Reconnaissance sandstone geochemistry, provenance, and tectonic setting of the lower Paleozoic terranes of the West Coast and Nelson, New Zealand

Abstract

Broad chemical characteristics of sedimentary rocks from the Buller and Takaka Terranes have been examined, and provenance and tectonic setting inferred, using whole‐rock major and trace element data from 208 samples. Buller Terrane rocks west of the Karamea Batholith (Greenland Group) have chemistry typical of turbidite sediments deposited at a weathered passive continental margin. Bulk chemical characteristics confirm earlier work, except that a small spatial chemical variation is seen, consistent with the presence of a small volcanogenic component in the south, decreasing to the north. East of the batholith, compositions of the Webb and Roaring Lion Formations overlap with those of the western Buller rocks, supporting equivalence (at least in part) with the Greenland Group. Passive margin chemical characteristics are maintained throughout the succeeding Ordovician formations.

Within the Takaka Terrane there are sharp contrasts in chemistry between the Cambrian and post‐Cambrian (Ordovician—Silurian) successions. Cambrian formations have low SiO₂/Al₂O₃, K₂O/Na₂O, Chemical Index of Alteration (CIA), Th/Sc and Ce/Sc ratios, and high Ti/Zr and V/La ratios. These features, together with provenance discriminant scores and enrichments in elements linked with mafic minerals, are compatible with deposition adjacent to an active volcanic arc. In the post‐Cambrian units, element abundances and indices change systematically up the column, consistent with cessation of arc activity and influx of quartzose clastic detritus in a passive margin setting.

Chemistry of samples in the Ordovician interval common to both Buller and Takaka Terranes is similar, indicating that the rocks probably both had Gondwana sources. Small anomalies, however, suggest that the two terranes may have had discrete source regions at the Gondwana margin.

Keywords:

• geochemistry
• sedimentary rocks
• Buller Terrane
• Takaka Terrane
• provenance
• tectonic setting