U–Pb and 40Ar/39Ar geochronological constraints on the tectonic evolution of the easternmost part...

Precambrian Research 98 (1999) 67–82www.elsevier.com/locate/precamres

U–Pb and 40Ar/39Ar geochronological constraints on thetectonic evolution of the easternmost part of the Zambezi

orogenic belt, northeast ZimbabweMichael L. Vinyu a, Richard E. Hanson b,*, Mark W. Martin c,Samuel A. Bowring c, Hielke A. Jelsma a, Michael A. Krol c,d,

Paul H.G.M. Dirks aa Geology Department, University of Zimbabwe, P.O. Box MP 167, Mount Pleasant, Harare, Zimbabweb Department of Geology, Texas Christian University, P.O. Box 298830, Fort Worth, TX 76129, USA

c Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USAd Department of Earth Sciences, Boston University, Boston, MA 02215, USA

Received 26 May 1998; accepted for publication 31 May 1999

Abstract

The Zambezi belt is a key segment of the network of Neoproterozoic/lower Paleozoic orogenic belts in southernAfrica that formed during amalgamation of central Gondwana. We present new geochronological data from theeasternmost Zambezi belt in northeast Zimbabwe, near the junction between that belt and the Mozambique belt.Allochthonous high-pressure granulite-facies migmatitic and mylonitic rocks at the highest exposed structural levelsin this part of the Zambezi belt are tectonically juxtaposed against amphibolite-facies supracrustal rocks, which areintruded by a regionally extensive, sheet-like composite granitoid batholith having peralkaline affinities. This batholithseparates the Zambezi supracrustal rocks from a zone of Archean basement that wraps the northeast margin of theZimbabwe craton and shows variable degrees of structural and thermal overprinting. U–Pb zircon and titanite and40Ar/39Ar hornblende geochronological data from the allochthonous granulites are interpreted to record high-grademigmatization at ca 870 to 850 Ma, with pervasive amphibolite-facies retrogression at ca 535 Ma during tectonicemplacement into mid-crustal levels. U–Pb zircon and titanite data from the peralkaline batholith indicate that itcrystallized at 805.2±11.1 Ma; partial thermal resetting of the U–Pb system occurred in the same time frame asretrogression of the allochthonous granulites. Hornblende from two samples in thermally overprinted Archeanbasement farther south yields 40Ar/39Ar plateau ages of 507.9±2.5 and 491.3±2.1 Ma. Together these new dataindicate that tectonic elements in the easternmost Zambezi belt have a protracted history, involving early, lowercrustal, granulite-facies metamorphism (ca 870–850 Ma) followed by intrusion of a peralkaline granitic batholith (ca800 Ma) into supracrustal rocks within the belt. The major tectonostratigraphic units in the easternmost Zambezi beltwere juxtaposed under amphibolite-facies conditions at ca 535 Ma, followed by relatively rapid cooling through Arclosure temperatures in hornblende. The 535 Ma event reflects deformation in the Zambezi belt in the same timeframe as widespread orogenesis that is recorded in other Pan-African belts in southern Africa and is related to finalstages in Gondwana assembly. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: Geochronology; Mozambique belt; Pan-African orogenesis; Zambezi belt; Zimbabwe

* Corresponding author. Tel.: +1 817 257 7996; fax: +1 817 257 7789.E-mail address: [email protected] (R.E. Hanson)

0301-9268/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.PII: S0301-9268 ( 99 ) 00039-X

68 M.L. Vinyu et al. / Precambrian Research 98 (1999) 67–82

1. Introduction the Zambezi belt appear to wrap the northeasternmargin of the Archean Zimbabwe craton (Fig. 2)to merge continuously with the Mozambique beltThe Zambezi belt forms the eastern portion

of a transcontinental system of Pan-African to the east (Johnson and Vail, 1965; Vail, 1966;Barton et al., 1991; Whittington et al., 1999),(~850-450 Ma) orogenic belts in southern Africa

that also includes the Lufilian arc and Damara although detailed structural and timing relationsbetween the two belts remain unclear.belt (Fig. 1). This transcontinental system sepa-

rates the Congo and Kalahari cratons and links The role of the transcontinental Zambezi–Lufilian–Damara orogenic system during Gond-north–south-trending belts exposed along the pre-

sent east and west margins of southern Africa. The wana amalgamation has been controversial. Somemodels infer the presence of a major ocean basineastern part of the Zambezi belt joins with the

Mozambique belt to the east (Fig. 1), which forms between the Congo and Kalahari cratons (e.g.Hoffman, 1991). Based on the nature of lithotec-part of the larger East African orogen (Stern,

1994) and represents one of the major suture zones tonic assemblages preserved in the Zambezi beltand correlations of older basement provinces oninvolved in Gondwana assembly (e.g. Hoffman,

1991; Kroner, 1993; Grunow et al., 1996; Dalziel, each side, it also has been argued that the beltdeveloped by closure of an intracontinental rift1997; Wilson et al., 1997). Structural trends, meta-

morphic zones, and major lithotectonic units in basin that involved, at most, only limited forma-tion of oceanic crust (Shackleton, 1973; Kroner,1983; Porada, 1989; Wilson et al., 1993; Hansonet al., 1994).

Determination of the plate-tectonic significanceof the Zambezi–Lufilian–Damara system duringNeoproterozoic evolution of the central part ofGondwana requires knowledge of the nature andtiming of depositional, magmatic, and orogenicevents from the various segments of the transconti-nental system. In this paper we report new U–Pbzircon and titanite and 40Ar/39Ar hornblende iso-topic age data from a critical part of the Zambezibelt in northeastern Zimbabwe near Rushinga(Fig. 3), close to the junction with theMozambique belt.

