MINERAL CHEMISTRY AND PHASE RELATIONS IN VOLCANOGENIC SEDIMENTS METAMORPHOSED TO PUMPELLYITE-ACTINOLITE FACIES, CHATHAM ISLAND, NEW ZEALAND PLATEAU, SOUTHWEST PACIFIC BY JOHN JEFFREY JOSEPHSON CONTRIBUTION NO. 55 DEPARTMENT OF GEOLOGY a GEOGRAPHY UNIVERSITY OF MASSACHUSETTS AMHERST, MASSACHUSETTS THE MINERAL CHEMISTRY AND PHASE RELATIONS IN VOLCANOGENIC SEDIMENTS METAMORPHOSED TO THE PUMPELLYITE-ACTINOLITE FACIES, CHATHAM ISLAND, NEW ZEALAND PLATEAU, SOUTHWEST PACIFIC (M.S. Thesis) by John Jeffrey Josephson Contribution No. 55 Department of Geology and Geography University of Massachusetts Amherst, Massachusetts June, 1985 iii TABLE OF CONTENTS Page ABSTRACT •.... 1 INTRODUCTION. 3 Regional Setting .. 3 Rock Types and Stratigraphy .. 5 Structural and Metamorphic History. 8 Purpose of Study ......•... 9 Acknowledgments. 9 PETROGRAPHY ...•.••. 9 Introduction. 9 Quartz-Albite Schists .. 15 Black Argillaceous Schist .. 1 7 Greenschists •.......... 1 7 Garnet-bearing Schist .. 20 Relations Between Tectonic Fabrics and Metamorphic Recrystallization .. 20 WHOLE ROCK GEOCHEMISTRY •. 22 Analytical Methods .. 22 Introduction ...... 22 Major Element Data .. 23 Trace Element Data. 23 Discussion. 26 MINERALOGY ..•.... 30 Analytical Procedure. 30 White Mica ......•..... 30 Thin section description ... 30 Chemistry .. 33 Chlorite ....... 40 Thin section description. 40 Chemistry ...•... 40 Epidote-Clinozoisite. 50 Thin section description •. 50 Chemistry .. 50 Pumpellyite ..... 54 Thin section description ... • •••••••••• 54 Chemistry •...............•.... 56 Actinolite-Hornblende .••..•... 66 Thin section description. 66 Chemistry ................. 66 Garnet . ....................... 69 Thin section description •. 69 Chemistry .. 69 Albite .••....... 72 Thin section description. 72 Chemistry .. 72 Sphene ..•.•.•.... 72 Thin section description .. 72 Chemistry ................... 72 iv Calcite ..................................................... 77 Thin section description ••••••.•.•.•..........••.••...• 77 Chemistry. 77 Quartz. 77 Minor minerals. 77 METAMORPHIC PHASE RELATIONS •.•...•.•.••••.•...•.••••.•..•...••.•• 79 Introduction ................................................ 79 [Al o +Fe o - Na o-K 0J - [CaO] - [FeO+MgO+MnO] •...•......• 79 2 3 2 3 2 2 Epidote Projection ••••..•.......•...•••...•..•.••..•.•.•.•.• 81 Chlorite Projection •.•..•....•...•.•••.•..••..••••....••..•. 86 Element Partitioning ........................................ 86 Mineral Reactions with Pumpelly i te. • • . • . • . 91 Phase Relations and Mineral Reaction Involving Garnet .•..... 98 [Al o +Fe o J - [KA10 J - [FeO+MgO+Mn0] ............•........ 105 2 3 2 3 2 Conditions of Metamorphism ••••...•..••...•..•...•..•.•...••. 110 CONCLUSIONS • •••••..•.•••••.•••.•••••••••.••••••.••.•••••••.••.••. 11 3 REFERENCES •• ••••.••••••••••.••••••••••••.•••••••••••••••••••••••• 11 6 LIST OF TABLES 1. Estimated modes of metamorphic rocks from Chatham Island •..• 11 2. Hand specimen description of analyzed schist samples .......• 12 3. Whole rock major and trace element analyses of Chatham Island schist samples ................................................ 24 4. Electron microprobe analyses of white mica •.•...••••...••... 34 5. Electron microprobe analyses of chlorite •.................•. 44 6. Relations between compositional parameters and some optical properties of some chlorites ...•............•...•........... 49 7. Electron microprobe analyses of epidote ...•.•..............• 51 8. Electron microprobe analyses of pumpellyite .•..•....•....... 62 9. Electron microprobe analyses of actinolite .••..•...•..•..... 67 10. Electron microprobe analyses of garnet ..•............•...... 70 11. Electron mlcroprobe analyses of al bite ......••.•.•...•.•.... 73 12. Electron microprobe analyses of sphene ••.••.•.....•......•.. 75 13. Electr~n microprobe antlyses of calcite .•...••.••..••....... 78 14. (Mg/Fe ) ac t vs. (Mg/Fe ) c hl and (Mn/Mg) pumpe ll y it e vs. 89 (Mn/Mg) chlor i te · · • · · · · · · · · · · · · · · • · · · · · · · · · · · · • · · · · · · • · · · · · · · 15. Electron microprobe analyses of carbonate, and garnet from sample 331 . 99 v LIST OF FIGURES Page 1 • Location map of Chatham Island ......•.•...............•.•..... 4 2. Generalized geologic map of the Chatham Islands .....•.......•. 6 3, Structure section across the western end of Chatham Island .... 7 4. Sample location map .......................................... 10 5. Metamorphic textures from two quartz-albite schists .......... 16 6. Metamorphic texture of sample 2248 ••.•.•.....•.••..••••..••..• 1 9 7. Metamorphic texture of sample 427 .•.•.•.•...•••...•.•.•••..•.. 21 8. Thin section drawing of sample 2108 garnet-bearing schist •.... 21 Whole rock major element variation vs. Si0 .....