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Isotopic evolution of the Middle to Late Proterozoic Awasib Mountain terrain in southern Namibia
Precambrian Research, 45 (1989) 175-189
175
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Isotopic Evolution of the Middle to Late Proterozoic Awasib Mountain Terrain in Southern Namibia B.G. HOAL Geological Survey of Namibia, P.O. Box 2168, Windhoek 9000 (Namibia)
R.E. H A R M E R and B.M. EGLINGTON National Physical Research Laboratory, CSIR, P.O. Box 395, Pretoria 0001 (Republic of South Africa) (Received February 12, 1988; revision accepted October 15, 1988)
Abstract Hoal, B.G., Harmer, R.E. and Eglington, B.M., 1989. Isotopic evolution of the Middle to Late Proterozoic Awasib Mountain terrain in southern Namibia. Precambrian Res., 45: 175-189. The Middle to Late Proterozoic Awasib Mountain terrain (AMT) is situated at the approximate contact between the supracrustal Sinclair Sequence and the Namaqualand Metamorphic Complex in southern Namibia. Tholeiitic to alkaline metabasalts from the Kairab Metamorphic Complex, which forms part of the basement to the Sinclair Sequence in the AMT, yield an Rb-Sr isochron age of 1461 _+169 Ma with an initial STSr/SeSr (Ro) of 0.70269 +_12. A +2°1 Ma for these metabasalts appears to confirm the Rb-Sr age. These preliminary Pb-Pb isochron age v~f 1497 - -215 metabasalts have been intruded by the calc-alkaline Aunis tonalite gneiss which yields an Rb-Sr isochron age of 1271 + 62 M a and Ro of 0.7029 _+3. The low R0 obtained for both of these basement units implies that the ages are primary and hence that the isotope systematics have not been disturbed by subsequent deformation and metamorphism. Andesitic and rhyolitic volcanics of the Haiber Flats Formation (HFF) are correlated with the Sinclair Sequence. Basaltic andesites from the H F F have shoshonitic affinitiesand m a y be classifiedas "medium- to high-K" and "orogenic" in character. One basaltic andesite unit yields an age of 1086 + 44 M a and Ro of 0.70305 + 17, the low Ro indicating derivation from a mantle source region that is slightly depleted relative to a "bulk earth" or "uniform reservoir" composition. The errorchron age estimate of 1038_+74 Ma and Ro of 0.718___15 obtained for a rhyolite porphyry from the HFF is within error of that deduced for the basaltic andesite. However, the high Ro of this largely ignimbriticunit is probably due to a significant component of melted crust. The younger, possibly subvolcanic, Awasib granite yields an errorchron with an apparent age of 934 _+70 Ma and Ro of 0.719-!-_12. Scatter of these data may be attributed to compositional variation in a postulated crustal source. The A-type characteristics exhibited by both the HFF rhyolite and Awasib granite are compatible with their derivation by crustal anatexis. Available isotopic and geochemical data, together with field evidence, suggest an extensional palaeoenvironment for much of the AMT and Sinclair Sequence. While isotopic data alone are insufficient to establish the nature of the rifting, the broadly contemporaneous "Namaqua Orogeny" suggests that rifting may be collision-induced. Furthermore, the mantle source of abundant "orogenic" lavas in the AMT and Sinclair Sequence appears to owe its imprint of subduction-related metasomatism to a more ancient event.
Introduction The regional geology of the Awasib Mountain terrain (AMT) has been reviewed and described by Hoal (1985). In summary, the ter0301-9268/89/$03.50
rain consists of a partly deformed and metamorphosed volcano-sedimentary succession with associated high-level plutons and dyke swarms which overlie, or have intruded, a metamorphic basement of suspected Proterozoic age.
© 1989 Elsevier Science Publishers B.V.
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Fig. 1. The location of the study area (Awasib Mountain terrain, AMT) relative to the major tectonic provinces and subprovinces of southern Africa (Hartnady et al., 1985 ).
The volcano-sedimentary succession is correlated with the Sinclair Sequence, the type area of which is situated immediately to the east of the AMT in the "Rehoboth Sub-province" (after Hartnady et al., 1985). Watters (1974, 1976) proposed, largely on the basis of voluminous igneous rocks that exhibit calc-alkaline and shoshonitic affinities, that the Sinclair Sequence constituted a continental margin magmatic arc related to the subduction of oceanic crust beneath the Kalahari Craton. However, KrSner (1977) argued that field evidence favoured the deposition of undeformed
Sinclair rocks in an intracratonic trough or aulacogen, probably related to tectonic activity in the Namaqualand Metamorphic Complex (NMC) to the south. Much of the confusion relating to the origin of the Sinclair Sequence, namely, whether it is "orogenic" or essentially "anorogenic", is the result of a paucity of data, especially geochemical and isotopic. The latter aspect, in the form of geochronology, has further clouded the issue with regard to the relationship between the Sinclair Sequence and its supposed basement, the NMC. The relatively undeformed Sinclair
177
MIDDLE TO LATE PROTEROZOIC AWASIB MOUNTAIN TERRAIN
Sequence, which apparently overlies the NMC, has yielded ages, determined by a variety of radiometric techniques, that are both similar to and older than ages determined for the NMC (age data reviewed in Cahen and Snelling, 1984). Lithologies of both Sinclair and NMC affinity occur in the AMT, which consequently provides a unique opportunity for the study of the geochemical and isotopic evolution of this portion of Middle to Late Proterozoic crust. The purpose of this paper is to report the initial findings of an isotopic study on five different suites of samples from the AMT. The samples are representative of lithostratigraphic units that form part of both the metamorphic basement (Kairab Complex metabasalt and Aunis tonalite gneiss) and supracrustal succession (HFF basaltic andesite, HFF rhyolite and Awasib granite) within the AMT. The location of the study area with regard to the various tectonic provinces and sub-provinces in southern Africa is illustrated in Fig. 1 (after Hartnady et al., 1985 ).
Petrography Kairab Complex metabasalt This metabasaltic orthoamphibolite is characterised by relatively undeformed pillow structures and large plagioclase phenocrysts which constitute between 20 and 40% of the rock. These phenocrysts of plagioclase can exceed 9 mm in length and commonly exhibit subhedral to euhedral crystal forms despite recrystallisation. Alteration of plagioclase, although widespread, still allows the identification of original anorthitic cores and normal zoning. The presence of primary mafic phenocrysts is difficult to assess in view of the high proportion of poikiloblastic green hornblende. Equilibrium intergrowths of hornblende, redbrown biotite and epidote suggest that the metamorphic grade is transitional between greenschist and amphibolite facies. These in-
tergrowths occasionally form elongate aggregates that define an anastomosing foliation. Rounded, quartz-filled amygdales rarely exceed 4% of the rock and may be entirely absent. The groundmass is largely obscured by poikiloblastic hornblende, but appears to be made up of a microcrystalline intergrowth of plagioclase, biotite, ilmenite + magnetite and a variety of alteration products, e.g., epidote, carbonate, sericite and chlorite. Aunis tonalite gneiss This leucotonalite gneiss is medium- to coarse-grained, sometimes massive, but usually displaying a strong foliation. Plagioclase typically forms porphyroclasts which make up between 35 and 50% of the rock, whereas K-feldspar (mostly microcline) rarely exceeds 5 % and is commonly absent. Despite widespread alteration, plagioclase exhibits strong normal zoning (andesine cores) and albite twinning. Quartz, either as large strained crystals or smaller strain-free recrystallised grains, makes up between 25 and 45% of the rock. Strongly pleochroic brown biotite and blue-green hornblende together constitute about 10% of the rock, but hornblende is absent from approximately half of the samples in the suite. The foliation is defined by quartz-rich layers, commonly ribbon structures, and anastomosing trails of biotite_+ hornblende which wrap around porphyroclasts of feldspar. Augen structures have developed in the more highly deformed gneisses. Accessory minerals include sphene, apatite, zircon and magnetite. Epidote, sericite, chlorite and carbonate are common secondary minerals. Haiber Flats Formation basaltic andesite Flows are always porphyritic, but only occasionally amygdaloidal. Phenocrysts make up between 5 and 15% of the rock and comprise augite, plagioclase and possibly olivine. Subhedral to euhedral augite has usually been re-
178
placed by pseudomorphous green actinolite. Relict cores of augite are fairly common and indicate an original glomeroporphyritic texture in which individual crystals display both twinning and zoning. Resorption, although present, is not a widespread feature. Subordinate plagioclase phenocrysts are typically tabular in form and exhibit both twinning (Carlsbad and polysynthetic) and normal zoning (labradorite cores) despite widespread saussuritisation. The presence of olivine phenocrysts is suggested by pseudomorphous aggregates of green-brown biotite and magnetite. In view of the degree of alteration, it is difficult to establish if magnetite, in addition, constituted a primary phenocryst phase. The presence of small amygdales is suggested by patchy aggregates of intergrown quartz, epidote and actinolite. The groundmass is crypto- to microcrystalline and comprises a pilotaxitic to trachytic intergrowth of plagioclase laths together with pseudomorphs of actinolite after augite and interstitial K-feldspar. The groundmass has a cloudy appearance due to the presence of numerous magnetite granules and secondary minerals, namely epidote, sericite, actinolite, biotite and sphene. The secondary mineral assemblage indicates greenschist facies metamorphism, but evidence for deformation is restricted to fairly rare kink bands in biotite and actinolite.
B.G. HOAL ET AL.
tion, but polysynthetic twinning of primary plagioclase crystals indicates original compositions ranging from albite to oligoclase. Quartz phenocrysts, although usually rounded and resorbed, may exhibit bipyramidal and angular outlines. The fractured and strained nature of both feldspar and quartz phenocrysts suggests greater deformation than would be expected from pyroclastic emplacement alone. Sparse aggregates of secondary magnetite +biotite + chlorite_+ sericite may represent pseudomorphs after mafic mineral phenocrysts. The groundmass consists predominantly of a micro- to cryptocrystalline granular intergrowth of quartz and feldspar which can probably be attributed to devitrification. A fabric of variable intensity is defined by trails of magnetite granules, quartzofeldspathic lenses, flattened lithic fragments and stringers of secondary minerals. Compositional banding, illustrated by a variation in dark mineral content and possibly grain size, may enhance the fluidal texture. Lithic fragments vary considerably in their character but are typically porphyritic and spherulite-bearing. Some disaggregation and flattening of these fragments may be attributed to secondary or tectonic effects. Albitisation, sericitisation and silicification are commonly present, particularly in the more deformed rhyolites. Accessory minerals include zircon, magnetite, allanite, biotite and rare fluorite.
