evolution of the northern somali basement: new constraints from zircon ages

Jarmd @African Earth Sciences. Vol. 22. No. I, pi. l-15. 1996

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Evolution of the northern Somali basement: new constraints from zircon ages

A. KR6NRR’ and F. I’. SASS12

%stitut fiir GeowissenschaftenJIniversiKt Mainz, 55099 Mainz, Germany 2Dipartimento di Mineralogia e Petrologia, UniversiK di Padova, Corso Garibaldi 37,351OO Padova, Italy

(Received 23 February 1995: revised version received 2 November 1995)

Abetract - The northern Somali crystalline basement (NSB) consists of seven major rock complexes, five of which are mainly metasedimentary sequences with some meta-igneous intercalations, while two are plutonic complexes. Using the single gram evaporation method, the authors have dated zircons of six samples from three of these complexes exposed in the Hargheisa-Sheikh-Burao area, the western part of the NSB.

The single zircon ages confirm that the western NSB is not a juvenile Pan-African terrain, but a composite basement consisting of pre-Pan-Atiican Proterozoic crust ranging in age between -1400 and -1820 Ma. This was affected by granitoi,d magmatism and associated tectono-thermal events at -840, -800-760 and -720 Ma, respectively, at the same time when farther north in the Arabian-Nubian shield, subduction-related magmatism led to the formation of h&a-oceanic island arcs and marginal basins. Pre-Pan-African Archaean to Proterozoic rocks similar in age to the western NSB also occur in eastern Ethopia and the two regions may be part of the same continental terrane, sandwiched in between juvenile domains farther west in Ethiopia and farther east in northern !So- malia.

R&nrme - L.e socle cristallin de la Somahe septentrionale (NSB) se compose de sept ensembles litho-tectoniques, comprenant cinq sequences m&as&iimentaires a intercalations metavoltiques et deux complexes plutoniques. L..es dorm&s g&xhronologiques obtenues par la m&ode d’evaporation de cristaux de zircon concement six echantilkms provenant de la partie occidentale du NSB (secteur Hargheisa-She&h-Burao).

Ces donn6es indiquent que la partie occidentale du NSB ne correspond pas a un domaine juvenile pan-africain, mats a tm complexe an&n, dont les composants crustaux sont d%ge pr6-pan-africain, compris entre 1820 et 1400 Ma. Ce socle a et6 le siege d’une importante activit6 magmatique (8404300 Ma) et thermo-tectontque (720 Ma) au corns du Prot6rozsoIque sup&ieur. Ces &&~ements sont contemporains des &+nements enregistres plus au nord, darts le bouclier arabo-nubien, oh les phQlom&nes geologiques sont lies aux processus de subduction et de formation d’arcs insulaires intraoceaniques et de bassins marginaux.

Des roches d’age arch&n a proterozdique sont connues en Ethiopie orientale. On suggere que ces deux regions appartiennent a une meme unite continentale ancienne (“terrane”) bord6e de deux ensembles a dominante juvenile FRIAR srtu&s respechvement a 1 ouest de cette mute en Ethiopre et a l’est de cette derniere en Somalie sep-

INTRODUCTION

The northern Somali crystalline basement (NSB for short) is relatively unknown as regards both the descriptive aspects and its interpretation on a wide regional scale. Specifically, few data exist in the literature concerning its structural setting, its tectono- stratigraphy and the sequence of events recorded in it.

The 24 geological sheets (scale 1:250 000) mapped by British geologists of the Geological Survey of the former Somaliland Protectorate in the 1950’s and the related explanatory reports are an important database concerning the rock types and sequences, although they do not reveal their age. More recent regional papers are by Sassi and Ibrahim (1981), Warden and Daniels 1(1984), Warden and Horkel (1984), Dal Piaz and Sassi (1986), Sassi et al. (1989) and Kriiner et al. (1989) and to the references quoted

therein, to which readers are referred for an extensive overview and interpretations and a review of the older literature. Papers on specific topics can be found in Abbate et al. (1989). In particular, readers interested in metallogenic problems in this area can find a review in Binda et al. (1989) and further data on Somali granitoids can be found in Lenoir et al. (1994).