•.....•....... 25 9. 2 10. Whole rock trace element variation vs. Si0 •...............•.. 27 2 11. Zr/Ti02 vs'. Ce discriminant plot.of Cha~ham Island schists .... 28 12. Na vs. S10 plot of quartz-alb1te schists •.................. 29 2o 2 1 3. Zr vs. Nb plot of Chatham Island schists ....•................. 31 14. Whole rock analyses plotted on an ACF diagram .. 2+········· +·· 32 15. White mica analyses in terms of Al(VI) - (Mg+Fe +Mn) - Fe 3 .. 37 16. Ternary plot of white mica analyses in terms of pyrophyllite­ muscovite - leucophyllite ..•............•..• +················ 39 17. Reciprocal prism diagram illustrating the Fe 3 substitution in Chatham Island white micas ..•....•...................•..... 41 18. KAl0 - Al 0 - NaAl0 - MgO+FeO+MnO diagram illustrating the 2 2 2 paragoni te sdbsti tut ion in white ~!cas + ••••.••••••••••••••••• 42 19. Mg~(Mg+Fe +Mn) vs. Altotal and Fe /Fe 3 +Al) vs. Altotal for white mica analyses •.....•.....•.•..•......•...... e·e·i······· 43 20. Chlorite microprobe analyses in terms of Mg/(Mg+Fe 0 a +Mn) vs. Al total. 48 21. Frequency diagram of epidote analyses in terms of Fe 3+/(Fe3++ Al) . 5 3 22. Crystal habit of type 1 and 2 pumpellyites .................... 55 23. Two different optic orientation diagrams of pumpellyite ..•.... 59 24. Pu~pellyite composition space in terms of molecular % Al- Fe - Mg.................... 60 25. Enlarged pumpellyite composition space illustrating pumpellyite microprobe analyses ................... 2+·········· 61 26. Pu~~e11y3~e analyses plotted in terms of Mg/(Mg+Fe +Mn) vs. Fe /(Fe +Al) ...•............................ 65 2+····2.+········ 27. Amphibole classification in terms of Si vs. Fe /~[e +Mg) .... 68 28. Garnet analyses plotted in terms of Mn - Ca - (Fe +Mg) ....... 71 29. Albite analyses plotted on a portion of the feld~gtr plane .... 74 30. Sphene analyses plotted in terms of Al - Ti - Fe a 1 ......... 76 31. ACF diagram illustrating phase relations of Chatham Island schist samples ................................................ 80 32. Geometry of the epidote projection ..............•.•........... 82 33, Greenschist microprobe analys3~ projected from epidote onto the ~asal plane of the [Al+Fe - Na-K] - [Ca] - [Mg] - [Fe 2 +Mn] tetrahedron ...•...........................•..•...... 83 vi 34. Quartz-albite schist microprobe analyses P§~jected from epidote on~~ the basal plane of the [Al+Fe -Na-K] - [Ca] - [Mg] - [Fe +Mn] tetrahedron ......................•.......... 84 35. Summary epidote projection of Chatham Island schist samples .. 85 36. Geometry of the chlorite projection .......................... 87 37. Modified chlorite projection illustrating phase relations for sc~ist samples ............ t .............................. 88 (Mg/Fe ) actinolite vs. (Mg/Fe ) hl .t ................•.... 90 38. c or1 e 39. (Mn/Mg) pumpe ll y1"t e vs. (Mn/Mg) c hl or1.t e •...................•.. 92 40. Modified chlorite projection illustrating ferric pumpellyite consuming reaction ...•....................................... 94 41. Chlorite projection used to illustrate ferric pumpellyite consuming reaction with microprobe data from sample 2248 ..... 94 42. Predicted change of Mg ratio for the assemblage pumpellyite­ chlorite- actinolite depicted on an epidote projection ...•... 95 43. Equilibrium domains schematically illustrated on an [Al 2o + Fe J - [CaO] - [MgO+FeO+MnO] - [KA10 J tetrahedron 3 2o 2 ( sampl~ rf7). • • • • • . • . • • • . • • • . • . • • • • • . • . • • • . • • . • 96 44. ACF diagram illustrating a possible reaction for the three phase assemblage chlorite-pumpellyite-actinolite .••.......... 97 45. Microprobe data from carbonate nodule (sample 331) and garnet-bearing schist sample (210) plotted in terms of Ca - Fe+Mg - Mn . •••.•....••....••.•..•.•.•....•....•...••..•.••... 1 00 46. ACF diagram illustrating phase relations of garnet-bearing schist sample 21 O...........................................• 102 47. Schematic [Al 0 + Fe o ] - [CaO] - [MnO] - [FeO +MgO] tetrahedron illdstrat1ng2 phase relations in garnet-bearing schist sample 21 OB ........................................... 103 48. Epidote projection of mineral analyses onto the basal plane of the [Al20 +Fe20 J - [Cao] - [FeO+MgO] - [MnO] tetrahedron fllust~ating phase relations and reaction direction for a garnet-forming reaction ...................... 104 49. AKF diagram illustrating common phases in low-grade metamorphic rocks ....................................•....... 106 50. AKF diagram illustrating white chlorite phase relations ...... 108 51. Averaged whit2+mica and chlorite analyses plotted in terms of the (Mg+Fe +Mn+ Si)= (Al+ Al) substitution ............ 109 52. Schematic chlorite projection illustrating the change in white mica composition during a continuous