Haiber Flats Formation rhyolite Awasib granite Rhyolites comprise two main groups, namely pyroclastic and non-pyroclastic, with the former group being predominant. A highly porphyritic unit of lithic crystal tuff forms a distinctive horizon characterised by flattened lithic fragments that wrap around the feldspar and quartz phenocrysts. Phenocrysts constitute between 10 and 35% of the rock and are made up largely of K-feldspar (perthite), plagioclase, quartz and minor pseudomorphs after mafic minerals. Original feldspar proportions are difficult to assess in view of widespread albitisa-
This granite displays textural variations from very fine-grained porphyritic to mediumgrained hypidiomorphic granular, but can most commonly be termed a microgranite porphyry. K-feldspar constitutes up to 60% of the rock and is typically perthitic, although replacement by albite is not uncommon. Phenocrysts of Kfeldspar show typical Carlsbad twinning and may be rimmed by albite to give the rock a rapakivi appearance. Plagioclase usually makes
MIDDLE TO LATE PROTEROZOIC AWASIB MOUNTAIN TERRAIN
up less than 10% of the rock and is typically saussuritised. Whereas primary albite is usually cloudy in appearance, secondary albite displays less alteration and often exhibits characteristic "chessboard" twinning. Quartz constitutes between 25 and 35% of the rock and occurs as both large bipyramidal phenocrysts and small secondary blebs. Granophyric intergrowths of quartz and feldspar are fairly widespread. Sparse brown biotite and even more rare green hornblende are usually ragged or poikilitic in appearance and generally altered to aggregates of chlorite + epidote + magnetite + sphene. Accessory minerals include apatite, allanite, zircon and rare fluorite. Magnetite appears to be mainly secondary and is commonly rimmed by sphene granules. Indications of deformation in this granite include bent twin and exsolution lamellae in feldspars, fracturing and marginal granulation in feldspars and quartz, and protomylonitic textures in the proximity of shear zones.
Analytical procedures Concentrations of major and trace elements used in geochemical variation diagrams were determined by the Geological Survey of South Africa. The X R F techniques used have been adapted from Norrish and H u t t o n (1969) and Nisbet et al. (1979). Modifications and standards used for calibration have been described by Walraven (1984). Rb and Sr concentrations were determined by X R F analysis at the University of Pretoria using a Siemens SRS- 1 spectrometer with a Mo tube; correction for matrix effects was made by monitoring the Mo Compton scattering peak. Replicate analyses of standards and samples determined by isotope dilution (ID) indicate an uncertainty of 1.5% in the R b / S r ratios for this technique. Rb and Sr concentrations for samples of the Kairab Complex metabasalt were analysed by ID because of the low levels of Rb involved. Replicate analyses of USGS stan-
179
dards indicate an uncertainty of 0.8% in ID measurements of R b / S r ratios. The analytical procedures applied at the National Physical Research Laboratory (NPRL) for Sr isotope analysis have been described by Harmer and Sharpe (1985) and Harmer (1985) and involve standard cation-exchange extraction of Sr following sample dissolution in H F HNO3-HC1. Blanks were of the order of I ng. Isotopic analyses were carried out on a VG 354 mass spectrometer equipped with a multi-collector array and automatic computer control of the analytical procedure. All ratios are normalised to SeSr/88Sr=0.1194 and are relative to a value of 0.71023 + 4 (2 standard deviations on 28 measurements) for NBS standard Sr salt SRM 987. Sample Sr ratios are reproducible to 0.01%. Data were regressed using the method of York (1969) with blanket weighting factors based on one-sigma uncertainties of 1.5 and 0.01% in X and Y respectively in the case of X R F analyses, and 0.8 and 0.02% in X and Y respectively in the case of ID determinations. Goodness-of-fit of the regression line was tested by the value MSUM=SUMS/(n-2) as described by Brooks et al. (1972); scatter in the data in excess of the analytical uncertainty is reflected by MSUM > 2.5. In cases of excess, or "geological scatter", the regression line is termed an "errorchron" and the errors for the age estimate are calculated by York (1966), which augments the errors to account for the scatter. Errorchron age estimates are identified in the text and figures by an asterisk (*). All errors are given at the two-sigma level. Sample preparation for Pb isotopes at the NPRL involved anion exchange in small quartzglass columns and elution using HBr and H20 xRx. Total method blanks of less than I ng were considered negligible. The Pb fraction was loaded on single Re filaments with 1 M HNO3 ~X and 1 N H3PO4, and isotopic analyses carried out on a VG MM30 mass spectrometer. Five runs of the NBS 981 Pb standard yielded mean values of 2°6pb/2°4pb = 16.897 + 5, 2°7pb/
180
B.G. HOAL ET AL.
2°4pb=15.441+7, and 2°sPb/2°4pb=36.539 +24. A reproducibility in 2°6Pb/2°4pb and 2°7pb/2°4pb of 0.09% was achieved and data were regressed using the technique of York (1969). A correlation coefficient of 0.946 was determined from five runs of NBS 981 and by using the formulation of Ludwig (1980).
Geochemistry Isotopic analyses form the main theme of this paper. For this reason, and in order to conserve space, tabulated geochemical data have not been included. Data for all of the rock suites analysed have been plotted on a conventional AFM diagram (Fig. 2) , with tholeiitic and calc-alkaline fields according to Irvine and Baragar (1971). However, identification of actual rock types using major elements may be problematic in view of widespread alteration and metamorphism. Using the approach of Winchester and Floyd (1977), we have utilized the immobile element ratios Nb/Y and Zr/TiO2 in order to interpret these suites meaningfully in terms of magma type and chemical affinity (Fig. 3). Although this diagram was originally intended for use with altered and metamorphosed volcanic rocks only, its use has subsequently been exFeO* + Awasib Granite (II) * Haiber Flats Rhyolite (I0] • Haiber Flats Andesite (8)
Aun,, Tooo,l,e n:,?o ,,o,
K20 + NazO
MgO
Fig. 2. AFM diagram showing the compositional variation of samples from the Awasib Mountain terrain. Tholeiitic (TH) and calc-alkaline (CA) fields are after Irvine and Baragar ( 1971 ).
tended to orthogneisses of plutonic origin, e.g., Winchester and Max (1984).
Kairab Complex metabasalt While data points on an AFM diagram (Fig. 2) indicate a predominantly tholeiitic character for this metabasalt (only two samples in the calc-alkaline field), the relatively high Nb/Y ratios reflect a strong alkaline tendency (Fig. 3). An average A1203 content in excess of 20% is attributed to the highly plagioclase phyric character of these flows. Low average contents of K20 (0.2%), TiO2 (0.4%), Zr (36 ppm), Y (9 ppm), Cr (39 ppm) and Ni (14 ppm) are in agreement with chemical characteristics described for island arc tholeiites (e.g., Pearce, 1982, 1983). Nb, however, at an average level of 8 ppm, appears to be enriched relative to the latter. Furthermore, the spread of data points across several fields in the discriminant diagrams of Pearce and Cann {1973) could suggest, instead, a continental setting for these metabasalts.
Aunis tonalite gneiss A calc-alkaline affinity and variation in composition from quartz diorite ("andesite") to granodiorite ("rhyodacite") are indicated for this gneiss (Figs. 2 and 3). On average this unit may be classified as a leucotonalite, although normative feldspar compositions suggest some overlap with calcic trondhjemite. Low average contents of high field strength elements (HFSE), e.g., Zr (98 ppm), Y (28ppm), Nb (6 ppm), and moderate to low contents of large ion lithophile elements (LILE), e.g., K20 (1.1%), Rb (26 ppm), are similar to chemical characteristics described for calc-alkaline arc granitoids (e.g., Brown et al., 1984). Data points for this tonalite gneiss also plot consistently in the "volcanic arc granite" field in the geochemical discriminant diagrams of Pearce et al. (1984).
MIDDLETO LATEPROTEROZOICAWASIBMOUNTAINTERRAIN
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181
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Alkali basalt
I
I
I
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NblY Fig. 3. Nb/Y versus Zr/TiO2 discrimination diagram (after Winchester and Floyd, 1977) showing the distribution of the various rock units discussed.
Haiber Flats Formation basaltic andesite The basic to intermediate character of this unit is illustrated by the slight overlap of data points from "andesite" to "sub-alkaline basalt" in Fig. 3. A calc-alkaline (Fig. 2) and "orogenic" chemical character (after Gill, 1981; Bailey, 1981 ) is defined by the low average contents of HFSE, e.g., TiO2 (0.9%), Zr (144 ppm ), Y (28 ppm), Nb (7 ppm), and low Cr (130 ppm). Relatively high contents of K20 (2.1% ), Rb (62 ppm), Sr (637 ppm) and Ba (639 ppm) are typical of medium- to high-K calc-alkaline andesites (Peccerillo and Taylor, 1976; Ewart, 1982 ). The data points plot consistently in the "calc-alkaline basalt" field in the discriminant diagrams of Pearce and Cann (1973). Haiber Flats Formation rhyolite Rhyolites which make up this unit are mildly peralkaline and metaluminous. Many of the chemical characteristics attributed to A-type granites (as defined by Collins et al., 1982;
Whalen et al., 1987 ) are demonstrated by these flows, e.g., relatively high K20/Na20 (about 1 ), high FeO*/(FeO* + MgO) of approximately 0.8, and significant enrichment in the HFSE Zr (289 ppm), Y (101 ppm) and Nb (17 ppm). Awasib granite Compositionally, this granite shows a remarkable similarity to the HFF rhyolite and may, therefore, represent a subvolcanic or comagmatic intrusion. However, data points for these two units show a slight separation in Fig. 3. A-type characteristics are similar to the HFF rhyolite, e.g., metaluminous, mildly peralkaline, high K20/Na20 (about 1), FeO*/ (FeO* + MgO) of about 0.8, and significant enrichment in the HFSE Zr (222 ppm), Y (130 ppm) and Nb (19 ppm). These chemical features are distinct from typical I-type granites (Collins et al., 1982; Whalen et al., 1987) and data points plot fairly consistently in the "within plate field" in the discriminant diagram of Pearce et al. (1984).
182
B.G. HOAL
Isotope results Results are shown in Tables 1 and 2 and on conventional isochron diagrams in Figs. 4-8.
Kairab Complex metabasalt The ten analysed samples of the Kairab Complex metabasalt define an isochron (MSUM--2.48) with an age of 1461 + 169 Ma and an initial SVSr/86Sr ratio (Ro) of 0.70269 + 12. The separation of sample BH 774 from the main cluster of data points in Fig. 4 clearly exerts a significant control on the slope of the regression line. Consequently, Pb separates were measured for four samples which fall within the main sample cluster on the R b - S r isochron. The inset in Fig. 4 illustrates that
ET AL.
these samples define a P b - P b isochron (MSUM = 1.1 ) with an age of 1467+22°11 Ma and /t = ~.ua ~7 +o.14_o.1,~The/z . value is slightly higher than the value of 9.74 preferred by Stacey and Kramers (1975) for the second stage of their twostage model of Pb isotope evolution. The Ro estimate for this unit is low, an indication that the source material of this metabasalt had a Rb/ Sr ratio similar to, or slightly lower than, a "bulk Earth" mantle composition.
