Download Thermocronology Age Determinations, Using Apatites and Zircons

UNIVER SIDAD
DE
CONCEPCIÓN
DEPARTAMENTO DE CIENCIAS DE LA TIERRA
10° CONGRESO GEOLÓGICO CHILENO 2003
FISSION TRACK THERMOCHRONOLOGY OF THE WESTERN
ANTARCTIC PENINSULA AND SOUTH SHETLAND ISLANDS: A
PROGRESS REPORT ON NEW ZIRCON DATA.
FAÚNDEZ, V.1; BRIX, M. R.2; HERVÉ, F.1; THOMSON, S. N.3; STÖCKHERT, B.2; and
LOSKE, W.4
1
Departamento de Geología, Universidad de Chile. [email protected], [email protected]
Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Germany. [email protected], [email protected]
3
Department of Geology & Geophysics, Yale University, New Haven, USA. [email protected]
4
Department Geo- und Umweltwissenschaften, Ludwig-Maximilians-Universität, München, Germany. wloske
@iaag.geo.uni-muenchen.de
2
INTRODUCTION
Tectonic processes affecting the western part of the Antarctic Peninsula and adjacent islands in
Mesozoic and Cenozoic times were related to the subduction of oceanic lithosphere from the
southeastern Pacific Ocean region. Sea floor magnetic patterns allow the differentiation of five
involved plates: the Pacific, Phoenix, Antarctica, Farallon, and South America plates (Anderson,
1999; Jabaloy, 2003; see Fig. 1). Since the Mesozoic times, the Phoenix plate was being
gradually subducted under South America and Antarctica. The resulting subduction complex is
represented by isolated fragments of metamorphic successions which crop out at Smith Island
and in the Elephant Island Group (Trouw et al., 1991). The corresponding magmatic arc is now
represented along the western coast of the Antarctic Peninsula by Triassic to Tertiary intrusive
and volcanic rocks (Pankhurst, 1983) that intruded into or overlaid a turbiditic complex (Trinity
Peninsula Group –TPG–) and correlative units (Smellie, 1991). As the ocean floor is subdivided
by NNW trending fracture zones, different segments of a spreading ridge, oriented obliquely to
the convergent plate boundary, may enter the subduction zone at specific times. Barker (1982)
suggested that such collisions occured along the western boundary of the Antarctic plate, moving
from South to North with time and being combined with the end of activity along the colliding
ridge segment and with the end of subduction in the affected area. This should have led to the
present situation with the Drake microplate as the last remnant of the Phoenix plate being
subducted and the lack of subduction further to the South.
Fission track (FT) analyses on zircons and apatites allow the reconstruction of the thermal
histories of rocks below about 280°C and 120° C respectively, including information on the time
of cooling for the last time through the closing temperature of the specific system, the cooling
rate or the occurence of a later reheating.
Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad.
Fig. 1: Supposed plate tectonic configuration
around the Antarctic Peninsula in Early Tertiary
times (after Barker, 1982). T = Tharp Fracture
Zone. At 85 Ma ago (Weissel et al. 1977), the
Campbell Plateau margin of the New Zealand
block began to separate from the Western
Elsworth/Marie Byrd Land block of Western
Antarctica. Tharp Fracture zone was the
effective southern boundary of Antarctic
Peninsula from then on, with subduction
ocurring to the E but not to the W (Barker
1982).
The aim of the current investigations is to use apatite and zircon fission track data to constrain the
low temperature geological histories of the Antarctic Peninsula and the islands to its West. The
new data will be used to test the ridge trench collision hypothesis of Barker (1982). In the
positive case, specific distributions of ages and cooling patterns should reflect different blocks
and the South to North migration of a thermal signal. The specific thermal records should help to
better understand the evolution of the different tectonic units related to the end of the subduction
process, thus during the tranformation from an active to a passive margin. If Barker´s (1982)
theory is not supported, the data will have to be used to model a different and more realistic
scenario.