According to Warden and Horkel(1984), the NSB is part of the northeast branch of the Mozambique belt. Sassi et al. (1989) considered the NSB as a Pan- African ensialic mobile belt, consisting of extensively rejuvenated Proterozoic crust (western sector) and juvenile Pan-African terrains (eastern sector), as pre- viously proposed by Sassi and Ibrahirn (1981) and Dal Piaz and Sassi (1986). Krijner et al. (1989) reported the occurrence of zircon xenocrysts up to 1816 Ma in age in rocks from the western part of the NSB and maintained that this basement is not an

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Evolution of the northern Somali basement 3

exclusively juvenile Pan-African terrain but consists of early to mid-Proterozoic crust affected by Neopro- terozoic magmatism.

However, the entire geochronological database, which is available in the literature, is rather poor so that any new radiometric age determinations on the NSB will contribute significantly to a better definition of its evolution. The present research was carried out withthisaiminmind.

GEOLOGICAL OUTLINE

General statement

According to Sassi et al. (1989), the NSB consists of seven main complexes, the approximate location and exposed extent of which are shown in Fig. 1. Five of these complexes are predominantly metasedimentary sequences and two consist of igneous intrusive suites. The metasedimentary complexes, the first four of which also include igneous protoliths, are:

i) the Qabri Bahar complex; ii) the Mora complex; iii) the Abdulkadir complex; iv) the Mait complex; and v) the Inda Ad complex.

The first two complexes make up the medium to high grade, polymetamolrphic basement of northern Somalia. The Abdulkadir and Mait complexes are composed of greenschist-facies, volcano-sedimentary sequences and are exposed in the western and cen- tral part of the NSB, respectively. The Inda Ad complex is a low- to very low-grade, essentially metasedimentary sequence, located in the eastern- most part of the NSB.

The igneous plutonic complexes are: (i) the Gab- bro-Syenite belt and (ii) the Younger Granites.

Predominantly metasedimentary complexes

Qabri Bahar compZex. This is a polymetamorphic sequence consisting of layered paragneisses, migmatites, felsic orthogneisses, amphibolites and rare talc-silicate rocks. Relics of gram&e metamorphism (Sassi and Vison21,1989),, a prevailing amphibolite fa- ties metamorphism associated to migmatite- producing anatexis and (a greenschist facies overprint have been detected in it.

Mora complex. The abundance of marbles, quart&es and amphibolites characterizes this complex, which otherwise &seems to be identical to the Qabri Bahar complex. Considering the peculiar pro- tolithology of the Mora complex (evolving from Al- rich sandstones to limestones, the former now being kyanite-bearing quartz&s frequently interlayered between the Qabri Bahar and Mora complexes), the

Mora complex could represent a younger suite deposited on the Qabri Bahar complex.

Abdulkudir complex. This is a low-grade volcano- sedimentary sequence consisting of quartz phyllites with minor intercalations of quartz& and marble, with which felsic and minor basic metavolcanics and metavolcanoclastics rocks are interlayered. According to Warden and Daniels (1984) and Sassi et al. (1989), this sequence was deposited unconformably on the underlying metamorphic basement after the latter had experienced high-grade metamorphism.

Mait complex. This is a low-grade metavolcanic belt interposed between the Inda Ad complex and the Qabri Bahar basement. The protoliths were pillowed to massive basalts and pyroclastics, pelitic to elastic sediments and thin carbonate beds (Dal Piaz ef al., 1989). An unconformity between the Inda Ad and the Mait complexes, and another unconformity between the Mait complex and the high-grade basement have been inferred (Abbate et al., 1981; Warden and Dan- iels, 1984). However, structural data and the lack of basal conglomerates at the bottom of the Inda Ad complex may indicate a tectonic or tectonized contact between the Inda Ad and Mait complexes.

Inda Ad complex. This is a low- to very low-grade sequence of metasedimentary rocks, the main protoliths of which were sandstones, siltstones, mudstones, intraformational conglomerates and marbles. Abbate et al. (1985) pointed out the turbiditic character of the Inda Ad sediments and related it to the erosion of an erogenic volcanic arc. Some problematical fossil findings (Abbate et al., 1981) and the radiometric age of the Younger Granites cross-cutting the Inda Ad complex constrain its sedimentation age to the uppermost Proterozoic to Cambrian.

