Trace element and isotopic (Sr, Nd, Pb, O) arguments for a mid-crustal origin of Pan-African garnet-bearing S-type granites from the Damara orogen (Namibia)

Precambrian Research 110 (2001) 325– 355 www.elsevier.com/locate/precamres

Trace element and isotopic (Sr, Nd, Pb, O) arguments for a mid-crustal origin of Pan-African garnet-bearing S-type granites from the Damara orogen (Namibia) S. Jung a,*, K. Mezger b, S. Hoernes c a

Max-Planck-Institut fu¨r Chemie, Abt. Geochemie, Postfach 3060, 55020 Mainz, Germany Institut fu¨r Mineralogie, Uni6ersita¨t Mu¨nster, Corrensstr. 24, 48149 Mu¨nster, Germany c Mineralogisch-Petrologisches Institut der Uni6ersita¨t Bonn, Poppelsdorfer Schloß, 53115 Bonn, Germany b

Received 17 September 1999; received in revised form 19 November 1999; accepted 23 January 2001

Abstract Geochronological data, major and trace element abundances, Nd and Sr isotope ratios, d18O whole rock values and Pb isotope ratios from leached feldspars are presented for garnet-bearing granites (locality at Oetmoed and outcrop 10 km north of Omaruru) from the Damara Belt (Namibia). For the granites from outcrop 10 km N% Omaruru, reversely discordant U–Pb monazite data give 207Pb/235U ages of 511 92 Ma and 51792 Ma, similar to previously published estimates for the time of regional high grade metamorphism in the Central Zone. Based on textural and compositional variations, garnets from these granites are inferred to be refractory residues from partial melting in the deep crust. Because P –T estimates from these xenocrystic garnets are significantly higher (800°C/9– 10 kbar) than regional estimates (700°C/5 kbar), the monazite ages are interpreted to date the peak of regional metamorphism in the source of the granites. Sm–Nd garnet–whole rock ages are between 500 and  490 Ma indicating the age of extraction of the granites from their deep crustal sources. For the granites from Oetmoed, both Sm – Nd and Pb– Pb ages obtained on igneous garnets range from 500 to  490 Ma. These ages are interpreted as emplacement ages and are significantly younger than the previously proposed age of 520 Ma for these granites based on Rb/Sr whole rock age determinations. Major and trace element compositions indicate that the granites are moderately to strongly peraluminous S-type granites. High initial 87Sr/86Sr ratios ( \0.716), high d18O values of \13.8‰, negative initial mNd values between −4 and −7 and evolved Pb isotope ratios indicate formation of the granites by anatexis of mid-crustal rocks similar to the exposed metapelites into which they intruded. The large range of Pb isotope ratios and the lack of correlation between Pb isotope ratios and Nd and Sr isotope ratios indicate heterogeneity of the involved crustal rocks. Evidence for the involvement of isotopically highly evolved lower crust is scarce and the influence of a depleted mantle component is unlikely. The crustal heating events that produced these granites might have been caused by crustal thickening and thrusting of crustal sheets enriched in heat-producing elements. Very limited fluxing of volatiles from underthrust low- to medium-grade metasedimentary rocks may have also been a factor in promoting partial melting. Furthermore, delamination of the lithospheric mantle and uprise of hot mantle could have caused localized high-T regions. The presence of coeval A-type granites at Oetmoed that have been * Corresponding author. Tel.: + 49-6131-305530; fax: +49-6131-371051. 0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 1 ) 0 0 1 7 5 - 9

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derived at least in part from a mantle source supports this model. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Crustal melting; Damara orogen; Namibia; Nd – Sr– Pb– O isotopes; S-type granites

1. Introduction The Pan-African orogeny, culminating with the collision of the Congo Craton and the Kalahari Craton, is the major early Phanerozoic orogenic event that affected southern Africa. Unlike other collisional belts (e.g. Scandinavian Caledonides, European Alps), the Damara orogen (Namibia) is characterized by the emplacement of large volumes of dominantly felsic magmas of various compositions. The petrogenesis of these granites, and their tectonic setting, are still poorly understood. The granitoids range in composition from gabbro/diorite:tonalite/granodiorite:granite in proportions of 2:2:96 with monzogranite being the most abundant (Miller, 1983). It has been suggested that some granites are remnants of a continental margin magmatic arc formed during northwest-dipping subduction of oceanic lithosphere beneath the Congo Craton (e.g. Kasch, 1983). Large volumes of granite magmatism are generated both in active continental margins (e.g. Andes) and broad ensialic orogens. However, the proportions of the different igneous plutonic rocks found in Namibia contrast markedly with those found in continental margins (gabbro/diorite:tonalite/granodiorite:granite in proportions of 16:58:26; Pitcher, 1978). Similarly, there is no clear geological evidence for a subduction zone environment (e.g. lack of eclogites and blueschists, lack of a high P– low T regime, no igneous rocks with a clear subduction zone signature). Therefore, the debate continues as to whether the orogen contains parts of a classical subduction zone environment or represents an intracontinental (ensialic) orogen that resulted from subcrustal delamination and continental subduction (e.g. Kro¨ ner, 1982). Others have suggested that the granites formed during the terminal phases of the collision between the Congo Craton and the Kalahari Craton, mainly by melting of metasedimentary rocks of the central Dam-

ara trough under water-saturated conditions (e.g. Winkler, 1983). Water-saturated melting of metasedimentary rocks under high-grade metamorphic conditions is no longer a satisfactory model for granite genesis and other models are needed to explain the large volumes of granites that occupy the central part of the Damara orogen. It has been suggested recently (Jung et al., 1998b) that some part of the heat necessary for crustal melting might have originated from delamination of the lithospheric mantle during collision, a model that explains the geochemical composition of some A-type granites as mantle– crust mixtures. One difficulty in evaluating models of Damaran granite petrogenesis is the lack of data that place limits on the ratio of components (lower crust, upper crust, mantle) involved in granite formation. Rare Earth Element (REE) abundances, whole-rock Nd isotopes, oxygen isotope values and Pb isotopes from leached feldspars are well suited for this purpose, yet few data are available. Initial Sr isotopes also provide important constraints on the nature of the source material. Many Damaran granites have been dated by the Rb/Sr whole rock method, thus initial 87Sr/86Sr ratios are available (e.g. Haack et al., 1980, 1982; Kro¨ ner, 1982; Allsopp et al., 1983; Hawkesworth et al. 1983; McDermott et al., 1996). However, the accuracy of the initial ratios determined from whole-rock isochrons depends upon all samples being cogenetic, a requirement difficult to meet. Furthermore, initial ratios obtained with this method erase the fine-scale differences in isotopic composition of crust-derived granites. Unfortunately, even in samples that did all have the same initial Sr isotope ratio, the Rb/Sr isotopic systematics of some granites can be modified by highgrade metamorphism (e.g. Jung et al., 1998b) and high-precision U–Pb or Sm –Nd mineral ages are needed to constrain precisely the timing of crustal magmatism within an orogenic cycle.

S. Jung et al. / Precambrian Research 110 (2001) 325–355

In this paper, we present major- and trace-element and Nd, Sr, Pb and O isotope data in order to derive a petrogenetic model for late-orogenic crustal (S-type) granites from two localities within the Damara Belt (Namibia). The timing of magmatism is constrained by Pb– Pb garnet ages and Sm –Nd whole rock – garnet ages for these granites which have either igneous garnets that crystallized from the melt or have incorporated restitic garnets from the deeper crust. In the Damara belt, the nature of the lower crust is unknown and hence, the relationship between orogenic granitoid magmatism and events deep within the crust cannot be directly investigated. Granites can be formed by a variety of processes including differentiation of mantle-derived magmas with or without interaction with crustal rocks and direct melting of meta-sedimentary and meta-igneous precursors (e.g. White and Chappell, 1977). The question then arises whether there is any chemical input from the mantle or whether they formed entirely by intra-crustal melting. Consequently, if

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the relationship between granite and source region cannot be directly studied, the geochemical and isotopic data of granites must be used to place limits on possible compositions of the unexposed sources of the granites and thus on the nature of the terranes through which the plutons ascended.

2. Geological outline The Damara orogen of Namibia comprises a deeply eroded section of a Pan-African mobile belt that can be divided (see inset to Fig. 1) into several zones based on stratigraphy, metamorphic grade, structure and geochronology (e.g. Miller, 1983). The compositions of the intrusive rocks range from syenitic, dioritic, granodioritic, granitic to leucogranitic and they crop out over an area of approximately 75 000 km2 (Fig. 1). Older (1.2–2.0 Ga) pre-Damara basement gneisses are overlain by the Neoproterozoic Nosib Group which comprises the Etusis Formation

Fig. 1. Geological map showing the study area within the Central Zone of the Damara orogen, Namibia. Abbreviations in inset: KZ: Kaoko Zone, NP: Northern Platform, NZ: Northern Zone, nCZ: northern central Zone, sCZ: southern Central Zone, SZ: Southern Zone, SMZ: Southern Margin Zone. Isograd map (Hartmann et al., 1983) gives the distribution of regional metamorphic isograds within the southern and central Damara orogen. Isograds: (1) biotite-in; (2) garnet-in; (3) staurolite-in; (4) kyanite-in; (5) cordierite-in; (6) andalusite Ž--- sillimanite; (7) sillimanite-in according to staurolite-breakdown; (8) partial melting due to: muscovite+plagioclase +quartz+ H2O Ž--- melt + sillimanite; (9) K-feldspar + cordierite-in; (10) partial melting due to: biotite + K-feldspar + plagioclase+ quartz+ cordierite Ž--- melt +garnet.

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(sandstones and arkoses) and the Khan Formation (quartzites, mica schists and calc-silicate rocks). Above this follows the main Damaran metasedimentary succession, the Neoproterozoic Swakop Group. The Swakop Group consists of the Ro¨ ssing Formation (marble, quartzite, conglomerate and mica schists), the Chuos Formation (metamorphosed glaciogenic diamictite, banded iron-stone formation, marble and quartzite), the Karibib Formation (marble, mica schist and calcsilicate rocks) and the Kuiseb Formation (Al-rich metapelite, carbonate, calcsilicate rocks, quartzite and conglomerate). In the Central Zone, estimates for the peak metamorphic temperatures yielded 560– 650°C at 3 9 1 kbar for impure marbles from the Karibib and Kuiseb Formations (Puhan, 1983). Metamorphic conditions based on oxygen isotope fractionation in meta-sedimentary and meta-igneous rocks yielded 570– 650°C (Hoernes and Hoffer, 1979). More recently, Hoffbauer et al. (1990) reported metamorphic temperatures between 600 and 700°C for the Central Damara orogen based on the carbon isotope fractionation between calcite, dolomite and graphite. The metamorphic grade increases from east to west reaching highgrade conditions with local partial melting in the coastal area (Hartmann et al., 1983). These highgrade conditions culminated in low-pressure hightemperature granulite-facies conditions with temperature estimated in excess of 700°C at 5– 6 kbar (Masberg et al., 1992; Bu¨ hn et al. 1995; Jung et al., 1998a). To the south-east, there is a gradation into the Okahandja Lineament Zone that separates the Central Zone from the Southern Zone. The Southern Zone is characterized by a Barrovian-type regional metamorphism with a general increase in the metamorphic grade from south to north. The metamorphic grade ranges from low to medium pressures and reaches up to 8 kbar at maximum temperatures of 600°C. The age of this metamorphism is not well constrained but Rb –Sr and K– Ar biotite ages indicate that the rocks had cooled to 300– 350° C by 480 to 460 Ma (Miller, 1983). Large volumes of granitic rocks are absent even in the highest grade part of the Southern Zone.

