Field and microstrucural observations of granulite facies rocks, Hamilton Downs, Mt. Hay Block, central Australia Zachary P. Montes Senior Integrative Exercise December 7, 2009 Submitted in partial fulfillment of the requirements for a Bachelor of Arts Degree from Carleton College, Northfield, Minnesota
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Field and microstrucural observations of granulite facies rocks, Hamilton Downs, Mt. Hay Block, central Australia
Zachary P. Montes Senior Integrative Exercise
December 7, 2009
Submitted in partial fulfillment of the requirements for a Bachelor of Arts Degree from Carleton College, Northfield, Minnesota
Table of Contents
Introduction………………………………………..……………………………….……1
Geologic Setting…………..……………………………………………………….……..2
Mt. Hay and Capricorn Ridge………………….……………………………………….4
Ceilidh Hill…………………………………..…………………………………………….6
Hamilton Downs……………………………………………………………………...…11
High Strain Zone……………………………………………………………………..…17
Microstructural and Mineralogical Characterization…………………………….….20
Hamilton Downs……………………………………………………………………….…21
High Strain Zone……………………………………………………………………..….24
Geothermobarometry………………………………………………………………….26
Discussion………………………………………………………………………………29
Conclusions…….………………………………………………………………………33
Acknowledgments.…………………………………………………………………….34
References Cited……………..………………………………………………………..35
Field and microstrucural observations of granulite facies rocks, Hamilton Downs, Mt. Hay Block, central Australia
Zach Montes Senior Integrative Exercise December 7th, 2009 Carleton College Advisors: Dr. Seth Kruckenberg – University of Wisconsin – Madison Dr. Sarah Titus – Carleton College Dr. Cam Davidson – Carleton College Abstract
This study reports the field and microstructural observations of a well-exposed, unretrogressed section of the lower continental crust in the Mount Hay Block, Central Australia. The Mount Hay Granulites (Mafic, Anorthositic, Intermediate, Quartzofeldspathic) experienced penetrative ductile deformation in the lower crust at ~800°C, ~8 kbar during the 1780-1720 Ma Strangways event. These rocks were later uplifted in the 1590-1570 Ma Chewings Event and the 400-300 Ma Alice Springs Orogeny. I studied the anorthositic granulites of Hamilton Downs, located on the far eastern margin of the Mount Hay Block. Compositions of the anorthositic granulites are distinct from the mafic/felsic granulites of Ceilidh Hill. In these rocks, lineation is steeply plunging and well developed. Foliation is variable. Fabrics including L=S, L>S, L>>S and L-tectonites are expressed in the shape-preferred orientation of pyroxene clots and segregated layers. The distribution of fabric types may indicate that Hamilton Downs is part of a larger sheath fold. A 1 km wide zone of high strain separates the lower strain anorthositic granulites of Hamilton Downs from the mafic granulites of the southeastern tongue of Ceilidh Hill. Transposition of compositional layering and foliation from Ceilidh Hill and Hamilton Downs indicates that the high strain zone is younger than surrounding exposures. Within the high strain zone, quartz and plagioclase deformed by grain-boundary sliding as inferred from microstrucural observations including amoeboid grain boundaries and variable grain size.
Keywords: Granulite, central Australia, Mount Hay Block, Hamilton Downs
1 Introduction
The Mount Hay Block is a particularly good place to study deformation of the
lower crust. Based on xenoliths (Chen et al., 2001) and seismic velocity analysis
(Christensen and Mooney, 1995; Rudnick and Fountain, 1995) we know the lower crust
is dominated by granulite facies rocks, consisting primarily of plagioclase and pyroxene.
Within the Mount Hay block, granulites with various compositions and structural fabrics
are preserved in a thick (~40 km laterally) nearly pristine structural section of the lower
continental crust. The Mount Hay block is therefore ideal for assessing the broad pattern
of deformation in the lower crust and for studying the variations in structural fabric as a
function of lithologic composition.
The Mount Hay Block has been the subject of many studies (Waters-Tormey, 2004;
Hallau, 2006; Waters-Tormey and Tikoff, 2007; Waters-Tormey et al., in press). Yet the
compatibility of structural elements, the localization of strain and the significance of
fabric variation continue to be poorly understood. In this study, I present preliminary
results from field and microstrucural observations of a largely unstudied exposure of
anorthositic granulites in the south and eastern portion of the Mount Hay Block just north
of the Hamilton Downs Cattle Station (Fig. 1C). Three important questions addressed in
this study include: (1) How is the degree and development of structural fabric (i.e.,
lineation and foliation) expressed in the rocks of Hamilton Downs which are primarily of
anorthositic composition? (2) How do the Hamilton Downs anorthositic granulites fit into
the larger geological context of the Mount Hay Block mafic granulites including what is
known of fabric formation on Ceilidh Hill just to the north (c.f., Hallau, 2006) (3) What
do microstructures in these rocks say about deformation mechanisms in the lower crust?
2 In order to address these questions, structural and fabric domains were defined
and identified in the field using a modification of the fabric classification criteria defined
by Hallau (2006) on Ceilidh Hill but more applicable to the anorothositic compositions at
Hamilton Downs. Mesoscale fabric intensity maps were constructed to illustrate the
distribution and intensity of strain and the structural connections to the better-studied
Ceilidh Hill to the north. Finally, microstructural observations in plagioclase and quartz
are presented to assess the likely deformation mechanisms operating in the Hamilton
Downs rocks, thereby providing important information on the mechanical characteristics
of the lower crust.
Geologic Setting
The Mount Hay Block is located within the Arunta Inlier of central Australia (Fig.
1) and is part of a 160 x 50 km2 EW oriented belt of granulites. (Shaw et al., 1984).
Granulites within the Mount Hay Block experienced six major tectonothermal events
(Shaw and Black, 1991; Dunlap and Teyssier, 1995; Hoatson et al., 2005). The protoliths
of the Mount Hay Block granulites were gabbros and charnokites emplaced in the lower
crust during the Stafford event (1810-1790 Ma) and subsequently deformed at granulite
facies conditions during the Yambah and Strangeways events (1780-1745 Ma and 1730-
1690 Ma) (Hoatson et al., 2005). Detailed geothermobarometric and isotopic studies
(Collins and Shaw, 1995; Staffier, 2007; Waters-Tormey et al., in press) constrain peak
metamorphic conditions of the Mount Hay granulites to conditions typical of the lower
continental crust (>600°C, >7 MPa).
77fo
liatio
n st
rike,
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Figu
re 1
. (A
) Gen
eral
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nta
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) Loc
atio
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ck w
ithin
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pli�
ed g
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t. H
ay B
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. Stu
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rea
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ers-
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2
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nes
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ral f
olia
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Amburla
Folds
3
4 The Mount Hay Block was partially exhumed during the Chewings event (1590-
1570 Ma) and later during the Paleozoic Alice Springs orogeny (400-300 Ma) which
resulted in the uplift and rotation of the entire Mount Hay Block along the Redbank thrust
zone (Claoue-Long and Hoatson, 2005). Isotopic data suggests that the Mount Hay Block
experienced amphibolite facies conditions during these major exhumation events, but the
retrogression to lower metamorphic facies (eg. greenschist) is restricted to discrete shear
zones bounding the Mount Hay Block (Waters-Tormey et al., in press).
