evolution of pan-african island arc assemblages in …precambrian research, 53 ( 1991 ) 99-118 99...

Precambrian Research, 53 ( 1991 ) 99-118 99 Elsevier Science Publishers B.V., Amsterdam

Evolution of Pan-African island arc assemblages in the southern Red Sea Hills, Sudan, and in southwestern Arabia as

exemplified by geochemistry and geochronology

A. Krrner a, p. Linnebacher a, R.J. Stern b, T. Reischmann a,c, W. Manton b and I.M. Hussein a.d a Institutj~r Geowissenschaften, Universitdt Mainz, Postfach 3980, 6500 Mainz, FRG

b Programs in Geosciences, University of Texas at Dallas, P.O. Box 688, Richardson, TX, 75083-0688, USA c Max-Planck-lnstitutf~r Chemie, Postfach 3060, 6500 Mainz, FRG

d Geological Research Authority, Red Sea Hills Office, P.O. Box 573, Port Sudan, Sudan

(Received April 15, 1990; revised and accepted December 1, 1990)

ABSTRACT

Kr6ner, A., Linnebacher, P., Stern, R.J., Reischmann, T., Manton, W. and Hussein, I.M., 1991. Evolution of Pan-African island arc assemblages in the southern Red Sea Hills, Sudan, and in southwestern Arabia as exemplified by geochemistry and geochronology. In: R.J. Stern and W.R. van Schmus (Editors), Proterozoic Crustal Evolution in the Late Protero- zoic. Precambrian Res., 53:99-117.

We report Rb-Sr whole-rock and zircon ages for metavolcanic and plutonic associations in the southeastern part of the Red Sea Hills, Sudan, and show that these rocks constitute one of the earliest Pan-African arc assemblages within the Arabian-Nubian shield. The remarkable similarity in geochemistry and age between these and comparable rocks in southwestern Arabia suggests that the southeastern Red Sea Hills and the Al-Lith area of Arabia probably formed part of one large arc complex that was probably accreted to the African continent about 720 Ma ago.

The Erkowit and nearby Dahand plutons SW of Suakin yield ages of 850-870 Ma and intrude older metavolcanic suites; one of these, exposed in Khor Ashat, provides an imprecise Rb-Sr age of 860+ 101 Ma. Several rhyolite suites from the hills SE of Tokar and near the border with Ethiopia yield variable Rb-Sr ages between ~ 670 and 770 Ma that we relate to partial resetting, whereas single zircons from these rocks gave 2°7pb/2°rpb ages of ~ 840-855 Ma. An early syn-tectonic granite infolded with the Tokar metavolcanic assemblage has a 2°7pb/2°6pb age of 827 + 33 Ma, while a post-tectonic granite, probably resulting from intracrustal melting after arc/continent collision, was dated at 652 + 14 Ma.

Introduction

The history of arc and microcontinent accretion in the central and southern Arabian shield (AS) during late Proterozoic times is now reasonably well understood as a result of systematic mapping (Delfour, 1981; Green- wood et al., 1982; Stoeser and Camp, 1985) and a wealth of geochemical and isotopic data (Roobol et al., 1983; see also references in Stoeser, 1986 ). The oldest arc terrains formed about 850-950 Ma ago, as documented by the widespread Baish and Baha volcano-sedimentary assemblages and associated calc-alkaline

intrusions of which the 901 + 37 Ma old Bidah pluton is the largest (Marzouki et al., 1982). Younger arc complexes include those of the 750-850 Ma period of which regionally extensive plutonic suites ranging in composit ion from gabbro to granodiorite are the most prominent (Stoeser, 1986) and best dated (e.g. Fleck et al., 1980; Fleck and Hadley, 1982 ). All these suites were generated in an oceanic en- vironment, without involvement of older continental crust (Pallister et al., 1990) and pre- sumably over subduction zones of unknown polarity and orientation. So far, at least one terrain has been identified in the southeastern

0301-9268/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.

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EVOLUTION OF PAN-AFRICAN ISLAND ARC ASSEMBLAGES IN THE SOUTHERN RED SEA HILLS | 01

AS that includes mid-Proterozoic crustal material (the southernmost Afif terrain) and has continental affinity on the basis of Pb isotopic data. This is presumed to have collided with the juvenile arc assemblage as a microcontinent some 700 Ma ago (Stoeser et al., 1984; Stoeser and Camp, 1985 ).

No such information has so far been available for the Nubian side of the shield that lies opposite southwestern Arabia in the southern Red Sea Hills ( R S H ) o f the Sudan. Although shown on published maps as part of the Pan- African magmatic province, the rock assemblages in the coastal region between Port Su- dan and the Ethiopian border and inland to the prominent escarpment marking the desert pla- teau of the RSH have only been described as part of reconnaissance surveys available in un- published reports in the offices of the Su- danese Geological Research Authority and in a few generalized papers that provide little detailed information. (e.g. Vail, 1976, 1983; Ahmed, 1979). Klemenic and Poole (1988) provide the only reliable geochronological data of the region.

Our previous geochemical and geochronological work in the A1-Lith region of Saudi Ara- bia (Kr6ner et al., 1983; Reischmann et al., 1983) led us to investigate the southern RSH as part of a comparative exercise initiated during IGCP Project 164 (Pan-African crustal evolution in the Arabian-Nubian shield). The purpose was to see whether the concept of juvenile arc accretion as established for Arabia can also be applied to this little known part of the Nubian shield and whether this region can be considered as a westward-extension of the Lith terrain, recently mapped by Pallister (1986). In this paper we present geochemical and geochronological data from the coastal area S of Port Sudan down to the Ethiopian border (see inset, Fig. 1 ) and compare these with previous results from southwestern Arabia.

Analytical procedures

Major elements were determined at Mainz University by DCP on a Spectraspan IV Spec- trometer, H20 and CO2 were analyzed gravi- metrically and FeO was found photometri- cally, using standard techniques. Values were within 1% or better for most elements in comparison to internal and international standards except for MnO and P205 (Linne- bacher, 1989). Trace elements were analyzed by XRF on a Philips PW 1404 and a Siemens SRS 200 X-ray fluorescence spectrometer, using standard software for the PW 1404 and a technique described by Willis et al. ( 1972 ) on the SRS 200. Values are better than 7% for most elements (Linnebacher, 1989).

