U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA 10° CONGRESO GEOLÓGICO CHILENO 2003

FISSION TRACK THERMOCHRONOLOGY OF THE WESTERN 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] 2 Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Germany. manfred.r.brix@ruhr-uni- bochum.de, [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

INTRODUCTION Tectonic processes affecting the western part of the Antarctic Peninsula and adjacent islands in Mesozoic and Cenozoic times were related to the 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 , 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.

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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.

Table 1: Previous fission track data Island Study Area or Formation (rocks) FT Data Reference King George Ap 32 Ma (1) Barton Peninsula (granites) Island Ap 45 Ma (2) Livingstone Island (granites) Ap 16 – 39 Ma (1) Livingstone Barnard Point (plutonic rocks) Ap 16 – 20 Ma (3) Island Hurd Peninsula, Miers Bluff Formation Ap 25 Ma (4) (metasedimentary rocks) LeMay Group Ap 29 – 57 Ma (metamorphic rocks) Zr 102 – 178 Ma Alexander Fossil Bluff Group Ap 27 – 62 Ma (5) Island (fore-arc sedimentary rocks) Zr 78 – 129 Ma Ap 25 – 81 Ma (intrusive rocks) Zr 46 – 80 Ma

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 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 – 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 Lithology Area FT Age EL – 4 Mica schist Elephant Island 81 ± 14 Ma DUB - 3 Xenolith Cape Dubouzet 80 ± 4 Ma DUB – 4 Metatonalite xenolith Cape Dubouzet 80 ± 13 Ma DUB – 10 Matrix granitoid Cape Dubouzet 87 ± 8 Ma 126 – 3 Granite pebble Horseshoe Island 89 ± 5 Ma 126 – 5 Granite pebble Horseshoe Island 91 ± 5 Ma REL – 1 Granitic gneiss Reluctant Island 91 ± 4 Ma SMY – 1 Granite Smyley Island 86 ± 4 Ma CAM Orthogneiss Campbell Ridge,Antarctic Peninsula 84 ± 4 Ma Sample locations are shown in figure 2. Analyst MRB.

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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