40Ar39Ar Thermochronological constraints on the structural evolution of the Mesoproterozoic Natal Metamorphic Province, SE Africa

Premmbriun Resenr(h ELSEVIER

Precambrian Research 86 (1997) 71-92

4°Ar/39ArThermochronological

constraints on the structural evolution of the Mesoproterozoic Natal Metamorphic Province, SE Africa J. Jacobs a,,, M. Falter

b R.J.

T h o m a s c, j. Kunz b, E.K. JeBberger b

a Universiti~t Bremen, FB5-Geowissenschaften, PF330440, 28334 Bremen, Germany b Max-Planck-Institutfi~r Kernphysik, P F 103980, 69029 Heidelberg, Germany c Council for Geoscience, P.O. Box 900, Pietermaritzburg, 3200, South Africa Received 13 May 1996; accepted 15 July 1997

Abstract

Ten 4°Ar/39Ar age spectra are presented for hornblende grains separated from mylonitized and weakly deformed amphibolites from the Mesoproterozoic ( ~ 1.2-1.0 Ga) Natal Metamorphic Province (NMP). Since the proposal of the SWEAT hypothesis, the NMP has become a crucial area in which to study Grenville-aged (locally termed 'Namaquan') accretion in this part of Rodinia/Gondwana. Unlike many segments of the World's ~ 1.1 Ga belts, the NMP is particularly suited to such studies since it is devoid of a high-temperature Pan-African ( ~ 500 Ma) overprint. In this study we have dated hornblendes from within the earliest (D1) NE-directed thrust-dominated structural domains of the belt. The D1 domains are characterized by NE-directed recumbent fold and thrust nappes and a pervasive, generally S- to SW-dipping, metamorphic foliation with down-dip stretching lineations. The new data constrain the minimum age of this early event (which includes obduction of the northernmost Tugela Terrane onto the Kaapvaal Craton) to 1135+_9 Ma. In the D1 domains of the Mzumbe Terrane to the south, cooling to below ~550°C had only been attained by ~ 1005 Ma. We also dated amphibolites from the later (D2) structural domains which are characterized by sub-vertical sinistral mylonite belts with sub-horizontal to oblique stretching lineations. Our data show that D2 oblique shearing commenced at 1050-1035 Ma in the Mzumbe Terrane and only terminated at ~ 980 Ma. The youngest movements also included reactivation of major D1 structures such as the Melville Thrust (Mzumbe-Margate Terrane boundary) at ~990 Ma. Finally, indications of minor Pan-African resetting was detected during the first ~5% of the A r ~ r spectra from mylonites of the Lilani-Matigulu shear zone (Tugela-Mzumbe Terrane boundary), indicating the polyphase nature of this major structure. It has been suggested that the D1 event records the early N E - S W directed arc-continent collision history of the belt whilst the D2 event represents a continuation of essentially the same convergence vectors after extensive crustal thickening. The new data place important time constraints on the major tectonic events in the NMP which appear comparable in many ways to other parts of the global 'Grenvillian' orogen. © 1997 Elsevier Science B.V.

Keywords: Natal Metamorphic Province; Grenville; Rodinia supercontinent; 4°Ar/39Ar; Geochronology

* Corresponding author. Tel: 0421 218 3995; Fax: 0421 218 3993; e-mail:[email protected] 0301-9268/97/$17.00 © 1997 ElsevierScience B.V. All rights reserved. PH S0301-9268 (97)00042-9

72

J. Jacobs et al. ," Precambrian Research 86 (1997) 71 92

I. Introduction

the N a m a q u a - N a t a l - F a l k l a n d - D r o n n i n g Maud Land ( N N F D ) belt have been pointed out by several authors (Martin and Hartnady, 1986; Grantham et al., 1988; Groenewald et al., 1991; Jacobs et al., 1993). The 'SWEAT' hypothesis (Moores, 1991) suggests that Palaeoproterozoic microcontinents were welded during the Grenville event into the Mesoproterozoic supercontinent of Rodinia and that the Grenville of Laurentia might have its natural continuation in the N N F D Belt of Africa and east Antarctica (Fig. 1 ). The Grenvilleaged crust of southern Africa, the Falkland Islands

The Mesoproterozoic ( ~ 1.1 Ga) Natal Metamorphic Province ( N M P ) forms part of a major orogenic belt, several thousands of kilometres in length, which was accreted onto the southern margin of the Archaean Kaapvaal Craton. Within Gondwana, this orogen has been traced eastwards from Namaqualand to Natal in South Africa, into the Falkland Islands and into Dronning Maud Land (east Antarctica) (Fig. 1). The close lithological and structural similarities of

"'~+++.

~!

+ 4 + + ÷ + + +

++++~z +++++~ ÷ ++++

India

,

Falkland

microplate Cape Meredith Complex Haag block

#/

iella

#

E-Antarctica

/

/

ii:i ii:i i!! ii:! !i:! iii Namaquan-Grenvillian crust (-1.1 Ga) Ebumian; Yavapai-Mazatzal crust (-1.8-2.0 Ga) Archaean crust ,~

Namaquan plate vector

Fig. 1. Position of the ~ 1.1 Ga NMP in a Rodinia reconstruction[modifiedafter Moores (1991)].

J. Jacobs et al. / Precambrian Research 86 (1997) 71-92

and Dronning Maud Lands thus form a key area in testing the SWEAT hypothesis. The southern African term for the orogenic episode which took place between ~1200 and 1000Ma is 'Namaqua-Natal' often shortened to 'Namaquan'. Since an exact corellations with orogenic belts from possible adjacent crustal blocks (e.g Grenvillian, Kibaran, Mozambiquean, Maud) cannot be unequivocally established, we use the local term 'Namaquan' for Grenville-age crust in the NMP. The western and eastern extremities of the NNFD belt were, to various degrees, overprinted during the Pan-African event (>_500 Ma) and in many of these areas it has remained a major problem to delineate the extent and significance of this later imprint (Jacobs et al., 1995). In contrast, it has recently been shown that the NMP underwent only low temperature (< 300°C) rejuvenation during the Pan-African, probably manifested as a period of relatively rapid uplift, as evidenced from titanite fission-track analyses (Jacobs and Thomas, 1996). The NMP is also one of the few zones in which no older crustal involvement has been recognized (e.g. Eglington et al., 1989). It is thus an appropriate area to study pristine Namaquan accretionary tectonism in this part of Rodinia. The NMP has been modelled as an oblique multi-arc/craton collision orogen (e.g. Matthews, 1972), with early NE-directed thrusting followed by ENE sinistral shearing (Jacobs et al., 1993; Thomas, 1989a; Jacobs and Thomas, 1994). Geochronological studies have demonstrated that the entire evolutionary history of the belt can be bracketed between 1200 and 1000 Ma (Thomas and Eglington, 1990; Thomas et al., 1993a,b). Whereas the timing of the various magmatic and metamorphic events is reasonably well constrained, the chronology of the structural evolution is less well constrained. The present study was undertaken to date the major structural events in order to constrain the timing and duration of the different Namaquan structural domains within the NMP. It was hoped that the integration of these data with the known magmatic history would enable the creation of a unified tectono-magmatic model for the evolution of the NMP which could

73

be extrapolated into those parts of the NNFD belt where the Namaquan history is to various degrees obscured by later Pan-African events. To this end, we undertook 4°Ar/39Ar-analyses of hornblende separates from carefully selected amphibolite facies mylonites and their undeformed equivalents associated with the different observed structural events.

2. Geological setting and previous geochronological studies The NMP comprises three distinct tectonostratigraphic terranes (Thomas, 1989a). These are known as, from N to S, the Tugela, Mzumbe and Margate Terranes (Fig. 2). The northernmost Tugela Terrane can be further subdivided into the northern 'Natal Thrust Belt', and the southern 'Natal Nappe Complex' (Matthews, 1972; Matthews and Charlesworth, 1981). The Natal Thrust Belt forms a 2-13 km wide zone of almost unmetamorphosed to greenschist grade passive continental margin sediments including limestones, conglomerates and shales (Ntingwe Group) and mafic metalavas (Mfongosi Group). The Natal Nappe Complex comprises four flat-lying nappes made up of layered amphibolite, tonalitic gneiss with serpentinite and ultramafic intrusions of greenschist to amphibolite grade. The soles of thrusts within the nappe complex are typically marked by thin talc-schist slivers which are interpreted as glide planes (Matthews, 1972, 1990). Mylonitic amphibolites are characteristically developed in the hanging walls of thrusts. SW to SSWplunging mylonitic stretching lineations are developed in these deformed rocks, indicating a top-tothe-NE displacement for the nappes. The Tugela Terrane has been interpreted as an ophiolite complex (Matthews, 1972) representing the remnant of the Tugela Ocean (Thomas and Eglington, 1990) which was obducted northwards onto the southern margin of the Kaapvaal Craton. It is bounded in the south by the Lilani Matigulu shear zone (LMSZ), a major ENE-trending sinistral transpression zone (Jacobs et al., 1993; Jacobs and Thomas, 1994). Gravity modelling by Barkhuizen and Matthews (1990) has shown that this structure also marks the southern extent of

"00"

:/)~'~

~

~ ~

~l'#elt,,171 e -- trl"°~ 16x'o~l ~

Transcurrent shear zones

Structural trends Thrusts Thrusting direction

Late tectonic granitoids

Nkomo nappe

5Okm

~.',J

+ !9" +

b)

Terrar~

Mzumt

)ietermadti

Tugela Terrane

• I

+

Transcurrent domains

Thrust dominated domains

Natal Metamorphic Province (~1.1 Ga)

~ Archean

....... MSZ3

~,,;-- MWR

. • QR2

VR8; HR2; SR2

:ig. 2. (a) Geological overview map of the NMP showing the Tugela, Mzumbe and Margate Terranes, the major terrane boundaries (LMSZ, Melville Thrust) as well s the distribution of the voluminous late-tectonic Oribi Gorge Granite Suite [after Thomas ( 1989a); Jacobs and Thomas (1994)]. (b) The NMP has been subdivided nto two major structural domains: i, thrust-dominated; and ii: transcurrent wrench-dominated [after Jacobs and Thomas (1994)]. Approximate sample localities ndicated, for more precise locations, see Table 1.

