Bahasa
Halaman
Hukum
ELSEVIER Precambrian Research 87 (1998) 87 115
Pre(umbriun Resenr(h
Unraveling the record of successive high grade events in the Central Zone of the Limpopo Belt using Pb single phase
dating of metamorphic minerals
L. Holzer a.,, R. Frei a, J.M. Barton, Jr b, J.D. Kramers a
a Gruppe Isotopengeologie, Min. Pet. Inst., Universitdt Bern, Erlachstrasse 9a, 3012 Bern, Switzerland b Department o f Geology, Rand Afi'ikaans University, P. 0 Box 524, Auckland Park 2006, Johannesburg, South A/i'ica
Received 28 November 1996: accepted 3 October 1997
Abstract
Dating of relic metamorphic assemblages can provide important information about the timing and character (metamorphic grade and/or PT-evolution) of early high grade episodes in polymetamorphic provinces. Using Pb stepwise leaching of metamorphic silicates, we have dated multiple granulite facies metamorphic episodes in the Central Zone (CZ) of the Limpopo Belt. Ages of 2.52 Ga were obtained from sillimanite and cogenetic garnet and ages of about 2.01 Ga from titanite, garnet and clinopyroxene. Together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data, these results lead us to a reinterpretation of the tectono-metamorphic history of the CZ. Three distinct high grade events at about 3.2-3.1 Ga, 2.65-2.52 Ga and 2.0+0.05 Ga are recognized. Each of these is suggested to correspond to a tectonic episode of distinct character: (a) for the 3.2 Ga event magmatic activity can mainly be identified (best represented, for example, by the Sand River Gneisses or the Messina Layered Intrusion). The field relationships concerning the tectono- metamorphic history of this Early-Archean event are largely erased by at least two high grade metamorphic overprints. (b) Late-Archean (~2.6-2.52 Ga) low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism. The anticlockwise P-T evolution of these granulites probably reflects deep crustal processes, associated with magmatic underplating (or in-plating), contemporaneous with vertical crustal growth of the Zimbabwe craton around 2.6 Ga. (c) During the Proterozoic event (~2.05-1.95 Ga) tectonic thickening was caused by the collision of the Kaapvaal and Zimbabwe cratons. The CZ was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise P T evolution. The structural, metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny. © 1998 Elsevier Science B.V.
Keywords." Archean: Geochronology; Granulite facies: Limpopo Belt: Palaeoproterozoic; Pb isotopes; Polymetamorphic
* Corresponding author. Tel: 0041 31 631 85 33; Fax: 0041 31 631 49 88: e-mail: [email protected]
0301-9268/98/'$19.00 c~ 1998 Elsevier Science B.V. All rights reserved. Pll S0301-9268 ( 97)00058-2
88 L. tlolz.cv ~'I ~11. Pr~'{anll~riaH Rc*{'orch ~'7 ' 199<¥/,S'7 115
I. Introduction
Granulite facies conditions may be reached in many difl'erent tectonic environments. Simplifying the large variety of granulite provinces, two endmembers can be distinguished with respect to the tectonic environment and the associated meta- morphic character (e.g. Harley, 1989: Percival, 1990 ): (a) belts that underwent high pressure gran- ulite metamorphism (~> 10 kbar) are characterized by clockwise pressure temperature (P T) evolu- tion: and {b) belts that underwent low to medium pressure granulite tilcies metamorphism are usually characterized by an anticlockwise P-T evolution. The first type of granulites is considered to form by tectonic crustal thickening. After peak meta- morphic conditions have been reached, a first phase of near isothermal decompression is in many cases initiated by a change to an extensional tec- tonic regime. Post-collisional exhumation of such buried granulites is the result of erosion following rapid uplift and leads to isostatic equilibration of the crust. Many of the large Archean granulite terranes pertain to the second type with low/medium pressure granulites. Their association with voluminous marie and (charno-) enderbitic intrusions suggests mantle derived magmas as an important heat source for their high T metamor- phism (e.g. Bohlen, 1987). Exhumation of these granulites usually occurs in distinct, later tectonic cycles. These features have characteristics similar to those m recent continental arc environments, but can in general hardly be explained with alpine- type tectonic processes.
In testing possible models of the formation of granulite belts, the reconstruction of the P T t evolution is of major importance. Therefore com- bined approaches involving metamorphic pet- rology and geochronology are necessary. A problem in such studies is that petrological data can often not be easily linked to conventional geochronological data. Whereas thermobarometry relies on thermodynamic equilibrium of major elements between metamorphic minerals, most geochronological data depend on the retrograde closmg behaviour of radiogenic trace element sys- tems of accessory minerals. Dating of minerals involved in metamorphic reactions can now be
used to overcome this problem. Age data from metamorphic minerals can be correlated with pet- rologically deduced temperature estimates in cases where the analysed mineral is a product of an observed metamorphic reaction and the closure temperature (To) is higher than the temperature at which the reaction takes place (Tgrow,h)- Mezger et al. (1989) for example dated the growth of garnet which formed from the breakdown of bio- tite during vapour-absent melting. Since P T con- ditions for this type of reaction are well established, the age derived from garnet could be correlated with a narrow T range during the progradc metamorphism.
The study of granulite belts is further compli- cated by their frequent localization at craton boundaries or within intracratonic zones of weak- ness. Such zones are prone to reactivation during possible subsequent tectono-metamorphic events. This overprint by a second high grade metamor- phic event could conceivably cause partial or com- plete resetting of chronometers and/or petrological thermobarometers. This may lead to wrong inter- pretations, particularly if chronology is based on different minerals (e.g. zircon) than those used for P T calculation (e.g. garnet, pyroxene, plagio- clase), and the need to obtain age data from metamorphic minerals is thereby strongly empha- sized. Then, the question of whether or not the radiogenic decay systems (e.g. U Pb) remain closed during later high grade metamorphic over- print is important.
In this study we address these problems for the case of the Central Zone of the Limpopo Belt in Southern Africa. This province is suggested to have undergone at least two high grade metamor- phic episodes, one prior to or synchronous with the Bulai intrusion at ca. 2.6 Ga (Watkeys. 1983) and one at ca. 2.0 Ga (Kamber et al., 1995b: Barton et al.. 1994, Holzer et al., 1996). We have, on the basis of paragenesis and texture, selected samples with mineral assemblages judged to belong to the earlier and the later event and carried out Pb/Pb stepwise leaching to constrain the age of metamorphic minerals. This method has been applied successfully to a wide range ot" silicates m metamorphic and hydrothermal environments: garnet, titanite, hornblende, clinopyroxene, epi-
L. Holzer et al. Precamhrian Research 87 (1998) 87 115 89
dote (Frei and Kamber, 1995), staurolite (Frei et al., 1995) and tourmaline (Frei and Pettke, 1996). A multi-experimental study was undertaken by Frei et al. (1997) in order to assess the leaching mechanisms.The aims of our investigations are:
1) To assess the retentivity of the U/Pb systems in high grade metamorphic minerals, given a second high grade overprint.
2) To delineate the P T- t evolution of the youn- gest high grade event in the Central Zone and to obtain an accurate age for the earlier meta- morphic episode.
3) To compare qualitatively the P T loops char- acteristic of each of the two events.
(4) To discuss possible tectonic models for the different granulite facies events recorded in the Limpopo Belt in the light of the results.
2. Geological setting
The Limpopo Belt (Figs. 1 and 2) is a high grade metamorphic province which has an elonga- tion of about 650 km ( E N E - W S W ) and a width of 200 km (N-S) , respectively. To the east it is cut by younger tectonic units. It covers parts of South Africa, Botswana and Zimbabwe and is wedged between two (mostly) Archean blocks: the Kaapvaal craton to the South and the Zimbabwe craton to the North. The margins of the Limpopo Belt are made up by two major thrust zones, along which granulite facies rocks and retrogressed gran- ulites have been thrust onto the adjacent cratons during the late Archean, i.e. the Hout River shear zone (e.g. Smit et al., 1992) in the South and the North Marginal Thrust Zone (Blenkinsop et al., 1995: Mkweli et al.. 1995) in the North. The Limpopo Belt itself is divided into three subzones: the Northern and Southern marginal zones (NMZ and SMZ) border the volumetrically dominant Central Zone (CZ). The marginal zones are consid- ered as lower crustal equivalents of the adjacent granite greenstone terranes (e.g. Du Toit et al., 1983) which underwent granulite facies metamor- phism during the late Archean. The CZ consists of a wide range of lithologies, including metamor- phosed platform sediments (Beit Bridge Complex). quartzo-feldspathic gneisses, tonalitic grey gneisses
(Sand River and Alldays Gneisses) and other intrusive rocks with variable composition and age. The polymetamorphic character of the CZ is docu- mented by the available age data and the complex structural pattern, both of which will be outlined below. The suture zones between the marginal zones and the CZ are defined by large shear zones: to the North the CZ is bounded by a set of ENE WSW trending dextral strike slip shear zones, including the Triangle, Lepokole and Magogaphate shear zones. The Triangle shear zone represents a belt of transpressive dextral deforma- tion which was active mainly under granulite facies conditions during the Proterozoic (2 .0Ga. Kamber et al., 1995a). The timing of movement in the Lepokole and Magogaphate shear zones is not yet determined. The kinematics at the southern boundary of the CZ is more complex. Almost the entire contact with the SMZ and the Kaapvaal craton is overlain by the Proterozoic Soutpansberg and Waterberg sediments. The high grade ~Tshipise Straightening Zone" (Bahnemann, 1972) is located to the North of the Soutpansberg trough (see Fig. 2). Foliations trend ENE WSW, similarly to those in the Triangle shear zone, but they dip more steeply towards SSE. In addition, the southern boundary of the CZ is defined by the 10 km wide Palala lineament (McCourt, 1983: Brandl and Reimold, 1990). The shear sense of this mylonite zone has been a subject of controversy: McCourt and Vearncombe (1992) have interpreted the Palala shear zone as a sinistral strike slip zone. Broekhuizen and McCourt (1995) defined two phases of movement with opposing lateral shear senses. The youngest transcurrent faulting in the Palala shear zone post-dates both the exhumation of the CZ granulites and the emplacement of the Bushveld complex 2060 Ma ago (Barton, 1995).
