Age and correlation of volcanic rocks of Campbell Island and Metamorphic basement of the Campbell Plateau, South-west Pacific

Abstract

Potassium-argon total-rock ages are presented for extrusive and hypabyssal volcanic rocks of Campbell Island Volcanics, which overlie Cretaceous-Oligocene pelagic marine (Campbell Island Group) and Upper Miocene volcaniclastic sediments (Shoal Point Formation). Ages of basalt, hawaiite, mugearite, and trachyte range from 11^.1 to 6^.5 Ma, but there is an important concentration at 7^.0-7-4 Ma (Upper Miocene). The earliest basalt flows (11^.1 Ma), seen only at Puiseux Peak, are overlain by about 300 m of hawaiite-mugearite-trachyte extrusives of alkali olivine basalt type (7^.0-7^.4 Ma). These in turn are capped by at least 100 m of similar but younger flows (6^.5-6^.7 Ma) that are best seen on the northern flanks of the island at Mt Faye. A trachyte at Smoothwater Bay, probably intrusive into Shoal Point Formation, is dated at 8 ^. 5 Ma and indicates an earlier phase of acid volcanism off the eastern coast of the island. Menhir Gabbro on the west coast is dated at 16 ±1 Ma (Middle Miocene) and represents an older pre-volcanic plutonic phase similar to Carnley Gabbro of Auckland Islands. The low grade metasediments, Complex Point Group, form the basement of the island and yield a maximum measured age of 443 ±6 Ma (Ordovician) that can be closely compared with ages of similar slate and phyllites of Greenland Group (Cambro-Ordovician) on the west coast of the South Island, that were formed during a Paleozoic regional metamophism of Tuhuan (Devonian-Carboniferous) Orogeny. The Miocene alkali olivine basalt volcanism of Campbell Island can be correlated on petrogenetic and chronological evidence with that of Auckland Islands, the Dunedin area and Banks Peninsula to the north, and central Marie Byrd Land in West Antarctica to the south-easi. This intra plate volcanism seems to have accompanied renewed fast spreading of the South-west Pacific in late Cenozoic time.