Combined apatite fission-track and single grain apatite (U–Th)/He ages from basement rocks of central Dronning Maud Land (East Antarctica) — Possible identification of thermally overprinted crustal segments?

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Abstract

Apatite fission-track (FT) and single grain (U–Th)/He ages from four vertical profiles in central Dronning Maud Land (East Antarctica) range from 312 ± 20 Ma to 135 ± 11 Ma and 304 ± 28 Ma to 104 ± 8 Ma, respectively. The combined age data allows to discriminate between undisturbed cooled (due to exhumation) and thermally overprinted crustal blocks. Profiles at the Zwieselhöhe and the Conradgebirge revealed unusual apatite FT vs. elevation relationships and (U–Th)/He ages older than the corresponding central apatite FT ages, possibly providing evidence for a Jurassic thermal overprint. Most probably Jurassic magmatism and associated advective heating led to total annealing of the apatite fission-tracks but helium only partially diffused. The model developed in this paper suggests that the (U–Th)/He ages from the Zwieselhöhe and Conradgebirge profiles are in part relicts of the pre-Jurassic cooling history.

Two thermally undisturbed vertical profiles are used to record the long-term cooling history of central Dronning Maud Land. Time-temperature paths derived from modelled apatite FT data of these profiles revealed two phases of accelerated cooling during the Late Carboniferous and the Early Jurassic. Both phases are followed by slow cooling which is also documented by the spread in apatite (U–Th)/He single grain ages. The cooling at the end of the Carboniferous is most probably related to far field effects associated to the prevailing convergent tectonics. During the initial separation between East Antarctica and Mozambique erosion along an evolving rift shoulder caused the Jurassic cooling. Denudation of the basement was simultaneous with volcanism with both pre-dating (c. 20–10 Ma) sea-floor spreading in the Riser Larsen Sea (c. 155 Ma). Post Jurassic cooling was restricted to the lowest temperature sensitivity of both methods. Combined inverse modelled apatite FT data and forward modelled (U–Th)/He data suggest an Eocene/Oligocene cooling step, possibly related to the flexural-isostatic response to the development of ice-sheets.

Introduction

Central Dronning Maud Land is a classic example of a highly elevated (up to c. 3200 m) passive continental margin (Fig. 1), which developed during the Mesozoic break-up of the Gondwana supercontinent. The mountain range comprises different types of basement rocks but lacks sufficient structures and sedimentary rocks to constrain the timing and magnitude of landscape development. In such a scenario low temperature thermochronological techniques such as fission-track (FT) and (U–Th)/He dating offer the unique possibility to reconstruct the upper crustal tectono-thermal history. The combination of apatite FT and (U–Th)/He thermochronology allows to improve and to cross check cooling histories derived only from one single method. Integrated studies of apatite FT and (U–Th)/He dating have been so far successfully applied to rapidly cooled rocks (> 10 °C/Myr) in different tectonic regimes (e.g. Arne, 1994, Bankwitz and Bankwitz, 1985, Barbarand et al., 2003, Bauer et al., 1997, Belton et al., 2004), for basin development (Berger et al., 2006, Bojar et al., 2005) and well data (Lorencak et al., 2004). However, combining these methods seems to be problematic for slowly cooled rocks (< 10 °C/Myr) (summarised in Hendriks and Redfield, 2005, Fitzgerald et al., 2006) or magmatic overprinted rocks (Stockli et al., 2000). Many previous (U–Th)/He studies used analytical routines whereby batches of apatite grains (c. 2–20) were outgassed in one aliquot thus making it difficult to detect excess helium in one of the grains that may yield greater than expected ages. The development of single grain (U–Th)/He analysis allows for detection of outliers more readily. Furthermore the spread in single grain ages with different grain radii can yield information about the progression of cooling through the 80 °C to 40 °C temperature regime (Reiners and Farley, 2001).