3. A compilation of available age data from the CZ
Table 1 is a compilation of the most reliable isotope data, indicating an apparent history for the CZ ranging from 3.8 until 2.0 Ga. These dates cover a nearly continuous spread of ages between 3.2 and 2.0 Ga. Considering only conventional zircon U/Pb concordia intercept ages from the CZ,
90
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L. Holzer el al. Precamhrian Res'earch 87 ( 1998j 87 115
Central Zone
v
,.--'. '-'"-;T: , 7~ M r l k "
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Fig. I. Geological map of the Limpopo Belt and the adjacent Kaapvaal and Zimbabwe cratons: curved lines in Central Zone mark the trend of fofiations. SMZ=Southern Marginal Zone, N M Z - N o r t h e r n Marginal Zone, Phanerozoic cover is shown in white: lntrus'ions: 1= Entabeni Pluton, 2-Schie l Alkaline Complex, 3 - M a t o k Pluton: Greens'lone Bells (GB): 4 = Rhenosterkoppie. 5 - Southerland GB, 6= Buhwa GB, 7 - G w a n d a GB, 8 = Umzingwane GB, 9-Bel ingwe GB: Proterozoic sedimentary has'ins: 10- Blouberg. 11 = Waterberg, 12 - Palapye: Localities': M = Mashwingo, R = Rutenga, T = Tshipise.
ttout R~ver P a l a l a Straightening Triangle Noah marginal shear zone shear zone zone shear zone thrust zone
' , ' " ~i- i i : ~ / , T ~" i!. 3~;2: /. ............................. ~ . . . . . . . . . . .
/ 5 ~ ~ O i Soutpansberg
Kaapvaal {rough Central Craton SMZ Zame Transiti~m zone NMZ Zimbabwe Craton
S S W [ 50 km ] N N E
Fig. 2. Geological profile across the Limpopo Belt. This geological interpretation is based o13 structural data {discussed in text) and on published geophysical profiles (De Beer and Stettler, 1992, and references therein). The trace of the profile is shown in Fig. 1. Signatures are consistent with those in Fig. 1. M = Matok Pluton, E + S -En taben i Granite and Schiel Alkaline Complex, B = Bulai Plutom R = Razi granite.
L. H o l z e r et a L / Pr e c ambr ian Research 87 ( 1 9 9 8 ) 87 115 91
QFG ~ ] undeformed leucosome :'
I SRG' ] Singelele gneiss '~ ] undefonned leucosome '~
I SRG' I Bulai granite ~ ] BBC:
I BBC~ I Alldays gran(~tiorite" I BBC"~
..................................... 7 ........................................... T 7 3 . 2 i 3.0 ,, 2.8 1 2.6 2.4 2.2 ~ 2.0 i 1.8 i_Gaj
Fig. 3. Histogram showing a compilation of published zircon concordia intercept ages from the CZ. Three peaks at 3.2, 2.6 and 2.0 mark three distinct geological events, which are discussed in the text. Labels correspond to references given in Table 1.
the data define three peaks at 3.2, 2.6 and 2.0 Ga (Fig. 3).
Early-Archean in the CZ (ca. 3.2 Ga): various isotopic data from the Sand River Gneisses, the Messina Layered Intrusion and the Zanzibar gneisses indicate important magmatic activity between 3.1 and 3.3 Ga (see Table 1). Structural relationships and metamorphic records are largely erased by later high grade tectono-metamorphic overprints. The pre-3.0 Ga geologic history of the CZ is thus still poorly understood.
Late archean in the CZ (2. ~2 .55 Ga): the zircon data from Jaeckel et al. (1997) give evidence for two late Archean magmatic pulses. A first period occurred around 2.65 Ga (granodioritic Alldays Gneiss); a second important phase of granitoid plutonism is identified between 2.6 Ga and 2.55 Ga, during which quartzo-feldspathic Singelele orthogneisses were emplaced. Furthermore the Early-Archean Zanzibar gneiss was intruded by granitic orthogneisses at 2.55 Ga (Barton and Key, 1983). The Bulai Pluton intruded at 2.6 Ga (enderbitic phase) and 2.57 Ga respectively (granitic phase; Barton et al., 1994).
Proterozoic in the CZ (2.05-1.95Ga): the recognition of a 2 Ga granulite metamorphism in the Triangle shear zone (Kamber et al., 1995b) initiated a detailed study of the Messina area during which it was recognized that this area was also affected by important Proterozoic metamor- phism (Barton et al., 1994; Holzer and Kamber, 1995: Holzer et al., 1996). U-Pb data from meta-
morphic zircons (Jaeckel et al., 1997; Barton and Sergeev, 1997) indicate that high grade metamor- phic conditions were reached between 2.06 and 2.03 Ga. Rb-Sr ages of ca. 1.97 Ga from biotite (Barton and van Reenen, 1992) and 1.92 Ga from retrograde muscovite (Barton et al., 1994) reflect the time of cooling in the CZ.
These three periods with increased magmatic and tectono-metamorphic activity during the Archean and early Proterozoic are largely con- firmed by new data from Kr6ner et al. (submitted).
4. Structural patterns in the CZ and their relations to metamorphic episodes
The Messina Beitbridge region is a classical study area, situated in the middle of the CZ (Fig. 1 ). This study area exemplifies a complicated, polyphase deformational history of the CZ, which is summarized below (see also Fig. 4 and Table 6). A more detailed description of the structures is given by Watkeys (1983, 1984).
For an interpretation of the structural evolution of this area the Bulai intrusion is an important time marker. This calc-alkaline pluton was emplaced at 2572+4 Ma (Barton et al., 1994) and reveals the following magmatic contact relation- ships: it intrudes a crystalline basement, consisting mainly of metamorphosed and multiply folded paragneisses (Beit Bridge Complex). The foliation of metapelitic xenoliths within the Bulai granite is
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al.
, 19
92
Coo
ling
age
M
3 19
70
30
Ar
Ar
hbl
Mea
n of
5 a
ge d
ata
fron
l N
MT
Z T
rian
gle
Kam
ber
el a
l..
1995
a S
ynki
n. c
ooli
ng
1959
32
R
b Sr
bt
wr
San
d R
i,,e
r G
neis
ses
Bar
ton
el a
l..
1983
C
ooli
ng a
ge
M3
z~
1900
10
R
b Sr
ms
kl~
Qua
rtzo
Fel
dspa
thic
Gne
iss
Bar
ton
et a
l.,
1994
R
elro
. gr
ox~t
h po
st M
3 "~
ap=a
patit
e, b
t=bi
otfle
, q~
x =c
linop
~rox
ene.
I~p
= fe
ldsp
ar,
grt =
garn
et,
hbl=
horn
blen
de,
tim ~
ilmen
ite,
ms=
mus
co',i
te,
pl=p
lagi
ocia
se, s
ill=s
illim
anile
, v,
r~w
hote
'~
' ro
ck,
zrll
=z
irc
on
.
94 L. Ho l ze r e* a/. P r e c o m h r i a n Res~arch <~'7 ( 197<~>~ ,~'7 115
d f " i i
z
/ . f ~
J
7
/
m
~: t~ l q t l h , /
~lDt t i t , ~)L- i / i
,~ IF*~} > . e l ]:}*.'l] <
i f
r
Fig. 4. Geological map ol" the study,' area betx~een Beitbridge and Tshipise in the CZ. The structural fc~ltures ~iihin the Buhli Pluton. the Shanzi and ('ampbell t\)lds and the Tshipise Straightening Zone are discussed in the text. Numbers indicate sample localities.
defined by granulite facies mineralogy (grt, sill, crd. bt) and is discordant to the foliation in the magmatic host rock. This implies that deformation under granulite facies conditions occurred prior to or during the emplacement of the Bulai Pluton. This intrusion also cuts migmatite structures in the lnetasediments. Locally, granodioritic Bulai
melt seems to be mingled with granitoid Singelele type melt (Holzer, 1995). The Singelele ortho- gneisses are interpreted as products of widespread anatexis and mobilization of leucosome which was contemporaneous with the Buhii emplacement. These field observations lead to the following temporal relationships: (a) the Bulai pluton post-
L. Holzer el a[. Pt'ecambrian Re,search 87 ( 1998/87- 115 95
dates high grade metamorphism (M1) which is associated with ductile shearing and folding (DI) ; (b) the emplacement of the Bulai pluton is contem- poraneous with migmatization (M2a) and occurred syntectonically (D2a). The emplacement age of 2.57 Ga of the Bulai pluton is interpreted as a minimum age for the first granulite facies episode in the CZ.
Several phases of high grade deformations post- date the emplacement of the Bulai Pluton. The entire Messina-Beit Bridge area exhibits highly ductile and multiple phase D3-deformations.
The main mechanism of deformation within the Bulai Pluton is ductile shearing. 'Miniature mobile shear belts' (Watkeys, 1984) with variable orienta- tion, size and shear sense are the earliest D3 deformations recorded within this pluton. Conjugate sets of shear zones also with variable orientation and shear sense--developed subse- quently. The early D3 deformations in the Bulai Pluton seem to be the product of a NNW SSE directed shortening (~1). The latest so-called "Limpopo trend shears" (ENE WSW) within the Bulai Pluton exhibit mainly dextral shear senses.
A distinct style of D3-deformation evolved in the well layered paragneiss sequences. Locally, EN E - W S W directed compressive stress regimes evolved and a flexural slip mechanism led to the formation of regional scale upright, isoclinal "crossfolds" (F3a). In the Messina area two impor- tant F3 folds have been described: the Campbell and the Shanzi structures. The latter is strongly affected by F3b deformations, during which earlier crossfolds have been refolded. This produced the 'top to the ENE'-shearing observed on Mount
Shanzi (one of our sample localities). F3b foldaxes and lineations in the Messina area are subparallel and dip with about 30 ~ towards 260. The contact of the Campbell and Shanzi fold structures with the Bulai body consists of highly ductile shear zones. Undeformed leucosome veins crosscut these D3 shear zones and they occur both in hinges of F3b secondary folds within the Campbell structure, and as post-tectonic mobilizates within the Sand River Gneisses at the Causeway locality (sampled by Jaeckel et al., 1997).
The section from Messina to Tshipise is charac- terized by a continuous transition into the Tshipise Straightening Zone, in which monotonous ENE WSW trending foliations dip steeply towards SSE. Fold axes and lineations dip moderately towards WSW, subparallel to the F3b-lineations in the Messina area. Fripp (1983) describes two sets of ductile shear zones trending towards 010 and 080 . They led to a clockwise rotation of older structures into the dominant ENE WSW struc- tural trend, suggesting a dextral simple shear com- ponent for the area south of Messina.