2. Geology of the Rushinga area

2.1. Major tectonostratigraphic units

Previous work in the Rushinga area by Talbot(1973, 1987) was expanded and revised by Barton

Fig. 1. Position of the Zambezi belt within the regional Pan- et al. (1991, 1993), and the details given belowAfrican tectonic framework of southern Africa. Pan-African come largely from their work. Barton et al. (1991)structural trends are highly schematic. Borders of Zimbabwe recognized three major tectonostratigraphic ter-are outlined, and area of Fig. 2 is indicated. WCB, West Congo

ranes in the Rushinga area (Fig. 3). Thebelt; LA, Lufilian arc; MB, Mozambique belt; ZB, ZambeziMigmatitic Gneiss Terrane separates the core ofbelt; KB, Kaoko belt; DB, Damara belt; GB, Gariep belt: SB,

Saldania belt. the Zambezi belt from the Archean Zimbabwe

69M.L. Vinyu et al. / Precambrian Research 98 (1999) 67–82

Fig. 2. Geological map of the Zambezi belt, modified from Hanson et al. (1994). Location shown in Fig. 1. Area of Fig. 3 is indicated.K, Katangan supracrustal rocks in Lufilian arc; MD, Mwembeshi dislocation; C, Chewore inliers; MMC, Makuti metamorphiccomplex; UK, Urungwe klippe; SG, Sijarira Group (Neoproterozoic–early Paleozoic?); UG, Umkondo Group (Mesoproterozoic).

craton to the south and consists predominantly of tural slices of basement gneisses interleaved withthe supracrustal rocks. Nearly everywhere in thepolyphase, migmatitic, amphibolite-facies Archean

gneisses. The gneisses have been strongly recrystal- area mapped by Barton et al. (1991), the contactbetween the Rushinga supracrustal rocks and thelized, and there is no clear break in metamorphic

grade with assemblages in the Marginal Gneiss underlying Migmatitic Gneiss Terrane is intrudedby a deformed, sheet-like, composite granitoidTerrane to the north. The latter terrane is divided

into the Rushinga and Chimanda Metamorphic batholith termed the Basal Rushinga IntrusiveComplex (BRIC).Suites. The Rushinga Metamorphic Suite is an

amphibolite-facies supracrustal assemblage inter- The Zambezi Allochthonous Terrane is interpre-ted by Barton et al. (1991, 1993) to represent apreted to record sedimentation along the northern

margin of the Zimbabwe craton prior to Pan- tectonically stacked thrust package emplaced ontothe Marginal Gneiss Terrane. The lower part ofAfrican orogenesis (Barton et al., 1991, 1993). The

Chimanda Metamorphic Suite consists of struc- the allochthonous terrane consists of the Masoso


Fig. 3. Geological map of the Rushinga area, after Barton et al. (1993). Sample localities are indicated. Locations of DZM2 andDZM3 thrusts are from Barton et al. (1993); for a different interpretation of the structure of this region, see Dirks et al. (1998).

Metamorphic Suite, which comprises a batholithic- part of the allochthonous terrane and containsvariably tectonized metagabbros, amphibolites,scale mass of granitic migmatite (Masoso

Leucomigmatite) imbricated along a series of shear and mafic garnet granulites. Barton et al. (1991,1993) interpreted the contact between the Masosozones represented by thinly layered, mylonitic

rocks termed Striped Mafic Gneiss. The and Mavuradonha Metamorphic Suites to be asouth-vergent thrust.Mavuradonha Metamorphic Suite forms the upper


2.2. Pan-African tectonothermal history In contrast, Dirks et al. (1998) argued that theMarginal Gneiss Terrane records progressive, non-coaxial, northeast- to east-directed shearing atBarton et al. (1993) recognized three Neopro-

terozoic–early Paleozoic deformational events in sillimanite grade that gave rise to fold geometrieswith complicated interference patterns. Theythe Rushinga area: DZM1; DZM2; and DZM3

(the ‘ZM’ refers to the possibility of combined inferred this to have occurred in an extensionaltectonic regime that preceded Pan-African con-Zambezi–Mozambique deformations). DZM1

structures are recognized only in the Masoso tractional deformation in the Zambezi belt. In thismodel, structural domes in the region describedMetamorphic Suite, where they are represented by

isoclinal folds and a variably developed mylonitic by Talbot (1973, 1987) and Barton et al. (1991,1993) are the result of increased heat flow, tectonicfoliation in the Masoso Leucomigmatite, and by

mylonitic fabrics in Striped Mafic Gneiss (Treloar unloading, and folding and transposition ofupward-rising, low-density material duringet al., 1990; Carney et al., 1991). Geothermo-

barometry indicates that DZM1 shearing in Striped regional shearing. The resulting structures corre-spond to DZM2 and DZM3 structures of BartonMafic Gneiss occurred under deep-crustal condi-

tions at temperatures of ~800°C and pressures of et al. (1993). Dirks et al. (1998) recognized ayounger, less penetrative set of crenulations and~12 kbar. Early tschermakitic amphibole inclu-

sions in cores of garnet in relict coarser grained leucosome-filled shear bands that developed atkyanite grade and overprinted the earlier fabricsgranulite pods within the shear zones suggest even

higher temperatures, with pressures as great as interpreted as extensional. They proposed that thisyounger deformation and higher pressure meta-15 kbar (Treloar et al., 1990; Carney et al., 1991).

Small, corona-textured metagabbro intrusions morphism resulted from thrust loading duringPan-African collisional orogenesis, although thetruncate the DZM1 migmatitic and mylonitic fab-

rics in the Masoso suite but show marginal shear- thrust sheets responsible for this loading areinferred to have been completely removed bying and amphibolitization, presumably reflecting

deformation during DZM2 and/or DZM3. erosion.Geothermobarometric studies indicate coronas inthe metagabbros formed at 625–700°C and 2.3. Previous geochronology~7 kbar, thereby recording uplift and decompres-sion following the main DZM1 event (Treloar Barton et al. (1991) reported a series of Rb–Sr

whole-rock errorchron dates from the Rushingaet al., 1990; Carney et al., 1991).DZM2 is interpreted by Barton et al. (1993) to area. In the Masoso Metamorphic Suite within the