Rb/Sr ratios for geochronology were determined on a Siemens SRS 200 X-ray fluorescence spectrometer at Mainz following the procedure of Pankhurst and O'Nions (1973) and outlined in Kr/Sner ( 1982 ). Reproducibil- ity (2 sigma) of the ratio is 1.5%. 87Sr/86Sr ratios were measured at UTD using either 12" or 6"-radius solid source mass spectrometers. To- tal processing blanks for Rb and Sr were ~< 0.1 ng and 3 ng, respectively. Zircons were sepa- rated using conventional methods such as Wil- fley table, magnetic separator, heavy liquids and final handpicking. For multigrain size- fraction work at UTD the zircon concentrates were cleaned in boiling HNO3 and HC1 before dissolution in Krogh-type bombs. Total processing blanks for Pb were 0.5-1.0 ng. The 12 "- radius instrument was used for isotopic analyses, and uncertainties were about 1.5% for Pb/ U and about 0.1% for Pb isotope ratios. Decay constants are those recommended by Steiger and J~iger (1977). Rb-Sr ratios were calculated using the York II model (York, 1969), while the zircon data were regressed using the procedure of Ludwig (1980). For Rb-Sr er- rorchrons (MSWD>F-variate) , York-II 2 sigma uncertainties on the age and initial 878r/

102 A. KRONER ET AL.

TABLE 1

Chemical data for samples of Erkowit trondhjemite (Su 13-18), Khor Dahand tonalite (To 36) and Khor Ashat metavolcanic rocks (To 24-30)

Sample Su13 Su14 Su15 Su18 To36 To24 To25 To26 To27 To28 To29 To 30

SiO~ 71.46 72.85 72.19 72.26 70.57 72.86 73.41 72.97 71.83 71.51 59.02 48.72 TiO2 0.32 0.27 0.22 0.22 0.24 0.45 0.47 0.51 0.56 0.66 1.02 0.62 A1203 14.35 14.00 14.30 14.34 14.80 13.56 13.18 13.91 13.76 14.09 15.32 14.92 FeO 1.27 1.07 1.18 1.04 1.73 0.91 0.65 0.99 0.65 1.09 3.92 5,26 Fe203 1.55 1.22 1.30 1.32 1.98 1.66 1.70 1.45 2.07 2.08 3.69 5,09 MnO 0.12 0.08 0.12 0.07 0.14 0.08 0.05 0.04 0.08 0.08 0.15 0.21 MgO 0.94 0.76 0.80 0.80 0.94 0.62 0.53 0.62 0.75 0.77 3.55 9.30 CaO 3.60 3.70 3.81 3.76 4.04 2.14 2.24 2.37 3.01 3.19 7.88 11.46 Na:O 4.08 4.06 4.14 4.04 3.60 5.14 4.72 6.09 4.94 5.05 4.04 2.15 K~O 1.38 1.48 1.29 1.31 1.17 1.46 2.22 0.42 1.29 0.90 0.17 0.29 P2Os 0.09 0.09 0.06 0.06 0.08 0.05 0.04 0.05 0.05 0.06 0.13 0.16 H20 + 0.24 0.50 0.42 0.40 0.54 0.82 0.71 0,49 0,68 0.55 0.96 1.67 H 2 0 - 0.10 0.15 0.10 0.12 0.08 0.11 0.13 0.12 0.11 0.07 0.09 1.12 CO2 0.22 <0.01 0.08 <0.01 0.04 0.11 <0.01 0.07 <0.01 <0.01 0.04 0.04

Total 99.72 100.23 100.01 99.74 99.95 99.97 100.05 100.10 99.78 100.10 99.98 100.02

Ba 294 1399 1591 696 1037 1266 274 223 Rb 40.8 28.4 35.4 29.9 31.2 24.1 20.0 5.2 12.1 10.7 1.2 3.5 Sr 275 275 265 288 323 276 317 396 397 435 582 398 Nb 4.5 2.8 4.5 3.8 3.0 4.7 4.4 4.2 3.5 4.1 3.0 < 1.0 Zr 62.0 67.2 66.2 64.3 83.3 145 164 141 152 144 89.9 29.0 Y 12.8 7.8 13.2 5.5 12.0 34.3 40.1 33.8 35.9 35.7 32.9 15.0 V 33.8 25.2 33.1 27.8 50.0 29.0 21.5 29.5 33.0 32.5 177 248 Co 89.1 96.8 96.8 116 23.5 3.0 4.0 4.0 4.5 2.5 19.0 36.5 Cr 8.1 5.0 5.6 4.1 6.5 10.0 12.0 12.0 11.5 10.5 37.5 491 Ni <5.0 <5.0 <5.0 <5.0 6.5 4.0 5.0 5.5 6.5 5.0 21.0 29.0 Cu <7.0 <7.0 <7.0 <7.0 7.0 3.0 6.5 4.0 1.5 1.5 9.0 23.0 Zn 43.0 26.5 37.3 28.9 55.0 31.0 24.0 16.5 28.5 31.0 45.5 96.5 La 10 3 7 3 31 23 33 21 21 13 6 Ce 20 10 15 8 50 40 49 37 34 35 9 Nd 12 7 9 6 36 31 35 28 31 24 7

Major elements in wt.%, trace elements in ppm.

86Sr ratio have been multiplied by the square root of the MSWD.

Sm-Nd analyses were performed on three whole-rock samples, using a Finnigan-MAT 261 mass spectrometer at the Max-Planck-In- stitut ffir Chemie in Mainz, and analytical procedures are described in White and Patchett (1984).