The igneous plutonic complexes

As mentioned above, metamorphic rocks with likely plutonic protoliths (e.g. the felsic gneisses) are intimately interlayered with supracrustal assemblages within the Qabri Bahar and Mora complexes in such a way that they may be considered as parts of these complexes., Metamorphic features prevail in these rocks and their original igneous nature is only inferred. In this section the authors only refer to plutonic associations for rocks which clearly display original igneous features, either because the metamorphic overprint is not very strong or because they are unmetamorphosed.

Gabbro-Syenite belt. Numerous bodies of layered gabbro occur in the NSB systematically associated with (rarely nepheline-bearing) syenites. Minor

A. KRiiNER and F. P. SASS1

MAIT YOUNGER COMPLEX GRANITES

G

G5

1820 1400

840

815780 810-760 720

(550)

(500)

Figure 2. Relative chronology of the main igneous and metamorphic events recorded in the different rock complexes of the NSB, slightly modified from Sassi et al. (1989). The last column on the right sche- matically shows the new zircon age data of the present paper, but the age data for Event IV are taken from Sassi et ul. (1989). M=metamorphic event, A=felsic volcanism, B=mafic volcanism, plutonism, Sy=syenite plutonism, Ga=gabbro plutonism.

G=granitoid

alkaline granitoids and rare dm to cm sized car- bonatitic dykelets and/or ovoidal patches also occur. Field relationships indicate that the gabbros were intruded into already deformed rocks (Daniels et al., 1965) and that the syenites and granitoids (ranging in composition from diorite to granite) are slightly younger than the gabbros. A foliation-producing metamorphism is recorded in the outer parts of the main gabbro and syenite bodies and penetratively af- fects the smaller bodies as well as shear zones cross- cutting the major bodies. Subsolidus pseudomorphic replacements of the igneous mineral grains totally preserving the igneous textures may also be observed within the gabbro bodies (Said, 1987, 1988). Accord- ing to Dal Piaz et al. (1985,19&V), these gabbros do not belong to an ophiolite suite but appear to reflect intracrustal magmatic activity, perhaps related to an underplating event and probably developed under extensional conditions.

Younger Granites. Two different groups of granitoids are included under this heading:

i) foliated granitoids: their foliation is well developed in the outer parts of the granitoid bodies but may be weak or lacking in the inner parts;

ii) unfoliated granitoids representing, with their aplitic and pegmatitic dykes, the youngest plutonic rocks in the NSB. Further analytical data are necessary in order to better understand all these granitoids since contrasting data exist in the literature concerning their geochemi- cal features (Bigioggero et al., 1989; Sassi et al., 1989)

and their isotopic age (Bigioggero et al., 1989; Lenoir et al., 1994).

Nature of boundaries between the complexes

Field evidence indicates that the plutonic bodies of the Gabbro-Syenite belt and the Younger Granites were emplaced when the respective country rocks were already metamorphosed, foliated and folded. The unfoliated Younger Granites intruded all metasedimentary sequences, while the foliated Younger Granites were never found in the Inda Ad complex. The Gabbro-Syenite intrusions only affect the Qabri Bahar and Mora complexes. In fact, the Mait greenstones do not belong to the Gabbro-Syenite belt (Dal Piaz et al., 1989).

As concerns the nature of the boundaries between the metasedimentaiy sequences, there are no conclu- sive data for establishing whether these rocks represent an originally unbroken stratigraphical sequence or whether they result from the tectonic juxtaposition of different blocks, or terranes, each having an independ- ent geological history. However, it is reasonable to assume that all these me&sedimentary sequences were side-by-side at least since the uppermost Proterozoic or lowermost Palaeozoic, when they were inject& by the Younger Granites. It is also reasonable to assume that the Qabri Bahar and Mora complexes shared a longer common history, at least since the time of emplace ment, in both of them, of the Gabbro-Syenite belt, and perhaps since the time at which they were both capped by the protoliths of the Abdulkadir complex.


/ Km 65

Figure 3. Geographical location of the samples dated in this study. Geological sketch maps of these sites can be found in Gatbz et al. (1981,1985) and in the Guide Book to the-Excursion B (1987).

Sequence of events

As turns out from the above descriptions, the NSB records a complex history of deformational, igneous and metamorphic activity. Sassi et al. (1989) largely based their interpretations on petrological reasoning supported by some radiometric data and subdivided this history into four major events as outlined below and as sketched in Fig. 2. However, the above authors clearly pointed out that the then available radiometric database was not sufficient enough to adequately portray and fully constrain the tectono-metamorphic evolution of the NSB. In the following discussions, and in Fig. 2, each event is Welled as explained in the caption of Fig. 2, the lower being the number, the higher the age.

