Age of metamorphism of Otago Schist in eastern Otago and determination of protoliths from initial strontium isotope characteristics

Abstract

Geochronological studies of the eastern Otago Schist and adjacent metagreywacke sequences are applied to their structural/metamorphic history and schist protoliths. Rb‐Sr whole‐rock isochron and K‐Ar total‐rock ages are inversely correlated with metamorphic grade (both in terms of mineral facies and textural zones), dating the time of peak metamorphism and/or postmetamorphic cooling. On the northern flank of the schists, older ages (200–230 Ma) date initial burial metamorphism and deformation in late Triassic ‐ early Jurassic times. Younger age patterns (to 115 Ma) reflect either (1) higher grade metamorphism during a single event followed by long‐continued uplift, or (2) early burial metamorphism at >200 Ma with a second, regional metamorphism between then and 115 Ma followed by rapid uplift. Initial Sr isotopic ratios derived from isochrons indicate that protoliths originate in two terranes. North of Dunedin, initial 87Sr/86Sr ratios range from 0.7064 to 0.7092, similar to Torlesse Terrane (Rakaia Subterrane) analogues in the Wellington region, and indicate a granitoid source dominated by I‐types. South of Dunedin, along the south Otago coast, schists have lower initial 87Sr/86Sr ratios, 0.7052 to 0.7064, unlike Caples‐type rocks elsewhere in Otago. They are thus of “uncertain” terrane affinity but indicate a dominantly silicic‐intermediate volcanic source.