°

~

i %°%<~'% [ ~

""-~+/j/ ~/~' I MARGATE ~ +31]~!y TERRANE i :

tu

',

/ c.~+

"" ~"-~-~'~,.-~--~q.-vz.~,r F^[#atS

~

Madidimanappe

{~7-~'~ Mandleni nappe

~

Durban

'~

TO

/ ~,

~i . . . .

~x,

/$'

+~+ ~-"~-/

~¢~',

~

~ / ++.

~

C=

~,~

+30"

L TERRANE

Pietermaritzburg

+ 29°S

I TERRANE

.....

I

7"

J. Jacobs et al. / Precambrian Research 86 (1997) 71-92

the Kaapvaal Craton beneath the thin-skinned Tugela Terrane. South of the LMSZ lies the Mzumbe Terrane which is a juvenile Mesoproterozoic arc complex to which no basement is recognized. The Mzumbe Terrane extends for some 200 km south to the Melville thrust zone, south of which lies the Margate Terrane. The oldest rocks of the Mzumbe Terrane are amphibolite-grade layered gneisses and migrnatites (Quha Formation) and fine-grained felsic gneisses of the Ndonyane Formation (Thomas, 1989a). These rocks have been interpreted as representing a calc-alkaline sequence of volcanic/volcaniclastie rocks and their sedimentary derivatives, formed in a major island-arc complex which developed as a result of subduction of the Tugela Ocean at 1250 Ma (Cornell et al., 1996). The older island-arc rocks were intruded by voluminous syn- to post-tectonic granitoid suites between ~ 1200 and ~1000 Ma, which now make up >90% of the exposed basement outcrop. The oldest and most distinct granitoid body of the Mzumbe Terrane is an extensive calc-alkaline, I-type tonalite-trondhjemite orthogneiss known as the Mzumbe Suite, which was dated at 1207+10 Ma (Thomas and Eglington, 1990). The Mzumbe Suite has been interpreted as representing partial melting of oceanic lithosphere at a convergent plate margin (Thomas, 1989b). At ~ 1100 Ma, sheet-like syntectonic granites were intruded and at ~ 1050 Ma the Mzumbe Terrane was intruded by large volumes of late-tectonic rapakivi-textured granitoid and charnockites (Oribi Gorge Suite). The latter are interpreted as having intruded sinistral transtensional pull-apart type shear zones during a late stage of the oblique collision of the Mzumbe arc and the Kaapvaal Craton (Jacobs and Thomas, 1994). In the southern granulite facies Margate Terrane the oldest supracrustal rocks (which only form 10% of the exposures) consist of metapelites, marbles and mafic granulites. As in the Mzumbe Terrane, these rocks were intruded by several distinctive granitoid/charnockite suites, including charnockites of the Oribi Gorge Granite Suite, suggesting that the Mzumbe and Margate terranes had been juxtaposed by ~ 1050 Ma.

75

3. Structural evolution of the N M P

An integrated structural model for the N M P as a whole was recently proposed (Jacobs and Thomas, 1994). The structures seen in the N M P can be attributed to two major progressive deformation phases. The earliest (D1) structures are the result of a phase of NE-directed nappe tectonics. D1 is characterized by the development of a penetrative foliation axial planar to NE-vergent tight to isoclinal folds and SW-inclined thrust planes associated with high-temperature mylonites and SW-plunging mylonitic stretching lineations (Fig. 3). These early thrust-related structures are extensively preserved in the Tugela Terrane, suggesting that it must have been obducted onto the southern margin of the Kaapvaal Craton early in the collision history of the belt. It has been argued that the cold, stable cratonic crust, over which the Tugela nappes were obducted isolated them from further pervasive tectonism. D1, thrust-related structures are locally preserved in the southern terranes at, for example, the Margate Mzumbe Terrane boundary (Melville thrust) and within the Mpambanyoni River thrust zone (southern Mzumbe Terrane) (Fig. 2). The dominantly subhorizontal D 1-structures are overprinted by a phase of pervasive sinistral shearing (D2) along steeply-inclined to vertical, cratonparallel shear zones. Structures of this second deformation are most strongly developed within the Mzumbe Terrane. The sinistral shear strain increases markedly in intensity towards the northern terrane boundary, reaching a peak along the LMSZ. It has been suggested that the period of sinistral shearing triggered the generation and emplacement of the voluminous late-tectonic Oribi Gorge Granite Suite in a transtensional, pull-apart setting, with deep shear zones tapping lower crustal source rocks (Jacobs and Thomas, 1994). Within a plate tectonic scenario, Jacobs et al. (1993) interpreted the accretion of the N M P onto the southern margin of the Kaapvaal Craton to have been the result of a prolonged phase of N E - S W directed arc-continent collision. In this model, the geometry of the structures observed, is seen as a function of the ENE-trending orientation

76

J. Jacobs et al.

Precambrian Research 86 (1997) 71 92

(a)

(b)

~cl Fig. 3. (a) (c) Show consecutive deformation stages within the Mandleni nappe of the Tugela Terrane (Wosi River section). (a) Tight N N E verging asymmetrical folds; (b) minor SW-dipping thrust developed within a fold limb in banded gneisses; and (c) SW inclined thrust with strongly sheared amphibolites above and relatively little deformed amphibolite with leucocratic mobilisates below the thrust plane. Sample WR8 comes from above the thrust plane; after Jacobs and Thomas (1994).

J. Jacobs et al. / Precambrian Research 86 (1997) 71-92

of the craton margin and the persistent (oblique) N E - S W directed plate convergence. From this working model several questions arise that we attempt to address in the current study: (1) What are the ages of the two major periods of deformation and what was the duration of D2 sinistral shearing? (2) When was the Tugela Terrane obducted onto the Kaapvaal Craton? (3) Is there any evidence of younger (e.g. PanA f r i c a n - - ~ 500 Ma?) rejuvenation of the N M P structures?

4. Samples and methods The structures associated with the structural evolution model described above developed under medium amphibolite to granulite facies conditions. Mylonites associated with both NE-vergent thrusts and sinistral transcurrent shear zones show recrystallized feldspars and amphiboles, indicating deformation temperatures of ~ 500°C. This temperature is similar to the estimated blocking temperature of 480-550°C for the 4°Ar/39Ar hornblende system (Harrison, 1981). We thus attempted to date hornblende separates using the 4°Ar/39Ar method from amphibolite mylonites developed in association with the different structures and from less deformed (non-mylonitic) amphibolites for comparative purposes. Ten appropriate samples were selected for analyses, making sure that each sample contained hornblende grains that were essentially free of inclusions, microcracking and alteration. The sample locations, lithologies etc. are given in Table 1. Hornblende was separated from the ten samples by conventional separation techniques. Selected grains from the 130-250/~m or 250-500 pm were carefully hand-picked for analysis. Between 12 and 116 mg of each sample hornblende were washed and cleaned several times in an ultrasonic bath in a mixture of HNO3 and afterwards in alcohol. Then the prepared grains were packed in high purity aluminium foil, stacked in two quartz ampoules which were then evacuated, and irradiated in a capsule behind a shield of 1 mm cadmium at the GKSS reactor in Geesthacht, Germany. For

77

the entire 10 day irradiation period, the capsule was rotated about its long axis at a rate of six revolutions per hour. In order to keep the gradient of the neutron flux as small as possible along the ampoules, the capsule was turned through 180 ° perpendicular to the axis of rotation after 5 days. The average effective neutron flux was 0.00343 (Jvalue), with a gradient along the ampoules of < 1%. This was detected with three aliquots of the interlaboratory age standard MNHb-1 which were included in each of the ampoules. The stepwise degassing and measurement routines are described by JeBberger et al. (1980). The errors of the plateau ages quoted are 1a, including the scattering of each fraction belonging to the plateau, the individual errors of these fractions and the uncertainties in the effective neutron flux received by the sample. As they do not include the contribution of the uncertainty in the monitor MNHb-1 age of 519_+1.7Ma, an additional error of 3 M a for comparison with data not obtained via this monitor has to be considered.

5. 4°Ar/39Ar results

The 4°Ar/39Ar results are presented as age spectra in Fig. 4 and their plateau ages, along with C a / K ratios from 39Ar/4°Ar ratios are presented in Tables 1-5 and Tables 6-11. Eight out of ten hornblende samples show pronounced A r - A r age plateaus over large portions of their age spectra and our data show no significant amounts of excess argon. Two slightly saddle shaped samples (L12, QR2) were checked for excess argon using argon correlation plots (Fig. 6); excess argon corrected ages were calculated (Tables 1, 5 and 8). The excess argon corrected ages of both samples are within error. The ages obtained can be separated into two groups: (1) An older age group (1077 1135 Ma) is made up of samples from the Tugela Terrane. (2) A younger age group (976-1005 Ma), which can be subdivided into less well constrained subgroups, was obtained from samples from the two terrane boundaries and from different structural units of the Mzumbe Terrane.

