Download Metamorphic and Thermal History of a Fore

JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 7
PAGES 1453–1465
2004
DOI: 10.1093/petrology/egh025
Metamorphic and Thermal History of a ForeArc Basin: the Fossil Bluff Group, Alexander
Island, Antarctica
S. MILLER1,2* AND D. I. M. MACDONALD1,3
1
BRITISH ANTARCTIC SURVEY, NERC, HIGH CROSS, MADINGLEY ROAD, CAMBRIDGE CB3 0ET, UK
2
PRESENT ADDRESS: NATIONAL MUSEUMS OF SCOTLAND, CHAMBERS STREET, EDINBURGH EH1 1JF, UK
3
PRESENT ADDRESS: DEPARTMENT OF GEOLOGY & PETROLEUM GEOLOGY, UNIVERSITY OF ABERDEEN, MESTON
BUILDING, ABERDEEN AB24 3UE, UK
RECEIVED NOVEMBER 27, 2002; ACCEPTED FEBRUARY 13, 2004
The Himalia Ridge Formation (Fossil Bluff Group), Alexander
Island is a 22-km-thick sequence of Upper Jurassic–Lower Cretaceous conglomerates, sandstones and mudstones, derived from an
andesitic volcanic arc and deposited in a fore-arc basin. The
metamorphic and thermal history of the formation has been determined using authigenic mineral assemblages and vitrinite reflectance
measurements. Metamorphic effects include compaction, pore-space
reduction, cementation and dissolution and replacement of detrital
grains by clay minerals (smectite, illite/smectite, corrensite and
kaolinite), calcite, chlorite, laumontite, prehnite, pumpellyite, albite
and mica, with less common quartz, haematite, pyrite and epidote.
The authigenic mineral assemblages exhibit a depth-dependence, and
laumontite and calcite exhibit a strong antipathetic relationship.
Detrital organic matter in the argillaceous layers has vitrinite
reflectance values (Ro) ranging from 23 to 37%. This indicates
considerable thermal maturation, with a systematic increase in
reflectivity with increasing depth. There is good correlation of metamorphic mineral assemblages with chlorite crystallinity and vitrinite
reflectance values—all indicating temperatures in the range of
140 20 C at the top of the sequence to 250 10 C at the
base of the sequence. The temperatures suggest a geothermal gradient
of 36–64 C/km and a most likely gradient of 50 C/km. It is
suggested that this higher-than-average gradient for a fore-arc basin
resulted either from rifting during basin formation or from a latestage arc migration event.
KEY WORDS: Antarctica; diagenesis; fore-arc basin; low-temperature
metamorphism; vitrinite reflectance
*Corresponding author. Telephone: 44 (0)131 247 4007. Fax: 44 (0)131
220 4819. E-mail: [email protected]
INTRODUCTION
Fore-arc basins are major sites of sediment accumulation
at active margins; their position between arc and
accretionary complex makes them prone to conflicting
tectonic influences (Dickinson, 1995). Fore-arc basins are
relatively cool, normally with a maximum geothermal
gradient no more than 30 C/km (Allen & Allen, 1990).
This low thermal gradient is partly because they overlie a
relatively cool accretionary complex and partly because
there is commonly no active mode of basin formation
(Dumitru, 1988, 1990): in their simplest form, they are
‘buckets’ lying in the topographic hollow between the arc
and the outer arc high (trench slope break). Although this
tectonic variation can cause variations in the composition
of clastic material being fed into fore-arc basins, the
essentially volcanically—or plutonically—derived sediments provide an ideal geochemical composition for the
study of thermally driven mineralogical and geochemical
features, such as authigenic mineral development, fission
track thermochronometry and stable isotope chemistry.
In addition, the presence of clastic organic debris shed
from subaerial arcs allows independent checking of other
palaeo-thermometers from vitrinite reflectance.
Before embarking on a study of the low-temperature
metamorphism of the fill of any arc-related basin,
there are several problems that should be confronted.
First, mixing and input processes on the arc (Sigurdsson
et al., 1980) can lead to strong lateral variation in
sediment composition (Macdonald, 1993) and, hence, in
Journal of Petrology 45(7) # Oxford University Press 2004; all rights
reserved.
JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 7
JULY 2004
Fig. 1. (a) Position of Alexander Island; area shown in (b) is shaded. (b) The northern outcrop of the Fossil Bluff Group, showing the outcrop of
the Himalia Ridge Formation and the location of the studied section.
metamorphism. Secondly, many ancient arc-related
basins have undergone very low-grade metamorphism.
