Geochemical characterization, evolution and source of a Mesozoic accretionary wedge: the Torlesse terrane, New Zealand

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Abstract

Whole-rock major and trace element compositions of quartzofeldspathic Permian–Cretaceous sandstones of the Torlesse terrane, New Zealand, display progressive changes which compare well with published petrographic data. Chemical indices in three Permian–early Late Triassic Rakaia subterrane petrofacies show small contrasts, as do the modes, reflecting original source compositions. A Late Triassic Rakaia petrofacies displays sharp increases in SiO 2 /Al 2 O 3 , K 2 O/Na 2 O, Th/Sc, La/Sc, Ce/Sc, La N /Y N and decreases in Ti/Zr and V/La, consistent with partial cannibalistic recycling and a small influx of lithic volcanic detritus. In the post-collisional Late Jurassic–Early Cretaceous Pahau subterrane, SiO 2 /Al 2 O 3 remains high, but other indices decrease to levels seen in the Permian, due to recycling of the earlier Rakaia petrofacies and influx of 10–30% mafic-intermediate volcaniclastic detritus derived from inboard volcanogenic terranes. The results are confirmed by additional suites, including mudstones, which cover a larger area, but also show that some spatial variation occurs within the terrane as a whole. Whole-rock data and Chemical Index of Alteration indices indicate that the Torlesse was derived from a relatively unweathered source with granodioritic bulk composition. Comparisons with data from a proposed Rakaia source in the New England and Hodgkinson orogens of eastern Australia suggest that the observed chemical and modal compositions cannot be simply derived from that area as presently exposed.

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