Mesozoic alkaline volcanism and mantle evolution of the Southwestern Sao Francisco Craton,Brazil.

234 Pages·2016·4.99 MB·English

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MESOZOIC ALKALINE VOLCANISM
AND MANTLE EVOLUTION OF THE
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SOUTHWESTERN SAO FRANCISCO
CRATON, BRAZIL.
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Luiz Augusto Bizzi

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U Thesis presented for the degree of
DOCTOR OF PHILOSOPHY
in the Department of Geological Sciences
University of Cape Town
1995.
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The copyright of this thesis vests in the author. No
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quotation from it or information derived from it is to be

published without full acknowledgeement of the source.
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The thesis is to be used for private study or non-
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commercial research purposes only.

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Published by the Universit y of Cape Town (UCT) in terms
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of the non-exclusive license granted to UCT by the author.
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To my brother, Serginho.
Table of Contents
Abstract
Preface
lV
Acknowledgements viii
Chapter I - Aspects of the geologic evolution of the SW Sao Francisco craton 001
1.1 Introduction 001
1.2 Regional geology, tectonic setting and isotopic ages 003
1.2.1 The southern Sao Francisco craton 005
1.2.2 The Tocantins Province 010
1.3 Isotope systematics of selected areas 016
1.3.1 The Congonhas Lineament 016
1.3.2 The Brasilia orogenic and foreland thrust belt 021
1.3.3 The Niquelandia mafic-ultramafic complex 025
1.4 A preliminary model for the evolution of the craton margin 027
1.5 Concluding remarks 031
Chapter II - Kimberlites and alkalic rocks of the southwestern
Sao Francisco craton 034
11.1 Introduction 034
ll.1.1 Geological setting 036
ll.1.2 Remote sensing and GIS 039
ll.1.3 Selection and sampling of occurrences 047
ll.2 Geology of selected occurrences 047
ll.2.1 Terminology and classification 047
ll.2.2 Brazilian kimberlites 050
ll.2.3 Alkalic rocks of kimberlitic affinity 053
ll.2.4 Rift-related alkalic rocks 056
11.2.5 Carbonatite complexes 059
11.3 Whole-rock geochemistry 060
11.3.1 Contamination 060
11.3.2 Major elements and compatible trace elements 061
11.3.3 Incompatible trace elements 066
11.3.4 Platinum-group-elements and gold 077
ll.4 lsotope geochemistry 081
11.4.1 Emplacement ages 081
11.4.2 Whole rock isotope geochemistry 084
11.5 Discussion 091
11.6 Concluding remarks 093
Chapter ill - Evidence for interaction of discrete mantle reservoirs
beneath the craton margin 101
m.1 Introduction 101
m.2 Evidence for mantle heterogeneity 102
m.2.1 Source characteristics 102
m.2.2 Further evidence from PGE for source heterogeneity 106
m.2.3 Use of PGE and isotope data in mixing and addition models 108
m.3 Component characterization 116
m.3.1 The "shallow-derived" mantle component, EMI and LTPGE 116
m.3.2 The "deep-derived" mantle component, LoNd, HIMU and HTPGE 117
m.3.3 The mixing process 118
m.4 Time-integrated Nd isotope evolution 120
m.4.1 Remobilization or recycling of continental and oceanic lithosphere 123
m.4.2 Entrainment by lower mantle materials 125
m.4.3 Contamination by granulites, C0 metasomatism and delamination 126
2
m.5 Concluding remarks 128
Chapter IV - The kimberlitic and rift-related magmatiSm in a
regional perspective 131
IV.l Introduction 131
IV.2 The tectonic framework of west Gondwana 134
IV.3 The Break-up of southwestern Gondwana 139
IV.3.1 Prolonged South Atlantic sedimentation and rifting precursors
to the igneous activity in Brazil 139
IV.3.2 Continental flood basalts, dike swarms and alkali volcanism 146
IV.4 Large-scale mantle heterogeneities of southwestern Gondwana 150
IV.5 Geochemical heterogeneities and volcanism along the
southwestern Sao Francisco craton margin 155
IV.6 Concluding Remarks 162
Chapter V - Summary and conclusions 165
References 171
Appendix: Analytical methods 206
Sample Preparation 206
X-Ray Fluorescence 207
Neutron Activation Analysis 207
Solid-source mass spectrometry 208
List of Figures
1.1 Tectonic framework of the Sao Francisco craton and the Tocantins Province.
1.2 Geological setting of the southern Sao Francisco craton area