According to seismic reflection studies, tectonic motion along the Redbank thrust
zone likely displaced the Moho by as much as 25 km vertically and at least 40 km
laterally (Wright et al., 1990; Wright et al., 1993; Korsch et al., 1998; Biermeier et al.,
2003). Therefore, in its present configuration, the Mount Hay Block displays a large,
pristine structural section through the lower crust with depth increasing from northeast to
southwest (Staffier, 2007).
Broadly speaking, the Mount Hay Block can be divided into four geologically
distinct regions: Mount Hay, Capricorn Ridge, Ceilidh Hill, and Hamilton Downs (Fig
1C). The dominant lithologies in all four regions consist of mafic (pyroxenitic and
gabbroic) and felsic (charnockitic) granulite with subordinate anorthositic, calc-silicate,
quartzofeldspathic, and pelitic granulites also present.
Mount Hay and Capricorn Ridge
Mount Hay and Capricorn ridge are located in the west and north Portion of the
Mount Hay Block and form the majority of granulite exposures (Fig 1C). Previous
mapping of the Mount Hay Block relied on aerial photography, limiting structural
5 relationship analysis to color contrast recognition. These geologic and petrologic
descriptions were compiled into the 1:250,000 Hermannsburg geologic map and notes
(Glikson, 1984; Watt, 1992; Warren and Shaw, 1995). Detailed ground-based mapping
and fieldwork of the study area is limited to Mount Hay and Capricorn Ridge (Waters-
Tormey, 2004; Staffier, 2007; Waters-Tormey and Tikoff, 2007). These workers defined
qualitative, field-based assessments of L versus S fabric development. Distinctions were
based on the strength of lineation and foliation based on consistency in linearity and
planarity respectively, and mesoscopic shape-preferred orientation in the defining phase
(most often plagioclase). With these data, it is possible to make interpretations of relative
strain in Mount Hay and Capricorn Ridge.
Mount Hay is inferred to be a low strain lens with lineation and foliation displaying
a 10 km scale fold (Fig. 1C). Fabric development and intensity is dominated by L>S
fabric, with less common L>>S, and L=S, and rare L-tectonites constrained to
topographic high points (Staffier, 2007).This large-scale structure with fold closures on
the east and west ends of the ridge, is interpreted to be a antiformal sheath like fold
(Glikson, 1984; Shaw et al., 1984; Staffier, 2007).
In contrast, Capricorn ridge displays discrete lithologic domains that are thinner,
laterally continuous and more planar than those found on Mount Hay. While there is
some heterogeneity in the fabric development across the discrete lithologic domains,
fabric is dominated by S>L tectonites (Waters-Tormey, 2004). Unlike fabric development
found throughout Mount Hay, the compositional layering displayed at Capricorn ridge is
everywhere parallel to a single primary east/west trending foliation. (Waters-Tormey et
6 al., in press) show that the consistently parallel orientation of the compositional layering
reflects the transposition of older foliations in the Mount Hay block and thus, Capricorn
ridge is inferred to be a 6 km wide zone high strain relative to Mount Hay.
Evidence that deformation of the Mount Hay Block occurred in the lower crust is
abundant. Sheath folds, transposition of compositional domains into the foliation, the
presence of L-tectonites, local boudinage and undulose to patchy extinction of
plagioclase in thin section, all indicate high temperature ductile deformation typical of
the lower crust (Waters-Tormey, 2004; Staffier, 2007; Waters-Tormey and Tikoff, 2007;
Waters-Tormey et al., in press). Extensive thermobarometric analyses of samples
throughout the Mount Hay Block constrain peak deformation conditions from 700-900 C
and 6.9-8.2 MPa (Collins and Shaw, 1995; Staffier, 2007; Waters-Tormey et al., in
press). Despite the range in depth, there is no significant variation in temperature
recorded in samples throughout Mt Hay and Capricorn ridge. The lack of systematic
variation in temperature suggests that deformation in the lower crust occurred at
relatively constant temperatures or that the mineral assemblage of Mount Hay re-
equilibrated when the Capricorn Ridge shear zone was active. Assuming a lithostatic
gradient of 27 MPa/km, constrained by the pressure calculations from Staffier (2007) of
690-820, MPa, Waters-Tormey et al. (in press) estimates that deformation of the Mount
Hay Block occurred at a depth between 26 and 30 km, with a geothermal gradient of
roughly 27-30°C.
Ceilidh Hill
Previous study of Ceilidh Hill and the southeastern portion of the Mount Hay Block
7 consists of structural mapping and a fabric study completed by Hallau (2006) as part of
an undergraduate thesis project at the University of Wisconsin-Madison. The lithology of
Ceilidh Hill is primarily fine-grained intermediate to mafic granulite, with a mineral
composition consisting of pyroxene, plagioclase, K-feldspar, and quartz. Compositional
layering is common and displayed as cm to m-scale compositional bands of charnockite
within fine-grained gabbro. These compositional bands are classified as the S0 fabric
(Fig. 2).
Hallau (2006) identifies four primary fabric domains on Ceilidh Hill: L-tectonites,
L>>S, L>S and L=S. Fabric is characterized by the shape-preferred orientation of the
plagioclase and quartz crystals found in the mm to cm-scale felsic segregations. Lineation
is consistently well developed and is most commonly defined by the long axes of
elongate quartz and plagioclase crystals of felsic segregations with up to 10:1 length to
width ratios. S1 foliation is defined by the consistency and planarity of shape-preferred
orientation of the felsic segregations in the plane perpendicular to lineation. L=S domains
display foliation and lineation that are approximately equally developed, with domains
between compositional layers showing sharp, well-defined margins. L>S domains retain
tabular margins between compositional layers, but display less well-developed flattening
in plane perpendicular to foliation. L>>S domains are characterized by a weak shape-
preferred orientation in foliation plane and non-tabular compositional banding.
Compositional layering and the shape-preferred orientation of felsic segregations are
often folded. L-tectonites are defined by rods of felsic segregation with a shape-preferred
orientation alignment almost exclusively in the direction of lineation. Compositional
layering in L-tectonites of Ceilidh Hill is largely absent.
8 In general, the fabric type of Ceilidh Hill is dominated by L>S fabric, with less
common L>>S and rare L-tectonites constrained to two topographical high points
(Hallau, 2006). L=S fabric is found along the northern boundary with Capricorn Ridge
and on the southeastern tip of along the boundary with Hamilton Downs. Foliation is
generally NW-SE striking, but begins to swing to an E-W orientation towards the
boundary with Hamilton Downs (Fig. 3). Lineation consistently plunges nearly vertically
(an average of 84°). In contrast to Capricorn Ridge, changes in fabric type are most
commonly gradual. Abrupt structural contacts between boundaries are rare (Hallau,
2006).
In the following sections, I add to this foundation of work with important detailed
field mapping and both macro and microstructural observations of the South Eastern
portion of Ceilidh Hill and Hamilton Downs. I attempt to integrate these data into
previous work in order to provide a more detailed picture of this regions tectonic history.
