the impact of accessory minerals on the distribution of trace elements in the continental crust

236 Pages·2013·5.86 MB·English

by  Jessica Matthews

THE IMPACT OF ACCESSORY MINERALS ON THE DISTRIBUTION OF TRACE
ELEMENTS IN THE CONTINENTAL CRUST: AN INTEGRATED
PETROLOGIC AND PHASE EQUILIBRIA MODELING
STUDY OF MIGMATITES

by
Jessica A. Matthews
A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of
Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy
(Geology).

Golden, Colorado

Date ____________________

Signed: ______________________
Jessica A. Matthews

Signed: ______________________
Dr. Nigel M. Kelly
Thesis Advisor

Golden, Colorado

Date ____________________

Signed:_________________
Dr. Paul Santi
Professor and Head
Department of Geology and Geological Engineering

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ABSTRACT

Partial melting of the lower continental crust and subsequent melt emplacement at higher
structural levels is an important crustal differential process that is predicted to produce an
enriched granitic upper crust and depleted, more mafic lower crust. However, the common
occurrence of accessory mineral-rich migmatite residues and upper crustal granites that are
depleted in incompatible elements (U, Th, and REE) suggests our understanding of the controls
on element distribution during recycling is limited. The behavior of key accessory minerals
during partial melting of metasedimentary rocks, is interpreted to impact the composition of
melts generated and extracted from mid-crustal migmatites, and therefore has important
implications for our understanding of the process of recycling in the continental crust.
Two broad approaches were used in this research project to assess the behavior of
accessory minerals in anatectic melts: a natural migmatite study, and modeling of theoretical
melts generated through migmatization of metasedimentary rocks.
Detailed in situ textural and mineral chemical characterization of migmatitic paragneiss
from the Larsemann Hills, east Antarctica, provides insight into the behavior of migmatitic rocks
during polycyclic metamorphism. Important trace element-bearing accessory minerals (zircon
and monazite) record complex trace element zoning patterns that are linked with major mineral
(garnet) growth and breakdown. The work provides insight into the utility of trace element
analysis in the interpretation of complex age relationships and constrains the relative extent of
melting during overprinting anatectic events. The extraction of melts generated during the first
partial melting event, D , left a predominantly residual composition, and the overprinting partial
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melting that occurred during D was localized to fertile domains. Results also suggest that the
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textural location and assemblage context affects accessory mineral growth and breakdown, and
as a result affect the flux of trace and heat producing elements from the site of melting into the
upper crust.
Modeled S-type granitic melts calculated from thermodynamic phase equilibria show that
pressure, temperature, and changes to melt composition (e.g. X ) affect the behavior of zircon
H2O
in partial melts. Under equilibrium conditions zircon will be saturated in the melt at the site of
melting. Slight decreases in X in the melt and/or in temperature will facilitate zircon
H2O

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crystallization. This suggests that zircon may grow close to the site of melting in the presence of
melt, and zircon in granulites does not necessarily reflect growth during cooling of the terrane
below the solidus. The lower crust will be enriched in Zr and therefore zircon, which is
supported by petrographic observations in some natural granulites. As a consequence of zircon
enrichment, trace elements (REEs, HPEs) will also not be depleted in residual lower crustal
rocks. Modeling also demonstrates that the major and accessory element compositions of melts
that are extracted from the deep crust and emplaced at higher structural levels are highly
dependent on melt segregation, extraction rate and interaction with wall rock.