Sedimentary rocks are commonly in the zeolite facies,
where the stability field of complex hydrous mineral
assemblages is not well understood. Low-grade metamorphism commonly obscures earlier stages of the paragenesis (Coombs, 1954). Thirdly, many ancient active
margin basin-fill sequences are only partly exposed and
in a highly deformed state (Zyabrev & Bragin, 1987) or
completely dismembered (Sengor and Natal’in, 1996),
making it difficult to assess the burial history prior to
tectonic deformation and uplift. Fourthly, there is little
information on the detailed thermal structure of arcrelated basins.
This is a study of the low-temperature metamorphism
of a single sedimentary sequence from a comparatively
little-deformed fore-arc basin of Mesozoic age in
Antarctica. A carefully planned sampling policy was
designed to allow related factors, such as grain size, facies
and stratigraphic height, to be assessed independently.
GEOLOGICAL SETTING
During Mesozoic times, the Antarctic Peninsula was the
site of an active volcanic arc, related to the eastwards
subduction of proto-Pacific oceanic crust (Suarez, 1976;
Pankhurst, 1982; Thomson et al., 1983; Leat et al., 1995).
Following Tertiary migration of the arc westward into the
inboard portion of the accretionary complex (Burn,
1984), subduction ceased as a result of a series of
Cenozoic ridge–trench collisions, which began off
Alexander Island at 50 Ma and became progressively
younger to the north (Barker, 1982; Larter & Barker, 1991).
Alexander Island (Fig. 1) is the largest of the many
islands that lie on the western (fore-arc) side of the
Antarctic Peninsula; it forms one of the best-exposed
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METAMORPHIC AND THERMAL HISTORY OF A FORE-ARC BASIN
Fig. 2. Lithostratigraphy of the Fossil Bluff Group (after Butterworth
et al., 1988; Doubleday et al., 1993; Moncrieff & Kelly, 1993). Ages of
stage bases in Ma from Harland et al. (1990).
ancient fore-arcs in the world (Doubleday et al., 1993).
The pre-Tertiary rocks can be divided into two main
units. The LeMay Group ( Jurassic–Tertiary; Burn,
1984; Thomson & Tranter, 1986) forms the structural
basement to Alexander Island and comprises greenschist-facies metasedimentary rocks. It is interpreted as
a Mesozoic accretionary prism (Doubleday et al., 1993;
Storey et al., 1996). The Fossil Bluff Group unconformably overlies and is faulted against the LeMay Group; it
represents the sedimentary fill of a coeval fore-arc basin
(Butterworth, 1985; Macdonald & Butterworth, 1990;
Moncrieff & Kelly, 1993).
The fore-arc basin deposits of the Fossil Bluff Group
crop out in eastern Alexander Island; they range in age
from Middle Jurassic to latest Early Cretaceous (Albian)
and have a total stratigraphic thickness in the order
of 7 km. The group consists of seven formations (Fig. 2).
The Selene Nunatak and Atoll Nunataks formations
(Bathonian–Tithonian) are clastic units, derived from
the accretionary complex, recording the transition in
time from trench-slope to fore-arc basin sedimentation
(Doubleday et al., 1993). The upper formations represent
a large-scale, shallowing cycle of Kimmeridgian to Albian
age, with a magmatic-arc provenance.
Of the seven formations, only the Himalia Ridge Formation is present basin-wide (Butterworth et al., 1988;
Crame & Howlett, 1988; D. I. M. Macdonald, unpublished data) and is the source of material for the work
recorded here. At its type locality (Himalia Ridge on
Ganymede Heights; Fig. 1), it is 22 km thick; elsewhere,
it is about 500–1000 m thick. The Himalia Ridge
Formation at Himalia Ridge consists of four major
conglomerate units associated with immature arkosic
arenites and subordinate lithic arenites and interbedded
with thick units of mudstone. Westerly palaeocurrents
indicate that sediments were derived from the magmatic
arc. There are also two thin andesitic flows at the base
of the formation. The formation was deposited as a
series of migrating conglomerate-filled inner-fan channels, with interchannel mudstone and sandstone facies
(Butterworth, 1988).
The plutonic content of the lithic arenites increases
towards the top of the formation; this is related to unroofing of the Antarctic Peninsula magmatic arc to the east
(Butterworth, 1985). The depositional setting of the forearc basin was discussed by Butterworth (1985) and
Butterworth & Macdonald (1991), and the formal lithostratigraphy was defined by Butterworth et al. (1988),
Doubleday et al. (1993) and Moncrieff & Kelly (1993).
The basin was inverted within a dextral strike-slip regime
in the mid-Cretaceous. Most strike-slip movement was
accommodated on the LeMay Range Fault (Fig. 1; Nell &
Storey, 1991); fore-arc basin strata adjacent to this fault
are near vertical, with dips shallowing eastward into a
broad synclinal monocline. There was associated thrusting, oblique to the axis of the basin and created by dextral
transpression (Doubleday & Storey, 1998). There are
three of these major thrusts, all in the central part of the
basin. Despite the presence of these thrusts, the majority
of the Fossil Bluff Group is only moderately deformed.
