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From Greenschist to Granulite: a Mineral Equilibria Approach to Melting and Melt Loss

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From Greenschist to Granulite: a Mineral Equilibria Approach to Melting and Melt Loss From greenschist to granulite: A mineral equilibria approach to melting and melt loss Thesis submitted in accordance with the requirements of the University of Adelaide for an Honours Degree in Geology Kiara Louise Bockmann November 2015 Kiara Bockmann Melt loss and melt reintegration modelling FROM GREENSCHIST TO GRANULITE: A MINERAL EQUILIBRIA APPROACH TO MELTING AND MELT LOSS RUNNING TITLE: MELT LOSS AND MELT REINTEGRATION MODELLING ABSTRACT Melt loss during regional high-grade metamorphism has important consequences for interpreting the metamorphic evolution of the lower crust and for understanding processes leading to the chemical differentiation of the crust. However, melt loss typically modifies the protolith; making it difficult to reconstruct the conditions of prograde metamorphism and the extent to which melt loss modified the rock composition. The Reynolds Range in central Australia preserves a rare example where a single melt-prone stratigraphic unit can be traced from greenschist to granulite grade conditions. Using this as a natural laboratory, P–T mineral equilibria forward models have been calculated to explore melt loss and melt reintegration where both the protolith and the residuum compositions are preserved. Incremental melt loss modelling from the protolith composition along an isobaric heating path at 5 kbar shows that the residual granulite facies rock composition is consistent with around 18% melt loss from the protolith. Large-scale, one-step melt loss from a closed rock system that had built up 18% melt resulted in a similar residual composition to incremental melt loss. The fertility of the open (incremental) system and the closed system showed the closed system produced 5.4% more melt along a heating path from 700–800 °C. Determination of the concentrations of K–U–Th with increasing metamorphic grade shows that K and U concentrations decreased with increasing metamorphic grade. Conversely, Th concentrations increased, resulting in a slight overall increase in heat production from the protolith to the residuum, despite around 18% volume loss associated with melt extraction. An implication for this is that for melt prone rocks such as metapelites, melt loss during granulite facies metamorphism does not deplete the concentration of heat producing elements in the lower crust as is typically assumed. KEYWORDS Metamorphic; Reynolds Range; pseudosection; melt loss; melt reintegration; THERMOCALC; catastrophic melting; heat production MINERAL ABBREVIATIONS bi, biotite; mu, muscovite; chl, chlorite; ma, margarite; g, garnet; opx, orthopyroxene; sill, sillimanite; ky, kyanite; and, andalusite; sp, spinel; mt, magnetite; cd, cordierite; liq, silicate melt; ksp, K-feldspar; q, quartz; pl, plagioclase; ep, epidote; ru, rutile; st, staurolite; hem, hematite; ilm, ilmenite. 3 Kiara Bockmann Melt loss and melt reintegration modelling TABLE OF CONTENTS From greenschist to granulite: A mineral equilibria approach to melting and melt loss .. 3 Running title: Melt loss and melt reintegration modelling ............................................... 3 Abstract ............................................................................................................................. 3 Keywords .......................................................................................................................... 3 Mineral abbreviations ....................................................................................................... 3 List of Figures and Tables ................................................................................................. 6 1. Introduction ................................................................................................................... 7 2. Geological background ................................................................................................. 8 2.1. Geological setting .................................................................................................................... 8 2.2. Field-based metamorphic geology ......................................................................................... 10 3. Analytical Methods ..................................................................................................... 13 3.1. Bulk rock and mineral chemistry ........................................................................................... 13 3.2. Mineral equilibria modelling ................................................................................................. 14 3.2.1 Melt loss and melt reintegration modelling ..................................................................... 16 3.3. KUTh concentration .............................................................................................................. 18 4. Results ......................................................................................................................... 19 4.1. Metamorphic geology ............................................................................................................ 19 4.1.1. Metamorphic petrography ............................................................................................... 19 4.1.2. Geochemistry and mineral chemistry ............................................................................. 21 4.2. Pressure-temperature pseudosections ...................................................................... 22 4.2.1. Melt loss .......................................................................................................................... 23 4.2.2. Melt reintigration ............................................................................................................ 30 5. Discussion ................................................................................................................... 36 5.1. Limitations of the modelling ................................................................................................. 36 5.2. Melt loss calculations ............................................................................................................ 38 5.2.1. Catastrohpic versus incremental melt loss calculations .................................................. 41 5.2.2. Melt Fertility ................................................................................................................... 43 5.2.3. Structural controls on the style of melt loss .................................................................... 44 5.3. Melt reintegration calculations .............................................................................................. 45 5.4. Implications for metamorphic induced differentiation of crustal heat production ................ 47 6. Conclusions ................................................................................................................. 47 7. Acknowledgments ....................................................................................................... 48 References ....................................................................................................................... 49 Appendix 1: Heat production .......................................................................................... 55 Methods ........................................................................................................................................ 57 Gamma Ray Spectrometers ...................................................................................................... 57 4 Kiara Bockmann Melt loss and melt reintegration modelling Results .......................................................................................................................................... 59 Outcrop determination of K-U-Th ............................................................................................ 59 Gamma ray spectrometer data .................................................................................................. 60 K–U–Th distribution and heat production ................................................................................ 66 Representative monazite microprobe analyses ......................................................................... 67 Discussion ..................................................................................................................................... 69 KUTh distribution with progressive metamorphism ................................................................ 69 Implications for metamorphic induced differentiation of crustal heat production ....................... 70 Appendix 2: Whole-rock geochemistry .......................................................................... 72 Appendix 3: Representative electron microprobe analyses ............................................ 78 Appendix 4: Melt loss compositions ............................................................................... 82 Appendix 5: Catastrophic melt loss ................................................................................ 83 Appendix 6: Melt fertility ............................................................................................... 84 Appendix 7: Melt reintegration compositions ................................................................ 84 5 Kiara Bockmann Melt loss and melt reintegration modelling LIST OF FIGURES AND TABLES Figure 1. Geology of the Reynolds Range ........................................................................... 10 Figure 2. Field photographs of the Pine Hill Formation
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