Single zircons were analyzed on a Finnigan- MAT 261 mass spectrometer at the Max- Planck-Institut t'tir Chemie in Mainz, using the direct evaporation technique of Kober ( 1986, 1987). In this method chemically untreated grains are embedded in the evaporation fila-

ment of a rhenium double-filament arrange- ment, and radiogenic Pb is then evaporated in the mass spectrometer after heating the grain to about 1550-1650°C. Kober (1986) has shown that the Pb components with the high- est activation energy reside in the undamaged crystalline zircon phase that shows no post- crystallization Pb-loss and therefore yields concordant 2°Tpb/2°6pb ages. Data acquisition was by magnetic peak switching using the elec- tron multiplier, and the detailed procedure is described in KfiSner and Todt (1988).The technique has been tested on isotope standards and natural zircons previously dated conven-

EVOLUTION OF PAN-AFRICAN ISLAND ARC ASSEMBLAGES IN THE SOUTHERN RED SEA HILLS 103

TABLE 2

Rb-Sr data

“Rbla6Sr

Erkowit trondhjemite: su 13 0.422 su 14 0.305 su 15 0.384 Su 18 0.296

a7Sr/a6Sr

0.70795 IL 13 0.70638 Ifr 13 0.70725 AZ 20 0.70631 f 18

Khor Ashat metavolcanics: To 24 0.246 To25 0.178 To 26 0.029 To 27 0.088 To 29 0.0032

0.705992 8 0.70486+ 9 0.70320+ 8 0.70375+ 7 0.70292? 6

Khor Sebat metavolcanics: su 68 9.61 Su 69 2.33 su 70 3.30 su71 1.42 Su 72 0.84 [Su 73 0.0616 [Su 74 0.178

Jebel Dirtet metarhyolites: To 1 6.57 To 2 6.53 To 3 4.59 To4 4.39 To 5 5.56 To 6 4.83 To 7 3.70 To 8 6.83 To 9 5.29 To 10 5.98 To 11 7.29

0.79786 + 14 0.72775+ 8 0.73584+ 7 0.71897i 9 0.71337? 8 0.70396? 131 0.70463* 71

0.776842 6 0.77125? 7 0.75372? 6 0.75254* 8 0.76489t 9 0.75817+ 8 0.79485+ 6 0.77323* 9 0.75953? 6 0.76665+ 7 0.77789f 11

Khor Aradib metarhyolites: To 14 1.50 To 15 1.89 To 16 1.80 To 17 1.63 To 18 5.10 To 19 7.29 To 20 1.48 To21 6.64 To 22 5.38

[ ] = omitted from isochron calculation.

0.72048t 7 0.72481 Z!X 6 0.72264* 20 0.72141+ 10 0.75895 + 11 0.778542 7 0.72257+ 9 0.77254+ 8 0.76039? 8

tionally and by ion microprobe (Kober, 1987; Compston and Kriiner, 1988; Kober et al., 1989; Kriiner et al., 1989), and the results are

ERKOWIT TRONDHJEMITE

.708- su 13 to IS ,“’

L ,p.m-

AGE = 906 t 103 Ma

MSWD = 0.4

Ri = 0.7024 +0.0005 ,702 -

Fig. 2. Rb-Sr isochron diagram for data points from samples of the Erkowit trondhjemite. Sample numbers are given in Table 2.

.14a - . ERKOWIT

.I44 -TRONOHJEMITE

w-54 8 .I40 79

.1x

.I32

i J %

%

-%

Fig. 3. Concordia diagram showing two zircon fractions from sample Su 54 of the Erkowit trondhjemite.

identical, within error, even for zircons with complex metamorphic histories.

The calculated ages and uncertainties are based on the means of all ratios evaluated and their 2-sigma errors after the Dixon-Test (Dixon, 1950) was employed as an outlier test in the statistical assessment of the measured values. A chi-square test on the data population for each grain or combination of grains was also performed to check on normal distribution.

Major and trace element data are presented in Tables 1, 6 and 7. Concentrations and iso-

104

TABLE 3

Conventional U-Pb zircon data

A. K R O N E R E T AL.

Concentrations Measured ratios

U (ppm) Pb (ppm) 206/204 207/206

Corrected ratios a

2°Tpb/235U 2°6pb/238U 2°7pb/2°6pb 207/206 age(Ma)

Erkowit(Su-54) 75-150#m 180.3 23.48 7300 0.06929 1.228

- 7 5 # m 240.3 29.81 5025 0.07002 1.174 .1322 .06733 849 ± 13 .1261 .06751 854± 13

" Corrected for common lead at 850 Ma (Stacey and Kramers, 1975).

80

60 0

[k co o

o. 4 0 r~ o c~

E z 2 0

Age in Ma ~25 850 875 840

I I I I

Su 52

Mean age: 856+10 Ma

~.0

To 36

85O I

Age in Ma 860 870 880

I I I I

Mean age: 871+10 Mal I

30~ 1 Grain 1, 60 ratios

[ ] G r a i n 2, 54 ratios

.~ [ ] Grain 3, 37 ratios

0.067 0.068 0.0670 0.0675 0.0680 (2°7pb/206pb)* (207pb/206pb)*

(a) (b)

0.0685

Fig. 4. Histograms showing distribution of 207 Pb/2°6pb ratios in single zircons from Erkowit trondhjemite sample Su 52 and Khor Dahand tonalite sample To 36.

topic data for the Rb-Sr study are listed in Ta- ble 2, and are graphically presented in Figs. 2, 5 and 7. S m - N d data are presented in Table 4. The zircon analyses are shown in Tables 3 and 5 and are graphically presented in Figs. 3, 4, 8 and 9. The evaporation data are shown in histograms, and the 2°7pb/2°6pb ratios for each grain are presented in different patterns.

Field relationships, geochemistry and geochronology

In view of the lack of any detailed regional mapping south of Lat. 19 °N we have undertaken reconnaissance traverses in the region around the summer resort of Erkowit, the escarpment region E of Erkowit and south to


Khor Ashat, and in the hills SE and SW of To- kar extending to the Ethiopian border (Fig. 1, inset). Although we are not able to directly link all the rock units found in these regions because of lack of mapping and continuous out- crop, our age data and geochemistry provide a consistent picture that we try to integrate into the emerging evolutionary scenario for the RSH (Kr6ner et al., 1987).

