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TitleArchaean Geochemistry: The Origin and Evolution of the Archaean Continental Crust
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Page 2

IU S
UNE 0

I

Final Report of the
IGCP-Project No. 92
(Archaean Geochemistry)

Page 148

Petrology and Geochemistry of Layered Ultramafic to Mafic Complexes 139

The CeN/YbN ratios correspond closely to those of Archaean layered anorthositic
gabbros from Canada and West Greenland. The late gabbros exhibit fraction-
ated REB patterns enriched by a factor of about 40 compared to chondrites.

The stratiform igneous bodies are comparable with the layered meta-igneous
complexes of the Lewisian in NW Scotland and the Limpopo belt of southern
Africa. The nature of the igneous complexes and the asscociated sediments
favours an intracrustal development of the Sargur Group rather than the forma-
tion by rifting and ocean opening processes. Conditions of granulite facies meta-
morphism (700± 50°C; 9± 1 kb) indicate a minimum crustal thickness of about
35 km at ca. 2.6 Ga ago.

1 Introduction

The study of Archaean ultramafic and mafic complexes has received much atten-
tion in recent years as these rocks represent important components of many
greenstone belts and of supracrustal series within high-grade gneiss terranes
(Windley and Smith 1976; Glickson 1977; Windley et al. 1981). These complexes
now occur as lenses, sheets and disrupted layers, tectonically interleaved, de-
formed and metamorphosed together with the associated supracrustal rocks.
Despite a strong tectonic and metamorphic overprint, the original igneous strati-
graphy, the process of generation and differentiation of the magmas and their
mode of emplacement may be elucidated in many cases by careful investigation
of the field relations, petrography and geochemistry of the complexes.

Apart from the layered igneous complexes, a distinctive group of ultramafic
to mafic rocks, the komatiite-tholeiite series, occurs in many Archaean green-
stone belts, either forming the basal members (Viljoen and Viljoen 1969; Arndt
et al. 1977; Blais et al. 1978) or overlying the sedimentary units (Bickle et al.
1975; Henderson 1981; Kroner et al. 1981). Komatiites were thought to be re-
stricted to Archaean greenstone belts, indicating unusually high geothermal
gradients in the upper mantle before 2.5 Ga ago. The recognition of Proterozoic
and Palaeogene komatiites (Gansser et al. 1979), however, indicated that such
conditions may not have been unique to the Archaean. In highly deformed and
metamorphosed Archaean terranes, the distinction between komatiites and
layered igneous complexes has major implications for models of crustal evolu-
tion.

In the Archaean Karnataka craton of southern India, deformed and meta-
morphosed ultramafic to mafic complexes occur within several belts of supra-
crustal rocks, e.g., the Nuggihalli, Holenarsipur and Sargur schist belts (Swami
Nath et al. 1974).

Viswanathan (1974), Naqvi and Hussain (1979), Hussain et al. (1982) and
Jafri et al. (1982) suggest that the ultramafic to mafic rocks in the Nuggihalli and
Holenarsipur belts represent komatiitic volcanic rocks that form the basal mem-
bers of the individual schist belts. Fine-grained anorthositic rocks associated with
the ultramafic complexes were compared to lunar anorthosites and interpreted as
primitive anorthositic basalt lavas (Drury et al. 1978; Naqvi and Hussain 1979).

Page 149

140 c. Srikantappa, P. K. HOrmann and M. Raith

Based on detailed geological and structural work in these belts, Swami Nath et al.
(1974), lanardhan et al. (1978) and Srikantappa (1979) have shown however, that
the ultramafic to mafic complexes are stratiform layered intrusives and are not
komatiite metalavas. The fine-grained anorthosites of the Holenarsipur belt are
now regarded as cumulates from basic magmas, their fine-grained texture being
attributed to intense shear deformation (Bhaskara Rao and Veerabhadrappa
1982).

These contrasting views regarding the origin of the ultramafic to mafic com-
plexes led to the postulation of different models for the evolution of the
Archaean crust in southern India. Formation of the greenstone belts by a process
of initial rifting and eventual ocean opening was proposed by Naqvi (1976), in
contrast to an intracrustal development implied by Swami Nath et al. (1974).

In order to identify the true nature of the complexes, the ultramafic to mafic
rocks from the high-grade Sargur schist belt in the Karnataka craton have been
investigated. The discussion of their petrogenesis is based on field observations
and on detailed petrographic and geochemical data. The significance of these
complexes in the Sargur high-grade terrane is discussed with regard to the pos-
sible nature of early continental crust in southern India. The P-T conditions of
the high-grade phase of metamorphism that affected these rocks are evaluated by
geothermobarometry on garnet-pyroxene bearing mafic rocks.

1.1 Analytical Procedure

Major element compositions were determined by colorimetry, atomic absorption
spectrometry (AAS) and X-ray fluorescence spectrometry (XRS). References are
given in Hormann et al. (1973). Rb was separated from the rocks by an Amberlite
IR 120 ion exchange method (Hormann and Eulert 1980) and measured sub-
sequently by AAS. The elements Sr, Ba, Ni, Cr, V, Sc, Y, and Zr were measured
by ICP spectrometry. The REE were isolated by means of ion exchange
chromatography with DOWEX 50WX8 (Broekaert and Hormann 1981) and
measured by ICP spectrometry.