LOW-TEMPERATURE THERMOCHRONOLOGY
PREVIOUS LOW-T THERMOCHRONOMETRIC RESULTS
Existing fission-track data from the study area cover some of the islands to the West of the
Antarctic Peninsula and reach from King George Island over Livingstone Island as far south as
Alexander Island.
On King George Island, thermochronologic data from 3 apatite and 3 zircon samples, and 3
length distributions of confined tracks in apatites have been obtained (Thomson et al., 2001) in
granodiorites near Noel Hill, Barton Peninsula. Zircon ages scatter around a weighted mean of
45.6 ± 1 Ma and apatite ages around 45.2 ± 2.5 Ma. This implies a very fast cooling in situ after
the emplacement of the intrusive rocks in Noel Hill. This is confirmed in the apatite track length
distribution between 14.3 and 14.8 µm indicative of fast cooling from 120 ºC to 60 ºC (the partial
annealing zone of tracks in apatites) just in few millions of years. However, apatite FT data
(Gonzalez-Cassado et al., 2001) obtained in plutonic intrusions of Barton Peninsula, yield a
different age of 32 Ma. The cause of this difference is not yet clear, probably related to a complex
history and it is presently being studied.
Apatites from Livingstone Island granites have FT ages from 39 Ma to 16 Ma (Gonzalez-Cassado
et al., 2001). Apatites from four tonalite samples of the Barnard Point pluton, Livingstone Island,
have FT ages which scatter between 16 and 20 Ma, with a concordant mean and standard error of
18.7 ± 1.2 Ma (Sell et al., 2000). The confined track length distributions show that the track
record starts about 40 – 35 Ma ago. The cooling of rocks, down to 120 ºC, seems to have
accelerated from Early Oligocene times on (about 30 Ma), coeval with an extensional tectonic
episode and a subsequent denudation process. Apatites from low-grade metamorphic
metasediments of Miers Bluff formation, in Peninsula Hurd, Livingstone Island, yield a single
age of 25.6 ± 0.9 Ma (Sell et al., 2001). Track lengths show that cooling following metamorphic
peak started at about 28 Ma. This coincides with the end of volcanic activity in this segment of
the volcanic arc (140 – 30 Ma). The modelling of fission tracks suggest that recent exhumation of
Hurd Peninsula started about 13 Ma ago.
Finally, in Alexander Island, apatites and zircons from 3 different units were dated (Storey et al.,
1996). In this region, a well-exposed Mesozoic accretionary prism (LeMay group) and forearc
basin (Fossil Bluff Group) are preserved, as well as a Tertiary igneous suite, which intruded and
erupted during the late stages of arc migration. Zircon FT central ages (weighted means) range
from 78 ± 11 to 129 ± 11 Ma for the Fossil Bluff Group and from 102 ± 8 to 178 ± 13 Ma for the
LeMay group, significantly younger than the depositional age of the host rocks. Apatite central
ages range from 27 ± 4 to 62 ± 10 Ma for the Fossil Bluff Group and from 29 ± 9 to 57 ± 7 Ma
for the LeMay Group. Samples from plutonic and porphyry dyke rocks yielded apatite central
ages ranging between 25 ± 4 Ma and 81 ± 42 Ma. Zircon ages show a smaller spread from 46 ± 6
to 80 ± 8 Ma. The similarity between the ranges of the apatite ages from the LeMay Group and
Fossil Bluff Group, as well as between youngest zircon ages (about 100 Ma), suggest a regional
pattern of thermal history for the fore-arc region. Cooling from maximum temperatures occurred
at approximately 100 Ma with a further accelerated cooling episode occurring at between 40 and
35 Ma. This episode appears to be coeval with the ridge-trench collision off-shore Alexander
Island and was likely caused by increased rates of denudation of topography created along the
inboard region of the fore-arc during ridge-trench collision. The magmatic rocks record data
independent of the rest of the fore-arc, with no evidence of a thermal pulse in Late Cretaceous or
Early Tertiary times, the time of magmatic activity recorded in apatites and zircons from
sedimentary units. On the other hand, the fission track ages are close to the emplacement ages of
their host rocks and may record the most precise dating of these events up to the present time
(Storey et al., 1996).
