Greenstone sucessions are defined as the nongranitoid component of granitoid-greenstone terrain and are linear to irregular in shape and where linear are termed belts. The chemical composition of greenstones is described. Also discussed are the continental environments of greenstone successions. The effects of contact with granitoids, geophysical properties, recumbent folds and late formation structures upon greenstones are examined. Large stratigraphy thicknesses are explained

Barton, J. M., Jr.

Phillips, G. N.

Vanreenen, D. D.

Vearncombe, J. R.

Wilson, A. H.

English

NASA Technical Reports Server

N 8 6 - 2 5 9 0 5
GREENSTONE BELTS: THEIR COMPONENTS AND STRUCTURE.
J.R. V e a r n c o m b e , J .M. B a r t o n , Jr . & D.D. van R e e n e n , Dept .
Geology, Rand Af r ikaans Universi ty, P.O. Box 524, Johannesburg
2 0 0 0 . G . N . Ph i l l ip s , Dept. Geology, U n i v e r s i t y of the
Wi twate r s rand , 1 Jan Smuts Av. Johannesburg 2001, and A.H.
Wilson, Dept. Geology, University of Natal at Pietermaritzburg,
P.O. Box 375, Pietermaritzburg 3200. N fy lj> I $~&
Although in common geological usage there is considerable
ambigui ty over the def in i t ion of greenstone belts which are
historically regarded as long and narrow in shape, Archaean in
age and composed of volcanic and sedimentary sequences at
greenschist facies. This definition remains true for many of what
are commonly regarded as greenstone belts but others d i f f e r
significantly, particularly in shape and metamorphic facies. For
this reason the term 'succession' is prefered for greenstones
which are not particularly linear. In the following discussion it
is our intention to main ta in 'greenstone1 as a use fu l term and
for that reason we specifically aim to exclude high-grade
supracrustal gneiss terrains such as those of the central zone of
the Limpopo belt and early Precambrian supracrustal sequences
such as the 3Ga Pongola, the 2.7Ga Witwatersrand and the 2.4Ga
Ventersdorp f rom any def in i t ion of greenstone successions. We
also aim to include all commonly accepted greenstone successions.
The following points are of relevance to the def in i t ion of
greenstone belts:
1. Most commonly accepted greenstone successions are of Archaean
age but a few younger belts have been repor ted f r o m
Wisconsin, USA (1) and northern Quebec, Canada (2) .
2. Al though many greenstone successions are long, linear and
narrow (e.g. Pietersburg and Murchison, Kaapvaal craton) many
others have more irregular shapes (eg. Bulawayan , Z imbabwean
craton and Pilbara, Western Australia). The word 'belt' therefore
is inappropriate for some greenstone successions.
3. Volcanic rocks are ubiquitous components whereas sediments may
be of secondary importance. The volcanics f requent ly include
komatii t ic rocks. Intrusive igneous rock units such as layered
complexes, dykes and sills may be present.
4. Greenstone successions occur at metamorphic conditions f r o m
sub-greenschist to granulite facies and the colour p re f ix ,
refering to the greenschist facies, is unfortunate.
5. Deformat ion intensity wi th in the greenstone successions is
variable.
6. Greenstone successions are always intimately associated
with and surrounded by trondh jemite-tonalite-granodiorite-granite
granitoids.
We tentatively suggest the following definition:
Greenstone successions are the non-granitoid component of
granitoid-greenstone terrains. Volcanic rocks are an essential
component, some of which are usually komatiitic. Sedimentary
rocks are commonly present and igneous intrusive units may exist.
The greenstone successions are linear to irregular in shape and
where linear they are termed belts. The greenstone successions
may occur at all metamorphic facies and are heterogenously
https://ntrs.nasa.gov/search.jsp?R=19860016433 2020-03-20T14:25:11+00:00Z
20
GREENSTONE BELTS: THEIR COMPONENTS AND STRUCTURE
Vearncombe, J.R. et al.
deformed. Most greenstone successions are Archaean in age.
Greenstone successions comprise a wide variety of rocks,
dominated by volcanics, which are usually altered and deformed.
Alteration of volcanic and other rock types is manifested b.y
hydration with variable silicification (3), carbonate-isation (4)