E r k o w i t - K h o r A s h a t region

The elevated region around the summer resort of Erkowit consists predominantly of granitoid rocks with interspersed amphibolite and hornblende-biot i te schist xenoliths and roof pendants that range in size from a few metres to hundreds of metres. Because of their higher metamorphic grade in comparison with the "normal" greenschist-grade Nafirdeib suite (sack name for greenschist-grade volcano-sedimentary assemblage found throughout the Red Sea Hills; Gabert et al., 1960; Ahmed, 1979) these inclusions have been considered as rem- nants of an older, possibly pre-Pan-African, supracrustal succession (Vail, 1985). How- ever, field relations west of Erkowit show that the higher grade is entirely a contact metamorphic effect related to the engulfing granitoids. There is a metamorphic gradation back to low- grade metavolcanics as the volume of plutonic rocks decreases towards the village of Summit. We therefore have little doubt that the mafic xenoliths belong to the same succession as found elsewhere in the southern RSH.

The oldest granitoid generation recognized is well exposed in abandoned quarries around Erkowit (Fig. 1) and consists of med ium to coarse-grained, well-foliated t rondhjemite (analyses Su 13, 14, 15 and 18 in Table 1; see also Klemenic and Poole, 1988). Where these rocks contain amphiboli te xenoliths, the foliation and lineation in both rock types is identical, and we have never observed clear, cross- cutting intrusive relationships, a feature common with early syn-tectonic granitoids whose

original contacts are overprinted by subse- quent deformation. We therefore conclude that the Erkowit pluton constitutes one of the oldest granitoid complexes intruding an older volcano-sedimentary succession.

The generally coarse- to medium-grained rocks consist of quartz, plagioclase, hornblende, biotite, minor microcline and acces- sory minerals. They plot in the tonalite and trondhjemite fields in an Ab-An-Or diagram (O'Connor, 1965) and have typical I-type characteristics such as high NazO/K20 and Fe 3 +/Fe z + ratios (Chappell and White, 1974; Hine et al., 1978 ).

Klemenic and Poole (1988) dated seven samples of the Erkowit pluton from unknown locations by the Rb-Sr whole-rock method and obtained an isochron age of 815 + 25 Ma with Ri (initial 87Sr/86Sr) of 0.7029+0.0002. We also dated five samples by the same method (Su 13-15 and 18, see Table 1 ), collected from three quarries near Erkowit over a distance of ~ 400 m, and the results are presented in Table 2 and Fig. 2. The data points are well aligned (MSWD=0.4 ) , corresponding to an age of 906 + 103 Ma and a 878r/86Sr initial ratio of 0.7024_+0.0005. Considering the large error (due to limited variation in Rb/Sr ) , this age is somewhat higher but still compatible, within error, with the data of Klemenic and Poole (1988). The low Sr-initial ratios indicate an isotopically primitive source, and the I-type character of the rock rules out a long crustal history. This is also supported by an end value ( T = 850 Ma) of + 6 for one granite sample and a depleted mantle model age of ~ 1000 Ma (Table 4 ).

Conventional multigrain zircon dating of two size fractions from sample Su 54, consisting of clear euhedral grains yielded near-concordant results with 2°7pb/2°6pb ages of 849 _+ 13 and 854 _+ 13 Ma, respectively (Table 3).In a concordia diagram, a regression line through these points and zero provides an upper intercept age of 852 _+ 30 Ma (Fig. 3 ), and the error is implausibly high since the data

106

TABLE 4

Sm-Nd data for Erkowit trondhjemite and metavolcanic rocks from S of Tokar

A. KRONERETAL.

Sample Rock Sm Nd 147Sin /144Nd 143Nd/144Nd ~NdCT) Model age no. type (ppm) (ppm) 2a error (850 Ma) (DM) in Ga

Su 13 Erkowit 2.02 9.6 0.1226 0.512556_+ 27 + 6.05 1.05 granite (0.512560_+34) +5.97 1.04

Su 72 Rhyolite 4.56 15.86 0.1741 0.512700_+ 16 +3.67 1.73 (0.512687_+ 18) +3.42 1.78

Su 73 Basalt 2.54 9.23 0.1662 0.512762 +_ 8 + 5.75 1.24

t43Nd/144Nd normalized to t46Nd/~44Nd = 0.7219. Chondritic ratios used are 1475m/144Nd = 0.1967, 143Nd/t44Nd = 0.512638. DM: depleted mantle model ages are based on the assumption of a linear evolution with ~ = 0 at 4.5 Ga and e = 10 today. Numbers in parentheses are duplicate analyses.

alignment of the two fractions alone would favour a negative intercept. We therefore consider 852 + 13 Ma as a realistic assessment of the zircon age. Evaporation of one 120 ~tm grain of the same population gave an identical result within analytical error at 856+ 10 Ma (Table 5, Fig. 4a) and testifies to the high pre- cision of this new technique. We consider the zircon ages as most accurately reflecting the t ime of intrusion of this rock unit and adopt a value of 855 Ma as the best estimate for emplacement of the Erkowit pluton. The data of Klemenic and Poole may reflect partial resetting during a later event, as also shown by their K-Ar biotite date of 800 ___ 18 Ma, and we suspect that their samples were collected over a larger region than our material.

Weakly foliated granodiorite to tonalite somewhat similar to that at Erkowit occurs in Khor Dahand E of the resort and below the escarpment and intrudes a sequence of variably deformed bimodal metavolcanics and tufts in the Khor Dahand-Khor Ashat area. Our sample To 36 (location see Fig. 1) consists of quartz, plagioclase (Ann0), > 5% biotite, mi- nor K-feldspar and accessories. The chemical data (Table 1 ) again reveal the calc-alkaline, I-type nature of the rock and classify it as tonalite. The trace element data are consistent with this and characterize the rock as volcanic arc or syncollisional granite (Pearce et al., 1984 ).

