Geochemistry and provenance of lower Permian sedimentary rocks, Nambucca Block, southern New England Orogen

Abstract

It is well documented that sediments record geochemical signatures that reflect the composition and tectonic setting of the rocks providing the detritus for them. This is true of the turbidites in the Nambucca Block that record compositions that vary from basaltic to rhyodacitic but are dominantly andesitic to dacitic. They show chondrite and N-MORB patterns, characteristic of calc-alkaline, continental arc rocks, namely light REE (av. La/Yb)_N = 6.3) and Th enrichment, Ti, Nb and Ta depletion, and substantial within plate components (Nb/Yb; 0.65–5.16; x = 3.15). Rare occurrences of rocks with intra-oceanic island arc affinities and felsic volcanic rocks showing tectonic fabrics occur, indicating more distal sources contributed detritus. Most detritus in the turbidites has been derived from the calc-alkaline, continental rocks of the Keepit arc, some from the subduction–accretion sequences of the Tablelands Complex. CIA [Al₂O₃/(Al₂O₃+Na₂O + K₂O + CaO)×100] values (48–83; x = 68) indicate that erosion of these arc sequences occurred during glacial, temperate and humid conditions at different times. Further, index of chemical variability values (0.3–1.85; x = 0.85) reveal a compositional maturity and a high percentage of clays in the source rocks suggesting a passive tectonic setting in which minimal uplift and extensive weathering took place.

KEY POINTS

1. Metasediments in the Nambucca Block record geochemical signatures typical of calc-alkaline, continental, arc rocks.

2. Detritus in metasediments was derived from the Keepit arc and Tablelands Complex, southern New England Orogen.

3. Source rocks were subjected to glacial, temperate and humid conditions at different times.

Keywords:

• Geochemistry
• metasediments
• provenance
• climatic conditions
• Nambucca Block
• southern New England Orogen

Acknowledgements

I thank Bill Landenberger for providing analyses of the calc-alkaline volcanic rocks from the Tamworth Belt and assistance with the preparation of Figure 2, Ryan Dwyer for preparing Figure 4 and modifying Figures 6–8, Paul Ashley for providing samples of the Halls Peak Volcanics and Songfa Liu for his excellent fieldwork and sampling. Constructive comments of the first draft by Covadonga Brime and Alan Moore, and by the reviewers Graziella Caprarelli and Uri Shaanan are greatly appreciated.

Disclosure statement

No potential conflict of interest was reported by the author.

Data availability statement

Data supporting the findings of this study are available in the Supplemental data files available at https://doi.org/10.1080/08120099.2021.1893219

Additional information

Funding

Funded in part by ARC Grant [A38920121] to W. J. Collins and R. Offler.