Aunis tonalite gneiss Nine samples of the Aunis tonalite gneiss define an isochron ( M S U M = 1.3) with an age of 1271 _+62 Ma and Ro of 0.7029 + 3. Sample BH 745 was excluded from the regression as it appears slightly weathered in thin section and deviates from the isochron line by more than three times the analytical uncertainty. Exclusion of
TABLE 1 Isotopic data, Awasib M o u n t a i n terrain ( b a s e m e n t ) Lithology and sample n u m b e r
Rb (ppm)
Sr (ppm)
STRb/S6Sr (atomic)
STSr/S%r (atomic)
2.01 12.1 1.37 1.10 0.89 0.75 7.92 3.92 2.30 5.38
307 254 384 265 290 328 395 313 274 360
0.0189 0.1372 0.0103 0.0120 0.0089 0.0067 0.0579 0.0362 0.0243 0.0433
0.70297 ± 2 0.70565 ± 1 0.70286 ± 1 0.70340 ± 1 0.70280 ± 1 0.70289 ± 1 0.7040 ± 1 0.70315 ± 1 0.70323 _+ 1 0.70337 _+1
29.8 30.1 25.3 19.9 25.9 25.1 25.4 28.9 24.6 19.7
121 205 214 227 193 200 208 190 181 210
0.713 0.425 0.342 0.254 0.388 0.368 0.353 0.440 0.393 0.271
0.71609 0.71131 0.70912 0.70766 0.70977 0.70951 0.70915 0.71082 0.71015 0.70782
2°~Pbff°4pb
2°7pb/2°4pb
2°SPbff°4pb
20.931 17.840
15.858 15.565
38.205 37.515
17.870
15.584
37.569
18.689
15.660
38.494
Kairab Complex metabasaIt BH BH BH BH BH BH BH BH BH BH
770 774 783 901 902 903 904 905 906 907
Aunis tonalite-gneiss BH BH BH BH BH BH BH BH BH BH
727 745 746 747 748 749 750 751 752 753
_+1 + 1 ±1 ±1 ± 1 ± 1 ±1 _+1 +_1 ± 1
183
MIDDLE TO LATE PROTEROZOIC AWASIBMOUNTAINTERRAIN TABLE 2 Isotopic data, Awasib M o u n t a i n terrain (cover a n d highlevel intrusions)
tion and crystallisation of the Aunis tonalite gneiss.
Haiber Flats Formation basaltic andesite Lithology a n d sample n u m b e r
Rb (ppm)
Sr (ppm)
STRb/ S6Sr (atomic)
STSr/S6Sr (atomic)
Haiber Flats Formation basaltic andesite BH BH BH BH BH BH BH BH
687 688 689 690 691 692 693 694
69.7 87.1 76.0 38.9 99.3 28.8 47.1 46.4
699 647 639 721 533 639 606 613
0.289 0.390 0.344 0.156 0.539 0.130 0.225 0.219
0.70763 _+1 0.70920_+ 1 0.70846 Jr 1 0.70544_+ 1 0.71122 _+ 1 0.70507 _ 1 0.70657 ___1 0.70639 _+ 1
Haiber Flats Formation rhyolite BH BH BH BH BH BH BH BH BH BH
504 513 585 611 697 705 706 707 708 709
163 152 182 185 181 162 160 290 249 252
48.7 37.8 30.1 46.9 46.4 32.4 25.0 16.5 47.0 26.0
9.84 11.8 14.9 11.6 11.5 14.8 19.0 55.0 15.7 29.3
0.87197 _+2 0.89284_+2 0.97459_+ 2 0.88677 _+2 0.88916 +_ 1 0.93042 +_ 1 0.98562 _+ 1 1.53662 _+2 0.96216_+ 1 1.17164_+ 1
119 135 269 292 288 296 164 225 247 281 179
71.7 40.7 22.8 10.3 12.8 26.6 52.2 13.5 15.4 10.0 31.0
4.84 9.74 35.8 91.6 71.4 33.8 9.21 50.8 49.7 91.0 17.1
0.78325 _+2 0.85780 + 2 1.21040_+ 2 1.90512 + 2 1.69006 _+3 1.20665 _+2 0.83989 _+2 1.25923 _+2 1.4389 __6 1.9284 +_5 0.92401 _+ 1
Awasib granite BH BH BH BH BH BH BH BH BH BH BH
618 619 677 678 679 680 681 682 683 684 685
this datum has little effect on the age and Ro estimates. Again the Ro estimate for the Aunis tonalite gneiss is low, an indication that the source material of this granitoid had a Rb/Sr ratio similar to a "bulk Earth" or "uniform reservoir" mantle composition. This precludes a significant crustal pre-history or "residence time" for this unit, and implies that the estimated age reflects the time of primary deriva-
The eight analysed samples define a precise isochron (MSUM=0.8) with an age of 1086 + 44 Ma and Ro of 0.70305_+ 17. The low Ro indicates a source region having slight timeintegrated depletion in Rb/Sr relative to a "bulk Earth" mantle composition.
Haiber Flats Formation rhyolite These data, which exhibit significant scatter, define an errorchron (MSUM = 7.0) with an age of 1038 + 74* Ma and Ro of 0.718 + 15". While it may be significant that both of these parameters are within error of those deduced for the basaltic andesite samples, the high Ro suggests a significant component of melted crust in the genesis of these rhyolites. A choice between these alternatives is precluded by the rather large (augmented) errors on the Ro estimate. Sample scatter may be attributed to one or more of the following: ( 1 ) the samples are not of the same age; (2) the samples have different values of Ro; or (3) the samples did not remain closed systems to Rb and Sr. Field relationships suggest that option (1) is unlikely, but the incorporation of exotic lithic fragments in the rhyolites could possibly account for differences in Ro during crystallisation of the lavas. However, as the rhyolites show extensive devitrification, the most likely explanation is option (3), i.e., loss of Rb and/or radiogenic Sr during post-crystallisation alteration, particularly during the formation of chlorite and epidote.
A wasib granite Ten samples of the Awasib granite also exhibit significant scatter (MSUM-- 15 ) and the apparent age of 957+50* Ma and Ro of 0.717 + 8* needs to be interpreted with caution. Sample BH 682 was excluded from the regres-
184
B.G. HOAL ET AL.
0.706 -
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sion because of its albitised appearance in thin section. Although not conclusive, the age estimate is consistent with field evidence, i.e., the granite is younger than the HFF volcanics. The Awasib granite is holocrystalline and contains only minor secondary material {especially when compared with the rhyolites), consequently it is unlikely that the large scatter is due entirely to secondary effects. The high apparent Ro may thus imply the involvement of pre-existing crustal material in the genesis of the Awasib granite. It is likely that this material would be compositionally variable and hence incomplete mixing of partial melt fractions extracted from the crustal material would give rise to Ro heterogeneity in the parent magma. If Awasib granite data are divided into two groups on the basis of R b / S r ratio, i.e., high ratios (samples BH 677,678, 679, 680, 683,684) and low ratios (samples BH 618, 619, 681, 685), two sub-parallel errorchrons are apparent (Fig. 8). These yield age estimates of 896 + 72* Ma (high ratios) and 895+ 180" Ma (low ratios), with R0
185
MIDDLE TO LATE PROTEROZOIC AWASIB MOUNTAIN TERRAIN
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186
B.G. HOALET AL.
estimates of 0.77 + 0.05* and 0.72_+ 0.02*, respectively. It is plausible, then, that scatter in this data set is attributable to variation in Ro at the time of crystallisation. Incomplete mixing can produce misleading ages in whole-rock isotopic dating systems (Juteau et al., 1984) and the deduced age for the Awasib granite must be viewed with caution.
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Discussion
Age of the AMT and Sinclair Sequence The age of ~ 1460 Ma obtained for the Kairab Complex metabasalt is considered to represent the time of extrusion of these flows in view of the agreement between Rb-Sr and Pb-Pb systems. A minimum age for the Kairab Complex basement is provided by the age of 1271 _+62 Ma obtained for the Aunis tonalite gneiss, since this
age is considered to represent a primary crystallisation event. This age further coincides with a likely maximum age of 1300 Ma for the D2 Namaqua event (Cahen and Snelling, 1984, p. 65) and is consistent with a previous correlation between the Kairab Complex and the NMC (Hoal, 1985). The age of the HFF volcanism is provided by the isochron date of 1086 + 44 Ma for the basaltic andesites. Open system behaviour during devitrification and alteration has given rise to errorchron systematics in the rhyolitic members but the estimated age of 1038 + 74 Ma is comparable with that of the basaltic andesites. Correlation of the HFF with the Barby Formation of the Sinclair Sequence (Hoal, 1985) would appear to be incorrect in view of the age of t392 +_33 Ma inferred for the latter by Watters (1982). However, Watters' (1982) Barby Formation data were recalculated with the NPRL regression program and found to yield an age estimate of 1238_+71. Ma and Ro of 0.7028_+ 5* with an MSUM of 3.9 (apparently Watters inadvertently quoted one-sigma errors on his fig. 3 ). Excluding the samples 1557 and 1565, which deviate from the regression line by more than three times the uncertainty (in "X"), the remaining twelve data points yield an isochron estimate (MSUM =0.9) of 1190 + 38 Ma and Ro value of 0.70298_+ 27. The two deviant samples 1557 and 1565 had Ro values of 0.7045 and 0.7043, respectively, at 1190 Ma (i.e., differing by more than the analytical uncertainty ) implying that these rocks were derived from a source of different Ro composition than the bulk of the Barby Formation samples. Independent support for the age of the Barby Formation is difficult to obtain as several granite bodies that have clearly intruded this formation paradoxically yield much older ages. These ages, which are largely based on U-Pb zircon determinations (reviewed by Watters, 1982), may reflect the existence of relict zircons in the approximate age range 1350-1250 Ma. It is further suggested that these inherited zircons were derived from a source of similar
MIDDLE TO LATE PROTEROZOIC AWASIB MOUNTAIN TERRAIN
age to the Kairab Complex or Aunis tonalite gneiss basement in the AMT. Whereas our estimate of 1190 Ma for the Barby Formation is also younger than KrSner's (1977) preferred age of 1264+23 Ma (wholerock Rb-Sr), it is still significantly older than the HFF and a direct time correlation between these formations is not justified. Similarity in Ro suggests, however, that the basic volcanics of the Barby Formation and HFF were both mantle-derived without any significant contribution from older crust. It is not possible to estimate confidently the true age of the Awasib granite from the available data since wide variations in Ro are suggested, probably due to incomplete mixing of crust-derived partial melts. However, field evidence and isotopic characteristics are consistent with a subvolcanic relationship between the Awasib granite and the HFF rhyolite.