I build on Hallau’s (2006) preliminary work, focusing on a much smaller area
encompassing the southeastern most tongue of Ceilidh Hill and a small 3.5 X 5 km
exposure called Hamilton Downs. Structural and lithologic geologic mapping of Ceilidh
Hill consisted of strike-perpendicular and strike-parallel traverses where significant fabric
and lithologic variations were recorded. Detailed measurements were collected,
documenting the orientation and degree of foliation, lineation, compositional layering and
various mesostructures. These measurements were compiled and overlaid on a
topographical map of the region (Fig. 2). Each site showing foliation and lineation was
classified on a spectrum of fabric intensity modified from Hallau (2006) (Fig. 3).
Ham
ilton
Dow
ns S
tatio
n30
m S
8261
8764
67 67
6771
6666 82
8284
62 80 75 83
7558
6059
70
6276
68
7178
80
81
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72
6362
7367
Hig
h St
rain
Zon
e
Ham
ilton
Dow
ns
Ceili
dh H
ill
Figu
re 2
. Str
uctu
ral m
ap o
f sou
thea
ster
n Ce
ilidh
Hill
and
Ham
ilton
D
owns
sho
win
g tr
ends
in li
neat
ion
and
folia
tion.
Loc
atio
n sh
own
in
Fig
1. To
pogr
aphy
from
the
Aust
ralia
1:5
0,00
0 G
eolo
gica
l Sur
vey.
St
ereo
nets
in u
pper
righ
t cor
resp
ond
to C
eilid
h H
ill (t
op le
ft),
high
st
rain
zon
e (t
op ri
ght)
, and
Ham
ilton
Dow
ns (b
otto
m le
ft).
Folia
tion
plot
ted
as p
lane
s (b
lue
lines
), lin
eatio
n as
poi
nts
(bla
ck d
ots)
.
Hig
h St
rain
Zone
Ceilid
h Hill
Ham
ilton
Dow
ns
no fa
bric
no fa
bric
S0 -
Com
post
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l lay
erin
g tr
end,
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tion
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ipLi
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ltram
ylon
ite la
yer
Folia
tion
tren
dlin
eTr
ack
Cont
our i
nter
val:
10 m
Lege
nd
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1
Scal
e (k
m)
59 807184
9
A’
A
24
23
25
26
24
23
25 26
00
01
99
01
8261
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67
67
677166
66
84
N0 10.5
Scale (km)
8282
628075
83
7558
60 59
59
70
6276
68
7178
80
80
8183
80
79
75
71
87
71
86
716784
84
84 S0 - Compostional layering trend, dip
S1 - Foliation trend, dip
Lineation trend, plunge
Ultramylonite layer
L versus S Fabric Development
L = S
L > S
L >> S
L - Tectonite
Shear zone
6657
86847672
706574
558179
626274727976
7978
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7171
686766
6367
6968
67
7675
7473
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7672
6362
7367
Legend
Figure 3. A) Generalized map of L versus S fabric development adapted from Fig. 2. Fabric transitions gradual in Ceilidh Hill, abrupt in Hamilton Downs. Note concentration of L>>S and L-tectonite fabric in topographical highs of Hamilton Downs. B) Composite cross section after A) showing penetrative foliation inclination and boundary of high strain zone. No vertical exaggeration.
Contour interval: 10 m
A
B
Hamilton Downs
High Strain
Zone
Ceilidh Hill
NE SW
Hamilton DownsCeilidh Hill
A’A
High Strain Zone
0
Foliation inclination High strain zone boundary
0 10.5
Scale (km)
no fabric
no fabric
10
11 Hamilton Downs
The composition of Hamilton Downs contrasts that of the largely interlayered
mafic and felsic granulites found on Ceilidh Hill to the north. Lithologies in Hamilton
Downs range from almost pure anorthosite (>15 percent pyroxene) to gabbronorite
(60/40 percent plagioclase/pyroxene) containing varying amounts of pyroxene but
consistently dominated by plagioclase. The Hamilton Downs exposure reflects the
weathering prone nature of plagioclase, exposing amphibole and orthopyroxene bearing
bands, rods, clots and aggregates that represent the regular to patchy cm to m-scale
compositional banding we define as the S0a fabric (Fig. 4). In addition to the S0a
compositional layering, local compositional bands of 10 cm to meter scale pure, fine-
grained anorthosites cut through pyroxene clotted anorthosites and gabbronorites. This
compositional layering has been designated as the S0b fabric, which is most often oblique
to the primary foliation (S1). Additional lithologic domains are represented by rare 1 to 3
mm seams of pyroxenite and 50 cm dikes of pure fine-grained gabbro (Fig. 4).
Exposures of mafic pyroxene clotted gabbro/anorthosite similar to Hamilton Downs
have been cited elsewhere within Mount. Hay Block, including the northern flank of
Capricorn Ridge and in discrete horizons and layers throughout the fault bound Amburla
Folds region between Mount Hay, Cap Ridge, and Ceilidh hill (Fig 1C). Both Glickson
(1984) and Waters-Tormey (2004, 2007) refer to this unit as the Mount Hay Anorthosite
(which Watt (1992) attempted to formally rename as the Amburla Meta-leuconorite). To
avoid further confusion, we shall refer to this unit simply as the anorthositic granulites.
A)
B)
C)D
)
Ham
ilton Dow
nsH
amilton D
owns
Ham
ilton Dow
nsCeilidh H
ill
Figure 4. Photographs showing di�erent types of com
positional layering. A) O
riginal igneous compositional layering (S0a)
shown by m
eter thick anorthosite band containing an isolated tabular layer of pyroxene clots (pencil for scale). Shape-preferred orientation of pyroxene clots (S1) of a L>S fabric is oblique to S0a. B) 3-5 cm
�ne-grained, pure plagioclase band (S0b) cutting SO
a and L>S S1 fabric (pencil for scale, oriented parallel to lineation de�ned by shape-preferred orientation of pyroxene clots). C) 8 m
m seam
of pyroxenite (S0c) cutting through L>>S fabric de�ned by rod like pyroxene clots (aussie dollar for scale). D)
Compositional layering com
mon in Ceilidh H
ill, showing 2 m
m to 5 cm
bands of charnockite within �ne-grained gabbro (pencil
for scale). Strong shape-preferred orientation of felsic segrations perpendicular to lineation make this an L=S fabric.
L=SL>>S
Lineation
L>SLineation
L>S
Lineation
Lineation
12
13
Given the significant difference in composition between the anorthositic
granulites of Hamilton Downs and the mafic granulites of Ceilidh Hill, major changes to
the fabric classification method developed by Hallau (2006) are necessary to accurately
assess the degree and development of fabric throughout Hamilton Downs. Felsic
segregations and compositional layering of charnockite and are absent in Hamilton
Downs and the shape-preferred orientation of plagioclase grains, vital to fabric
classification in Ceilidh Hill, is often weak to non-existent. The degree and development
of the S1 foliation within Hamilton Downs is primarily based on the shape-preferred
orientation of pyroxene grains (Fig. 5). Lineation is likewise defined by shape-preferred
orientation in pyroxene grains (and occasionally, polycrystalline ribbons of plagioclase).
