The Mavuradonha Layered Complex: Neoproterozoic ... - ArchiMeD

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3 Analytical methods 32 higher 204 Pb counts than the CZ3 204 Pb counts (Nelson, 1997). Data regression followed the method described in Nelson (1997). Reported ages for single spot analyses are given at 1 σ error. The errors are based on counting statistics. Concordia intercept calculations were prepared using the computer program of Ludwig (1994). Single zircon evaporation method Zircon evaporation was carried out after Kober (1986, 1987) with minor modifications after Kröner & Todt (1988) and Kröner & Hegner (1998). Zircons were embedded into Re-filaments and measured in double filament arrangements. The evaporation method involves repeated measurements of untreated zircons with gradually increased temperature for evaporation and ionisation. Deposition of the evaporated Pb isotopes takes place on a second filament from which the Pb isotopes are ionized. In this study only data without changes in isotopic ratios were considered for age determinations. The ages and their 2 σ mean error were integrated from all measured 207 Pb/ 206 Pb ratios; the distribution of these ratios is shown in histograms. Pb mass fractionation is estimated around 0.3 ‰ per atomic mass unit (W. Todt, pers. comm. 1999). This fractionation is significantly less than the relative standard deviation of the measured 207 Pb/ 206 Pb ratios and therefore no corrections were made for mass fractionation. The common lead correction was performed following the two-stage lead evolution model (Stacey & Kramers, 1975). Single zircon U-Pb vapour transfer method Single zircons were dissolved in a Teflon ® capsule with six separate holes following the procedure described in Parrish (1987) with modifications after Wendt and Todt (1991). The zircons were picked and ultrasonically washed in a Savilex ® beaker in a two-step procedure. The first step involved washing the zircons for 30 min with 7 N HCl, and the second step included washing for 5 min with 3 N HNO3. After each washing step the zircons were rinsed three to four times with Millipore ® water. Single or up to three (only for the metagabbros) zircons were handpicked and transferred into each hole. An enriched mixed 265 U- 205 Pb tracer was added prior to dissolution of the zircons with HF for up to 5 days at 200 °C. The samples were dried, and 3 µl 6 N HCl was added, and the sealed capsule was again heated for 12 hours. Lead and U of some zircons were separated using 0.5 ml Teflon ® columns with the HCl-HBr method using DOWES AG 1X8 anion exchange resin. The separation minimized the matrix effects on Pb and U, which led to a stable and strong signal for those isotopes during measurement. Zircon analyses, in which U and Pb were separated, are marked with a ‘c’ in Table V-1 and V-3, appendix V. Concordant data points are given as 206 Pb/ 238 U ages because of the more precisely determined isotopic ratios.
3 Analytical methods 33 U-Pb rutile analyses Three fractions of 4-8 mg rutile were handpicked from sample ZZB 150 in order to obtain optically inclusion-free separates. The rutile separates were washed in a mixture of 0.5 N HF/1N HCl or 6 N HCl respectively. The fractions were spiked with a 233 U/ 205 Pb mixed tracer and were dissolved in Teflon ® beakers using a H2SO4-HF mixture for four days in a Parr ® bomb. Subsequently, the rutile samples were dried on a hot plate at 200 °C for at least three weeks to remove H2SO4. Uranium and lead were separated using 0.5 ml Teflon ® columns. Lead was separated using the HCl-HBr method and using DOWES AG 1X8 anion exchange resin. Uranium was separated on UTEVA exchange resin (100-150 µm) using the 2.0 N HNO3 – 0.02 N HNO3. For U and Pb measurements of zircon and rutile the samples were taken up with three ml silica gel and loaded on single Re-filaments. Lead and U were ionised at different temperatures (Pb: 1180-1250 °C; U: 1350-1450 °C) and measured on a Finnigan MAT 261 mass spectrometer in peak jumping mode with a secondary electron multiplier (zircons) and in static, multicollector mode (rutile) at the Max Planck Institut für Chemie in Mainz. Blank values were 3.5 pg for lead in which the lead isotopic composition was 206 Pb/ 204 Pb: 18.210, 207 Pb/ 204 Pb: 15.162 and 208 Pb/ 204 Pb: 36.890. Common lead corrections for the zircons and rutile of the Mavuradonha Layered Complex were performed with measured Pb isotope compositions from coexisting plagioclase. For zircons of the Ocellar Gneiss, common Pb corrections were undertaken for an age of 1000 Ma, using the value of Stacey and Kramers (1975). Lead fractionation (2.9 to 3.2 ‰ per amu during this study) was corrected each time by measuring a standard NBS 981 under the same conditions. All isotopic results were corrected for spike, common lead, blank and Pb fractionation. Regression lines were calculated after York (1969). Since the zircons were not weighed, no U and Pb concentrations could be calculated. However, the use of a mixed spike allows the calculation of a total U over radiogenic lead ratio. Mineral chemistry Electron microprobe analyses were performed using a JEOL JXA 8900 RL electron microprobe (EPMA) at the Institut für Geowissenschaften, Mainz. This was to identify mineral chemical characteristics within different layered sequences in the Mavuradonha Layered Complex and for the purpose of geothermobarometry. Operating conditions for SilUMainz, used for all minerals except garnet, were 15 kV accelerating voltage and 12 nA probe current. Chlorine was measured as an additional element for scapolite and amphibole analysis. For garnet analyses the method GranUMainz with 20 kV accelerating voltage and 20 nA probe current was used. All analyses were performed in
Page 1 and 2: The Mavuradonha Layered Complex: Ne
Page 3 and 4: All views and results presented in
Page 5 and 6: Table of contents Acknowledgements
Page 7 and 8: 6.2.3.4 Summary and interpretation
Page 9 and 10: This project was part of the Mainz-
Page 11 and 12: utile ages ranging from 526 ± 7 to
Page 13 and 14: Alter von 899 ± 46 Ma untermauert.
