A project submitted in partial fulfilment of the requirements for the Degree of Bachelor of Science with Honours in Geology at the Department of Geological Sciences, University of Canterbury. Katherine L. Bodger August, 2004 Frontispiece ___________________________________________________________________________ Lake Ohakuri observed from the west on the airplane from Rotorua to Christchurch. Atiamuri deposits lie in the forest to the right of the photograph. ___________________________________________________________________________ Abstract: The Atiamuri pyroclastic sequence (229 + 12 ka) is part of the Maroa Volcanic Centre, Taupo Volcanic Zone (TVZ), New Zealand. Data from stratigraphy, petrography, geochemistry and comparisons with other research have been gathered to produce a description of the Atiamuri deposits and their eruptive history. Units from this pyroclastic sequence occupy an area of <1 km3 and includes basal phreatomagmatic airfall layers, three ignimbrite units with different degrees of welding separated by shower beds, and overlying airfall layers with alternating ash and lapilli. The Atiamuri pyroclastics are suggested to have erupted from a single vent beneath Mandarin Dome from a small, un-zoned discrete chamber, or from partial evacuation of a larger chamber. Comparisons with other known small ignimbrite-forming events (in particular, the ~AD 1315 Kaharoa eruptive episode at nearby Tarawera Volcanic Centre) have determined that although the Atiamuri is similar in extent and was followed by dome extrusion, it differs in lithic content, absence of block-and-ash flows, interaction with water, general chemical homogeneity and lack of evidence for a mafic eruption trigger. A suggested model for the eruptive sequence at Atiamuri is as follows: (a) Small phreatomagmatic airfall eruption into a swampy lakeshore environment with possible early extrusion of a small proto-dome. (b) Emplacement of pyroclastic flows by a small laterally directed eruption from the side of the proto-dome. Pyroclastic flows are of HARI-type (high aspect ratio) and are topographically controlled. (c) Post-ignimbrite airfall with alternating small eruptions from the vent. Some further minor interaction with water possible. (d) Extrusion of Mandarin Dome from vent, pushing through the ignimbrite deposits. i ___________________________________________________________________________ Abstract i Table of Contents ii List of Figures vi CHAPTER 1 - INTRODUCTION 1 1.1 Overview of the Taupo Volcanic Zone……………………………………… 1 1.2 Regional Geology of the Maroa Volcanic Centre…………………………… 3 1.3 General characteristics of ignimbrites and pyroclastics…………………….. 6 1.4 Field area geology…………………………………………………………... 9 1.5 Previous work………………………………………………………………. 11 1.6 Project objectives…………………………………………………………… 12 1.7 Research methods………………………………………………………….. 12 CHAPTER 2 - STRATIGRAPHY 14 2.1 Introduction………………………………………………………………… 14 2.1.1 Atiamuri stratigraphic column………………………………….. 15 2.2 Relationship to Pukeahua and Ohakuri deposits…………………………… 19 2.3 Airfall layers and shower beds……………………………………………… 19 2.4 Welding……………………………………………………………………… 27 2.4.1 Welding in Atiamuri…………………………………………….. 27 2.5 Pumice and lithic distribution………………………………………………. 30 2.5.1 Unwelded ignimbrite at base…………………………………… 30 2.5.2 Welded ignimbrite……………………………………………… 31 2.5.3 Unwelded ignimbrite at top…………………………………….. 31 2.6 Ignimbrite and dome interaction……………………………………………. 33 ii ___________________________________________________________________________ 2.6.1 Tuahu Dome contact……………………………………………. 33 2.6.2 Mandarin Dome contact………………………………………… 33 2.7 Overview……………………………………………………………………. 35 CHAPTER 3 - PETROGRAPHY 36 3.1 Introduction…………………………………………………………………. 36 3.2 Dome lavas………………………………………………………………….. 36 3.2.1 Kemp Dome…………………………………………………….. 36 3.2.2 Tuahu Dome…………………………………………………….. 37 3.2.3 Mandarin Dome…………………………………………………. 38 3.3 Atiamuri pyroclastics……………………………………………………….. 39 3.3.1 Phenocrysts……………………………………………………… 39 3.3.2 Textures………………………………………………………….. 40 3.3.2.1 Glomeroporphs………………………………………… 40 3.3.2.2 Eutaxitic texture and degree of welding………………... 41 3.3.2.3 Clast interactions……………………………………….. 42 CHAPTER 4 - GEOCHEMISTRY 45 4.1 Introduction………………………………………………………………….. 45 4.2 Dome lavas………………………………………………………………….. 45 4.3 Ignimbrite…………………………………………………………………… 46 4.3.1 Pumice………………………………………………………….. 46 4.3.1.1 Major and minor elements……………………………. 46 4.3.1.2 Trace elements………………………………………… 46 4.3.2 Lithics…………………………………………………………… 46 iii ___________________________________________________________________________ 4.3.2.1 AM8…………………………………………………… 46 4.3.2.2 AM31…………………………………………………. 47 4.3.2.3 AM43…………………………………………………. 