As shown in Figure 6, there is substantial range in texture across Hamilton Downs,
making it difficult to create a universal fabric classification scheme. We used the
following classification method to make field based decisions of fabric development
shown in Figure 3: L=S fabric are not found on Hamilton Downs. L>S fabrics are
characterized by well-defined margins within the S0a compositional layering and a clear
shape-preferred orientation of pyroxene clots perpendicular to the plane of lineation (Fig.
4B). L>>S fabrics are defined by weak shape-preferred orientation of pyroxene
perpendicular to plane of lineation. L-tectonites are difficult to classify, but are most
often characterized by highly elongate pyroxene clots in the direction of lineation with a
complete lack of shape-preferred orientation perpendicular to lineation (Fig. 5A and B,
Fig 6B).
Figure 5. A) Photograph show
ing shape-preferred orientation of 6cm pyroxene clots parallel to lineation (rock ham
mer for scale).
B) Photograph showing sam
e pyroxene clots in the plane perpendicular to lineation (Aussie dollar for scale). Note com
plete lack of shape-preferred orientation, de�ning an L-tectonites fabric. C) Photograph of m
uch more strongly developed shape-preferred
orientation in felsic segregations parallel to lineation in L>S fabric on Ceilidh Hill (pencil for scale). D
) Photograph of same felsic
segregations in the plane perpendicular to lineation showing w
eaker shape-preferred orientation perpendicular to lineation (Aussie dollar for scale).
A)
B)
C)D
)
Ham
ilton Dow
nsH
amilton D
owns
Ceilidh Hill
LineationL-tectonite
L-tectoniteLineation
L>SLineation
Ceilidh Hill
L>SLineation
14
A)
B)
C)D
)
Figu
re 6
. Var
ious
pyr
oxen
e an
d pl
agio
clas
e te
xtur
es fo
und
thro
ugho
ut H
amilt
on D
owns
. A) P
hoto
grap
h fr
om th
e so
uth-
wes
tern
kno
b sh
owin
g co
mpl
ete
lack
of f
abric
in b
oth
pyro
xene
and
pla
gioc
lase
(Aus
sie
dolla
r for
sca
le).
B) P
hoto
grap
h fr
om s
outh
east
ern
knob
sho
win
g la
rge
1-3
cm p
yrox
ene
clot
s w
ith s
tron
g sh
ape-
pref
erre
d or
ient
atio
n pa
ralle
l to
linea
tion
(bla
ck p
enci
l for
sca
le).
C) P
hoto
grap
h fr
om s
outh
wes
tern
kno
b sh
owin
g �n
e-gr
aine
d py
roxe
ne a
nd p
lagi
ocla
se c
ryst
als,
both
with
str
ong
shap
e-pr
e�er
ed o
rient
atio
n pa
ralle
l to
linea
tion
(pen
cil t
ip p
aral
lel t
o lin
eatio
n fo
r sca
le).
D) P
hoto
grap
h fr
om n
orth
ern
knob
sho
win
g py
roxe
ne c
lot s
eggr
egat
ion
into
pyr
oxen
e ric
h an
d py
roxe
ne p
oor l
ayer
s th
at d
e�ne
the
S0a
com
posi
tiona
l lay
erin
g (b
lack
pen
cil f
or s
cale
).
No
Fabr
icLi
neat
ion
L-te
cton
ite
Line
atio
nLi
neat
ion
L>S
L>>S
15
16
The degree and development of foliation and lineation varies substantially across
Hamilton Downs (Fig. 3, Fig 5A, B and C). In contrast to Ceilidh Hill, contacts between
fabric types are occasionally gradual, but more often abrupt. The most southeastern
portion of Hamilton Downs is defined by a weak L>>S fabric striking SW-NE and
dipping steeply an average of 80° (Fig. 3). Fabric changes abruptly from an anorthosite
rich L>S containing very limited, small <20mm pyroxene clots, to L-tectonite defined by
large pencil like rods of pyroxene that range in size from 0.5cm to 6 cm parallel to
lineation with no shape-preferred orientation perpendicular to lineation (Fig 4C). Further
west, pyroxene clots show less overall strain, but display a definite shape-preferred
orientation perpendicular to lineation, and are therefore classified as L>S. The primary
foliation trend changes from a SW-NE orientation (in the L>>S and L-tectonite fabrics)
to an EW orientation (in the L>S fabrics) with lineation plunging an average of 71°. As
shown in Figure 2, this region displays both S0a and S0b compositional banding that is
oblique to the S1 primary foliation.
Near the topographical high point on the southwestern knob of Hamilton Downs,
there are 5 m2 regions that display little to no fabric at all (Fig 3). Crystals of pyroxene
are not consolidated into clots and segregations, and are randomly oriented (Fig. 6A). Just
15 meters north of this knob, there is an abrupt change into course-grained gabbronorite
with a clear shape-preferred orientation parallel and perpendicular to lineation in
individual pyroxene crystals and plagioclase (Fig. 6C). This L>S fabric is cut by multiple
consistently oriented 6.5 cm S0b fine-grained anorthosite bands. The distribution and
17 consolidation of pyroxene crystals is highly variable along the ridge towards Ceilidh
Hill. In a small area of 6m2, it is possible to find randomly oriented, 20 mm clots, as well
as substantially smaller 4-8 mm clots segregated into obvious layers defining the S0a
fabric.
The northernmost knob of Hamilton Downs is characterized by an L>>S fabric
defined most often by the shape-preferred orientation of large, 3-4 cm pyroxene clots
(parallel to lineation) in S0a segregated layers. The trend of the primary foliation is
oriented SW-NE, dipping steeply an average of 79°. Lineation is region is highly
variable, trending NW to NE and plunging an average of 65°.
High Strain Zone
On the western boundary of Hamilton Downs, towards the southeastern most
portion of Ceilidh Hill, S-fabric intensity begins to surpass lineation (Fig. 3). There is an
extensive 1 km wide L=S domain located between the boundary of the southeastern
portion of Ceilidh Hill and Hamilton Downs. This horizon of s-dominated fabric strikes
southwest and dips steeply towards the northwest an average of 78°. Lineation is
consistently well developed trending almost due east and plunging an average of 69°
(Fig. 3).
The lithology of this region differs from that of Hamilton Downs and Ceilidh Hill.
Composition abruptly changes from a heavily weathered course grained gabbronorite
(50/50 plagioclase/pyroxene) to a fine-grained (>1mm grain size) leucogranite, with no
visible pyroxene, but with abundant quartz and large 5mm subhedral garnets (Fig. 7). The
18 steeply pitching lineation is defined by stretched quartz and plagioclase crystals. As
shown in Figure 7, the compositional layering between plagioclase layers and quartz and
k-spar layers is tabular and well defined, everywhere parallel to the primary foliation.
A)
B)
Lineation
Folia
tion
Figu
re 7
. Pho
togr
aphs
from
the
high
str
ain
zone
sep
erat
ing
Ceili
dh H
ill fr
om H
amilt
on D
owns
. A) S
how
s st
rong
line
atio
n in
hig
hly
stre
tche
d qu
artz
and
pl
agio
clas
e po
lycr
ysta
lline
ribb
ons.
Not
e ab
senc
e of
pry
oxen
e an
d la
rge
8mm
euh
edra
l gar
net t
o th
e rig
ht o
f pen
cil.