U/Pb monazite ages and Sm/Nd garnet–whole rock ages (Jung et al., 2000b) indicate that the time span of high-grade metamorphism in the Central Zone ranges at least from  540 to  480 Ma with an age of  508 Ma for the (main) peak of regional metamorphism. These results are compatible with the view of Miller (1983) who suggested that metamorphism in the Central Zone started at  550 Ma and culminated at  500 Ma. The age of  508 Ma for peak metamorphism is also indicated by some previous geochronological studies on syntectonic granites using U/Pb monazite ages (Briqueu et al., 1980; Kukla et al., 1991). Syn- to post-collisional granites in the Central Damara orogen have Rb–Sr whole rock ages between  570 and  460 Ma (Haack et al., 1982; Kro¨ ner, 1982; Miller, 1983). Most are monzogranites and granodiorites. Beside rare occurrences of more mafic rock types (e.g. quartz diorites) several types of leucogranites can be distinguished, including some containing garnet. The origin of the Damaran granitoids has been the subject of several isotope studies but none has included combined trace element, Sr, Nd, Pb and O isotope analyses in combination with precise mineral ages. From Sr and O isotope analyses, Haack et al. (1982) concluded that most of the granites have crustal sources containing metasedimentary and metavolcanic rocks. McDermott et al. (1996) presented Sr and Nd isotope evidence indicating at least three different groups of granites. One group has elevated High Field Strength Elements (HFSE) content similar to A-type granites. These granites have initial mNd values as low as −17 and low initial 87Sr/86Sr ratios indicating old, LREE-enriched sources with low Rb/Sr ratios. More recently, Jung et al. (1998b) presented Sr, Nd, Pb, and O isotope evidence, indicating that some of these A-type granites may also contain a component from the lithospheric mantle. Another group, containing mafic quartz diorites and their derivatives, also originate from sources with low Rb/Sr ratios but have a much more restricted range in initial mNd values from 0 to − 8 (McDermott et al., 1996). Hawkesworth et al. (1981) suggested that some quartz diorites are melts from the lithospheric mantle, whereas

S. Jung et al. / Precambrian Research 110 (2001) 325–355

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Fig. 2. Generalized geological map showing the distribution of granites and country rocks within the Central Zone of the Damara orogen, Namibia.

Haack et al. (1982) suggested, based on elevated d 18 O values and initial 87Sr/86Sr ratios, that others represent mantle melts variably contaminated by crustal material. A third group of granites are clearly peraluminous crustal melts (McDermott et al., 1996; Jung et al., 1999) with a large variation of mNd values between −3 and − 18 and initial 87 Sr/86Sr ratios between 0.712 and 0.742. The granites with low mNd values (−10 to − 18) and high initial 87Sr/86Sr ratios are spatially restricted to basement outcrops (Jung et al., 1998b; McDermott et al., 1996) which crop out in the most westerly part of the orogen near the Atlantic coast. Other granites within the central part of the orogen (Fig. 2) intruded into metasedimentary rocks of the Proterozoic Kuiseb Formation and their derivation from unexposed basement or metasedimentary rocks is as yet unconstrained. According to the subdivision of gran-

ites from the Damara orogen proposed by Miller (1983), the granites studied here belong to the group of syn-tectonic medium- to fine-grained leucogranites which are thought to be  520 Ma old. For the locality of Oetmoed (Fig. 2), early S-type granites have U/Pb monazite ages of  530 Ma indicating that the intrusion is syn-collisional (Jung et al., 2000a). However, post-collisional A-type granites have U/Pb monazite and titanite ages of  490 Ma, indicating that some granite complexes consist of several distinct plutonic bodies (Jung et al., 1998b). The garnet-bearing granites from Oetmoed are slightly younger ( 480 Ma) and provide evidence for a third phase of plutonism at this locality. For the post-collisional S-type granites that are exposed at the Omaruru–Kalkfeld roadcut (10 km N% of Omaruru, Fig. 2), an intrusion age of 478 Ma has been postulated (Haack et al., 1982).

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3. Mineralogy and petrography The granites studied here are white to light-grey and homogeneous. Grain sizes are medium to coarse (2–5 mm). Textures are mainly equigranular but some samples contain K-feldspar megacrysts up to 10 mm long. The granites consist of 35–40% quartz, 25– 40% K-feldspar and 15–30% plagioclase. K-feldspar is anhedral untwinned orthoclase or rarely microcline. At Oetmoed, the granites contain 1 – 2% biotite and  1–3% garnet. Plagioclase is albite with An2 –An10 (Table 1). These granites contain a limited amount of apatite, monazite and zircon. Garnet composition is almandine-rich. Generally, spessartine and grossular components increase and the pyrope component decreases from core to rim (Fig. 3 and Table 1). Garnet forms small euhedral or slightly rounded grains with diameters of 1 – 5 mm and tiny inclusions of biotite and apatite needles. All samples contain minor

amounts of Fe-rich biotite (a= pale straw yellow, b= g = dark reddish brown) with XFe (XFe = molar Fe/Fe + Mg) between 0.64 and 0.72. At the outcrop 10 km north of Omaruru (from now on referred to as Omaruru) the granites are richer in biotite and garnet, with  5–10% biotite and  5% garnet. Plagioclase is oligoclase with An30 –An32 (Table 1). Individual garnet grains are large (5–10 mm) and irregularly shaped with abundant inclusions of quartz. The garnets differ from those at Oetmoed and have a core with a low grossular component and a rim enriched in grossular (Fig. 3). Elemental profiles are generally flat, but there is a tendency for the pyrope to decrease and the spessartine component to increase from core to rim (Table 1). Biotite has less FeO, TiO2 and Al2O3 than at Oetmoed, but is also Fe-rich with XFe (XFe = molar Fe/Fe + Mg) between 0.70 and 0.73. Accessory minerals include apatite; some occur as large (0.5–1 mm) rounded grains, zircon and monazite.

Fig. 3. Chemical composition of garnets from Oetmoed (26.5, 12.2, 89.52) and from Omaruru (S 81, S 84, S 85). C, core; R, rim.

Table 1 Mineral composition (garnet, biotite, plagioclase) from garnet-bearing granites (central Damara orogen, Namibia) 12.2

SiO2 TiO2 Al2O3 FeO MnO MgO Na2O K 2O H2O* Total X Fe

34.23 3.84 19.44 22.24 0.20 6.93 0.14 9.53 3.99 100.54 0.64

Grt

26.5

89.53

89.52

S 85

34.00 3.33 19.74 21.95 0.12 6.35 0.15 10.36 3.99 99.99 0.66

34.13 3.22 19.29 22.32 0.26 6.00 0.08 10.53 3.99 99.82 0.68

33.59 0.33 21.07 25.12 0.26 5.40 0.00 10.20 3.99 99.96 0.72

33.90 2.80 17.46 25.36 0.41 5.28 0.05 10.76 3.99 100.03 0.73

SiO2 Al2O3 FeO MnO MgO CaO

12.2 Core 36.10 20.15 33.20 7.51 2.40 0.33

Rim 36.33 20.47 32.98 7.99 1.91 0.41

26.5 Core 36.81 20.25 34.08 5.58 2.50 0.57

Rim 35.76 20.82 34.40 6.44 1.80 0.68

Total

99.69

100.09

99.79

Alm Spess Pyr Gross Fe/Fe+Mg

72.9 16.7 9.4 0.9 0.89

73.3 18.0 7.6 1.2 0.91

75.9 12.6 9.9 1.6 0.88

S 81

S 84

Plg

12.2

26.5

89.53

89.52

S 85

S 81

S 84

34.62 2.39 17.99 25.74 0.36 5.37 0.05 9.37 3.99 99.88 0.73

34.94 2.54 18.51 25.75 0.37 5.24 0.04 8.76 3.99 100.14 0.73

SiO2 Al2O3 CaO Na2O K 2O

65.24 21.78 2.04 10.06 0.20

63.84 22.77 3.29 9.73 0.25

63.50 23.20 3.83 9.67 0.24

62.46 22.73 4.07 9.11 0.29

60.59 23.89 6.63 7.61 0.29

59.37 24.95 6.77 7.92 0.17

59.09 25.21 7.10 7.62 0.22

Total

99.32

99.88

100.44

98.66

99.01

99.18

99.24

Ab An Or

88.9 10.0 1.2

83.1 15.5 1.4

81.0 17.7 1.3

78.9 19.5 1.7

66.4 32.0 1.7

67.3 31.8 1.0

65.5 33.6 1.2

89.53 Core 36.61 20.94 33.70 6.14 2.57 0.61

Rim 37.08 20.86 32.78 6.46 2.16 0.66

89.52 Core 36.44 21.08 33.17 5.86 2.64 0.64

Rim 36.41 21.08 32.89 6.30 2.58 0.67

S 85 Core 37.10 21.31 34.18 2.71 3.76 1.16

Mid 37.17 21.62 33.35 2.54 3.76 2.13

Rim 36.42 21.30 33.37 2.52 4.10 1.63

S 81 Core 36.87 21.82 33.91 3.60 2.80 1.46

Mid 36.44 21.85 33.56 3.60 2.77 2.17

Rim 36.69 21.88 33.64 3.78 2.60 2.01

S 84 Core 36.36 21.7 34.25 2.74 2.92 1.21

Mid 36.61 21.77 33.97 3.68 2.79 1.77

Rim 36.39 21.87 33.28 3.76 2.56 2.09

99.90

100.57

100.00

99.83

99.93

100.22

100.57

99.34

100.46

100.39

100.60

99.18

100.59

99.95

76.4 14.5 7.1 1.9 0.91

74.4 13.7 10.1 1.7 0.88

74.4 14.9 8.8 1.9 0.89

70.4 22.2 5.7 1.7 0.93

69.1 23.6 5.5 1.8 0.93

75.7 6.1 14.9 3.3 0.84

73.5 5.7 14.8 6.0 0.83

73.5 5.6 16.1 4.6 0.82

76.3 8.2 11.2 4.2 0.87

74.7 8.1 11.0 6.2 0.87

75.3 8.6 10.4 5.8 0.88

78.2 6.3 11.9 3.5 0.87

75.6 8.3 11.1 5.0 0.87

75.1 8.6 10.3 6.0 0.88

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Bt

H2O*, assumed to be 3.99 wt%.