Zambezi Allochthonous Terrane, Striped Mafichave involved strong internal deformation of theZambezi Allochthonous Terrane under amphibo- Gneiss and Masoso Leucomigmatite yielded

errorchron dates of 823±54 and 823±57 Ma,lite-facies conditions during thrusting onto theRushinga supracrustal rocks, with pervasive refoli- respectively. Two-mica augen granite intruding the

upper part of the Migmatitic Gneiss Terrane andation of DZM1 fabrics in the Masoso suite. Thiswas accompanied by formation of a penetrative, inferred to have been emplaced at the same time

as the BRIC yielded an errorchron date ofamphibolite-facies foliation in the supracrustalrocks, along with structural and thermal overprint- 829±78 Ma. Samples from a migmatitic zone

within the BRIC itself yielded an errorchron dateing of the underlying Migmatitic Gneiss Terrane.DZM3 is regarded by Barton et al. (1993) as a of 868±55 Ma. Barton et al. (1993) suggested that

the errorchron dates from the Masosomajor backfolding and backthrusting event thatwas most intense in the easternmost part of the Metamorphic Suite most likely record DZM2

amphibolite-facies overprinting in those rocks,Zambezi belt, northeast of Rushinga. The easternpart of the sole thrust to the Zambezi Allo- whereas the other two errorchron dates were inter-

preted to record igneous crystallization of thechthonous Terrane was reactivated as a south-dipping backthrust at this time. BRIC.


More recently, Mariga et al. (1998) reported a Allochthonous Terrane, the BRIC, and theU–Pb zircon igneous crystallization age of Migmatitic Gneiss Terrane. Sample locations are849±2 Ma for one of the small, corona-bearing given in Table 1 and are shown in Fig. 3. Zircon,metagabbro bodies that truncates DZM1 fabrics titanite, and hornblende were extracted from 5 toin the Masoso suite. This result indicates 10 kg bulk-rock samples using standard crushingthat DZM1 deformation and high-pressure and heavy liquid and magnetic separation tech-granulite-facies metamorphism in the Zambezi niques. Results of the U–Pb and 40Ar/39Ar isotopicAllochthonous Terrane occurred prior to ca analyses are presented in Tables 2 and 3, respec-850 Ma. tively. All errors are reported at ±2s.

A series of younger Rb–Sr errorchron dateswere obtained by Barton et al. (1991) from otherunits in the area, primarily the Rushinga 3.1. Masoso Metamorphic Suitesupracrustal rocks. Two separate suites of samplesfrom feldspathic gneisses within the supracrustal 3.1.1. Masoso Leucomigmatitesequence yielded errorchron dates of 669±44 and The regionally extensive Masoso Leuco-468±35 Ma, and garnet–kyanite–biotite schist migmatite is characterized by discontinuous, per-yielded an errorchron date of 481±30 Ma. Barton vasively developed, centimeter scale felsic segre-et al. (1991, 1993) suggested that the younger gation layers ( leucosomes) within a mesocratic,errorchron dates mark final closure of the Rb–Sr biotite- and amphibole-bearing orthogneiss host.system resulting from uplift and cooling following The migmatization and much of the deformationthe cessation of Pan-African orogenesis.

in this unit have been interpreted by Treloar et al.In addition, Vail and Dodson (1969) reported(1990), Barton et al. (1991) and Carney et al.two K–Ar dates from micas in and near the Idol(1991) to have occurred at high-pressure granulite-pegmatitic muscovite deposit (Fig. 3), which isfacies conditions during the DZM1 event.hosted by Rushinga supracrustal rocks just north

Sample Zim-616 is representative of the Masosoof the contact with the BRIC (Barton et al., 1991).Leucomigmatite. At the sample locality, felsicMuscovite from the pegmatite yielded a date ofsegregation layers define small-scale, rootless iso-468±20 Ma, whereas biotite from the adjacentclinal folds that are refolded by later tight folds,pelites yielded a date of 519±25 Ma [recalculatedthe limbs of which pass into mylonitic shear zones.according to constants recommended by SteigerIn thin section, Zim-616 contains oriented biotiteand Jager (1977); error bars come from originaland amphibole and granoblastic quartz, plagio-reference].clase, and microcline. The amphibole is a sodicvariety similar to that present in the BRIC (seebelow). Titanite occurs as abundant, anhedral to3. U–Pb and 40Ar/39Ar geochronologyeuhedral grains that form part of the granoblasticmatrix of the rock. Zircon occurs as small crystalsSamples for this study were collected from the

Masoso Metamorphic Suite within the Zambezi with elongate, prismatic habits and is present along

Table 1Detailed sample locations

Sample Description Locationa

Zim-169 Amphibolite in Migmatitic Gneiss Terrane East side of Mazowe River, grid ref. E969N388, Sheet 1632C3Zim-610 Basal Rushinga Igneous Complex Mukotwe River bed, grid ref. E976N507, Sheet 1632C1Zim-611 Amphibolite in Migmatitic Gneiss Terrane Mukotwe River bed, grid ref. E979N505, Sheet 1632C1Zim-615 Striped Mafic Gneiss Mudzi River bed, grid ref. E085N736, Sheet 1632C1Zim-616 Masoso Leucomigmatite Mudzi River bed, grid ref. E087N734, Sheet 1632C1

a Locations given by grid references on Zimbabwe Government topographic map sheets (1:50 000).


Table 2Zircon and titanite U–Pb isotopic data

Sample Weight Concentration Error 2s (%) Age (Ma) Correlation Pbd(mg) coefficient (pg)

U Pba (206Pb/ (208Pb/ (206Pb/ % (207Pb/ % (207Pb/ % 206Pb/ 207Pb/ 207Pb/(ppm) (ppm) 204Pb)b 206Pb)c 238U )c Error 235U )c Error 206Pb)c Error 238U 235U 206Pb