Three clear to yellowish, euhedral and inclu- sion-free zircons without optically recognizable zonation from sample To 36 were evaporated and yielded a mean 2°7pb/2°6pb age of 870 _+ 5 Ma (Table 5, Fig. 4b). This rock is thus slightly older than the Erkowit pluton but be- longs to the same 840-895 Ma age range as also found for dioritic complexes in the southwestern AS (Fleck, et al., 1980).

The bimodal sequence of Khor Ashat must be older than the Khor Dahand tonalite and consists of interlayered basaltic to andesitic and flow-banded rhyolitic metalava (Table 1 ), locally highly sheared and intruded by dolerite dykes and granodiorite-tonalite-gabbro stocks. The rocks are medium to fine-grained, have predominantly porphyritic texture and are variably but only slightly altered with plagioclase generally fresh. Chemically this suite is calc-alkaline and has trace element characteristics of modern intra-oceanic arc volcanics which is particularly evident from Cr/Y, V/Ti , T i /Z r and N b / Z r ratios (Linnebacher, 1989 ). These rocks also compare remarkably well with similar rocks in the A1-Lith area of the southwestern AS (Reischmann et al., 1983; Pallis- ter, 1986).

We were unsuccessful in separating zircons from available samples of this suite in spite of reasonably high Zr-contents. Our Rb-Sr data for 5 whole-rock samples (Table 2) scatter

EVOLUTION OF PAN-AFRICAN ISLAND ARC ASSEMBLAGES IN THE SOUTHERN RED SEA HILLS

TABLE 5

Isotopic data from single zircon evaporation

107

Sample Grain Mass Evaporation MeanS°7/pb/2°6pb 2°Tpb/2°6pb age number scans a temp. in °C ratio band 2aerror and 2 ae r ror

Su52 1 106 1530 0.06758± 36 856±10

To 36 1 60 1420 0.06806± 18 870+10 2 54 1440 0.06807± 26 871± 8 3 37 1450 0.06809± 44 871±14 1-3 151 0.06807± 34 8 7 1 £ 1 0

Su 69

To 7

To 13

To 23

1 49 1490 0.06703 ± 72 839±22 2 57 1420 0.06708± 39 840± 12 3 75 1550 0.06705± 55 839±18 1-3 181 0.06706± 54 840± 16

1 87 1460 0.06753± 38 854±12 2 41 1480 0.06753± 58 8542 18 3 81 1430 0.06752± 78 8542 24 1-3 209 0.06753± 60 854±18

1 64 1550 0.06663 ± 110 826 ± 34 2 48 1510 0.06661 ± 123 826 ± 36 3 36 1520 0.06667± 120 828 ± 36 1-3 148 0.06664± 105 827± 33

1 64 1615 0.06136± 27 6 5 2 2 1 0 2 66 1580 0.06135± 35 651±14 3 46 1520 0.06137± 38 652±14 4 65 1530 0.06137± 54 652±18 1-4 241 0.06137± 40 652±14 5 45 1550 0.06381± 52 735±17

a 207 206 Number of Pb / Pb ratios evaluated for age assessment. b Observed mean ratio corrected for non-radiogenic Pb where necessary. Errors based on uncertainties in counting statistics.

considerably, and regression ( M S W D = 3 . 4 ) .707 yields an errorchron age o f 860 + 101 Ma ( Fig. 5). We consider this to approximate the time .,o6 of lava emplacement and suggest that the isotopic disturbance shown by the scatter o f data ~ .7o5 points is due to significant low-temperature al- ~ teration as can be seen in thin section. The low ® .,o4 Sr initial ratio of 0.7028_+ 0.0002 is again in- 7o3 dicative of a primitive source and corroborates our suggestion o f an intra-oceanic origin 7O2o for the entire terrain between Erkowit and Khor Ashat farther SE. We consider it likely that the Khor Ashat sequence is part o f an arc complex intruded, between ~ 870 and 850 Ma

KHOR ASHAT S U I T E / " TO 24 to 29

= 8 6 0 + 1 0 1 M o

, J R,o"oSG" to%oo2 / - .

I 0.1 012 0.3

87Rb/86Sr

Fig. 5. R b - S r isochron diagram for data points f rom samples of the Khor Ashat bimodal suite.

tory of the ANS and contains the oldest arc rocks so far known in the shield.

ago, by large volumes of early syntectonic granitoid material that locally caused contact- metamorphic overprinting and was subse- quently deformed together with the supracrustal assemblage.

The above ages for the Erkowit-Khor Ashat area in the Sudan compare well with those N of A1-Lith in the Asir terrain of Saudi Arabia (Fleck et al., 1980; Kr/Sner et al., 1983) and suggest that the region from Erkowit in the W to the escarpment NE of A1-Lith probably constituted one single arc terrane early in the his-

Area southeast of Tokar

r 18 ° 30' N

The region between the Tokar Delta and the Eritrean border is occupied by a hilly terrain consisting of a volcano-sedimentary assemblage intruded by a variety of grey to yellow- brown foliated, syntectonic granitoids and conspicuous coarse-grained, unfoliated, red to pink K-feldspar-rich granites (Fig. 6) that make up distinct, rugged mountain peaks.

18'

I 37 ° 30' E

- - ~ o r

138 °

Marafit v \ ~ / ' - ~ x

138o30'

River gravel and sand

Post-tectonic granites

~ ] Syn-to late-tectonic granitoids

Volcarto - sedimentary sequence

Faults and major fractures

inferred from photo-interpretation

eTa 13 Sample locality

17'

N

20

I


I I

30 km

SCH tg90

Fig. 6. Generalized geological map of the area south of Tokar, showing location of studied samples (modified from Lin- nebacher, 1989).