Precision of the major element analyses is better than 10/0. The standard
deviation of the Rb measurements is 10%, those of Sr and Ba 1 %. The standard
deviation of the Ni, Cr, V, Sc, Y, and Zr measurements is 10% for contents
lowert han 10 ppm, 5% at the 10 ppm level and smaller than 1 % for contents
higher than 100 ppm. The precision of the REE measurements is better than 5 %,
except for Ho where it is about 10%.

Compositional data of minerals reported in this paper are based on micro-
probe analyses carried out on a wavelength dispersive microprobe (Siemens) at
the Mineralogical Institute, University of Kiel. Detailed analyses are reported in
Srikantappa et al. (1984).

1.2 Regional Geological Setting

The Archaean Karnataka craton of southern India is a classical example of a
greenstone-granite terrane which grades progressively into a high-grade granu-

Page 296

Springer-Verlag
Berlin
Heidelberg
New York
Tokyo

Crustal Evolution
of Southern Africa
3.8 Billion Years of Earth History

By A.J. Tankard, M.P.A.Jackson, K.A.Eriksson,
D.K.Hobday, D.R.Hunter, W.E.L.Minter

With a contribution by S. C. Eriksson

1982. 182 figures. XV, 523 pages
ISBN 3-540-90608-8

Contents: Tectonic Framework. - Archean Crustal Evolu-
tion: Granite Greenstone Terrane: Kaapvaal Province.
Granulite-Gneiss Terrane: Limpopo Province. - Early
Proterozoic Supracrustal Development: The Golden Pro-
terozoic. The Transvaal Epeiric Sea. The Bushveld
Complex: A Unique Layered Intrusion; The Vredefort
Dome: Astrobleme or Gravity-Driven Diapir? The Earliest
Red Beds. - Proterozoic Orogenic Activity: Namaqua-Natal
Granulite-Gneiss Terranes. The Pan African Geosynclines.
- The Gondwana Era: The Cape Trough: An Aborted Rift.
The Intracratonic Karoo Basin. - After Gondwana: Frag-
mentation and Mesozoic Paleogeography. Kimberlites and
Associated Alkaline Magmatism. Changing Climates and
Sea Levels: The Cenozoic Record. - References. - Index.

The southern African subcontinent is a key segment of the
Earth's crust, providing a record of geological processes that
occurred over the entire span of terrestrial history. This
volume, of interest to a broad spectrum of geologists study-
ing every time period, is a contemporary analysis of the
crustal evolution of this unique region. Site of the discovery
of the earliest evidence oflife to date, this area is known for
its remarkable geological formations and great mineral
wealth, particularly gold, diamonds, and strategic minerals.
Six experts have collaborated to make this book a modem,
process-oriented approach, focusing on the dynamics of
crustal structure. "In the book", B. F. Windley writes in the
foreword, "the reader will find a detailed review of factual
data, together with a balanced account interpretive models."
With outstanding maps and illustrations prepared especially
for this volume. The Crustal Evolution of Southern Mrica is
an authoritative, multidisciplinary approach to a region of
great geological importance.

Page 297

Springer-Verlag
Berlin
Heidelberg
New York
Tokyo

L.J.Salop

Geological
Evolution of the
Earth During the
Precambrian
Translated from the Russian by V. P. Grudina
1983. 78 figures. XII, 459 pages. ISBN 3-540-11709-1

Contents: General Problems in Division of the Precambrian. -
The Katarchean: Rock Records. Geologic Interpretation of Rock
Record. - The Paleoprotozoic (Archeoprotozoic): Rock Records.
Geologic Interpretation of Rock Record. - The Mesoprotozoic:
Rock Records. Geologic Interpretation of Rock Record. - The
Neoprotozoic: Rock Records. Geologic Interpretation of Rock
Record. - The Epiprotozoic: Rock Records. Geologic Interpreta-
tion of Rock Record. - The Eocambrian (Vendian sensu stricto):
Rock Records. Geologic Interpretation of Rock Record. -
Geologic Synthesis. - References. - Subject Index. - Index of
Local Stratigraphic Units and of Some Intrusive Formation.

Geologic evolution from approximately 3.7-109 y up to the
beginning of the Cambrian period is the subject of this study.
The author, head of the Precambrian Geology Department of
the All-Union Research Geology Institute in Leningrad,
proposes herein a new, detailed division of the Earth's early
history. His geologic conclusions are based on rock record data
collected the world over, as well as on new methods for studying
older formations and the refinement of existing methods, in
particular the division, dating and correlation of "silent" meta-
morphic strata.
The scope of the book embraces such problems as
- the evolution of sedimentogenesis, of tectonic structures and

of larger elements of the Earth's crust
- the periodicity of tectogenesis
- the relation between tectonic processes and magmatism
- the origin of the oldest astroblemes and their appearance on

the Earth's surface, as well as possible causes for changes in
organic evolution.

The author concludes with a consideration of a directed and
irreversible geologic evolution of the planet.
With the great significance new data on the Precambrian bear
for the theoretical and philosophical foundations of science,
showing as they do previously unknown general regularities and
unexpected evolutionary relations of different geologic phenom-
ena and processes, this book will prove required reading for the
Precambrian specialist and advanced student and will also inter-
est the researcher in other geologic branches.

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