Major Event I (Palaeo- to Mesoproterozoic) is only recorded by relics of gram&e-facies metamorphism Ml and the basic and granitoid protoliths &, &, Gl, from which rocks of the Qabri Bahar complex were derived. It may represent the vestiges of a pre- Pan-African continental crust, consistent with Kr6ner et al. (1989).

Major Event II (older than cu 700 Ma) includes amphibolite-facies metamorphism Mz,, anatexis GP and associated deformation. The emplacement of post-tectonic granitoids GS was also referred to this event, but it could also be referred to Major Event III.

Major Event III (cu 700-640 Ma) includes crustal thinning, extension and aborted lithospheric rupture; deposition of a Neoproterozoic seqtience with basic volcanic activity (BI: Abdulkadir complex; B5: Mait

6 A. KRCiNER and F. P. SASS1

complex); related emplacement of the Gabbro-Syenite suite through the thinned crust, followed by regional heating, i.e. M metamorphism.

biotite-rich variety.

Major Event IV (cu 600-500 Ma) includes emplacement of granites Gb deposition of the Inda Ad sequence, a further thermal pulse with regional heating MJ accompanied by deformational activity and finally emplacement of granites Gs (at -500 Ma, but at -630 Ma according to Lenoir et aZ., 1994).

So 14 (Fig. 3c, d; from the Mora complex): leucosome of the Caadda migmatites; it is interpreted, on field evidence, as a leucogranite occurring within a semipelitic metasediment, now a cordierite- sillimanite-garnet-spine1 paragneiss. Both were tightly folded together and subsequently intruded by rocks of the Gabbro-Syenite complex.

Several authors, including Abbate ef al. (1981), Bigioggero et al. (1989), Dal Piaz and Sassi (1986), Dal Piaz et al. (1989), Sassi et al. (1989), Warden and Dan- iels (1984) and Warden and Horkel(l984) pointed out specific features suggesting a “juvenile” character for the Inda Ad complex, i.e. of Major Event IV. As pre- viously mentioned, an Early Palaeozoic sedimentation age was suggested for the Inda Ad complex on the basis of the occurrence of poorly preserved fossils in the marbles, specifically the sponges spiculae and pelmatozoa (Abbate et al., 1981). Although such an age assessment has been questioned (Bigioggero et al., 1989, pp95), it is well established that the Inda Ad, Mait and Abdulkadir complexes display a significantly shorter and simpler deformational history in comparison to the Qabri Bahar and Mora complexes and also record a simple metamorphic evolution.

SINGLE ZIRCON EVAPORATION

Kober (1986) has shown that the Pb components with the highest activation energy normally reside in the undamaged crystalline phase of zircon that did not undergo post-crystallization Pb loss and therefore yields concordant 207pb/~Pb ages. Lead phases due to radiation damage (metamict zones) have low activation energy and are removed during low- temperature evaporation. The method involves re- peated evaporation and deposition of Pb isotopes from chemically untreated single zircons in a double- filament arrangement (Kober, 1987). The kinetics of Pb release during zircon evaporation has been inves- tigasted by Chapman and Roddick (1994).

SAMPLES ANALYZED

Single zircons from six rock samples have been analyzed during the present study. These come from the western sector of the NSB and their location is shown in Fig. 3. Considering the petrography of these samples and the classification of the NSB rocks into the above seven complexes, the analyzed samples can be classified as follows:

So 1 (Fig. 3a, b; from the Gabbro-Syenite belt): weakly foliated diorite dyke intruding the foliated Sheikh gabbro body.

The laboratory procedures, as well as comparisons with conventional and ion-microprobe zircon dating, are published elsewhere (Kroner and Todt, 1988; Krijner et al., 1991). Isotopic measurements were carried out on a Finnigan-MAT 261 mass spectrometer at the Max-Planck-Institut fitr Chemie in Mainz. No cor- rection was made for mass fractionation, which is on the order of one per mil (Kober, 1987), significantly less than the relative standard deviation of the measured 207Pb/2~Pb ratios (see Table 1) and insignificant at the age range considered in this study. Measure- ment of a chip of Curtin University SHRIMP II zircon standard C/Z3 yielded a 2’JrPb/2’J6Pb age of 564.8kl.4 Ma, identical to the adopted age of 564 for this standard (Pidgeon et al., 1994).