At its type section, the Himalia Ridge Formation is
exposed as a single sequence, dipping southeast at about
30 (Fig. 3a). The upper part of the formation is repeated
by the most northerly of the ENE-directed thrusts. There
have been no previous diagenetic studies, but, based on a
study of thin sections of samples from the Himalia Ridge
Formation, Elliot (1974) reported the presence of authigenic laumontite, prehnite, calcite and chlorite.
MATERIALS AND METHODS
Sampling
One hundred and twenty-five samples from mudstone,
sandstone and conglomerate units of the Himalia Ridge
Formation were selected from the collections of the
British Antarctic Survey. The sampling strategy was
designed to test the effects of grain size, depositional facies
and stratigraphic height on the mineral paragenesis.
Sandstone samples were selected in groups of three to
six from a particular stratigraphic height. These groups
contained the range of grain sizes found at that height,
plus representatives from each of the major facies associations. In addition, a suite of mudstone samples was collected for vitrinite analysis to provide parallel but
independent evidence of the thermal maturity of the
succession. Figure 3b shows the main sedimentological
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section through the Himalia Ridge Formation, with the
positions of the samples indicated.
Methods of investigation
Thin sections of all the samples were examined and point
counting of the sections was completed to determine
detrital sandstone composition. Four hundred points
were counted per section.
Freshly fractured rock fragments were viewed under
the scanning electron microscope (SEM) with semiquantitative mineral analysis by EDAX.
The semi-quantitative mineralogy of the cleaned and
crushed bulk samples was determined by X-ray diffraction (XRD). Results were compared with the XRD traces
of standard mixtures of the major mineral phases in the
rocks (identified by optical methods).
The <2 mm fraction of 34 of the mudstone and sandstone samples was separated by gravity settling according
to Stokes’ Law, smeared onto glass slides and analysed by
XRD, using Ni-filtered Cu–Ka radiation. Clay minerals
were identified by comparing the reaction of the major
peaks following XRD analysis of the samples that were
air-dried, glycerolated, and heated to 440 C and 550 C.
Results were compared with standard tables produced by
Brindley & Brown (1980) and Elsinger & Pevear (1988).
The chlorite ‘crystallinities’ (ChC) of selected mudstones
from the Himalia Ridge Formation were also determined.
In order to determine thermal maturity, vitrinite reflectance measurements were made on the organic material
extracted from 11 mudstone and siltstone samples,
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selected from various depths in the succession (Table 1).
Organic concentrates were obtained by standard
demineralizing techniques, involving HCl and HF. The
residues were mounted and polished according to the
method of Hillier & Marshall (1988). A Zeiss UMSP
microscope was used to measure the mean vitrinite reflectance of samples under oil immersion.
Results of fission track age determinations (Storey et al.,
1996) and oxygen isotope measurements from authigenic
cements and replacement phases (Kelly et al., 1995) were
used to compare the temperatures indicated by the
petrological studies described in this work.
RESULTS
Sandstone petrology
The Himalia Ridge Formation largely comprises clastic
sedimentary rocks locally containing interbedded volcanic rocks. Sandstones range in particle size from very fine
to very coarse sand grade. Most of the sandstones are
poorly sorted and grains are generally sub-rounded to
angular, although fragments in the very coarse-grained
rocks are commonly rounded.
The detrital mineralogy is relatively simple, consisting
mostly of plagioclase and recognizable volcanic and plutonic fragments, with minor quantities of quartz, biotite,
muscovite, hornblende, augite, zircon, sphene, apatite
and allanite. The relative proportions of these constituents, especially volcanic and plutonic rock fragments,
vary depending on stratigraphical position of the unit; the
(a)
Fig. 3. The Himalia Ridge Formation at Ganymede heights. (a) Photograph of the main section on Himalia Ridge, looking south; the bases of the
four conglomerate units are arrowed and the position of the thrust at the top of the section is shown. (b) Sedimentological log through the Himalia
Ridge Formation at Himalia Ridge (after Butterworth, 1985), showing the sample positions for XRD (s) and vitrinite reflectance (v).
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(b)
Fig. 3. Continued
proportion of plutonic fragments increases upwards at
the expense of the volcanic component (Butterworth,
1991).
Sandstones were classified according to Folk (1968);
most are arkosic arenites with less common lithic arenites.
All lithic arenites plot within the volcanic arenite field in
the rock fragment daughter triangle. Andesitic rock
fragments are by far the most common volcanic rock
fragments, although minor basaltic and pumice fragments were also observed and some volcanic fragments
contain plagioclase laths set in a groundmass of volcanic
glass. Plutonic rock fragments are mostly of granodioritic
composition.