Table 1 shows the apatite and zircon fission track data yielded in Western Antarctica and their
references.
As the above mentioned data up to now do not display any clear regional trend, further
investigations are required until consistent regional or structural patterns are observed which
allow the definition of gradients or of blocks with a shared geological history.
Island
King George
Island
Livingstone
Island
Alexander
Island
Table 1: Previous fission track data
Study Area or Formation (rocks)
Barton Peninsula (granites)
Livingstone Island (granites)
Barnard Point (plutonic rocks)
Hurd Peninsula, Miers Bluff Formation
(metasedimentary rocks)
LeMay Group
(metamorphic rocks)
Fossil Bluff Group
(fore-arc sedimentary rocks)
(intrusive rocks)
FT Data
Ap 32 Ma
Ap 45 Ma
Ap 16 – 39 Ma
Ap 16 – 20 Ma
Reference
(1)
(2)
(1)
(3)
Ap 25 Ma
(4)
Ap 29 – 57 Ma
Zr 102 – 178 Ma
Ap 27 – 62 Ma
Zr 78 – 129 Ma
Ap 25 – 81 Ma
Zr 46 – 80 Ma
(5)
FT data: Ap Apatites, Zr zircons. References: (1) González-Cassado et al. (2001); (2) Thomson et al., 2001; (3) Sell
et al. (2000); (4) Sell et al. (2001); (5) Storey et al. (1996)
NEW LOW-T THERMOCHRONOMETRIC RESULTS
69 samples are at present included in the study; 44 having recently been sampled along the
western coast of the Antarctic Peninsula between the chilean base Bernardo O´Higgins and on the
adjacent islands down the southern coast of Adelaide Island; 16 having been taken from the
collection of the Universidad de Chile and 9 from the collection of the Universität München.
Up to date, only zircon fission track analyses have been finished on nine samples from Western
Antarctica. The locations are shown in Fig. 2 and the results are reported in Table 2. Between
about 4 and 6 kg of suitable material were taken and processed using conventional techniques by
crushing, sieving, separation by Wilfley table, heavy liquids, and final hand-picking. Zircon
populations for fission track analyses were prepared following the techniques outlined by
Hurford et al. (1991). The zircons were embedded in FEP-Teflon, polished, and etched in a
KOH-NaOH eutectic melt at 214 °C in steps varying between 1 and 4 hours, until the majority of
grains was fully etched. Total etch time was 10 to 19 h. Thermal neutron irradiation was
performed in the thermal neutron facility of the TRIGA reactor at the Oregon State University in
15
-2
Corvallis/Oregon, U.S.A., with a neutron fluence of 1 x 10 ncm . All samples were dated
using the external detector method (Naeser, 1976; Gleadow, 1981). The detector mica was etched
for 45 minutes in 40% HF at room temperature. Counting was performed on a ZEISS Axioplan
microscope with a total magnification of 1250x using an oil immersion 100x objective. Ages
were determined following the zeta approach (Hurford and Green, 1983). No length histograms
for zircons have been produced yet, but their determination for a selection of representative
samples is in progress.
Different units in different areas from Western Antarctica were dated using zircon fission track
analysis. The samples have been used in the past for isotopic determinations to constrain their
genesis and potential later metamorphism ages. This study is concentrated on their cooling under
low temperature conditions.
PLUTONIC ROCKS:
A granitic gneiss sample from Reluctant Island with a crystallisation age between 264 – 156 Ma,
probably close to 200 Ma (Loske et al., 1997), yields an age of 91 ± 4 Ma.