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S. Jung et al. / Precambrian Research 110 (2001) 325–355

332

Table 2 Sm–Nd isotope results for garnets and whole rocks from garnet-bearing leucogranites from the Damara orogen (Namibia) 143

147

Sm

Nd

Age

0.513587 9 10 0.512468 910 0.5121559 13

0.593 0.238 0.153

2.17 8.02 12.28

2.25 21.91 48.55

497 9 7

Leached garnet Unleached garnet Whole rock

0.513905 908 0.512402 914 0.512213 911

0.682 0.231 0.158

2.10 11.46 11.79

1.86 33.75 45.28

493 96

Leached garnet Unleached garnet Unleached garnet Whole rock

0.514034 920 0.512625 909 0.512651 908 0.512214 911

0.731 0.287 0.298 0.160

1.24 6.66 4.48 15.54

1.02 14.53 9.64 58.68

488 9 7

Loc. Oetmoed 12.2 Leached garnet Unleached garnet Whole rock

0.517260 9 31 0.512844 934 0.512167 914

1.722 0.352 0.124

0.15 0.33 0.14

0.05 0.57 0.69

486 9 4

Leached garnet Unleached garnet Whole rock

0.5156669 28 0.5127679 25 0.512153 919

1.229 0.340 0.133

0.09 0.18 0.18

0.04 0.31 0.83

489 97

89.52

Leached garnet Unleached garnet Whole rock

0.516549 930 0.513000 957 0.512150 924

1.490 0.411 0.126

0.09 0.23 0.09

0.04 0.33 0.63

492 9 5

26.5

Leached garnet Leached garnet Whole rock

0.5139229 31 0.5138679 35 0.512221 926

0.681 0.677 0.144

0.17 0.17 0.15

0.15 0.15 0.63

484 9 13

Loc. Roadcut Omaruru-Kalkfeld S 81 Leached garnet Unleached garnet Whole rock S 84

S 85

89.53

Nd/144Nd

4. Geochronology For the garnet-bearing granites from Omaruru, Haack et al. (1982) suggested an emplacement age of 478 Ma. For the garnet-bearing granites from Oetmoed no age constraints were available. A Rb –Sr whole rock age for the granites from Omaruru gives an age of 498939 Ma, whereas the samples from Oetmoed define an age of 4909 8 Ma. U –Pb monazite data (samples S 84, S 85, S 86) are reported in Table 3. Some monazites are reversely discordant. The most widely invoked mechanism for producing reverse discordance involves the accumulation of 206Pb that is unsupported by 238U (Scha¨ rer, 1984). Scha¨ rer (1984) explained the excess 206Pb by the decay of 230Th, an intermediate daughter in the 238U decay chain, which was incorporated into the crystal during

Sm/144Nd

monazite crystallization. Because this mechanism does not affect the 207Pb/235U ratio, the 207Pb/235U age is considered to be the most reliable age (e.g. Parrish, 1990). The 207Pb/235U ages of the different monazite fractions indicate growth of monazite at 5119 2 and 5179 2 Ma (Fig. 4). Sm–Nd (samples 89.52, 89.53, 12.2, 26.5) and Pb –Pb isotope analyses (samples 12.2, 26.5, 89.52) have been performed on garnets from the granites from Oetmoed. Additionally, Sm–Nd isotope analyses on garnets (samples S 81, S 84, S 85) from Omaruru have also been performed. For Sm–Nd analyses, mineral separates of unleached and leached garnet were used in combination with the whole rock in order to establish two- or three-point isochrons. In some samples different size fractions of garnet were used for analyses. Details of the analytical techniques, the leaching technique and the mass spectrometric procedures

S. Jung et al. / Precambrian Research 110 (2001) 325–355

are given in the appendix. For the samples from Oetmoed, the Sm and Nd contents of the unleached garnet separates vary between 0.174 and 0.334 ppm for Sm and between 0.151 and 0.567 ppm for Nd. Concentrations in the leached samples are lower and vary between 0.089 and 0.165 ppm for Sm and 0.036 and 0.147 ppm for Nd. These values are substantially lower than Sm and Nd concentrations for garnet reported in the literature (Irving and Frey 1978; van Breemen and Hawkesworth, 1980; DuBray, 1988; Henson and Zhou, 1995; Maboko and Nakamura, 1995; Zhou and Henson, 1995; DeWolf et al., 1996; Moyes and Groenewald, 1996; Stowell and Goldberg, 1997), but are similar to the values obtained by ion microprobe studies (Hickmott et al., 1987; Harris et al., 1992; Sevigny, 1993). For Oetmoed, Sm –Nd ages calculated for garnet– whole rock pairs are 4689 4, 48997, 49295 and 484 9 13 Ma (Table 3). Pb–Pb isotope sytematics of some garnets are reported in Table 2 and, together with the feldspar data, indicate a similar age of 4839 10 Ma (Fig. 5). Unleached garnet fractions from the granites from Omaruru have low Nd isotope ratios coupled with high Sm and Nd concentrations. We suggest that garnet analyses with high Nd concentrations and low 147Sm/144Nd ratios indicate contamination with minute REE-rich inclusions. The observation that high Nd concentrations are cou-

Fig. 4. U/Pb concordia diagram showing monazite analyses from granite samples S 84, S 85 and S 86 from Omaruru. Data sources from Table 3.

333

Fig. 5. 207Pb/206Pb versus 204Pb/206Pb diagram of garnet separates from granite samples 12.2, 26.5 and 89.52 from Oetmoed. Data sources from Table 3.

pled with low Nd isotope compositions supports this view (Fig. 6). The unleached garnets fall close to or on the isochron defined by the whole rock samples and the leached garnet separates (Fig. 7) suggesting that either the inclusions are synchronous with the garnet growth or that the Sm– Nd isotope system of the inclusions has been reset and equilibrated during garnet growth. For the

Fig. 6. Plot of 143Nd/144Nd versus Nd concentration for garnet analyses from this study (open dots: Oetmoed samples; open diamonds: granite samples from Omaruru) and from the literature (Zhou and Henson, 1995; Henson and Zhou, 1995; Maboko and Nakamura, 1995; Moyes and Groenewald, 1996; Stowell and Goldberg, 1997). Nd concentrations above 40 ppm have been omitted for clarity. Note the increase in 143 Nd/144Nd isotope ratio with decreasing Nd concentration in garnets indicating that the bulk of the Nd isotope composition is governed by REE-rich inclusions.

334

Table 3 U–Pb monazite data and Pb–Pb garnet data from Pan-African granites (Central Damara orogen, Namibia) 10 km N’Omaruru

10 km N’Omaruru

10 km N’Omaruru

10 km N’Omaruru

10 km N’Omaruru

10 km N’Omaruru

Oetmoed

Oetmoed

Oetmoed

Sample 84.1 Mineral Mnz U (ppm) 4691 Pb (ppm) 2214 206 1723 Pb/204Pb 208 Pb/206Pb 5.4842 207 Pb/206Pb 0.057389 Error (abs.) 0.000039 207 Pb/235U 0.6548 ratio Error (abs.) 0.0021 206 Pb/238U 0.08276 ratio Error (abs.) 0.00035 206 513 Pb/238U age (m.y.) 207 Pb/235U age 511 (m.y.) 207 Pb/206Pb age 507 (m.y.)

84.2 Mnz 4349 1686 1688 4.2545 0.057584 0.000034 0.6638

85.1 Mnz 4284 2085 2003 5.6119 0.057627 0.000035 0.6659

85.2 Mnz 4366 2000 1789 5.2623 0.057258 0.000035 0.6565

86.1 Mnz 4168 1686 1653 4.5650 0.057446 0.000033 0.6529

86.2 Mnz 3712 1697 1507 5.2199 0.057685 0.000120 0.6634

12.2 Grt n.d 0.073 93.6 0.0120 0.056669 0.00159 n.d

26.5 Grt n.d 0.057 131.1 0.0022 0.057192 0.00024 n.d

89.52 Grt n.d 0.062 137.1 0.0174 0.057192 0.00076 n.d

0.0012 0.08361

0.0013 0.08381

0.0013 0.08316

0.0011 0.08243

0.0032 0.08341

n.d n.d

n.d n.d

n.d n.d

0.00028 518

0.00029 519

0.00029 515

0.00028 511

0.00044 516

n.d n.d

n.d n.d

n.d n.d

517

518

512

510

517

n.d

n.d

n.d

514

516

501

509

518

479

499

508

S. Jung et al. / Precambrian Research 110 (2001) 325–355

Locality

Table 4 Whole-rock major- (in wt%) and trace-element (in ppm) composition of garnet-bearing granites from outcrop 10 km N’Omaruru (samples S 81–S 86) and Oetmoed (samples 89.52, 26.5, 9.1, 13, 89.53, 12.2) from the Damara orogen, Namibia Sample

S85

S82

S81

S80

S86

72.80 0.09 14.01 0.10 1.06 0.06 0.19 1.98 2.26 6.22 0.04 1.10

73.29 0.07 13.88 0.31 0.87 0.06 0.18 1.88 2.55 4.93 0.02 1.16

73.46 0.08 13.60 0.10 1.13 0.07 0.19 1.37 2.05 6.51 0.03 0.99

73.55 0.07 13.57 0.36 1.01 0.08 0.22 1.60 2.21 5.85 0.03 0.95

73.72 0.08 13.61 0.27 0.91 0.07 0.16 1.38 2.02 6.53 0.03 1.20

Total

99.91

99.20

99.58

99.50

99.98

Sc V Cr Co Ni Zn Ga Rb Sr Y Zr Nb Ba Pb Th U ASI K2O/Na2O Rb/Sr Rb/Ba Sr/Ba

13 4 n.d. 2 n.d. 12 22 169 54 126 88 8 108 42 55 10

5 5 n.d. 24 n.d. 14 23 216 48 96 89 9 149 86 39 5

7 2 n.d. 15 n.d. 18 20 198 54 127 101 7 146 79 41 4

9 5 n.d. 2 n.d. 10 17 217 53 111 87 7 152 85 40 4

26.5

9.1

13

89.53

12.2

74.09 0.07 13.62 0.18 0.98 0.07 0.17 1.67 2.26 5.60 0.03 0.88

73.10 0.01 15.22 0.10 0.64 0.07 0.20 0.89 3.10 5.73 0.19 0.47

73.36 0.02 14.85 0.01 0.53 0.04 0.13 0.88 3.11 5.73 0.23 0.42

73.34 0.01 14.41 0.01 0.28 0.01 0.03 0.49 2.34 8.81 0.26 0.33

73.13 0.04 14.85 0.01 0.34 0.01 0.03 0.57 2.32 8.97 0.24 0.19

73.97 0.01 14.89 0.20 1.36 0.22 0.16 0.60 3.18 4.69 0.22 0.39

75.16 0.01 14.40 0.01 0.58 0.08 0.08 0.33 2.65 5.55 0.17 0.53

99.62

99.72

99.31

100.32

100.70

99.89

99.55

2 25 3 3 2 13 11 95 97 10 25 10 203 39 l.l.d. l.l.d.