Vapour absent melting, observed in a multitude of lithologies (including the Bulai body), indicates that granulite facies conditions were reached during M3. The youngest granulite event (M3) is characterized by a clockwise P-T path. Spectacular reaction textures in metapelites have been the subject of various metamorphic studies (Chinner and Sweatman, 1968; Droop, 1989; Horrocks, 1983a; Windley et al., 1984). Peak conditions are constrained to 8 2 5 C ( _ 2 5 : C ) and 10 12kbar. After a phase of near isothermal decompression (ITD) pressures of 5 6 kbar were reached. During
Table 2 Modal mineralogical composition
Sample Mineralogy vol.% Accessories
93048 matrix 30 Crd 22 Grt 20 Sill 10 Bt 10 Qtz 5 P1 3 Mag+Spl Rtl, Mon 93048 pseudomorphs 84 Sill 11 Grt 4 Crd 1 Bt Rtl, Spl, Mon 93,083 45 Cpx 35 P1 10 Qtz 5 Tit 4 Kfs I Mag+Spl 93167 core 40 P1 24 Grt 20 Cpx 8 Qtz 5 Tit 2 Kfs I Mag 93167 rim 60 PI 20 Hbl 7 Cpx 7 Tit 4 Mag 2 Kfs
Ap, AIn, Scp. Mon Ap, Zrn. Mon
Abbreviations: aln = allanite, ap = apatite, bt = biotite, crd = cordierite, cpx = clinopyroxene, grt =garnet. hbl = hornblende, k f s - K feldspar, mag=magneti te, mon-monaz i t e , pl-plagioclase, q t z -qua r t z , r t l - rut i le , scp-scapoli te, sill-sillimanite, spl-spinell , tit = titanite, zrn - zircon.
#
Tab
le 3
A
ge d
ata,
Pb
isot
opic
com
posi
tion
s an
d ex
peri
men
tal
data
of
leac
h ex
peri
men
ts
Sam
ple
Min
. S
lep
Tim
e A
cid
-'""P
b -'"
4pb
+_2
s '-°
:Pb
2"q~
b +
2s
>sP
b ~
'~'4
pb
+2
s rl
r2
9304
8 M
etap
elite
si
ll P
bSL
:llO
15
(l~,
tnl,
191
me
bulk
sill
iman
itc.
II0
15
0t.t
m.
176
me
bulk
ear
ner,
I l
O
1651
tm.
108m
e gr
l P
bSL
: 11
0 16
5 !-
u11.
85
nlg
sill
I 4
h
4N
ll
Br
72.2
0.
4 24
.6
0.1
160.
1 0.
9 0.
995
0.99
4 si
ll 2
18 h
8.
8 N
HB
r 36
1.4
3.0
72.8
0.
6 77
1.5
6.5
0.99
9 11
.998
si
ll 3
24 h
14
.5 N
HN
O3
203.
7 4.
0 47
.8
0.9
313.
8 6.
1 0.
999
tl.99
9 si
ll 4
72 h
It
F c
onc.
35
.1
0.1
18.9
0.
1 44
.5
0.2
0.96
7 0.
900
sill
bulk
tt
F c
one.
47
.9
0.9
20.8
(}
.4
76.3
1.
4 0.
998
0.99
9 ~z
gr
t bu
lk
11F
cone
. 27
.4
0.4
17.5
(.
l 47
.0
0.8
0.99
6 (I
.998
gr
t 1
1 h
1.5
N t
lBR
II
CI
mix
N
ot m
easu
red
grt
2 5
h 4
N t
tBr
85.7
5.
9 26
.6
1.8
231}
.8
15
.9
(I.9
99
1.
0011
gr
t 3
19 h
8.
9 N
HB
r 10
8.5
2.7
3(1.
7 0.
8 29
6,8
7.4
0.99
9 0.
999
grt
4 26
h
14 N
HN
O
32.4
5.
3 18
.0
3.0
67.3
11
.1
0.99
8 (I
.99
9
grt
5 48
h
tt F
' con
e.
65. I
0.
4 23
.6
(}. I
42
.3
0.3
0.99
0 (I
.995
m
-t 6
6d
H
F c
one.
21
.8
0.1
16.3
0.
1 39
.8
0.2
0.98
9 tl
.98
8
grt
7 6
d l I
F c
onc.
18
.6
0.2
15.6
(I
,2
38.6
0.
4 0.
994
11.9
94
Pb
Pb
2 po
int
age:
252
4_+
5 M
a, c
alcu
lale
d fr
om l
each
ste
ps I
and
2 o
f si
ll P
b ~,
P
b 2,v
",-a
93(1
48 a
ggre
gate
93
/048
agg
rega
le
93/0
48 a
ggre
gate
9
30
48
agg
rega
te
93"t
148
aggr
egat
e 9
30
48
agg
rega
te
93/0
48 m
atri
x 93
/048
mat
rix
93/0
48
mat
rix
93/0
48 m
atri
x 93
/048
mat
rix
93
04
8 m
atri
x 93
/048
m
atri
x si
ll ag
greg
ate
grt
mat
rix
grt
and
sill
Pb
isoc
hron
age
: 25
18_+
35 M
a, M
SW
D
1.42
, ca
lcul
ated
fro
m l
each
ste
ps 2
.3,
4, 6
and
7 o
f gr
t P
b is
ochr
on a
ge:
2521
+4
Ma.
MS
WD
1.
43.
calc
ulat
ed
fi'o
m l
each
ste
ps 2
, 3,
4,
6, 7
of
grt
and
leac
h st
eps
1.2
of
sill
93/0
83 c
alcs
ilica
te
laye
r 93
/083
9
30
83
93
/083
93
,083
9
30
83
cp
x
cpx
PbS
L:
100
200
)un,
12
0 m
g cp
x 1
15 r
ain
1.5
N t
tBr
IIC
I m
ix
28.1
0.
3 17
.2
0.2
39.0
(I
.6
(I.8
44
0.
775
cpx
2 3
11
4 N
HB
r 32
.3
0.3
17.7
0,
2 4(
I.5
0.5
0.91
9 0.
912
cpx
3 15
h
8.8
N I
tBr
27.6
0.
4 17
.0
0.3.
38
.9
0.6
0.94
0 (I
.96
9
cpx
4 24
II
14.5
N H
NO
.~
Nol
nle
asur
ed
cpx
5 48
h
HF
con
c.
20.4
0.
2 16
.3
0.2
36.3
(I
.5
0.91
7 0.
952
Pb
Pb
iso
chro
n a
ge:
18
60
+3
20
Ma.
M
SW
l) O
.329
, ca
lcul
ated
fr
om l
each
ste
ps 1
3 a
nd 5
ofc
px
Tab
le 3
(cm
ltim
wd)
Sam
ple
Min
. S
tep
Tim
e A
cid
-'~"P
b '-'
"a P
b +
2s
-'°:P
b -'°
'lPb
_+
2s
-'c~q
)b '-'°f
l~b
_+2s
rl
r2
93'1
67
calc
silic
ate
encl
ave
grt
PbSL
: 9(
1 12
5 ~.
tm, 3
1 tr
ig g
rl b
ulk:
9(/
12
5 ,r
an,
5(1
mg
fsp
>1
50
I-t
in 2
0(
lag'
no
n >
125
I-tin
, 11.
37 m
g tit
PbS
L:
II0
16
5 J.
tm, 6
7 na
g'bu
lk t
it:
175
31t(t
~.tm
, 73
mg
ap f
ract
ion
a:<
125
/am
. 39
.2 m
gb
: 12
5 16
5 .u
rn, 6
3.7
mg
c:
> 1
6(I g
in,
55.4
mg
d:
125
160
e-m
a, 2
3.9
rag
e:
125
160
I.Un,
42.
1 m
g 93
'167
cor
e gr
t I
15 r
ain
1.5
N H
Br
ItC
I m
ix
164.
1 1.
8 33
.9
11.4
58
11.9
6.
4 1t
.961
t 0.
985
93
16
7 c
ore
grt
2 3
h 4
N t
tBr
978.
5 31
.1
135.
1/
4.3
1197
.9
38.2
0.
998
0.99
6 93
,'167
cor
e gr
t 3
211
h 8.
8 N
[tB
r 18
24.9
88
.2
~t9
~
I 1.6
1
01
0.6
48
.9
1.01
10
I.(10
0 ~"
93
,'167
cor
e gr
t 4
24 h
14
.5 N
ttN
O:
792.
3 37
.6
1111
.9
5.3
454.
5 21
.6
11.9
93
0.99
7 93
/167
cur
e gr
t 5
24 h
H
F c
one.
13
4.7
4.0
33.7
1.1
1 81
.2
2.5
(I.9
69
(I
.98
3
..~
93,'1
67 c
ore
grt
bulk
H
F c
one.
13
51.8
2.
1 18
1.3
0.3
1146
.8
1.9
0.96
2 11
.984
93
,'167
rim
tit
I
I h
1.5
N H
Br
HC
I m
ix
294.
3 2.
6 50
.9
0.5
781.
2 7.
(1
0.99
8 0.
997
93/1
67
rim
tit
~
18 h
8.
8 N
HB
r 18
01.7
1
30
.7
236.
8 1
7.2
1
92
3.7
13
9.6
1.11
00
1t.9
99
93
16
7 r
im
tit
3 24
h
14.5
N t
tNO
3
472.
7 73
.4
75.6
1
1.7
44
11.7
68
.5
1.00
11
1.00
0 ~
93/1
67
rim
tit
4
24 h
tt
F c
one.
12
114.
4 1
44
.9
202.
4 24
.3
337.
4 41
1.6
1.00
11
1.0
00
93
,'167
rim
tit
bu
lk
ItF
con
e.
1271
.4
19.8
17
1.5
2.7
1420
.2
22.3
0.
997
11.9
95
93,'1
67 r
im
ap a
bu
lk
7 N
ttC
I 10
3.3
11.4
26
.6
0.1
1011
.7
0.4
0.95
2 (I
.95
9
93/1
67
rim
ap
b
bulk
7
N H
CI
121.
8 11
.9 29
.11
11.2
11
4.0
(1.8
(I
.99
8
(I.9
97
93
'167
rim
a
pc
bu
lk
7 N
HC
I 12
5.3
0.9
29.4
0.
2 11
7.2
11.8
0.86
9 11
.993
"q
93
/167
cor
e ap
d
bulk
7
N I
tC1
62.5
0.
2 21
.7
0.1
78.3
11
.3 0.
994
11.9
85
93
16
7 c
ore
ap e
bu
lk
7 N
ttC
I 12
3.1
0.7
29.1
11
.2 11
19.5
0.
6 11
.984
11
.993
93
/167
ri
m
mo
n
bulk
14
N t
tNO
3 |
IF m
ix
642.
7 28
.5
92.7
4.
2 11
/1111
.6 44
.5
11.9
88
0.99
7 c~
",
.1
93
16
7 c
ore
fsp
bulk
H
F c
one.
18
,9
t).0
3 16
.4
0.03
44
.6
0,07
11
.982
t)
.981
1 ,..