Tectonic setting of the A M T and Sinclair Sequence Application of tectonic discrimination diagrams has met with varying degrees of success. The Kairab Complex metabasalt, in particular, has produced results indicative of a heterogeneous source rather than any specific tectonic setting. It is likely that this metabasalt, together with associated felsic volcanics, represents a bimodal mafic-silicic suite of continental or oceanic affinity. Whereas bimodal volcanism appears to be most characteristic of continental rifts, it also occurs in a variety of continental and oceanic settings, e.g., cratonic areas of minor compression, collision zones, back-arc basins and oceanic ridges (Ewart, 1979; Condie, 1982). The alkaline character of the Kairab Complex metabasalt is similar to "anomalous" volcanic arc basalts, e.g., Grenada {Pearce, 1982), but the paucity of associated calc-alkaline andesites is more compatible with a back-arc setting. Unfortunately, field relations do not indicate whether or not this metabasalt is underlain by older continental crust. The calc-alkaline Aunis tonalite gneiss, which
187
has an age within error of the Kairab Complex metabasalt, exhibits chemical characteristics which closely resemble those of continental arc granitoids (after Brown et al., 1984; Pearce et al., 1984). There do not, however, appear to be any volcanic equivalents of the Aunis tonalite gneiss within the AMT and this granitoid may, therefore, represent the root zone of an ancient arc. Within the cover sequence of the AMT, the HFF basaltic andesite displays many of the geochemical characteristics thought to be typical of an island arc or active continental margin setting (Pearce, 1982, 1983). Voluminous pyroclastic rhyolites of the HFF exhibit A-type characteristics (after Collins et al., 1982; Whalen et al., 1987) and a high R0 indicative of crustal anatexis. However, as with the compositionally similar Awasib granite, it must be stressed that A-type characteristics are not diagnostic of any specific tectonic setting (W.J. Collins, personal communication, 1987). The bimodal nature of interbedded basaltic andesites and rhyolites in the HFF probably excludes their extrusion in a graben-hosted arc, but is not incompatible with a back-arc setting. Geological evidence indicates that the HFF volcanics were extruded in fault-bounded troughs in the Kairab Complex basement, and have undergone relatively mild deformation and metamorphism. We conclude that available isotopic, geochemical and field data are consistent with an extensional tectonic setting for the supracrustal part of the AMT and the Sinclair Sequence. Broadly contemporaneous collisional tectonics between 1.2 and 0.9 Ga in the adjacent Gordonia Belt of the NMC further suggest that this extension may be related to continental convergence, although not necessarily associated with an active continental margin (cf. Keweenawan "impactogen" described by Gordon and Hempton, 1986). However, the geochemical character of HFF basaltic andesite indicates subduction-related metasomatism in its mantle source region which could, therefore, have
188
been inherited from a more ancient event. Evidence for such an event may be provided by the presence of tholeiitic to alkaline metabasalts in the Kairab Complex and/or the calc-alkaline Aunis tonalite gneiss.
Acknowledgements Financial support for the XRF and isotopic analyses was provided by the Geological Survey of Namibia. We thank D.J, Farrow for technical assistance with the isotopic analyses. A.J. W a l k e r a n d S. S c h e r m a c h e r a r e t h a n k e d f o r drafting the diagrams. Helpful comments from an anonymous reviewer are gratefully acknowledged.
References Bailey, J.C., 1981. Geochemical criteria for a refined tectonic discrimination of orogenic andesites. Chem. Geol., 32: 139-154. Brooks, C., Hart, S.R. and Wendt, I., 1972. Realistic use of two-error regression treatments applied to rubidium data. Rev. Geophys. Space Phys., 10: 551-577. Brown, G.C., Thorpe, R.S. and Webb, P.C., 1984. The geochemical characteristics of granitoids in contrasting arcs and comments of magma sources. J. Geol. Soc. London, 141: 413-426. Cahen, L. and Snelling, N.J., 1984. The geochronology and evolution of Africa. Clarendon Press, Oxford, 512 pp. Collins, W.J., Beams, S.D., White, A.J.R. and Chappell, B.W., 1982. Nature and origin of A-type granites with particular reference to southeastern Australia. Contrib. Mineral. Petrol., 80: 189-200. Condie, K.C., 1982. Plate Tectonics and Crustal Evolution. Pergamon Press, New York, 310 pp. Ewart, A., 1979. A review of the mineralogy and chemistry of Tertiary-Recent dacitic, latitic, rhyolitic and related salic volcanic rocks. In: F. Barker (Editor), Trondhjemites, Dacites, and Related Rocks. Elsevier, Amsterdam, pp. 13-122. Ewart, A., 1982. The mineralogy and petrology of TertiaryRecent orogenic volcanic rocks: with special reference to the andesitic-basaltic compositional range. In: R.S. Thorpe (Editor), Andesites. Wiley, Chichester, pp. 2595. Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics. Springer-Verlag, New York, 390 pp. Gordon, M.B. and Hempton, M.R., 1986. Collision-induced rifting: the Grenville Orogeny and the Keweenawan Rift of North America. Tectonophysics, 127: 1-25.
B.G. HOAL ET AL.
Harmer, R.E., 1985. Rb-Sr isotopic study of units of the Pienaars River Alkaline Complex, north of Pretoria, South Africa. Trans. Geol. Soc. S. Afr., 88: 207-214. Harmer, R.E. and Sharpe, M.R., 1985. Field relations and strontium isotope systematics of the marginal rocks of the Eastern Bushveld Complex. Econ. Geol., 80: 813837. Hartnady, C.J., Joubert, P. and Stowe, C.W., 1985. Proterozoic crustal evolution in southwestern Africa. Episodes, 8: 236-244. Hoal, B.G., 1985. Preliminary report on the geology of the south-eastern part of Diamond Area No. 2, South West Africa/Namibia. Commun. Geol. Surv. S.W. Afr./Namibia, 1: 9-21. Irvine, T.N. and Baragar, W.R.A., 1971. A guide to the chemical classification of the common volcanic rocks. Can. J. Earth Sci., 8: 523-548. Juteau, M., Michard, A., Zimmerman, J. and Albarede, F., 1984. Isotopic heterogeneities in the granitic intrusion of Monte Capanne (Elba Island, Italy ) and dating concepts. J. Petrol., 25: 532-545. KrOner, A., 1977. The Sinclair aulacogen - a late Proterozoic volcano-sedimentary association along the Namib Desert of southern Namibia (SWA). Abstr. 9th Colloq. Afr. Geol., University of GSttingen, pp. 82-83. Ludwig, K.R., 1980. Calculation of uncertainties of U-Pb isotope data. Earth Planet. Sci. Lett., 46: 212-220. Nisbet, E.G., Dietrich, V.J. and Essenwein, A., 1979. Routine trace element determination in silicate minerals and rocks by X-ray fluorescence. Fortschr. Mineral., 57: 264279. Norrish, K. and Hutton, J.T., 1969. An accurate X-ray spectrographic method for the analysis of a wide range of geological samples. Geochim. Cosmochim. Acta, 33: 431-453. Pearce, J.A., 1982. Trace element characteristics of lavas from destructive plate margins. In: R.S. Thorpe {Editor), Andesites. Wiley, Chichester, pp. 525-548. Pearce, J.A., 1983. Role of subcontinental lithosphere in magma genesis at active continental margins. In: C.J. Hawkesworth and M.J. Norry (Editors), Continental Basalts and Mantle Xenoliths. Shiva Publishing, Nantwich, pp. 230-249. Pearce, J.A. and Cann, J.R., 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth Planet. Sci. Lett., 19: 290-300. Pearce, J.A., Harris, N.B.W. and Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J. Petrol., 25: 956-983. Peccerillo, A. and Taylor, S.R., 1976. Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastasmonu area, northern Turkey. Contrib. Mineral. Petrol., 58: 63-81. Stacey, J.S. and Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two stage model. Earth Planet. Sci. Lett., 26: 207-221.
MIDDLETO LATEPROTEROZOICAWASIBMOUNTAINTERRAIN
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Walraven, F., 1984. Geochemistry of the Timbavati Gabbro of the Eastern Transvaal Lowveld, South Africa. Trans. Geol. Soc. S. Aft., 87: 211-223. Watters, B.R., 1974. Stratigraphy, igneous petrology and evolution of the Sinclair Group in southern South West Africa. Bull. Precambrian Res. Unit. Univ. Cape Town, 16:235 pp. Watters, B.R., 1976. A possible late Precambrian subduction zone in South West Africa. Nature (London), 257: 471-473. Watters, B.R., 1982. A St-isotopic study of a suite of Precambrian shoshonites from the Sinclair Group in southern South West Africa. Trans. Geol. Soc. S. Aft., 85: 81-86. Whalen, J.B., Currie, K.L. and Chappell, B.W., 1987. Atype granites: geochemical characteristics, discrimina-
tion and petrogenesis. Contrib. Mineral. Petrol., 95: 407419. Winchester, J.A. and Floyd, P.A., 1977. Geochemical discrimination of different magma series and their differentiation products using immobile elements. Chem. Geol., 20: 325-343. Winchester, J.A. and Max, M.D., 1984. Geochemistry and origins of the Annagh Division of the Precambrian Erris Complex, N.W. County Mayo, Ireland. Precambrian Res., 25: 397-414. York, D., 1966. Least-squares fitting of a straight line. Can. J. Phys., 44: 1079-1086. York, D., 1969. Least-squares fitting of a straight line with correlated errors. Earth Planet. Sci. Lett., 5: 320-324.
175
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Isotopic Evolution of the Middle to Late Proterozoic Awasib Mountain Terrain in Southern Namibia B.G. HOAL Geological Survey of Namibia, P.O. Box 2168, Windhoek 9000 (Namibia)
R.E. H A R M E R and B.M. EGLINGTON National Physical Research Laboratory, CSIR, P.O. Box 395, Pretoria 0001 (Republic of South Africa) (Received February 12, 1988; revision accepted October 15, 1988)
Abstract Hoal, B.G., Harmer, R.E. and Eglington, B.M., 1989. Isotopic evolution of the Middle to Late Proterozoic Awasib Mountain terrain in southern Namibia. Precambrian Res., 45: 175-189. The Middle to Late Proterozoic Awasib Mountain terrain (AMT) is situated at the approximate contact between the supracrustal Sinclair Sequence and the Namaqualand Metamorphic Complex in southern Namibia. Tholeiitic to alkaline metabasalts from the Kairab Metamorphic Complex, which forms part of the basement to the Sinclair Sequence in the AMT, yield an Rb-Sr isochron age of 1461 _+169 Ma with an initial STSr/SeSr (Ro) of 0.70269 +_12. A +2°1 Ma for these metabasalts appears to confirm the Rb-Sr age. These preliminary Pb-Pb isochron age v~f 1497 - -215 metabasalts have been intruded by the calc-alkaline Aunis tonalite gneiss which yields an Rb-Sr isochron age of 1271 + 62 M a and Ro of 0.7029 _+3. The low R0 obtained for both of these basement units implies that the ages are primary and hence that the isotope systematics have not been disturbed by subsequent deformation and metamorphism. Andesitic and rhyolitic volcanics of the Haiber Flats Formation (HFF) are correlated with the Sinclair Sequence. Basaltic andesites from the H F F have shoshonitic affinitiesand m a y be classifiedas "medium- to high-K" and "orogenic" in character. One basaltic andesite unit yields an age of 1086 + 44 M a and Ro of 0.70305 + 17, the low Ro indicating derivation from a mantle source region that is slightly depleted relative to a "bulk earth" or "uniform reservoir" composition. The errorchron age estimate of 1038_+74 Ma and Ro of 0.718___15 obtained for a rhyolite porphyry from the HFF is within error of that deduced for the basaltic andesite. However, the high Ro of this largely ignimbriticunit is probably due to a significant component of melted crust. The younger, possibly subvolcanic, Awasib granite yields an errorchron with an apparent age of 934 _+70 Ma and Ro of 0.719-!-_12. Scatter of these data may be attributed to compositional variation in a postulated crustal source. The A-type characteristics exhibited by both the HFF rhyolite and Awasib granite are compatible with their derivation by crustal anatexis. Available isotopic and geochemical data, together with field evidence, suggest an extensional palaeoenvironment for much of the AMT and Sinclair Sequence. While isotopic data alone are insufficient to establish the nature of the rifting, the broadly contemporaneous "Namaqua Orogeny" suggests that rifting may be collision-induced. Furthermore, the mantle source of abundant "orogenic" lavas in the AMT and Sinclair Sequence appears to owe its imprint of subduction-related metasomatism to a more ancient event.