or silica loss (5) as well as isochemical metamorphism.
Alteration itself is temporally and spatially variable, Smith and
Erlank (6) have described possible early sea floor alteration of
komatiitic rocks from Barberton and carbonate-isation in
Murchison is patchy and syn- to post-tectonic. This alteration
constrains identification of original rock-types and the use of
whole rock chemistry. This restriction added to the problems of
equating area of surface outcrop with rock volume means that
estimates of greenstone lithological proportions must be treated
circumspectly. However, greenstone successions commonly comprise
the following primary lithologies: komatiitic, mafic and felsic
volcanics, cherts, banded iron formations, shales, graywackes and
quartz arenites. Less commonly limestones (including
stromatolites), arkose, ultramafic and mafic layered complexes,
quartz-feldspar porphyries and quartz tholeiite dykes are
present.
The identification of the environment of emplacement of
greenstone igneous rocks is highly problematic. Subvolcanic
intrusions exhibit many features almost indistinguishable from
true lavas. Skeletal crystal growths, commonly grouped under the
all-embracing term of 'spinifex1, are an important textural form
in these rocks and these textures, in abundance, are restricted
to Archaean greenstone successions. These textures are indicative
of rapid crystal growth under supersaturated conditions (7) and
need not be restricted to lava flows. In fact, the inordinatly
thick cumulate zones associated with some spinifex-bearing rock-
types preclude these being lava flows in the currently accepted
sense and the non-genetic term 'cooling unit1 has been used to
describe these layered rocks which may represent lava flows or
subvolcanic intrusions. The recognition of crescumulate type
crystal growth and rhythmically developed spinifex units indicate
a variety and complexity of mechanisms which have given rise to
these textures and criteria should be established to permit the
environment of emplacement to be determined more precisely.
Symmetry of structures and spinifex textures encountered in some
units may be indicative of dyke emplacement.
Until recently, greenstone research was largely oriented
towards deducing a unifying model, subsequently heterogeneity has
become the key-word. In essence, greenstone belts are of
different ages and formed in different tectonic situations.
Groves and Batt (8) recognise both younger and older greenstone
successions in Western Australia in two distinct environments,
determined on the basis of volcanic constituents, sedimentary
facies, mineral deposits and tectonic style, to which they gave a
21
GREENSTONE BELTS: THEIR COMPONENTS AND STRUCTURE.
Vearncombe, J.R. et al.
genetic in te rpre ta t ion as r i f t -phase or p l a t f o r m - p h a s e
greenstones. Whereas this is a major development in understanding
Aus t r a l i an greenstones the d ivis ion of other greens tone
successions into r i f t - and p l a t f o r m - p h a s e is t e n u o u s ,
particularly for those of the Kaapvaal craton. The Murch ison
greenstone belt, for instance, has characteristics of both rift-
and platform-phase greenstones.
The Barberton greenstone belt, compr i s ing the lower
komatiitic to felsic units of the Onverwacht Group and overlying
deep water sediments of the Fig Tree Group, probably represents a
rift-phase (8) and the overlying Moodies Group with shallow water
quartzites and banded iron formation is typical of a p la t form-
phase greenstone belt. However, herewithin lies an important
observat ion on greenstone successions: the environment of
formation can vary within a greenstone. This variation may be due
to either:
1. A progressive evolution in environment. (Eriksson (9) has
described the Fig Tree to Moodies group evolut ion of the
Barberton greenstone belt in terms of an evolving back-arc, or
passive continental margin.)
2. The superposi t ion of d i f f e r e n t env i ronmen t s which are
tempora l ly separate and m a n i f e s t e d in the f ie ld by an
unconformity.
or 3. Some or all of the units are allochthonous and represent
spatially and/or temporally diverse environments now tectonically
juxtaposed.