B) P
lane
per
pind
icul
ar to
line
atio
n,
show
ing
stro
ng L
=S fa
bric
de�
ned
by h
ighl
y st
retc
hed
quar
tz a
nd p
lagi
ocla
se c
ryst
als
and
tabu
lar m
argi
ns o
f S0
com
posi
tiona
l ban
ding
par
alle
l to
prim
ary
folia
tion
(pen
cil f
or s
cale
). N
ote
extr
eme
redu
ctio
n in
gra
in s
ize
com
pare
d to
gra
in s
ize
with
in H
amilt
on D
owns
and
Cei
lidh
Hill
.
19
20 Towards the topography of southeastern Ceilidh Hill, the lithology changes from
sillimanite bearing gneiss to fine-grained gabbro containing the same centimetric felsic
segregations and 3-5 cm charnockite layers that are found throughout Ceilidh Hill.
Despite the gradual change in composition, these rocks continue to be dominated by a
strong S-fabric, and are still classified as L=S. The shape preferred orientation of felsic
segregations and 4 cm layers of S0 charnockite are tabular, showing well defined margins
parallel to the primary S1 foliation. Orientation of foliation changes from southwest to
nearly due west, and begins to dip more shallowly, an average of 68°.
Microstructural and Mineralogical Characterization
To compliment outcrop-scale observations, thin sections were made for
microstructural and mineralogical analysis. Hand samples were collected from a variety
of locations and fabric types within Hamilton Downs and the high strain zone. All thin
sections were cut parallel to lineation and perpendicular to foliation so that the
microstructural elements could be evaluated within the kinematic reference frame.
Analysis of thin sections included detailed notes of mineral assemblage, grain shape and
size, deformation textures, and grain-boundary relationships.
Deformation microstructures of plagioclase are generally in agreement with high
temperature deformation textures described by (Lafrance et al., 1996). Undulose
extinction, tapered deformation twins and healed microfractures are the most common
microstructures indicative of deformation. The deformation twins, which follow the
pericline and albite twin laws, are often gently curved, and terminate at grain boundaries,
tapering to a fine point within the grain (Deer et al., 1992). Larger, primary plagioclase
21 grains are commonly more deformed than smaller recrystalized grains. These
secondary recrystalized grains show straight extinction and are generally absent in
deformation twins (Fig. 8 and Fig. 9). Grain boundaries throughout the study area range
in texture, from very irregular (interlobate to amoeboid in HD09-27), to gently curved,
with common triple point junctions (Fig. 9A and B). The presence of dihedral 120°
anneals suggests that at a minimum, rocks in this region reached partial textural
equilibrium (Lafrance et al., 1996; Passchier and Trouw, 2005). The highly interlobate
grain boundaries and dissected recrystalized plagioclase grains shown in Figure 9 may be
indicative of grain boundary migration. In order to confirm the presence of grain
boundary migration, additional studies of lattice-preferred orientation are needed.
According to (Passchier and Trouw, 2005) the most conclusive evidence of grain
boundary migration is a complete lack of lattice-preferred orientation.
Hamilton Downs:
Field-based assessments of lithology are largely correct and are supported by the
microstructural observations. The composition of samples from Hamilton Downs is
largely anorthositic (containing 5-30% mafic materials), consistent with the field calls of
lithology. Typical mineralogy consists of plagioclase + orthopyroxene + hornblende +
illmenite (Fig. 8). In general, shape fabric in these samples is weak to nonexistent.
Extinction families (groups of grains that are extinct in the same position under cross
polarized light) are not observed (Waters-Tormey, 2004). The more abundant plagioclase
grains are approximately equal in size (0.8-1.2 mm), with grain boundaries that are gently
curved to interlobate. Shape-preferred orientation is weak to non-existent both
22 perpendicular and parallel to lineation. Larger plagioclase grains display undulose to
sweeping extinction. Tapered deformation twins are ubiquitous and often smoothly bent.
Plagioclase grains occasionally exhibit 120° triple junctions (Fig. 8).
Orthopyroxene grains are generally smaller than plagioclase, and range in size from
0.1-0.3 mm, with grain boundaries that are gently curved to interlobate (especially in
contact with plagioclase grains). Rarely, there is a weak shape-preferred orientation of
pyroxene parallel to lineation in the form of football shaped grains. Pyroxene grains
exhibit 120° triple junctions but are less common than in plagioclase. Most pyroxene
grains exhibit characteristic weak pink to green pleochroism. Smaller orthopyroxene
grains commonly form subgrains within plagioclase and other, larger orthopyroxene
grains. These subgrains may be evidence of dynamic recrystalization, and more
specifically bulging recrystalization (BLG), or even high temperature grain boundary
migration (GBM) (micro-tectonics, 2005).
Hornblende grains, like orthopyroxene, are much smaller than the dominant
plagioclase crystals, ranging in size from 0.05-0.3 mm. Grain and phase boundaries are
most commonly straight to gently curved. Hornblende is distinguished from other phases
by its dark green pleochroism and high birefringence.
hbl plag
opx
opx
AB
CD
Para
llel t
o lin
eatio
n
Perpendicular to foliation
plag
opx
Figu
re 8
. P
hoto
mic
rogr
aphs
of s
ampl
es H
D09
-11
and
HD
09-0
4 cu
t par
alle
l to
linea
tion
and
perp
endi
cula
r to
folia
tion.
A)
HD
09-1
1und
er c
ross
-pol
ariz
ed li
ght s
how
ing
inte
rloba
te g
rain
bou
ndar
ies
and
rare
120
° ann
eals
(red
arr
ow).
B) S
ame
area
as
sho
w in
A) u
nder
pla
in-p
olar
ized
ligh
t sho
win
g w
eak
shap
e-pr
efer
red
orie
ntat
ion
in fo
otba
ll sh
aped
ort
hopy
roxe
ne
para
llel t
o lin
eatio
n. C
) HD
09-0
4 un
der c
ross
-pol
ariz
ed li
ght s
how
ing
gent
ly c
urvi
ng ta
pere
d de
form
atio
n tw
ins
and
patc
hy
to u
ndul
ose
extin
ctio
n in
larg
e pl
agio
clas
e gr
ain
(cen
ter)
. Not
e su
bgra
ins
with
in la
rger
ort
hopy
roxe
ne g
rain
s. D
) Sam
e ar
ea
as s
how
n in
C) u
nder
pla
in p
olar
ized
ligh
t sho
win
g pl
eoch
rois
m in
ort
hopy
roxe
ne. M
iner
al a
bbre
viat
ions
from
Kre
tz (1
983)
.
23
24 High Strain Zone:
Two hand samples were collected from the high strain zone separating Ceilidh Hill
from Hamilton Downs. HD09-29 was taken from the transitional fabric on the
southeastern boundary of Ceilidh Hill from what we designated a felsic segregated
gabbro with a strongly developed L=S fabric. As shown in Figure 8A and B, the sample
consists entirely of 50/50% plagioclase/pyroxene composition. Well-developed shape-
preferred orientation is present in both phases, defined by elongate plagioclase grains and
distinctive football shaped pyroxenes. Plagioclase crystals are largely equant size,
ranging from 0.8-1.4 mm. Orthopyroxene crystals are smaller, ranging in size from 0.2-
0.5 mm (lengthwise). Grain and phase boundaries are straight to gently curved, never
interlobate. Grain boundaries are dominated by foam texture (abundant 120° triple
junctions). In contrast to the samples from Hamilton Downs, patchy to undulose
extinction in plagioclase is not observed.