Zim-616z2m 38.8 1497.1 223.8 62705.71 0.151 0.14250 (0.09) 1.33663 (0.10) 0.06803 (0.04) 858.8 861.8 869.5 0.91 8.4z3c1 58.5 121.6 17.7 3269.27 0.147 0.13709 (0.12) 1.27673 (0.19) 0.06754 (0.15) 828.2 835.4 854.5 0.63 18.6z5cl 17.5 185.6 26.7 7971.29 0.163 0.13607 (0.11) 1.26183 (0.14) 0.06726 (0.09) 822.4 828.7 845.7 0.76 3.6z8cl 71.9 44.4 6.3 3516.46 0.152 0.13419 (0.11) 1.24092 (0.12) 0.06707 (0.06) 811.7 819.3 839.9 0.87 7.7z1m 114.5 260.7 35.3 8628.00 0.112 0.13278 (0.08) 1.23645 (0.09) 0.06754 (0.04) 803.7 817.3 854.5 0.89 29.4z4cl 29.2 53.0 7.4 3241.01 0.158 0.13205 (0.15) 1.21367 (0.19) 0.06666 (0.10) 799.6 806.9 827.0 0.84 4.1z7cl 70.3 62.7 8.6 3892.54 0.149 0.12979 (0.17) 1.19157 (0.18) 0.06658 (0.06) 786.7 796.7 824.8 0.94 9.2z6cl 27.4 142.2 19.4 2703.61 0.146 0.12851 (0.11) 1.17408 (0.12) 0.06626 (0.06) 779.4 788.6 814.6 0.88 11.6z3m 12.3 569.5 72.0 8186.48 0.099 0.12552 (0.07) 1.16949 (0.08) 0.06758 (0.05) 762.2 786.4 855.6 0.81 6.9z2cl 22.0 99.8 13.3 2312.81 0.127 0.12762 (0.19) 1.16545 (0.22) 0.06623 (0.12) 774.3 784.5 813.8 0.85 7.7z1cl 30.3 84.3 11.2 2866.53 0.145 0.12583 (0.20) 1.14812 (0.27) 0.06618 (0.17) 764.0 776.4 812.0 0.78 7.2z4m 6.9 596.4 75.7 5142.94 0.122 0.12329 (0.09) 1.14750 (0.12) 0.06750 (0.08) 749.5 776.1 853.3 0.76 6.3s1 17.0 39.2 4.3 202.65 0.100 0.08488 (0.43) 0.68349 (0.62) 0.05840 (0.42) 525.2 528.9 544.8 0.74 19.6s3 25.3 13.2 1.6 133.01 0.083 0.08432 (0.86) 0.68338 (1.68) 0.05878 (1.36) 521.9 528.8 558.8 0.60 15.7

Zim-615bz1m 24.1 47.4 15.5 3968.32 0.173 0.29291 (0.21) 4.70580 (0.22) 0.11652 (0.09) 1656.1 1768.3 1903.5 0.92 5.4z6m 14.2 327.1 54.0 1826.58 0.107 0.15175 (0.08) 2.55187 (0.10) 0.12196 (0.05) 910.8 1287.0 1985.2 0.84 24.9z5m 26.3 42.0 9.7 2285.61 0.240 0.20077 (0.24) 2.29488 (0.38) 0.08290 (0.28) 1179.4 1210.7 1266.9 0.68 6.2s1 162.0 27.8 3.1 251.73 0.112 0.08782 (0.12) 0.71324 (0.26) 0.05891 (0.22) 542.6 546.7 563.6 0.52 105.0s3 55.0 23.9 3.5 116.41 0.181 0.08685 (0.21) 0.71295 (0.59) 0.05954 (0.53) 536.9 546.5 586.8 0.46 71.9z4a 15.3 60.3 8.9 103.04 0.148 0.08590 (0.30) 0.69732 (0.87) 0.05887 (0.79) 531.3 537.2 562.5 0.42 57.9z3b 25.3 27.7 2.2 1908.94 0.006 0.08686 (0.16) 0.69538 (0.38) 0.05806 (0.34) 536.9 536.0 532.2 0.46 2.1z4b 15.1 32.2 2.5 711.95 0.004 0.08580 (0.65) 0.69236 (1.15) 0.05853 (0.90) 530.6 534.2 549.5 0.63 3.9z2b 13.9 20.1 1.6 905.12 0.015 0.08615 (0.26) 0.68989 (0.59) 0.05808 (0.50) 532.7 532.7 532.7 0.54 1.8z4 14.2 62.4 4.8 2233.19 0.009 0.08457 (0.11) 0.68404 (0.21) 0.05866 (0.17) 523.3 529.2 554.6 0.58 2.2z3 20.3 89.0 4.4 3408.18 0.009 0.05399 (0.11) 0.43796 (0.17) 0.05883 (0.13) 339.0 368.8 560.8 0.65 1.9

Zim-610z4 26.9 27.3 3.8 1791.85 0.199 0.12844 (0.20) 1.15884 (0.22) 0.06544 (0.10) 779.0 781.4 788.4 0.90 3.4z5 36.3 32.2 4.4 2719.29 0.192 0.12676 (0.21) 1.14205 (0.27) 0.06535 (0.17) 769.3 773.5 785.5 0.79 3.5z1 70.9 33.2 4.6 2039.95 0.181 0.12681 (0.18) 1.14134 (0.22) 0.06528 (0.13) 769.6 773.1 783.3 0.82 9.2s2 13.2 33.7 5.0 363.09 0.177 0.12462 (0.49) 1.12157 (0.60) 0.06527 (0.32) 757.1 763.7 783.2 0.85 10.0z1a 16.3 50.5 6.7 2030.45 0.170 0.12413 (0.21) 1.11104 (0.26) 0.06492 (0.14) 754.3 758.7 771.6 0.83 3.3z3 16.2 25.1 3.3 791.13 0.162 0.12330 (0.54) 1.10089 (0.61) 0.06476 (0.27) 749.5 753.8 766.4 0.90 4.2z2 30.6 37.8 5.9 297.90 0.175 0.12273 (0.21) 1.09763 (0.34) 0.06486 (0.25) 746.3 752.2 769.9 0.67 31.3

a Radiogenic Pb.b Measured ratio corrected for spike and fractionation only. Mass fractionation correction of 0.15% amu−1±0.04% amu−1 (atomic

mass unit) was applied to single collector Daly analyses and 0.12% amu−1±0.04% amu−1 to dynamic Faraday–Daly analyses.c Corrected for fractionation, spike, blank and initial common Pb.d Total common Pb in analyses. Total procedural blank ranged from 0.65 to 3.7 pg for Pb and was <1.0 pg for U. Blank isotopic

composition: 206Pb/204Pb=19.10±0.1, 207Pb/204Pb=15.71±0.1, 208Pb/204Pb=38.65±0.1. Sample weights were estimated using avideo monitor and are known to within 40%. Age calculations are based on the decay constants of Steiger and Jager (1977).Common-Pb corrections were calculated using the model of Stacey and Kramers (1975) and the interpreted age of the sample. Zirconand titanite preparation, chemistry and ID-TIMS analytical procedures follow those of Bowring et al. (1998) and Hawkins andBowring (1999).

grain boundaries or as inclusions within the rock- [Table 1, Fig. 4(a)]. All of the zircon crystals wereeuhedral with subrounded terminations. Eight offorming minerals.