47 4.4 Airfall pumices……………………………………………………………… 47 4.4.1 Major and minor elements……………………………………… 47 4.4.2 Trace elements………………………………………………….. 48 4.5 Normalised multi-element abundance diagram…………………………… 48 4.6 Figures…………………………………………………………………….. 48 CHAPTER 5 - DISCUSSION 56 5.1 Introduction…………………………………………………………………. 56 5.1.1 Brief overview of Kaharoa……………………………………… 56 5.2 The Atiamuri pyroclastics………………………………………………….. 57 5.2.1 Stratigraphy…………………………………………………….. 57 5.2.1.1 Pumice and lithic trends……………………………… 57 5.2.1.2 Lithic content in the upper airfall deposits…………... 58 5.2.1.3 Water interaction and accretionary lapilli…………… 58 5.2.1.4 Origin of grey pumices……………………………… 59 5.2.1.5 Welded facies……………………………………….. 60 5.2.1.6 Ignimbrite characteristics…………………………… 61 5.2.2 Petrography…………………………………………………… 62 5.2.3 Geochemistry…………………………………………………. 63 5.2.3.1 Weathering………………………………………….. 64 5.2.3.2 AM4 airfall…………………………………………. 64 5.2.3.3 Spider diagram……………………………………… 64 iv ___________________________________________________________________________ 5.2.3.4 Sr compositional trends…………………………….. 64 5.2.3.5 Using geochemistry for dome and pyroclastic relationships 65 5.3 Source and dome relationships……………………………………………. 65 5.3.1 Mandarin Dome contact………………………………………. 66 5.3.2 Tuahu Dome contact…………………………………………... 67 5.4 Atiamuri emplacement mechanisms………………………………………. 68 5.4.1 Ignimbrite type………………………………………………… 68 5.4.2 Block-and-ash flow deposits………………………………….. 69 5.4.3 Eruptive sequence…………………………………………….. 69 CHAPTER 6 - CONCLUSIONS 72 6.1 Conclusions………………………………………………………………… 72 6.2 Further Research…………………………………………………………… 73 ACKNOWLEDGEMENTS 75 REFERENCES 76 APPENDIX A: SEDIMENTARY DESCRIPTIONS 81 APPENDIX B: PETROGRAPHIC DESCRIPTIONS 90 APPENDIX C: GEOCHEMISTRY DATA 105 v ___________________________________________________________________________ LIST OF FIGURES ___________________________________________________________________________ INTRODUCTION Fig. 1.1 Map of the Taupo Volcanic Zone …………………………………………… 2 Fig. 1.2 a Digital Elevation Map (DEM) of the Maroa Volcanic Centre ……………. 5 b Oblique DEM of the Maroa Volcanic Centre …………………………….. 6 Fig. 1.3 Idealised flow unit of an ignimbrite …………………………………………. 7 Fig. 1.4 Atiamuri field area …………………………………………………………… 10 Fig. 1.5 Photo panorama of the Atiamuri field area…………………………………… 11 STRATIGRAPHY Fig. 2.1 Type-section of Atiamuri at AM2 …………………………………………… 15 Fig. 2.2 a Representative Atiamuri stratigraphic column.…………………………….. 17 b Representative Atiamuri stratigraphic column ……………………………. 18 Fig. 2.3 a Atiamuri welded cliffs ……………………………………………………. 19 b Ohakuri welded cliffs …………………………………………………….. 19 Fig. 2.4 a AM4 airfall layers ………………………………………………………… 21 b AM4 airfall in detail with fine ash………………………………………… 21 c AM33 airfall below welded cliffs…………………………………………. 21 d AM41 airfall showing paleosols…………………………………………… 21 Fig. 2.5 Airfall layers at AM2………………………………………………………… 22 Fig. 2.6 Accretionary lapilli in airfall at AM41………………………………………. 22 Fig. 2.7 a Shower bed between ignimbrite flows at AM8……………………………. 23 b Shower bed detail at AM15……………………………………………….. 23 Fig. 2.8 a Airfall layers cut by small fault, AM19……………………………………. 24 b Shower beds above ignimbrite at AM23………………………………….. 24 c Lapilli airfall at AM21…………………………………………………….. 24 Fig. 2.9 a Stratigraphic columns of basal airfall units at AM2 and AM41…………… 25 b Upper airfall and shower beds at AM19, AM23 and AM31……………… 26 Fig. 2.10 a Welded, columnar-jointed ignimbrite cliff at AM2……………………… 28 vi ___________________________________________________________________________ b Welded cliffs at AM3…………………………………………………… 28 Fig. 2.11 Welded flow units at AM1 showing degrees of welding………………….. 29 Fig. 2.12 a Unwelded basal ignimbrite at AM2……………………………………… 32 b Grading from unwelded to welded in ignimbrite at AM3……………….. 32 Fig. 2.13 Grading from partially welded to welded with columnar jointing at AM3... 32 Fig. 2.14 Contact of Atiamuri with Tuahu Dome at AM31………………………….. 33 Fig. 2.15 a Contact of Atiamuri with Mandarin Dome rhyolite at AM14…………… 34 b Large fragments of rhyolite dome material in ignimbrite at AM17…….. 34 Fig. 2.16 Glassy fiamme at AM16…………………………………………………… 35 PETROLOGY Fig. 3.1 a Plane-polarised light (PPL) thin section of Kemp Dome………………….. 36 b Cross-polarised light (CPL) thin section of Kemp Dome………………… 36 Fig. 3.2 a PPL thin section of Tuahu Dome………………………………………….. 37 b CPL thin section of Tuahu Dome…………………………………………. 37 Fig. 3.3 a b PPL thin section of Mandarin Dome…………………………………….. 38 c d CPL thin section of Mandarin Dome……………………………………. 38 Fig. 3.4 a Orthopyroxene…………………………………………………………….. 39 b Plagioclase………………………………………………………………… 39 c Quartz……………………………………………………………………… 39 d Ilmenite…………………………………………………………...……….. 39 Fig. 3.5 a Compositional zoning, AM13……………………………………………... 40 b Compositional