2 5 10 3 3 10 14 130 106 3 26 2 245 55 l.l.d. l.l.d.

2 3 10 3 7 114 18 270 60 5 3 3 278 70 l.l.d. l.l.d.

2 3 3 4 4 1 16 220 160 4 3 2 1075 188 l.l.d. l.l.d.

9 3 35 10 20 10 12 90 106 7 30 5 290 50 l.l.d. l.l.d.

2 3 9 5 3 4 14 154 17 11 16 3 85 56 l.l.d. l.l.d.

12 6 n.d. 21 n.d. 10 22 191 53 122 113 8 118 81 39 6

1.00 2.75 3.83 1.48 0.39

1.07 1.93 3.13 1.56 0.50

1.05 3.18 4.50 1.45 0.32

1.05 2.65 3.67 1.36 0.37

1.05 3.23 4.09 1.43 0.35

1.06 2.48 3.60 1.62 0.45

1.18 1.85 0.98 0.47 0.48

1.15 1.84 1.23 0.53 0.43

1.01 3.76 4.50 0.97 0.22

1.02 3.87 1.38 0.20 0.15

1.31 1.47 0.85 0.31 0.37

1.31 2.09 9.06 1.81 0.20

69.30 148.00 52.40

51.30 103.80 38.20

61.00 122.10 46.40

39.80 83.10 29.60

53.00 113.70 41.60

49.90 103.00 37.00

1.70 3.70 1.20

1.70 3.90 0.90

2.80 4.80 1.10

0.90 1.80 0.80

3.00 3.80 1.10

1.30 2.90 1.10

335

La Ce Nd

9 3 n.d. 17 n.d. 13 20 207 54 130 127 6 140 86 54 11

89.52

S. Jung et al. / Precambrian Research 110 (2001) 325–355

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI

S84

336

Sample Sm Eu Gd Dy Er Yb Lu REE total LaN/SmN GdN/YbN LaN/YbN Eu/Eu*

S85

S82

S81

S80

S86

S84

89.52

26.5

9.1

13

89.53

12.2

15.10 0.59 16.14 19.13 11.50 12.60 1.65

11.32 0.42 12.27 17.37 12.60 12.20 2.18

13.60 0.49 12.44 14.45 8.80 9.20 1.70

9.58 0.53 9.60 13.72 10.60 10.70 1.80

12.74 0.45 11.86 14.77 9.90 10.50 1.57

12.66 0.36 12.62 17.39 11.10 12.00 2.21

0.20 0.63 0.22 0.32 0.25 0.40 0.10

0.18 0.52 0.30 0.40 0.30 0.40 0.06

0.26 0.48 0.30 0.40 0.40 0.70 0.12

0.21 0.95 0.20 0.30 0.20 0.30 0.08

0.20 0.40 0.22 0.50 0.40 0.50 0.10

0.27 0.55 0.20 0.40 0.40 0.66 0.14

346 2.89 1.03 3.71 0.11

262 2.85 0.81 2.83 0.10

290 2.82 1.09 4.47 0.12

209 2.61 0.72 2.51 0.16

270 2.62 0.91 3.40 0.11

258 2.48 0.85 2.80 0.09

9 5.35 0.44 2.87 4.60

9 5.94 0.61 2.87 3.34

11 6.77 0.35 2.70 2.82

6 2.70 0.54 2.02 7.14

10 9.44 0.36 4.05 2.92

8 3.03 0.24 1.33 3.84

LOI, loss on ignition; ASI, alumina saturation index (molar Al2O3/Na2O+K2O+CaO); Eu/Eu*, Eu-anomaly; n.d., not determined; l.l.d., lower limit of detection.

S. Jung et al. / Precambrian Research 110 (2001) 325–355

Table 4 (Continued)

S. Jung et al. / Precambrian Research 110 (2001) 325–355

337

samples from Omaruru, Sm– Nd garnet– whole rock analyses yielded ages of 49797, 493 9 6 and 4889 7 Ma (Table 3).

5. Geochemistry The garnet-bearing granites have similarly high SiO2 at both localities (72.8– 74.1 wt% at Omaruru and 73.1– 75.2 wt% at Oetmoed) and, given the small variation in the other major elements, a direct distinction is precluded. Abundances of CaO, TiO2, Fe2O3, Zr and Y are lower but abundances of Al2O3 and P2O5 are higher in the granites from Oetmoed (Table 4). Both suites have similar high K2O/Na2O ratios, ranging from 1.9 –3.2 at Omaruru and from 1.5–3.9 at Oetmoed. Granites from Omaruru are metaluminous to slightly peraluminous with A.S.I. values ranging from 1.00–1.06, whereas the granites from Oetmoed are more peraluminous with A.S.I values between 1.01 and 1.31. These features are shown in the Al – (K +Na + 2Ca) versus Fe+ Mg +Ti diagram (Fig. 8, Debon and LeFort, 1983). Granites from Omaruru plot close to the intersection of primitive S-type granites and fractionated I-type granites whereas the granites from Oetmoed are peraluminous and poor in Mg, Fe and Ti. The Rb/Sr and Rb/Ba ratios are high and Sr/Ba ratios are low, typical for crustal granites (Harris and Inger, 1992) and the ratios overlap within the two suites. Granites from Omaruru have a restricted range in these ratios (Rb/Sr: 3.13 –4.50, Rb/Ba: 1.36– 1.62, Sr/Ba: 0.32–0.50) whereas the granites from Oetmoed show a pronounced spread in these values (Rb/Sr: 0.85– 9.06, Rb/Ba: 0.02–1.81, Sr/Ba: 0.06– 0.48). Granites from each locality have distinctive REE patterns. Granites from Omaruru have high total REE abundances (209– 346 ppm) and a flat chondrite-normalized pattern (Fig. 9) with low Lan /Ybn ratios (Lan /Ybn : 2.5 – 4.5) and a pronounced negative Eu anomaly (Eu/Eu: 0.09 – 0.16). Granites from Oetmoed have low REE abundances (6– 11 ppm), but also low Lan /Ybn ratios (Lan /Ybn : 1.3– 4.1) and a pronounced positive Eu anomaly (Eu/Eu: 2.82 –7.14; Fig. 9).

Fig. 7. 143Nd/144Nd versus 147Sm/144Nd diagram of the analyzed garnets from Damaran granites from (a) Omaruru and (b) Oetmoed. Note that the unleached garnet fractions plot on or close to the garnet-whole rock isochron indicating equilibrium between monazite-controlled garnet fractions (low 147 Sm/144Nd ratios) and monazite-free garnet fractions (high 147 Sm/144Nd ratios).

Compared with the average unfractionated S-type granite from the Lachlan Fold Belt (Chappell and White, 1992), the granites from Omaruru show LREE enrichment, whereas the granites from Oetmoed show relative depletion in LREE.

6. Isotopic composition

6.1. Nd, Sr and Pb isotopes The results of the Sr, Nd, Pb and O isotope analyses are reported in Table 5. The garnet-bear-

338

S. Jung et al. / Precambrian Research 110 (2001) 325–355

ing granites at Omaruru have Nd isotopic composition with initial mNd values ranging from − 5.9 to − 6.7 and initial 87Sr/86Sr (488 Ma) ratios ranging from 0.71820 to 0.71911. Garnet-bearing granites from Oetmoed have Nd isotopic composition with initial mNd values from − 4.7 to −5.9 and initial 87 Sr/86Sr (483 Ma) ratios between 0.71647 and 0.71906 (Fig. 10). A feature of the granites is their uniform initial 87Sr/86Sr isotope ratios which contrasts other S-type granites from the Damara orogen (e.g. Jung et al., 1998b) and elsewhere (e.g. Villasecca et al., 1998). The 206Pb/204Pb and 207Pb/204Pb ratios for the samples from Omaruru range from 18.536 to 18.881 and 15.632 to 15.645, respectively. At Oetmoed, Pb isotopes are less radiogenic than at

Fig. 9. Chondrite-normalized rare earth element plots for granites from samples from Omaruru and Oetmoed. Normalization factors according to Boynton (1984).

Omaruru with 206Pb/204Pb and 207Pb/204Pb isotope ratios from 18.212 to 18.439 and 15.615 to 15.630, respectively. The 208Pb/204Pb ratios are also higher

Fig. 8. Al-(K +Na+2Ca) versus Fe + Mg+ Ti diagram (Debon and LeFort, 1983) showing distribution of the granites from Oetmoed and from Omaruru relative to S-type granites, A-type granites and I-type granites from the Damara orogen (McDermott et al., 1996; Jung et al. 1998b, 1999; unpublished data).

Fig. 10. Initial mNd versus initial 87Sr/86Sr diagram. Fields represent isotope analyses of peraluminous S-type granites, basement-derived granites and metasedimentary rocks from the Kuiseb and Khan Formations, Central Zone of the Damara orogen (Jung and Mezger, unpublished data).

Table 5 Rb–Sr, Sm–Nd, Pb and oxygen isotope data for garnet-bearing granites from the Proterozoic Damara orogen (Namibia) (see appendix for analytical procedures) 87

Sr/86Sr (meas.)