78

J. Jacobs et al. / Precambrian Research 86 ( 19971 71 92

Table 1 Summary of analytical results Lithology (sample)

Locality

Fraction analysed

Amount analysed

Plateau ages_+ lo

28'54'58": 30 59'17"

250 500/~m

55.2 mg

1135_+9 Ma

30'50'47"; 31 00'52"

120 250/~m

23.9 mg

1080_+6 Ma

Ca (wt%)

K (wt%)

9.11

0.354

8.26

0.617

13.36

0.273

9.32

0.401

6.96 8.79

0.318 0.597

8.68

1.23

9.61

0.373

8.43

0.621

8.17

0.601

Tugela Terrane

Wosi River mylontic amphibolite ( WR8 ) Slaty road outcrop mylonitic amphibolite (SR2)

1112_+6 Ma Tolwane River mylonitic amphibolite (HR2)

28°48'30"; 31 06'28"

120 250 Itm

22.0 mg

1077 + 11 Ma

Lilani gabbro (LI2)

2906'21"; 30~52'04 "

120 250 #m

24.5 mg

Lilani mylonitic amphibolite (LI4) Lilani mylonitic amphibolite (LI 5.1 )

29'06'23"; 3052'05" 29°06'23"; 3052'05"

120 250/~m 120 250/~m

11.5 mg 17.3 mg

1013_+5 Ma 1009_+5 Ma" 9 7 6 + 8 Ma 978_+7 Ma

29~16'12"; 3101'40"

250 500 #m

24.1 mg

984_+5 Ma

30~20'00"; 3019'52"

120 250/~m

23.7 mg

30°27'44";

"

250-500 elm

115.7 mg

1004_+4 Ma 1002_+4 Ma a 1005_+4 Ma

30'38'57"; 3(131'37"

250 500 F~m

53.8 mg

988_+3 Ma

Lilani-Matigulu Shear Zone

Mzumbe Terrane

Mylonitic amphibolite (MVI); Mvoti shear zone Mylonitic amphibolite (QR2); Jolivet shear zone Undeformed Equeefa dike ( M W R )

30°33'38

Melville Thrust Zone

Melville mylonitic Equeefa dyke (MSZ3) aExcess argon corrected.

The three hornblende samples from the Tugela Terrane were collected from mylonitic amphibolites near the base of thrust planes. Their spectra are shown in Fig. 4a. The oldest ages were obtained from a sample from the Wosi River ( W R 8 ) which gave a plateau age of 1135_+9 Ma (Fig. 4a). The youngest age comes from a sample from the Tolwane River ( H R 2 ) . This sample shows a pronounced plateau age of 1077_+ 12 Ma during the first 80% of degassing, with younger ages recorded from the last heating steps, probably caused by minor inclusions in the hornblende crystals. The third sample (SR2, from a road cutting in the Madidima Nappe, Fig. 4a), yielded a two stage plateau. The lower part derived from the first third of degassing reveals an age of 1080_+6Ma, the same age as HR2. The last two-third of gas in SR2 shows an age of 1112_+6 Ma, similar to sample WR8. Three samples were dated from the LMSZ, the

T u g e l a - M z u m b e terrane boundary (Fig. 4b). These include two mylonitic amphibolites (L14, L15.1) and one sample from the undeformed margin of this shear zone (L12, Fig. 5). Hornblendes from the undeformed sample gave a plateau age of 1013_+5 Ma and an excess argon corrected age of 1009_+5 M a (Fig. 6). The mylonites gave two identical plateau ages of 976_+8 and 978-+ 7 Ma, respectively (Fig. 4b). During the first 5% of degassing both samples show a significantly younger age component of 500 600 Ma. Hornblendes from two additional sinistral shear zones from the Mzumbe Terrane were dated. A hornblende separate from the Mvoti shear zone (MV1, Fig. 5), some 20 km south of Lilani, gave a plateau age of 984-+ 5 Ma (Fig. 4c). The highest age from a sinistral shear zone was obtained from hornblende in the Jolivet shear zone (QR2), some 175 km south of Lilani. This sample gave an age of 1004-+ 4 Ma, defined by a plateau covering over 70% of the spectrum Fig. 4c. Excess argon correc-

79

J. Jacobs et al. / Precambrian Research 86 (1997) 71-92

(a) SR2

",t 0

WR8

. . . .

l

HR2

"r__

"~'11,

~

. . . . . .

1135+9Ma

~Iooo

ol '¢~ 9o0

1080 :l: 6 Ma

1112+6Ma

1077 ± 11 Ma

i

800

I

.700 6OO

(b)

L15.1

5

4 b(

L14

0 2

~1ooo ,<

L12 i

" ~ - - - - ~

900

tJ::::KZ2.

~

5 Ma

~,oo!

976 ± 8 Ma

]

978 + 7 Ma

I,

~. 700 L i

500 ~ - .

(c)

0

ii.a-

QR2

~, ~

~ .......

~

"E 9ooL ~

......

1 L=

1002 ± 3 Ma

984 ± 5 Ma

~o. 8OO

~7oof

L

600

t

5O0 . . . . . . .

"N

~

M~

'] (d)

I MSZ3

110o

~'looolL

9ooLf

O_ So°

~ -700 600

20

40

60

80

Fractional 39Ar release [%]

100 0

20

40

60

80

Fractional 39At release

[%]

100

Fig. 4. 4°Ar/39Ar-spectra and K/Ca-ratios for the samples. (a) Tugela Terrane, (b) LMSZ, (c) Mvoti and Jolivet shear zones, (d) undeformed and deformed Equeefa dyke.

80

J. Jacobs et aL / Precambrian Research 86 (1997) 71 92

Table 2 4 ° A r / 3 9 A r analyses for WR8 (J = 0.003436 _+0.000008 )

Temp. (°C)

36Ar 10 12

38Ar 10 12

400 800 950 1010 1050 1060 1070 1080 1090 1100 1110 1120 1130 1140 1050 1160 1170 1180 1200 1240 1300 1350 1400 1460 1530 1531

35__+4 59__+4 15-+4 10_+ 3 87_+4 66_+6 28_+5 23 _+4 29_+5 27_+6 27-+5 11_+5 12_+6 11 _+5 24-+6 54_+6 24_+9 0-+0 0_+0 0_+0 12_+9 54_+ 10 181 _+ 16 208_+20 363 ± 39 0__+0

2±1 18_+ 1 27-+ 1 69-+ 1 223_+2 444-+4 397_+3 483 __-+5. 646_+6 632_+7 687__+7 394-+2 489-+3 663_+3 1003-+5 1199-+4 362-+2 71 _+ 1 117_+ 1 363_+2 597_+3 382_+2 755_+4 842+3 232_+ 3 61 _+2

10-+1 22_+2 5_+ 1 3-+ 1 21 _+3 19_+4 9_+3 2 _+2 5_+5 1 _+ 1 7-+5 0_+0 0-+0 0__+0 0__+0 0_+0 2 __+2 1 _+1 0-+0 0_+0 0_+0 12+4 26_+6 34_+8 68 _+8 0_+0

28__+2 836_+4 548_+6 519+3 1524__+9 3161 -+ 15 2887_+ 16 3602 _+23 4814_+34 4713_+39 5033_+32 2949_+12 3610_+ 15 4892_+23 7507_+31 9029_+ 34 2852_+ 14 542_+5 857-+6 2697_+ 13 4510_+ 19 3027_+ 14 5972_+23 6528_+27 1826-+ 20 482+ 18

24__+1 233_+ 1 99__+ 1 118 _+ 1 391 __+1 801 +2 735_+2 920 _+4 1237_+6 1208_+7 1294_+6 758±1 931 _+2 1261 +3 1923__+4 2344-+2 732-+3 134_+2 218 __+2 689+2 1153 _+3 787 _+3 1576_+5 1731 _+6 583 _+ 11 93 ± 11

1.738__+0.142 1.144__+0.007 0.837-+0.013 1.021 _+0.010 1.081 _+0.006 1.107_+0.004 1.122_+0.005 1.128 _+0.004 1.133_+0.004 1.132_+0.005 1.134_+0.004 1.135__+0.004 1.139_+0.004 1.140+0.004 1.133__+0.003 1.142__+0.003 1.129__+0.006 1.105_+0.013 1.130_+0.009 1.135_+0.005 1.132_+0.004 1.131 +0.006 1.133__+0.005 1.135__+0.005 1.150_+ 0.030 0.916_+0.091

Total

1359_+ 53

11 160_+ 17

246_+ 17

84 950± 100

21 9 7 4 _ + 2 3

1.129_+0.001

37Ar10-10

39At 10 -12

4°Ar 10 -9

App. age (AE)

Notes on Ar- analyses: 1. Mass discrimination and sensitivity were determined by pipetted amounts of air argon. 2. Blank amounts are corrected for. The signals were <0.3 x 10 9ccmSTP for 1000°C, <1.2 x 10 9ccmSTP for 1400°C and < 3 x 10 9 in 4°Ar ccmSTP for 1530°C in 1 h of degassing time. The blank isotopic composition was essentially atmospheric with always negligible--additions of 37 Ar and 39Ar. 3. The interfering contributions from K and Ca were determined by the irradiation of CaF 2 and hornblende samples. The correction factors w e r e (38Ar/39Ar)K= 0.0231 + 0.0008, (39Ar/3VAr)c. = 0.001065 - 0.000145, (38Ar/37Ar)ca = 0.000344 + 0.000016 and (36Ar/3VAr)c~=0.000289 ± 0.000011. In addition we used (4°Ar/39Ar)K-0.0123 ± 0.0024 from Brereton (1970) and (4°Ar/37Ar)ca= 0.003 + 0.003 from Turner ( 1971 ). 4. Absolute Ar amounts are listed before subtraction of atmospheric contributions. This correction, however, was made for the calculation of apparent ages. The samples LI2 and QR2 contain small amounts of excess 4°Ar (Fig. 6). Their ages after correction for 4°Ar...... are given in parentheses and are also used in age spectra (Fig. 4). 5. If 36Ar = 0 in the tables, the measured amount after blank correction was below 1 x 10-12 ccmSTP/g.

tion

for

this

sample

( F i g . 6)

gave

an

age

of

1002_+4 M a . C o m p a r i n g t h e d a t a o b t a i n e d f r o m t h e s i n i s t r a l s h e a r z o n e s s a m p l e d , it is n o t e w o r t h y that their ages decrease from south to north, with the lowest age c o m i n g f r o m the T u g e l a - M z u m b e terrane boundary

(LMSZ).