Kinked biotite flakes and strained plagioclase
and quartz provide evidence of post-depositional
crushing.
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Table 1: Vitrinite reflectance data for
samples from the Himalia Ridge Formation
Sample no.
Height1
n
Mean IR
SD
3.62
3.32
3.71
0.73
0.52
KG2869.18
50
80
KG2877.9
225
40
KG2916.2
50
36
KG2924.2
800
36
KG2933.2
1500
39
KG2937.1
1700
41
KG2941.1
1000
38
KG2944.1
2150
40
KG3217.25
1700
28
3.54
3.18
3.35
3.65
2.46
2.98
0.89
0.45
0.36
0.43
0.47
0.34
0.26
1
Height above base of section (m).
n, number of measurements.
The unit features extensive development, sporadic distribution and selective growth of secondary minerals in
pore spaces, fractures and within vesicles in volcanic rock
fragments; secondary minerals have also grown after
primary plagioclase, hornblende and pyroxene. This
porosity-controlled mineral growth, together with compactional features, such as bending or kink-banding in
micas, and the lack of evidence of strong shear stress or
of widespread penetrative deformation indicates that
the sequence has undergone low-temperature burial
metamorphism.
Authigenic mineral assemblages
We were able to verify the composition of authigenic
cements and replacement minerals within the succession
from both sandstone petrography and XRD analyses of
sandstones and mudstones from the formation. The
cement phases range from clay coats (smectite, corrensite,
mixed illite/smectite and minor kaolinite) on detrital
grain surfaces to complete pore-fill by calcite, laumontite,
prehnite and chlorite with much less common silica.
Replacement of relict phases, particularly plagioclase,
by laumontite, prehnite, calcite or albite is common.
Grains vary from being embayed to nearly totally dissolved. Clinopyroxene is commonly chloritized. Other
authigenic phases include pumpellyite, pyrite, haematite
and epidote. There was no obvious correlation between
authigenic mineralogy and features such as sandstone
grain size or depositional facies. Timing of these phases
is discussed further below.
Clay minerals
Clay mineral rims, 50–150 mm thick, are present on
volcaniclastic lithic grains and complete pore-fill by
fibrous chlorite is common, particularly in the tuffaceous
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sandstones. Five separate clay mineral phases were identified, with the following characteristics:
1. Changes in the shape of the 10 Å maxima were
attributed to expandable material that is referred to
as illite/smectite.
2. Smectite was identified on XRD traces by a variable
basal reflection between 14 and 15 Å. This peak
collapsed on heating at 440 C to approximately
125 Å, with loss of the peak on heating to 550 C.
The minor quantities of illite/smectite and smectite in
the sandstones appear as thin coats and rim cements
on volcaniclastic grains.
3. Chlorite was recognized by XRD peaks at approximately 14, 7 and 35 Å; these peaks are unaffected by
glycerolation and heating to 440 C, but disappear on
heating to 550 C. Chlorite was also identified in thin
section and under the SEM. The chlorite crystallinity
ChC(001) is the half-height width of the 14 Å XRD
peak and the ChC(002) is the half-height width of the
7 Å XRD peak. Authigenic chlorite was recognized
throughout the succession, in thin section, under the
SEM and by XRD, commonly present as pore-filling
intergranular cement exhibiting a radiating fibrous
fabric or as an alteration product of detrital
hornblende, of volcanic glass and, to a lesser extent,
of ferromagnesian minerals. The 7 and 35 Å peaks
may also indicate kaolinite, but it was recognized
under the SEM in two samples only.
Because no pure illite was found in the succession,
it is possible to use chlorite ‘crystallinities’ (ChC) as an
alternative to illite ‘crystallinity’ as a measure of
the metamorphic grade of a rock (Arkai, 1991). This
method is of particular use in units where illite–
muscovite is lacking. Measurements of ChC have been
undertaken infrequently (e.g. Duba & Williams-Jones,
1983; Weaver et al., 1984; Arkai, 1991), but such studies
have shown that there is generally a good linear relation between ChC and illite crystallinities (Frey, 1987).
The Himalia Ridge Formation mudstones exhibit
ChC(001) values of between 045 and 065 (D 2q),
and ChC(002) values between 03 and 05 (D 2q).
4. Corrensite (low-charge mixed chlorite/smectite) was
identified by the 001 peak between 29 and 31 Å in the
untreated samples, which expands to 32 Å on glycerolation. On heating, the diffraction pattern is much
reduced and diffuse peaks at approximately 13 and 8 Å
represent the 002 and 003 peaks, respectively. The
variable basal reflection for the air-dried samples is at
14–15 Å, expanding on glycerolation to 15–177 Å,
and collapsing on heating to 14 Å (typically producing
an asymmetric peak, skewed towards the high 2q side).