1.3 Geological setting of the Arai rhyolites and associated granitoids
1.4 Geological setting of the Western Goias area
1.5 Geological setting of the Congonhas do Campo area.
1.6 Rb-Sr and Sm-Nd isotopes diagrams for the greenstone association and granitoids in
the Congonhas do Campo area
I. 7 Geological setting of the Western Minas Gerais area
1.8 Rb-Sr and Sm-Nd isotopes diagram for granitoids in Western Minas Gerais
1.9 Rb-Sr isotopes diagram for samples from the Niquelandia Complex
1.10 versus time of emplacement of crustal sequences
eNd
11.1 Geological setting of kimberlite-related occurrences within the southwestern Sao
Francisco craton area.
II.2 Bouguer anomaly map and interpretative structural map of SW Minas Gerais
II.3 Set of maps digitized from 1:250000 scale geology maps, remote sensing
interpretation both on screen and on 1:100000 hard-copy images, and field data
compiled using GIS technology.
II.4 Photographs illustrating the use of thematic mapping (TM) imagery in the thesis.
II.S Photomicrographs of kimberlitic rocks
II.6 Photomicrographs of alkalic rocks of kimberlitic affinity
II.7 Photomicrographs of rift-related alkalic rocks
II.S Photomicrographs of xenolith materials
II.9 Spinel and mica composition of Brazilian kimberlites
II.10 Silica contents related to isotopic signatures and total alkalis content
II.ll Selected major element variation diagrams
II.12 Chondrite-normalized REE
IT.13 Mantle-normalized trace element compositions
II.14 U and Pb relationship to other trace elements
II.15 Nb/La histograms in mantle and crustal rocks
ll.16 Chondrite-normalized PGE patterns
ll.17 Inter PGE ratios and Pt versus Au contents
ll.lS Emplacement ages for kimberlites and kimberlite related rocks obtained by Rb-Sr
analysis of phlogopite separates
ll.l9 87Sr/86Sr versus 143Nd/144Nd isotopic ratios of the studied alkaline rocks compared to
kimberlites, Walvis Ridge OIB and Parana CFB.
ll.20 87Sr/86Sr versus Rb/Sr and 143Ndf144Nd versus Sm/Nd
ll.21 Pb isotopic signatures of the alkaline rocks
ll.22 87Sr/86Sr versus 207Pbf204Pb and 207Pbf204Pb versus 143Nd/144Nd isotopic ratios
Ill.l 87Sr/86Sr versus 143Nd/144Nd isotopic ratios of the alkaline rocks
m.2
Summary of source depth profiles related to isotopes and PGE characteristics
ID.3 Mixing Systematics
ID.4 Ba/Nb versus 143Nd/144Nd isotopic ratios
Ill.5 Plots of Pt versus La and Pt versus Th comparing the studied rocks to transitional and
Group II kimberlites
ID.6 PGE variations with Sr and Nd isotopes
m.
7 versus time of emplacement
ENd
m.s
Tectonic zones and a model for cratonic evolution
IV.l Photographs illustrating the manipulation of the GO-GEOID relational database using
GIS technology
IV .2 Cratons and major chrono-tectonic provinces of Gondwana
IV .3 GIS generated map for western Gondwana
IV .4 Tectonic evolution of Gondwana
IV .5 Lithostratigraphy of the southern sector of Gondwana
IV .6 Early Aptian reconstruction of the Angolan and Brazilian rifted margins
IV. 7 Fission related magmatism and the breakup
IV .8 HTZ and LTZ zonation
IV .9 Tectonic elements of the Sao Francisco and Kaapvaal cratons at the same scale
List of Tables
Table 1.1 Isotope compositions of studied crustal rocks
Table 11.1 Summary of characteristics of alkalic rocks
Table 11.2 XRF data for alkalic rocks
Table 11.3 INAA data (REE) for alkalic rocks
Table 11.4 INAA data (PGE) for alkalic rocks
Table 11.5 Isotope data for alkalic rocks
Table 11.6 Isotope data for phlogopites
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List of Abbreviations
AFFZ Agulhas-Falklands/Malvinas Fault Zone
BSE Bulk Silicate Earth
CFB Continental Flood Basalts
CHUR Chondritic Uniform Reservoir
DMM Depleted Mantle Material
EMI/EMII Enriched Mantle I/II
EFZ Equatorial Fault Zone
GIS Geographic Information System
HIMU High 238Uf204Pb
MBL Mechanical Boundary Layer
MES Maximum Entropy Scaling
Mg# Magnesium Number
MORB Mid Ocean Ridge Basalts
MSWD Mean Sum of the Weighted Deviates
NAA Neutron Activation Analysis
OIB Ocean Island Basalts
P&T Pressure and Temperature
Pers.comm. Personal Communication
PGE Platinum Group Elements
HTPGE High-temperature Platinum Group Elements
LTPGE Low-temperature Platinum Group Elements
PREMA Prevalent Mantle
REE Rare Earth Elements
RGWR Rio Grande-Walvis Ridge
Initial Isotopic Ratios
Neodymium Model Age Relative to Chondritic Reservoir
TcHUR
Neodymium Model Age Relative to Depleted Mantle
ToM
TBL Thermal Boundary Layer
Wt% Weight Percent
XRF X-Ray Fluorescence