Age and isotopic characterisation of metasedimentary rocks from the Torlesse Supergroup and Waipapa Group in the central North Island, New Zealand

Abstract

Detrital zircon U‐Pb ages in grey wackes and initial ⁸⁷Sr/⁸⁶Sr ratios are reported for low‐grade metasedimentary rocks from Torlesse Supergroup, Waipapa Group, and Kaimanawa Schist in the central North Island, New Zealand. The data reveal the presence of a hitherto unsuspected, areally extensive, Jurassic part of the Torlesse composite terrane in the Kaimanawa, Kaweka, and Ruahine Ranges, which we name the Kaweka Terrane. Kaweka rocks have initial ⁸⁷Sr/⁸⁶Sr ratios (at metamorphism) and detrital zircon patterns that in part are transitional between Rakaia and Pahau Terrane rocks and in part similar to Waipapa Terrane rocks. Combined detrital zircon age data for all Torlesse and Waipapa Terrane data reveal an essential unity, with a long persistence (260–120 Ma) of predominant Permian‐Triassic sources in the form of a major Cordilleran‐style batholith, a decline in major early Paleozoic‐Precambrian sources between 260 and 220 Ma, and presence of minor Early Carboniferous to Late Devonian sources between 180 and 120 Ma. Rb‐Sr and K‐Ar ages indicate latest Triassic to Early Cretaceous metamorphism in an evolving accretionary wedge.

Keywords:

• New Zealand
• Torlesse Supergroup
• Waipapa Group
• Torlesse Terrane
• Waipapa Terrane
• Kaweka Terrane
• U‐Pb dating
• Rb‐Sr dating
• Sr isotopes
• detrital zircon
• petrographic modes
• new structural name