5. Mica-group minerals were recognized by a basal
spacing at 99 Å, with higher-order spacings at about
50 and 33 Å, which are unaffected by the routine
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METAMORPHIC AND THERMAL HISTORY OF A FORE-ARC BASIN
treatments. Commonly, the 50 Å peak was weak or
absent, indicative of Fe-rich micas, and biotite was
seen to replace plagioclase grains under the SEM.
Carbonates and aluminosilicates
Calcite is a common early cement phase in the sandstones
and mudstones and also replacive of a number of early
phases, with a number of the samples comprising 60–70%
calcite (whole rock). These early cements occur as porerim cements, but more usually as pore-fill cements exhibiting micritic or granular poikilotopic fabrics. This calcite
appears to have dissolved and embayed many of the
detrital grains, and may have removed finer interstitial
fractions, resulting in the clean, well-sorted appearance of
the detrital fraction of the calcite-rich units.
Prehnite is recognized by its optical properties and
under the SEM. The diffraction peak at 38 Å was attributed to prehnite. It is widely, although not abundantly,
distributed throughout the arkosic sandstones and occurs
as granular patches, radiating fibres in pore spaces and as
grains within plagioclase crystals. Prehnite was also precipitated in narrow veins, cutting the bedding of sandstones and mudstones. The association of prehnite and
calcite is common.
Calcite and prehnite are commonly intergrown, exhibiting sutured grain boundaries. Prehnite is also found
replacing plagioclase grains, particularly the calcic cores
of the grains.
Laumontite is the only zeolite recognized in the
Himalia Ridge Formation sandstones, identified by
optical methods, under the SEM, and by XRD. It occurs
as both a cement phase that forms discrete patches, and
as an alteration product of plagioclase grains.
Pumpellyite had not previously been recognized in these
rocks, but was identified in minor quantities in thin section in two samples from the base of the section. It occurs as
pale brown–green pleochroic, fine-grained radiating crystals, replacing plagioclase grains, associated with prehnite.
Albite commonly replaces plagioclase grains (and
phenocrysts in volcanic fragments) in association with
laumontite.
Distribution of diagenetic phases
Authigenic mineral distribution
Depth variation of metamorphic features ranges from
moderate to strong changes down section in the Himalia
Ridge Formation; the distribution of authigenic minerals
with stratigraphic height is shown in Fig. 4. All elevations
are given as metres above the base of the formation. All
grades and depositional facies of sandstone at any one
stratigraphic height had identical authigenic mineralogy,
with the exception of calcite and laumontite, which
exhibit an antipathetic relationship, as discussed below.
Fig. 4. Distribution of authigenic minerals with stratigraphic height in
the Himalia Ridge Formation.
It is, therefore, concluded that the major control on
metamorphic products was burial depth.
Chlorite rim and pore-fill cements were observed
throughout the section, although chlorite is scattered in
its distribution. Smectite rim cements were observed
above 1300 m, whereas corrensite was observed between
500 and 1400 m. The mixed illite/smectite phase was
recognized in mudstones above 600 m. Kaolinite was
observed in the upper 350 m of the section only.
Laumontite, prehnite and calcite were observed
throughout the succession, although, as mentioned
above, laumontite and calcite are seldom found in the
same specimens and thus exhibit an antipathetic relationship. Maximum laumontite contents generally decrease
with increasing height in the succession (Fig. 4), with
samples from the base of the formation containing up to
40% laumontite (whole rock). The maximum calcite contents (up to 70% whole rock) do not exhibit any obvious
relationship to depth but are generally found in mudstone
units, possibly a result of increased organic content of
such units compared with volcaniclastic sandstones. This
observation is consistent with the suggestion of Surdam &
Boles (1979) that the amount and distribution of organic
matter in sediments will be the limiting factors in diagenetic carbonatization reactions.
The alteration of plagioclase grains to albite does not
appear to be depth-related, whereas both pumpellyite
and mica were recognized only in the lowermost 200 m
of the succession.
A plot of ChC(001) and ChC(002) versus depth in the
succession is shown in Fig. 5. The ChC values generally
increase from the base to the top of the section (i.e.
increasing crystallinity with increasing depth).
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Fig. 5. Variation in chlorite crystallinity with stratigraphic height in
the Himalia Ridge Formation.
Fig. 6. Paragenetic sequence inferred for the authigenic minerals of the
Himalia Ridge Formation.
Authigenic mineral paragenesis
reduced porosity in rocks that have abundant early
calcite cement, inhibiting the mobility of pore fluids
from which the later laumontite is thought to have
been precipitated. Hoare et al. (1964) and Hay (1966)
have recorded other examples of an inverse relationship between authigenic silicates and calcite cement.