Introduction The regional geology of the Awasib Mountain terrain (AMT) has been reviewed and described by Hoal (1985). In summary, the ter0301-9268/89/$03.50
rain consists of a partly deformed and metamorphosed volcano-sedimentary succession with associated high-level plutons and dyke swarms which overlie, or have intruded, a metamorphic basement of suspected Proterozoic age.
© 1989 Elsevier Science Publishers B.V.
176
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The volcano-sedimentary succession is correlated with the Sinclair Sequence, the type area of which is situated immediately to the east of the AMT in the "Rehoboth Sub-province" (after Hartnady et al., 1985). Watters (1974, 1976) proposed, largely on the basis of voluminous igneous rocks that exhibit calc-alkaline and shoshonitic affinities, that the Sinclair Sequence constituted a continental margin magmatic arc related to the subduction of oceanic crust beneath the Kalahari Craton. However, KrSner (1977) argued that field evidence favoured the deposition of undeformed
Sinclair rocks in an intracratonic trough or aulacogen, probably related to tectonic activity in the Namaqualand Metamorphic Complex (NMC) to the south. Much of the confusion relating to the origin of the Sinclair Sequence, namely, whether it is "orogenic" or essentially "anorogenic", is the result of a paucity of data, especially geochemical and isotopic. The latter aspect, in the form of geochronology, has further clouded the issue with regard to the relationship between the Sinclair Sequence and its supposed basement, the NMC. The relatively undeformed Sinclair
177
MIDDLE TO LATE PROTEROZOIC AWASIB MOUNTAIN TERRAIN
Sequence, which apparently overlies the NMC, has yielded ages, determined by a variety of radiometric techniques, that are both similar to and older than ages determined for the NMC (age data reviewed in Cahen and Snelling, 1984). Lithologies of both Sinclair and NMC affinity occur in the AMT, which consequently provides a unique opportunity for the study of the geochemical and isotopic evolution of this portion of Middle to Late Proterozoic crust. The purpose of this paper is to report the initial findings of an isotopic study on five different suites of samples from the AMT. The samples are representative of lithostratigraphic units that form part of both the metamorphic basement (Kairab Complex metabasalt and Aunis tonalite gneiss) and supracrustal succession (HFF basaltic andesite, HFF rhyolite and Awasib granite) within the AMT. The location of the study area with regard to the various tectonic provinces and sub-provinces in southern Africa is illustrated in Fig. 1 (after Hartnady et al., 1985 ).
Petrography Kairab Complex metabasalt This metabasaltic orthoamphibolite is characterised by relatively undeformed pillow structures and large plagioclase phenocrysts which constitute between 20 and 40% of the rock. These phenocrysts of plagioclase can exceed 9 mm in length and commonly exhibit subhedral to euhedral crystal forms despite recrystallisation. Alteration of plagioclase, although widespread, still allows the identification of original anorthitic cores and normal zoning. The presence of primary mafic phenocrysts is difficult to assess in view of the high proportion of poikiloblastic green hornblende. Equilibrium intergrowths of hornblende, redbrown biotite and epidote suggest that the metamorphic grade is transitional between greenschist and amphibolite facies. These in-
tergrowths occasionally form elongate aggregates that define an anastomosing foliation. Rounded, quartz-filled amygdales rarely exceed 4% of the rock and may be entirely absent. The groundmass is largely obscured by poikiloblastic hornblende, but appears to be made up of a microcrystalline intergrowth of plagioclase, biotite, ilmenite + magnetite and a variety of alteration products, e.g., epidote, carbonate, sericite and chlorite. Aunis tonalite gneiss This leucotonalite gneiss is medium- to coarse-grained, sometimes massive, but usually displaying a strong foliation. Plagioclase typically forms porphyroclasts which make up between 35 and 50% of the rock, whereas K-feldspar (mostly microcline) rarely exceeds 5 % and is commonly absent. Despite widespread alteration, plagioclase exhibits strong normal zoning (andesine cores) and albite twinning. Quartz, either as large strained crystals or smaller strain-free recrystallised grains, makes up between 25 and 45% of the rock. Strongly pleochroic brown biotite and blue-green hornblende together constitute about 10% of the rock, but hornblende is absent from approximately half of the samples in the suite. The foliation is defined by quartz-rich layers, commonly ribbon structures, and anastomosing trails of biotite_+ hornblende which wrap around porphyroclasts of feldspar. Augen structures have developed in the more highly deformed gneisses. Accessory minerals include sphene, apatite, zircon and magnetite. Epidote, sericite, chlorite and carbonate are common secondary minerals. Haiber Flats Formation basaltic andesite Flows are always porphyritic, but only occasionally amygdaloidal. Phenocrysts make up between 5 and 15% of the rock and comprise augite, plagioclase and possibly olivine. Subhedral to euhedral augite has usually been re-
178
placed by pseudomorphous green actinolite. Relict cores of augite are fairly common and indicate an original glomeroporphyritic texture in which individual crystals display both twinning and zoning. Resorption, although present, is not a widespread feature. Subordinate plagioclase phenocrysts are typically tabular in form and exhibit both twinning (Carlsbad and polysynthetic) and normal zoning (labradorite cores) despite widespread saussuritisation. The presence of olivine phenocrysts is suggested by pseudomorphous aggregates of green-brown biotite and magnetite. In view of the degree of alteration, it is difficult to establish if magnetite, in addition, constituted a primary phenocryst phase. The presence of small amygdales is suggested by patchy aggregates of intergrown quartz, epidote and actinolite. The groundmass is crypto- to microcrystalline and comprises a pilotaxitic to trachytic intergrowth of plagioclase laths together with pseudomorphs of actinolite after augite and interstitial K-feldspar. The groundmass has a cloudy appearance due to the presence of numerous magnetite granules and secondary minerals, namely epidote, sericite, actinolite, biotite and sphene. The secondary mineral assemblage indicates greenschist facies metamorphism, but evidence for deformation is restricted to fairly rare kink bands in biotite and actinolite.
B.G. HOAL ET AL.
tion, but polysynthetic twinning of primary plagioclase crystals indicates original compositions ranging from albite to oligoclase. Quartz phenocrysts, although usually rounded and resorbed, may exhibit bipyramidal and angular outlines. The fractured and strained nature of both feldspar and quartz phenocrysts suggests greater deformation than would be expected from pyroclastic emplacement alone. Sparse aggregates of secondary magnetite +biotite + chlorite_+ sericite may represent pseudomorphs after mafic mineral phenocrysts. The groundmass consists predominantly of a micro- to cryptocrystalline granular intergrowth of quartz and feldspar which can probably be attributed to devitrification. A fabric of variable intensity is defined by trails of magnetite granules, quartzofeldspathic lenses, flattened lithic fragments and stringers of secondary minerals. Compositional banding, illustrated by a variation in dark mineral content and possibly grain size, may enhance the fluidal texture. Lithic fragments vary considerably in their character but are typically porphyritic and spherulite-bearing. Some disaggregation and flattening of these fragments may be attributed to secondary or tectonic effects. Albitisation, sericitisation and silicification are commonly present, particularly in the more deformed rhyolites. Accessory minerals include zircon, magnetite, allanite, biotite and rare fluorite.
Haiber Flats Formation rhyolite Awasib granite Rhyolites comprise two main groups, namely pyroclastic and non-pyroclastic, with the former group being predominant. A highly porphyritic unit of lithic crystal tuff forms a distinctive horizon characterised by flattened lithic fragments that wrap around the feldspar and quartz phenocrysts. Phenocrysts constitute between 10 and 35% of the rock and are made up largely of K-feldspar (perthite), plagioclase, quartz and minor pseudomorphs after mafic minerals. Original feldspar proportions are difficult to assess in view of widespread albitisa-
This granite displays textural variations from very fine-grained porphyritic to mediumgrained hypidiomorphic granular, but can most commonly be termed a microgranite porphyry. K-feldspar constitutes up to 60% of the rock and is typically perthitic, although replacement by albite is not uncommon. Phenocrysts of Kfeldspar show typical Carlsbad twinning and may be rimmed by albite to give the rock a rapakivi appearance. Plagioclase usually makes
MIDDLE TO LATE PROTEROZOIC AWASIB MOUNTAIN TERRAIN
up less than 10% of the rock and is typically saussuritised. Whereas primary albite is usually cloudy in appearance, secondary albite displays less alteration and often exhibits characteristic "chessboard" twinning. Quartz constitutes between 25 and 35% of the rock and occurs as both large bipyramidal phenocrysts and small secondary blebs. Granophyric intergrowths of quartz and feldspar are fairly widespread. Sparse brown biotite and even more rare green hornblende are usually ragged or poikilitic in appearance and generally altered to aggregates of chlorite + epidote + magnetite + sphene. Accessory minerals include apatite, allanite, zircon and rare fluorite. Magnetite appears to be mainly secondary and is commonly rimmed by sphene granules. Indications of deformation in this granite include bent twin and exsolution lamellae in feldspars, fracturing and marginal granulation in feldspars and quartz, and protomylonitic textures in the proximity of shear zones.