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TABLE OF CONTENTS
ABSTRACT ..................................................................................................................................iii
LIST OF FIGURES .......................................................................................................................ix
LIST OF TABLES .........................................................................................................................xi
ACKNOWLEDGMENTS ............................................................................................................xii
CHAPTER 1 INTRODUCTION ................................................................................................. 1
1.1 Aim of the research ................................................................................................ 2
1.2 Background to the problem .................................................................................... 2
1.2.1 Migmatites ................................................................................................. 2
1.2.2 Crustal melts .............................................................................................. 4
1.2.3 Melt extraction and ascent ......................................................................... 5
1.2.4 S-type granites ........................................................................................... 5
1.2.5 Role and importance of accessory minerals ............................................... 6
1.2.6 Trace element geochemistry ...................................................................... 7
1.3 Approach and methodology ................................................................................... 8
1.4 Natural sample study – Broknes Paragneiss, Larsemann Hills .............................. 9
1.4.1 Mineralogy and microtextures ................................................................. 12
1.4.2 Major and trace element characterization and U-Pb isotopic analysis .... 13
1.5 Phase equilibria modeling .................................................................................... 13
1.6 Dissertation organization ..................................................................................... 15
1.7 References cited ...……........................................................................................ 17
CHAPTER 2 CONSTRAINTS ON THE TIMING AND INTENSITY OF MIGMATIZATION
IN THE LARSEMANN HILLS, PRYDZ BAY, EAST ANTARCTICA: AN
INTEGRATED U-PB AND TRACE ELEMENT STUDY OF ZIRCON,
MONAZITE, AND GARNET ............................................................................. 27
2.1 Abstract ................................................................................................................ 27
2.2 Introduction .......................................................................................................... 28
2.3 Geological context ............................................................................................... 30
2.4 Analytical methods .............................................................................................. 34
2.5 Sample descriptions – Broknes Paragneiss .......................................................... 35
2.5.1 Stüwe Hill – low D strain zone ............................................................... 36
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2.5.1.1 Microtextures and mineral chemistry .......................................... 38
2.5.1.2 Zircon analysis ............................................................................. 41
2.5.1.3 Monazite analysis ......................................................................... 54
2.5.1.4 Garnet analysis ............................................................................. 64
2.5.2 Seal Cove – high D strain zone ................................................................. 65
2
2.5.2.1 Microtextures and mineral chemistry .......................................... 65
2.5.2.2 Zircon and monazite analysis ....................................................... 75
2.5.2.3 Biotite-felsic gneiss ...................................................................... 75
2.5.2.4 Cordierite-felsic gneiss ................................................................ 76
2.5.2.5 Late syn-D leucocratic veins ...................................................... 79
2
2.5.2.6 Garnet analysis ............................................................................. 82
2.6 Discussion .................................................................................................................. 85
2.6.1 Using trace elements to interpret complex age data ................................ 86
2.6.2 Stüwe Hill ................................................................................................ 87
2.6.3 Seal Cove: Biotite-felsic gneiss ............................................................... 94
2.6.4 Seal Cove: Cordierite-felsic gneiss .......................................................... 95
2.6.5 Seal Cove: Late syn-D leucocratic veins ................................................ 96
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2.6.6 Migmatites in the Larsemann Hills: implications for melting in
polycyclic terranes ................................................................................... 97
2.6.7 Implications for REEs in migmatites ..................................................... 101
2.7 Conclusions .............................................................................................................. 103
2.8 References cited ....................................................................................................... 106
CHAPTER 3 INTRODUCTION TO THERMODYNAMIC MODELING OF PARTIAL
MELTING .......................................................................................................... 114
3.1 Thermodynamic modeling ................................................................................. 114
3.2 Phase diagram calculation with THERMOCALC ............................................. 115
3.3 Quantitative features of THERMOCALC ......................................................... 118
3.4 Modeling melts .................................................................................................. 118
3.4.1 Modeling zirconium in melts ................................................................. 121
3.5 Application of THERMOCALC to this study ................................................... 121
3.6 References cited ................................................................................................. 124

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CHAPTER 4 PRESSURE, TEMPERATURE, AND X CONTROLS GROWTH OF
H2O
ZIRCON IN S-TYPE GRANITIC MELTS ....................................................... 128
4.1 Abstract .............................................................................................................. 128
4.2 Introduction ........................................................................................................ 129
4.3 Partial melting and the role of water in mineral crystallization ......................... 131
4.4 Methods .............................................................................................................. 133
4.5 Results – modeling melt-zircon evolution ......................................................... 140
4.6 Caveats and imitations of the models ................................................................ 150
4.7 Discussion .......................................................................................................... 153
4.7.1 Growth of zircon in anatectic melts ....................................................... 154
4.7.2 Zircon controls on trace element budgets .............................................. 156
4.7.3 Implications for dating high temperature metamorphic terranes ........... 160
4.8 Conclusions ........................................................................................................ 161
4.9 References cited ................................................................................................. 163
CHAPTER 5 THE ROLE OF MELTING, MELT EXTRACTION AND EVOLUTION ON
THE COMPOSITION OF S-TYPE GRANITES .............................................. 172
5.1 Abstract .............................................................................................................. 172
5.2 Introduction ........................................................................................................ 174
5.3 Methods .............................................................................................................. 176
5.3.1 Generating modeled melts ..................................................................... 180
5.3.2 Geochemical analysis ............................................................................. 184
5.4 Results ................................................................................................................ 185
5.4.1 Modeled melts ........................................................................................ 185
5.4.2 Whole rock geochemistry of migmatites of the Brattstrand Paragneiss 189
5.5 Discussion .......................................................................................................... 197
5.5.1 The migmatite-granite link .................................................................... 200
5.5.2 Further testing the entrainment hypothesis with modeled melt
compositions .......................................................................................... 205
5.6 Conclusions ........................................................................................................ 210
5.7 References cited ................................................................................................. 211
CHAPTER 6 CONCLUSIONS AND FUTURE WORK ........................................................ 217