The degree of pore-fill by laumontite generally
decreases with increasing stratigraphic height in the
Himalia Ridge Formation succession.
4. Replacement of detrital grains by calcite, laumontite
or prehnite appears to have occurred concurrently
with dissolution of the grains and precipitation of
these pore-fill cements.
The distribution of diagenetic changes permits the reconstruction of a mineral paragenetic sequence for the
Himalia Ridge Formation (Fig. 6). Most of the observed
changes show a dependence on stratigraphic height in the
section. Although different mineral phases are present at
different heights in the succession, there is a common
order of formation from rim cements, to radiating porefill, to poikilotopic pore-fill. Grain replacement occurred
in parallel with these stages.
1. The early rim cements comprise calcite, smectite and
chlorite. Smectites occur in units above 1200 m,
whereas calcite and chlorite rim cements are present
throughout the succession.
2. The next phase of diagenesis is radiating pore-fill
cement. Fibrous chlorite is present throughout the
section. Pore-fill smectite is present up to 1300–
1400 m; corrensite occurs between 1300 m and
approximately 1700 m. Minor quantities of mixed
illite/smectite are also present in the mudstones
above 600 m, with the appearance of mica at around
200 m.
3. The clay rim and early pore-fill cementation was
followed by precipitation of either calcite pore-fill and
replacement cement, or laumontite pore-fill (particularly in silica-rich, calcite-poor rocks such as tuffaceous sandstones). These cements significantly reduced
porosity. This is consistent with the observation that
zeolites require a high silica concentration in solution
to form (Hay, 1966). The antipathetic relationship
between laumontite and calcite may be due to
Vitrinite reflectance
The samples for vitrinite reflectance measurements were
taken from the localities indicated in Fig. 3b. Vitrinite
reflectance results for the Himalia Ridge Formation are
presented in Table 1. There appears to be little reworked
material in the selected mudstones (each sample
exhibiting fairly strong unimodal distributions). The
reflectance–depth plot shown in Fig. 7 is a composite
sequence profile for the Himalia Ridge Formation. The
average reflectance in oil for the formation varies from
23 to 37%. According to the calibrations of other
workers (e.g. Castano & Sparks, 1974; Teichmuller,
1987) these reflectances are indicative of semi-anthracite
to anthracite coal rank.
There is an overall decrease in vitrinite reflectance
from the base of the formation to the top. However, this
trend is poorly developed and is not linear through the
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MILLER AND MACDONALD
METAMORPHIC AND THERMAL HISTORY OF A FORE-ARC BASIN
100–150 C. The occurrence of mica in the mudstones
at the base of the section indicates maximum temperatures of around 250 10 C. The authigenic mineral
assemblages in the sandstones (i.e. laumontite or calcite,
with prehnite, epidote, albite, smectite, chlorite and corrensite) are common low-temperature metamorphic
mineral assemblages of andesitic volcanics (Liou et al.,
1987). The Himalia Ridge Formation sandstones are
therefore akin to the ‘volcanics’ of Hoffman & Hower
(1979). Comparisons of the authigenic assemblages of the
sandstones with the phase diagrams of Liou et al. (1987)
indicate minimum temperatures of 150–180 C towards
the top of the section.
Vitrinite reflectance and burial history
Fig. 7. Variations in vitrinite reflectance with stratigraphic height in
the Himalia Ridge Formation.
succession. This may be due to the undetected reworking
of vitrinite from other sources. Alternatively, the vitrinite
records peak temperatures attained after deformation,
relating more to topographic height. This cannot be
determined for the Himalia Ridge section, as we do not
have an accurate record of the topographic heights of the
samples.
INTERPRETATION OF AUTHIGENIC
MINERAL ASSEMBLAGES AND
VITRINITE REFLECTANCE
Authigenic mineral assemblages
The mineral assemblages presented here define two
metamorphic grades, based on the classic work on metapelitic assemblages (e.g. Coombs, 1954; Utada, 1965;
Seki, 1969, 1976; Kawachi, 1975). The uppermost
2000 m belong to the zeolite facies, whereas the bottom
200 m of the Himalia Ridge Formation are prehnite–
pumpellyite grade. However, there are more subtle variations that are obscured by the somewhat simplistic
application of facies names; Surdam & Boles (1979)
have made a similar point.