Sample HD09-27 was taken from the middle of the high strain zone, from an
exposure with a particularly well-developed s-fabric (L=S). As shown in Figure 9C and
D), the average grain size is extremely reduced, with the largest quartz and plagioclase
crystals rarely exceeding 0.05 mm. This sample is lithologically distinct from both
Ceilidh Hill and Hamilton Downs, with a composition of quartz + k-feldspar + biotite +
kyanite + (sillimanite) + orthopyroxene + plagioclase + garnet + illmenite. Grain
boundaries of quartz and plagioclase are interlobate to amoeboid, and exhibit patchy to
undulose extinction. Subhedral garnets are strongly associated with sillimanite and
biotite, and are most often found growing around stretched sillimanite and biotite grains.
B
plag
opx
plag
Para
llel t
o Li
neat
ion
Perpendicular to foliation
A CD
opx
qtz
btgr
t
qtz
ky
Figu
re 9
. P
hoto
mic
rogr
aphs
of s
ampl
es H
D09
-29
and
HD
09-2
7, c
ut p
aral
lel t
o lin
eatio
n an
d pe
rpen
dicu
lar t
o fo
liatio
n. A
) HD
09-2
9 un
der c
ross
-pol
ariz
ed li
ght s
how
ing
equa
nt g
rain
siz
e an
d fo
am te
xtur
e (1
20° a
nnea
ls).
B) S
ame
area
sho
wn
in A
) und
er p
lain
-pol
ariz
ed
light
sho
win
g fo
otba
ll sh
ape
of o
rtho
pyro
xene
, ind
icat
ive
of s
hape
-pre
ferr
ed o
rient
atio
n pa
ralle
l to
linea
tion.
C) H
D09
-27
unde
r cr
oss-
pola
rized
ligh
t sho
win
g pa
tchy
to u
ndul
ose
extin
ctio
n in
qua
rtz
(upp
er ri
ght)
and
sha
pe-p
refe
rred
orie
ntat
ion
of q
uart
z pa
ralle
l to
line
atio
n. D
) sam
e ar
ea a
s C)
und
er p
lain
-pol
ariz
ed li
ght s
how
ing
elon
gate
bio
tite.
Not
e ex
trem
ely
redu
ced
grai
n si
ze a
nd in
terlo
-ba
te to
am
oebo
id g
rain
bou
ndar
y in
qua
rtz
and
plag
iocl
ase.
Not
e su
bhed
ral f
ace
of g
arne
t. M
iner
al a
bbre
viat
ions
from
Kre
tz (1
983)
.
25
26 Geothermobarometry
Preliminary work on sample HD09-27 was performed on the Carleton College
Hitachi S-3000N Scanning Electron Microscope (SEM) with an acceleration voltage of
20 kV, and beam current of 98 µA equipped with an Oxford INCA microanalysis system.
Microprobe analysis allowed for detailed mineral identification, confirming the
assemblage identified in thin-section. The peak metamorphic mineral assemblage of
HD09-27 is qtz + kfs + bt + ky + (sil) + opx + plag + grt (Kretz, 1983). The positions in
P-T space of all calculated equilibria are made using the computer program TWQ 2.34
(Berman, 2007) and the thermodynamic database of (Berman, 1991) with revisions
through February, 2007. Reactions were considered in the seven-component system K 2O
-CaO-FeO-MgO- Al2O3- SiO2- H2O (KCaFMASH) (Table 1) allowing for the calculation
of seven reactions, three of which are independent (Fig. 10). Taken together, these
reactions suggest that peak metamorphic conditions recorded by sample HD09-27 range
from ~8-13 kbar and 880-980° C. However, the well documented GASP (Spear,
1993) and garnet-biotite (Ferry and Spear, 1978) geothermometers intersect at 980°C and
11.9 kbar (Fig. 10).
Table 1. Representative mineral analyses for HD09-27
Grt Bt Pl Opx
SiO2 46.78 37.30 57.07 50.64
Al2O3 22.09 15.21 27.15 6.21
TiO2 0.00 5.39 0.00 0.00
FeO 18.55 12.64 0.00 20.71
MgO 10.85 14.67 0.00 22.44
MnO 0.00 0.00 0.00 0.00
CaO 1.73 0.00 9.83 0.00
K2O 0.00 9.80 0.19 0.00
Na2O 0.00 0.00 5.76 0.00
total: 100.00 95.00 100.00 100.00
Basis: 12 O 11 O 8 O 6 O
Si 3.38 2.77 2.56 1.86
Al 1.88 1.33 1.44 0.27
Ti 0.00 0.30 0.00 0.00
Fe2+ 1.12 0.78 0.00 0.64
Mg 1.17 1.62 0.00 1.23
Mn 0.00 0.00 0.00 0.00
Ca 0.13 0.00 0.47 0.00
K 0.00 0.93 0.01 0.00
Na 0.00 0.00 0.50 0.00
cations: 7.68 7.73 4.98 4.00
27
300 400 500 600 700 800 900 1000
8
10
12
14
2 Alm + Gr +
3 aQz
6 Fsl + 3 An
3 Ky + G
r + 3 Fsl
Alm + 3 An
Gr + 2 Ky + aQz
3 An
Alm
+ P
hl
Py
+ A
nn
2 Ann + Gr + 2 Py + 3 aQz
2 Phl + 6 Fsl + 3 An aQ
z + Py + Ann
3 Fsl + Ky + Phl
Temperature (°C)
2
4
6Pres
sure
(kba
r)
Figure 10. Geothermobarometry results for sample HD09-27. Mineral equilibria in the KCaF-MASH system calculated using TWQ (Berman, 1991). The yellow shaded area depicts the likely range of possible P-T conditions for this sample. Al2SiO5 phase diagram is shown for reference.
Phl +
3 K
y + G
r + 3
Fsl
Ann
+ 3
An +
Py
15
And
Sil
Ky
1100
28
29 Discussion
The Hamilton Downs exposure is lithologically and structurally distinct from the
neighboring Ceilidh Hill. Composition is dominated by plagioclase with varying amounts
orthopyroxene (0-30%) and limited hornblend. In contrast to Ceilidh Hill, there is little to
no quartz and no observed felsic segregations or compositional layers of charnockite.
Shape fabric is defined most frequently by the shape-preferred orientation of
orthopyroxene clots. Unlike what we see in Ceilidh Hill, shape-preferred orientation in
plagioclase is weak to non-existent both in field observation and in thin section. Whereas
lineation is mostly consistent in orientation and magnitude throughout Ceilidh Hill, there
are 5 m2 regions within Hamilton Downs that lack a visible macroscopic fabric
measurable in the field. With this abundant evidence, it is reasonable to assert that the
rocks of Hamilton Downs have experienced less deformation than the nearby exposure of
Ceilidh Hill as well as the high-strain shear zone of Capricorn Ridge described by
Waters-Tormey (2004) and Waters-Tormey et al. (2007, in press) and the Mount Hay
sheath fold (Glikson, 1984; Shaw et al. 1984; Staffier, 2007).