So 7 (Fig. 3a, b; from the Gabbro-Syenite belt): weakly foliated syenite dyke intruding both mon- zonite and gabbro of the She&h gabbro body.

So 8 (Fig. 3a, b; from the Qabri Bahar complex): layered, partly migmatitic and isoclinally folded sillimanite-bearing paragneiss within the Hudiso migmatites; this is the country rock of the Sheikh gabbro body.

S 12A/80 (Fig, 3a; from the Qabri Bahar complex): granodioritic gneiss making up the country rocks in which the Daymoleh Younger Granite is intruded. These gneisses are intimately associated with and interlayered within the Hudiso paragneisses and migmatites.

In these experiments, evaporation temperatures were gradually increased in 20-30°C steps during re- peated evaporation-deposit cycles until no further changes in the a7pb/aPb ratios were observed. Only data from the high-temperature runs or those with no changes in the Pb isotope ratios were considered for geochronological evaluation after testing for statisti- cal outhers (Dixon, 1950). The calculated 207pb/BPb ratios and their standard errors are based on the means of all measurements evaluated and are presented in Table 1. The 207pb/aPb spectra are shown in histograms that permit visual assessment of the data distribution from which the ages are derived.

So 10 (Fig. 3c, e; from the Mora complex): well fo- The reliability of the single zircon evaporation liated unit of layered granitoid orthogneiss, which method in comparison to conventional single grain can be classified as granitoid G3. It consists of alter- U/Pb dating and ion microprobe analyses has been nating dark, biotite-rich bands and leucocratic, quart- demonstrated in several recent studies (e.g. Kober et zofeldspathic bands; the sample comes from the al., 1989; Kroner et al., 1991; Paquette et al., 1994). In


Table 1. Isotopic data from single grain zircon evaporation.

Sample Zircon colour and Grain # Mass scansi Evaporation temp. Mean *07pb/*06Pb 207pb/%Pb age number morpholo,gy in”C ratid and lo error and lo error

Gabbro-Syenite complex io 1 long-prismatic, <clear 1 50 1580 0.06621*22 813f 7

as above 2 101 1590 0.06623&28 814k 9 long-prismatic, yellow 3 60 1580 0.0662&15 813k 4 long-prismatic, clear 4 57 1600 0.06627i20 815k 6

nean 1-4 268 0.06623&23 81&7 stubby, idiomorphic, 5 59 1604 0.10583*23 172% 4 clear as above 6 51 1590 0.10930*27 1788k 4 as above 7 62 1585 0.11137&32 1822k 5

io 7 long-prismatic, light 1 99 1600 0.06509&24 777k8 yellow-brown, euhe 2 73 1610 0.0651&22 778k 7 dral as above 3 88 1608 0.06510*28 778k 9

nean l-3 260 0.0651&24 778*8

io 8 long-prismatic, clear Qabri Bahar complex

1 72 1600 0.06607&18 SOS& 6 idiomorphic 2 52 1584 0.06606i22 SOS* 7 as above 3 51 1592 0.06606*18 SOS& 6

nean l-3 175 0.06606k19 SOS*6 oval, grey-brown 4 57 1608 0.1047Oi56 1709ilO as above 5 22 1600 0.1048%36 1712k 6

nean 4&5 79 0.10475f52 1710f9 long-prismatic, yellow 6 32 1595 0.11102k35 1816k 5

i 12A/80 long-prismatic, ends 1 65 1585 0.06461*12 761f 4 rounded, yellow-brown 2 66 1590 0.06462kll 762 4 as above 3 66 1580 0.06462k12 762k 4 as above 4 66 1585 0.06460*10 761k 3

nean l-4 263 0.06461ill 761k4 Mora complex

io 10 long-prismatic,, euhe 1 61 1580 0.06327*15 717* 5 dral, yellow 2 50 1580 0.06328&28 718k 9 as above 3 42 1570 0.06341&31 722ilO

nean l-3 153 0.0633Oi27 718k9 long, ends round 4 55 1600 0.08888*23 1402 5

io 14 long-prismatic, clear 1 75 1610 0.06715i 8 842i2 euhedral 2 52 1590 0.06713k16 842k5 as above 3 36 1600 0.06717i13 843* 4 as above 4 30 1595 0.06712k 7 8422

nean l-4 193 0.06714*12 842k4 stubby, grey, euhedral 5 57 1610 0.08984&25 1422k 5

