Pyroclastic deposits and volcanic history of Mayor Island

Abstract

The emergent summit of Mayor Island, Bay of Plenty, New Zealand, is a peralkaline rhyolite volcano constructed by: a sequence of lava flows, the Tutaretare Rhyolite Formation (new); and pyroclastic deposits, the Oira Pyroclastite Formation (new). These 2 formations constitute the Mayor Island Group (new). The pyroclastic deposits mantle most of the outer slopes of the island, in places exceeding 100 m in thickness, and also occur interbedded with lava flows of the main cone. The pyroclastics have been informally assigned on the basis of their compositional, welding and textural, and sedimentary structural characteristics to one or other of 15 lithotypes which may be related to palrticular modes of eruption and emplacement, of both airfall (phreatic, phreatomagmatic, phreatoplinian, and plinian types) and pyroclastic flow (ignimbrite, nuee ardente, and base surge types) origins. A sixteenth lithotype comprises epiclastic deposits formed possibly by catastrophic overspill from an ancestral crater lake. Two new radiocarbon dates on logs from the pyroclastic deposits are recorded: (Wk105) 8000 ± 70 years B.P., and (Wk77) 6340 ± 190 years B.P. Recognition of the calcalkaline Rotoehu and possibly Rotoma Ashes on Mayor Island, together with the new radiocarbon dates, enables definition of 8 phases of major volcanic activity, each separated by relatively quiescent periods with erosion and paleosol formation. Volcanism commenced sometime prior to 42 000 years ago and has continued intermittently up to the eruption of the young dome lavas, possibly less than 1000 years ago. At present, only 1 Mayor Island-derived tephra has been identified on the mainland of the North Island, namely the Tuhua Tephra dated (Wk77) at source as 6340 ± 190 years B.P. However, the character and magnitude of several of the pyroclastic units on Mayor Island is such that recognition of other peralkaline tephras is anticipated in northern North Island.