There is virtually no written information on these rocks, and we undertook two major reconnaissance traverses along Khor Sebat and tributaries south of Marafit and along the dirt road from Aqiq to Karora and into Khor Ara- dib. The supracrustal sequence appears to be bimodal in composition with the felsic vari- eties dominating and making up the major topographic relief of the region. The mafic rocks consist of appreciably altered metabasalt and mafic tuff, almost everywhere strongly ep- idotized and sheared. These beds are rarely more than 50 m thick and are intercalated in massive to sheared rhyolite as well as felsic tuff and pyroclastic beds. The massive rhyolites preserve flow-banding and ignimbritic tex- tures and have considerable similarities with the Baish felsic flows of the A1-Lith region of Saudi Arabia described by Reischmann et al. (1983) and Pallister (1986). Deformation is highly variable and seems to be concentrated in distinct zones where tight to isoclinal folds were observed that locally led to almost com- plete transposition of the primary layering.

Minor alteration in the felsic metavolcanics produced sericite, epidote, chlorite, pumpel- lyite and calcite, documenting a low-grade greenschist-facies event that may have af- fected the rocks immediately after deposition or during later deformation and regional metamorphism.

Chemically, the Tokar felsic metavolcanics are subalkaline to peralkaline alkali-rhyolites to rhyolites (Tables 6 and 7) that follow a tholeiitic trend in the Jensen (1976) cation diagram. Strong differentiation is indicated by high Zr and Nb and low TiO2-contents. Most discrimination diagrams (e.g. TiO2/Zr, Pearce, 1980; Lear et al., 1986; Nb/Zr, Leat et al., 1986) classify these rocks as subduction-related rhyolites, and they may have formed along an active continental margin (Ewart, 1979) or in an intra-oceanic arc with thick- ened crust such as the situation in Papua-New Guinea (Smith and Johnson, 1981). Since peralkaline rhyolites commonly occur in rift

settings (Berberi et al., 1975), and the Tokar rhyolites are associated with basalts showing marginal basin characteristics (Linnebacher, 1989), we interpret these rocks as an early expression of extensional tectonics in an evolving arc complex with associated back-arc basin, in analogy with the Cenozoic evolution of Papua-New Guinea (Smith and Johnson, 1981).

The remarkable chemical similarity of the Tokar rhyolites with similar Zr- and Y-rich rocks in the A1-Lith area of Saudi Arabia (Ku- lada suite, Reischmann et al., 1983), dated at 847_+34 Ma (KriSner et al., 1983) suggests a similar tectonic setting for both areas.

The felsic metavolcanics selected for isotopic dating are five suites of porphyritic rhyolites each representing sequences about 50- 60 m thick and collected within several metres of each other at localities shown in Fig. 6. These rocks are remarkably fresh, often with pre- served flow-banding, and consist of a micro- crystalline groundmass of quartz and K-feldspar and phenocrysts of alkali-feldspar, 2-6 mm in size and showing magmatic corrosion. Several samples contain idiomorphic primary garnet in the groundmass and enclosed in alkali-feldspar phenocrysts and large grains of xenomorphic quartz intergrown with K-feldspar. Examples of garnet-bearing felsic volcanic rocks are known from Phanerozoic arc sequences in New Zealand (Wood, 1974), Ja- pan (Tagiri et al., 1975; Ujiki and Onuki, 1976) and Australia (Birch and Greadow, 1974), and experimental petrology suggests a primary magmatic origin (Kano and Yash- ima, 1976).

Analytical data for the five suites of felsic Tokar metavolcanics dated by Rb-Sr are presented in Table 2. The first suite consists of interlayered basalt and rhyolite collected in Khor Sebat (SU 68-72) The five felsic samples are reasonably well aligned (Fig. 7a) (MSWD= 1.5), but the calculated age of 666 + 16 Ma is difficult to assess, and the 87Sr/ 865r initial ratio of 0.7054 _+ 0.0004 is elevated

TA

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met

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8-7

2,

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) an

d m

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ts (

SU

73,

74)

fro

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sui

tes

of

the

area

SW

of

Tok

ar,

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ls,

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Su

74

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2

To

3

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4

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5

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6

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7

To

8

To

9

SiO

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.46

75.1

0 80

.17

75.9

6 78

.63

49.4

6 50

.55

75.2

6 76

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75.1

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75.0

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82.9

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75.4

5 T

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0.46

0.

31

0.20

0.

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2.54

0.

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0.15

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0.

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A12

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2.2

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12.3

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11.8

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11.4

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FeO

0.

77

0.59

0.

26

0.78

0.

24

4.39

4.

92

0.46

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37

0.46

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0.60

1.

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0.70

0.

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0.73

Fe

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1.09

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64

1.03

1.

90

2.32

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91

7.41

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1.34

1.

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1.17

1.

63

0.06

1.

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3.69

0.

96

Mn

O

0.05

0.

05

0.02

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05

0.09

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28

0.27

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02

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0.07

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0.08

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0.12

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0.13

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8.

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8.56

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0.79

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734

187

482

443

173

149

259

263

256

264

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299

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42.5

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75.5

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76.6

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Tot

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.86

99.6

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01

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179

1327

87

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131

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72

767

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142

169

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59

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55

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70.6

99

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96.4

94

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44.6

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r 69

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67.8

73

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92.4

80

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97.5

39

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39.0

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0 42

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51.3

56

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214

Nb

12.0

13

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13.3

13

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15.7

13

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9.8

13.7

9.

2 14

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13.3

15

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3.7

Zr

235

273

580

589

713

586

433

570

320

586

582

339

80.1

Y

37

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35.8

10

5 ll

0

119

100

68.5

10

0 26

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92.9

11

9 76

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9.5

V

4.5

5.0

6.5

5.0

3.5

4.0

6.5

3.5

5.0

5.5

8.0

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2.5

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2.5

1.5

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70

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12

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34

77

84

92

78

51

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56

56

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r"

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z 2~

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2~

,'r

m

"-r

Z

m N

Maj

or e

lem

ents

in w

t.%

, tr

ace

elem

ents

in p

pm

.

1 1 2 A. K R O N E R ET AL.