Chlorite, smectite and chlorite/smectite dominate the
diagenetic clay mineral assemblages. The occurrence of
similar clay assemblages in deep wells and geothermal
systems offers perhaps a provisional estimate of the minimum temperature of formation. The correlation of
mineral alterations and temperatures of Hoffman &
Hower (1979) would suggest minimum temperatures
for the Himalia Ridge Formation in the range of
It is widely recognized that temperature is the critical
factor in the process of coalification of organic material
and that the rate of rise in temperature, and therefore
increase in rank, with depth depends on the geothermal
gradient. Typical coalification temperatures for semianthracites and anthracites are in the range of 150–
250 C (Teichmuller, 1987). However, in addition to
temperature, the reactions involved in the thermal
maturation of organic matter are also time-dependent
(Bostick, 1973; Castano & Sparks, 1974; Lopatin &
Bostick, 1974; Bostick et al., 1978; Waples, 1980;
Teichmuller, 1987). In order to estimate metamorphic
temperatures from vitrinite reflectance measurements, it
is necessary to estimate the burial path and duration of
the maximum heating phase (Buntebarth & Stegena,
1986). Figure 8 shows the decompacted burial curve for
the whole Fossil Bluff Group, based on published stratigraphic work (Butterworth, 1988; Crame & Howlet,
1988; Butterworth & Macdonald, 1991; Kelly &
Moncrieff, 1992; Doubleday et al., 1993; Moncrieff &
Kelly, 1993) and on unpublished field observations; the
burial history is based on the timescale of Harland et al.
(1990). This curve is based on the maximum thickness of
each formation. It suggests that the Himalia Ridge
Formation, which was deposited between earliest
Tithonian and latest Berriasian times (152–135 Ma;
Harland et al., 1990), was buried to between 45 and
7 km by the end of sedimentation in latest Albian times
(97 Ma). Thus, the base of the formation has been buried
for at least 55 Myr and the top for at least 38 Myr. Regional geological evidence suggests that central Alexander
Island was exhumed by about 50 Ma—a conclusion
supported by fission track studies (Storey et al., 1996).
By using the curves of Sweeney & Burnham (1990), we
find that the temperature for Ro of 23% (equivalent to
semi-anthracite coal rank) and a time factor of 41 Myr is
160 C. Similarly, the temperature for Ro of 37%
(equivalent to anthracite coal rank) and a time factor of
55 Myr is 235 C.
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NUMBER 7
JULY 2004
Fig. 8. Decompacted burial curve for the Fossil Bluff Group, based on the stratigraphic information given by Butterworth et al. (1988), Crame &
Howlet (1988), Kelly & Moncrieff (1992), Doubleday et al. (1993) and Moncrieff & Kelly (1993). Absolute ages from Harland et al. (1990).
However, the less-than-perfect correlation between
vitrinite reflectance and stratigraphic height in the
Himalia Ridge Formation samples has implications for
the timing, and therefore duration, of heating. Poor correlation between vitrinite reflectance and stratigraphic
height has also been noted in other parts of the Fossil
Bluff Group by Doubleday & Storey (1998), who inferred
that iso-reflectance contours cut across bedding. Alternatively, isotherms from a localized asymmetric heat source
could cross-cut burial depth contours. Either scenario
suggests that the strata were deformed prior to the time
of maximum heating, with further tilting occurring after
that time.
Correlation among authigenic mineral
assemblages, vitrinite reflectance and
chlorite crystallinities
Correlation of authigenic mineral assemblages, vitrinite
reflectance and chlorite crystallinities for the Himalia
Ridge Formation is shown in Fig. 9. The upper part of
the Himalia Ridge Formation lies within the laumontite
zone of the zeolite facies, with a progression downwards
in the sandstones from laumontite (or calcite) þ
prehnite þ chlorite assemblages to laumontite þ
prehnite þ pumpellyite þ chlorite assemblages at the base
of the succession. The transition between laumontite–
prehnite and laumontite þ prehnite þ pumpellyite
assemblages in the sandstones corresponds to Ro values
of about 35% and thus to probable maximum
temperatures of 250 C (Liou, 1979). The occurrence of
mica in the mudstone clay mineral assemblages also
indicates maximum temperatures of approximately
250 C. These observations are in good agreement with
the findings of Diessel & Offler (1975) in their study of
relationships between mineral metamorphic grade and
organic catagenesis. The chlorite crystallinities are similar to those found by Arkai (1991) in his ‘diagenetic zone’,
which he correlated to the laumontite zone for metabasites. There is general agreement for minimum and
maximum temperatures for the Himalia Ridge Formation between the authigenic mineral assemblages and the
vitrinite reflectance.
The metamorphic temperatures estimated are corroborated by fission-track results from Himalia Ridge Formation samples (Storey et al., 1996). The apatite fission
tracks are totally annealed, indicating that temperatures,
even at the top of the section, were in excess of 105 C.
The zircon tracks, however, are only partially annealed,
limiting the maximum possible temperatures at the base
of the formation to around 240 C. The apatite ages vary
between 27 and 62 Ma, with temperatures of less than
40–50 C since 40 Ma (Storey et al., 1996). The zircon
ages reflect cooling after about 100 Ma, with no variation
in ages between the sampled areas (Storey et al., 1996).