87 Sr/86Sr (init.)

S 80

0.788778 909 0.779246 910 0.787436 909 0.790183 910 0.795899 909 0.796687 912 0.741963 916 0.741794 918 0.806722 912 0.743144 919 0.736329 922 0.919485 917

S 81 S 82 S 84 S 85 S 86 89.52 26.5 9.1 13 89.53 12.2

87

Rb/86Sr

Rb (ppm)

0.718982

9.99

175.76

51.27

0.718425

8.71

156.61

52.38

0.719110

9.79

175.65

52.33

0.718199

10.31

192.33

54.41

0.719057

11.00

199.48

52.89

0.718713

11.17

201.07

52.54

0.718047

3.41

119.80

101.90

0.717810

3.45

128.40

108.00

0.718839

12.58

266.81

61.98

0.716468

3.81

214.48

163.54

0.718472

2.56

97.70

110.60

0.719059

28.94

163.30

17.30

Sr (ppm)

143 Nd/144N 143Nd/144N 147Sm/144N TDM d (meas.) d (init.) d

o Nd (init.)

o Nd (today)

Sm (ppm)

Nd (ppm)

206

0.512187 910 0.512155 913 0.512194 907 0.512213 9 11 0.512214 911 0.512181 910 0.512150 924 0.512207 926 0.512134 912 0.512160 911 0.512153 919 0.512167 914

Pb/204Pb

207

Pb/204Pb

Pb/204Pb d

208

18

O

0.511689

0.155

1.55

−6.2

−8.8

12.63

49.22

18.790

15.639

37.919

15.15

0.511664

0.153

1.50

−6.7

−9.4

12.28

48.55

18.619

15.632

37.885

14.86

0.511697

0.155

1.54

−6.0

−8.7

10.23

39.99

18.536

15.643

38.022

14.96

0.511707

0.158

1.53

−5.9

−8.3

11.79

45.28

18.785

15.636

38.062

15.04

0.511696

0.160

1.54

−6.1

−8.4

15.54

58.68

18.705

15.645

38.025

15.02

0.511684

0.155

1.56

−6.3

−8.9

11.76

45.99

18.881

15.645

37.946

15.10

0.511697

0.126

1.47

−5.1

−9.5

0.09

0.63

18.212

15.615

37.740

14.20

0.511675

0.144

1.47

−5.0

−8.4

0.16

0.63

18.244

15.618

37.775

14.70

0.511777

0.113

1.37

−4.7

−9.8

0.06

0.34

18.195

15.616

37.773

15.10

0.511717

0.141

1.53

−5.9

−9.3

0.07

0.30

18.210

15.631

37.816

15.10

0.511657

0.133

1.50

−5.5

−9.8

0.18

0.83

18.232

15.627

37.839

13.80

0.511719

0.124

1.43

−4.7

−9.2

0.14

0.69

18.439

15.626

37.854

13.98

S. Jung et al. / Precambrian Research 110 (2001) 325–355

Sample

Uncertainties for the 87Sr/86Sr and 143Nd/144Nd isotope ratios are 2 | (mean) errors in the last two digits. mNd values at the time as obtained from the Sm–Nd garnet-whole rock geochronology (see Table 2) are calculated relative to CHUR with present day values of 143Nd/144Nd=0.512638 and 147Sm/144Nd=0.1966 (Jacobsen and Wasserburg, 1980). Nd model ages calculated with a depleted mantle reservoir and present-day values of 144 Nd/143Nd=0.513144 and 147Sm/144Nd=0.222 (Michard et al., 1985).

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S. Jung et al. / Precambrian Research 110 (2001) 325–355

Pb ratios and the initial mNd values (Fig. 12). There is no obvious correlation between the Sr isotopic composition and the Pb isotopic composition (Fig. 13). The lack of correlation between Nd, Sr and Pb isotopes indicates that the isotope systems are decoupled from each other. This may be caused by different processes controlling the various isotope systems or the end-members which controlled the isotope variations, being themselves variable. However, the range of Pb isotope values of the garnet-bearing granites from Oetmoed is consistent with the correlation given by the older S-type granites and metasedimentary country rocks from Oetmoed, but the granites from Omaruru plot off this trend.

204

6.2. Oxygen isotopes

Fig. 11. (a) Plot of 207Pb/204Pb and (b) 208Pb/204Pb 6s 206Pb/ 204 Pb isotope ratios of leached K-feldspar from the granites. The curves represent the upper part of the Stacey and Kramers (1975) U – Pb and Th– Pb growth curve. Tick marks represent 250 Ma intervals. Fields represent isotope analyses of peraluminous S-type granites and metasedimentary rocks from Oetmoed (Jung and Mezger, unpublished data).

in samples from Omaruru (208Pb/204Pb: 37.885– 38.062) than in samples from Oetmoed (208Pb/ 204 Pb: 37.740–37.854). All samples plot above the U/Pb but below the Th/Pb growth curve (Fig. 11a and b) of Stacey and Kramers (1975). There is no apparent correlation between 206Pb/204Pb and 207 Pb/204Pb ratios and initial mNd values, except for the garnet-bearing granites from Omaruru, which show a positive correlation between the 208Pb/

Garnet-bearing granites from Omaruru have whole rock d 18O values between 14.86 and 15.15 ‰ (relative to SMOW), whereas garnet-bearing granites from Oetmoed have d 18O values ranging from 13.80 to 15.10 ‰ (Table 4). Generally, both S-type suites define different d18O-initial mNd-initial 87 Sr/86Sr relationships (Fig. 14). Samples from Oetmoed show a negative correlation between d 18 O values and the initial 87Sr/86Sr isotopic composition and a positive correlation between initial mNd values and the d 18O values. Granites from Omaruru show no visible correlation between d 18 O values and the initial 87Sr/86Sr and the 143Nd/ 144 Nd isotopic composition. For the granites from Oetmoed, the correlation between 207Pb/204Pb and 208 Pb/204Pb isotope ratios and d 18O values is similar to the fields occupied by the earlier S-type granites and metasedimentary country rocks from the Oetmoed locality (Fig. 15). Granites from Omaruru plot off this trend. The negative correlation of O isotope values with initial 87Sr/86Sr ratios is a common feature of granites from the Damara orogen (Haack et al., 1982; Jung et al., 1998b), but is only rarely observed elsewhere (Sheppard, 1986; Taylor, 1986). However, positive correlations between O isotope values and initial Sr isotope compositions have also been reported for some S-type granites from the Damara orogen (Haack et al., 1982; Jung et al., 1998b) and elsewhere (Taylor, 1986).

S. Jung et al. / Precambrian Research 110 (2001) 325–355

In summary, the geochemical characteristics (enrichment of Ba and Rb relative to Sr, high K2O, low CaO) of the garnet-bearing granites indicate a crustal source rock. Low Na2O ( B3.5 wt.%) and CaO (B2.0 wt.%) but high K2O ( \ 5.0 wt.%) contents, the LILE contents (Rb: 90– 270 ppm, Sr: 17–106 ppm, Ba: 85 – 1075 ppm), Rb/Ba \0.25, high 87Sr/86Sr and d18O values indicate a pelitic parent (Haack et al., 1982; Miller, 1985; Williamson et al., 1997). Moderate negative mNd values and evolved Pb isotope compositions also argue for a crustal source rock. Consequently, these granites can be classified as moderately aluminous to peraluminous S-type granites. 7. Discussion

7.1. Constraints on the le6el of segregation and emplacement Thermobarometric constraints for the segrega-

341

tion of granitic magmas from their source regions are semi-quantitative at best, because the rock system, and hence the minerals (e.g. garnet, biotite, plagioclase) that can be used for thermobarometric calculations, can undergo considerable modification as the solid– liquid mush moves through the crust. Nevertheless, if the calculated pressures and temperatures are different from those obtained for the peak of regional metamorphism, the estimates indicate minimum pressure– temperature conditions for the segregation of felsic melts from their crustal sources. The Oetmoed samples give temperature estimates that range from 720 to 820oC using the calibration of Perchuk and Lavrent’eva (1983). Model pressures are between 5.9 and 6.2 kbar (Table 6) according to the calibration of Powell and Holland (1988). The garnets show high Mn contents, which increase from core to rim, whereas the Mg contents are low and decrease from core to rim. Such elemental profiles are

Fig. 12. Plot of (a) 206Pb/204Pb, (b) 207Pb/204Pb and (c) 208Pb/204Pb isotope ratios versus initial mNd values. Fields represent isotope analyses of peraluminous S-type granites and metasedimentary rocks from Oetmoed (Jung and Mezger, unpublished data).

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S. Jung et al. / Precambrian Research 110 (2001) 325–355

Fig. 13. Plot of (a) 206Pb/204Pb, (b) 207Pb/204Pb and (c) 208Pb/204Pb isotope ratios versus initial 87Sr/86Sr ratios. Fields represent isotope analyses of peraluminous S-type granites and metasedimentary rocks from Oetmoed (Jung and Mezger, unpublished data).

different from those observed in garnets that have undergone prograde growth zoning showing bellshaped Mn profiles and an increase in Mg from core to rim (e.g. Tracy, 1982). Instead, such profiles (e.g. increase in Mn and decrease in Mg from core to rim, flat Ca distribution) are observed in high-grade metamorphic garnets that have undergone retrograde zoning and homogenization (Yardley, 1977; Tracy, 1982). However, the high Mn contents and low Mg contents together with the zoning pattern and the euhedral shape of the garnets argue for a magmatic origin of these garnets. The late-orogenic setting of the granites further suggests that the elemental profiles of the garnets are not modified by high-temperature diffusion processes. Pressure– temperature (P – T) estimates obtained from the adjacent high-grade migmatites at Oetmoed are  5 kbar at  700°C (Jung et al., 2000b), which are lower than the values obtained from the garnet-bearing granites. Therefore, the P –T estimates obtained on the

Oetmoed samples probably represent the closure conditions of Fe–Mg –Ca in garnet, biotite and plagioclase during ascent of the magma. The samples from Omaruru give estimated pressures that are significantly higher ( 9–10 kbar) and indicate temperatures in excess of  800oC (Table 6). The garnet elemental profiles are flat suggesting that they became homogenized during volume diffusion. However, there is a tendency for Mg to decrease from core to rim. The Ca distribution is unusual and suggests a twostage process. Low Ca contents are confined to the core of the garnets, whereas the rims are notably enriched in Ca. This feature suggests that the garnets grew within or equilibrated with the granitic magma. Consequently, the pressure and temperature estimates from the granites at Omaruru are interpreted as the maximum conditions of formation of the felsic melts in the crust. There is an apparent positive correlation between the P–T estimates and the ages for the

S. Jung et al. / Precambrian Research 110 (2001) 325–355

samples from Oetmoed (Fig. 16). The data indicate that the post-collisional felsic plutonism started immediately after the peak of metamorphism and lasted for up to 20 million years. Post-collisional A-type granites from the same outcrop record similar ages and similar high temperatures and pressures of equilibration (Jung et al., 1998b), but are mineralogically, chemically and isotopically unrelated to the granites. The occurrence of A-type granites probably marks the onset of uplift after continental collision. This suggestion is consistent with the isotopic composi-

Fig. 14. Plot of d18O versus (a) initial 87Sr/86Sr ratios and (b) initial mNd values. Fields represent data for peraluminous Stype granites and metasedimentary rocks from Oetmoed (Jung et al., 1999; Sr and Nd isotope, unpublished data).

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tion of the A-type granites which contain a significant contribution from the subcontinental lithospheric mantle (Jung et al., 1998b). Therefore, uplift of the lithosphere initiated a second melting event within the metasedimentary crust leading to the generation of garnet-bearing felsic melts with S-type signatures.