Twelve single zircon crystals and two single the zircon crystals were clear and inclusion-free,whereas the remaining four crystals were cloudy,titanite crystals were analyzed from this sample


Table 340Ar/39Ar isotopic dataa

Temperature Duration 40Ar/ 38Ar/ 37Ar/ 36Ar/ 39Ark 39Ar 40Ar1 Age ±2s( °C) (min) 39Ar 39Ar 39Ar 39Ar (moles) Released (%) (Ma) (Ma)

Zim-601 Hornblende (weight: 0.040 g; J=0.01881±1.89E-4)702 10 46.581 0.04040 1.39845 0.13399 2.26E-14 0.0061 15.29 227.09 47.63827 10 36.587 0.03584 1.67691 0.07273 1.19E-14 0.0094 41.72 455.99 22.24877 10 17.852 0.01429 1.81798 0.00512 1.72E-14 0.0140 92.51 488.78 36.74902 10 17.103 0.01113 1.81818 0.00011 9.38E-15 0.0166 100.00 507.57 35.83927 10 16.806 0.01476 1.84364 0.00010 9.49E-15 0.0192 100.00 500.27 24.67952 10 17.377 0.01512 2.11008 0.00034 2.05E-14 0.0247 100.00 513.80 26.17977 10 16.762 0.02994 2.15435 0.00102 2.82E-14 0.0324 99.47 492.99 18.981002 10 16.561 0.02862 2.55552 0.00159 2.82E-13 0.1091 98.70 484.78 7.491027 10 16.641 0.03067 2.66916 0.00111 1.16E-12 0.4243 99.63 490.77 4.861077 10 16.654 0.03086 2.69023 0.00097 1.23E-12 0.7572 99.89 492.31 4.891277 10 16.654 0.03050 2.70689 0.00101 8.79E-13 0.9959 99.83 492.02 4.911697 10 19.452 0.02898 2.78713 0.00015 1.50E-14 1.0000 100.00 569.55 50.51

Zim-611 Hornblende (weight: 0.0331 g; J=0.01881±1.89E-4)702 10 546.567 0.32000 2.78461 1.73742 2.20E-15 0.0014 6.12 882.59 170.64827 10 136.316 0.07305 3.34034 0.38925 3.95E-15 0.0038 15.86 617.49 154.09877 10 35.048 0.00011 3.41013 0.01543 1.73E-15 0.0049 87.94 827.17 283.54902 10 31.901 0.00023 4.01097 0.00113 8.70E-16 0.0054 100.00 851.84 505.32927 10 26.238 0.00017 5.49027 0.00086 1.14E-15 0.0061 100.00 733.30 352.26952 10 23.825 0.00008 6.11953 0.00045 2.19E-15 0.0075 100.00 682.50 242.03977 10 20.586 0.00958 6.75066 0.00267 7.01E-15 0.0118 99.41 590.83 70.521002 10 18.276 0.01844 6.29144 0.00438 1.98E-14 0.0240 96.34 518.78 33.411027 10 17.441 0.01446 5.79993 0.00212 7.50E-14 0.0703 99.70 512.99 8.291077 10 17.147 0.01488 5.78188 0.00220 8.64E-13 0.6042 99.55 504.72 6.541277 10 17.093 0.01461 5.89814 0.00192 6.27E-13 0.9916 100.00 506.01 6.661652 10 19.692 0.01618 6.23963 0.00992 1.37E-14 1.0000 88.25 512.90 41.95

Zim-615A Hornblende (weight: 0.0371 g; J=0.01881±1.89E-4)702 10 457.030 0.15220 2.76895 0.91015 3.44E-15 0.0011 41.22 2735.00 84.01827 10 193.783 0.08405 4.39678 0.36176 2.66E-15 0.0020 45.05 1757.60 206.93877 10 120.419 0.00007 1.99321 0.07679 3.31E-15 0.0031 81.32 1886.57 203.63902 10 53.950 0.03060 1.63101 0.00049 2.00E-15 0.0038 100.00 1265.51 155.04927 10 44.308 0.00010 2.32229 0.00054 1.82E-15 0.0044 100.00 1096.55 159.01952 10 39.459 0.05192 3.11592 0.00044 2.21E-15 0.0051 100.00 1007.45 414.82977 10 36.990 0.09696 3.35558 0.00049 1.98E-15 0.0058 100.00 959.12 137.921002 10 31.173 0.06976 3.31368 0.00032 3.04E-15 0.0068 100.00 839.74 190.301027 10 21.932 0.03957 3.08126 0.00005 1.92E-14 0.0132 100.00 631.60 13.511077 10 18.005 0.03522 3.20694 0.00121 5.83E-13 0.2075 99.79 526.74 5.391277 10 17.802 0.03494 3.22309 0.00111 2.37E-12 0.9976 99.97 522.31 5.271652 10 19.634 0.02919 4.41814 0.00656 7.34E-15 1.0000 92.37 531.41 59.18

a Hornblende samples were hand-picked to remove contamination, packaged in Al foil packets, and loaded into aluminum disksalong with flux monitor MMhb-l hornblende [520.4 Ma; Samson and Alexander (1987)] and salts (CaF2, K2SO4 and KCl ). Irradiationof the samples was performed in the McMaster University nuclear reactor in Ontario, Canada, for 75 h in the 5C position. Detailedinformation regarding the extraction systems and analytical techniques is given by Hodges et al. (1994).

metamict, and ruby-red in color. The two titanite sions through analyses for either population arenot statistically significant, though both yieldgrains were cloudy, euhedral, yellow and inclusion-

free. Analyses of the two zircon populations fall upper intercepts of ca 875 Ma. These isotopicsystematics suggest that both zircon populationsalong different Pb-loss trajectories. Linear regres-