Another aspect of the heterogeneity is the recognition of
both cont inental and oceanic env i ronments . The M b e r e n g w a
greenstone belt of Zimbabwe rests unconformably on granitic rocks
(10, 11, 12). Basement has also been inferred to exist beneath
other greenstone belts in Austral ia , Canada and India (13, 14,
15). Major layered igneous complexes such as Dore Lake (16) and
the Rooiwater, Murchison greenstone belt (17), are a significant
component of some greenstone belts. These complexes have minor
u l t ramaf ic components, anorthosite-gabbro layers, magnetit i te
layers and a highly differentiated and sodic granite. These
complexes are analogous to bodies such as the Bushveld and are
intrusions in a continental environment..
In contrast to the cont inenta l env i ronmen t of some
greenstone successions no proven continental basement exists at
the base of the Barberton greenstone belt and the Onverwacht
Group may be partially of oceanic origin (18). In addition, some
u l t r amaf i c complexes may also be ophiolitic (19). De Wit and
Stern ( 2 0 ) have recognised a possible sheeted-dyke complex in the
Onverwacht group. Support for the obducted oceanic origin for
some greenstone rocks comes f rom the recognition of pod i fo rm
alpine-type chromites at Shurugwi (Z imbabwe) (21, 22) and at
Lemoenfontein (Kaapvaal craton) ( 2 3 ) . These have textural and
chemica l charac ter i s t ics s i m i l a r to those recognised in
ophiolitic complexes of Phanerozoic age.
22
GREENSTONE BELTS: THEIR COMPONENTS AND STRUCTURE
Vearncombe, J.R. et al.
Historically greenstone structures were regarded as simple
synformal belts between sub-circular r imming granitoid domes.
This relationship has given rise to genetic interpretations that
greenstone belts are pinched-in synformal keels between domal or
diapiric granitoids or between granitoid domes which are the
result of interference folding ( 2 4 ) . Unfortunately the paucity of
detailed structural observations and accurately determined
stratigraphic successions mean that few of the assumed synforms
are proven.
In the Kaapvaa l craton the M u r c h i s o n , P ie te r sburg ,
Sutherland, Rhenosterkoppies, Amalia and Muldersdrift belts lack
a gross s y n f o r m a l s t ruc ture . At Barber ton the g reens tone
succession comprises several syn fo rma l structures separated by
steep reverse f a u l t s ( 2 5 ) . De Wi t ( 2 6 ) and Lamb ( 2 7 ) have
recently described thrusts, some of which eniplace Onverwacht
volcanics over Moodies sediments. The suggestion of Anhaeusser
(28) that deformation structures within the Barberton greenstone
belt can mostly be related to granitic diapirism is at variance
with the observed thrust structures and evidence presented by
R a m s a y ( 2 5 ) , Roer ing ( 2 9 ) and Burke e t a l . ( 3 0 ) who note
deformation structures prior to granite intrusion, intrusive
granite contacts oblique to deformation structures and an absence
of deformation structures within the greenstone directly related
to those in the surrounding granitoids.
We suggest that whereas broadly synformal belts may exist
this is not a characteristic of greenstone belts. Many of the
intrusive granitoids are undoubtedly domal but intervening
greenstone belts are not necessarily synformal and the role of
diapirism in controlling the structure of greenstone successions
may be over-emphasised.
In d e d u c i n g t he overa l l l a r g e - s c a l e s t r u c t u r a l
characteristics of greenstone successions the following general
observations may be relevant:
1. Contacts with the surrounding granitoids can be either
tectonic (31) or intrusive with dykes and veins of granitic rock
in the greenstone belts and a static high T/low P metamorphism
near the greenstone contact with the granitoids suggesting
contact metamorphism by igneous intrusion.
2. Geophysical evidence from a number of belts suggests they are
shallow with vertical depth extents rarely more than 10km and
usual ly less than 5km (32 , 33) , f igures considerably less than
the proposed stratigraphic thicknesses of these belts. This
shallow depth extent suggests no simple rotation of the usually
upright greenstone belt but instead a truncation which may be a