7.2. Petrogenetic processes Geochemical studies on the petrology of leucocratic granites are still limited (e.g. Kistler et al., 1981; Vidal et al., 1982; Bernard-Griffiths et al., 1985; White et al., 1986; D’Lemos et al., 1992; Williamson et al., 1996). In general, the geochemical evolution trends depicted by many leucogranitic suites could result from mixing of two different magma batches, from partial melting processes including the restite unmixing process (e.g. White and Chappell, 1977) and from fractional crystallization processes with (assimilationfractional crystallization) or without assimilation (fractional crystallization). For the Oetmoed samples, mixing of melts seems to be unlikely although three major granite types were observed (Jung et al., 1999, 2000a). Mixing of felsic melts requires the presence of an extremely SiO2-rich end-member, but such endmembers have not been observed. Mixing of melts cannot be ruled out for the samples from Omaruru. However, on the outcrop scale no distinctly more mafic or felsic rock types have been observed, suggesting that the intrusive body represents a rather homogeneous batch of magma. The most obvious geochemical features are the contrasting REE compositions of the two suites. The samples from Oetmoed are depleted in LREE and HREE (and CaO, Fetotal, TiO2, Y, Zr) and we use the geochemical compositions of these granites to place constraints on fractional crystallization processes. The granites from Omaruru have enriched REE patterns and their major- and trace element composition are best explained by partial melting processes with only limited fractional crystallization. Early syn-collisional S-type granites at Oetmoed have U/Pb monazite ages of  530 Ma and post-collisional A-type granites have U/Pb mon-

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Fig. 15. Plot of d 18O versus (a) 206Pb/204Pb, (b) 207Pb/204Pb and (c) 208Pb/204Pb isotope ratios. Fields represent data for peraluminous S-type granites and metasedimentary rocks from Oetmoed (Jung et al. unpublished).

azite and titanite ages of 490 Ma (Jung et al., 1998b, 2000a). The garnet-bearing granites could be, in principle, derived from the early S-type granites by fractional crystallization processes; however, the large time interval of  40 Ma makes this possibility unlikely. A derivation from the roughly coeval A-type granites is also unlikely based on geochemical and isotopic evidence. The main evolutionary trend for the early syn-collisional S-type granites from Oetmoed, especially the linear variation in CaO, Fetotal, and TiO2 as well as the decreasing REE content with increasing SiO2 content, indicates igneous fractionation involving plagioclase, biotite, Ti– Fe oxides and REE-rich accessory phases. Fractionation of plagioclase is also indicated by the positive correlation between Sr contents and the size of the negative Eu anomaly in which Sr decreases from 100 to 25 ppm while the size of the negative Eu anomaly increases from Eu/Eu* =0.66 to Eu/ Eu* =0.14 (Jung et al., 1999). Such trends with

decreasing LREE abundances and development of strong negative Eu anomalies are typical for leucocratic granites. Generally, leucocratic granites worldwide have low total REE contents with LREE concentrations below 100× chondritic and a flat LREE/HREE pattern with HREE abundances below 5–10× chondritic. Additionally, they have strong negative Eu anomalies with Eu/ Eu* B 0.5 (Kistler et al., 1981; Vidal et al., 1982; Bernard-Griffiths et al., 1985; Downes and Duthou, 1988; Williamson et al., 1996). These REE patterns have been explained by fractional crystallization of accessory phases and plagioclase in the presence of a fluid phase (Vidal et al., 1982; Williamson et al., 1996). The major-and trace element characteristics of the garnet-bearing granites (high K2O, Ba, Pb and Rb contents, low Zr contents, very low total REE and a pronounced positive Eu anomaly) suggest that they could represent strongly fractionated feldspar-rich granitic rocks (e.g. Miller and Mit-

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tlefehldt, 1982). The presence of a fluid phase during differentiation is unlikely for the garnetbearing granites from Oetmoed, because they do not have either primary magmatic muscovite nor tourmaline, which would be expected if the magmas were nearly fluid-saturated at emplacement. Instead, they crystallized euhedral magmatic garnet indicating that the magma was relatively hot and fluid-undersaturated (Clemens and Wall, 1988; Vernon and Collins, 1988). Differentiation involved mainly monazite and zircon as indicated by the low LREE and HREE abundances. Accumulation of magmatic garnet may account for the increase in HREE abundances. The positive Eu anomaly may be explained by accumulation of K-feldspar under high-temperature conditions in which K-feldspar incorporates much more Eu than at low temperatures (Bea et al., 1994). Granitic rocks with positive Eu anomalies and low amounts of LREE and HREE have been recorded from many granite terranes and are commonly associated with migmatitic rocks. Some of these rock types have been interpreted as disequilibrium partial melts (migmatite leucosomes) originating by water-undersaturated partial melting of pelitic metasedimentary rocks. The Oetmoed migmatite– granite complex is no exception and such leucosomes have also been described from this terrane (Jung et al., 1999). However, the granites studied here have substan-

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tially lower CaO and Na2O, but higher K2O contents than the migmatite leucosomes from the same complex. Rb/Sr, Rb/Ba ratios and d18O values are higher and Sr/Ba ratios are lower than those of the migmatite leucosomes. Furthermore, (La/Sm)N and (Gd/Lu)N ratios and the size of the positive Eu anomaly are also distinct from those of the migmatite leucosomes (Jung et al., 1999). Geochronological data indicate formation of the migmatites around  510 Ma (Jung et al., 2000b) whereas the granites studied here are 20 Ma younger. In conclusion, there is no evidence supporting a connection between the granites with positive Eu anomalies and the migmatite leucosomes. The granites from Omaruru show only limited variation in their geochemical composition, which would not be expected if they were part of a fractionation trend. It is therefore suggested that this granite body originated by partial melting in the deeper crust. Their trace element composition (Rb/Ba: 1.4–1.6; Rb/Sr: 3.1–4.5; Sr/Ba: 0.3–0.5) suggest that they may be derived from partial melting of metapelitic rocks (Rb/Ba \ 0.2; Miller, 1985) under H2O-undersaturated conditions through biotite-dehydration melting (Rb/Sr: 2 –6, Sr/Ba: 0.2–0.7; Harris and Inger, 1992). However, CaO contents are higher (CaO: 1.4–2.0 wt%) than those predicted by partial melting of pelites (CaOB1.2 wt%; Miller, 1985) and are also higher

Table 6 Pressure–temperature estimates from garnet–biotite, garnet–plagioclase geo-thermobarometry, and temperature estimates using empirical equations for element saturation for Zr, P, and LREE: errors on the grt–bt temperatures and grt–plg pressures are believed to be950°C (Kleemann and Reinhardt, 1994) and 9 1 kbar (Essene, 1989) Sample

12.2

26.5

89.52

89.53

S 85

S 81

S 84

A B C D E F G H

716 680 5.3 6.2 620 570 750 560

742 701 6.0 6.2 650 580 910 565

815 859 6.7 6.2 650 580 865 569

761 748 6.1 5.9 660 590 780 580

824 1043 10.7 9.9 775 830 850 828

800 921 10.0 9.3 745 815 790 815

800 895 9.7 8.8 765 800 780 800

A, Perchuk and Lavrent’eva (1983) garnet–biotite geothermometer; B, Thompson (1976) garnet–biotite geothermometer; C, Ghent (1976) garnet–plagioclase geobarometer; D, Powell and Holland (1988) garnet–plagioclase geobarometer; E, Watson and Harrison (1983) saturation temperature for Zr; F, Rapp et al. (1987) saturation temperature for LREE; G, Bea et al. (1992) saturation temperature for P2O5; H, Montel (1993) saturation temperature for LREE at 4 wt% H2O.

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ture estimates on the origin of these granites using trace-element saturation equations yield temperatures of 775°C for Zr, 815°C for P2O5 and 830°C for LREE (Table 6). The low MgO (0.16–0.22 wt%) and FeOtotal (1.14–1.31 wt%) contents indicate that they might represent unfractionated melts because at T= 800°C and X H2O 0.5–1.0, the solubilities of MgO and FeO in peraluminous granitic melts are 0.20–0.30 wt% and 1.20–1.30 wt%, respectively (Clemens and Wall, 1981; Puziewicz and Johannes, 1988; Holtz and Johannes, 1991). It is therefore concluded that the melting conditions for the garnet-bearing granites at Omaruru were  800°C. These temperature estimates are in good agreement with the P–T estimates using garnet-biotite-plagioclase equilibria which indicate temperatures in excess of 800°C at pressures of 9–10 kbar. The conclusion is also supported by experimental results (Vielzeuf and Montel, 1994), which indicate that the P/T domains between 800°C at 2 kbar and 900oC at 10 kbar are the regions where most crustal metasedimentary rocks can melt under fluid-absent conditions.

7.3. Constraints on the sources for Damaran late-orogenic granites Fig. 16. Plot of (a) temperature and (b) pressure versus age for the garnet-bearing granites. Temperature and pressure estimates from biotite-garnet-plagioclase equilibria (see Table 6). Crosses represent pressure-temperature estimates and ages from coeval A-type granites from Oetmoed (Jung et al., 1998b). Filled dot represents P–T–t estimate for the main peak of regional metamorphism at Oetmoed (Jung et al., 2000b).

than in typical unfractionated pelite-derived granites from Oetmoed (CaO: 0.9– 1.1 wt%; Jung et al., 1999). Surprisingly, Sr and Eu contents are lower ( 50 and  0.5 ppm, respectively) than in typical pelite-derived granites ( 70 – 100 and  0.5 –1.0 ppm, respectively; Jung et al., 1999). High CaO but low Sr and Eu contents cannot be explained by restitic plagioclase at the site of melting or fractionation of plagioclase during ascent of the granite body. It is therefore probable that the metasedimentary source rock had a composition different from the hypothetical metapelite discussed by Miller (1985). Tempera-

Both suites of granite exhibit similar negative but distinctive initial mNd values. This suggests that LREE-enriched sources with different Sm/Nd fractionation and/or residence ages were involved in the genesis of these granites. A narrow range in initial 87Sr/86Sr ratios is unusual among leucocratic granites from worldwide studies (e.g. Vidal et al., 1982; Pin and Duthou, 1990; Villasecca et al., 1998) and is the opposite of what is observed in metasedimentary and pre-existing igneous rocks which show rather homogeneous mNd values with a considerable variation in initial 87Sr/86Sr ratios due to the more pronounced spread in Rb/Sr than in Sm/Nd ratios (Fig. 10). Bickle et al. (1988) have shown that the isotopic composition of potential source rocks of granites are strongly affected by high-grade metamorphism and anatexis. Meta-pelitic and meta-igneous lower crustal sources tend to shift their Sr–Nd isotope composition towards Bulk Earth values relative to their