Fig. 4. Concordia diagrams for samples from the Rushinga area. Individual analyses are depicted as 2s error ellipses (where errorellipses are too small due to scale of plot, small filled circles are used). Numbers by error ellipses correspond to individual zircon (z)or titanite (s) grains analyzed (Table 2). In (a), numbers are shown only for cloudy, metamict zircon grains. A linear regression isshown through the eight clear zircon grains in this sample (see discussion in text). (c) shows an enlarged view of the outlined partof the concordia diagram in (b).

crystallized at ca 875 Ma. The best estimate of the the metamict zircon population do not define asimple mixing line between 870 Ma and 535 Ma,zircon crystallization age is interpreted as the

207Pb/206Pb date of the most concordant zircon, because of complex Pb-loss related to high Uconcentrations.869.5±1.0 Ma. Bearing in mind other U–Pb

zircon geochronological evidence noted above that The two titanite analyses are discordant andfall along the linear regression of the clear zirconDZM1 in the Masoso Leucomigmatite must have

occurred prior to 850 Ma, the zircon crystallization population near the lower intercept at ca 535 Ma.We interpret this behavior to indicate nearly com-age for Zim-616 could be interpreted either as the

protolith age for the leucomigmatite, or as the plete thermal resetting of older titanite at this time.time of high-grade DZM1 migmatization. This isdiscussed further below. 3.1.2. Striped Mafic Gneiss

Striped Mafic Gneiss was sampled from a later-The lower intercept of the analyses for the eightclear zircons is at ca 535 Ma [Fig. 4(a)], which we ally continuous horizon of this lithology that is

traceable for ~10 km and is tens of meters thick.interpret to indicate partial thermal resetting ofthe U–Pb system in the clear zircons during a The sample (Zim-615) was collected ~200 m from

Zim-616 and shows the pronounced planar band-metamorphic event at ca 535 Ma. The analyses of


ing defined by mafic and felsic layers that is However, it is uncertain if this age represents acooling age (cooling through 500°C), or whethercharacteristic of Striped Mafic Gneiss. At the

sample locality, the banding is deformed by an it records growth during the retrograde amphibo-lite-facies metamorphic event.early set of noncylindrical, mesoscopic isoclinal

folds that are refolded by recumbent folds; both Sample Zim-615b comes from a representativefelsic layer in the Striped Mafic Gneiss. Thesets of structures are warped by broad, open folds

with steep axial surfaces and wavelengths of several sampled layer is concordant with adjacent layersand lacks igneous textures or other evidence ofmeters. Carney et al. (1991) interpreted the pro-

nounced layering in these rocks to reflect a combi- originating as partial melt. It may represent meltthat was strung out in the layering and thennation of metamorphic segregation and intro-

duction and progressive shearing of granitic melt underwent solid-state recrystallization, or it mayhave formed by metamorphic segregation duringfractions during development of deep-crustal

DZM1 shear zones. shearing. The sample consists of granoblasticquartz, plagioclase, and microcline, with minorSample Zim-615a is representative of the mafic

layers in the gneiss and is a partly retrogressed aligned biotite and sparse poikiloblastic horn-blende. Titanite forms subhedral to euhedral,mafic granulite containing abundant garnet porph-

yroblasts up to >3 cm in diameter. Foliation in wedge-shaped crystals aligned in the foliation.Zircon occurs as small, prismatic crystals alongthe matrix is defined by thin, discontinuous layers

of granoblastic quartz and plagioclase ( locally grain boundaries or as inclusions in quartz andfeldspar.showing antiperthitic texture), together with a

relatively poorly defined, shape-preferred orienta- Nine single, euhedral zircon crystals with sub-rounded terminations and two single, large,tion of titanite, biotite, hornblende, and clinopyro-

xene. The clinopyroxene forms elongate, cloudy, euhedral titanite crystals were analyzedfrom this sample [Table 2, Fig. 4(b) and (c)].poikiloblastic crystals containing numerous tiny

quartz and plagioclase inclusions. Carney et al. Three reddish, metamict zircons yielded discordantanalyses with Meso- to Paleoproterozoic 207Pb/(1991) attributed similar poikiloblasts in Striped

Mafic Gneiss to prograde mineral growth during 206Pb dates, indicating significant inheritance. Theremaining six zircon grains were clear, colorless,granulite-facies metamorphism. The hornblende is

in textural equilibrium with titanite and biotite but and inclusion-free. Analyses for two of these zir-cons (z2b, z3b) are concordant at 536 and 532 Ma.shows clear replacement relations against older

clinopyroxene. Garnet shows marginal recrystalli- The remaining zircons have 207Pb/206Pb dates thatrange from 562.5 to 549.5 Ma. The two titanitezation to fine-grained aggregates of biotite, plagio-

clase, and quartz, and the smaller garnet blasts are analyses are discordant, with 207Pb/206Pb dates of586.8 and 563.6 Ma, probably reflecting incom-partly to completely replaced by this assemblage.

The textures as a whole are interpreted to indicate plete resetting of older titanite during meta-morphism.initial granulite-facies metamorphism followed by

pervasive retrogression in the amphibolite facies. In the context of other geochronological datapresented herein, we interpret the ages of the40Ar/39Ar hornblende data for Zim-615a are

shown as an age spectrum diagram in Fig. 5(A). concordant zircons to date post-DZM1 amphibo-lite-facies overprinting of Striped Mafic Gneiss.Incremental heating of Zim-615a yielded a well-

defined plateau age of 524.5±3.2 Ma. The39Ar/37Ar ratios for Zim-615a are relatively con- 3.2. Basal Rushinga Intrusive Complexstant over the majority of the gas released, indicat-ing little intra-sample compositional variation Sample Zim-610 was collected from a typical

part of the BRIC where it forms a relatively thin,[Fig. 5(A)]. Also, there is no distinct correlationbetween apparent age and the 39Ar/37Ar values. tabular sheet intruded along the interface between

the Migmatitic Gneiss and Marginal GneissWe interpret the 524.5±3.2 Ma age to be the bestestimate for the timing of final Ar closure. Terranes. At the sample locality, the contact