Page 15 and 16: 1 Introduction 1 1 Introduction 1.1
Page 17 and 18: 1 Introduction 3 1993). In the West
Page 19 and 20: 1 Introduction 5 Moreover, the stru
Page 21 and 22: 1 Introduction 7 Zambezi Belt Mount
Page 23 and 24: 1 Introduction 9 metamorphosed unde
Page 25 and 26: 1 Introduction 11 on minerals with
Page 27 and 28: 2 Geology of the study areas 13 Zam
Page 29 and 30: 2 Geology of the study areas 15 The
Page 31 and 32: Plate I Plate I-1: View at the land
Page 33 and 34: Plate I Plate I-3: Layering in the
Page 35 and 36: Plate I Plate I-5: Compositional la
Page 37 and 38: Plate I Plate I-7: Shearzone within
Page 39 and 40: 2 Geology of the study areas 25 def
Page 41 and 42: 2 Geology of the study areas 27 Two
Page 43 and 44: 3 Analytical methods 29 3 Analytica
Page 45: 3 Analytical methods 31 Mineral sep
Page 49 and 50: 3 Analytical methods 35 microscopic
Page 51 and 52: 4 Petrology and metamorphic evoluti
Page 57 and 58: Am II-1 II-2 Pl II-3 Grt Pl Cpx Cpx
Page 63 and 64: Plate III 49 Plate III-8: Corona te
Page 65 and 66: Pl III-4 Am III-5 III-6 Am Pl Am Op
Page 93 and 94: 5 Geochemistry 79 a) Gabbro / Norit
Page 95 and 96: 5 Geochemistry 81 Chimwaya Hill Inl
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31 28 25 22 19 16 13 10 2.5 1.6 1.4
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5 Geochemistry 85 25 20 15 10 5 Ga
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5 Geochemistry 87 The trace element
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5 Geochemistry 89 1000 900 800 700
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5 Geochemistry 91 600 500 400 300 2
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5 Geochemistry 93 proposed for gabb
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5 Geochemistry 95 1000 900 800 700
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6 Geochronology and zircon-cathodol
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6 Geochronology and zircon cathodol
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6 Geochronology and zircon cathodol
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7 Sm-Nd, Rb-Sr and Pb-Pb isotopic s
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8 Petrogenetic and geotectonic impl
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9 Interpretation in the regional co
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References 164 References ARANOVICH
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References 166 COMPSTON, W., WILLIA
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References 168 GRAHAM, C.M. and POW
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References 170 JOHN, T., SCHENK, V.
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References 172 KROGH RAVNA, E. (200
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References 174 MEZGER, K., HANSON,
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References 176 POLLER, U. (1999): A
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References 178 TEERTSTRA, D.K. and
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Appendix 180 Appendix A-I Geology o
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Appendix A-I. Geology of the study
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Appendix A-I. Geology of the study
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Appendix A-II. Analytical methods v
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Appendix A-II. Analytical methods v
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Appendix A-III. Petrology and metam
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Appendix A-III. Petrography and met
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Appendix A-IV. Geochemistry xlviii
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Appendix A-IV. Geochemistry l Table
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Appendix A-IV. Geochemistry lii Tab
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Appendix A-IV. Geochemistry liv Tab
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Appendix A-IV. Geochemistry lvi Tab
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Appendix A-IV. Geochemistry lviii T
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Appendix A-V. Geochronology and cat
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Appendix A-VI. Sm-Nd, Rb-Sr and Pb-
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Appendix A-VI. Sm-Nd, Rb-Sr and Pb-
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Curriculum vitae 181 Curriculum Vit
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