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6.1 Petrology of natural migmatites ......................................................................... 218
6.2 Modeled melts .................................................................................................... 219
6.3 Broader implications .......................................................................................... 219
6.4 Future work ........................................................................................................ 220
6.5 References cited ................................................................................................. 222
APPENDIX A SUPPLEMENTAL ELECTRONIC FILES ...................................................... 223

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LIST OF FIGURES
Figure 1.1 Geological map of Prydz Bay and adjacent regions, East Antarctic Shield ........ 10
Figure 1.2 Migmatitic outcrop in the Larsemann Hills ......................................................... 11
Figure 1.3 Photomicrograph of xenoblastic garnet surrounded and embayed by cordierite . 12
Figure 2.1 Location and regional geological map of the Larsemann Hills, east Antarctica . 31
Figure 2.2 Outcrop pictures from Stüwe Hill, Broknes Peninsula ........................................ 37
Figure 2.3 Microtextures from Stüwe Hill migmatite ........................................................... 40
Figure 2.4 Representative Backscattered Electron (BSE) images for in situ zircon grains
from migmatitic semipelite and metapelite at Stüwe Hill and Seal Cove ........... 45

Figure 2.5 U–Pb isotope and REE data from zircon grains in melanosome, selvage, and
leucosome domains from Stüwe Hill ................................................................... 51

Figure 2.6 Representative SEM images (BSE) and X-ray element maps for monazite
grains from migmatite at Stüwe Hill and Seal Cove ............................................ 56

Figure 2.7 U–Pb isotope and REE data from monazite grains in melanosome, selvage and
leucosome at Stüwe Hill ...................................................................................... 62

Figure 2.8 Chondrite-normalized “Matsuda” diagrams for REE in garnet ........................... 66
Figure 2.9 Outcrop pictures from Seal Cove migmatite ....................................................... 71
Figure 2.10 Microtextures from Seal Cove migmatite ............................................................ 73
Figure 2.11 U–Pb isotope and REE data from zircon and garnet grains in biotite-felsic
gneiss .................................................................................................................... 78

Figure 2.12 U-Pb isotope and REE data from zircon and garnet grains in cordierite-felsic
gneiss .................................................................................................................... 81

Figure 2.13 U–Pb isotope and REE data from zircon, monazite, and garnet grains from late
syn-D leucocratic vein ........................................................................................ 84
2

Figure 2.14 D (zrc/grt) patterns calculated for combinations of average zircon rims with
REE
garnet domains ..................................................................................................... 92

Figure 2.15 Zircon, low Th/U monazite, and high Th/U monazite 206Pb/238U
crystallization ages associated with D at Stüwe Hill and Seal Cove ................ 100
2

Figure 3.1 Petrogenetic grid for the KFASH and KMASH systems ................................... 116
Figure 3.2 P-T pseudosection for the system NCKFMASHTZr ......................................... 117
Figure 4.1 Images that give textural context for accessory migmatites and accessory
mineral growth in migmatites ............................................................................ 130

Figure 4.2 P-T pseudosections calculated for an average pelite and average greywacke ... 138
Figure 4.3 T-X pseudosections calculated for an average greywacke and average pelite ... 143

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Figure 4.4 Enlargement of T-X pseudosections 4.3a-c ........................................................ 145
Figure 4.5 Composite T-X grid with first appearance of zircon in two greywacke melts
with different X at 10, 7, 5.5 and 3 kbar ....................................................... 148
H2O

Figure 4.6 Molar proportions of phases as melt crystallizes ............................................... 149
Figure 4.7 Molar proportions of phases as melt crystallizes with decreasing temperature . 157
Figure 5.1 Location map of the Larsemann Hills and Brattstrand Bluffs, Prydz Bay, east
Antarctica ........................................................................................................... 178

Figure 5.2 Migmatites from the Larsemann Hills and Brattstrand Bluffs ........................... 179
Figure 5.3 Chondrite normalized “Matsuda” diagrams for REE in migmatites and S-type
granites ............................................................................................................... 191

Figure 5.4 Major and trace element variation diagrams for natural migmatites, S-type
granites, and experimental glasses, truncated scale ........................................... 196

Figure 5.5 Major and trace element variation diagrams for pelite and greywacke modeled
melts 2, 3 and 4 and Residue 2, natural migmatites, S-type granites, and
experimental glasses, full scale .......................................................................... 199

Figure 5.6 Major and trace element variation diagrams for pelite and greywacke modeled
melts 2, 3 and 4 and Residue 2, natural migmatites, S-type granites, and
experimental glasses, truncated scale ................................................................ 203

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