Archean crustal evolution in the northern Yilgarn Craton: U–Pb and Hf-isotope evidence from detrital zircons
Introduction
We can gain insights into the nature of tectonics and crust–mantle interaction in the early Earth by studying the processes of crustal generation in Archean time. This information can help us to evaluate how far the current plate tectonic paradigm, based on data and observations from Phanerozoic environments, can be projected back in time.
To understand the genesis of a block of crust, we need to know not only the age distribution of the magmatic rocks but also the source (juvenile or recycled) of the magmatic material. The problem commonly has been approached by a combination of U–Pb dating of zircons, and Sm–Nd analysis of their host rocks to define the source materials. This method has several limitations: it is relatively slow and hence expensive, and the Sm–Nd system is prone to metamorphic disturbance with consequent errors in the determination of initial ratios, εNd values, and model ages. The problems of Sm–Nd disturbance can be largely circumvented by using the closely related Lu–Hf system, and analysing the Hf-isotope composition of zircon of known U–Pb age. Zircon is very resistant to metamorphic recrystallisation even up to high metamorphic grades, and it essentially preserves the initial of its source magma at the time of crystallisation.
This latter approach has been used in several studies of crustal genesis (Patchett et al., 1981, Smith et al., 1987, Vervoort and Patchett, 1996) but until recently has been limited by the need to use zircon composites (which may contain different generations of zircon) in order to obtain enough Hf for analysis. However, the multi-collector ICPMS laser microprobe (LAM-MC-ICPMS) now makes it possible to obtain high-precision Hf-isotope analyses on small portions of single zircon grains (Griffin et al., 2000, Thirlwall and Walder, 1995), while U–Pb ages can be obtained on the same grains by LAM-ICPMS techniques (see below). In addition, trace-element data obtained by electron microprobe analysis and as an adjunct to the U–Pb and Hf-isotope analyses give useful information on the composition of the magma from which each zircon crystallised.
This integration of different analytical datasets makes it possible to survey the magmatic history of a crustal block in terms of age, rock types and sources of material by analysing detrital zircons in sediments derived from that block. In this report, we present the results of a reconnaissance study of zircons taken from modern drainages across the northern part of the Yilgarn Craton and the adjacent Capricorn Orogen. We use these data to evaluate the relative importance of mantle and crustal sources in the generation of Archean and Proterozoic magmas within major structural provinces of the northern Yilgarn Craton. The magmatic events also mark important tectonic episodes in crustal evolution of the region studied.
Section snippets
Regional geological setting
There have been numerous attempts to divide the Yilgarn Craton into provinces, terranes or other major structural units; a review of these is beyond the scope of this paper. In a major synthesis and discussion of previous work, Myers (1993) proposed a series of units with distinct geological histories (Fig. 1). The West Yilgarn Superterrane includes the Narryer terrane in the NW, which contains crust as old as 3.7–3.8 Ga, intruded by 3.0 Ga granitoids, and the Murchison and SW Yilgarn terranes
Sampling and analytical methods
Trace-element and isotopic (Lu/Hf and U/Pb) in situ analyses were carried out on about 550 zircon grains in this study. The full dataset is available Appendix A.
Samples are heavy mineral concentrates of sand samples from modern drainages, originally taken by DeBeers Australia Exploration Ltd. for diamond exploration purposes. Sampling covered 21 individual drainages in 15 areas spread across the region of interest; these drainages were selected to give a broad coverage of the different
Results
The analytical data, and observations on external morphology and internal structure of each grain, are given in Appendix A. Zircons have been distinguished as possibly metamorphic (rather than magmatic) when they display a combination of anhedral or rounded external form, little or no internal structure, low Th and U contents and Th/U<0.1 (Rubatto et al., 1999, Hoskin and Black, 2000). However, these criteria are not invariably indicative of metamorphic origin, and the identification should be
Appearance of the Depleted Mantle reservoir
The Depleted Mantle reservoir is broadly complementary to the continental crust, and the earliest appearance of this reservoir is an important parameter in understanding the evolution of the early Earth. Most zircons older than 3.2 Ga analysed in this study have εHf=0 (Fig. 7); they crystallised from magmas with Hf-isotope compositions similar to the chondritic unfractionated reservoir (CHUR), or from magmas derived from older crust. The two oldest grains are from the Yeelirrie Province. They
Conclusions
U–Pb and Hf-isotope analysis of >500 zircons from modern drainages across the northern part of the Yilgarn Craton and the adjacent Capricorn Orogen provides a broad view of crustal evolution in Archean and Proterozoic time. The oldest crustal components (3.7 Ga) are identified in the Yeelirrie geophysical domain that runs N–S down the middle of the craton; these components are represented by ancient zircons and also are reflected in the Hf model ages of younger magmas. This block of ancient
Acknowledgements
We thank Suzie Elhlou, Carol Lawson, and Ashwini Sharma for expert and cheerful assistance with the analytical work, Tom Bradley for preparation of many zircon grain mounts, and David Nelson, David Champion and Kevin Cassidy for useful discussions about Yilgarn magmatism. We are grateful to A. Whitaker for providing both a pre-publication copy of his new domain analysis, and guidance in its use. Useful reviews were contributed by R. Stern, J. Ketchum, M. Norman and an anonymous referee. This
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