Lithic inclusions in the Taupo pumice formation

Abstract

The Taupo Pumice Formation is a product of the Taupo eruption of about 1800a, and consists of three phreatomagmatic ash deposits, two plinian pumice deposits and a major low-aspect ratio and low grade (unwelded) ignimbrite which covered most part of the central North Island of New Zealand. The vent area for the eruption is located at Horomatangi Reefs in Lake Taupo. Lithics in the phreatoplinian ash deposits are negligible in quantity, but the plinian pumice deposits contain 5-10% lithics by volume in most near-vent sections. Lithics in the plinian pumice deposits are dominantly banded and spherulitic rhyolite with minor welded tuff, dacite and andesite. The ground layer which forms the base of the ignimbrite unit consists of dominantly lithics and crystals and is formed by the gravitational sedimentation of the 'heavies' from the strongly fluidized head of the pyroclastic flow. Lithic blocks in the ground layer are dominantly banded and spherulitic phenocryst-poor rhyolite, welded tuff with minor dacite and andesite. Near-vent exposures of the ground layer contain boulders upto 2 m in diameter. Friable blocks of hydrothermally altered rhyolite, welded tuff and lake sediments are found fractured but are preserved intact after transportation. This shows that the fluid/pyroclastic particle mixture provided enough support to carry such blocks upto a distance of 10 km from the vent. The rhyolite blocks are subdivided into hypersthene rhyolite, hypersthene-hornblende rhyolite and biotite-bearing rhyolite on the basis of the dominant ferromagnesian phenocryst assamblage. Hypersthene is the dominant ferromagnesian phenocryst in most of the rhyolite blocks in the ground layer and forms the major ferromagnesian crystal of the Taupo Sub-group tephra. The rhyolite blocks have similar whole rock chemistry to the Taupo Sub-group tephra and are probably derived from lava extrusions associated with the tephra eruptions from the Taupo Volcanic Centre in the last 10 ka. Older rhyolite domes and flows in the area are probably represented by the intensely hydrothermally altered rhyolite blocks in the ground layer. The dacite blocks contain hypersthene and augite as a major ferromagnesian phenocryst. Whole rock major and trace element analyses shows that the dacite blocks are distinct from the Tauhara dacites and from the dacites of Tongariro Volcanic Centre. The occurrence of dacite inclusions in significant quantity in the Taupo Pumice Formation indicates the presence of other dacite flows near the vent area. Four types of andesite blocks; hornblende andesite, plagioclase-pyroxene andesite, pyroxene andesite and olivine andesite occur as lithic blocks in the ground layer. The andesites are petrographically distinct from those encountered in deep drillholes at Wairakei (Waiora Valley Andesites), and are different from the Rolles Peak andesite in having lower Sr content. The andesite blocks show similar major and trace element content to those from the Tongariro Volcanic Centre. The roundness of the andesite blocks indicates that the blocks were transported as alluvium or lahars in to the lake basin before being incorporated into the pyroclastic flow. Two types of welded ignimbrite blocks are described. The lithic-crystal rich ignimbrite is correlated with a post-Whakamaru Group Ignimbrite (ca. 100 ka ignimbrite erupted from Taupo Volcanic Centre) which crops out to the north of Lake Taupo. The crystal rich ignimbrite is tentatively correlated with the Whakamaru Group Ignimbrites. The lake sediment boulders, pumiceous mudstone and siltstone in the ground layer probably correlate to the Huka Group sediments or younger Holocene sediments in the lake basin. A comparative mineral chemistry study of the lithic blocks was done. A change in chemistry of individual mineral species was found to accompany the variation in wholerock major element constituents in the different types of lithics. The large quantity of lithic blocks in the ground layer suggests extensive vent widening at the begining of the ignimbrite eruption. A simple model of flaring and collapse of the vent area caused by the down ward movement of the fragmentation surface is presented to explain the origin of the lithic blocks in the ground layer. The lithics in the Taupo Pumice Formation are therfore produced by the disruption of the country rock around the vent during the explosion and primary xenoliths from depths of magma generation were not found. Stratigraphic relations suggest that the most important depth of incorporation of lithics is within the post-Whakamaru Group Ignimbrite volcanics and volcaniclastic sedimentary units.