Fig. 5. 40Ar/39Ar incremental release age spectra for hornblendes from samples Zim-615a (A), Zim-601 (B) and Zim-611 (C). Allerrors are reported at the 95% confidence level and include the error in the J parameter. Also shown are the corresponding39ArK/37ArCa ratios, indicating that none of the samples display significant intra- or inter-sample compositional variations. Ageplateaux in the incremental release spectra are defined for the purposes of this paper as the error-weighted mean age of contiguoussteps that define 50% or more of the 39Ar released and are statistically indistinguishable at the 2s confidence level, exclusive ofuncertainty in the J value.


between the BRIC and migmatitic basement gneiss Gneiss Terrane. Sample Zim-601 comes from acan be located within 1 m in the bed of the thin garnet amphibolite layer within migmatitic,Mukotwe River and dips steeply to the south. The sillimanite–kyanite paragneiss, south of the zonecontact is sharp, and banding in the gneiss becomes of obvious Pan-African structural overprinting inparallel to the base of the BRIC as the contact is the Migmatitic Gneiss Terrane. The amphiboliteapproached. The BRIC contains a gneissic banding is discordant to the main banding in the gneiss,parallel to the contact and defined by thin suggesting that it represents a metamorphosedmafic/felsic layers; at least some of the felsic layers mafic dike. The amphibolite is folded with theappear to be derived from discontinuous, sheared- gneissic layering and has a mineral lineation paral-out pegmatite veins. A foliation parallel to the lel to that in the host. Hornblende from this samplebanding is defined by oriented amphibole and yields an 40Ar/39Ar release spectrum with a well-biotite. In thin section, quartz, microcline, and defined plateau age of 491.3±2.1 Ma [Fig. 5(B)].plagioclase show granoblastic texture, reflecting Sample Zim-611 comes from an amphibolitemetamorphic recrystallization of original igneous layer within polydeformed Archean migmatitictextures. The amphibole is a sodic variety, based biotite gneiss ~300 m south of the contact withon its strong pleochroism, intense absorption, and the BRIC at the locality of Zim-610. The amphibo-low 2V. A similar amphibole is typical of the lite shows a thin, well-developed internal layeringBRIC as a whole and was identified as ferrohas- defined partly by quartzofeldspathic segregations.tingsite by Talbot (1973). Titanite forms small, The hornblende within individual layers, however,anhedral to subhedral crystals, and zircon is rela- is randomly arranged, implying post-deforma-tively abundant as small prismatic crystals with tional recrystallization or static new growth.rounded pyramidal terminations. Hornblende from this sample yields an 40Ar/39Ar

Six individual zircon crystals and one titanite release spectrum with a well-defined plateau agecrystal were analyzed from this sample (Table 2). of 507.9±2.5 Ma [Fig. 5(C)]. Combined with theThese data define a linear regression that yields an

post-deformational fabric, this age provides aupper intercept of 805.2±11.1 Ma [Fig. 4(d)],lower limit for the timing of deformation of thiswhich we interpret as the crystallization age forunit.this part of the BRIC. The lower intercept at

520.7±71.3 Ma reflects partial Pb-loss due to par-tial thermal resetting of the U–Pb system, consis-

4. Discussion and conclusionstent with U–Pb zircon and titanite and 40Ar/39Arhornblende data from the Masoso Metamorphic

4.1. Deep-crustal rocks in the ZambeziSuite and with 40Ar/39Ar hornblende data fromAllochthonous Terranenearby thermally overprinted basement discussed

below. The single titanite analysis plots within theStructural and metamorphic data presented byzircon data array, consistent with crystallization

Treloar et al. (1990), Barton et al. (1991) andof titanite at about the same time as zircon, withCarney et al. (1991) indicate that the batholithic-subsequent minor thermal resetting of the titanitescale Masoso Leucomigmatite in the lower part ofU–Pb system. These data suggest that this part ofthe Zambezi Allochthonous Terrane formed underthe BRIC cooled below the closure temperature inhigh-pressure, granulite-facies conditions during atitanite [~650°C; Cherniak (1993)] relativelyDZM1 event that preceded Pan-African tectonicquickly after igneous crystallization and subse-exhumation. U–Pb zircon age data reported byquently was not metamorphosed at high enoughMariga et al. (1998) require that DZM1 occurredgrade to significantly affect the titanite.prior to 850 Ma. As noted earlier, it is unclearwhether the zircon crystallization age of ca 870 Ma3.3. Archean Migmatitic Gneiss Terranefor Masoso Leucomigmatite reported herein(Zim-616) reflects the protolith age of the migma-40Ar/39Ar hornblende data have been obtained

from two amphibolites within the Migmatitic tite, or the time of granulite-facies migmatization


during DZM1. Either scenario requires that part within-plate, peralkaline geochemical affinities(Munyanyiwa et al., 1997) like the BRIC.of the history of the unit is not recorded in the

U–Pb zircon isotopic systematics in the analyzed Munyanyiwa et al. (1997) interpreted the gneissesto be derived from felsic volcanic rocks, whereassample. The available data constrain DZM1 to

have occurred between ca 870 and 850 Ma. Dirks et al. (1997) and Sithole et al. (1997) haveargued that the gneisses represent mylonitizedIsotopic systematics of zircons from the sampled

felsic layer within Striped Mafic Gneiss (Zim-615b) granites. Clearly much more geochronological andgeochemical data from these various felsic rocksdo not record the DZM1 event. It is thus possible

that some of the felsic layers in the Striped Mafic in the Zambezi belt are needed before their tectonicsignificance becomes fully understood. Based onGneiss formed after this event, during amphibolite-

facies reactivation of DZM1 ductile shear zones, the within-plate, peralkaline affinities for some ofthese rocks, one possible interpretation is that theywhich is consistent with the structural complexity

of these rocks. 207Pb/206Pb dates of metamict zir- represent one or more episodes of extension-relatedmagmatism occurring prior to final stages of Pan-cons within Zim-615b suggest that these zircons

are inherited from a Meso- to Paleoproterozoic African orogenesis in the Zambezi belt (e.g.Munyanyiwa et al., 1997).source. This is evidence that older crust was

involved in the petrogenesis of the MasosoMetamorphic Suite. 4.3. Orogenesis at ca 535 Ma