1: Number of W’b/mPb :ratios evaluated for age assessment. 2: Observed mean ratio corrected for non-radiogenic F% where necessary. Errors based on uncertainties in counting statistics.

rare cases, however, such as in high U zircons from some granulites, Pb loss may be severe and, as a con- sequence, the qb/mPb ratios measured by evaporation do not correspond to concordant compositions and yield ages which are too low (Kroner and Wil- hams, 1993). The Tb/aPb ratios from a significantly discordant zircon population are not repro-

ducible and such cases are therefore easily recognized and were not met in the present study.

DATA PRESENTATION AND DISCUSSION

Diorite sample So 1 was collected on the main asphalt road along the Sheikh-Hudiso escarpment (Fig.

A. KRONER and F. P. SASS1

Age in Ma Age in Ma 750 775 800 825 1700 1750 1800 1850 120 II 80 1' I I I

Mean age: 814k7 Grain 5

59 ratlos Zircon xenocrysts

Mean age: 1729f4 Ma Grain 8 Grain 7 51 ratlos 82 ratios

g 90 Grain 2, 101 ratios 45 .- E

% Mean age: Mean age:

1788k4 Ma 1822f5 Ma : 2 P 80 30 k (u ‘t5 Gabbro-Syenite

t 2 30

Igneous zircons 15

O.OS/ ’ ’ b.Ok’ ’ ’ b.688' ’ ’ i).O870.104 &lb8 ’ b.i'O8' ’ b.r'lO' ’ b.i'l2' ’

(207PbP"6Pb)* (207PbP06Pb)*

(a) (b)

Figure 4. Histograms showing the distribution of radiogenic Pb isotope ratios derived from the evaporation of single zircons from diorite sample So 1, Gabbro6yenite complex. (a) Spectra for four igneous grains, integrated from 268 ratios and representing the magmatic phase of diorite. (b) Xenocqstic grains derived from older basement, integrated from ratios as shown. Mean ages are given with standard error.

Agein Ma 700 725 750 775 800

120r ’ I 1 I

Mean age: 778k8 Ma

@$Grain 3, 88 ratios -

so 7, Gabbro-Syenite 1

0.083 0.084 0.085 0.088

3a, b). The zircons can be grouped into two morphological types. Type 1 predominates and consists of long-prismatic, euhedral and clear to light yellow grains with no observable core/overgrowth relationships. Type 2 is much less abundant and consists of stubby, idiomorphic, clear grains with no obvious older cores. Four grains of type 1 were analyzed and yielded almost identical 207pb/mPb ratios, giving a mean age of 81&7 Ma (Table 1, Fig. 4a). This age is interpreted as reflecting the tune of igneous crystallization of the diorite magma. Three type 2 zircons had variable and much higher 207pb/mPb ratios corre- sponding to ages of 1729k4,1788*4 and 1822k5 Ma, respectively (Table 1, Fig. 4b). In spite of their idiomorphic shape, these grains are interpreted as xenocrysts which were probably incorporated in the diorite magma during its ascent through the crust. Zircons and their U-Pb isotopic systematics are known to survive very high temperatures above lOOO“C, as demonstrated by xenocrysts in basalt (Compston et al., 1986).

Figure 5. Histogram showing the distribution of radiogenic Pb iso- toperatiosderivedfromtheevaporationoftbreezircongrains from syenite sample So 7, Gabbro-Syenite complex. The spectrum has been integrated from 260 ratios. Mean age is given with stan- darderror.


Aae in Ma Ag0 in Ma

750 775- 800 825

Mean age: 808k8 Ma

So S,, Qabrl Bahar

Glrain 1, 72

Grain 2, 52

Grain 3, 51

Igneous 211

1850 1700 1750 1800 1850

801

I Grain 4, 57 ratios

%fI Grain 5, 22 ratios

80 Mean age: 1710f9 Ma

zircon xenocrysts

Grain 8. 32 ratios

/ Mean age: 1818*8 Ma

0.084 0.085 0.088

(207PbPo6Pb)*

(a)

0.105 0.110

(207PbPo6Pb)*

(b)

Figure 6. Histograms showing the distribution of radiogenic Pb isotope ratios derived from the evaporation of single zircons from migmatitic gneiss sample So 8, Qabri Bahar complex. (a) Spectra for 3 grains, ink grated from 175 ratios and interpreted to reflect the time of leucosome formation. (b) Spectra for 3 xenocrystic grains, integrakd from ratios as shown. Mean ages are given with standard error.