In o t h e r w o r d s , the

a g e s t e n d t o d e c r e a s e as t h e d e g r e e o f s t r a i n increases towards the north. H o r n b l e n d e grains were also s e p a r a t e d f r o m the amphibolitic Equeefa dyke swarm (southern Mzumbe Terrane). These dykes form a valuable t i m e - m a r k e r in t h e s o u t h e r n M z u m b e T e r r a n e ,

81

J. Jacobs et al. / Precambrian Research 86 (1997) 71-92

Table 3 4°Ar/39Ar analyses for HR2 (J=0.003433_+ 0.000008) Temp. (°C)

36Ar 10 -12

3TAr 10 -1°

38Ar 10 -12

39Ar 10 11

400 800 1000 1060 1090 1120 1140 1160 1180 1200 1220 1250 1280 1300 1330 1360 1390 1420 1450 1490 1530 1531

21_+3 161 _+6 72_+7 59 _+10 194 _+10 114 _+29 316-t-34 0___0 15±14 0_+0 0-+0 4_+4 0_+0 0+_0 0_+0 0_+0 0_+0 0_+0 0+_0 0_+0 0+0 23_+23

1_+1 26_+ 1 915_+7 1093 _+5 4556 _+11 3750 -+240 318±21 276___ 18 354_+4 186_+3 402_+4 1101 _+4 596_+2 580_+2 324___2 332-+3 421 _+2 484±3 183-+2 186+2 199_+2 83 _+2

12±2 62±6 85_+6 119 _+9 432 _+18 344 _+42 102+7 58,1,7 68_+7 43+4 30_+8 91 _+7 61 _+7 39_+6 35-+6 46-+6 46_+7 49-t-11 12-+8 24_+8 23_+ 12 32_+ 10

1_+1 8+ 1 366_+2 413 _+3 1790 _+10 1484 _+34 135_+3 114±8 148_+2 78_+1 161 _+2 440_+3 242_+2 241 +2 139-+ 1 141 -+ 1 179_+2 196_+3 75_+2 75_+2 82_+2 37_+2

Total

979 _ 55

16 370 ___250

1813 + 56

6544 + 38

4°Ar 10 -9 14_+1 163-+ 1 891 ± 1 997 -+_2 4320 ,1,6 3595 _+5 415_+2 288-+2 346_+4 179±3 381 _+3 1050_+3 576_+4 549_+6 311 _+5 321 _+5 411 _+6 447±11 159-+9 162-+ 10 174_+ 15 94_+ 13 15 842 + 31

App. age (AE) 2.111_+1.231 3.193_+0.155 1.074+0.006 1.073 ___0.006 1.076 ± 0.005 1.082 _+0.019 1.079_+0.033 1.127-+0.056 1.048_+0.016 1.046+0.017 1.071 _+0.012 1.077+0.006 1.077+0.008 1.042±0.011 1.030-t-0.015 1.039_+0.014 1.047_+0.013 1.040,1,0.022 0.982_+0.048 1.004_+0.051 0.982_+0.070 1.071 _+0.175 1.074 + 0.005

See Table 2 for notes.

having been intruded between D 1 and D2 (Thomas et al., 1992a). Their age of emplacement is constrained as younger than the Mzumbe gneiss (1207 Ma), but older than the Oribi Gorge Suite (1050 Ma) (Thomas et al., 1993a). However, the dykes are also deformed in the D1 Melville thrust zone, suggesting that this structure has had a polyphase history. Consequently, hornblende separates were obtained from an undeformed Equeefa dyke, which cross-cuts the D1 fabric in the Mzumbe gneiss, from the Mtwalume River, southern Mzumbe Terrane (MWR, Fig. 2). This sample yielded a plateau age of 1005 ___4Ma (Fig. 4c). For comparison, a sample of an Equeefa dyke deformed by the Melville thrust zone (MSZ3) gave a plateau age of 988 ___3 Ma (Fig. 4c).

6. Interpretation and discussion of

4°Ar/39Arresults

The three oldest 4°Ar/39Ar ages of 10771135 Ma were obtained from hornblendes in D1

mylonitic amphibolites of the Tugela Terrane. These oldest ages are considered to represent the minimum age of the oblique thrusting of the Tugela Terrane onto the southern margin of the Kaapvaal Craton. The plateau age of ~ 1080 Ma seen in samples HR2 and SR2 probably marks a younger thermal event. This could either be associated with late alkaline magmatism within the Tugela Terrane (Nicolaysen and Burger, 1965) or with the emplacement of syntectonic sheet-like granites in the Mzumbe Terrane at that time (Thomas et al., 1995). No younger age components are seen in the Tugela age spectra, content that this terrane was preserved and isolated from the subsequent sinistral shearing event by the underlying cold, thick and rigid Kaapvaal Craton (Jacobs and Thomas, 1994). In the Mzumbe Terrane, post-tectonic cooling of hornblende from the D1 (thrust-dominated) domains of the Mzumbe terrane, to below the 4°Ar/39Ar blocking temperature, occurred around

82

J. Jacobs et al. / Precambrian Research 86 (1997) 71 92

Table 4 4°Ar/39Ar analyses for SR2 (J = 0.003427 -+0.000028 ) Temp. CC)

36At 10 12

3TAr 10 11

3BAr 10 12

39At 10 11

4°Ar I0 ~

App. age(AE)

400 800 950 1000 t050 1065 1075 1085 1095 1105 1175 1200 1240 1280 1330 1390 1430 1480 1530 1531 1532 1533

22±4 127-+4 35_+6 48_+6 71 +7 39-+7 26_+7 30-+7 9-+8 4-+4 33_+9 34-+10 10-+ 10 0-+0 10-+ 10 26+_ 15 5-+5 5-+5 0±0 0-+0 0-+0 3_+3

19-+3 442+ 10 1071 + 15 3731 + 16 10 650-+47 8024_+39 6237-+42 4615_+27 3260-+21 3283+26 11 372-+59 9919_+59 7418 -+41 1387_+ 13 13 608_+47 3913 -+40 2524_+25 3349-+37 2822+34 1113-+26 463-+22 464_+22

9-+2 47-+3 10_+3 11 ±6 6-+6 0_+0 0-+0 6-+6 0-+0 4+4 0+0 0-+0 0-+0 0_+0 0+0 0+0 2_+2 4-+4 0±0 4-+4 7-+7 2_+2

3_+ 1 108+ 1 154-+ 1 481 -+2 1448-+5 1117-+4 891 +3 668-+2 481 _+_2 487_+2 1675+5 1466+4 1116-+4 213_+2 2138_+4 616_+2 386_+2 522-+3 44(1_+3 170-+2 67-+2 70±2

22+ 1 349+ 1 313_+ I 1150-+2 3496-+3 2689_+3 2168-+4 1654-+2 1184+2 1202-+3 4200-+3 3639-+3 2794-+3 530_+3 5378_+4 1555 +4 975+8 1312-+ 15 1100-+ 13 431 -+ 13 170-+ 13 180-+ 13

1.770_+(I.168 1.234 + 0.009 0.925 + 0.008 1.064 _+0.003 1.078 +_0.003 1.077 + 0.003 1.086 + (/.002 .099 _+0.003 .097 _+0.004 .101 _+0.004 .112+0.002 .103 +0.002 .112+0.003 .106 + 0.007 .116-+0.002 .116+0.004 .118+_0.011 .115+0.015 .112+0.011 .121 +0.028 .128 _+0.069 .130 + 0.090

Total

538+32

99 680+ 160

112+ 15

14 717+ 13

36 493 -+33

1.101 -+0.001

See Table 2 for notes.

