LITHIC INCLUSIONS in the TAUPO PUMICE FORMATION by Tadiwos
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LITHIC INCLUSIONS IN THE TAUPO PUMICE FORMATION by Tadiwos Chernet A thesis submitted for the degree of Master of Science in Geology at the Research School of Earth Sciences Victoria University of Wellington January, 1987 FRONTISPIECE Subscene of Pel 18821 colour mosaic (courtesy of Physics and Engineering Labratory, D.SJ.R.) consists of photos taken by Landsat 1 on 22 Dec 1975 and 15 Feb 1976. The picture shows most part of the TVZ, with Lake Taupo Volcanic Centre in the middle. The greywacke ranges and ignimbrite plateau to the east and west of the TVZ are distinct from the active volcanic region which runs in the north-east direction. Products of the 1800a Taupo eruption have covered most part of the area shown. \/\CTORIA UN~' ,,--- -':Y CF WELLlNGTON' i i ABSTRACT The Taupo Pumice Formation is a product of the Taupo eruption of about 1800a, and consists of three pt'batomagmatic 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-]0% 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 kID 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 hydrothe'inally 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-<:rystal 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. iii A comparative mineral chemistry study of the lithic blocks was done. A change in chemistry of individual mineral species was found to a.ccompany the variation in whole rock 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. IV ACKNOWLEDGEMENTS Many people have contributed directly or indirectly to this work and I can not hope to mention them all. I thank all the academic and non-academic staff and graduate students of the Research School of Earth Sciences (Victoria University of Wellington). First and foremost I am very grateful to my supervisor Dr Jim Cole without whose instruction and constant encouragement this work could have not been possible. I am very grateful to Dr John Gamble, Dr Rodney Grapes and Dr Russele Howorth for their help in many problems related to this project. I am also very grateful to Mr Ken Palmer for instructions and assistance given in using the analytical facilities at the Research School of Earth Sciences and for providing his computer programmes used for most of the calculations in this work. I am very grateful for Dr Paul Froggatt (Center of Continuing Education, Victoria University of Wellington ) for his interest in the work, important comments and help in reviewing the text. Special thanks are due to Dr Bruce F. Houghton of the New Zealand Geological Survey and Dr Colin J. N. Wilson (University of Auckland) for the invaluable advice and discussions concerning the field work for this study. I thank all the staff of Geological Survey of New Zealand (Rotorua branch) for the material support (maps, sample bags etc.) and moral encouragment in the course of this work. I am very grateful to Mr Philip White for help in many ways during my labratory work and writing of this thesis and Mr John Keale for many important discussions about some of the topics in this thesis and for proof reading most of the text. I am very grateful to Mr John Carter for the preparation of thin sections and polished sections and Mr Stev~n Eagar for his help in getting maps, slides and photos in the department. Most of the thesis is typed by the author and I am responsible for most of the typing mistakes however I am very grateful for Mrs. Va.) Herbert and Ms. Betty Finlayson for help with typing and preparation of the this thesis. v CONTENTS Frontispiece ........................................... i Abstract ....... ii Acknowledgements iv Table of Contents v INTRODUCTION 1 REGIONAL GEOLOGY 2.1 Tectonic setting 4 2.2 The Taupo Volcanic Zone 6 2.3 The Taupo Volcanic CentrE 10 2.3.1 Structures ....... 10 2.3.2 Eruption History ....... 12 PRE-TAUPO PUMICE FORMATION STRATIGRAPHY 3.1 Introduction 14 3.2 Ohakuri Group 17 3.3 Whakamaru Group Ignimbrite 19 3.4 Waiora Formation 22 3.5 Waiora Valley Andesite 24 3.6 Huka Falls Formation 25 3.7 Tauhara Dacite 28 3.8 Haparangi Rhyolite 29 3.9 Lake Taupo Group ............. 30 3.9.1 Okai Sub-group ............ 32 3.9.2 Kawakawa Tephra Formation 32 3.9.3 Lake Taupo Sub-group 37 THE TAUPO PUMICE FORMATION 4.1 Introduction 38 4.2 Eruptive History 40 4.3 Hatepe Tephra 43 4..4 Rotongaio Ash 43 4.5 Taupo Plinian Pumice .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ........ 45 4.6 Taupo Ignimbrite .................................................................. 45 vi 4.7 The Ground Layer ............................. 48 4.7.1 General .................................. 48 4.7.2 Field occurrence ................................ 50 4.7.3 Grainsize characteristics ........................... 52 4.7.4 Origin and mechanisms of formation . 54 4.7.5 Bulk composition and sampling ...................... 59 PETROGRAPHY OF LITHIC BLOCKS 5.1 Introduction 62 5.2 Rhyolites ..... 62 5.2.1 Hypersthene Rhyolite 65 5.2.2 Hypersthene-hornblende Rhyolite 66 5.2.3 Biotite bearing Rhyolite 66 5.3 Dacites 67 5.4 Andesites ................ 69 5.4.1 Hornblende Andesite ..... 71 5.4.2 Plagioclase-pyroxene Andesite 71 5.4.3 Pyroxene. Andesite 73 5.4.4 Olivine Andesite 73 5.5 Ignimbrites .................. 74 5.5.1 Type 1 Lithic crystal-rich ignimbrite ..... 74 5.5.2 Type 2 Crystal-rich ignimbrite .......... 75 5.6 Non-volcanic Rock types ....... 75 5.6.1 Lake sediments ....... ...... 75 5.6.2 Greywacke ....................... 75 MINERAL CHEMISTRY OF LITHIC BLOCKS 6.1 Introduction 77 6.2 Olivine 77 6.3 Pyroxene ..... 80 6.3.1 Orthopyroxene 84 6.3.2 Clinopyroxene 86