A reconnaissance titanite, zircon and apatite FT study in central Dronning Maud Land (Meier et al., 2004) revealed diverse Phanerozoic phases of enhanced exhumation. Jurassic mafic dykes (Wand et al., 1988, Drewry et al., 1982) cross-cutting the basement rocks appear to exert important influence on the thermo-tectonic history (Meier, 1999). In this paper we re-evaluate apatite FT data from Meier (1999) and present new apatite FT and single grain (U–Th)/He ages from four vertical profiles across the elevated margin of central Dronning Maud Land. We use this combined approach to further constrain the pre-Jurassic cooling history and to assess the thermal influence of Jurassic magmatism on the evolving continental margin.

Section snippets

Geological background

The basement of central Dronning Maud Land, within the East Antarctic craton, is composed of polymetamorphic, medium to high grade gneisses and late to post-tectonic granitoids (Ehlers and Farley, 2003, Jacobs et al., 2003).

U–Pb SHRIMP ages indicate that the oldest metasedimentary rocks of central Dronning Maud Land were deposited at c. 1130 Ma (Jacobs et al., 1998). Thereafter, the basement experienced at least two major regional metamorphic events. During the early Grenvillian, these rocks

Previous fission-track results from Dronning Maud Land

Meier (1999) conducted a reconnaissance fission-track study of central Dronning Maud Land and reported titanite, zircon and apatite FT ages obtained from basement rocks ranging between 531 ± 51 Ma to 293 ± 28 Ma, 364 ± 47 Ma to 237 ± 31 Ma and 315 ± 18 Ma to 83 ± 3 Ma, respectively. Based on the FT data set, Meier (1999) suggested that central Dronning Maud Land experienced significant crustal denudation. Meier (1999) proposed that during the Early Jurassic, mantle plume related denudation initiated

Apatite fission-track analysis

Apatites were separated using conventional crushing, sieving, magnetic and heavy liquid techniques. Populations of several hundred grains were embedded in epoxy, and thereafter ground and polished. Apatites were etched for c. 20 s. in 5N HNO3 at room temperature to reveal spontaneous tracks (e.g. Gleadow et al., 1984). Samples were irradiated at the FRM-II reactor at Technische Universität München in Garchingen (Germany) using a total thermal neutron flux of 1 × 1016 ncm 2. Detectors were etched

Results

Apatite FT and (U–Th)/He data come from four sample profiles. (a) All apatite FT samples from Meier (Meier, 1999) were recalculated (supplementary data file). All apatite FT ages are reported as central ages with their two-sigma errors (Table 1). (b) Single grain apatite (U–Th)/He ages corrected for α ejection (Farley et al., 1996) are quoted with two sigma errors (Table 2). Within one sample the variations in most single grain (U–Th)/He ages exceed the analytical error (Fig. 3). However, if no

Identification of single grain (U–Th)/He age outliers

Despite a very careful pre-examination of apatites and the exclusion of obvious outliers samples yield apparently “older than expected” for α ejection corrected apatite (U–Th)/He single grain ages. Furthermore, c. 80% of (U–Th)/He single grain ages overlap within their two-sigma errors with their corresponding apatite FT ages (Table 2). This could indicate miscalculated α corrections (Farley et al., 1996; Ketcham et al., 1999, Lippolt et al., 1994) of the raw data using the conventional method

Conclusions

Integrated data-sets of apatite FT and single grain (U–Th)/He analysis were used to constrain the cooling history of central Dronning Maud Land in East Antarctica. Combined thermochronological data sets allowed the deduction of periods of basement cooling. In the low temperature regime (c. < 120 °C) only the combination of these two methods enables the discrimination of undisturbed cooled (caused by exhumation) and a simultaneous thermal overprint caused by volcanism and associated thermal

Acknowledgements

We acknowledge U. Schwarz-Schampera for his help with and the use of the cathodoluminescence facility at the BGR Hanover. Thanks to A. Läufer for the supply of samples from the Mühlig–Hofmann–Gebirge. Also thanks go to N. Evans and B. McDonald at CSIRO Exploration and Mining, Perth for the U and Th analysis. This study was founded in parts by the DFG project JA 617/24. The paper benefited by reviews of T.L. Riley and an anonymous colleague.

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