A Geochemical and Mineralogical Investigation of the M O Bility of Uranium and Other Lith O Ph Ile Elements During High-Grade Me
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A GEOCHEMICAL AND MINERALOGICAL INVESTIGATION OF THE MOBILITY OF URANIUM AND OTHER LITHOPHILE ELEMENTS DURING HIGH-GRADE METAMORPHISM. b y Michael Bradford Fowler A thesis submitted for the degree of Doctor of Philosophy, U niversity o f London. Department o f Geology, Im perial College of Science and Technology, Prince Consort Road, L o n d o n SW7 M a y , 1 9 8 5 ABSTRACT Two case-studies were selected for their contrasting characteristics. Regional granulite-grade element depletion was studied in an amphibolite to granulite facies transition, identified at Gruinard Bay, by adopting a comparative approach. Literature data were used for the metamorphic end members, having firs t established magmagenetic comparability. Despite pervasive retrogression, Gruinard Bay gneisses are thought to have suffered hornblende-granulite facies Badcallian metamorphism. The intracrustal partial m elting model for granulite genesis is inappropriate for the specific conditions leading to such a hornblende-bearing residuum. K and Rb abundances at Gruinard Bay intersect those of the end members, related to the continued sta b ility of hornblende and b iotite. Radioelement abundances remain sig n ifica n tly higher than those of granulites, substantially due to the presence of a lla n ite . Minor minerals exert a crucial control on U (and Th?) abundances both in Gruinard Bay gneisses and in granulites. Together with the high mineral-melt partition coefficients for radioelements of even rock-forming minerals when in equilibrium with acid melts, this suggests that partial melting would not result in the observed extensive radioelement depletion. Therefore a fluid is preferred, I^O -rich rather than carbonic because of the advanced stage of depletion reached in rocks which retain significant hydrous m inerals. In contrast, the Glen Dessarry syenite provides a potential example of local element m obility during am phibolite-facies metamorphism/deformation. However, fractional crystallisation coupled with local crustal contam ination clearly remains the first-o rd e r control on element distribution, regardless of the degree of deform ation. A more rigorous study of deformation in the leucocratic syenite confirmed its isochemical and isovolum etric nature on the scale of a whole-rock sample. Nevertheless, several styles of microscopic m obility of selected incompatible elements were detected. Most could be related to pegmatite production in the last stages of a protracted magmatic history. However, in tra crysta llin e m obility of U, Th and possibly lig h t REEs occured as a direct result of deformation, which adequately defines the scale w ithin which any deform ation-related m obility might be sought in this particular environment. ACKNOWLEDGEMENTS The author thanks especially Professor Janet Watson* and Dr Jane Plant for their supervision of this project. Dr G.F. Marriner helped with XRF analyses at Bedford College; Mr P.R. Simpson (BGS London) w ith fa c ilitie s and advice on fission-track analysis; Dr D.I.Sm ith (BGS Edinburgh) w ith early work in the fie ld ; Dr C.T.W illiam s (BMNH) w ith identification and analysis of zirconolite, and Drs J.V.P. Long and R.W. Hinton (University of Cambridge) w ith ion-probe fa c ilitie s . Other menbers of technical and academic staff in the Department of Geology at Imperial College and the M etalliferous M inerals and Applied Geochemistry U nit (BGS London) are too numerous to mention individually, but without their ready help and advice this thesis would never have been completed. Lastly, thanks to Christine for putting up w ith the single-minded chaos necessary for w riting up, w ith confidence that she w ill get her own back one day. ♦Shortly before this thesis was submitted, Professor Watson died. She w ill be sadly missed; this work is dedicated to her memory. LIST OF CONTENTS Chapter_______Title____________________________________________ Page Chapter 1 General Introduction .................................................................... 15 Chapter 2 Introduction to Gruinard Bay. ................................................. 24 2.1 H is to ry o f Research......................................................................25 The Lewisian Complex ........... ....................................................... 25 Anphibolite-G ranulite Transitions Elsewhere .................28 2.2 Geological Setting and Sanple Collection ....................... 29 2.3 Approach...................... 31 2.4 P resentation o f R esults.............................................................32 Chapter 3 Magmagenesis: Comparisons w ith Metamorphic End Members......................................................................................33 3.1 Geochemical Data for "Inrnbbile" Elements ....................... 33 Major Elements ................................................................................ 34 T ra n s itio n M eta ls.......................................................................... 37 "Irnnobile" Large-Ion Lithqphile Elements ....................... 37 High F ie ld -S tre n g th Elem ents..................................................37 Rare E a rth Elem ents......................................................................39 3.2 Magmagenetic M odels.......................................................................42 3.2.1 A C alc-A lkaline In tru sive S uite...........................................43 P artial M elting of a Wet Basaltic Source Leaving an E clogitic Residue ................................................................... 44 Fractional C rystallisation from a Basaltic P a re n t.................................................................................................. 50 3.2.2 A Lower-Crustal Residue after the Abstraction of a G ra n itic P a rtia l M e lt.............................................................54 Chapter 4 The Transitional Nature of the Badcallian Metamorphism a t G ruinard B ay.................................................59 4.1 Mineralogy .......................................................................................... 59 G ran ulite-F acie s M ineralogy....................................................59 Ham blende-G ranulite Facies M ineralogy............................60 Anphibolite-Facies Mineralogy ............................................... 69 G reenschist-Facies M ineralogy................................................71 4.2 Fission-Track Determination of the D istribution o f U T a n iu n ......................................................................................... 71 Gruinard Bay Gneisses ................................................................. 72 G ra n u lite -F a cie s G neisses.........................................................75 4.3 Large-Ion Lithcphile Element Interfacies V a ria tio n s ......................................................................................... 76 Data far the Whole Suite .......................................................... 79 A Comparison o f S pecific Rock Groups ................................. 85 Chapter 5 Large-Ion Lithcphile Element Depletion Mechanisms............................................................................ 94 5.1 Rubidium and Potassium ...............................................................94 5.2 Uraniun and Thorium............................................ 96 5.3 The Phase Responsible far Depletion: Fluid or M e lt? .................................................................................................... 98 Chapter 6 Conclusions to G ruinard Bay..................................................105 Chapter 7 Introduction to Glen D essarry..............................................107 7.1 History of Research ................................................................... 107 7.2 Geological Setting and Sample Collection ....................... 109 7.3 Approach............................................................................................112 7.4 P resentation o f R esults...........................................................114 Chapter 8 Mineralogy and Petrographic Variations. ........................ 115 8.1 M ineralogy........................................................................................115 F e ld sp a rs..........................................................................................115 Pyroxenes and Anphiboles ........................................................ 116 B io tite s ............................................................................................118 Accessory M in e ra ls ......................................................................118 8.2 Petrographic V a ria tio n s ...........................................................120 C um ulates..........................................................................................120 M a fic S ye n ite s...............................................................................121 Leucocratic Syenites ................................................................. 123 Metasedimentary X enoliths ...................................................... 123 S yenite-H ost Rock R eactions..................................................124 8.3 The O rig in o f Am phibole...........................................................125 Chemical Comparisons w ith Igneous and M etamorphic A n phiboles.............................................................128 Oxygen Is o to p e s .............................................................................129