. gr
t P
b P
b is
ochr
on a
ge:
2010
_+17
Ma,
MS
WD
2.2
4, c
alcu
late
d fr
om l
each
st
eps
I 4
of
grt
tit
Pb
Pb
isoc
hron
age
: 20
07_+
5 M
a, M
SW
D 0
.05,
cal
cula
ted
from
lea
ch s
teps
I,
2 an
d bu
lk t
itan
ite
ap
Pb
Pb
erro
rch
ron
age
: 19
92+
21
Ma,
MS
WD
9.9
, ca
lcul
ated
fr
om a
pati
le f
ract
ions
a
e an
d I~
p 5
phas
es,
14 p
oint
s P
b P
b is
ochr
on a
ge:
2008
_+ 1
0 M
a, M
SW
D
13.6
, ca
lcul
ated
fro
m g
rt
1 to
4;
tit
1.2,
bul
k: a
p a
e: f
sp a
nd m
on
Abb
revi
atio
ns:
ap
= a
pati
te,
cpx
=cl
ino
py
rox
ene,
fsp
= f
elds
par,
grt
:g
arn
et,
mo
n =
mon
azit
e, s
ill =
sill
iman
ite,
tit :
ti
tani
te r
l. r
2 =
co
rrel
atio
n co
effi
cien
ts
acco
rdin
g to
L
udw
ig (
1988
).
98 L. t t o l z c r ~'t a/. P r c c o m h r i a H Re,~ear{ /i ,~¢7 g 199N) h' 7 115
the subsequent isobaric cooling gedrite was locally l\~rmed at the expense of orthopyroxene. This initial rehydration occurred at about 700 800 C and 5 6 k b a r ( H i s a d a a n d Miyano. 1996).
5. Samples and sample localities
From structural and regional criteria, based on the above framework, samples whose metamorphic parageneses were formed specifically in either the D2 M2 or the D3~M3 event were collected, and these were selected l\)r Pb stepwise leaching ( PbSL- dating) of metamorphic minerals. Modal mineral- ogical compositions are listed in Table 2 and the sample localities are shown in Fig. 4.
Sam~de 93048:1.5 km NE of ~Three Sisters" (Farm Boston). Strongly elongated, up to several km long slices of leucogranitic orthogneiss and metapelite occur within the Bulai Pluton. tn one of these metapelitic xenoliths ( 93048 ). rectan- gular sillimanite pseudomorphs afler andalusite (probably chiastolite) are up to I cm in diameter. Between the coarse grained, perfectly oriented sillimanite needles (84%) cogenetic cordierite and garnet occur as interstitial phases (15%) within the pseudomorphic aggregates. Rutile and hercynite are further accessory minerals. These p s e u d o m o f phic sillimanite aggregates are embedded in a fine-grained matrix consisting of cordierite, biotite, garnet, sillimanite and minor rutile, spinel and magnetite. The matrix-foliation anastomoses around the sillimanite pseudomorphs. Sillimanite in the matrix is aligned within the foliation. Garnet forms elongated, irregularly shaped grains and is partly broken. The youngest deformation recorded in this rock thus post-dates growth of both the coarse sillimanite-nests and matrix garnet. We interpret the pseudomorphic aggregates as relics that grew during an early phase of granulite meta- morphism. As they are probably after andalusite, they might reflect the prograde phase of an early metamorphic episode.
Sample 93 083 is a calcsilicate gneiss, which was sampled close to the hinge of the Campbell fold structure, 11.5 km W of Messma (Farm Plaatje). This calcsilicate layer was strongly attenuated during the formation of the 'Campbell cross fold'.
The intense mtergrowth of plagioclase, clinopyro- xene and titanite in marie bands parallel to the F3 axial planar surface give evidence that these miner- als recrystallized synkinematically during the D3 event.
Sample 93167:4 km NW of Messina. on the NE-side of Mount Shanzi (Farm Uitenpas), a sequence from porphyritic Bulai gneiss at the base which "merges into a migmatitic zone, contami- nated by assimilation of supracrustal xenoliths, overtopped by quartzofeldspathic gneisses and magnetite quartzites" (Watkeys, 1984. p. 156ff.) is exposed. These paragneisses on top of the sequence mark the easternmost boundary of the D3-Shanzi fold structure. The degree of delBrmation within the Bulai granite increases towards the contact with the overlying metasediments. Shear bands indicate a transport direction "top to the ENE'. The mineral elongation lineations are subparallel to the fold axes and dip moderately towards WSW (ca. 265'35). These structures are ascribed to the D3 F3b deformation. The migmatitic zone at the contact itself is characterized by ductile shear zones ~ith calcsilicate enclaves. A slight compositional zoning is observed in these enclaves: the core of one (93 167 core) consists of plagioclase, garnet. clinopyroxene, titanite and quartz (Table 2). Abundant green hornblende, an increased plagio- clase content and lack of garnet and quartz charac- terize the bright, decimeter wide rim (93,167 rim). The calcsilicate enclaves have an internal composi- tional banding (mafic and felsic layers) parallel to the shear planes in the mylonitic host rocks. In the more mafic layers garnet is mtergrown with plagioclase, titanite and cpx. indicating a strong synkinematic (D3) recrystallization. In the more fclsic layers post-kinematic recrystallization produced coarse grained, polygonally shaped plagioclase.
6. Analytical technique s
Pb stepwise leaching (PbSL) was carried o u t Oll 93:048 sillimanite and garnet, 93083 clinopyro- xene, 93167 (core) garnet and 93"167 (rim) titanite and followed the procedure described in Frei and Kamber (1995) with slight modifications. Table 3
L. Holzer el a/. Precanthriatt Rc",'earc/t 37 ( 199,¥: 87 115 99
summarizes the experimental parameters as well as the Pb isotope results. Subsequent to hand picking of the mineral separates under a binocular microscope, mineral leaching was performed in 7 ml Savillex" screw-top beakers. Individual stepleach solutions were decanted and then dried on a hot plate. Pb was separated using conven- tional HCI-HBr anion exchange procedures. during which the blank level was less than 130 pg. Pb was loaded on single Re-filaments and mea- sured on an AVCO '" 9 0 , 35 cm radius single collector mass spectrometer. Pb isotope ratios were corrected for fractionation using the values obtained fl'om repetitive NBS SRM 981 runs under similar operating conditions ( 0.001 frc./amu + 4%. n = 10). Isochrons and error correlations are calcu- lated after York (1969), using lsoplot (Ludwig, 1994). Errors assigned to the isochrons are 20.
Conventional bulk U Pb analysis was per- formed on monazite and apatite from sample 93'167. Apatite was dissolved in 7 N HCI in a Savillex beaker for 3 h on a hot plate. Monazite was dissolved for 3 h in a mixture (3:1) of 14 N HNOs and HF conc. A mixed 23sU-2°Spb tracer was used for both. Chemical separation and meas- urement of Pb was the same as applied for PbSL. U was extracted using a conventional HCI HNO3 anion exchange procedure and measured from a Ta Re-Ta triple filament configuration.
No absolute quantities of lead were determined in our leach experiments. The relative quantities released during the individual leach steps can be estimated from the signal intensities. Although the leaching behaviour is not the same for all host minerals, for most experiments highest signal intensities are obtained for the first step(s). The amount of lead released during the final steps is strongly dependent on the presence of microinclu- sions and on their leaching behaviour.
The lack of absolute amounts of Pb released during the single leach steps is a potential problem for the blank corrections. The laboratory total procedure blank is below the 130pg level (gen- erally 80 pg) and its composition is unradiogenic (2°~pb'2°apb 18.7+0.2 2c~ abs, 2°~pb'2°4pb / . _ _ / .
15.67 _+ 0.3, 2°8pb/z°4pb: 38.45 -I- 0.4). From signal intensities, quantities of Pb in fractions were always > 20 ng. therefore blank contributions were
r---
e -
.q
7
-%
g
- r
Y,
Y~
£
7,
~r
7j
- r - z
o ~ 0 0
._J
, : ¢ . . . . . . :~ i i.s
I 0 0 L. H o / z c r el a/. t ' i 'ecaml~riast R e s e a r c h / ~ ' 7 ( 1997,', {%'7 115
38 " ' --- , r - - ~ - - .
93/048 (matrix): garnet 4 34 L
! 3 70 ~. 3o r / i
m 26 ~- 5 ] 7 ~. - ! ~ 5O
22 j 4
?] 18 steps 2, 3, 4.6, 7 ~ i 7 2518 +/- 35 Ma ' "~0 L MSWD 1.42 J - 14!
I0 i L
400
300
2 0 0
10 30 50 70 00 I I0 130
:<' Pb / :'= Pb
' ~ ~ . . . . . . . . ~- ' = ] 800 /
• c 71 ~ J 600
i
## / 2 I -,
100 ~
7 2OO
I :~6 5 [ (} ___~_ , i _. i i !
I0 3{I 50 70 90 110 130 0 _ _ _ _ _ _
:" Pb / ~u Pb
i r r
93/048 (aggregate): 2 i sillimanite ~ 4
i
i 1
i I
I steps I and 2 4 . age: 2524 +/ 5 Ma
/ > sill bulk gn bulk I
1 O0 200 300 400 : " Pb / :"* Pb
X E
T - -
i
. g~t bulk " /3 i
"':1 I
sill bulk ' ~ 4
_ , i I oo 200 qo0 400
( a ) ( b ) .... Pb / ~"~ Pb
F ig . 5. Pb i s o t o p i c d i a g r a m s o f P b S L d a t a . F o r e a c h s a m p l e b o t h . t he u ranogc l l iC ( > ' P b 2napb \ . e r sus - '< 'Pb '~ 'aPb) a n d the tho roge l f iC
(2<'SPb 2/'4pb x e r s u s 2""pb > a P b ) Pb i s o t o p i c c o m p o s i t i o n s a re s h o w n . E x p e r i m e n t a l d a t a . i s o t o p i c c o m p o s i t i o n s a n d m i n e r a l a b b r e v i -
a t i o n s a r e g i v e n in T a b l e 3.
< 1%. This, and the close proximity of the blank composition to the regression lines, means that the ages calculated from the leach data are not percep- tibly affected by the blank.
7. Results
The PbSL and U Pb isotope data are given in Tables 3 and 4. The Pb spectra from leach experi- ments are plotted in conventional uranogenic (2° 'pb2°4pb versus '°<'pb/2°4pb)) and thorogenic (2°~pb "2°4pb versus 2°t~Pb :2°4pb)) diagrams ( Fig. 5(a) tel) . U Pb and Pb Pb data fi'om apatite are plotted in Figs. 6 and 7.
93/048 matrix garnet: increasingly radiogenic Pb was recovered for the first steps (step 1 could not be measured due to a weak signal). After step 3 the radiogeneity again decreased and the least radiogenic Pb was measured for steps 6 and 7,
which indicates that leaching was nearly complete. Step 5 marks an irregularity in this pattern, because it is again more radiogenic than step 4. In the thorogenic plot (see Fig. 5(a)) this step lies below the garnet reference line, indicating the influence of a second, low Th:U Pb-source. From our experi- ence with stepleaching we suggest zircon microin- clusions as possible contaminants. Zircon is a uranium rich phase which is only attacked effi- ciently by fluoric acid {step 5). In the uranogenic plot steps 2 4, 6 and 7 define sin isochron with an age of 2518 _+ 35 ( MSWD 1.42 ). All steps together. including step 5, also define an isochron with a slightly higher age (2546+_41, MSWD 2.08). However, as discussed, step 5 may be dominated by zircon inclusions and we therel\~re consider the first isochron to give a more accurate age for garnet growth.
