The Park Volcanics group: Field relations of an igneous suite emplaced in the Triassic‐Jurassic Murihiku Terrane, South Island, New Zealand

Park Volcanics Group is proposed for igneous rocks, either shallow intrusive or extrusive, emplaced in the Murihiku Terrane during Triassic‐Jurassic times. The term replaces Park Intrusives of Mutch, some members of which are shown to be extrusive rather than intrusive. Formation status within the group is given to Gowan Andesite and Pinney Volcanics (new names) in western Southland, Glenham Porphyry in eastern Southland, and Barnicoat Andesite (new) in the Richmond area, Nelson. Gowan Andesite is a porphyritic feldspar two‐pyroxene andesite with a glassy or microcrystalline groundmass. A suite of low‐grade metavolcanic rocks which forms the main mass of Malakoff Hill and which has formerly been included in the “Park Intrusives” is here excluded and ascribed to the Takitimu Group; representative chemical data are given. Glenham Porphyry is typically a porphyritic feldspar two‐pyroxene andesite texturally similar to the Gowan Andesite but with significant geochemical differences. Two volumetrically minor members are recognised, Habukinini Trachydacite and Kenilworth Rhyolite. In the north of its outcrop area, Glenham Porphyry is emplaced on or into Late Triassic terrestrial beds; in the middle it overlies Kaihikuan (Middle Triassic) and is overlain by Otapirian (latest Triassic) marine beds; and in the southeast it is directly overlain by Ururoan (late Early to early Middle Jurassic) conglomerates and marine sandstones. Pinney Volcanics are restricted to a very few, probably one, massive conglomeratic horizon in the Oretian Stage. The commonest rock type is a two‐pyroxene trachydacite, modified by very‐low‐grade burial metamorphism. Auto‐brecciation is characteristic and rock types change over short distances. Hornblende‐rich variants occur as well as more felsic varieties including rhyolite ignimbrite. These may have been erupted onto a bouldery floodplain or shallow‐marine surface, but alternatively may have been mass‐emplaced by debris avalanche resulting from flank collapse of a volcano lying beyond the present terrane boundary. In its unaltered condition, Barnicoat Andesite was an olivine two‐pyroxene andesite, distinctly more tholeiitic than the Gowan and Glenham andesites. A previously reported occurrence of “Park Intrusives” in the southern Taringatura Hills is discredited, as is the presence of Jurassic strata in that area. Weetwood Formation, in the eastern flanks of the Takitimu Mountains, is provisionally excluded from the Park Volcanics. Radiometric data indicate a Late Triassic or Early Jurassic age for the Gowan, Glenham, and Barnicoat andesites. Excluding the Pinney Volcanics, no more than 5 unequivocal volcanic events are recognised during more than 80 million years of sedimentation in the Murihiku Basin. A minimum volume of 7.5 km 3 is estimated for the Glenham Porphyry and about one‐third of that figure for the Gowan Andesite. The volume of Pinney Volcanics and Barnicoat Andesite appears to be much less. Medium to high K 2 O in the Park Volcanics suggests that the locus of Murihiku sedimentation was a back‐arc or possibly intra‐arc rather than a fore‐arc basin.