University of Wollongong Research Online Faculty of Science, Medicine & Health - Honours Theses University of Wollongong Thesis Collections 2016 Mineralogy and geochronology of high-pressure, low-temperature metamorphic rocks using a combination of field studies, petrography and zircon U-Pb dating Ruby R. Stovin-Bradford Follow this and additional works at: https://ro.uow.edu.au/thsci University of Wollongong Copyright Warning You may print or download ONE copy of this document for the purpose of your own research or study. The University does not authorise you to copy, communicate or otherwise make available electronically to any other person any copyright material contained on this site. You are reminded of the following: This work is copyright. Apart from any use permitted under the Copyright Act 1968, no part of this work may be reproduced by any process, nor may any other exclusive right be exercised, without the permission of the author. 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Recommended Citation Stovin-Bradford, Ruby R., Mineralogy and geochronology of high-pressure, low-temperature metamorphic rocks using a combination of field studies, petrography and zircon U-Pb dating, IBSci Hons, School of Earth & Environmental Sciences, University of Wollongong, 2016. https://ro.uow.edu.au/thsci/133 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] Mineralogy and geochronology of high-pressure, low-temperature metamorphic rocks using a combination of field studies, petrography and zircon U-Pb dating Abstract The serpentinite mélange at Port Macquarie hosts an assortment of island-arc and accretionary complex rocks including chert, shale, felsic to ultramafic dykes and high- pressure, low-temperature (HP/LT) blueschist facies rocks that together represent a dismembered ophiolitic sequence. Rare blocks of blueschist conglomerate from Rocky Beach reported for the first time here contain felsic, basaltic and pelitic clasts, within an arkose matrix. Dominant assemblages in both the clasts and matrix established through petrography and geochemistry are glaucophane + phengite + alkali feldspar ± lawsonite ± chlorite ± pyroxenes ± hornblende. The coexistence of glaucophane and lawsonite confirms that this conglomerate underwent HP/LT blueschist facies metamorphism. Due to the low metamorphic temperature (<400˚C) overgrowths did not develop on the protolith zircons therefore a direct age of metamorphism was not obtained. Nonetheless the U-Pb zircon dates presented in this thesis help to resolve an ongoing controversy over an Ordovician or younger Palaeozoic age of metamorphism. The sample of blueschist conglomerate gave a small yield of zircons with variable rounding and pitting associated with abrasion within sedimentary systems. This morphology highlights the detrital nature of the zircons, therefore allowing the maximum age of metamorphism to be established by the youngest detrital grain. U-Pb zircon ages range from Archean (2936 ± 14 Ma) to Carboniferous (343 ± 8 Ma). These results constrain the maximum age of metamorphism to be less than 343 ± 8 Ma, revealing a much younger HP metamorphic event than the Ordovician 470 Ma age obtained through K-Ar dating of phengite micas, a technique prone to giving excessive ages due to retention of radiogenic argon in HP/LT metamorphic rocks. These results are compatible with a tectonic model for the eastern margin of Gondwana which incorporates periods of accretion and “Andean-type” magmatism with episodic events of island-arc collisions (Gympie terrane). A possible interpretation using this model is that this Carboniferous detrital zircon came from the erosion of material from a Carboniferous continental arc to the west. Following which it was deposited into the fore-arc basin and trench as a deep marine, mass-flow deposit before being subducted and subjected to blueschist facies metamorphism. These blueschist conglomerates were exhumed rapidly as serpentinite diapirs prior to or during the docking of the Gympie terrane onto Gondwana at the Permo-Triassic transition. Degree Type Thesis Degree Name IBSci Hons Department School of Earth & Environmental Sciences Advisor(s) Allen Nutman Keywords Port macquarie, Rocky beach, bluechrist, bluechrist conglomerate This thesis is available at Research Online: https://ro.uow.edu.au/thsci/133 Mineralogy and geochronology of high-pressure, low-temperature metamorphic rocks using a combination of field studies, petrography and zircon U-Pb dating A thesis submitted in partial fulfilment of the requirements for the award of the degree of INTERNATIONAL BACHELOR OF SCIENCE (HONS) From The University of Wollongong By RUBY ROSE STOVIN-BRADFORD School of Earth & Environmental Sciences, Faculty of Science, Medicine and Health OCTOBER, 2016 ii The information in this thesis is entirely the result of investigations conducted by the author, unless otherwise acknowledged, and has not been submitted in part, or otherwise, for any other degree or qualification. Ruby Rose Stovin-Bradford 9th October, 2016 i ABSTRACT The serpentinite mélange at Port Macquarie hosts an assortment of island-arc and accretionary complex rocks including chert, shale, felsic to ultramafic dykes and high- pressure, low-temperature (HP/LT) blueschist facies rocks that together represent a dismembered ophiolitic sequence. Rare blocks of blueschist conglomerate from Rocky Beach reported for the first time here contain felsic, basaltic and pelitic clasts, within an arkose matrix. Dominant assemblages in both the clasts and matrix established through petrography and geochemistry are glaucophane + phengite + alkali feldspar ± lawsonite ± chlorite ± pyroxenes ± hornblende. The coexistence of glaucophane and lawsonite confirms that this conglomerate underwent HP/LT blueschist facies metamorphism. Due to the low metamorphic temperature (<400˚C) overgrowths did not develop on the protolith zircons therefore a direct age of metamorphism was not obtained. Nonetheless the U-Pb zircon dates presented in this thesis help to resolve an ongoing controversy over an Ordovician or younger Palaeozoic age of metamorphism. The sample of blueschist conglomerate gave a small yield of zircons with variable rounding and pitting associated with abrasion within sedimentary systems. This morphology highlights the detrital nature of the zircons, therefore allowing the maximum age of metamorphism to be established by the youngest detrital grain. U-Pb zircon ages range from Archean (2936 ± 14 Ma) to Carboniferous (343 ± 8 Ma). These results constrain the maximum age of metamorphism to be less than 343 ± 8 Ma, revealing a much younger HP metamorphic event than the Ordovician 470 Ma age obtained through K-Ar dating of phengite micas, a technique prone to giving excessive ages due to retention of radiogenic argon in HP/LT metamorphic rocks. These results are compatible with a tectonic model for the eastern margin of Gondwana which incorporates periods of accretion and “Andean-type” magmatism with episodic events of island-arc collisions (Gympie terrane). A possible interpretation using this model is that this Carboniferous detrital zircon came from the erosion of material from a Carboniferous continental arc to the west. Following which it was deposited into the fore-arc basin and trench as a deep marine, mass-flow deposit before being subducted and subjected to blueschist facies metamorphism. These blueschist conglomerates were exhumed rapidly as serpentinite diapirs prior to or during the docking of the Gympie terrane onto Gondwana at the Permo-Triassic transition. ii TABLE OF CONTENTS 1 Overview .......................................................................................................................... 13 1.1 Introduction ............................................................................................................... 13 1.2 Study Area ................................................................................................................. 14 1.3 Geological Setting ..................................................................................................... 15 1.3.1 Regional Setting ................................................................................................. 15 1.3.2 Local Setting: Port Macquarie Block ................................................................. 19 1.4 Suprasubduction Setting ............................................................................................ 21 1.5 Aims and Objectives ................................................................................................. 21 1.6 Significance ............................................................................................................... 22 2 Background .....................................................................................................................