Since each xenocrystic grain yielded a different age, one could either suspect these ages to be mix- tures between old cores and younger overgrowth or that these grains are strongly discordant. The first possibility is ruled out by the fact that the grains are clear and do not exhibit any microscopically dis- cernible cores. Discordancy is possible and cannot be detected by the evaporation method. However, since none of the grains showed any statistically significant increase in the measureId ro7pb/T’b ratios with in- creasing evaporation temperature, it can at least be assumed that Pb loss, if any, occurred in recent times, in which case the 2mpb/rMPb age is not affected. Nev- ertheless, it may be safe to assume that the xenocryst ages are minimum ages. Since the xenocrysts are euhedral, the authors do not consider them to be of detrital origin, but suggest that they are derived from an igneous precursor. The three zircon xenocrysts analyzed, although not necessarily representative of the entire age spectrum in their host rock, document the presence of a mid-Proterozoic crust below the Gab- bro-Syenite complex.

Syenite sample So 7 was collected some 2 km north of sample So 1, d.ownhill on the main asphalt road along the Sheikh-Hudiso escarpment (Fig. 3a, b). It contained only one morphological type, which is long-prismatic, euhedral, transparent and light yellow-brown in colour. Three grains were evaporated.

They yielded a mean 2mpb/aPb age of 77&8 Ma (Table 1, Fig. 5) which, as in the case of So 1, is interpreted as approximating the time of syenite intrusion. The zircon ages for So 1 and So 7 are significantly different and prove that several stages of syenite-diorite intrusions affected the Sheik gabbro, consistent with field observations reported by Visor& (1989) for the Daba Shabely gabbro.

Sample So 8, the migmatitic sillimanite-bearing Hudiso paragneiss (for location see Fig. 3a, b), yielded two distinct zircon morphologies. One consists of long-prismatic, clear, idiomorphic grains of magmatic habit and is derived from the leucosome veins of the migmatite. The other consists of oval, brown to grey-brown grains of likely detrital origin. Three grains of the former type provided near- identical Tb/mPb ratios, which combine to a mean age of 808&6 Ma (Table 1, Fig. 6a). The authors consider this to reflect the time of leucosome veining, i.e. the peak of the thermal event which produced the migmatite. The oval grains provided much higher ages and the data for two grains combine to a mean age of 17lO&9 Ma while the third grain has a 2Lnpb/mPb age of 181&6 Ma (Table 1, Fig. 6b). These latter ages are thought to characterize the source terrain from which the original Hudiso meWediments were derived. It is noteworthy that these ages are in the same range as the ages determined for the xenocrysts of So 1 and this

10 A. KR6NER and F. P. SASS1

750 Age in Ma

760 770 760 I””

Mean age: 761k4 Ma

Grain 1, 65 ratios

Grain 2, 66 ratios “0 75 ‘Z= Grain 3, 66 ratios

I I I - Grain 4, 66 ratios

0. L 0640

Sl2Al60, Qabrl-Bahar Gneiss

/ .0650 0.0655

Figure 7. Histogram showing the distribution of radiogenic Pb isotope ratios derived from the evaporation of 4 zircon grains from granodioritic gneiss sample S 12/A80, Qabri War complex. The spectrum has been integrated from 263 ratios. Mean age is given with standard error.

would support the contention that the Sheikh-Hudiso area is underlain by mid-Proterozoic crust.

Abbate et al. (1985) reported a three-point Rb-Sr whole-rock error&on age of 828il70 Ma for weakly migmatized sillimanite-bearing paragneiss from the same area. This is identical, within error limits, to the zircon age calculated here and was interpreted by these authors as reflecting the migmatite-producing event.

Zircons from sample S 12A/80 (for location see Fig. 3a) are uniformly yellow-brown, long and prismatic with distinct rounding at their terminations, suggesting a metamorphic overprint. Evaporation of four grains yieided a mean iqjbjiKFb age of 76lf4 Ma (Table 1, Fig. 7), which is interpreted as the age of the magmatic-anatectic protolith of this gneiss.