The compositions and textures displayed at Hamilton Downs are most similar to
the anorthositic granulites found in the northeastern portion of Capricorn Ridge and
Amburla Folds region (Glickson, 1984; Watt, 1992; Waters-Tormey, 2004; Waters-
Tormey et al., 2007, 2008 in review). Similar to what has previously been documented at
Capricorn Ridge and Amburla Folds, Hamilton Downs displays no observable
migmatization and intrusion by charnockitic compositional layers. The presence of
distinct lithologies of anorthosite in the Mount Hay block that record different structural
30 fabrics likely indicates that compositional differences between the mafic granulites and
the anorthosites also represent rheological differences between these rock types during
deformation and thereby affect the fabrics they record during deformation.
Similar to previous interpretations of Mount Hay and Ceilidh Hill, the fabric types
displayed throughout Hamilton Downs likely express different states and types of strain.
Domains of L-tectonite and L>>S typically imply unidirectional stretching and
constriction whereas L=S fabrics imply flattening and plain strain (Davis and Reynolds,
1996). The distribution of fabric types in the Hamilton Downs region suggests that plane
strains dominated near the boundary with Ceilidh Hill and constrictional strains
dominated the topographical highs found in the middle of Hamilton Downs. These spatial
variations in fabric type may indicate that Hamilton Downs is part of a large-scale sheath
fold.
Alternatively, fabric variation may be indicative of overprinting relationships of
varying strain intensity. Throughout the study area, with the exception of regions
showing little to no fabric, lineation is generally well developed. Relative strain is
therefore estimated by the development of foliation. Regions with strong S-fabrics can be
interpreted as zones of relative higher strain than L>>S and L-tectonite fabrics. The km
wide L=S domain that separates Hamilton Downs from the southeastern tongue of
Ceilidh Hill is therefore inferred to be the region of highest strain. Hallau (2006) suggests
that there is significant overprinting of three separate foliations within Ceilidh Hill. In our
focus area of the southeastern most tip of Ceilidh Hill, two of Hallau (2006) foliation
trends are observed, one striking NE-SW, and a second striking WNW-ESE. The L>>S
31 fabric in the northwestern portion of our study area displays the latter foliation trend
(Fig. 3). On the southeastern boundary of Ceilidh Hill, towards the high strain zone
separating Ceilidh Hill from Hamilton Downs, foliation and compositional layering are
transposed onto a single, strongly developed NE-SW trending S-dominated fabric. This
complete transposition may indicate that the zone of high strain is younger than the
surrounding exposures of Ceilidh Hill and Hamilton Downs.
The zone of high strain may be related to deformation structures of the (400-300
Ma) Alice Springs orogeny found elsewhere in the Mount Hay Block, including the
amphibolite and greenschist facies mylonite zones bounding the northern and southern
portions of Capricorn Ridge. It is important to note however that the mineral assemblage
and geothermobarometry work within this zone of high strain is indicative of much
higher-grade deformation.
Previous geothermobarometry work on the Mount Hay Granulites constrains peak
deformation conditions from 700-900° C and 6.9-8.2 kbar (Collins and Shaw, 1995;
Staffier, 2007; Waters-Tormey et al., in press). Preliminary geothermobarometric data on
sample HD09-27 from the high strain zone records a range from ~8-13 kbar and 880-
980° C, with the well-documented GASP and garnet-biotite geothermometer intersecting
at 980°C and 11.9 kbar (Fig. 10).
Berman (1991) suggests that it is possible to reasonably assess the state of
equilibrium given the convergence of all equilibria in a single P-T region. As shown in
Figure 10, mineral reactions do not intersect at a single point. The intersection of the
GASP and garnet-biotite geothermometer occurs at pressures and temperatures well
32 above peak deformation conditions found elsewhere in the Mount Hay Block. Faulty
microprobe data may be to blame. As shown in Table 2, the mineral garnet in particular
displays unusual cation sums: Si cations should be about 3.0 as opposed to the 3.38, Al
should be 2.0 instead of 1.88, and total cations should add up to 8.0 instead of 7.68.
Scattered reactions may also indicate that the mineral assemblage in sample HD09-27 is
in a state of disequilibrium. Disequilibrium is supported by the presence of kyanite
coexisting with sillimanite, which is difficult to account for, given that the calculated
range of pressure and temperature conditions falls entirely within the sillimanite stability
field (Fig. 10).
The deformation microstructures of samples throughout Hamilton Downs and the
region of high strain are indicative of high-temperature, high pressure conditions present
in the lower crust (Lafrance et al., 1996). Undulose extinction, tapered deformation twins
and healed microfractures are the most common microstructures indicative of
deformation. Microstructures such as the highly interlobate grain boundaries and
dissected recrystalized plagioclase grains present in HD09-27 (Fig. 9C and D) have
previously been used to classify high-temperature grain boundary migration (Passchier
and Trouw, 2005; Staffier, 2007; Waters-Tormey et al., in press). If GBM is indeed the
primary deformation mechanism, it would be an important contribution to the growing
consensus that grain boundary sliding makes a more significant contribution to the bulk
rheology of the lower crust than previously thought (Waters-Tormey, 2004).
33 Conclusions
The primary goals of this research are to fit the Hamilton Downs exposure into the
larger geological context of the Mount Hay Block granulites, to explore how fabric
variation is expressed in the anorthositic granulites of Hamilton Downs and to
characterize the microstructural relationships in an attempt to better understand large-
scale rheology of the lower continental crust. The following results presented in this
paper provide valuable insight into this important question:
1. Hamilton Downs is more weakly deformed than Ceilidh Hill
2. Hamilton Downs is lithologically distinct from the neighboring Ceilidh Hill and is
most similar in composition texture to the northernmost exposure of Capricorn
Ridge, and the Amburla Folds region.
3. Based on the distribution of fabric types, with high strain S-fabrics on the outer
boundaries and constrictional L-tectonites near the central topographical highs,
Hamilton Downs may be part of a larger sheath fold.
4. Separating Hamilton Downs from Ceilidh Hill is a 1 km wide zone of solid state
deformation that we describe as a high strain shear zone
5. The shear zone is most likely younger than surrounding exposures given complete
transposition of both compositional layering and primary foliation from Ceilidh
Hill and Hamilton Downs onto a single L=S fabric trending SW-NE.
6. Preliminary geothermobarometry work on a sample within the shear zone
indicates peak metamorphic conditions in the range of ~8-13 kbar and 880-980°
C. However, presence of kyanite and interlobate grain boundaries is indicative of
34 chemical disequilibrium, thus providing an inaccurate estimation of P-T
conditions. Further study on a professionally polished thin section is required.
7. Deformation microstructures such as interlobate to amoeboid grain boundaries
and dissected recrystalized plagioclase grains may be indicative of grain boundary
sliding, an important component in bulk rheology of the lower crust.
