Abstract
Lamprophyres, occurring as dikes and high-level diatremes, intrude greenschist facies Haast Schist in the southern part of the Alpine Dike Swarm, near Wanaka, western Otago. Of 12 lamprophyres dated by conventional K-Ar techniques, 10 give whole-rock ages ranging between 25.2 and 31.9 Ma, with kaersutites separated from four samples forming a tighter group at 22.9-27.8 Ma. A late Oligocene-early Miocene age of intrusion is compatible with field evidence of dike emplacement postdating a major postmetamorphic fold phase that, in the Lake Wakatipu area. deforms Oligocene Bobs Cove Beds along the trace of the Moonlight Fault. Two samples from a dike in the Matukituki River area yield discordant kaersutite, K-feldspar, ocellus, and whole-rock ages that are older (43-126 Ma) than those from compositionally similar dikes elsewhere in the swarm. Similar anomalous ages have been determined previously from lamprophyres and associated intrusives of the northern part of the swarm, at Haast River, and are attributed to incorporation of excess argon into the lamprophyre magma, especially during the later, intratelluric, stages of crystallisation. The late Oligocene-early Miocene Alpine Dike Swarm can be interpreted either as the westernmost and oldest of a series of linear, NNE to NE trending zones of alkaline volcanic rocks, related to the passage of the Campbell Plateau over a narrow mantle melting zone, or to alkaline magmatism in a transtensional tectonic regime related to the propagation of a new plate boundary through the South Island, and the inception of the Alpine Fault.