Provenance of Permian‐Triassic volcaniclastic sedimentary terranes in New Zealand: Evidence from their radiogenic isotope characteristics and detrital mineral age patterns

Abstract

Detrital mineral age patterns, Rb‐Sr age, and initial strontium isotope ratio variations are reported for metasediments of the Brook Street, Dun Mountain‐Maitai, and Murihiku Terranes (informally grouped as “central arc terranes") of New Zealand. U‐Pb SHRIMP detrital zircon age populations in Late Permian Tramway Formation, sandstones of the Dun Mountain‐Maitai Terrane in Nelson and Southland, New Zealand, show restricted Late Permian (253–268 Ma) and broader Devonian‐Carboniferous (330–373 Ma) age components in about equal proportions, with a minor (<20%) proportion of early Paleozoic and Mesoproterozoic ages. The Permian component is related to contemporaneous volcanic inputs, whereas the older Devonian‐Carboniferous component is derived from a mixture of exhumed continental crust of Lachlan Fold Belt, and possibly New England Fold Belt age characteristics. A detrital zircon age population of Jurassic Barretts Formation sandstone of the Brook Street Terrane in Wairaki valley, Southland, carries a major (75%), dominantly Middle Jurassic to Late Triassic (170–230 Ma) component derived from contemporary volcanic inputs of uncertain origin, and a minor (<25%) Middle Triassic to Late Permian (229–254 Ma) component.

Rb‐Sr isochron ages (t) of Brook Street, Dun Mountain‐Maitai, and Murihiku Terrane metasediments (and their initial ⁸⁷Sr/⁸⁶Sr ratios (i) at the time of metamorphism), reveal cooling age patterns consequent upon low‐grade metamorphism in latest Triassic and earliest Jurassic times. The (t)‐(i) data define a central arc terranes group consistently less radiogenic than Permian and Triassic time correlatives in the Torlesse Supergroup of the Rakaia Terrane. This feature suggests a provenance that includes sediment detritus dominantly from less radiogenic I‐type igneous sources in both Lachlan and New England Fold Belts in similar proportions, and with little contribution from their Precambrian and early Paleozoic hinterlands.

Keywords:

• Permian
• Triassic
• volcaniclastic sediments
• U‐Pb geochronology
• Rb‐Sr
• strontium isotopes
• terranes
• provenance