The volcanic geology of Matahana Basin

Abstract

Matahana Basin and environs cover a 70 sq km area between the Mamaku Plateau and Guthrie graben at the western margin of the Taupo Volcanic Zone. The oldest previously described formation that can be identified is the Marshall Ignimbrite. Stratigraphically below are three new ignimbrite formations which are formally named 'Pukerimu Ignimbrite', 'Tikorangi Ignimbrite' and 'Rahopaka Ignimbrite'. The oldest, Pukerimu Ignimbrite, is strongly welded but non-lenticular and quartz-free. It is in gradational contact with the crystal-poor, strongly welded, Tikorangi Ignimbrite, a lenticulite characterised by large fiamme. Rahopaka Ignimbrite is moderately welded, crystal-rich and contains abundant hornblende. Ten thin pyroclastic flows overlying and interbedded with these formations are also described. The Marshall Ignimbrite is redefined and three subunits are recognised largely by welding characteristics. The previously described 'Taupaki Ignimbrite' is no longer recognised. Size distribution of lithic clasts in the basal pumice-fall shows a general decrease in clast size and thickness from north-east to south-west across Matahana Basin. A source for the ignimbrite is postulated beneath the Mamaku Plateau north-east of Matahana Basin. Stratigraphically above the Marshall Ignimbrite are the Waiotapu and Mamaku Ignimbrites. The Waiotapu Ignimbrite is texturally and mineralogically homogeneous throughout but four sub-units (I-IV) are recognised within the Mamaku Ignimbrite. In the basal Sub-unit I there is an increase in the abundance of quartz and decrease in ratio of quartz to feldspar from base to top. In Sub-unit II there is an upward increase in porosity, in size of quartz phenocrysts, in amount of resorption of both quartz and feldspar phenocrysts, in abundance of feldspar and in total phenocryst content. Vertical variations in mineralogy are inferred to reflect a reversal of the pre-eruptive compositional variation within the magma body. The increased resorption of quartz and feldspar at the top of Sub-unit II is inferred to have resulted from the sudden release of pressure in the magma chamber following rapid eruption of the overlying magma. Evidence for past hydrothermal activity in the Matahana area is described. Matahana Basin, although partly fault bounded, is considered to be an erosional, rather than volcanic or tectonic, feature. Paleomagnetic results are presented for nine pyroclastic units underlying the Marshall Ignimbrite. Pukerimu, Tikorangi, and Rahopaka Ignimbrites and overlying pyroclastics are reversely magnetised while a thin pyroclastic flow underlying Rahopaka Ignimbrite is normally magnetised. These results extend the previous paleomagnetic results stratigraphically downwards and are considered to detect the Jaramillo Event within the Matuyama Epoch. Provisional fission-track dates are given for the Tikorangi (1.1 M.a.), Rahopaka (0.74 M.a.), Marshall (0.56 M.a.), and Mamaku (0.17 M.a.), Ignimbrites. The fission-track dates are not statistically valid but are in agreement with the magnetic polarity of the rocks. It is concluded that ignimbrite eruptions commenced on the western margin of the Taupo Volcanic Zone at least 1.1 M.a. ago.