1005 Ma. This age is considerably younger than equivalent rocks in the Tugela Terrane (above), the intrusion of the Equeefa dykes ( ~ 1150 Ma) and the emplacement of the voluminous Oribi Gorge Granitoids (1050 1035 Ma). Clearly therefore, the juvenile Mzumbe Terrane has had a hightemperature thermal history which continued long after that of the Tugela Terrane. The apparent 4°Ar/39Ar ages of hornblendes from the sinistral shear zones at the Mzumbe Tugela terrane boundary, as well as those within the Mzumbe Terrane range from 976 to 1002 Ma. Comparison with the 4°Ar/39Ar-ages of the significantly older unmylonitized country rock ( > 1005 Ma) indicates that the mylonite ages have to be interpreted as deformation ages. These deformation ages are surprisingly young. Thomas and Eglington (1990) and Jacobs and Thomas (1994) suggested that the Oribi Gorge Suite granitoids (1050-1035 Ma) were generated from partial melting of lower crust, tectonically thickened

during D1 thrusting and emplaced during the ensuing sinistral transtensional pull-apart type regime. It is envisaged that the major shear zones acted as deep conduits enabling the tapping and ascent of the voluminous deep crustal melts. This in turn suggests that the initiation of sinistral shearing may have commenced between 1050 and 1035 Ma. The new 4°Ar/~gAr data presented here do not give any clear evidence of the upper age limit to the sinistral shearing, but it is possible that it continued until ,-~980 Ma, so that the largescale oblique shearing episode was a major deformational event which may have continued for some ~ 6 0 Ma. In terms of the younger age limit of deformation, it is significant that the two mylonite samples from the Tugela-Mzumbe terrane boundary display ages of 500-600 Ma (Pan-African) during the first 5% of total gas release. The two possible explanations are that either the mineral phase bearing this young age component (e.g. inclusions or skins

83

J. Jacobs et al. /Precambrian Research 86 (1997) 71-92

Table 5 4°Ar/39Ar analyses for L 12 (J = 0.003452 +_0.000028) Temp. (°C)

36Ar 10 -12

37Ar 10 - ' °

3BAr 10 -12

39Ar 10 -12

400 800 950 1010 1050 1060 1070 1080 1090 1100 1130 1160 1200 1250 1310 1370 1440 1530 1531

39+6 464.-,10 90--- 10 90_+9 157_+10 195_+15 49_+11 30-+10 24.-,11 21_+11 0+0 9-+9 2_+2 78-+22 0_+0 0_+0 46---46 23_+23 124--,94

2.-,1 259.--3 170_+3 526_+6 1996--,9 2724.-,14 1059.-,8 653---4 442.--4 296___4 350___5 732.--6 526.--7 430---5 391_+4 286---5 318--,6 77.--7 7___7

12.-,3 159.-,3 50+4 195_+5 865--,16 12824-19 499.--8 308.--5 196.-,5 129_+4 166.-,5 353.--9 218+8 169--,8 130_+7 92_+8 135_+10 35_+14 22--,19

28_+7 950_+9 1090-+ 10 3567--,21 15949___60 24922--73 10025_+36 6004_+26 4032+21 2523--,15 3185_+16 6848.--32 4457_+26 3200+27 2920_+23 2302.+23 2646---26 699---30 98.--42

Total

1440.+110

11 245-+27

5016--,43

95450--,140

4°Ar 10 -9 25.-,1 433+2 231 ---2 848---3 3657---3 5594___6 2232___3 1341.-,3 888_+3 564.--3 706.--4 1513_+8 979+6 728---7 640---8 478.-,11 589_+14 166+20 61.-,27 21 673-+42

App. age (AE)

Corr. age

1.749_+0.333 1.305+0.014 0.891 +0.014 1.045___0.006 1.031--,0.003 1.017-+0.002 1.013.-,0.003 1.016.-,0.004 1.004--,0.005 1.012_+0.008 1.015.-,0.006 1.011 .--0.006 1.007_+0.007 1.010+0.012 1.006--,0.011 0.966+0.018 0.999_+0.027 1.038.+0.142 1.087_+1.375

(1.001_+0.538) (1.002+0.037) (0.831 _+0.024) (1.028__.0.008) (1.025.-,0.003) (1.011+0.003) (1.010_+0.004) (1.012.-,0.005) (1.000_+0.008) (1.007-+0.012) (1.015.-,0.006) (1.010_+0.011) (1.007+0.011) (0.993_+0.019) (1.006_+0.011) (0.966_+0.018) (0.988_+0.048) (1.017+_0.256) (0.007---4.464)

1.018--,0.002

(1.008-+0.004)

See Table 2 for notes.

Table 6 4°Ar/39Ar analyses for L14 (J=0.003413.+0.000008 Temp. (°C)

36Ar 10 - ' z

37Ar 10 11

3aAr 10 -'2

39Ar 10 12

400 700 800 900 1000 1050 1070 1080 1090 1100 1115 1150 1200 1270 1340 1400 1470 1530 1531

144+8 877+ 12 108+8 39+9 327 +- 14 173 +- 14 152--, 15 191 +- 15 73 + 13 60 + 13 100+20 43+ 15 111 + 18 145 ___19 58 +_30 150--,55 0+-0 0_+0 0_+0

57+7 852___ 15 219__ 11 652+__11 3797 ---40 6325 + 53 12 3274-67 10 308 +67 6359 + 34 4466 +-28 6097---49 4910--,51 8018 _+43 6694 + 55 3842 ---40 5248_+68 3199_+51 1662+66 218+59

43+3 204+4 25.--3 12--,4 133 +- 6 193 +- 8 413 +- 14 351 + 12 219 +-9 139 +- 7 190--,13 168_+8 262 + 15 220 +-9 125 _+10 192_+16 79_+17 33---27 0+-0

58+6 2324+ 18 816.-, 10 651+11 2372 +-20 5901 + 34 10 996---50 9138 +42 5627 + 28 3944 +-24 5310+27 4245---29 6886 _+34 5409 _+28 3439 _+28 4822_+45 2645_+43 1451 .+62 163_+60

Total

2751 -+ 82

85 250 +_210

3003 + 50

76 200 +- 150

See Table 2 for notes.

4°At 10 -9 45+2 361 +2 77+2 77+2 566 + 4 1120 ---4 2329 ±4 1973 - - - 4 1195 + 4 839 + 4 1113.-,6 882.--4 1496 +- 5 1223 ___6 746 +- 8 1101.+15 585---23 260_+39 0+0 15 988 _ 51

App. age (AE) 0.195+0.293 0.252+0.010 0.316.-,0.021 0.534.--0.025 0:933 + 0.010 0.871 +0.005 0.970+0.004 0.977---0.004 0.972 -t-0.005 0.971 -I-0.007 0.956+0.007 0.959_+0.007 0.987 _ 0.005 1.007 +-0.006 0.985 + 0.014 1.010_+0.018 1.017.+0.033 0.862--,0.106 0.000-+ 0.000 0.938 +-0.004

84

J. Jacobs et al. /' Precambrian Research 86 (1997) 71 92

Table 7 4°A r/39Ar analyses for L 15.1 (J = 0.003416 -+ 0.000008 ) Temp. ( C )

36Ar 10 i1

3VAr l0 -1°

3BAr 10 12

39At 10 lz

400 700 800 900 1000 1050 1080 1090 1100 1110 1120 1140 1160 1195 1230 1280 1350 1440 1530

7± 1 24 ± 1 13± 1 5± 1 4± 1 12±2 14 ± 2 9±2 9±2 0±0 3 +2 0-+0 0_+0 0±0 0±0 0±0 0±0 0_+0 0± 0

2_+1 57 _+ 1 36± 1 52±2 232_+3 1141 ± 8 2396 ± 14 1864±11 2005 ± 8 407_+3 512±7 239±4 229±3 348±5 451 _+6 248±3 368 ± 4 117±3 60 _+5

20±4 74 ± 5 30±4 0±0 42±6 156± 15 275 ± 24 194±20 195 ± 23 54±7 51 -+ 10 32_+6 19_+6 44_+7 3 1 ± 18 23±8 36 ± 1 l 0_+0 0 _+0

5x 1 237 _+ l 149_+2 161 ± I 334±2 1453±6 3025 ± 9 2442±8 2770 _+7 554+3 690+4 315±2 281 + 2 419±3 534_+5 288_+2 453 + 4 142+3 69 + 5

Total

100 ± 4

10 764 + 26

1276 ± 51

14 322 ± 19

4°Ar l0 9

App. age (AE)

23±2 349 ± 2 216±2 188-+3 522_+3 2949±5 6444 ± 7 5168±5 5909 ± 8 1135±5 1437_+6 644_+5 577 _+5 865+6 1118± 13 604+7 932 ± 10 266_+ 15 62 _+35

0.159_+0.313 0.610 ± 0.008 0.619+0.012 0.565±0.011 0.760±0.007 0.943_+0.004 0.984 ± 0.002 0.980 ± 0.003 0.986 + 0.002 0.959_+0.005 0.967_+0.006 0.958_+0.008 0.961 _+0.008 0.964_+0.007 0.975±0.010 0.977±0.0t0 0.963 _+0.009 0.895_+0.042 0.487 + 0.245

29 409 _+46

0.953 ± 0.001

See Table 2 for notes.

