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Catalogue of Crustal Xenoliths from the St. Heliers Volcanoes, Auckland Volcanic Field, New Zealand K.B

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Catalogue of Crustal Xenoliths from the St. Heliers Volcanoes, Auckland Volcanic Field, New Zealand K.B Catalogue of Crustal Xenoliths from the St. Heliers Volcanoes, Auckland Volcanic Field, New Zealand K.B. Spörli and P.M. Black IESE Report 1-2013.01 | November 2013 ISBN: [print] 978-0-9876566-4-3 [pdf] 978-0-9876566-5-0 Catalogue of Crustal Xenoliths from the St. Heliers Volcanoes, Auckland Volcanic Field, New Zealand K.B. Spörli*1, 2 and P.M. Black1 1 Geology Programme, School of Environment, University of Auckland 2 Institute of Earth Science and Engineering (IESE), University of Auckland *[email protected] IESE Report 1-2013.01 | November 2013 This report was prepared by IESE as part of the DEVORA Project. Disclaimer: While the information contained in this report is believed to be correct at the time of publication, the Institute of Earth Science and Engineering and its working parties and agents involved in preparation and publication, do not accept any liability for its contents or for any consequences arising from its use. Copyright: This work is copyright of the Institute of Earth Science and Engineering. The content may be used with acknowledgement to the Institute of Earth Science and Engineering and the appropriate citation. Photos, from left to right: 1) thin section of amphibolitic schist (plain polarized light). 2): Typical boulder of tuff with xenoliths. 3) Thin section of rodingite with garnet (black) replacing a crack-seal vein (x polarisers). TABLE OF CONTENTS ABSTRACT .................................................................................................................................... 1 INTRODUCTION ........................................................................................................................... 2 GEOLOGICAL FRAMEWORK .................................................................................................... 2 THE AUCKLAND VOLCANIC FIELD .................................................................................... 2 BASEMENT ............................................................................................................................... 7 Murihiku terrane ...................................................................................................................... 7 Maitai terrane .......................................................................................................................... 7 Caples/Pelorus terrane ............................................................................................................. 7 Waipapa terrane....................................................................................................................... 8 Cenozoic cover sediments ....................................................................................................... 8 Northland Allochthon ............................................................................................................ 10 Tectonic history ..................................................................................................................... 10 Present day structural state .................................................................................................... 10 THE SOURCE VOLCANOES ................................................................................................. 11 Glovers Park (St. Heliers) Volcano (Whakamuhu) ............................................................... 11 Taylors Hill Volcano (Taurere) ............................................................................................ 12 Source of fragment collections .............................................................................................. 12 Mode of occurrence of the fragments ................................................................................... 12 DESCRIPTION OF XENOLITHS ............................................................................................... 14 NON- OR LITTLE DEFORMED METABASITES ................................................................ 14 GABBROS ................................................................................................................................ 15 PORPHYRITIC VOLCANIC ROCKS ..................................................................................... 16 Foliated metabasic rocks ....................................................................................................... 17 Opaque–rich coarse metabasites: .......................................................................................... 26 Semi-opaque rich rocks ......................................................................................................... 30 SEDIMENTARY ROCKS ........................................................................................................ 31 Metamorphic sediments ........................................................................................................ 31 Non-metamorphic sandstones ............................................................................................... 34 Metamorphic sandstone xenoliths from other volcanoes: ..................................................... 34 SUMMARY AND DISCUSSION ................................................................................................ 35 TYPES OF XENOLITHS IN THE AUCKLAND VOLCANIC FIELD .................................. 35 THE ROCK ASSEMBLAGE EJECTED BY THE ST. HELIERS VOLCANOES ................. 35 Components ........................................................................................................................... 35 Present state of the ejecta ...................................................................................................... 35 STRUCTURAL SYNTHESIS AND SEQUENCE OF EVENTS ............................................ 36 How many rock suites? ......................................................................................................... 36 Foliated metabasites .............................................................................................................. 36 Ductile, semi-mylonitic faulting in the foliated metabasites ................................................. 37 Non- or little- foliated metabasites ........................................................................................ 37 Metasomatism of the metabasites ......................................................................................... 37 Cataclastic faulting in the metabasites .................................................................................. 37 Veining in the metabasites .................................................................................................... 37 Metamorphism of the metabasites......................................................................................... 38 Metasediments ....................................................................................................................... 38 CORRELATION WITH TERRANES AND STRATIGRAPHIC UNITS ............................... 38 The metabasic rocks .............................................................................................................. 38 The metamorphic sandstones ................................................................................................ 40 Red chert ............................................................................................................................... 42 Source of the metamorphic metabasites and sediments ........................................................ 42 ERUPTION MODEL ................................................................................................................ 43 FURTHER WORK ................................................................................................................... 44 The metabasites ..................................................................................................................... 44 The metamorphic sedimentary rocks .................................................................................... 44 The non-metamorphic xenoliths............................................................................................ 44 The enclosing lavas and tuffs ................................................................................................ 45 Field work at Glovers Park ................................................................................................... 45 Elsewhere in the Auckland Volcanic Field ........................................................................... 45 CONCLUSIONS ........................................................................................................................... 46 ACKNOWLEDGMENTS ............................................................................................................. 47 REFERENCES .............................................................................................................................. 48 APPENDICES ............................................................................................................................... 54 APPENDIX I: list of samples in the Searle collection .............................................................. 54 APPENDIX II: list of samples collected subsequently. ............................................................ 60 ABSTRACT We describe a collection of xenoliths ejected by phreato-magmatic eruptions and preserved in tuff from Glovers Park and Taylors Hill volcanoes in St. Heliers. These rocks are different from mantle-derived xenoliths and semi-assimilated
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