Metamorphic and tectonic geochronology of the Torlesse Terrane, Wellington, New Zealand

Rb-Sr and K-Ar whole-rock ages of low-grade zeolite - prehnite-pumpellyite facies metasedimentary sequences are reported from the Torlesse Terrane in the Wellington region, North Island, New Zealand. These studies reveal a latest Triassic to earliest Jurassic (225-200 Ma) age of burial metamorphism for the older Rakaia Subterrane, with postmetamorphic uplift and cooling continuing through to Middle Jurassic (c. 170 Ma). A small (c. 20 m.y.) age decrease from the west to east (i.e. to the western boundary of the Esk Head Subterrane, which divides the Rakaia Subterrane from the younger Pahau Subterrane) is related to eastward-migrating imbrication of the accretionary prism. Narrow zones of semischistose (textural zone IIA-B) slates and phyllites within the Rakaia Subterrane are consistently associated with the youngest radiometric ages (135-100 Ma). These zones of later tectonism were associated with local thermal overprints in Early Cretaceous times, related to imbrication of the Pahau Subterrane. The Pahau Subterrane developed in Late Jurassic - Early Cretaceous times and underwent low-grade metamorphism 100-85 m.y. ago. Initial Sr-87/Sr-86 ratios for Rakaia Subterrane meta-turbidites are in the range 0.7065-0.7098 over the 225-170 m.y. age range, and are consistent with the postulated continental arc/cratonic source for the older part of the Torlesse Terrane. Similar initial Sr-87/Sr-86 ratios (0.7065-0.7088) for the Pahau Subterrane over a younger age range (145-85 m.y.) are inconsistent with the Rakaia Subterrane being a dominant sediment source for the Pahau Subterrane through reworking.