on hornblende) could have been removed from the other hornblende separates by complete mineral separation or it may even be originally absent from the other samples. If this is the case, there is a Pan-African overprint restricted to the samples from the LMSZ. This may indicate that the LMSZ was a zone along which the N M P was exhumed during the Pan-African. A phase of Pan-African exhumation in the range of ~ 6 - 1 0 km of the southern Mzumbe Terrane has previously been suggested from titanite fission-track analyses (Jacobs and Thomas, 1996). Alternatively, the LMSZ was the site of Pan-African fluid circulation, causing limited resetting of the Grenvillian ages. It is relevant in this regard that the LMSZ was reactivated in still younger times as a Post-Karoo brittle fault and is today the site of thermal sulphur springs (Gevers, 1963). The terrane boundary between the Mzumbe and Margate terranes (Melville thrust zone), is viewed as an old (D1) structure which developed early during the accretionary history of the N M P (Jacobs and Thomas, 1994). This is evident from

the early structures seen within the zone and the contrasting lithological assemblages that occur on either side of it. It was thus interpreted as having developed during the early thrust-dominated tectonism preceding sinistral shearing (Jacobs et al., 1993; Jacobs and Thomas, 1994). The new 4°Ar/39Ar data indicate that the Melville thrust must have remained active and/or was reactivated at ~990 Ma, during sinistral shearing. Weakly developed E W trending lineations, locally observed in the thrust zone, might have developed at this stage, and may explain the scarcity of expected down-dip (D1) lineations in the Melville Thrust. It is thus significant that the two major terrane boundaries exposed in the N M P yielded the youngest ages and that they are statistically coeval. This indicates that the locus of final Namaquan movements in the N M P was concentrated along the major terrane boundaries, even though those boundaries are different in nature and age of initiation. Many rock units of the N M P show evidence of a late hydration, manifested in the

85

J. Jacobs et al. / Precambrian Research 86 (1997) 71-92

Table 8 4°Ar/39Ar analyses for QR2 ( J = 0.003430 _+0.000008) Temp. (°C)

36Ar 10 -12

3TAr 10 -1°

38Ar 10 -12

39At 10 -12

400 700 900 1000 1040 1060 1075 1085 1095 1105 1115 1130 1150 1170 1190 1210 1240 1280 1320 1360 1400 1440 1480 1530 1531

28_+5 88 -+4 92_+5 45 _+6 28 _+6 33 _+8 30 _+5 39 _+8 45_+7 28_+7 9 +7 8 _+7 18 _+8 15_+7 13_+8 9_+9 15 _+8 0_+0 15 _+ 11 26_+ 14 50_+ 13 12_+ 12 46_+40 56+_40 57_+40

1 _+ 1 65 -+ 1 95_ 1 102_+ 1 286_+2 645 ± 4 766 _+4 1068 _+5 1050_+4 746-+5 518 -+4 429 _+3 383 _+3 363_+3 448_+4 531 _+3 657 _+4 754_+4 779 _+4 745-+4 519_+3 307 -+2 316_+3 160_+3 38_+2

9_+2 48 -+ 2 25_+3 17_+3 12_+5 13 _+6 14 _+6 l 5 _+8 11 + 9 10_+6 21 _+6 24 -+4 25 _+5 27_+7 17_+6 7_+4 10 _+5 10_+6 16 _+7 15_+7 20_+7 11 _+6 11_+9 17_+9 15-+8

24_+8 903 _+9 647_+8 964_+8 2080_+ 14 4678 _+22 5700 _+29 7878 _+32 7996+38 5718__.22 3881 _+21 3211 _+21 2942 _+ 17 2713_+18 3428+21 3991 _+22 4946 _+26 5755_+26 5948 _+25 5685_+22 3985_+22 2340_+21 2511_+25 1247_+21 301_+19

Total

807_+79

11774_+ 17

421 _+31

89 480_+ 110

4°Ar 10 -9 18 _+ 1 205 _+ I 224_+ 1 207_+ 1 468+ 1 1055 _+2 1262 _+2 1740 _+2 1745_+2 1249+_2 849+_2 705 + 2 647 _+2 596_+2 746_+2 870_+2 1077 _+2 1253_+3 1296 _+3 1253_+4 888_+4 522_+6 572_+11 295_+12 86_+12 19 830_+23

App. age (AE)

Corr. age

1.608-+0.376 0.935 _+0.009 1.285_+0.014 0.945+0.010 1.016_+0.006 1.025 +_0.004 1.013 _+0.004 1.011 ± 0.003 1.001 _+0.004 1.003_+0.003 1.006_+0.005 1.009 _+0.006 1.006 _+0.006 1.007_+0.006 1.001 _+0.006 1.004+0.005 1.002 _+0.005 1.005+__0.004 1.003 _+0.004 1.010_+0.004 1.010_+0.006 1.019-+0.014 1.021_+0.024 1.024_+0.048 1.047_+0.196

0.799_+0.698 0.852+0.040 1.185_+0.050 '0.906 _+0.023 1.006+0.010 1.020 _+0.006 •1.009 _+0.005 1.007 _+0.004 •0.997 + 0.005 •0.999 +_0.004 1.004 _+0.006 • 1.007 +0.007 1.001 _+0.008 1.003 _+0.009 (0.998 _+0.008 ( 1.002_+0.007 ( 1.000 -+ 0.006 ( 1.005 _+0.004 ( 1.001 _+0.006 ( 1.006 _+0.007'

1.010_+0.002

(1.002_+0.003

(1.000_+ 0.011

( 1.014_+0.024 ( 1.007 +- 0.045 (0.988 -+O.094 (0.893 _+0.408

See Table 2 for notes.

growth of post-tectonic muscovite, dated by K - A r at ~ 9 0 0 M a (Jacobs and Thomas, 1996). This event is clearly a low-temperature phenomenon which does not appear in any of the 4°Ar/39Ar-spectra.

A summary of the major so far recognized tectono-magmatic events in the N M P and their geochronological record are listed in Table 12.

6.1. The Grenville-Maud Natal-Mozambique(Kibaran ) connection The 1200-1000 Ma Grenville Province of Laurentia forms a ~400 km-wide orogenic belt which can be traced for over 4000km from Ontario(Canada) down the eastern flank of the U.S.A. (under Phanerozoic cover rocks) and into Mexico. It is one of the world's most extensive

metamorphic belts which many authors have suggested might represent the deeply eroded remnants of a Himalayan-type collision orogen (e.g. Jamieson et al., 1992). Reconstructions of the proposed Proterozoic supercontinent of Rodinia show the Grenville Province extending from the southern U.S.A. and Mexico into East Antarctica [the so-called 'SWEAT' hypothesis of Moores (1991)] and thence into southern Africa (Namaqua-Natal and Mozamique belts). In view of this, it is pertinent to assess how our data fit into this scenario. It is clearly beyond the scope of this article to review the geology of such a vast and complex dismembered orogen, but it is nevertheless useful to broadly compare some basic characteristics of the Grenville-aged orogens of this part of Rodinia. The situation in Natal, where the boundary between the N M P and its cratonic

86

J. Jacobs et al. / Precambrian Research 86 ( 19971 71 92

Table 9 4°Ar/39Ar analyses for MV1 (,/=0.003426_+0.000008) Temp. ( C )

36Ar 10 j'~

3TAr 10 lo

3BAr 10 -12

39Ar 1 0 11

4°Ar 10 9

App. age (AE)

400 700 900 1000 1040 1050 1060 1070 1080 1090 1100 1110 1120 1135 1160 1180 1195 1220 1260 1330 1400 1530

1500+ 1500 950 _+230 1180+ 1030 910_+ 110 800 _+71 635-+94 430 -+ 110 700 _+ 120 325 -+ 80 200 -+ 110 99_+99 42_+42 0 -+ 0 0 Jr 0 0_+0 0_+0 270_+ 130 3_+3 0_+0 0_+0 0_+0 0-+0

1 -+ I 23 _+5 54+46 338_+3 857 _+4 1083_+5 1082 -+ 5 1442 ± 9 1006 -+ 6 1020 _+ 120 849_+5 717_+5 442 -+ 2 453 _+4 631 -+3 385_+4 114_+2 27_+1 18_+ 1 26_+ 1 33_+1 32+3

41 +41 55 -+ 13 71 _+61 761 _+ 11 1752 ± 21 2135-+25 2106 -+ 27 2766 _+33 1946 _+24 1970 _+240 1660+22 1381 -+ 17 860 _4_13 877 ± 10 1221 + 16 735_+ 11 194_+6 44_+4 25_+3 31 + 4 25_+6 6+6

6+6 167 -+ 38 420_+360 1148+3 2440 ± 7 2972-+7 2928 _+8 3882 + 15 2722 -+ 8 2780 + 340 2316-+8 1959-+5 1227 _+3 1260 _+4 1727_+5 1025 + 3 277_+2 58_+1 37+ 1 60_+ 1 66_+1 77+2

170+ 170 880 _+200 580_+500 2415_+3 5238 _+8 6415_+ 10 6270 _+ 11 8295 _+ 15 5787 -+ 7 5900 ± 710 4864_+ 10 4131 + 6 2576 + 2 2663 -+ 5 3656_+3 2170_+4 598_+4 127_+3 74_+3 120-+4 131 + 5 112+ 15

3.832_+0.323 1.827 _+0.009 0.666_+0.010 0.971 +0.003 0.991 _+0.002 0.996-+0.001 0.991 + 0.002 0.989 _+0.003 0.986 -+ 0.002 0.984 + 0,002 (I.977_+0.002 0.981 -+0.002 0.978 _+0.002 0.982 _+0.002 0.984_+0.002 0.983 _+0.002 0.989_+0.008 1.013_+0.027 0.93t _+0.034 0.944_+0.027 0.930+0.030 0.727_+0.082

Total

8040_+ 1860

10 6 3 0 + 130

20 660_+260

29 560_+490

63 180+900

0.988-+0.007

See Table 2 for notes.

