New constraints from U–Pb, Lu–Hf and Sm–Nd isotopic data on the timing of sedimentation and felsic magmatism in the Larsemann Hills, Prydz Bay, East Antarctica
Highlights
► Basement orthogneiss is dated at 1126 ± 11 million years. ► Sedimentary cover was deposited at circa 1000 million years in back arc basin. ► Basin was inboard of 1000 Ma continental arc along Indo-Antarctic craton leading edge. ► Cover and basement were metamorphosed and deformed at ca. 900 and 530 million years.
Introduction
Granulite-facies metamorphic rocks exposed in Larsemann Hills (Fig. 1) are unique for the abundance of boron- and phosphorus-rich units such as tourmaline quartzite and a variety of gneisses containing the high-temperature borosilicate minerals grandidierite and prismatine as well as tourmaline (Carson and Grew, 2007, Carson et al., 1995a, Grew et al., 2006a, Ren et al., 1992). The protoliths of these rocks are thought to be clastic and volcanogenic rocks that were altered and enriched in boron by submarine hydrothermal processes (Grew et al., 2006a, Grew et al., 2011, Grew et al., in press), perhaps analogous to the precursors proposed for tourmaline-rich rocks associated with Pb–Zn–Ag deposits in Broken Hill, Australia (e.g., Slack et al., 1993). In order to test such an analogy, a better understanding of age relationships within the complex rocks in the Larsemann Hills is required. Data of this kind would contribute to improved chronologies of sedimentation, igneous activity and metamorphic events in Prydz Bay, which are important in any proper evaluation of the region's place in the evolution of both Gondwana and the pre-Gondwana Indo-Antarctic and Australo-Antarctic cratons.
This paper presents U–Pb zircon ages, zircon–Hf isotope ratios and Sm–Nd whole rock data for several key lithologic units in the Larseman Hills, southern Prydz Bay. These data provide constraints on (1) the age and nature of granitic protoliths of the Søstrene Orthogneiss, the major orthogneiss unit within the basement underlying the paragneiss units, (2) the chronology of the Blundell Orthogneiss, a major gneiss unit thought to correlate with the earlier of two metamorphic events, and (3) the depositional age and provenance of the Tassie Tarn metaquartzite, a unit in the Brattstrand Paragneiss. Preliminary results and interpretations of these data have been presented in Carson et al. (2007) and Grew et al. (2008a).
Section snippets
Geologic context
The Brattstrand Paragneiss is the collective term applied by Fitzsimons (1997) for the metasedimentary units exposed in the southern Prydz Bay region (e.g., Carson et al., 1995b, Fitzsimons, 1997, Stüwe et al., 1989, Zhao et al., 1995). These metasediments include not only metapelitic gneisses, but also several boron- and phosphate-rich units such as tourmaline quartzite, gneisses rich in the the borosilicate minerals prismatine and grandidierite and gneisses with apatite nodules (Carson and
Søstrene Orthogneiss
Sample 122901 of the Søstrene Orthogneiss, selected for zircon analysis was collected from northeast McLeod Island (Fig. 2), about 200 m northeast of the sample site of the same unit selected by Wang et al. (2008a). The sample site consists of layered quartzofeldspathic gneiss with abundant folded and attenuated layers of mafic granulite. Sample 122901 contains orthopyroxene, biotite, trace clinopyroxene, and accessory apatite, zircon and allanite. Sparse hornblende is mostly fine-grained and
Analytical methods
Details of the analytical methods used can be found in the Supplementary Material. Briefly, the rocks were analyzed for major and trace elements at the Research School of Earth Sciences (incorporating the former Department of Earth and Marine Sciences), Australian National University. U–Pb zircon ages were obtained over several years, using the SHRIMP ion microprobe at the Australian National University (ANU), Canberra. Lu–Hf isotope compositions of zircons were measured on a HELEX 193 nm
Søstrene Orthogneiss
Forty-two analyses were conducted on 38 zircon grains from sample 122901 over two analytical sessions. Zircon morphology ranges from euhedral prisms to subequant, rounded grains, mostly exhibiting complex zoning patterns. Oscillatory zoning is characteristic of the euhedral prisms and some cores (Fig. 4A–C) that are inferred to be magmatic. These magmatic grains return 207Pb*/206Pb* (asterisk refers to radiogenic Pb after correction for common Pb) between ca. 1109–1190 Ma (Table 2), giving a
Lu–Hf (zircon) and Sm–Nd (whole rock) isotopes
Zircons from the Søstrene Orthogneiss, mostly unassigned except for two magmatic grains, yield initial ɛHf from −2.8 to +3.6, a range equivalent to 150–200% of the analytical reproducibility. These ɛHf values correspond to two-stage Hf model ages (TDM2) of 1.53–1.93 Ga (Table 5, Fig. 11A). Magmatic zircons from the Blundell Orthogneiss have homogeneous Hf isotope ratios (ɛHf = −1.0 to −3.7) with the exception of a possible xenocryst (ɛHf = +8.7, Table 5, Fig. 11B). Corresponding hafnium model ages
Late Proterozoic igneous activity
Liu et al. (2009) recognized five episodes of igneous activity in the Prydz Bay-Eastern Amery Ice Shelf area. The oldest two, dated at ca. 1380 Ma and ca. 1210–1170 Ma have not yet been reported in the Larsemann Hills or nearby areas. Evidence for igneous activity during the third episode at ca. 1130–1120 Ma is widespread. For example, our intrusion age of 1126 ± 11 Ma for the precursor to the Søstrene Orthogneiss on McLeod Island falls within this interval, as does the identical age of 1124 ± 24 Ma
Summary
Ion microprobe U–Pb data on zircon indicate an 1126 ± 11 Ma age for the igneous protolith of Søstrene Orthogneiss, a prominent unit of the basement complex in the Larsemann Hills. In the Tassie Tarn Metaquartzite, a unit of the overlying Brattstrand Paragneiss, the youngest recognized detrital population of core analyses (n = 3) gives a weighted mean 207Pb*/206Pb* age of 1023 ± 19 Ma. Metamorphic rims on the detrital zircons define a broad discordia array between ca. 530 Ma and ca. 900 Ma, ages which
Acknowledgements
Fieldwork during the 2003/2004 field season and laboratory expenses were supported by NSF grants OPP-0228842 and EAR-0837980 to University of Maine and ASAC 2350 to C.J.C. and E.S.G. C.J.C. conducted ion microprobe (SHRIMP) analysis for this research whilst a holding a honorary position of Visiting Fellow at the Research School of Earth Sciences, Australian National University, Canberra, Australia. Simon Harley is thanked for giving us permission to cite unpublished information on his find of
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