Geology of the plutonic basement rocks of Stewart Island, New Zealand

Abstract

Exposures of basement rocks on Stewart Island provide a c. 70 km long by 50 km wide map of part of the Median Batholith that spans the margin of the Western Province. Because of their distance from the present plate boundary, these rocks are relatively unaffected by Cenozoic tectonism, allowing examination of unmodified Carboniferous‐Cretaceous relationships within the Median Batholith. Thirty individual plutons (>c. 20 km2) have been mapped along with numerous relatively small intrusions (<c. 5 km2). The large plutons form 85–90% of the Median Batholith on Stewart Island while the many smaller intrusions comprise 10–15%, mostly in the north. Lithologies include: biotite ± minor hornblende granodiorite, granite and leucogranite with accessory titanite ‐ magmatic epidote and allanite (c. 50%); biotite ± muscovite ± garnet granite with S‐type affinities (c. 10%); alkaline quartz monzonite, granite, and alkali feldspar granite with rare aegirine and blue‐green amphibole (c. 3%); quartz monzodiorite and diorite with hornblende > biotite (c. 23%); gabbro and anorthosite (c. 12%) and ultramafic rocks (c. 2%). U‐Pb zircon and monazite dating indicates that c. 12% of these plutonic rocks were emplaced during the Carboniferous between 345 and 290 Ma, c. 20% in the Early‐Middle Jurassic at c. 170–165 Ma, c. 30% in the latest Jurassic to earliest Cretaceous between 152 and 128 Ma, and c. 38% in the Early Cretaceous between 128 and 100 Ma. The distribution of Pegasus Group schists and peraluminous granitoid rocks indicates that the northern limit of extensive early Paleozoic Western Province basement is located either within the Gutter Shear Zone or at the Escarpment Fault, 10–15 km south of the Freshwater Fault System previously thought to mark this boundary. Carboniferous and Middle Jurassic magmatism extended plutonic basement northwards as far as the Freshwater Fault System, while further magmatism during the latest Jurassic and earliest Cretaceous produced the basement north of the Freshwater Fault System. The focus of Early Cretaceous plutonism then returned southwards into the Western Province, although the older basement in this area was only involved in the genesis of subordinate peraluminous plutonism at this time and not the more extensive metaluminous rocks. The Escarpment Fault disrupted this c. 40 km wide section across the margin of the Western Province at c. 110–100 Ma.