foreland are exposed, is not a c o m m o n one. In the Grenville Province, one has to go to southern Canada to find an analogous situation. Further south (e.g. Llano Uplift, Oaxaca) Grenville-age rocks are exposed only as small fragmented inliers with no contacts to the foreland, though possible ophiolitic rocks with protolith ages of ~ 1300 M a are recorded from the Llano Uplift in Texas (Garrison, 1981; Roback, 19961. At Georgian Bay (Canada), however, a continuous section through the Grenville Front zone is exposed comprising parautochthonus Palaeo- (1800-1600 Ma) to Mesoproterozic (1450-1350Ma) rocks of the Laurentian craton. These were thrust northwestwards during Grenville deformation and underwent upper amphibolite to granulite facies polyphase metamorphism at ~ 1150 M a and 1070 1050 Ma, with a more localized high-T event between 1000-980 M a [e.g. Krogh ( 1989); Mezger et al. (1991)]. A r - A r hornblende plateau ages

range from 1206_+10 to 979_+9Ma, with the majority falling between 1120 and 990 Ma [e.g. Haggart et al. (1993) and Reynolds et al. (1995)]. Further into the orogen A r - A r hornblende ages range from ~ 1005_+8 M a to 974+5 Ma (Reynolds et al., 1995), thereby dating the latest convergent movements, followed by tectonicallycontrolled exhumation. Thus, a comparison of transects over the Grenville Province Laurentian craton boundary in North America and over the N M P Kaapvaal Craton boundary reveals both similarities and major disparities. The Grenville Province is a linear orogen where orthogonal plate motions prevailed throughout the collision history and large volumes of parautochthonous pre-Grenville cratonic crust were reworked [e.g. Reynolds et al. (19951]. In Natal, there is a very narrow zone of reworking of the K a a p v a a l Craton, limited to isotopic resetting of minerals to ~ 9 5 0 M a up to 25 M a north

87

J. Jacobs et al. / Precambrian Research 86 (1997) 71 92 Table 10 4°Ar/39Ar analyses for M W R (J=0.003423 _+0.000008) Temp. (°C)

36At 10 -12

3VAr 10 - l °

3SAt 10 -13

39Ar 10 -11

400 800 950 1010 1050 1070 1080 1090 1100 1110 1120 1130 1140 1150 1170 1200 1240 1270 1300 1350 1400 1440 1480 1530 1531 1532 1533

14_+ 1 85 _+2 18 _+2 8 _+2 8 _+2 22 _+2 43 _+3 24 _+2 33_+3 21 _+3 26 -+3 12 _+3 6± 2 3 _+2 3 _+3 21 -+ 5 19_+4 2 _+2 1 _+ 1 35 _+5 3 _+3 0_+0 0 +_0 0_+0 0_+0 0_+0 0_+0

1 -+ 1 35 _+ 1 37 _+ I 36_+ 1 125 _+ 1 404 _+2 829 _+ 5 584 _+4 728_+5 730_+4 1458 + 6 643 -+ 2 248 _+ 1 117_+ ! 166_+ 1 807 _+3 956_+2 305 _+ 1 178 _+ 1 937 _+3 267 _+2 126_+ 1 224 _+ 1 217_+ I 57_+ 1 57_+ 1 52_+2

49_+3 266_+ 7 31 _+ 11 32_+ 13 29 _+ 17 97 _+42 213 _+86 132 _+64 96_+79 40_+40 120+_ 120 44 -+ 44 17 _+ 17 16_+ 15 63+21 58 _+58 64_+64 40 _+34 44 _+24 151 _+ 100 26 _+26 0_+0 38 _+35 0-+0 0_+0 0_+0 0_+0

1 _+ 1 24_+ 1 38 _+ 1 43 _+ 1 175 _+ 1 568 _+2 1169 _+4 838 _+3 1060_+3 1068_+3 2124 -+8 942 _+2 358 _+ 1 167_+ 1 241 -+ 1 1165 _+3 1387_+3 442 _+ 1 259 _+ 1 1363 _+3 388 _+ 1 187_+ 1 335 _+ 1 321 _+1 84_+ 1 85_+ 1 76_+ 1

Total

407_+ 13

10 323 _+ 13

1670+250

14 910_+ 12

4°Ar 10 -9

App. age (AE)

23_+ 1 238 _+ 1 77 _+ 1 100 _+ 1 385 _+ 1 1241 _+ 1 2572 _+7 1836 _+2 2317_+2 2330-+2 4663 _+ 13 2053 -+ 2 775 _+ 1 358_+ 1 518+ 1 2528 _+3 3016_+3 961 +_2 558 _+ 1 2978 _+2 843 _+2 399_+3 723 _+3 682-+5 173 _+5 176_+5 155_+8

3.434_+0.134 2.528 _+0.012 0.894_+0.008 1.027 _+0.007 1.007 _+0.003 1.003 _+0.002 1.009 _+0.002 1.007 _+0.002 1.005_+0.002 1.005-+0.002 1.010+0.002 1.005 _+0.002 1.000 _+0.003 0.989_+0.003 0.994_+0.002 1.001 _+0.002 1.003_+0.002 1.003 _+0.002 0.998 _+0.003 1.005 _+0.002 1.002 _+0.003 0.991 _+0.007 0.999 _+0.004 0.987-+0.006 0.964_+0.022 0.968 _+0.022 0.954+0.039

32 678_+21

1.007_+0.001

See Table 2 for notes.

of the Natal thrust front and lacking penetrative deformation (B.M. Eglington, personal communication). However, in the N M P itself, the rocks show no evidence of significant quantities of reworked older crust (Eglington et al., 1989) and an oblique juvenile arc-continent collision setting has been proposed [e.g. Thomas and Eglington (1990)]. Nevertheless, A r - A r hornblende ages are comparable, with the majority of those of the Grenville Front zone ranging between 1120 and 990 Ma, and those from the Tugela Terrane falling within the range 1135 to 1077 Ma. Ar/Ar muscovite ages range from 934 to 930 Ma in the Grenville Front zone, whilst a muscovite sample from a latetectonic pegmatite in the northern Mzumbe Terrane gave a similar K/Ar age of 954_+ 23 Ma (Jacobs and Thomas, 1996). In both the Grenville

and Natal Provinces hornblende and muscovite ages ( ~ 9 0 0 Ma) decrease towards the centre of the orogen. Thus, while the A r - A r hornblende ages presented here are generally similar to those recorded from the better-studied parts of the Grenville Province, the lithological, tectono-metamorphic and magmatic histories are very different. This is hardly surprising considering the vast distance separating the two areas within proposed Rodinia reconstructions. Nevertheless, it may be significant that the only other possible ophiolitic rocks recorded from the Grenville Province are from the Llano Uplift in what has been interpreted as an island arc-continent collision setting (Garrison, 1981). Within Rodinia recontructions, the Grenville-

88

J. Jacobs et al. / Precambrian Research 86 (1997) 71 92

Table 11 4°Ar/39Ar analyses for MSZ3 (J=0.003414+-0.000028) Temp. (°C)

36Ar 10 12

3VAr 10 10

38Ar 10 13

39Ar 10 -11

4°Ar 10 -9

App. age (AE)

400 800 950 1000 1040 1050 1060 1070 1080 1090 100 ll0 120 130 150 170 180 190 1200 1220 1240 1260 1300 1340 1400 1440 1480 1530 1531 t532 1533

12+_2 80 _+3 30_+2 7+_2 5 +_3 11 +- 4 10+_3 6 +_4 16+_3 28+_5 13 _+3 14+_4 4 +_4 4 _+4 0_+0 9 _+5 2 _+2 0+_0 0+_0 3+_3 0 +_0 18_+8 0+_0 0+_0 5 +_5 0_+0 13+_ 13 0+_0 0+_0 0+_0 0+_0

2+_ 1 40 +_ 1 32+ 1 20_+ 1 109 +- 1 233 +- 2 280_+2 372 _+3 518-+4 618+_4 586 +_4 599_+4 427 _+3 437 +_3 558+_4 1044 +_7 632 +_2 160_+ 1 246_+ 1 280_+2 205 +_ 1 250+_1 318-+2 787_+3 553 +_2 293+_2 253+_2 188+_2 103_+2 69+_2 22+_2

30+_7 257 +_ 18 47+_ 13 20+_ 14 48 _+28 63 Jr 41 138+_47 180 +_59 175+_78 259+_97 283 -+ 84 231 +_91 183 +_64 197 +_63 270+86 270 _+ 160 262 +_94 30_+30 94+_43 105+_44 59 +_40 94_+46 111 _+52 220_+ 110 224 +_84 110+_58 101 _+72 20+_20 0+_0 0+_0 0+_0

4_+ 1 149 _+ 1 90+_ 1 61 +_ 1 285 +- 1 559 +_2 646_+2 841 _+2 1179+_4 1377+_4 1308 +_4 1336_+4 960 _+3 975 _+3 1255+_4 2343 +_6 1426 + 3 357+_ 1 556+_ 1 632_+2 464 _+ ! 563+_1 703+_2 1764+_4 1249 +_3 662_+2 572_+2 426_+2 240+_ 1 151 _+ 1 50_+ 1

8+ 1 348 _+ 1 311 _+ 1 129+ 1 621 _+ 1 1192 +_ 1 1373+ 1 1794 +_ 1 2513+_2 2941 _+3 2782 + 2 2850+_2 2048 +_ 1 2082 _+2 2696_+2 5057 + 4 3059 +_3 761 -+ 1 1188+_ 1 1348+_2 994 +_2 1213+_2 1514_+2 3791 _+4 2678 +_3 1421 +_4 1229+_8 903+_8 504+_8 313+_8 99+8