93/04~; Sillimanite: a similar leach spectrum as for matrix garnet was obtained for sillimanite,
L. Hol-er et al. / Precambrian Research 87 (1998) 87 115 101
I9
17
15
13
41
39
f. 3 7
35
33
(c)
i i i i i i
93/083: clinopyroxene
1
Points 1 - 3 and 5 Age: 1860 +1- 320 Ma (MSWD 0.3)
J i i i i i
14 18 22 26 30 34 38
~o, Pb / ~ Pb
i , ~ , i , i , i , i
i i ~ i i i
14 18 22 26 30 34 38
z~ Pb / 20, Pb
Fig. 5.
240
2OO
.~ 160
120
80
40
0
2000
1600
1200
800
400
0
(d)
(continued)
93/167 (core): garnet
/
Y J steps 1 - 4
age: 2010 +/- 17 Ma (MSWD 2.24)
I J I r [ , I ~ I
0 400 800 1200 1600 2000 2'* Pb / 2o, Pb
2 grt bulk
t,~ 4
f s p ~ I J I ~ I , I - -
400 800 1200 t 600 2000
2~ Pb / 2~ Pb
which forms aggregates pseudomorphic after anda- lusite. Increasingly radiogenic Pb was recovered as leaching progressed, and the less radiogenic Pb from the residue indicates that step leaching was nearly complete. Regression of all sillimanite steps did not result in an isochron (MSWD 44.1 ). Also, scattering values of the individual step solutions in the thorogenic plot indicate at least two sources of Pb. In the uranogenic plot steps 3, 4 and bulk sillimanite and bulk associated garnet (intergrowth within the sillimanite aggregates) all plot slightly above the tie line between steps 1 and 2. It is apparent from the thorogenic plot (2°8pb/2°4pb versus 2°6pb/2°4pb) that the four datapoints (steps 3,4, bulk sillimanite, bulk associated garnet) are characterized by a highly uranogenic Pb compo- nent whereas leach steps 1 and 2 have more thorogenic Pb components. The data thus define two linear trends and the question arises which of them has age significance. Steps 3 and 4 (together with the associated bulk garnet) plot on a line with an apparent age of 2573_+15 Ma (MSWD
1.59). Alternatively, the tie line between steps 1 and 2 has a slope equivalent to an age of 2524+5 Ma. The combination of both leach spectra (matrix garnet and sillimanite) gives evi- dence for the second approach: leach steps 1 and 2 of sillimanite together with all steps of matrix garnet (excluding step 5) define a perfect isochron with an age of 2521 +4 Ma (MSWD_+ 1.43), which is considered to reflect the time of both sillimanite (aggregates) and garnet (matrix) growth. The slightly older and more uranogenic Pb signatures of steps 3 and 4 from sillimanite are explained to be contaminated by Pb components from zircon microinclusions. The age of 2573 _+ 15 is considered as a minimum age for these inclusions.
93/167 (core) garnet: very radiogenic Pb compo- nents were leached from garnet within the core of a calcsilicate enclave. Except for the residue, PbSL data almost define an isochron with an age of 2010_+17 Ma (MSWD 2.24). The scatter in the thorogenic diagram suggests the presence of three different Pb components: the first two steps are
102 L. Holzer eta/. Precambrian Research 87 (1998) 87 115
280
240 f 200
g. L ~ 16o[ ~- 120
80
4(1 k I J ' ~
: f sp (c re) o o
0 400
2000 I
1600
~ ~ 1200 [
i i i i i
93/167 (rim): titanite ~ 2
4 ( r e s i d u <
• 7" titanite bulk
m o n a z i t e
3 l each steps I, 2 + bulk t itanite 2007 +/- 5 Ma (MSWD 0.05)
I I , i , I _ 800 1200 1600 2000
20~ Pb / x~, Pb i i
~bulk titanite
4 (res idue)
I , L , i 120(1 1600 2000
t m o n a z i t e
800 ~ ~ 1 i
400 i is ~ 3 e )
0 ~ " -, 1) 400 8()0
(e) v~ Pb / :~" Pb
Fig. 5. (('ontim,,d)
dominated by thorogenic Pb that probably derived from monazite inclusions (data points lie above the garnet reference line in the thorogenic plot). Their colinearity with other fractions in the urano- genic plot shows that these microinclusions are cogenetic with garnet. This is compatible with the age of 2011 _+ 20 Ma obtained for a monazite frac- tion from the same sample (see below). Steps 3 and 4 are interpreted to be dominated by garnet hosted Pb. The residue revealed a uranogenic Pb component (data point lies below the garnet refer- ence line in the 2°Spb/Z°4pb versus 2°6pb/z°4pb diagram), which we interpret to derive from zircon inclusions, attacked during the final dissolution in HF. These inclusions were not in initial isotope equilibrium with garnet as the residual data point lies above the isochron in the uranogenic diagram. Pb from bulk feldspar is in near isotope equilib- rium with Pb from garnet, which is reflected by an errorchron (2003,+18 Ma, MSWD 6.86), defined by garnet steps 1 4 and bulk feldspar. The published two point garnet-feldspar isochron of
150
1 3 0
110
90
a.. 70
50
3O
10
i
Apatite 93/167
d
/
L
I O 0
° 1 b ~ e a
4
2047 +/- 240 Ma (MSWD 82.71 j :'~Pbf~Pb.: 16.78 +A 4.6 (2~) 1
I I ] 200 300
34
3O
26
~" 22
18
14
IO I
0 0.4
d [q
b c
V~
/ 2028 +A 71Ma (MSWD 17) ~'"Pbf~Pb,: 16.11 +/ 0.9 (2~)
i ~ i I I
0.8 1.2 1.6 2.0 2.4 2.8
~" U / ~'~ Pb
Fig. 6. Convent ional U Pb i s o c h r o n d i a g r a m s f o r f i v e a p a t i t e
f r a c t i o n s o f s a m p l e 9 3 / 1 6 7 . T h e r e s u l t s a r e d i s c u s s e d in t h e text. E x p e r i m e n t a l d a t a . i s o t o p i c c o m p o s i t i o n s a n d m i n e r a l abbrevi- ations a r e g i v e n in T a b l e s 3 a n d 4.
0.38
? 0.36
0.34
0.32
4 .8
i i i ~ ; , !
Apatite 93/167 a ~ ~
2 ~ ~
1 9 2 0 ~ d i z
1880 , ~ 4 . J y Intercepts at 1983 +! 14 Ma
18 and 0 +/- 5 Ma MSWD 2.0
i I I L i
5.2 5.6 6 .0 6 .4 6.8
~'" Pb* / ~'~ U
Fig. 7. C o n c o r d i a d i a g r a m f o r f i v e a p a t i t e f r a c t i o n s o f s a m p l e
93/167. U Pb isotopic data a r e g i v e n in Table4.
2011,+6Ma (Holzer et al., 1996) is compatible with the above PbSL result. The position of bulk garnet relative to the garnet reference line in the thorogenic diagram implies that its signature is
L. Hol;er et a/. ,' Precamhrian Research 87 (1998) 87 115 103
more effectively dominated by Pb from cogenetic monazite than from older zircon inclusions. A minimum age of 2337 -+ 47 Ma for the zircon inclu- sions can be estimated from the feldspar-step 5 garnet tie line.
93/167 (rim) titan#e: similarly to the above described garnet, PbSL of titanite from the rim of the calcsilicate enclave also released very radio- genic Pb components. The scatter of data in the thorogenic diagram again suggests the presence of three different Pb sources, i.e. host titanite and monazite- and zircon-inclusions: in Step 1 effec- tively all Pb was leached from cogenetic monazite inclusions, as shown by its very thorogenic Pb signature. In contrast, steps 3 and 4 are dominated by more uranogenic Pb (low 2°8pb/2°4pb ratios relative to a titanite reference line). The final dissolution in HF (step 4) particularly revealed a predominant influence of Pb from zircon inclu- sions. Steps 1, 2 and bulk titanite define a perfect isochron of 2007-+5Ma (MSWD 0.05). This isochron is defined by two cogenetic Pb sources: titanite hosted Pb dominates step 2, monazite hosted Pb dominates step 1. In the bulk titanite the zircon source seems to be hardly expressed. In addition, near isotopic equilibrium with titanite is also indicated for monazite and feldspar (93/167 core), which define an errorchron with an age of 2014_+ 15 Ma (MSWD 10.2). For the zircon inclu- sions a minimum age of 2422_+ 15 Ma can be calculated from the tie line between feldspar and 'titanite" residue.
Initial isotopic near-equilibrium for all measured minerals (titanite, garnet, monazite, apatite and feldspar) from core and rim of the calcsilicate enclave 93/167 is demonstrated by a Pb Pb errorchron (2008 _+ 10 Ma, M SWD 13.6), which is defined by 14 datapoints. Only steps 3 and 4 of titanite, bulk and step 5 of garnet have been excluded. For these steps the influence of non- cogenetic contaminants has been detected in the leach spectra.
93/167 apatite: the lead budget of all five apatite fiactions is characterized by relatively high propor- tions of initially incorporated Pb (ca. 35%). The initial Pb isotopic compositions can be determined from conventional isochron diagrams for each decay system (Fig. 6). However, the five apatite
fractions only define errorchrons and thus the isotopic ratios determined by y-axis intercepts in conventional isochron diagrams have large errors (2°6pb/2°4Pbi: 16.78+4.8 20- abs; 2°Tpb/z°4pbi: 16.11 + 0.9). Within error these values are identical with the isotopic ratios of feldspar (93/167 core). We therefore used the feldspar composition as a best approximate for the initial Pb component and justify the appropriate correction by the fact that apatite and feldspar are nearly in isotopic equilib- rium. The slight scatter may be due to non- cogenetic microinclusions. In thin sections for example zircon and monazite inclusions within apatite were observed.
The U Pb analyses of five apatite grain size fractions yielded two nearly concordant and three slightly reversely discordant data points (Fig. 7). All five fractions together define a discordia line (forced through 0_+5 Ma) with an upper intercept at 1983_+14 Ma (MSWD 2.0).
93/167 (rinT) monazite: a U-Pb analysis o fmon- azite yielded a concordant data point with a 2°Tpb/z°~Pb age of 2011 _+20 Ma (Table 4). This age is consistent with the ages from garnet and titanite of the same sample and partly justifies our interpretation of the Pb leach spectra that monazite microinclusions are cogenetic with the host minerals.