Analytical procedures Concentrations of major and trace elements used in geochemical variation diagrams were determined by the Geological Survey of South Africa. The X R F techniques used have been adapted from Norrish and H u t t o n (1969) and Nisbet et al. (1979). Modifications and standards used for calibration have been described by Walraven (1984). Rb and Sr concentrations were determined by X R F analysis at the University of Pretoria using a Siemens SRS- 1 spectrometer with a Mo tube; correction for matrix effects was made by monitoring the Mo Compton scattering peak. Replicate analyses of standards and samples determined by isotope dilution (ID) indicate an uncertainty of 1.5% in the R b / S r ratios for this technique. Rb and Sr concentrations for samples of the Kairab Complex metabasalt were analysed by ID because of the low levels of Rb involved. Replicate analyses of USGS stan-
179
dards indicate an uncertainty of 0.8% in ID measurements of R b / S r ratios. The analytical procedures applied at the National Physical Research Laboratory (NPRL) for Sr isotope analysis have been described by Harmer and Sharpe (1985) and Harmer (1985) and involve standard cation-exchange extraction of Sr following sample dissolution in H F HNO3-HC1. Blanks were of the order of I ng. Isotopic analyses were carried out on a VG 354 mass spectrometer equipped with a multi-collector array and automatic computer control of the analytical procedure. All ratios are normalised to SeSr/88Sr=0.1194 and are relative to a value of 0.71023 + 4 (2 standard deviations on 28 measurements) for NBS standard Sr salt SRM 987. Sample Sr ratios are reproducible to 0.01%. Data were regressed using the method of York (1969) with blanket weighting factors based on one-sigma uncertainties of 1.5 and 0.01% in X and Y respectively in the case of X R F analyses, and 0.8 and 0.02% in X and Y respectively in the case of ID determinations. Goodness-of-fit of the regression line was tested by the value MSUM=SUMS/(n-2) as described by Brooks et al. (1972); scatter in the data in excess of the analytical uncertainty is reflected by MSUM > 2.5. In cases of excess, or "geological scatter", the regression line is termed an "errorchron" and the errors for the age estimate are calculated by York (1966), which augments the errors to account for the scatter. Errorchron age estimates are identified in the text and figures by an asterisk (*). All errors are given at the two-sigma level. Sample preparation for Pb isotopes at the NPRL involved anion exchange in small quartzglass columns and elution using HBr and H20 xRx. Total method blanks of less than I ng were considered negligible. The Pb fraction was loaded on single Re filaments with 1 M HNO3 ~X and 1 N H3PO4, and isotopic analyses carried out on a VG MM30 mass spectrometer. Five runs of the NBS 981 Pb standard yielded mean values of 2°6pb/2°4pb = 16.897 + 5, 2°7pb/
180
B.G. HOAL ET AL.
2°4pb=15.441+7, and 2°sPb/2°4pb=36.539 +24. A reproducibility in 2°6Pb/2°4pb and 2°7pb/2°4pb of 0.09% was achieved and data were regressed using the technique of York (1969). A correlation coefficient of 0.946 was determined from five runs of NBS 981 and by using the formulation of Ludwig (1980).
Geochemistry Isotopic analyses form the main theme of this paper. For this reason, and in order to conserve space, tabulated geochemical data have not been included. Data for all of the rock suites analysed have been plotted on a conventional AFM diagram (Fig. 2) , with tholeiitic and calc-alkaline fields according to Irvine and Baragar (1971). However, identification of actual rock types using major elements may be problematic in view of widespread alteration and metamorphism. Using the approach of Winchester and Floyd (1977), we have utilized the immobile element ratios Nb/Y and Zr/TiO2 in order to interpret these suites meaningfully in terms of magma type and chemical affinity (Fig. 3). Although this diagram was originally intended for use with altered and metamorphosed volcanic rocks only, its use has subsequently been exFeO* + Awasib Granite (II) * Haiber Flats Rhyolite (I0] • Haiber Flats Andesite (8)
Aun,, Tooo,l,e n:,?o ,,o,
K20 + NazO
MgO
Fig. 2. AFM diagram showing the compositional variation of samples from the Awasib Mountain terrain. Tholeiitic (TH) and calc-alkaline (CA) fields are after Irvine and Baragar ( 1971 ).
tended to orthogneisses of plutonic origin, e.g., Winchester and Max (1984).
Kairab Complex metabasalt While data points on an AFM diagram (Fig. 2) indicate a predominantly tholeiitic character for this metabasalt (only two samples in the calc-alkaline field), the relatively high Nb/Y ratios reflect a strong alkaline tendency (Fig. 3). An average A1203 content in excess of 20% is attributed to the highly plagioclase phyric character of these flows. Low average contents of K20 (0.2%), TiO2 (0.4%), Zr (36 ppm), Y (9 ppm), Cr (39 ppm) and Ni (14 ppm) are in agreement with chemical characteristics described for island arc tholeiites (e.g., Pearce, 1982, 1983). Nb, however, at an average level of 8 ppm, appears to be enriched relative to the latter. Furthermore, the spread of data points across several fields in the discriminant diagrams of Pearce and Cann {1973) could suggest, instead, a continental setting for these metabasalts.
Aunis tonalite gneiss A calc-alkaline affinity and variation in composition from quartz diorite ("andesite") to granodiorite ("rhyodacite") are indicated for this gneiss (Figs. 2 and 3). On average this unit may be classified as a leucotonalite, although normative feldspar compositions suggest some overlap with calcic trondhjemite. Low average contents of high field strength elements (HFSE), e.g., Zr (98 ppm), Y (28ppm), Nb (6 ppm), and moderate to low contents of large ion lithophile elements (LILE), e.g., K20 (1.1%), Rb (26 ppm), are similar to chemical characteristics described for calc-alkaline arc granitoids (e.g., Brown et al., 1984). Data points for this tonalite gneiss also plot consistently in the "volcanic arc granite" field in the geochemical discriminant diagrams of Pearce et al. (1984).
MIDDLETO LATEPROTEROZOICAWASIBMOUNTAINTERRAIN
1.00
-I- + + +
0.10 C~J o F--
181
Rhyodocite
+: ~ drl(~ ~.~ ~ .~ Rhyolite yolite -F ~
Oooi,e
o
+ Awasib Granite (11) * Hoiber Flats Rhyolite (10) • Hoiber Flats Andesite (8) ~ A Aunis Tonolite Gneiss (10) , ~ o 2 " " Koirab Metabasolt (,0)
I
N 0.01 S u b ~
0.001 0.01
basalt
Alkali basalt
I
I
I
I
0.10
1.00
tO.O0
NblY Fig. 3. Nb/Y versus Zr/TiO2 discrimination diagram (after Winchester and Floyd, 1977) showing the distribution of the various rock units discussed.
Haiber Flats Formation basaltic andesite The basic to intermediate character of this unit is illustrated by the slight overlap of data points from "andesite" to "sub-alkaline basalt" in Fig. 3. A calc-alkaline (Fig. 2) and "orogenic" chemical character (after Gill, 1981; Bailey, 1981 ) is defined by the low average contents of HFSE, e.g., TiO2 (0.9%), Zr (144 ppm ), Y (28 ppm), Nb (7 ppm), and low Cr (130 ppm). Relatively high contents of K20 (2.1% ), Rb (62 ppm), Sr (637 ppm) and Ba (639 ppm) are typical of medium- to high-K calc-alkaline andesites (Peccerillo and Taylor, 1976; Ewart, 1982 ). The data points plot consistently in the "calc-alkaline basalt" field in the discriminant diagrams of Pearce and Cann (1973). Haiber Flats Formation rhyolite Rhyolites which make up this unit are mildly peralkaline and metaluminous. Many of the chemical characteristics attributed to A-type granites (as defined by Collins et al., 1982;
Whalen et al., 1987 ) are demonstrated by these flows, e.g., relatively high K20/Na20 (about 1 ), high FeO*/(FeO* + MgO) of approximately 0.8, and significant enrichment in the HFSE Zr (289 ppm), Y (101 ppm) and Nb (17 ppm). Awasib granite Compositionally, this granite shows a remarkable similarity to the HFF rhyolite and may, therefore, represent a subvolcanic or comagmatic intrusion. However, data points for these two units show a slight separation in Fig. 3. A-type characteristics are similar to the HFF rhyolite, e.g., metaluminous, mildly peralkaline, high K20/Na20 (about 1), FeO*/ (FeO* + MgO) of about 0.8, and significant enrichment in the HFSE Zr (222 ppm), Y (130 ppm) and Nb (19 ppm). These chemical features are distinct from typical I-type granites (Collins et al., 1982; Whalen et al., 1987) and data points plot fairly consistently in the "within plate field" in the discriminant diagram of Pearce et al. (1984).
182
B.G. HOAL
Isotope results Results are shown in Tables 1 and 2 and on conventional isochron diagrams in Figs. 4-8.
Kairab Complex metabasalt The ten analysed samples of the Kairab Complex metabasalt define an isochron (MSUM--2.48) with an age of 1461 + 169 Ma and an initial SVSr/86Sr ratio (Ro) of 0.70269 + 12. The separation of sample BH 774 from the main cluster of data points in Fig. 4 clearly exerts a significant control on the slope of the regression line. Consequently, Pb separates were measured for four samples which fall within the main sample cluster on the R b - S r isochron. The inset in Fig. 4 illustrates that
ET AL.
these samples define a P b - P b isochron (MSUM = 1.1 ) with an age of 1467+22°11 Ma and /t = ~.ua ~7 +o.14_o.1,~The/z . value is slightly higher than the value of 9.74 preferred by Stacey and Kramers (1975) for the second stage of their twostage model of Pb isotope evolution. The Ro estimate for this unit is low, an indication that the source material of this metabasalt had a Rb/ Sr ratio similar to, or slightly lower than, a "bulk Earth" mantle composition.
Aunis tonalite gneiss Nine samples of the Aunis tonalite gneiss define an isochron ( M S U M = 1.3) with an age of 1271 _+62 Ma and Ro of 0.7029 + 3. Sample BH 745 was excluded from the regression as it appears slightly weathered in thin section and deviates from the isochron line by more than three times the analytical uncertainty. Exclusion of
TABLE 1 Isotopic data, Awasib M o u n t a i n terrain ( b a s e m e n t ) Lithology and sample n u m b e r
Rb (ppm)
Sr (ppm)
STRb/S6Sr (atomic)
STSr/S%r (atomic)
2.01 12.1 1.37 1.10 0.89 0.75 7.92 3.92 2.30 5.38
307 254 384 265 290 328 395 313 274 360
0.0189 0.1372 0.0103 0.0120 0.0089 0.0067 0.0579 0.0362 0.0243 0.0433
0.70297 ± 2 0.70565 ± 1 0.70286 ± 1 0.70340 ± 1 0.70280 ± 1 0.70289 ± 1 0.7040 ± 1 0.70315 ± 1 0.70323 _+ 1 0.70337 _+1
29.8 30.1 25.3 19.9 25.9 25.1 25.4 28.9 24.6 19.7
121 205 214 227 193 200 208 190 181 210
0.713 0.425 0.342 0.254 0.388 0.368 0.353 0.440 0.393 0.271
0.71609 0.71131 0.70912 0.70766 0.70977 0.70951 0.70915 0.71082 0.71015 0.70782
2°~Pbff°4pb
2°7pb/2°4pb
2°SPbff°4pb
20.931 17.840
15.858 15.565
38.205 37.515
17.870
15.584
37.569
18.689
15.660
38.494
Kairab Complex metabasaIt BH BH BH BH BH BH BH BH BH BH
770 774 783 901 902 903 904 905 906 907
Aunis tonalite-gneiss BH BH BH BH BH BH BH BH BH BH
727 745 746 747 748 749 750 751 752 753
_+1 + 1 ±1 ±1 ± 1 ± 1 ±1 _+1 +_1 ± 1
183
MIDDLE TO LATE PROTEROZOIC AWASIBMOUNTAINTERRAIN TABLE 2 Isotopic data, Awasib M o u n t a i n terrain (cover a n d highlevel intrusions)
tion and crystallisation of the Aunis tonalite gneiss.
