Systematics of zircon crystallisation in the Cretaceous Separation Point Suite, New Zealand, using U/Pb isotopes, REE and Ti geothermometry |
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| Authors: | Robert Bolhar Steve D Weaver J Michael Palin Jim W Cole Lorraine A Paterson |
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| Institution: | (1) Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, 8020, New Zealand;(2) Department of Geology, University of Otago, P.O. Box 56, Dunedin, 9054, New Zealand |
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| Abstract: | Plutonic zircons from the Cretaceous Separation Point Suite (SPS) were analysed by LA-ICPMS for U–Pb isotope ratios and trace
element concentrations. Pooled 206Pb/238U ages range from 112 to 124 Ma. Cathodoluminescence imaging reveals minor inheritence and textural evidence of repeated dissolution
and re-precipitation of zircon. Core and rim spot analyses, however, document zircon growth during extended periods of time
(>2 myr). Protracted crystallisation histories for simple plutonic systems are inconsistent with generalised thermal constraints,
which predict cooling below the solidus within <1 myr. Consequently, we conclude that the SPS granitoids sampled in this study
were not emplaced rapidly but incrementally over extended time periods. Zircon Th/U and Zr/Hf ratios are positively correlated
with crystallisation temperatures, consistent with crystallisation from evolving melts. However, highly variable trace element
concentrations, along with temperature reversals are indicative of complex crystallisation histories involving continuous
fractional crystallisation repeatedly punctuated by hotter, more mafic magma recharge. Normalised abundances of the redox-sensitive
elements Eu and Ce in zircon vary systematically with degrees of whole rock differentiation, pointing to evolutionary trends
in magmatic oxidation states coupled with feldspar crystallisation. |
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| Keywords: | ICPMS Pluton Separation Point Suite Ti-in-zircon thermometry Trace elements Western Fiordland Orthogneiss Zircon |
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