0.687+_0.083 1.000 +_0.004 1.377+_0.006 0.963 +_0.008 0.998 +- 0.004 0.983 +_0.003 0.981 -+0.003 0.985 +_0.002 0.983+_0.002 0.984+_0.002 0.982 +_0.002 0.984-+0.002 0.985 _+0.002 0.986 +_0.002 0.991 +_0.002 0.994 +_0.002 0.989 -+ 0.001 0.985+_0.003 0.987+_0.002 0.985+_0.003 0.988 +_0.002 0.989_+0.003 0.992+_0.002 0.991 +_0.002 0.989 +_0.002 0.990+_0.003 0.988+_0.008 0.980+_0.008 0.973_+0.013 0.964+_0.021 0.929+_0.062

Total

289_+22

10 239_+ 15

4080_+360

23 184+ 15

49 759+_21

0.989_+0.001

See Table 2 for notes.

aged belt is believed to continue into the Mesoproterozoic orogen o f East Antarctica (Maud Province). This belt appears to have a very similar geological history to that described in Natal [e.g. Jacobs et al. (1993)]. However, the late tectonic Mesoproterozoic history usually recorded by K/Ar and Ar/Ar mineral data is reset by the later PanAfrican tectono-thermal overprint (Jacobs et al.,

1995). Within Africa, the broad age correlation of the N a m a q u a - N a t a l belt and the Kibaran orogen o f Central Africa has long been noted. The Kibaran belt has been modelled as a polyphase intracratonic

orogeny, with two distinct orogenic climaxes [e.g. Pohl ( 1994)]. Early rifting o f the Palaeoproterozoic Congo-Tanzanian Craton at ~ 1400 Ma led to the deposition of > 1 0 k m o f volcanosedimantary rocks which were deformed at ~ 1300 Ma during the first (main) Kibaran orogeny. Subsequent 'Katangan' rifting at ~ 1275 Ma was accompanied by acid and mafic plutonism and molasse sedimentation. The second orogenic phase t o o k place at ,-~ 950 Ma, manifest as deformation and granite intrusion. Other workers [e.g. Tack et al. (1994)], prefer to see the two episodes as separate events, with only the older event (i.e. ~ 1300 Ma) referred

J. Jacobs et al. / Precambrian Research 86 (1997) 71-92

89

(a)

(b)

(c)

(~)

Fig. 5. Selected microphotographs from 4°Ar/39Ar-dated samples: (a) none-mylonitizedgabbro (L12)just outside the LMSZ showing in part primary magmatic textures; (b) mylonitic gabbro (L14) within the LMSZ showing strongly deformed amphiboles; (c) amphibolite mylonite (SR2) which formed within a NE-vergent thrust plane within the Tugela Terrane; (d) mafic dyke (MV1), which was strongly deformed in the sinistral Mvoti shear. Same scale in all microphotographs. Length of each photograph: 3 cm.

to as the Kibaran. Putting this aside, however, from the above discussion, it is clear that the geological history of the N M P has more in c o m m o n with that of the Grenville and M a u d Provinces than that of the Kibaran. The entire history of the Grenville, M a u d and Natal Provinces appears to be contained within the period 1 2 5 0 - ~ 9 5 0 Ma, not coeval with the two orogenic episodes of Central Africa which took place between 1400 and 1000 Ma. Furthermore, whereas the Grenville and N a m a q u a - N a t a l Provinces have been modelled in terms of collisional accretionary orogens (continent-continent and arc-continent, respectively), the Kibaran orogen has traditionally been regarded as a intracratonic belt [e.g. Pohl (1994)]. In east Africa, the problem of the southern part of the Mozambique belt remains. This too, appears

to be a juvenile Mesoproterozoic orogenic belt, formed at ~ l l 0 0 M a , but it. was extensively re-worked during Pan-African times [e.g. Pinna et al. (1993)]. Consequently, much work needs to be done to unravel the early history of this belt to ascertain its status within Rodinia. Similarly, further work needs to be carried out on other Mesoproterozoic microcontinental fragments such as the Falkland microplate and the H a a g block (West Antarctica), dispersed during the break-up of Gondwana, in order to more regorously test the SWEAT hypothesis.

7. Conclusion

Hornblende separates from mylonitized amphibolites from different shear zones within the two

90

J. Jacobs et al. / Precambrian Research 86 (1997) 71 92

30000

L12 Intercept:

,Y N

(2)

4°Ar/3BAr=487+1 6 20000

J

<

(3)

10000

I

100

150

39Ar/a6Ar

QR2 Intercept:

40000

4OAr/36Ar=520 + 99 .~30000

(4)

~ 20000 10000

0

i

0

50

100

i

i

150

200

39Ar/36Ar

Fig. 6. Ar-isotope correlation plots of samples LI2 and QR2 revealing small amounts of excess argon. Data were fitted according to York (1969) giving an 4°Ar/36Ar intercept of 520_+99 for QR2 and 487_+ 16 for L12.

contrasting tectonic domains ( D I thrust-dominated; D2 shear-dominated) of the Tugela and Mzumbe terranes, N M P have been dated by the 4°Ar/39Ar method. For comparative purposes, adjacent weakly- to undeformed amphibolites have also been dated. This was undertaken to test the hypothesis that the two structural domains evolved during subsequent tectonic events in a continuous, N a m a q u a n oblique collision orogeny. The ten new 4°Ar/39Ar ages generally support this model, with some new findings permitting certain refinements. The data also allow a more precise and detailed interpretation of the structures observed. ( 1 ) The minimum age for the emplacement of the Tugela terrane onto the southern margin of the Kaapvaal Craton can be constrained to 1135_+9Ma. From this time onwards, the

(5)

(6)

(7)

Tugela Terrane was isolated from further major deformation. Cooling to below 500-600°C of the pre-sinistrally sheared domains within the Mzumbe Terrane post-dates the intrusion of the voluminous late-tectonic Oribi Gorge Granite Suite ( ~ 1050 Ma) and occurred at ~ 1005 Ma. Oblique sinistral shearing within the Mzumbe Terrane, which probably commenced at 1050-1035 Ma, terminated at ~ 980 Ma. This is consequently a very important, long-lived tectonic event in the evolution of the NMP. Sinistral shearing terminated in the south earlier ( 1 0 0 2 + 4 M a ) , and was concentrated (and continued later) along the high strain zones ( L M S Z ) at the Tugela Mzumbe terrane boundary until ~ 980 Ma. The Melville thrust was initiated during the early (D1) thrust event. The new 4°Ar/39Ar ages indicate that this structure was active until, or more probably, was reactivated at ~ 9 9 0 Ma. It is significant that the latest Grenvillian-age movements occurred at 980 M a along both terrane boundaries, notwithstanding their contrasting structural characteristics. A Pan-African age component is seen during the first ~ 5 % of stepwise heating from the mylonite samples of the LMSZ. Titanite fission-track analyses on the N M P have shown that the N M P was rapidly exhumed by 6 10 km during the Pan-African event. Since a young age component was not evident in the other A r - A r samples analysed, one of the possible explanations is that the L M S Z could have been a zone along which the N M P was exhumed during the Pan-African. The A r - A r spectra do not record evidence of a post-tectonic hydration event which caused widespread muscovite growth at ~ 900 Ma. Our new age data from the N M P are similar to those reported from the Grenville Province in southern Canada. However, their respective tectonic histories are very different: the N M P represents an oblique juvenile arc-continent collison with very minor reworking of older pre-Mesoproterozoic crust, whilst the Grenville orogen is an orthogonal orogen in

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J. Jacobs et aL / Precambrian Research 86 (1997) 71 92

Table 12 Summary of the geological evolution of the NMP Tectonothermal events Exposure of basement complex to weathering, deposition of clastic Natal Group Rapid exhumation from ~ 6 - 1 0 km Localized growth of post-tectonic muscovite Widespread oblique sinistral shearing. Tight folding, lowpressure amphibolite to granulite grade metamorphism

NE-directed collision, refolded isoclinal folds, thrusts (regional pervasive fabric); amphibolite-greenschist grade regional metamorphism

Magmatic events

Gieochronology

Localized pyroclastic volcanism

~490 Ma" ~ 500 Ma bc ~900 Ma b

Oribi Gorge Suite plutons (Mzumbe and Margate terranes) Equeefa mafic dyke swarm (Mzumbe terrane) Syntectonic granite sheets (Mzumbe and Margate terranes), Alkaline granitoids (Tugela terrane)

Obduction of Tugela Ophiolite Subduction of Tugela Ocean, formation of juvenile arcs (Mzumbe and Margate terranes), Deposition/extrusion of supracrustal rocks in all terranes (on oceanic crust?) Tugela Ocean south of Kaapvaal Craton

Hot-spot mafic volcanism in Tugela Ocean Quha volcanics, pre-tectonic Mzumbe Suite (Mzumbe terrane)

~980-1050 Ma c'd 1090-1050 Ma 1090 Ma ° 1100 Maf ~ 1135 Ma c 1180 Ma g 1180 M a h ~ 1200 Ma i ~3500 to ~ 1300 Ma?

aThomas et al. (1992b). bJacobs and Thomas (1996). cPresent study. dThomas et al. (1993a). eThomas et al. (1995). fNicolaysen and Burger (1965). gWilson ( 1991 ). hCornell et al. (1996). Thomas and Eglington (1990).

which large blocks of parautochthonous Grenville cratonic crust was reworked.

pre-

Acknowledgement This project was supported in part by Deutsche Forschungsgemeinschaft G r a n t J a 617/1 + 2 . W e acknowledge the constructive reviews of M. de W i t a n d P . H . R e y n o l d s . T h i s is a c o n t r i b u t i o n t o I G C P P r o j e c t s 348 a n d 368.

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