamorphism, the
features of this complex, which has an oval shape with the minor axis
about one mile and the major axis about two miles, are a leucocratic
syenite surrounded by a more mafic syenite with an association of
ultramafic rocks whose precise field relationship to the syenites
is not obvious from the map. The mafic syenites have been intruded
in a complex way, by granitic pegmatites whose time sequence has not
been determined. The contacts of the various rock units are very
sharp and are considered igneous intrusions on this basis.
There is evidence of some feldspathic rocks intermediate between
the ultramafic rocks and the mafic syenites. The syenites are very
slightly oversaturated with free quartz rare and feldspathoids are
absent. Richardson considers the sequence to be one of differentiation
beginning with the ultramafic rocks through the mafic syenites to the
leucocratic syenites. The main differences between the two syenites
is the relative amounts of alkali feldspar to the dark minerals
(principally biotite, pyroxene, hornblende and magnetite). The ultra-
mafic rocks are composed mainly of pyroxene and hornblende. No complete
gradation to mafic syenite can be seen but up to 25% feldspar may be
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present in some ultramafic rocks. Pyroxene and hornblende occur in
the syenites. Pyroxenes are aegirine augites.
Typical analyses of rocks from Glen Dessary:
Mafic Syenite Leucocratic Syenite Ultramafic Rocks
Si02 56.23 59.82 43.72
A1203 15.50 16.77 7.00
TiO2 1.03 0.80 1.40
Fe205 2.59 2.38 5.62
FeO 4.80 1.89 10.95
MnO 0.15 0.04 0.34
MgO 3.20 1.06 10.88
CaO 6.03 2.44 15.08
Na20 3.10 5.52 2.04
K20 5.49 6.16 1.03
P205 0.63 0.16 0.98
H2O+ 0.39 - 1.00
99.14 97.04 100.14
SYENITES IN LAYERED INTRUSIONS
Klokken Gabbro-Syenite Complex, South Greenland
This complex is part of the Gardar Province of southwestern
Greenland. It is a small stock, about 4 km in diameter and is
described in part by Parsons (1979). Essentially the syenites have
had a relatively complex history but nevertheless represent a
differentiated gabbroic parent. The initial ratio for strontium
isotopes of this complex is 0.7031 ± 0.003 which is considered to be
consistent with an origin from primitive mantle material. The age
of this complex is given at 1159 ± 11 Ma (Rb-Sr isochron, Blaxland
and Parsons, 1975). The liquid line of descent suggested by Parsons
for these rocks, is shown in Fig. VI-
Kiglapait Layered Intrusion, Labrador
Morse (1969) describes this as a Precambrian layered intrusion
of probably lopolithic form. The rocks are mainly gabbroic types