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metamorphic upper crustal equivalents (e.g. Downes and Duthou, 1988; Turpin et al., 1990) and this displacement of the isotopic composition seems to be common place (e.g. Ruiz et al., 1988). For the garnet-bearing granites such a displacement is not observed and we have to infer non-depleted crustal rocks located at structural deeper levels. A non-depleted source is also indicated by the high d18O values and radiogenic Pb isotope compositions (see below). Nd model ages give values around 1.5 Ga (Table 5), similar to the Nd model ages of the metasedimentary rocks from the Kuiseb Formation and pre-existing igneous rocks, (Jung et al., unpubl.) which suggest mid-Proterozoic sources. However, these ages do not refer to any specific crust formation age, but they provide an estimate of the average crustal residence age of the REE in the granite protolith, which presumably resulted from multiple mixing of recycled and newly formed components in pre-Damaran times. The apparent Nd model age equilibrium signature shown by the granites and metasedimentary and pre-existing igneous rocks indicate that there was no large-scale juvenile addition to this segment of the crust during the Pan-African orogeny. Previous studies have shown that Pb isotopes from leached feldspars provide useful information for the understanding of crustal evolution and often record evidence for ancient crustal memory. Magmatic differentiation and metamorphic events can produce important variations in 238U/204Pb (v values) and 232Th/206Pb (s values) ratios which lead to highly variable Pb isotope ratios. Lead isotope ratios from leached feldspars are useful because feldspar has low v and s values and their Pb isotope composition can be used to estimate the initial Pb isotope ratios at the time of the last equilibration event. For the S-type granites from the Damara orogen, the Pb isotope ratios are homogeneous for each suite but the range of variation indicates heterogeneity between the suites. In 207Pb/204Pb versus 206Pb/204Pb space, the samples plot above the Pb evolution curve whereas in 208Pb/204Pb versus 206Pb/204Pb space, the samples plot below the Pb evolution curve (Fig. 11). In general, the 207Pb/204Pb and 206Pb/ 204 Pb isotopic composition of the feldspars plot

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closer to the composition of pelagic sediments than to the composition found in island-arc volcanic rocks or MORB (e.g. Sun, 1980). This supports the view that the granites originate from sedimentary sources. Because the samples plot above the U/Pb evolution curve, slightly elevated U/Pb ratios in that source can be inferred. In contrast, the sources seem to have had significantly lower Th/Pb ratios because the samples plot below the ‘normal’ Th/Pb evolution curve of Stacey and Kramers (1975). The sources for the granites from Omaruru have higher U/Pb ratios, but similar Th/Pb ratios to the sources of the Oetmoed granites. This feature is monitored by their higher 207Pb/204Pb and 206Pb/204Pb ratios, but similar 208Pb/204Pb ratios and suggests that the sources of the granites from Omaruru is depleted in Th relative to U. Dehydration and partial melting during ancient high-grade metamorphic events in the deep crustal source of these granites probably cannot account for the depletion of Th relative to U, because U is more soluble than Th and is preferentially incorporated into a fluid phase or melt (e.g. Taylor and McLennan, 1985). Thus, the low 208Pb/204Pb ratios relative to the 206 Pb/204Pb ratios recorded by the feldspars must be a primary feature of the source which evolved with a low Th/U ratio between the time of crust formation and partial melting. Low Th/U ratios are observed in volcanogenic sedimentary rocks (e.g. Taylor and McLennan, 1985) which further supports the presence of a sedimentary component in the source of the granites. The high whole-rock d18O values fall within a narrow range between 13.80 and 15.15‰. Interaction with hydrothermal systems leading to considerable changes in the oxygen isotopic composition has been ruled out, based on a number of arguments (Jung et al., 1998b, 1999). Therefore, the oxygen isotope composition of the granites most probably reflects the nature of the sources. The granites from Oetmoed define a negative correlation between the oxygen isotope composition and the initial 87Sr/86Sr ratios (Fig. 14). There is also an apparent negative correlation between the oxygen isotope values and the Pb isotope composition for metasedimentary rocks and the granites studied here, indicating that the 18O-enriched

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sources have less radiogenic Pb isotope compositions (Fig. 15). Granites from Omaruru also have enriched oxygen isotope compositions which define a weak positive correlation between the d18O values and the initial mNd values (Fig. 14b). Such features are monitored only by basementderived granites from the Damara orogen (Jung et al., 1998b) and we suggest that the granites probably contain a minor contribution from material slightly older and probably more evolved than the metasedimentary rocks. This conclusion is supported by slightly older Nd model ages, slightly more negative mNd values and slightly more radiogenic Pb isotopic compositions. Furthermore, the results from thermobarometric calculations (Table 6) also indicate a deeper crustal level for the generation of these granites. Overall, the crustal contribution to the chemical composition of the granites is striking, but massive reworking of old basement rocks seems unlikely. Haack et al. (1982) observed two trends in d18O-initial 87Sr/86Sr space. One trend exhibits a positive correlation between the two parameters. This correlation was explained as a mixture of mantle-derived material probably in the form of altered volcanic rocks with metasedimentary material. Some other granites display an obvious negative correlation between d18O values and initial 87Sr/86Sr ratios similar to the trend observed in this study. This negative correlation has been interpreted to have resulted from a mixture of mafic volcanic rocks (high d18O, low initial 87Sr/ 86 Sr) and silicic volcanic rocks (low d18O, high initial 87Sr/86Sr) in the source. Because metasedimentary rocks from the Damara orogen also exhibit this negative correlation and approach the values of the granites (Jung et al., 1998b) there is no need to argue for large amounts of altered volcanic rocks in this part of the crust of the Damara orogen. In summary, several arguments favour an entirely crustal (metasedimentary) origin for these granites. The major and trace elements clearly identify the granites as S-type granites (Chappell and White, 1974). This conclusion is confirmed by high initial 87Sr/86Sr ratios and initial negative mNd values but also by the high oxygen isotope values, which clearly indicate the major involvement of a

metasedimentary component (Sheppard, 1986; Taylor, 1986). Lead isotope evidence also suggests metasedimentary sources. Older crustal sources were not involved and an involvement of depleted mantle material can be ruled out.

7.4. Suggestions for a geodynamic framework for the Pan-African plutonism in the Damara orogen and consequences for crustal e6olution Three geological environments can be proposed for the Pan-African plutonism in the Damara orogen with the magma sources located in a subducted oceanic crust, the lower continental crust or the upper continental crust metamorphosed at amphibolite facies to lower granulite facies conditions. The first model would fit a classical platetectonic scenario, but the scarcity of features which are characteristic of modern active continental margins, namely andesitic to granodioritic magmatism, linear batholiths with high-pressure/ low-temperature environments, ophiolites, etc. rule out such a scenario for the granitic magmatism. Based on the isotopic evidence presented above (high initial 87Sr/86Sr ratios, moderately negative mNd values, high d18O values and high 207 Pb/204Pb and 206Pb/204Pb ratios but moderate 208 Pb/204Pb ratios) the sources of the granites from the present study must lie within the crust. The basement of the Damara orogen does not seem to be a viable source region because basementderived granites have negative initial mNd values ranging from −7 to − 20, low d18O values between 10 and 15‰, high initial 87Sr/86Sr ratios in excess of 0.725 (Jung et al., 1998b) and elevated 207 Pb/204Pb and 206Pb/204Pb ratios (Jung, Mezger and Hoernes, unpubl.). Geochronological constraints suggest that metamorphism and felsic magmatism are contemporaneous and result from the same thermal disturbance. A compilation of recently published pressure and temperature estimates (Masberg et al., 1992; Bu¨ hn et al., 1995; Jung et al., 1998a, 2000b) reveals an apparently smooth metamorphic gradient, which ranges from lower granulitefacies conditions in the southeast (coastal area and Khan-Swakop area) to mid-amphibolite-facies conditions in the northeast (Omaruru and

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Otjosundo areas). Distribution of U/Pb monazite and Sm/Nd garnet ages from metasedimentary rocks and migmatites are roughly the same over an area of several 1000 km2 (Jung et al., 2000b and unpubl.). It is suggested that high-grade metamorphism lasted for at least 60 Ma from  540 to 480 Ma with the main peak of metamorphism occurring around 510 – 520 Ma (Jung et al., 2000b). The persistence of felsic crustal plutonism for at least 50 Ma is satisfactorily explained by a thickened crust in which several crustal segments could reach melting conditions (e.g. Patin˜ o Douce et al., 1990). As granitic rocks with different geochemical affinities intruded before and after the peak of metamorphism, the thermal anomalies that accompany metamorphism and plutonism must have involved the entire continental crust. The degree of crustal melting required to generate the large amount of granitic material demands temperatures in excess of 900oC in the deep crust (e.g. Vielzeuf and Montel, 1994). Mafic plutonism is extremely rare in the Damara orogen and cannot account for elevated temperatures at the peak of metamorphism because most of these quartz diorites appear to be much older than the felsic plutonism (Miller, 1983). If the continental crust is thickened by stacking several nappes, crustal rocks will be depressed to depths in which partial melting is likely. This suggestion is confirmed by the present results, since the granites from Omaruru have initial mNd values of  −6 and record pressures of 9– 10 kbar corresponding to depths of 27–30 km. Metasedimentary rocks from the Kuiseb Formation have rather similar initial mNd values of  − 5 (Jung et al., unpublished work), but record pressures of not more than 5 kbar (Jung et al., 2000b). It therefore seems very likely that metapelitic material similar in composition to the Kuiseb metapelites has been depressed into the deeper crust. The huge volumes of granitic material in the Damara orogen could be the result of an unusual abundance of fertile material in the pre-Damara crust, combined with special thermal circumstances during orogenesis. Petrological and experimental investigations have shown that sources containing abundant hydrous minerals can pro-

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duce large volumes of felsic melt, whereas fluidpoor lithologies such as high-grade crystalline basement rocks cannot generate large volumes of granite under conditions of crustal melting (e.g. Clemens and Vielzeuf, 1987; Vielzeuf and Montel, 1994, and many others). There is no evidence for extensive reworking of 1.2–2.0 Ga-old basement in the central Damara orogen and it can be suggested that metasedimentary rocks, enriched in Th, U and K were subjected to melting during the Pan-African orogeny. As a consequence, the crust was characterized by high radioactive heat production. Haack et al. (1983) calculated average heat production ratios of 2.5– 8.8 vW m − 3 for meta-igneous rocks and 1.7–2.1 vW m − 3 for metapelites from the Damara orogen. These values are much higher than those used in theoretical models of anatexis in tectonically-thickened crust (England and Thompson, 1986; 0.9–1.3 vW m − 3). According to England and Thompson, a 50% increase in internal heat production raises peak temperatures by 150–200°C at the bottom of the crust and thickening of the crust by 1.5 to 2 times must result in widespread anatexis at mid-crustal levels. It is also predicted that the melting event lags behind initiation of thrusting several tens of millions of years (Zen, 1987; Patin˜ o Douce et al., 1990). This view is also compatible with the observation that this period of crustal melting in the Damara orogen occurred  30–40 Ma after the peak of metamorphism. It is very likely that the Damaran Pan-African continental crust was enriched in heat-producing elements, making it a good candidate for the massive production of granite during an orogeny. Existing temperature gradients will be steepened by the high radioactivity of subducted upper crustal rocks. Melting can begin in-situ or can be triggered by the development of gravitational instabilities followed by rapid adiabatic decompression. Collision tectonics could have contributed to the generation of granites in several ways including thickening of fertile metasedimentary piles, fluxing of aqueous fluids introduced by continuous underthrusting of low-grade metasedimentary rocks (e.g. Kro¨ ner, 1982), stacking of crustal units with different thermal conductivities (metasedi-

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mentary rocks vs. dry basement lithologies) and shear heating (e.g. Jaupart and Provost, 1985). In addition to these intracrustal processes, delamination of the sub-continental lithosphere could have occured, giving rise to the unusual geochemistry of syn- to post-collisional A-type granites (Jung et al., 1998b). The concomitant rise of hot mantle material close to the lower crust could account for the granulite-facies metamorphism recently observed (Masberg et al., 1992) and late-stage granite production.