93,/083 clinopyroxene: only a small data spread was obtained by PbSL of diopside from calcsilicate 93,/083. Due to a weak signal step 4 could not be analysed. PbSL data define an isochron of 1860_+320Ma (MSWD 0.329) (Fig. 5(c)). The linear array in the thorogenic diagram suggests an inclusion-free sample. Mixing two different compo- nents would result in a line only in the case where both sources are cogenetic and when they have an identical U/Th ratio. The age result excludes an Archean age for the cpx, which therefore most likely also formed, or recrystallized during the 2.0 Ga event.
In this study the range of minerals to which PbSL dating (Pb stepwise leaching) has been applied (garnet. titanite, hornblende, clinopyro- xene, epidote, staurolite, tourmaline) is extended to sillimanite. As in previous studies using this method, the leaching technique has shown the great advantage, compared to bulk mineral dating,
104 L. Holzer et al. / Precambrian Research 87 (1998) 87 115
that the different components of Pb (host mineral, microinclusions, contaminants) can be detected through their varying uranogenic and thorogenic Pb signatures. In most cases consistent ages could be obtained for minerals with non-cogenetic inclu- sions by disregarding the leach steps with mixed isotopic signatures. In cases where the total Pb budget is dominated by microinclusions and most steps are a mixture of Pb components from host mineral and inclusions, the age of the host mineral could be determined under the assumption that the microinclusions are cogenetic. A complicated leach spectrum is obtained for siUimanite 93/048 (aggregates pseudomorphic after andalusite), which can be interpreted with confidence because steps 1 and 2 are in isotopic equilibrium with garnet from the matrix of the same sample. However, it can not clearly be distinguished whether the first two steps are dominated by Pb components from the host mineral itself. The high radiogeneity of these steps might indicate the influence of cogenetic microinclusions such as monazite.
8. Discussion
8. i. Implications jo t geochronology
The results from sillimanite and garnet (93/048) provide evidence for a highly retentive character of the U Pb system in these two minerals. In our example the U-Pb system was not affected during the high grade metamorphic overprint at 2.0 Ga, where peak metamorphic temperatures of 800 850~C were reached. Pb isotopic data from garnet and sillimanite may therefore rather reflect the time of mineral growth or recrystallization than that of a closure during cooling. Garnet and sillimanite are thus important geochronometers in polymetamorphic terranes. Nevertheless, it cannot be uncritically assumed that the P T conditions recorded by the mineral chemistry of, for example, garnet in metamorphic assemblages would be those dated by the PbSL chronometer, as major element systems might have at least partly reequilibrated in the later metamorphic event (Mezger et al., 1989). Information about the P-T evolution during
early metamorphic episodes can, however, come from the identification and dating of relic meta- morphic mineral assemblages and reaction textures, involving alumosilicates and other meta- morphic minerals.
In the following sections the different geological events will be discussed. In Table 5 a simplified summary of the complex relationships in the CZ is presented, in which the structural, magmatic and metamorphic events are correlated with each other.
8.2. The late Archean/early Proterozoie event in the CZ (~2 .6 Ga)
A long period of geological activity in the CZ must be assumed for the Archean-Proterozoic. Magmatism occurred from ~>2655 Ma (Alldays granodiorite) to <~2550 Ma (Singelele and other granitoids). The Bulai intrusion (2570 Ma) post- dates high grade structures of the D2a phase and associated anatectic features. Mobilization of large amounts of anatectic melt was contemporaneous with the Bulai intrusion and mingling of the calcal- caline magmas with leucogranitic material has been described. However, the Bulai gneisses underwent several phases of high grade deformations, provid- ing evidence for more than one 'post-Bulai' overprint.
Our data from metapelite 93/048 give evidence for growth of sillimanite and garnet during a high grade episode at 2521 +4 Ma. The textural rela- tionships indicate sillimanite (+cordierite+ garnet) growing at the expense of chiastolitic anda- lusite (+inclusions), thereby implying a prograde metamorphic reaction at high temperatures and low pressures. The dating of these relic metamor- phic assemblages thus documents a first episode of 'post-Bulai' high grade metamorphism. It seems that the CZ underwent several pulses of thermal disturbance associated with magmatism and (tectono-)metamorphism between 2.7 and 2.5 Ga.
The late Archean history of the CZ thus resem- bles that described for the NMZ by Berger et al. (1995) and Kamber and Biino (1995) in that (a) magmatic activity is constrained between 2.7 and 2.62 Ga (charno-enderbites in the NMZ, granodio- ritic Alldays Gneiss in the CZ; Jaeckel et al., 1997):
Tab
le 5
Su
mm
ary
of
geol
ogic
al e
vent
s in
the
Cen
tral
Zon
e, L
impo
po B
elt (
mod
ifie
d af
ter
Wat
keys
, 19
83)
Ma
Igne
ous
and
sedi
men
tary
eve
nts
Def
orm
atio
nal
even
ts
Met
amor
phic
eve
nts
3.36
3.8
Ga
Det
rita
l zi
rcon
s in
Bei
t B
ridg
e C
ompl
ex
3200
± 1
00
Firs
t ph
ase
of
TT
G f
orm
atio
n:
- S
and
Riv
er G
neis
s S
RG
Z
anzi
bar
Gra
nodi
orit
e M
ess±
ha L
ayer
ed I
ntru
sion
ML
I F
irst
fab
ric
form
ing
even
t in
the
CZ
: >
300
0 D
1 ls
oclin
al f
oldi
ng a
nd d
uctil
e sh
eari
ng
M I
whi
ch a
ppea
r to
be
uniq
ue t
o th
e B
asem
ent
gnei
sses
(S
RG
)
3000
M
afic
dyk
es i
ntru
ding
SR
G
Ear
ly h
igh
grad
e m
etam
orph
ism
pr
edat
ing
ca.
3 G
a ol
d m
afic
dyk
es
2654
± 1
5 >
2600
25
75 ±
25
Seco
nd p
hase
of
TT
G f
orm
atio
n:
Alld
ays
Gne
iss
D2a
B
ulai
Plu
ton
calc
alka
line
suite
: sy
ntec
toni
c D
2 Si
ngel
ele
type
gra
nito
ids
Gra
niti
c dy
kes
intr
udin
g Z
anzi
bar
Gne
iss
Duc
tile
recu
mbe
nt f
oldi
ng (
F2a
) Po
lyph
ase
duct
ile f
oldi
ng (
F2a
and
F2b
) M
2a
Mig
mat
izat
ion
pred
atin
g th
e B
ulai
int
rusi
on
2520
D
2b
Fir
st f
abri
c fo
rmin
g ev
ent
in t
he B
ulai
Plu
ton
M2b
L
ow P
/hig
h T
eve
nt
2500
-200
0
2000
_+40
G
rani
tic
Plu
tons
in
Mah
alap
ye C
ompl
ex
D3
Gen
eral
ly N
NW
-SS
E d
irec
ted
shor
teni
ng
M3
Gra
nuli
te f
acie
s ev
ent
with
clo
ckw
ise
Cha
rnoc
kite
s in
the
Koe
does
rand
win
dow
L
ocal
var
iati
ons
of
stre
ss a
nd s
trai
n PT
-evo
lutio
n:
Tra
nsit
ion
to E
NE
-WS
W d
irec
ted
mov
emen
ts
and
righ
t la
tera
l di
spla
cem
ents
F3
a: O
pen,
upr
ight
'cr
ossf
olds
' with
N
NW
-SS
E t
rend
ing
fold
axi
s pl
ane
'Min
iatu
re m
obil
e sh
ear
belt
s' i
n B
ulai
F3
b: E
NE
-WS
E d
irec
ted
thru
stin
g (L
3b)
Rot
atio
n o
f ea
rlie
r st
ruct
ures
D
extr
al ~
Lim
popo
tre
nd"
shea
ring
(E
NE
-WS
W)
-.q
2027
±6
2010
±5
2005
± 5
2000
19
70
Peak
: ca
. 82
5 C
, >
10
kbar
M
igm
atiz
atio
n
Rap
id d
ecom
pres
sion
is n
earl
y is
othe
rmal
Pa
rtia
l m
elti
ng d
urin
g de
com
pres
sion
po
stda
tes
F3b
def
orm
atio
n S
ubse
quen
t is
obar
ic c
ooli
ng a
t 5-
6 kb
ar
t~
106 L. t to / :er el a/. Precamhrian Research ,¥7 (1998) 87 115
(b) porphyritic granites and charnockites were emplaced between 2.62 and 2.58 Ga in the NMZ, whereas in the CZ the porphyritic Bulai Pluton (Barton et al., 1994) and the granitoid Singelele orthogneiss intruded between 2.6 and 2.55 Ga (Jaeckel et al., 1997): (c) low to medium pressure granulite facies conditions prevailed in the N M Z between 2.62 and 2.58 Ga, whereas our data give evidence for (M2-)granulite facies conditions in the CZ prevailing until about 2.52 Ga. Anticlockwise P-T evolution is well constrained for the N M Z and is suggested for the CZ from the occurrence of sillimanite pseudomorphs after chiastolitic andalusite: (d) N N E S S W directed compression was the main foliation producing tectonism in the NMZ, which has its CZ-equivalent in the D2 deformations described by Watkeys ( 1983 ).
There is no evidence for important geological activity between 2.5 and 2.0 Ga in the CZ (or SMZ and NMZ) , or for retrogression and rehydration of the presently exposed CZ granulites. They prob- ably resided in mid to lower crustal levels during this period. A reason for lack of exhumation after 2.6 Ga might be due to the fact that the late Archean low pressure granulites in the CZ were not produced primarily by tectonic thickening and thus erosion and associated isostatic equilibration of the crust was not an efficient mechanism for post-collisional exhumation.
<%3. Ideltt(/ication oJProterozoic structures bt the CZ
hi order to date the D3 deformational event, we have chosen samples with mineral assemblages that recrystallized synkinematically (samples 93:083 and 93/167). The data from clinopyroxene 93/083 give an imprecise age of 1860_+320Ma, which nevertheless indicates that the synkinematic recrystallization of cpx, which occured during the formation of the Campbell crossfold (F3a), is not the product of Archean tectonic activity, as pre- viously suggested. The data obtained from calcsili- cate enclaves in a shear zone on Mount Shanzi give more precise ages of 2010+ 17 Ma (garnet), 2 0 0 7 + 5 M a (titanite), 2 0 1 1 + 2 0 M a (monazite) and 1983+ 14Ma (apatite). The deformation on
Mount Shanzi belongs to a phase of refolding (F3b) of the regional scale crossfold structures (F3a). During this phase subparallel fold axes and mineral elongation lineations (ductile feldspars and strings of garnet) were produced, which dip with about 30 towards 260 (Watkeys, 1983: Holzer. 1995). The lineations and fold axes in the Tshipise Straightening Zone, South of Messina, have the same orientation and we therefore suggest that this structure was also produced during the Proterozoic event. It is important to note that apparently there exist domains in the CZ where the Proterozoic D3 deformations are not predomi- nanL such as in the Three Sisters area 20 km NW of Messina.
