Geochemistry of a metabasite — chert — coloured-argillite — turbidite association at Red Rocks, Wellington, New Zealand

Abstract

A sequence of Late Permian pillow lava, chert, and coloured argillite (red, green) occurs within Late Triassic greywacke-argillite turbidites and associated grey and red argillite at Red Rocks, south Wellington coast. The lower contact of the Late Permian rocks is a fault parallel to bedding, while the upper contact is sedimentary and conformable. Relic clinopyroxenes, bulk rock Y/Nb, Ti-Zr-Y, Ti-Zr, Ti-V, and mantle-normalised values of hygromagmatophile elements indicate that the pillow lava is tholeiitic and was erupted in an oceanic (oceanic island) setting. The geochemistry of the associated coloured argillites and chert is a function of: (1) a dominant terrigenous component similar to that of the enclosing turbidite argillite; (2) siliceous biogenic (radiolarian) detritus that is locally important in the cherts and some siliceous red argillites, and subordinate in coloured argillites; and (3) a minor metalliferous component (volcano-hydrothermal and hydrogeneous), accounting for slight enrichment in Fe, Mn, Cu, Ni, and Zn, that is most important in the red and green argillites and chert, and insignificant in the grey argillites. High Fe-oxidation ratios of the Late Permian coloured argillites and cherts are compatible with deposition of finegrained material in an oceanic setting on the flanks of a seamount. Relatively high oxidation ratios and increasing MnO/TiO₂ ratios in the direction of younging through the Late Triassic grey and red argillite compared with the normal turbidite argillite, and the presence of calcareous concretions, also indicate a period of slow to negligible sedimentation but within the area of turbidite deposition. Major and trace element chemistry and detrital mineralogy of the Late Permian and Late Triassic coloured argillites indicate that the terrigenous source did not change significantly during this time. Lower SiO₂/Al₂O₃ and K₂O/Na₂O ratios of the turbidites to the west of the metabasalt compared with those to the east suggest that the eastern set could have been derived from a more felsic source. The western turbidites were deposited over the pillow lava, after the Late Permian sequence was emplaced as a gravity slide.