major decollement zone, recumbent syntectonic granite or a late
intrusive contact.
3. Recumbent fold structures and possible thrusts are relatively
common and have been described from greenstone successions of the
Zimbabwean craton (34 , 35) , of the Kaapvaal craton (25, 26, 2 7 ) ,
23
GREENSTONE BELTS: THEIR COMPONENTS AND STRUCTURE
Vearncombe, J.R. et al.
of the Western Australia shield (36, 37) and the north Amer ican
shield (38) .
4. Greenstone successions occur as either linear belts or as
irregular shaped units comprising arcuate arms.
5. Late-deformation structures and the present disposition of
primary layering structures in the greenstone successions are
usually upright.
Greenstone successions are composed of deformed and
metamorphosed (including metasomatised) rocks. However despite
the obvious d i f f i c u l t i e s , m a n y au thors have proposed
stratigraphies for greenstone belts, but some have deduced total
stratigraphic thicknesses dramat ica l ly in excess of those
predicted by currently accepted models for basin formation (39,
4 0 ) . Greenstone successions such as Barberton with 17 to 23km
(41) , Pietersburg with 21.4km (41) and Abitibi with over 30km
( 4 2 ) or up to 45km ( 4 3 ) total stratigraphic thickness contrast
with both thinner sequences f r o m other greenstone and non-
greenstone early Precambrian supracrustal sequences such as the
Witwatersrand. It is the greenstone successions with large
stratigraphic thicknesses which are invariably at sub-greenschist
or greenschist facies and without the high grades of metamorphism
that would be expected at the base of these sequences. These
thicknesses represent one of the cha l lenging problems in
greenstone geology.
Possible explana t ions for the large s t ra t ig raphic
thicknesses are as follows:
1. They are an artifact of combining separate sections into a
composite section or are oblique sections.
2. That incorporated within the greenstone belt and incorrectly
interpreted as part of the stratigraphy are layered igneous
complexes, sills and tectonically rotated dykes.
3. The stratigraphic sequences are in fact related to two or more
spatially superimposed but temporally separate and essentially
unrelated events. In the Barberton greenstone belt granite
cobbles in a Moodies Group conglomerate have yielded zircons
giving ages of 3.15Ga ( 4 4 ) contrasting with ages of 3.54Ga ( 4 5 )
for the stratigraphically lower Onverwacht volcanic "rocks. A
major phase of granite emplacement separates these two dates and
a major unconformity may exist at the base of the Moodies Group.
4. They are not true stratigraphic sections but are structurally
repeated by imbricate thrusting and/or folding. To achieve
significant structural repetition by thrusting, folding or both
requires major recumbent tectonics on or above a decollement
plane.
Whi l s t e x p l a i n i n g large s t r a t ig raph ic repetition the
recumbent thrust-fold model also predicts metamorphic conditions
at the base of the pile in i t ia l ly at- high P/low T and wi th
thermal relaxation to medium pressure facies. Bickle et al. ( 4 6 )
have reported such rocks from the Yilgarn and similar staurolite-
24
GREENSTONE BELTS: THEIR COMPONENTS AND STRUCTURE
Vearncombe, J.R. et al.
kyanite-bearing rocks occur in the Murchison greenstone belt.
However the very large apparent stratigraphic thicknesses with
associated sub-greenschist or greenschist metamorphism remain
unexplained by horizontal thrust-nappe tectonics. These may
however be explained by repetition above a flat decollement in an
imbricate stack with associated folding. In this situation the
stratigraphy is turned on end and multiply repeated but the
structure remains shallow. Zones of cyclic repetition should be
investigated to determine if the cyclicity is real or the result
of imbricate stacking. Examples of this type of structural
stacking resulting in repetition are provided by Coward et al.
(35) from Matsitama, Zimbabwean eraton, Botswana and Martyn (37)
from the Kalgoorlie area in the Norseman-Wiluna greenstone belt
(Western Australia).
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Greenstone belts: Their components and structure

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