Authigenic monazite and detrital zircon dating from the Proterozoic Rocky Cape Group, Tasmania: Links to the Belt-Purcell Supergroup, North America
Introduction
Constraining the age of sedimentation in Proterozoic basins is a challenge, particularly those formed on passive margins, where interbedded volcanic horizons are rare and detrital zircon commonly yields maximum ages older than the actual age of deposition (Cawood et al., 2012). The Proterozoic Rocky Cape Group (RCG) in NW Tasmania, Australia, is an example of a particularly poorly dated marine-shelf sequence (Fig. 1). This succession contains the oldest rocks exposed on the island state, but is only constrained in age to between c. 1450 and 750 Ma. The RCG provides a record of passive margin sedimentation and as such has the potential to constrain the position of Tasmania within Meso-Neoproterozoic supercontinent reconstructions (Burrett and Berry, 2000, Black et al., 2004, Berry et al., 2008). The recent discovery of fossil remains of one of the oldest macroscopic organisms (Horodyskia – ‘string of beads’) (Calver et al., 2010) provides further impetus for improving the age constraints on these rocks.
In this study we present authigenic monazite and detrital zircon ages from sub-greenschist facies sandstones and siltstones from the RCG, encompassing the oldest exposed units to the youngest strata, in order to constrain the timing of deposition. Dating authigenic monazite in sedimentary rocks is a relatively recent step forward (Rasmussen et al., 2001, Evans et al., 2002), which eliminates problems associated with inheritance of detrital components and later isotopic resetting that can plague more traditional techniques (e.g. Rb–Sr, K–Ar and Ar–Ar analysis of clays and micas). Furthermore, authigenic monazite can be both texturally and chemically distinguished from detrital monazite (Milodowski and Zalasiewicz, 1991, Rasmussen et al., 2007, Rasmussen and Muhling, 2007, Rasmussen and Muhling, 2009, Alipour-Asll et al., 2012). Here we present a method for the identification via semi-automated scanning electron microscope (SEM) techniques, and characterisation and U–Pb dating via LA-ICPMS, of low temperature monazite in sedimentary rocks that is widely applicable to other terranes. The combined zircon and monazite data presented here demonstrate that the majority of the NW Tasmanian siliciclastic sequence was deposited within the Mesoproterozoic, much earlier than previously envisaged. On the basis of this revised chronology we make correlations with adjacent King Island and wider tectonic links with the Belt-Purcell Basin in western North America in the context of Proterozoic supercontinent reconstructions.
Section snippets
Stratigraphy and depositional environments
The RCG (Spry, 1957, Gee, 1967, Gee, 1968) consists almost entirely of quartz arenite, siltstone, and mudstone, deformed and metamorphosed at prehnite-pumpellyite to lower greenschist facies conditions (Chester, 2006), with an estimated thickness of over 10 km. The RCG stratigraphy includes the original four units defined by Gee, 1967, Gee, 1968, extended by Bell (1972) and supplemented by recent mapping that defines conformable units below the Cowrie Siltstone (Everard et al., 2007) (Fig. 1).
Sampling rationale
Low temperature monazite shows particular promise for constraining “minimum” depositional ages in sedimentary successions due to its ability to grow over a broad range of conditions including diagenesis (Burnotte et al., 1989, Milodowski and Zalasiewicz, 1991, Evans and Zalasiewicz, 1996, Alipour-Asll et al., 2012), low grade metamorphism (Cabella et al., 2001, Rasmussen et al., 2001, Rasmussen and Fletcher, 2002, Wing et al., 2003, Rasmussen and Muhling, 2007, Wilby et al., 2007, Čopjaková et
Monazite petrography
Rare detrital monazite grains found in quartzite samples (Lagoon River Quartzite and Detention Subgroup) are rounded (Fig. 2a) to sub-equant (Fig. 2b) grains found as inclusions within sedimentary quartz grains. No detrital monazite was found outside detrital quartz, which suggests any grains originally in the matrix have since been destroyed in these samples. In the quartzite samples authigenic monazite typically occurs as irregular grains amongst the fine-grained interstitial
Zircon geochronology
Five sandstone samples were chosen for detrital zircon geochronology to provide a maximum age of deposition for these sequences and to investigate shared provenance throughout the RCG. Detrital zircon spectra for >80 grains in each sample are presented in Fig. 5a and summarised in Table 3, arranged in stratigraphic order, and supplemented by detrital zircon data from the upper parts of the RCG in the eastern sector and from King Island (Black et al., 2004).
Depositional ages and stratigraphic relationships within the Rocky Cape Group
The new constraints on both the minimum (authigenic monazite) and maximum (zircon) ages for the RCG can be combined to constrain the likely depositional window for this thick Proterozoic package (Fig. 5c). The lower parts of the RCG (Lagoon River Quartzite to Cowrie Siltstone) have indistinguishable maximum deposition ages as derived from the youngest detrital zircon populations (Table 3 and Fig. 5c). However, minimum depositional constraints fall into two statistically significant populations:
Conclusions
Our new detrital zircon and authigenic monazite data sampled throughout the Rocky Cape Group siliciclastics of NW Tasmania allow us to: (1) vastly improve on depositional age constraints, including constraining the age of Horodyskia-bearing strata; (2) make regional basin-scale correlations, and (3) speculate on the tectonic correlations between proto-Australia and Laurentia at c. 1.45–1.1 Ga.
The ∼9 km thick lower-middle units (Pedder River Siltstone, Lagoon River Quartzite, Balfour Subgroup,
Acknowledgements
We are grateful to S. Feig (Central Science Laboratory, University of Tasmania) for assistance with FE-SEM work. S. Pisarevsky (Curtin University) generously provided for the Mesoproterozoic reconstruction files used in the making of Fig. 6. E. Stewart (Texas A&M University) is thanked for providing LA-ICPMS detrital zircon data and stratigraphic information for the upper Belt-Purcell Supergroup. We thank C. Spencer and an anonymous reviewer for their constructive reviews and R. Parrish for
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