8. Conclusions The major- and trace-element composition (high SiO2, Al2O3, A.S.I. values, Rb/Sr, Rb/Ba and Ba/Sr ratios) and Nd, Sr, Pb and O isotope composition of the late-orogenic garnet-bearing granites from the Damara orogen (Namibia) is consistent with formation of the granites by melting of metasedimentary crustal rocks similar to those they intrude. Thus, no new juvenile material is added to the crust. Some granites have mNd values too low to have formed by melting of Kuiseb-type metasedimentary material indicating that the crust is isotopically more evolved with depth. Thermal models consistent with an intracrustal origin for the granites involve crustal heating due to the insulating effect of stacked thrust sheets with high abundances of U, Th and K. Whatever the thermal history adopted for the Pan-African orogeny in the central Damara orogen, it seems to be dominated by events located within the continental crust with only limited input from the underlying lithospheric mantle. Therefore, the Damara orogen should be viewed as a special type of an intracontinental orogen that formed by different processes from the modern-circum Pacific-type orogenic belts that form along convergent margins.

Acknowledgements The analytical work was supported by grants from the Max-Planck-Society and the senior author greatly appreciates the financial support pro-

vided during preparation of the manuscript. A.W. Hofmann is thanked for hospitality and free access to mass spectrometry facilities while S. Jung held a post-doctoral position in Mainz. The geochronological work was supported under grant Ju 326/1-1 to S. Jung. Special thanks go to D. Dohle from the Bonn fluorine-lab crew for determination of the oxygen isotope ratios. E. Hoffer (Marburg) is thanked for free access to the samples from Omaruru and for ICP facilities. S. Go¨ beler (Marburg) did the Fe2 + measurements. E. Macsenaere and J. Pfa¨ nder (Max-Planck-Institut, Mainz) assisted with the microprobe work and I. Bambach (Max-Planck-Institut, Mainz) did a superb job in managing the line drawings. We appreciate the formal reviews by A.B. Kampunzu and an anonymous reviewer and the editorial work by C.McA. Powell. This paper is a contribution to IGCP 440: Assembly and Breakup of Rodinia.

Appendix A. Analytical methods Minerals were analysed with a Camebax microprobe (WDS system). Operating conditions were 15 kV and 15 nA, with a counting time of 20 s. The ZAF correction procedure was applied to the data; errors in the major oxides are estimated to be about 1–2% relative. Whole rock powders were prepared using a jaw crusher, a ball mill and an agate mortar. Major and some trace elements (except for REE) were determined on fused lithium–tetraborate glass beads using standard XRF techniques. REE have been analyzed by inductively coupled plasma emission spectrometry following separation of the matrix elements by ion exchange (Heinrichs and Herrmann, 1990). LOI (loss on ignition) was determined gravimetrically after heating the samples at 1050°C for 1 h (Lechler and Desilets, 1987). FeO was measured titrimetrically with standard techniques. Accuracy has been controlled by repeated measurements against several international and in-house standards and the results are in agreement with the recommended values. For the Rb – Sr and Sm–Nd whole rock isotope analyses, the samples were spiked with a 149Sm/

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Nd and a 85Rb/84Sr tracer and digested in concentrated HF–HNO3 in 3 ml screw-top Teflon vials inside Krogh-style Teflon bombs at 200°C for 3 days. After complete dissolution the samples were dried down and redissolved in 2.5N HCl. Rb, Sr and REE were separated by using standard cation exchange columns with a DOWEX AG 50 W-X 12 resin using 2.5N HCl for Rb and Sr and 6N HCl for the REE. Nd and Sm were separated from the other REE by using HDEHP coated teflon columns and 0.12 N HCl for Nd and 0.3 N HCl for Sm. Isotope analyses were carried out at the Max-Planck-Institut fu¨ r Chemie at Mainz using thermal ionization mass spectrometry with a Finnigan MAT 261 multicollector mass spectrometer operating in the static mode. Rb, Sm and Nd were run on Re double filaments and Sr was run on W single filaments. Nd isotopes were normalized to 146Nd/144Nd = 0.7219. The total procedural blank for Nd wasB 40 pg and is considered to be negligible. Repeated measurements of the La Jolla Nd standard gave 143 Nd/144Nd = 0.511848 90.000021 (2 |; n= 28). The reproducibility of the Sr standard (NBS 987) is 87Sr/86Sr = 0.710224 90.000024 (2 |; n = 14) and the fractionation was corrected to 86Sr/88Sr: 0.1194. Uncertainties in the 87Sr/86Sr and 143Nd/ 144 Nd are reported in the last two digits. Typical analytical errors in the 87Rb/86Sr and 147Sm/144Nd ratios are equal to or better than 0.5 and 0.1%, respectively. Thirty– fifty mg of high-purity Kfeldspar separates were washed with a mixture of 3:1 HCl/HNO3 to remove surface contamination, and were subsequently rinsed three times with ultrapure water. After this treatment the separates were leached three times in a mixture of concentrated HF/HNO3 which resulted in a weight loss of 70–80%. The Pb was extracted using conventional HBr/HCl techniques and was loaded on Re single filaments following the H3PO4-silica gel method. Pb analyses were corrected for mass fractionation by a factor of 0.11% per amu. The reproducibility of the standard NBS 982 was estimated to be 0.068, 0.064 and 0.071% for the 206 Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratio, respectively. The total procedure blank isB 100 pg Pb and is therefore considered negligible.

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Oxygen isotope analyses were performed at the University of Bonn on  10 mg aliquots of powdered whole-rock samples, using purified fluorine for oxygen extraction, followed by conversion to CO2 (Clayton and Mayeda, 1963). 18O/16O measurements were made on a SIRA-9 triple-collector mass spectrometer by VG-Isogas. Analytical uncertainties areB 0.2%o. The sieved garnet separates were purified by magnetic separation, separation by heavy liquids (methylen iodide) and handpicking which produced high-purity separates. Rigorous attention was paid to exclude fragments that contained visible inclusions. However, initial Sm–Nd analyses of the hand-picked samples yielded anomalous results with Nd contents\10 ppm and 147Sm/ 144 Nd ratiosB0.3 indicating a significant number of undetected REE-rich inclusions. It was therefore decided to eliminate these inclusions by chemical leaching. Roughly 500–1000 mg of garnet were ground in a ultra-clean agate mortar for 20–30 min which reduced the grain size toB 5 v and effectively exposed most submicroscopic inclusions. This extremely fine-grained sample was then subjected to a step-wise leaching procedure. The samples were leached by means of 6N HCl, 11.8 N HCl and a mixture of 2:1 7N HNO3/6N HCl. Each leaching step was performed for at least 1 h on a hot plate. Because of the finegrained nature of the sample 30–50% weight loss was observed. The samples were rinsed three times with ultrapure water and separation of the residue from the leaching solution was performed by centrifuging. The leachates were preserved and treated separately. The garnet samples were dried down and before digestion a mixed 149Sm/150Nd spike was added to the samples. The samples were dissolved by means of concentrated HF/HNO3 in 10 ml screw-top Teflon vials inside Krogh-style Teflon bombs at 200°C for 24 h. After dissolution, the samples were dried down on a hot plate. After drying the samples were not soluble in 6N HCl and a white precipitate (?Al–Mg –Fe fluoride) was observed. In this case, the sample was boiled three times in concentrated HNO3 in 10 ml screw-top Teflon vials on a hotplate at 120°C for 2 h. Generally, after this treatment the sample was soluble in 6 N HCl. The samples were

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again heated and periodically placed in an ultrasonic bath to aid dissolution. Before chemical separation the sample was converted to 2.5 N HCl and extraction of Sm and Nd from the REE and isotope analyses was performed as described above. For Pb isotope analyses ca. 200 mg of garnet were used. The Pb was extracted using conventional HBr/HCl techniques and was loaded on Re single filaments following the H3PO4-silica gel method. The monazite was washed in warm deionized water to remove surface contamination and was spiked with a 205Pb/235U tracer before digestion in a 1:1 mixture of 6N HCl and 7N HNO3 in 3 ml screw-top Teflon vials inside Krogh-style Teflon bombs at 200°C for several days. After evaporation, the monazite was dissolved in 2.5 N HCl for ion exchange chromatography. The Pb was separated using HCl– HBr chemistry and the U was separated using HCl– H2O chemistry. The U and Pb isotope analyses were carried out at the Max-Planck-Institut fu¨ r Chemie at Mainz using thermal ionization mass spectrometry with a Finnigan MAT 261 multicollector mass spectrometer operating in the static mode. The Pb and U were loaded on Re single filaments with a mixture of H3PO4 and silica gel. Pb analyses were corrected for mass fractionation with a factor of 0.12% per amu. Total blanks for U were B20 pg and the U analyses were corrected for 0.04% per amu based on the U 500 standard. Uncertainties for the U/Pb and Pb/Pb ratios, ages and corresponding uncertainties were calculated according to the method of Ludwig (1991a,b) using the PBDAT and ISOPLOT regression programs. References Allsopp, H.L., Barton, E.S., Kro¨ ner, A., Welke, H.J., Burger, A.J., 1983. Emplacement versus inherited isotope age patterns: a Rb – Sr and U –Pb study of Salem-type granites in the central Damara belt. Spec. Publ. Geol. Soc. South Africa 11, 281 – 288. Bea, F., Fershtater, G., Corretge´ , L.G., 1992. The geochemistry of phosphorus in granite rocks and the effect of aluminium. Lithos 29, 43 –56. Bea, F., Pereira, M.D., Stroh, A., 1994. Mineral/leucosome trace-element partitioning in a peraluminous migmatite (a laser ablation-ICP-MS study). Chem. Geol. 117, 291 –312.

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