The new geochronological data indicate that an4.2. Age and tectonic significance of the BRICorogenic event affected the Zambezi belt at ca535 Ma in the Rushinga area. This event isThe U–Pb crystallization age of 805±11 Ma

for the BRIC is consistent with the Rb–Sr recorded by concordant zircons from a felsic layerin Striped Mafic Gneiss, by discordant titaniteserrorchron dates obtained by Barton et al. (1991)

for this unit. Barton et al. (1993) argued that and partial thermal resetting of zircon in MasosoLeucomigmatite, and by partial thermal resettingemplacement of the BRIC was broadly synchron-

ous with major DZM2 thrusting in the Rushinga of zircon and titanite in the BRIC. 40Ar/39Arplateau ages of 525 to 491 Ma for hornblendearea, which is consistent with structural relation-

ships shown by granites with similar ages in the from Striped Mafic Gneiss and from Archeanbasement in the Migmatitic Gneiss Terrane areZambian part of the Zambezi belt (Barr et al.,

1978; Hanson et al., 1988). However, trace-element consistent with cooling below the closure temper-ature for Ar diffusion in hornblende following thiscontents of the BRIC are comparable to within-

plate granitoids (Barton et al., 1991; Carney et al., event and with the previously published Rb–Srand K–Ar dates from the area. The differences1991), and peralkaline affinities are indicated by

the widespread occurrence of aegirine–augite and between hornblende dates could be explained byvariation in the timing of metamorphism/ferrohastingsite within this unit (Talbot, 1973;

Barton et al., 1991). These are not typical features deformation, by differential exhumation followingthis event, and/or by variations in closure temper-of granitoids formed in compressional settings,

and Carney et al. (1991) have suggested that the atures caused by compositional variations in theanalyzed hornblendes. Additional work is neededBRIC was emplaced in an extensional environment

prior to Zambezi contractional deformation. Dirks before we can distinguish between these possibilit-ies, although 39Ar/37Ar ratios for the analyzedet al. (1998) suggested that the BRIC is syntectonic

with respect to extensional shearing that they samples (Fig. 5) indicate lack of significant inter-sample compositional variations.inferred occurred during this time period in the

Rushinga area. The Makuti metamorphic complex Based on the present data, we are unable toshow that the DZM2 and DZM3 events recognizedin northwest Zimbabwe (Fig. 2) contains felsic

gneisses that are similar in age to the BRIC by Barton et al. (1993) were separated by asignificant time interval. The available data are(Dirks et al., 1997; Hanson et al., 1998) and have


consistent with a model in which DZM2 and (1991, 1993) and Dirks et al. (1998). In theZambezi belt to the west of the Rushinga area, aDZM3 structures formed during a single, pro-

gressive, amphibolite-grade deformational event at regional metamorphic event at 550 to 530 Ma issuggested by sparse U–Pb zircon and titanite iso-ca 535 Ma. This conflicts with the interpretation

of Barton et al. (1993) that the Rb–Sr errorchron topic age data (Hanson et al., 1998), and bySHRIMP U–Pb zircon data from the Cheworedates of 823±54 and 823±57 Ma from the Striped

Mafic Gneiss and Masoso Leucomigmatite date inliers (Fig. 2; Goscombe et al.,. 1998). K–Ar,40Ar/39Ar, and Rb–Sr mineral dates from samplesDZM2 thrusting onto the Rushinga supracrustal

rocks. scattered throughout the belt fall in the range 650to 410 Ma (Hahn et al., 1991; Hanson et al., 1994,Relatively rapid cooling after the ca 535 Ma

event is indicated by the close similarity in U–Pb and references therein; Goscombe et al., 1997).Furthermore, deformation and plutonism in thezircon and titanite ages and 40Ar/39Ar hornblende

ages from the Masoso Metamorphic Suite, given inner part of the Lufilian arc, along strike to thewest in central Zambia (Fig. 1), occurred in thisthe differences in closure temperatures for these

three phases. We infer that this rapid cooling time frame (Hanson et al., 1993), as did majororogenesis in the Damara belt in Namibia (e.g.reflects erosional and tectonic unroofing of thick-

ened crust during and/or immediately following Miller, 1983; Buhn et al., 1994). These new andexisting data suggest that broadly coeval tectonicPan-African contractional deformation. Cooling

following thermal overprinting of Archean base- activity affected large parts of the Pan-Africanorogenic framework in southern Africa and adja-ment in the Migmatitic Gneiss Terrane also is

indicated by 40Ar/39Ar hornblende ages of 491 and cent regions during final amalgamation of centralGondwana.508 Ma from the Migmatitic Gneiss Terrane, con-

sistent with previous studies which have shownthat Pan-African amphibolite-facies metamor-phism was superimposed on Archean metamorphic Acknowledgementsassemblages within this terrane (Barton et al.,1991; Dirks et al., 1998). The authors gratefully acknowledge support for

Our conclusion that major orogenesis affected this research from the following grants: Universitythe Rushinga area at ca 535 Ma allows the evolu- of Zimbabwe Research Board Granttion of the easternmost segment of the Zambezi No. 3YYH-10/3784 to Vinyu, Jelsma and Dirks;belt to be related more clearly to the regional Pan- Stichting Schurmannfond Grants Nos. 1995/13African tectonic framework. Farther east, a large and 1996/13 to Jelsma and Dirks; MINRESTamount of geochronological evidence from funds to Vinyu; Fulbright African Regionaldifferent parts of the Mozambique belt [East Research Grant to Hanson; National ScienceAfrican orogen of Stern (1994)] indicates that the Foundation Grant No. EAR 95-08013 to Hanson.culminating orogenic events occurred there at ca Thorough reviews by J.M. Barton and D.R.600 to 520 Ma (e.g. Grunow et al., 1996; Wilson Nelson resulted in significant improvements in theet al., 1997; Kroner et al., 1997; Paquette and manuscript.Nedelec, 1998). Similar isotopic ages also occur inDronning Maud Land in East Antarctica, alongthe inferred continuation of the Mozambique belt

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U–Pb and 40Ar/39Ar geochronological constraints on the tectonic evolution of the easternmost part...

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