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

Abstract

Whole-rock major and trace element compositions of quartzofeldspathic PermianCretaceous 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 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. chemistry of Torlesse sediments overall with terranes of similar age or tectonic setting (e.g. Graham, 1985;

Provenance and geochemistry of exotic clasts in conglomerates of the Oligocene Torehina Formation, Coromandel Peninsula, New Zealand

“…This is also noted from a plot of Th/Sc versus Zr/Sc (Fig. 13) (Roser & Korsch 1999) wherein the fields for TA and UA clasts fall within an overlap of Torlesse and more volcanogenic terrane rocks fields.…”

“…1) occurred by Early Cretaceous time (Black 1994;Mortimer 1995a;King et al 1999). These terranes range in age from Permian to ?Early Cretaceous, and, as now found across North Island, there is a general west-to-east gradient of decreasing volcanogenic facies from a westerly magmatic arc (Black 1994;Mortimer 1995a;Roser & Korsch 1999).…”

“…8) are shown relative to trends calculated and observed for weathering of granodiorite (Nesbitt & Young 1989) and rocks representative of the Torlesse Terrane New Zealand Journal of Geology andGeophysics, 2003, Vol. 46 (Roser &Korsch 1999). An increase in weathering is defined, first by a path directed toward the Al 2 O 3 -K 2 O tie line, then toward the Al 2 O 2 endmember (Nesbitt & Young 1989).…”

“…Fig. 13 Distribution of Torehina clast types (see text for definition) and sediment relative to Th/Sc versus Zr/Sc, along with fields for some tectonic terranes in New Zealand, and endmember intersections for rhyolite (RHY) and dacite (DAC) (Roser & Korsch 1999). See text for details.…”

“…Indeed, from east and south of Coromandel Peninsula, potential metamorphosed basement sources reveal increased availability of quartzofeldspathic material of granodioritic bulk composition (Roser & Korsch 1999), as well as a greater variation in abundance of chert and volcanic detritus (Skinner 1978;Kear 1992;Mortimer 1995b).…”