8.4. The timing ojnletamorphism during tire Prolerozoic event (ca. 2.05 1.95 Ga)
The clockwise P-T path of the youngest meta- morphic event (M3) in the CZ was determined by various authors (Chinner and Sweatmam 1968: Droop, 1989: Horrocks, 1983a,b; Windley et al., 1984; Harris and Hollan& 1984; Hisada and Miyano, 1996). In the following section we reinter- pret the timing of this clockwise P T evolution in the light of recently published age data and those from this study. The error-envelopes in the P T diagram (Fig. 8) are not based on quantitative error calculations. The temperature time data and
kbkl
12
0
8
4
2
2 0 3 1 + / 7 Ma I I I l]]C[klllll wphik" III k't I11
11 201 I+1.20 Ma m~mazitc I VI -til5 2{}1I}+1 17 Ma garlic* l 21ln7+/ 5 Ma litgtnit¢ ~,. ~
2005+/ .~ Ma/ircoll tlndef{lrnled iTlelt palche,
I r 19X~+i 14 Ma apame
ill7 +/-5 )Rb-S~ hioute
IO{l 21}0 ~ } 40{} 50(I [}00 ?0{} S{}0 un(l ("
Fig. 8. Pressure temperature diagram: combination of new age data with the PT path defined for the youngest high grade event in the Messina area. CZ. The timing of a Proterozoic PT-evolution is discussed in the text. Labels correspond to refer- ences given ill Table 1.
L. Holzer et al. / Precambrian Research 87 (1998) 87-115 107
the temperature errors follow commonly accepted values of the respective (thermo-)chronometers (e.g. Tc of R b -S r Biotite=350_+50°C; Dodson, 1979), whereas the pressure errors are speculative.
Metamorphic zircons from metapelites yield an age of 2026 ± 7 Ma (Jaeckel et al., 1997). This date is interpreted to reflect the time of high pressure granulite facies metamorphism. Our PbSL data from garnet and titanite with ages of 2010 __. 17 Ma and 2007+5 Ma, respectively, show that synki- nematic recrystallization of garnet and titanite (in calcsilicate enclaves from Mount Shanzi) during M3/D3 occurred almost synchronously and they probably mark the time when near-isothermal decompression was initiated. The PbSL data is confirmed by a concordant datapoint with an age of 2011_+20 Ma from monazite of the same sample. Textural evidence for equilibrium between garnet, titanite, clinopyroxene and plagioclase imply that their dynamic recrystallization occurred under granulite facies conditions. Subsequent static recrystallization of plagioclase shows that high temperatures persisted after 2010 Ma. At some localities a late phase of partial melting is observed. The crystallization of these melt patches post-dates the latest ductile deformations (F3b). Undeformed leucosome from within the Sand River Gneiss (Causeway locality) yielded zircon ages of 2005_+8 Ma (Jaeckel et al., 1997). These authors interpreted the melt patches as the product of decompression melting around 2005 Ma. The time of the subsequent near isobaric cooling (IBC) is constrained by a U Pb concordia intercept age of 1983_+14Ma from five apatite fractions. Furthermore the time of retrograde metamorphism is constrained by Rb-S r biotite-whole rock cool- ing ages from throughout the CZ (Barton et al., 1992), which scatter widely but concentrate around 1970 Ma. This scatter may signify regional vari- ance of uplift in the ca. 600 km long CZ. However, it may partly be due to Sr isotopic disequilibrium between biotite and the whole rock, caused e.g. by retrograde recrystallization.
8.5. Towards a Proterozoic tectonic model for the Limpopo Belt--a working hypothesis
This model is based mainly on structural obser- vations from the Messina area (summarized pre-
viously in this paper) and on the data discussed above. Further we try to delineate large scale tectonic relationships of the 2 Ga event, including available structural and P-T- t data from the entire CZ. The model is merely an outline and should therefore be considered as no more than a working hypothesis for future research. Four phases of a Proterozoic event are distinguished.
8.5.1. Phase 1. Tectonic thickening of the CZ (>2.03 Ga) (Fig. 9(a))
The CZ was squeezed between the Kaapvaal and Zimbabwe cratons and as a consequence the CZ was thickened and high pressure granulite facies conditions in the presently exposed crustal levels were reached at or before 2.03 Ga. Early D3-deformations within the Bulai Pluton suggest a NNW-SSE directed shortening. The exact rela- tive positions of the CZ and the adjacent marginal zones (amalgamated to the cratons) during this early stage of the Proterozoic orogeny are unknown. Information about duration and P-T path of the prograde metamorphism are also lack- ing. The Proterozoic orogeny in the Limpopo Belt appears to have a close relationship with the Kheis and Magondi fold belts at the western margins of the cratons, which were formed at about 2.0 Ga. A better understanding of the link with these orogenic belts could provide important additional information about the geodynamic evolution in the Limpopo Belt.
8.5.2. Phase 2. Dextral transpression ( ~2.03 2.01 Ga) (Fig. 9 (b))
In the Messina area the formation of large scale crossfolds occurred at or shortly before 2.01 Ga. ENE WSW directed movements along D3 struc- tures (e.g. thrusting on Mount Shanzi or Limpopo- trend shears) indicate a transition from NNW-SSE directed shortening (with strong pure shear compo- nent) to dextral transpressive tectonics. However, the shear strain was concentrated in the suture zones bordering the CZ. The moderately SSE dipping Triangle shear belt (Kamber et al., 1995b) to the North and the steeply SSE dipping Tshipise Straightening Zone to the South (Bahnemann, 1972) are both regarded as the product of such right lateral transpressive tectonics. Strike
108 L. Holzer et al. Precambrian Research 87 (1998) 87 115
Figure 9a: Tectonic thickening of the CZ (> 2.03 Ga)
........ iii
" f
v ' \ N ~ I 50k in I "F" ~ Kaap~-aal c r a t o n v V ' ' '
(a)
Figure 9b: Dextral Transpression (-2,03 - 2.01 Ga)
Zimbabwe craton
. . . . .
........... o \ *
; 1 - ' / " f Su~g~ater6~g zone
Mahalapye
Kaapvaal craton
(b)
Fig. 9. Sketch maps illustrating the four-stage tectonic evolution during the Proterozoic 'strike slip orogeny'. The tectonic model is discussed in the text. For legend and abbreviations see Fig. 1. (a) 1 -Mo lopo Farms Complex, 2 - Nebo Granite, 3 = Soutpansberg Graben, 4=Waterberg Basin, 5=Palapye Graben, 6-Murchison Thabazimbi Lineament. (d) P=Palala Granite, E=Entabeni Granite, S = Schiel Alkaline Complex.
L. Holzer et al. / Precambrian Research 87 (1998) 87-115 109
Fi ure 9c' I Zimbabwe craton _ Ufflifl and cooling t ~ -'4"-" " (-2.01 - 1.95 Ga) ~ ~ ~ " ~ "
~ o ~ ~ ' ° ~ ~ ____2~~ / " ~ / ~ ~ e n ~ Z o n e
Kaapvaal craton
(c)
Figure 9d: Final exhumation of the CZ Transtensional phase (Soutpansberg rifting) associated with postorogenic transcurrent faulting along the Palala (ca. 1.95 - 1.85 Ga)
i Zimbabwe craton
A,f~
PalapyeGroup f ' f ! ~'4' . I - - ~ " ~ / - ~
a shear zone 1 ~ ~2' ~ ' Soutpansberg " trou h palai ~ g
Kaapvaal craton
( d )
I 10 L. Holzer el aL , PrecanThrian Research 87 (1998) 87 115
slip movement in these 30 50 km wide shear zones occurred under granulite facies conditions. Transpressional tectonics where strike slip and thrusting were contemporaneous may explain the existence of both subhorizontal and down-dip lin- eations in the Triangle shear zone. Steep lineations are also observed in the Sunny Side shear zone and in the northern domain of the Palala shear zone (McCourt and Vearncombe, 1992). They reflect subvertical movements in the southern and southwestern part of the CZ as a response to the transpressive tectonics. Schaller et al. (submitted) document continuous deformation under changing metamorphic conditions along the Palala shear zone in the Koedoesrand area. Thrusting in the Koedoesrand window occured under granulite facies conditions at /> 2.03 Ga, changing to dextral strike slip shearing associated with the exhumation of the CZ shortly after 2.0 Ga. Important migmati- zation and formation of voluminous granitic bodies at about 2.0 Ga occured in the westernmost part of the CZ, the Mahalapye migmatite complex in Botswana (Chavagnac et al., submitted).
8.5.3. Phase 3. Upl(['t and eooling (2.01- 1.95 Ga; (F~,. 9 (e))
The right lateral movements of the Zimbabwe and Kaapvaal cratons relative to each other pre- vailed and thus strike slip tectonics in the Triangle (e.g. Kamber et al., 1995a) and Straightening shear zones lasted until after 2.0 Ga. The various cooling ages between 2.0 1.95 Ga from throughout the CZ (e.g. Barton et al., 1992) indicate that the uplift and cooling of the entire CZ was associated with these strike slip movements. Two large shear belts are suggested to have bounded the tectoni- cally thickened CZ at that time (Holzer et al., 1997). Both shear belts are characterized by a set of interconnected shear zones that operated simul- taneously: the Triangle-Lepokole-Magogaphate shear system to the North and the Tshipise Straightening Zone Sunny Side-Palala shear system in the Southern part of the CZ. The detailed kinematics of these shear systems are not yet fully understood. Whereas the northern shear system is predominated by dextral strike slip at high grade conditions, the southern system is much more complex, including strike slip, oblique slip and
subvertical movements under peak to retrograde conditions. The near isothermal uplift from high to low/medium pressure granulite facies conditions in the CZ might be the product of a rapid change from a transpressive to a transtensive tectonic environment. Subsequent to the decompression, thermal relaxation resulted in a phase of near isobaric cooling (pressure=5 6 kbar). By about 1.95 Ga temperatures had decreased to ca. 350:C.