In Smyley Island, a granite sample (Loske and Miller, 1995) yields 86 ± 4 Ma.
In the Campbell Ridge, a Paleozoic orthogneiss (Harrison and Loske, 1988) yields 84 ± 4 Ma.
CLASTS OF PLUTONIC ROCKS AND XENOLITHS IN SEDIMENTARY ROCKS:
Two granite pebbles from a Mesozoic conglomerate from Horseshoe Island with a Paleozoic
provenance (Loske et al., 1997; Loske et al., 1998) yield 89 ± 5 and 91 ± 5 Ma.
At Cape Dubouzet, Triassic – Jurassic granitic clasts in a moraine with xenoliths of a minimum
Late Carboniferous age (Loske et al., 1990) yield zircon FT ages of 80 ± 4 and 80 ± 13 Ma in
xenoliths and 87 ± 8 Ma in the matrix granitoid.
METAMORPHIC ROCKS:
A Paleozoic mica schist from Elephant Island, where a Cretaceous metamorphism close to 120
Ma has been recorded (Loske et al., 1985), yielded an age of 81 ± 14 Ma.
Table 2 shows the zircon fission track ages as well as the sample number, location, and lithology.
Figure 2 shows the results from fission track dating in apatites and zircons of the different units
along the Western Antarctica coast and their references, as well as the new zircon fission track
data.
Table 2: New zircon fission track data from rocks of West Antarctica.
Sample
EL – 4
DUB - 3
DUB – 4
DUB – 10
126 – 3
126 – 5
REL – 1
SMY – 1
CAM
Lithology
Mica schist
Xenolith
Metatonalite xenolith
Matrix granitoid
Granite pebble
Granite pebble
Granitic gneiss
Granite
Orthogneiss
Area
Elephant Island
Cape Dubouzet
Cape Dubouzet
Cape Dubouzet
Horseshoe Island
Horseshoe Island
Reluctant Island
Smyley Island
Campbell Ridge,Antarctic Peninsula
Sample locations are shown in figure 2. Analyst MRB.
FT Age
81 ± 14 Ma
80 ± 4 Ma
80 ± 13 Ma
87 ± 8 Ma
89 ± 5 Ma
91 ± 5 Ma
91 ± 4 Ma
86 ± 4 Ma
84 ± 4 Ma
Figure 2: Thermochronologic ages in the study area. age boxes represent apatite FT data and age hexagons, zircon
FT data. White boxes represent new zircon FT data. The rest of the ages are from the literature, see Tab. 1.
DISCUSSION AND PRELIMINARY CONCLUSIONS
The new zircon fission track ages, which are the first in the northern part of the Antarctic
Peninsula, show a low spread with values between 80 ± 13 and 91 ± 5 Ma despite the fact that the
samples come from different geological units (metamorphic rocks, igneous rocks, clasts in
metasedimentary rocks) and from a wide area. This indicates regional Late Cretaceous cooling,
concomittant with the accretion of the Scotia Metamorphic Complex (80 – 120 Ma, Elephant
Island; 63 – 68 Ma, Smith Island; Feraud et al, 2000). The data at present do not display any
significant indication of a South to North directed migration of a thermal signature east of the
subduction zone.
Apatite data which have been published after the start of the project show a range and regional
spread of ages that as well do not easily correspond to what would be predicted for the South
Shetland sector from Barker´s (1982) hypothesis of ridge trench collision.
To judge from the above mentioned unexpected observations, the ongoing low-temperature
thermochronometric investigations promise a more detailed understanding of the plate tectonic
evolution of the western Antarctica region.
ACKNOWLEDGEMENTS
Field Work was made possible by INACH Project 01–99 and by Polarstern Ant 11/3 expedition
(Alfred – Wegener -Institut). FT dating was performed at the Ruhr-Universität, Bochum.
Programa Institucional Antártico, Universidad de Chile provided complementary funds for the
study.
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