Acknowledgments
I offer my sincerest gratitude to my advisor Sarah Titus, without whom, I’d have never found my way to Australia. Despite being on leave, she couldn’t help but dispense wisdom and cheer. I thank Seth Kruckenberg for taking me on as a field assistant, preparing excellent meals, and telling great stories by the fire. Thank you Basil Tikoff for rustling up funding through the University of Wisconsin-Madison and for keeping me on track. To Bereket for patiently teaching me how to grind and polish thin sections, and perhaps more importantly, forever reminding of the important things in life. Thank you Cam Davidson for the five-minute questions turned to hour long, illuminating conversations. To Tim Vick for graciously putting up with late time cards and messy senior desks. And last but certainly not least, I am forever indebted to the entire Carleton Geology department and all of its students. Thank you for the backrubs, the fresh pressed cider, venison and the willingness to fruit golf. Truly, this experience has made me realize all that is good about Carleton College and it’s people.
References
Berman, R. G., 1991, Thermobarometry using multi‐equilibrium calulations – a new technique, with petrological applications: Canadian Mineralogist, v. 29, p. 833‐855.
Biermeier, C., Wiesinger, M., Stuwe, K., Foster, D. A., Gibson, H. J., and Raza, A., 2003, Aspects of the structural and late thermal evolution of the Redbank Thrust system, central Australia: constraints from the Speares Metamorphics: Australian Journal of Earth Sciences, v. 50, no. 6, p. 983‐999.
Chen, S., O'Reilly, S. Y., Zhou, X., Griffin, W. L., Zhang, G., Sun, M., Feng, J., and Zhang, M., 2001, Thermal and petrological structure of the lithosphere beneath Hannuoba, Sino‐Korean Craton, China: evidence from xenoliths: Lithos, v. 56, no. 4, p. 267‐301.
Christensen, M. I., and Mooney, W. D., 1995, Seismic velocity structure and composition of the continental‐crust: A global view: Journal of Geophysical Research‐Solid Earth, v. 100, no. B6, p. 9761‐9788.
35 Claoue‐Long, J. C., and Hoatson, D. M., 2005, Proterozoic mafic‐ultramafic
intrusions in the Arunta Region, central Australia Part 2: Event chronology and regional correlations: Precambrian Research, v. 142, no. 3‐4, p. 134‐158.
Collins, W. J., and Shaw, R. D., 1995, Geochronological constraints on orogenic events in the Arunta Inlier: a review: Precambrian Research, v. 71, no. 1‐4, p. 315‐346.
Davis, G. H., and Reynolds, S. J., 1996, Structural Geology of Rocks and Regions: New York, John Wiley & Sons, Inc, 776 p.
Deer, W. A., Howie, R. A., and Zussman, J., 1992, An introduction to the rock forming minerals: New York, Pearson Education Limited 696 p.
Dunlap, W. J., and Teyssier, C., 1995, Paleozoic deformation and istopic disturbance in the southeastern Arunta Block, central Australia: Precambrian Research, v. 71, no. 1‐4, p. 229‐250.
Ferry, J. M., and Spear, F. S., 1978, Experimental calibration of partitioning of Fe and Mg between biotite and garnet: Contributions to Mineralogy and Petrology, v. 66, no. 2, p. 113‐117.
Glikson, A. Y., 1984, Granulite‐gneiss terranes of the southwestern Arunta Block, central Australia: Glen Helen, Narwietooma, and Anburla 1:100,000 sheet areas: Bureau of Mineral Resources.
Hallau, D. G., 2006, Fabric variation in granulite tectonites from Ceilidh Hill, Arunta Block, central Australia: University of Wisconsin‐Madison, 22 p.
Hoatson, D. M., Sun, S.‐S., and Claoue‐Long, J. C., 2005, Proterozoic mafic‐ultramafic intrusions in the Arunta Region, central Australia: Part 1: Geological setting and mineral potential: Precambrian Research, v. 142, no. 3‐4, p. 93‐133.
Korsch, R. J., Goleby, B. R., Leven, J. H., and Drummond, B. J., 1998, Crustal architecture of central Australia based on deep seismic reflection profiling: Tectonophysics, v. 288, no. 1‐4, p. 57‐69.
Kretz, R., 1983, Symbols for rock‐forming minerals: American Mineralogist, v. 68, no. 1‐2, p. 277‐279.
Lafrance, B., John, B. E., and Scoates, J. S., 1996, Syn‐emplacement recrystallization and deformation microstructures in the Poe Mountain anorthosite, Wyoming: Contributions to Mineralogy and Petrology, v. 122, no. 4, p. 431‐440.
Passchier, C. W., and Trouw, R. A. J., 2005, Microtectonics: Berlin, Springer 366 p. Rudnick, R. L., and Fountain, D. M., 1995, Nature and composition of the continental‐
crust: A lower crustal perspective: Reviews of Geophysics, v. 33, no. 3, p. 267‐309.
Shaw, R. D., and Black, L. P., 1991, The history and tectonic implications of the Redbank Thrust Zone, central Australia, based on structural, metamorphic and Rb‐Sr isotopic evidence: Australian Journal of Earth Sciences, v. 38, no. 3, p. 307‐332.
Shaw, R. D., Stewart, A. J., and Black, L. P., 1984, The Arunta Inlier ‐ A complex ensialic mobile belt in central Australia: 2. tectonic history: Australian Journal of Earth Sciences, v. 31, no. 4, p. 457‐484.
36 Spear, F. S., 1993, Metamorphic Phase Equilibria and Pressure‐Temperature‐Time
Paths: Washington, D.C., Mineralogical Society of America, 799 p. Staffier, K., 2007, Field and microstructural investigation of plagioclase deformation
in granulite facies rocks, Mt. Hay, central Australia [Master of Science thesis]: University of Wisconsin ‐ Madison, 88 p.
Warren, R. G., and Shaw, R. D., 1995, Hermmansburg, Northern Territory 1:250,000 sheet areas and explanatory notes: Bureau of Mineral Resources.
Waters‐Tormey, C., 2004, Structural geology of a ten‐kilometer scale lower crustal shear zone: Mt. Hay Granulites, central Australia [Doctor of Philosphy thesis]: University of Wisconsin‐Madison, 188 p.
Waters‐Tormey, C., Goodwin, L. B., Tikoff, B., Staffier, K., and Kelso, P., in press, A granulite‐facies normal shear zone exposed in the Arunta inlier of central
Australia: Implications for deep‐crustal deformation during oblique divergence p. 60.
Waters‐Tormey, C., and Tikoff, B., 2007, Characteristics of a kilometer‐scale high strain zone in the lower continental crust: Mt. Hay block, central Australia: Journal of Structural Geology, v. 29, no. 4, p. 562‐582.
Watt, G., 1992, Geology of Mount Hay ‐ Mount Chapple massif (Arunta Block, Hermannsburg 1:250,000 Sheet area, central Australia: Bureau of Mineral Resources, Geology and Geophysics.
Wright, C., Goleby, B. R., Collins, C. D. N., Korsch, R. J., Barton, T., Greenhalgh, S. A., and Sugiharto, S., 1990, Deep seismic profiling in central Australia: Tectonophysics, v. 173, no. 1‐4, p. 247‐256.
Wright, C., Goleby, B. R., Shaw, R. D., Collins, C. D. N., Kennett, B. L. N., and Lambeck, K., 1993, Seismic structure and continuity of the Redbank Thrust Zone, central Australia: BMR Journal of Australian Geology & Geophysics|BMR Journal of Australian Geology & Geophysics, v. 13, no. 4, p. 359‐68.
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