Haiber Flats Formation basaltic andesite Lithology a n d sample n u m b e r
Rb (ppm)
Sr (ppm)
STRb/ S6Sr (atomic)
STSr/S6Sr (atomic)
Haiber Flats Formation basaltic andesite BH BH BH BH BH BH BH BH
687 688 689 690 691 692 693 694
69.7 87.1 76.0 38.9 99.3 28.8 47.1 46.4
699 647 639 721 533 639 606 613
0.289 0.390 0.344 0.156 0.539 0.130 0.225 0.219
0.70763 _+1 0.70920_+ 1 0.70846 Jr 1 0.70544_+ 1 0.71122 _+ 1 0.70507 _ 1 0.70657 ___1 0.70639 _+ 1
Haiber Flats Formation rhyolite BH BH BH BH BH BH BH BH BH BH
504 513 585 611 697 705 706 707 708 709
163 152 182 185 181 162 160 290 249 252
48.7 37.8 30.1 46.9 46.4 32.4 25.0 16.5 47.0 26.0
9.84 11.8 14.9 11.6 11.5 14.8 19.0 55.0 15.7 29.3
0.87197 _+2 0.89284_+2 0.97459_+ 2 0.88677 _+2 0.88916 +_ 1 0.93042 +_ 1 0.98562 _+ 1 1.53662 _+2 0.96216_+ 1 1.17164_+ 1
119 135 269 292 288 296 164 225 247 281 179
71.7 40.7 22.8 10.3 12.8 26.6 52.2 13.5 15.4 10.0 31.0
4.84 9.74 35.8 91.6 71.4 33.8 9.21 50.8 49.7 91.0 17.1
0.78325 _+2 0.85780 + 2 1.21040_+ 2 1.90512 + 2 1.69006 _+3 1.20665 _+2 0.83989 _+2 1.25923 _+2 1.4389 __6 1.9284 +_5 0.92401 _+ 1
Awasib granite BH BH BH BH BH BH BH BH BH BH BH
618 619 677 678 679 680 681 682 683 684 685
this datum has little effect on the age and Ro estimates. Again the Ro estimate for the Aunis tonalite gneiss is low, an indication that the source material of this granitoid had a Rb/Sr ratio similar to a "bulk Earth" or "uniform reservoir" mantle composition. This precludes a significant crustal pre-history or "residence time" for this unit, and implies that the estimated age reflects the time of primary deriva-
The eight analysed samples define a precise isochron (MSUM=0.8) with an age of 1086 + 44 Ma and Ro of 0.70305_+ 17. The low Ro indicates a source region having slight timeintegrated depletion in Rb/Sr relative to a "bulk Earth" mantle composition.
Haiber Flats Formation rhyolite These data, which exhibit significant scatter, define an errorchron (MSUM = 7.0) with an age of 1038 + 74* Ma and Ro of 0.718 + 15". While it may be significant that both of these parameters are within error of those deduced for the basaltic andesite samples, the high Ro suggests a significant component of melted crust in the genesis of these rhyolites. A choice between these alternatives is precluded by the rather large (augmented) errors on the Ro estimate. Sample scatter may be attributed to one or more of the following: ( 1 ) the samples are not of the same age; (2) the samples have different values of Ro; or (3) the samples did not remain closed systems to Rb and Sr. Field relationships suggest that option (1) is unlikely, but the incorporation of exotic lithic fragments in the rhyolites could possibly account for differences in Ro during crystallisation of the lavas. However, as the rhyolites show extensive devitrification, the most likely explanation is option (3), i.e., loss of Rb and/or radiogenic Sr during post-crystallisation alteration, particularly during the formation of chlorite and epidote.
A wasib granite Ten samples of the Awasib granite also exhibit significant scatter (MSUM-- 15 ) and the apparent age of 957+50* Ma and Ro of 0.717 + 8* needs to be interpreted with caution. Sample BH 682 was excluded from the regres-
184
B.G. HOAL ET AL.
0.706 -
4 KAIRAB COMPLEX METABASALT
~
Age = 1461 4- 169 Me R o = 0 . 7 0 2 6 9 4- 12 MSUM = 2 . 4 8
0.705
-~ (~ r--
•
77
0.704
9
0
~
J
16.1
/ 901•
o,7o3
/
.a 907
906 • - 903~077
= 1467 Mu - 9 . 8 7 MSUM = I I
15.8
•905
o
-
o
Ma (+201-215 (+0.14-0.15)
Ma)
j_//~/'9°0"~ 90,5111907
s5 ~ 9 ~ z
902 15.2
I 18
17
0.702
I 0.02
0
I 0,04
I 0.06
I 0.08 87
Rb/
I OAO
I ~ 19 20 2 0 6 / 2 0 4 Pb
I 0.12
I 0.14
I 21
I 0.16
86
Sr
Fig. 4. Kairab Complex metabasalt: whole-rock (ten-point) Rb-Sr isochron. The inset illustrates a preliminary whole-rock (four-point) Pb-Pb isochron for selected samples.
0 . 7 2 0 --
AUNIS T O N A L I T E
GNEISS
Age = 1271 ± 6 2 Mo Ro = 0 . 7 0 2 9 ± 3 7 MSUM = 1.3
2
7
=
/
0.715
7450 ~
"
0.710
0.705
0.700 0
I OI
I 02
I 03
I 04
I 05
I 06
I 07
S7Rb / S6Sr
Fig. 5. Aunis tonalite gneiss: whole-rock (nine-point) RbSr isochron. The open symbol BH 745 was excluded from the regression line calculation.
sion because of its albitised appearance in thin section. Although not conclusive, the age estimate is consistent with field evidence, i.e., the granite is younger than the HFF volcanics. The Awasib granite is holocrystalline and contains only minor secondary material {especially when compared with the rhyolites), consequently it is unlikely that the large scatter is due entirely to secondary effects. The high apparent Ro may thus imply the involvement of pre-existing crustal material in the genesis of the Awasib granite. It is likely that this material would be compositionally variable and hence incomplete mixing of partial melt fractions extracted from the crustal material would give rise to Ro heterogeneity in the parent magma. If Awasib granite data are divided into two groups on the basis of R b / S r ratio, i.e., high ratios (samples BH 677,678, 679, 680, 683,684) and low ratios (samples BH 618, 619, 681, 685), two sub-parallel errorchrons are apparent (Fig. 8). These yield age estimates of 896 + 72* Ma (high ratios) and 895+ 180" Ma (low ratios), with R0
185
MIDDLE TO LATE PROTEROZOIC AWASIB MOUNTAIN TERRAIN
0.711
HAIBER FLATS FORMATION 0.710
f
--"
B A S A L T I C ANDESITE Aoe : 1086 + 4 4 Ma Ro 0 . 7 0 3 0 5 ± l? MSUM = 0.82
0.709
f
sea
~e-"~89
0.708 0.707 693 ¢o
0.706 0.705
j~e 690 692
•
0.704 0.703
I
1
oJ
0.2
0,702
1
0.3 87Rb/ e6Sr
I
I
0.4
0.5
Fig. 6. Haiber Flats Formation basaltic andesite: whole-rock (eight-point) Rb-Sr isochron.
1.5
HAIBER FLATS FORMATION
/
RHYOLITE PORPHYRY Age = t 0 3 8 ± 7 4 ~ Me
1.4 1.3 1.2
I,I 1.0 0.9 0.8 0.7 0
I
I0
I
20
I
30
87Rb/e6Sr Fig. 7. Haiber Flats Formation rhyolite: whole-rock (ten-point) Rb-Sr errorchron.
I
4.0
I
50
/
186
B.G. HOALET AL.
estimates of 0.77 + 0.05* and 0.72_+ 0.02*, respectively. It is plausible, then, that scatter in this data set is attributable to variation in Ro at the time of crystallisation. Incomplete mixing can produce misleading ages in whole-rock isotopic dating systems (Juteau et al., 1984) and the deduced age for the Awasib granite must be viewed with caution.
1.9 • 18
AWASIB GRANITE
zff/,./"67e
/
Age = 957 ± 50 * Me Ro = 0,717 ¢ 8 "~ MSUM = q5
17
6
16
/
,
/ //
1.5 683
u)
•/
13 0682 i.2
6 / /
I0
/I /I / l ,/v7
0.9
//
0.8
~81
~6e5
O.7 0
io
20
30
4.0
50
60
70
80
90
8TRb/~Sr Fig. 8. Awasib granite. The solid line represents a wholerock (ten-point) R b - S r errorehron; the broken lines are possible parallel errorchrons of about 895 Ma age (see text). The open symbol BH 682 was excluded from the regression line calculation.
Discussion
Age of the AMT and Sinclair Sequence The age of ~ 1460 Ma obtained for the Kairab Complex metabasalt is considered to represent the time of extrusion of these flows in view of the agreement between Rb-Sr and Pb-Pb systems. A minimum age for the Kairab Complex basement is provided by the age of 1271 _+62 Ma obtained for the Aunis tonalite gneiss, since this
age is considered to represent a primary crystallisation event. This age further coincides with a likely maximum age of 1300 Ma for the D2 Namaqua event (Cahen and Snelling, 1984, p. 65) and is consistent with a previous correlation between the Kairab Complex and the NMC (Hoal, 1985). The age of the HFF volcanism is provided by the isochron date of 1086 + 44 Ma for the basaltic andesites. Open system behaviour during devitrification and alteration has given rise to errorchron systematics in the rhyolitic members but the estimated age of 1038 + 74 Ma is comparable with that of the basaltic andesites. Correlation of the HFF with the Barby Formation of the Sinclair Sequence (Hoal, 1985) would appear to be incorrect in view of the age of t392 +_33 Ma inferred for the latter by Watters (1982). However, Watters' (1982) Barby Formation data were recalculated with the NPRL regression program and found to yield an age estimate of 1238_+71. Ma and Ro of 0.7028_+ 5* with an MSUM of 3.9 (apparently Watters inadvertently quoted one-sigma errors on his fig. 3 ). Excluding the samples 1557 and 1565, which deviate from the regression line by more than three times the uncertainty (in "X"), the remaining twelve data points yield an isochron estimate (MSUM =0.9) of 1190 + 38 Ma and Ro value of 0.70298_+ 27. The two deviant samples 1557 and 1565 had Ro values of 0.7045 and 0.7043, respectively, at 1190 Ma (i.e., differing by more than the analytical uncertainty ) implying that these rocks were derived from a source of different Ro composition than the bulk of the Barby Formation samples. Independent support for the age of the Barby Formation is difficult to obtain as several granite bodies that have clearly intruded this formation paradoxically yield much older ages. These ages, which are largely based on U-Pb zircon determinations (reviewed by Watters, 1982), may reflect the existence of relict zircons in the approximate age range 1350-1250 Ma. It is further suggested that these inherited zircons were derived from a source of similar
MIDDLE TO LATE PROTEROZOIC AWASIB MOUNTAIN TERRAIN
age to the Kairab Complex or Aunis tonalite gneiss basement in the AMT. Whereas our estimate of 1190 Ma for the Barby Formation is also younger than KrSner's (1977) preferred age of 1264+23 Ma (wholerock Rb-Sr), it is still significantly older than the HFF and a direct time correlation between these formations is not justified. Similarity in Ro suggests, however, that the basic volcanics of the Barby Formation and HFF were both mantle-derived without any significant contribution from older crust. It is not possible to estimate confidently the true age of the Awasib granite from the available data since wide variations in Ro are suggested, probably due to incomplete mixing of crust-derived partial melts. However, field evidence and isotopic characteristics are consistent with a subvolcanic relationship between the Awasib granite and the HFF rhyolite.