8.5.4. Phase 4. Final exhumation o[the CZ and post-orogenic transeurrent jaulting (~1.95 1.85 Ga) (Fi,~. 9(d))
Strike slip movements persisted after the uplift and cooling of the CZ. However, strain was local- ized along the southern margin of the CZ, mainly in the Palala lineament. This phase of late- to post-orogenic transcurrent faulting led to the formation of the Soutpansberg and Palapye graben structures. The clastic sediments of the Soutpansberg sequence were transported from the North (CZ). The transtensive tectonics and the contemporaneous sedimentation in the graben structures are thus closely associated with tectonic thinning and final exhumation of the CZ (Barker, 1983). The timing of this transtensional phase is bracketed by the ages of the Entabeni granite (1950 Ma; Barton et al., 1995), which is uncon- formably overlain by the Soutpansberg basal con- glomerates, and the Schiel Alkaline Complex (1850 Ma; Barton et al., 1996), which reflects the final stages of the Soutpansberg rifting tectonics. The formation of low grade mylonites in the Palala shear zone post-dates the uplift and cooling of the CZ and is thus most probably related to the described transcurrent tectonics. A dextral shear sense is described for the late strike slip movements in the Palala shear zone (Broekhuizen and McCourt, 1995).
8.6. hnplications for the teetono-metanlorphic history oJthe entire Limpopo Belt
The view presented above of the CZ as a poly- metamorphic province is in strong conflict with many of the published tectonic models that include all three subzones of the Limpopo Belt, and with terrane interpretations of the whole of Southern
Tab
le 6
Su
mm
ary
of g
eolo
gica
l ev
ents
in t
he t
hree
sub
zone
s o
f th
e L
impo
po B
elt
Age
in
Ma
Sou
ther
n M
argi
nal
Zon
e S
MZ
C
entr
al Z
one
CZ
N
orth
Mar
gina
l Z
one
NM
Z
3200
31
00_+
100
Fi
rst
high
gra
de e
vent
(M
I)
reco
rded
in
the
Bas
emen
t G
neis
ses
of th
e C
Z
(San
d R
iver
Gne
isse
s)
2720
262
0 H
igh/
med
ium
pr
essu
re
Mag
mat
ic a
ctiv
ity i
n th
e C
Z:
gran
ulite
th
cies
IT
D f
ollo
wed
by
IBC
26
55
Alld
ays
Gra
nodi
orit
e an
d an
d re
hydr
atio
n T
hrus
ting
at
Hou
t R
iver
s.z
. >
2600
A
nate
xis
(M2a
) 26
70
Mat
ok p
luto
n (s
ynte
cton
ic
intr
usio
n)
:z:
2620
255
0
2720
262
0
Mag
mat
ic a
ctiv
ity i
n th
e N
MZ
Intr
usio
n o
f vo
lum
inou
s ch
arno
ckit
es
and
ende
rbite
s.
Hig
h T
eve
nt i
n th
e C
Z,
anat
exis
(M
2)
Low
to
med
ium
pre
ssur
e gr
anul
ite
faci
es
rg
2.6/
2.57
Ga
2568
2520
Syn
tect
onic
por
phyr
itic
Bul
ai P
lulo
n an
d Si
ngel
e[e
gran
itoi
ds
Pol
ypha
se d
uctil
e fo
ldin
g
Lat
e ep
isod
e lo
w P
/hig
h T
m
etam
orph
ism
(M
2b)
2620
258
0 A
ntic
lock
wis
e PT
evo
luti
on
Synt
ecto
nic
porp
hyri
tic
char
nock
ite
and
gran
ite
Rev
erse
thr
usti
ng a
t N
orth
Lim
popo
T
hrus
t Z
one
NN
W-S
SE
com
pres
sion
"--1
.L
2000
_+ 50
2.
05
1.95
Ga
Fina
l ex
hum
atio
n "S
trik
e sl
ip o
roge
ny'-
dext
ral t
rans
pr.
Hig
h pr
essu
re g
ranu
lite
faci
es (
M3)
20
27
Peak
met
amor
phis
m
2.01
1.
97 G
a IT
D f
ollo
wed
by
IBC
and
re
hydr
atio
n
ca.
2.0
Ga
< 2
.0 G
a
Low
gra
de o
verp
rint
in
NM
Z a
nd
Tra
nsit
ion
Zon
e Fi
nal
exhu
mat
ion
< 1
950
Pahf
la t
rans
curr
ent
faul
ting
and
Sou
tpan
sber
g ri
flin
g Fi
nal
exhu
mat
ion
of t
he C
Z
112 L. Holzer et al. / Precambrian Research 87 (1998) 87 115
Africa, which argue that the final juxtaposition of the Kaapvaal and Zimbabwe cratons is the product of a suggested late Archean 'Limpopo Orogeny' (e.g. Treloar et al., 1992). Our study leads to the following implications concerning the history of the Limpopo Belt (Table 6).
Different types of granulite facies events: In the Limpopo Belt several granulite facies events are recorded. The different types of granulite facies metamorphism signify different tectono-metamor- phic genetic relationships: high pressure granulites with clockwise P-T paths were produced in oro- genic environments including tectonic thickening of the crust (SMZ at 2.7 Ga, CZ at 2.0 Ga). Low to medium pressure granulites with anticlockwise P T evolution seem to be confined to the Archean-Proterozoic boundary (2.6 Ga in NMZ and 2.52 Ga in CZ). These low pressure granulite facies events, in which mantle derived magmas provided the overlying crust with heat, might reflect vertical crustal growth during the late Archean.
Two episodes of late Archean metamorphism: two episodes of late Archean activity from 2.72 to 2 .62Ga and from 2.62 down to 2 .52Ga are recorded in the Limpopo Belt. During the first period high to medium pressure granulites were produced in the SMZ, regarded as a product of orogenic tectonics (Van Reenen et al., 1987). Magmatic activity at the same time is also iden- tified in the CZ and NMZ; however, the appertain- ing tectono-metamorphism has not yet been characterized sufficiently and therefore the tectonic relationships remain unclear. During a second period shortly after 2.62 Ga granulite facies meta- morphism with anticlockwise P T evolution occured in the NMZ (Kamber and Biino, 1995). The voluminous charnockitic and granitic magma- tism in the NMZ has its analogy in the Chilimanzi- type granites, which at the same time intruded the adjacent Zimbabwe craton. The features observed in the NMZ probably illustrate processes at lower crustal levels which were important during the late Archean cratonization. Granulite facies conditions associated with important magmatic activity between 2.6 and 2.52 Ga are also recorded in the CZ. Relic sillimanite aggregates pseudomorphic after andalusite give evidence for a late high grade
metamorphic episode with an anticlockwise P-T evolution at 2.52 Ga.
Although the previously discussed similarities between the late Archean histories of the NMZ and the CZ may suggest a common geologic history at about 2.6 Ga, the tectonic relationships are difficult to assess because high grade overprint of the CZ and important strike slip movements along the Triangle shear zone at about 2.0 Ga have erased much of the field evidence and obscured the regional context. The relationships between the CZ and the SMZ are even more diffuse. The timing and style of metamorphism recorded in the late Archean granulites from the SMZ differ markedly from those in the CZ and NMZ: clockwise P T evolution led to the forma- tion of high pressure granulites at about 2.7 Ga (Van Reenen et al., 1992), which is 100 Ma earlier than the anticlockwise episodes in the CZ and NMZ. Uplift of the SMZ was contemporaneous with the syntectonic Matok intrusion around 2.67 Ga (Barton et al., 1992). No evidence for high/medium grade metamorphism younger than 2.62 Ga has yet been identified in the SMZ. A correlation of high temperature events in all three subzones during the late Archean can thus neither be denied nor justified convincingly.
Juxtaposition of tile dfff'erent tectonic units: the marginal zones were thrust onto the adjacent cra- tons mainly in the late Archean. However, it is quite uncertain whether the five tectonic units (Kaapvaal craton, SMZ, CZ, NMZ, Zimbabwe craton) formed a connected continental mass. It cannot be precluded convincingly that some of these tectonic units were brought together only during the Proterozoic event. The relative positions of the cratons and the three subzones of the Limpopo Belt before the Proterozoic transpressive orogeny (at ca. 2.0 Ga) are uncertain.
9. Conclusions
The Limpopo Belt 1s an intercratonic suture zone, which has been repeatedly reactivated during the late Archean and Proterozoic. However, the three subzones are characterized by distinct tec- tono-metamorphic histories, which cannot easily
L. Holzer et al. / Precambrian Research 87 (1998) 87 115 113
be corre la ted , because style and t iming o f the recorded granul i te events are signif icantly different f rom each other. The m e t a m o r p h i c h is tory o f the L i m p o p o Belt is thus much more complex than mos t pub l i shed tectonic mode l s imply. The L i m p o p o Belt remains an interes t ing object for fu ture research where possible genetic models for granul i te provinces can be tested, such as con t inen t col l is ion and m a g m a t i c arc env i ronments or crust f o rma t ion by m a g m a t i c unde rp la t ing in the Archean .
The h is tory o f the L i m p o p o Cent ra l Zone is dep ic ted by repea ted high grade m e t a m o r p h i s m assoc ia ted with de fo rma t iona l and m a g m a t i c events which occured at abou t 3.2-3.1 Ga , 2.65 2.52 G a and 2.0_+0.05 Ga . The younges t high grade event is descr ibed as a dextra l t ranspress ive orogeny, dur ing which the K a a p v a a l and Z i m b a b w e cra tons coll ided. The CZ, which was tec tonical ly th ickened dur ing this coll is ion, is b o u n d e d by two large s tr ike slip shear systems which were active dur ing peak m e t a m o r p h i s m and exhumat ion .
This s tudy has conf i rmed the impor t ance o f age da t a f rom m e t a m o r p h i c silicates, because they p rov ide i m p o r t a n t i n fo rma t ion that can be used to unravel the geologic his tor ies o f p o l y m e t a m o r - phic high grade terranes. Lead stepwise leaching (PbSL) is a tool with which i m p o r t a n t minera ls such as garnet , t i tani te , c l inopyroxene and even s i l l imanite can be da ted . These age da t a can be di rect ly l inked with the pe t ro log ic in te rp re ta t ion ( P - T condi t ions) o f the da t ed minera ls or minera l assemblages and are thus i m p o r t a n t for the deter- m ina t ion o f P - T - t paths . This s tudy has also shown tha t PbSL on highly retent ive minera ls like s i l l imanite and garne t can p rov ide in fo rma t ion abou t the P T evolu t ion o f ear ly m e t a m o r p h i c events tha t have been overp r in ted by a la ter granu- lite facies m e t a m o r p h i s m .
Acknowledgment
Discuss ions and field tr ips with Tom Blenkinsop, D i rk van Reenen, A n d r e Smit and B.K. Paya have con t r ibu ted much to this work. D i rk and A n d r e are t hanked for their gen t lemanly a t t i tude towards
geological controvers ies . F inanc ia l suppor t f rom the Schweiz. N a t i o n a l f o n d s ( G r a n t 20 40442.94) is grateful ly acknowledged . L.H. would also like to t hank Al f red K r 6 n e r for the m a n y helpful discussions. Klaus Mezger is t ha nke d for his con- s truct ive review, which great ly improved an earl ier version o f the manuscr ip t .
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