These observations are consistent with maximum heating and cooling post-dating deformation. In addition,
oxygen and carbon isotope data from late-stage calcite
cements and replacement phases of selected samples from
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METAMORPHIC AND THERMAL HISTORY OF A FORE-ARC BASIN
°C/
C/k
36 °
m:
: 64
km
imu
um
xim
Min
Ma
m
Most likely palaeo
geothermal gradient:
50 °C/km
Fig. 9. Correlation of various thermal indicators for the Himalia Ridge Formation. Cc: calcite; Ch: chlorite; Corr: corrensite; I/S: illite/smectite;
Lm: laumontite; Mc: Mica group; Pm: Pumpellyite; Pr: Prehnite; Sm: Smectite.
the Himalia Ridge Formation indicate precipitation temperatures in excess of 120 C (Kelly et al., 1995).
The maximum and minimum temperature ranges
calculated for the Himalia Ridge Formation yield a
geothermal gradient for the sequence of between 36
and 64 C/km (most likely 50 C/km) that is rather
higher than the average burial geothermal gradient
(20–25 C/km; Frey, 1987). Although high heat flow is
common in basins with large sediment thickness
(Watanabe et al., 1977), fore-arc basins tend to be
hypothermal, exhibiting 30–70% of average heat flow
values (average of 35 mW/m2; Allen & Allen, 1990).
The data presented here indicate that the Himalia
Ridge Formation was subjected to a much higher heat
flow than would have been expected for such a tectonic
setting. Indeed, given that the iso-reflectance contours for
the Fossil Bluff Group as a whole indicate that maximum
heating occurred after the strata were folded, the calculated geothermal gradients are minimum values.
There are three possible causes for the anomalously
high heat flow. First, it has been suggested that, unlike
other fore-arc basins, there was an active rift mechanism
within the fore-arc basin, coeval with deposition of
the Himalia Ridge Formation (Macdonald et al., 1999).
A second possibility is connected with arc relocation from
the Peninsula area to western Alexander Island, which
occurred during early Tertiary times (Pankhurst, 1982;
Hole et al., 1991). This would result in increased heat flow
in the fore-arc area and may therefore explain the higher
metamorphic gradient in the fore-arc basin. Finally, the
presence of Cenozoic high-magnesian andesites in
central Alexander Island (McCarron & Smellie, 1998)
suggests that magmatism in the area was a result of
ridge subduction, with successive ridge–trench collisions,
producing a temporal migration of the magmatism and
therefore producing high geothermal gradients in an
anomalously hot fore-arc region. The evidence for peak
temperatures being attained after deformation suggests
that the last two mechanisms may be more important.
CONCLUSIONS
The mineralogy of the mudstones and volcaniclastic
sandstones from the Himalia Ridge Formation indicates
exposure to diagenetic/low-temperature conditions.
Diagenetic/metamorphic assemblages (laumontite or
calcite with prehnite, chlorite and smectite) are found in
rocks towards the top of the sequence, which underwent
less burial. The effects of diagenesis increase from the top
towards the base. Corrensite-bearing clay assemblages
are present in an intermediate zone in the laumontite
facies. The highest grades are recorded in the older strata,
where laumontite or calcite with prehnite, chlorite and
pumpellyite have formed in the sandstones, whereas
laumontite or calcite with prehnite, chlorite and mica
dominate the authigenic assemblages in the mudstones.
The interpretation from these mineralogies is that the
probable diagenetic/metamorphic temperature range
was between 140 20 C and 250 10 C. This temperature range is in agreement with the temperatures
indicated by vitrinite reflectance of between 23 and
37% Ro. The temperature range indicates a geothermal
gradient that is certainly in excess of 36 C/km and, more
probably, around 50 C/km.
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The observations on the thermal history of the Himalia
Ridge Formation indicate that the top of the section was
buried by 3–35 km of younger sediment—rather less
than the maximum possible overlying sediment thickness
of close to 5 km (determined from the total stratigraphical
thickness of the Fossil Bluff Group). Examination of the
burial history of the Himalia Ridge Formation thus provides evidence that sediment thickness varied considerably across the basin.
It is also evident that, although the authigenic/
metamorphic mineralogy is stratigraphically controlled,
suggesting burial metamorphism, the maximum heating
of the sequence was related not to maximum burial depth
but to a late-stage thermal event: arc relocation and/or
ridge subduction.
ACKNOWLEDGEMENTS
We gratefully acknowledge the field mapping and use
of samples collected by Drs P. J. Butterworth and
P. J. Howlett, British Antarctic Survey, and the helpful
comments of Drs P. A. Doubleday and M. R. A. Thomson.
We thank Dr C. V. Jeans for his assistance with the XRD
analyses. We are especially grateful to Ray Ingersoll and
Ian Moxon for their constructive reviews.
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