Tectonic setting of the A M T and Sinclair Sequence Application of tectonic discrimination diagrams has met with varying degrees of success. The Kairab Complex metabasalt, in particular, has produced results indicative of a heterogeneous source rather than any specific tectonic setting. It is likely that this metabasalt, together with associated felsic volcanics, represents a bimodal mafic-silicic suite of continental or oceanic affinity. Whereas bimodal volcanism appears to be most characteristic of continental rifts, it also occurs in a variety of continental and oceanic settings, e.g., cratonic areas of minor compression, collision zones, back-arc basins and oceanic ridges (Ewart, 1979; Condie, 1982). The alkaline character of the Kairab Complex metabasalt is similar to "anomalous" volcanic arc basalts, e.g., Grenada {Pearce, 1982), but the paucity of associated calc-alkaline andesites is more compatible with a back-arc setting. Unfortunately, field relations do not indicate whether or not this metabasalt is underlain by older continental crust. The calc-alkaline Aunis tonalite gneiss, which
187
has an age within error of the Kairab Complex metabasalt, exhibits chemical characteristics which closely resemble those of continental arc granitoids (after Brown et al., 1984; Pearce et al., 1984). There do not, however, appear to be any volcanic equivalents of the Aunis tonalite gneiss within the AMT and this granitoid may, therefore, represent the root zone of an ancient arc. Within the cover sequence of the AMT, the HFF basaltic andesite displays many of the geochemical characteristics thought to be typical of an island arc or active continental margin setting (Pearce, 1982, 1983). Voluminous pyroclastic rhyolites of the HFF exhibit A-type characteristics (after Collins et al., 1982; Whalen et al., 1987) and a high R0 indicative of crustal anatexis. However, as with the compositionally similar Awasib granite, it must be stressed that A-type characteristics are not diagnostic of any specific tectonic setting (W.J. Collins, personal communication, 1987). The bimodal nature of interbedded basaltic andesites and rhyolites in the HFF probably excludes their extrusion in a graben-hosted arc, but is not incompatible with a back-arc setting. Geological evidence indicates that the HFF volcanics were extruded in fault-bounded troughs in the Kairab Complex basement, and have undergone relatively mild deformation and metamorphism. We conclude that available isotopic, geochemical and field data are consistent with an extensional tectonic setting for the supracrustal part of the AMT and the Sinclair Sequence. Broadly contemporaneous collisional tectonics between 1.2 and 0.9 Ga in the adjacent Gordonia Belt of the NMC further suggest that this extension may be related to continental convergence, although not necessarily associated with an active continental margin (cf. Keweenawan "impactogen" described by Gordon and Hempton, 1986). However, the geochemical character of HFF basaltic andesite indicates subduction-related metasomatism in its mantle source region which could, therefore, have
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been inherited from a more ancient event. Evidence for such an event may be provided by the presence of tholeiitic to alkaline metabasalts in the Kairab Complex and/or the calc-alkaline Aunis tonalite gneiss.
Acknowledgements Financial support for the XRF and isotopic analyses was provided by the Geological Survey of Namibia. We thank D.J, Farrow for technical assistance with the isotopic analyses. A.J. W a l k e r a n d S. S c h e r m a c h e r a r e t h a n k e d f o r drafting the diagrams. Helpful comments from an anonymous reviewer are gratefully acknowledged.
References Bailey, J.C., 1981. Geochemical criteria for a refined tectonic discrimination of orogenic andesites. Chem. Geol., 32: 139-154. Brooks, C., Hart, S.R. and Wendt, I., 1972. Realistic use of two-error regression treatments applied to rubidium data. Rev. Geophys. Space Phys., 10: 551-577. Brown, G.C., Thorpe, R.S. and Webb, P.C., 1984. The geochemical characteristics of granitoids in contrasting arcs and comments of magma sources. J. Geol. Soc. London, 141: 413-426. Cahen, L. and Snelling, N.J., 1984. The geochronology and evolution of Africa. Clarendon Press, Oxford, 512 pp. Collins, W.J., Beams, S.D., White, A.J.R. and Chappell, B.W., 1982. Nature and origin of A-type granites with particular reference to southeastern Australia. Contrib. Mineral. Petrol., 80: 189-200. Condie, K.C., 1982. Plate Tectonics and Crustal Evolution. Pergamon Press, New York, 310 pp. Ewart, A., 1979. A review of the mineralogy and chemistry of Tertiary-Recent dacitic, latitic, rhyolitic and related salic volcanic rocks. In: F. Barker (Editor), Trondhjemites, Dacites, and Related Rocks. Elsevier, Amsterdam, pp. 13-122. Ewart, A., 1982. The mineralogy and petrology of TertiaryRecent orogenic volcanic rocks: with special reference to the andesitic-basaltic compositional range. In: R.S. Thorpe (Editor), Andesites. Wiley, Chichester, pp. 2595. Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics. Springer-Verlag, New York, 390 pp. Gordon, M.B. and Hempton, M.R., 1986. Collision-induced rifting: the Grenville Orogeny and the Keweenawan Rift of North America. Tectonophysics, 127: 1-25.
B.G. HOAL ET AL.
Harmer, R.E., 1985. Rb-Sr isotopic study of units of the Pienaars River Alkaline Complex, north of Pretoria, South Africa. Trans. Geol. Soc. S. Afr., 88: 207-214. Harmer, R.E. and Sharpe, M.R., 1985. Field relations and strontium isotope systematics of the marginal rocks of the Eastern Bushveld Complex. Econ. Geol., 80: 813837. Hartnady, C.J., Joubert, P. and Stowe, C.W., 1985. Proterozoic crustal evolution in southwestern Africa. Episodes, 8: 236-244. Hoal, B.G., 1985. Preliminary report on the geology of the south-eastern part of Diamond Area No. 2, South West Africa/Namibia. Commun. Geol. Surv. S.W. Afr./Namibia, 1: 9-21. Irvine, T.N. and Baragar, W.R.A., 1971. A guide to the chemical classification of the common volcanic rocks. Can. J. Earth Sci., 8: 523-548. Juteau, M., Michard, A., Zimmerman, J. and Albarede, F., 1984. Isotopic heterogeneities in the granitic intrusion of Monte Capanne (Elba Island, Italy ) and dating concepts. J. Petrol., 25: 532-545. KrOner, A., 1977. The Sinclair aulacogen - a late Proterozoic volcano-sedimentary association along the Namib Desert of southern Namibia (SWA). Abstr. 9th Colloq. Afr. Geol., University of GSttingen, pp. 82-83. Ludwig, K.R., 1980. Calculation of uncertainties of U-Pb isotope data. Earth Planet. Sci. Lett., 46: 212-220. Nisbet, E.G., Dietrich, V.J. and Essenwein, A., 1979. Routine trace element determination in silicate minerals and rocks by X-ray fluorescence. Fortschr. Mineral., 57: 264279. Norrish, K. and Hutton, J.T., 1969. An accurate X-ray spectrographic method for the analysis of a wide range of geological samples. Geochim. Cosmochim. Acta, 33: 431-453. Pearce, J.A., 1982. Trace element characteristics of lavas from destructive plate margins. In: R.S. Thorpe {Editor), Andesites. Wiley, Chichester, pp. 525-548. Pearce, J.A., 1983. Role of subcontinental lithosphere in magma genesis at active continental margins. In: C.J. Hawkesworth and M.J. Norry (Editors), Continental Basalts and Mantle Xenoliths. Shiva Publishing, Nantwich, pp. 230-249. Pearce, J.A. and Cann, J.R., 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth Planet. Sci. Lett., 19: 290-300. Pearce, J.A., Harris, N.B.W. and Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J. Petrol., 25: 956-983. Peccerillo, A. and Taylor, S.R., 1976. Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastasmonu area, northern Turkey. Contrib. Mineral. Petrol., 58: 63-81. Stacey, J.S. and Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two stage model. Earth Planet. Sci. Lett., 26: 207-221.
MIDDLETO LATEPROTEROZOICAWASIBMOUNTAINTERRAIN
189
Walraven, F., 1984. Geochemistry of the Timbavati Gabbro of the Eastern Transvaal Lowveld, South Africa. Trans. Geol. Soc. S. Aft., 87: 211-223. Watters, B.R., 1974. Stratigraphy, igneous petrology and evolution of the Sinclair Group in southern South West Africa. Bull. Precambrian Res. Unit. Univ. Cape Town, 16:235 pp. Watters, B.R., 1976. A possible late Precambrian subduction zone in South West Africa. Nature (London), 257: 471-473. Watters, B.R., 1982. A St-isotopic study of a suite of Precambrian shoshonites from the Sinclair Group in southern South West Africa. Trans. Geol. Soc. S. Aft., 85: 81-86. Whalen, J.B., Currie, K.L. and Chappell, B.W., 1987. Atype granites: geochemical characteristics, discrimina-
tion and petrogenesis. Contrib. Mineral. Petrol., 95: 407419. Winchester, J.A. and Floyd, P.A., 1977. Geochemical discrimination of different magma series and their differentiation products using immobile elements. Chem. Geol., 20: 325-343. Winchester, J.A. and Max, M.D., 1984. Geochemistry and origins of the Annagh Division of the Precambrian Erris Complex, N.W. County Mayo, Ireland. Precambrian Res., 25: 397-414. York, D., 1966. Least-squares fitting of a straight line. Can. J. Phys., 44: 1079-1086. York, D., 1969. Least-squares fitting of a straight line with correlated errors. Earth Planet. Sci. Lett., 5: 320-324.