.800

.780

.760

. 7 4 0

.720

~.D .700 .% .73C

¢0

.720

.710

.70C

SU 68 t o ~

/ - AGE = 6 6 6 "1"16 Me / - MSWD - 1.5

/ " R i = 0 ,7054 + 0 .0004 , / -Q ) i i 2 4 ~ ~ -70

TO 14 to 17

~ 270Mo 4.0

-'2 0.0066 c)

I I 0.5 1.0 1.15

1800~ TO I to II / . . ~ /

.780 I- TO I - 8 ~ ~ / / A G E ' 7 2 4 - + I 2 8 M o ~ ' ~ - / - / M S W D " 4 7 /-~" 760 + " ,~

" ~i=0.7071_0.0093 d'7

,4 , - , , / . ~ AGE = 702 +57Me ,7201- ~ MSWD = 1.7

. 7 0 0 I / / / b ) , Ri = 0"7061 "1" 0 "0753

0 2 4 6 8

.780

.760

.740

• 72° t

.700 0

j ~

TO 18 to 22 /

" MSWD = 0.9 R i =0.7082 + -0 .0007

d) I I I 2 4 6

87Rb/e6Sr

Fig. 7. Rb-Sr isochron diagrams for rhyolite suites from the area S of Tokar. (a) Khor Sebat suite, Su 68-72. (b) Jabal Dirtet suites To 1-6 and To 7-11. (c) Khor Aradib suite To 14-17. (d) Khor Aradib suite To 18-22.

as compared to most dated suites elsewhere in the ANS. This may either mean derivation from a crustally contaminated source or isotopic resetting after emplacement, in which case the above age does not reflect the time of rock formation. The two altered basaltic samples have very low Rb /Sr and 87Sr/86Sr (Table 2) and therefore fall significantly below the above regression line. This underlines the con- clusion that these rocks are genetically unre- lated to the adjacent rhyolites and are derived from a chemically more primitive source. Sm- Nd data for one basalt and one rhyolite sample from this suite (Table 4) support this conclu- sion and show significantly different ~NO values of +5.75 (basa l t ) and +3.42-3.67 (rhyolite) for an assumed emplacement age of

850 Ma (based on zircon data, see below). The resulting depleted mantle model ages of 1.24 Ga for the basalt and 1.73-1.78 Ga for the rhyolite are substantially higher than that for the Erkowit granite and support involvement of some older crustal component in the generation of these rocks as already inferred from the 87Sr/86Sr initial ratio.

Rhyolite samples To 1-6 from Jabal Dirtet are poorly aligned (MSWD=4.7, Fig. 7b), and the regression line calculated corresponds to an age of 724 + 126 Ma with Sri = 0.7071 _+ 0.0093. Again, assessment of age and elevated Sq is difficult due to the scatter in the data. The sec- ond rhyolite suite collected some 4 km farther S (To 7-11) is somewhat better aligned (MSWD--1.7, Fig. 7b), and the corresponding age of 702 _+ 57 Ma and 87Sr/86Sr initial ratio of 0.7061_+0.0053 are indistinguishable, within error, from the adjacent suite, but both data sets are aligned along distinctly different chords. Four additional samples of felsic metavolcanics from Khor Aradib (To 14-17) yield an errorchron (MSWD=4.0 , Fig. 7c) "age" of 769_+ 270 Ma and a 875r/86Sr initial ratio of 0.7037_+0.0066, while four further samples from the same suite (To 18-22) collected some 4 km farther S in Khor Aradib, are significantly better aligned (MSWD = 0.9, Fig. 7d) and define an age of 684-+ 16 Ma with Sri = 0.7082 -+ 0.0007.

The above Rb-Sr data are inconclusive and suggest considerable scatter in age and initial

EVOLUTION OF PAN-AFRICAN ISLAND ARC ASSEMBLAGES IN THE SOUTHERN RED SEA HILLS 1 13

ratio although, within error, all five regression lines yield similar results. We have therefore evaporated single zircons with the Kober ( 1986, 1987) technique from two samples of rhyolite. Su 69 is from the Khor Sebat suite and contains a homogeneous population of clear, pink to redbrown, idiomorphic zircons of ig- neous origin showing no optically recognizable zoning or inclusions. Three grains were ana-

lyzed and yielded identical 2°7pb/2°6pb ratios that combine to a mean age of 840_+ 16 Ma (Table 5, Fig. 8a). To 7 is representative of the sequence S of Aqiq and also contained euhedral zircons with identical 2°7pb/a°6pb ratios that yield an age of 854+ 18 Ma (Table 5, Fig. 8b). These two ages are indistinguishable within their errors and suggest emplacement of the acid metavolcanics some 845-850 Ma ago,

..o (3_ (.D

..o Q.

"6

.O E

Age in Ma Age in Ma 800 850 900 950 1000 775 800 825 850 875 900

I I I I I I i I I

J t Mean age: 854+18 Ma 120 60-'t

Su 69

--4

--I

Mean age: 840_+16 Ma q

100 50-t • Grain 1, 87 ratios " 4

" [ ] Grain 2, 41 ratios -1 ' D Grain 3, 81 ratios

80-_ • Grain 1, 49 ratios 40-1.t

[ ] G r a i n 2, 57 ratios -4 [] -4

Grain 3, 75 ratios -4

60- 30-1 - -4

" I

- 4

40- 20-~ - ..-i

,-4

,-4

- 4 10-- L 0.0650 0.0670 0.0690 0.0710 0.065 0.066 0.067 0.068 0.069

(207pb/206pb)" (207pb/206pb)-

(a) (b)

To 7

Fig. 8. Histograms showing distribution of 2°7pb/2°6pb ratios in single zircons from Tokar rhyolite samples Su 69 and To 7.


significantly earlier than indicated by the Rb- Sr data but virtually contemporaneously with the rocks of the Erkowit-Khor Ashat area.

