Heavy minerals in the Torlesse Terrane of the Wellington area, New Zealand

Abstract

Heavy minerals from dated sandstones of the Torlesse Terrane in the Wellington area show several distinct assemblages. Samples from the Rakaia Subterrane (n = 19) have assemblages of two types: one dominated by secondary minerals (mainly in the Terawhiti area), and one by detrital biotite (mainly in the eastern part of the subterrane). Both types were present in the Kapiti Island area. Pahau (n = 3) and Esk Head samples of Pahau affinity (n = 3) are distinct from Rakaia samples in containing more than trace amounts of garnet, and suggest a different provenance (i.e. little reworking). The remaining two Esk Head samples resemble the biotite‐rich assemblage of the Rakaia but contain minor pyroxene and no epidote minerals. The distinction between epidote‐rich Pahau and biotite‐rich Rakaia assemblages has a parallel in North Canterbury.

Semi‐opaque debris and pumpellyite indicate low‐grade metamorphism of Pahau rocks and of some Rakaia rocks from near Cape Terawhiti. However, the difference between the two Rakaia assemblages is unlikely to be due solely to this metamorphism, as zircon morphology and garnet compositions suggest different provenances. Many of the garnet compositions in Pahau and Esk Head samples are distinct from those in Rakaia samples, suggesting that the Rakaia Subterrane was not the provenance.

Systematic distinctions such as these in Torlesse rocks offer potential for refining provenances of overlying Cretaceous‐Cenozoic sandstones.

Keywords:

• Torlesse
• Wellington
• heavy minerals
• provenance
• metamorphism