Clearly, then, the Rb-Sr data reflect disturbance of the isotopic system considerably later than emplacement, and this could either be the combined effect of post-depositional alteration and metamorphic overprinting associated with regional deformation or only reflect the latter. We are unable to decide on this but note the rather close agreement of the reset Rb-Sr "ages" from the Tokar area with similar values elsewhere in the RSH (Klemenic, 1985; Reischmann, 1986), suggesting one widespread event or several distinct events some

I00

Age in Ma 500 600 700 800 900

I I I I I

#. (D O

..{3 n r,..

o c-J

E z

80

60

40-

20

To 13 Mean age: 827_+33 Ma

, G r a i n 1, 64 ratios


r- ' ]Grain 3, 36 ratios

670-770 Ma ago that may have been associated with lateral arc accretion and/or late- to post-tectonic granitoid plutonism. Based on a mean zircon age of 850 Ma for the Tokar rhyolites we recalculated the 875r/86Sr initial ratios of the dated suites and found that most yield implausibly low values, indicating post- depositional Rb gain or Sr-loss. We relate the ubiquitous, though not severe, sericitization of the rhyolite groundmass in virtually all samples analyzed to secondary potassium-gain as shown by Linnebacher (1989), and therefore favour Rb-gain as the main mechanism lower- ing the 875r/86Sr initial ratios. The calculations also suggest that the original 875r/86Sr initial

101

80

Age in Ma 575 600 625 650 675

I I I I I

Mean age: 652_+14 Ma

To 23

, Grain 1, 64 ratios


D Grain 3, 46 ratios


0.060 0.065 0.070 0.059 0.060 0.061 0.062 (2o7Pb/2O6Pb)* (2o7Pb/2O6Pb)*

(a) (b)

Fig. 9. Histograms showing distribution of 2°7pb/2°6pb ratios in single zircons from trondhjemitic gneiss To 13 and granite To 23.


ratios must have been uniformly low, comparable to those reported from the Arabian shield and ruling out significant involvement of much earlier continental crust in the generation of the rhyolites.

Two distinct generations of granitoid rocks were recognized during our reconnaissance survey. The older type was deformed together with the supracrustal assemblage and displays a penetrative foliation with strong mineral lineation. Original intrusive relationships are therefore difficult to recognize but were clearly established at Jabal Debir Anka near Aqiq (Fig. 6 ) where we collected trondhjemite sample To 13 (Table 7). Some of these early granitoids may have been sheet-like intrusions and do not seem to constitute distinct, large plutons, while other granitoid masses such as SW of Aqiq are more batholithic in dimension and form elongated bodies (Fig. 6). The post-tectonic granite generation consists of medium- to coarse-grained unfoliated, pink granite composed of quartz, K-feldspar, plagioclase, little biotite and accessories. It forms oval- to circular-shaped plutons clearly cross-cutting the foliation in the supracrustal rocks, and our sample To 23 (Table 7 ) was collected at Jabal Um Achabe.

Zircons from the foliated granite To 13 are non-transparent light-yellow to brown, euhedral with slightly rounded ends and consist of only one population of rather small grains between 40 and 80/2m in length. These grains were difficult to evaporate and yielded weak signals below 1 V for 2°6pb (low original U- contents), so that scatter in the isotopic ratios was higher than usual and resulted in imprecise ages. The combined mean 2°6pb/a°7pb age for 3 grains is 827 _+ 33 Ma (Table 5, Fig. 9a), indistinguishable, within error, from the zircon age of the Tokar rhyolites and also compatible with the granitoid ages from the Er- kowit-Khor Ashat region. Our sample To 23 from the post-tectonic Jabal Um Achabe granite has euhedral zircons, and 4 grains evaporated yielded a mean 2°6pb/a°Tpb age of

652___ 14 Ma (Table 5, Fig. 9b) that we interpret as reflecting the time of granite emplacement. One grain provided a distinctly higher 2°7pb/2°6pb age of 735_ 12 Ma (Table 5, not shown in a diagram) and may be a xenocryst of uncertain derivation. We suspect that this grain has a --, 850 Ma core, surrounded by new growth that took place in the Um Achabe granite melt, and that the above "age" reflects a mixture of isotopic components and therefore has no geological significance. The intrusion of this and similar granites in the Tokar area may have been responsible for resetting of the Rb- Sr systems in the felsic metavolcanics dis- cussed above.

Conclusions

Our chemical and geochronological data for plutonic and felsic metavolcanic rocks from the southeastern RSH, together with a compre- hensive study of additional samples by Linne- bacher(1989) demonstrate a close affinity to rock associations in the AI-Lith region of the southwestern Arabian shield (Reischmann et al., 1983; Kr6ner et al., 1983; Pallister, 1986) and suggest that these two regions constituted part of the same island arc terrain in late Pre- cambrian times. The primitive isotopic and chemical character of both the plutonic and metavolcanic associations in the Erkowit-Khor Ashat area corroborates an intra-oceanic origin as already pointed out before (Stoeser and Camp, 1985; Kr6ner et al., 1987), but the rocks S of Tokar signify the involvement of some older crustal component in their generation and may therefore have evolved near the ancient continental margin of Africa. We relate the remarkably bimodal character of both the Khor Ashat and Tokar suites and those in the A1-Lith area of Saudi Arabia to extensional tectonics in an early Pan-African island arc setting. The arc terrane defined here is one of the oldest so far identified in the entire ANS and records considerable growth of juvenile crust some 840-870 Ma ago. We suggest that

116 A. KRONERETAL.

this terrane collided with the African continent some 720 Ma ago and caused extensive crustal thickening at the continental margin, indicated by the formation of granulites now exposed W of the Red Sea Hills near Khar- toum (Kr/Sner et al., 1987). The intrusion of much younger S-type post-tectonic granites in the present study area at around 650 Ma sig- nifies a considerable t ime gap between the crustal thickening event and the onset of intracrustal melting, as also found in modern collision belts.

Acknowledgements

This study was undertaken as part of the Mainz Consort ium "Accretion and Differen- tiation of the Earth", funded by the Deutsche Forschungsgemeinschaft under grant Kr 590/ 11 and the Federal Ministry of Economic Co- operation. R.J.S. was funded by NASA grant NAGW-1169. We gratefully acknowledge the logistic support of the Geological Research Authority, Red Sea Hills Office, Port Sudan.

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