Zircons from the biotite-rich orthogneiss sample So 10 of the Mora complex (for location see Fig. 3c, e) are predo minantly long-prismatic, clear to yellowish, euhedral with sharp terminations, typical of granitoid rocks. Evaporation of three grains of this population vielded almost identical 207pb/mPb ratios which ,------- provide a mean age of 7l8i9 Ma (Table 1, Fig. 8a). This is interpreted as reflecting the time of granitoid emplacement. One grey-brown, long-prismatic grain with round terminations, interpreted as a xenocryst,

has a significantly higher *wPb/2C’6Pb age of 1402k5 Ma (Table 1, Fig. 8b).

Sample So 14 (for location see Fig. 3c, d), a leucosome emplaced into, and tectonically interlayered with, gram&e facies paragneisses, contains almost exclusively long-prismatic, clear, idiomorphic zircons of typical magmatic morphology with sharp terminations. This suggests that the leucosome postdates the gram&e metamorphism in the metasediments since the zircons would otherwise show the typical rounding at their termination as usually found in high-grade rocks. Four grains of this type were evaporated and have virtually identical 207Pb/2wPb ratios with a mean age of 8424 Ma (Table 1, Fig. 9a). This is interpreted as the formation age of the leucosome. Granulite metamorphism in the associated naraonks must therefore k n&r fi_m_ fi_& age’i &_p T-_-p_‘-__ -_-__ grey-brown, long-prismatic grain with slightly rounded terminations yielded a much higher age of 1422f5 Ma (Table 1, Fig. 9b), almost identical to a similar grain from sample So 10 and also interpreted as a xenocryst. The fact that samples So 10 and So 14 both contain xenocrysts of virtually the same age suggests that a crust of this age must be present un- derneath the region north and northwest of Hargeisa.

DISCUSSION

As shown in the last column of Fig. 2, the zircon ages presented above indicate a sequence of events which can be summarized as follows (from oldest to youngest):

i) The existence of a mid-Proterozoic continental crust below the currently exposed rock sequences of the Qabri Bahar and the Gabbro-Syenite belt is documented in the northwestern Somali basement by zircon xenocryst ages ranging from 171W Ma to 18m5 Ma (So 1 and So 8). Two further xenocryst ages just above 1400 Ma were obtained from magmatic rocks of the Mora complex (So 10, So 14) and suggest that the pm-Pan-African basement in the NSB is chronologically heterogeneous.

Xenocrystic zircon ages between 1997*4 and 2948&5 Ma were reported from gneisses and migmatites in the Hima-Harar area of eastern Ethiopia (Teklay et al., 1993) and it is therefore considered possible that pre-Pan-African Proterozoic crust extends from northwestern Somalia to eastern Ethiopia and may represent a continental terrain surrounded in the east and west by juvenile material of the Ara- bian-Nubian Shield (ANS for short).

ii) The earliest Pan-African event is marked by a zircon age of m4 Ma (So 14){ which reflects granitoid magmatism related to high-grade metamorphism and anatectic processes (Mz in the Caadda migmatites of the Mora complex). Intense post- migmatization deformation must postdate this age.


Age in Ma Age in Ma

675 700 725 750 1400 1450

6o r I 1 L

So 10, Mora Gnelss

g 60 = E

Mean age: 71833 Ma

Grain 1,

Grain 2,

??Grain 3,

Igneous

61 ratios

50 ratios

0.062 0.663 0.084 0.066 0.090

(207PbPo6Pb)* (207p’bP0sPb)’

(a) (b)

Figure 8. Histograms showing the dishibution of radiogenic Pb isotope ratios derived from the evaporaticm of single zircons from granitoid orthogneiss sample So 10, Mora complex. (a) Spectra for 3 grains, integrated from 153 ratios and interpreted to approximate the age of magmatic emplacement. (b) Spectrum for a xenocrystic grain, integrated from 55 ratios. Mean ages are given with standard error.

Calc-alkaline juvenile magmatism in the southern ANS of southeastern Sudan, western Ethiopia and Saudi Arabia is equivalent in time with this event (Stoser et al., 1984; Ayallew et al., 1990; Kriiner et al., 1991). This may suggest that crustal thickening and intracrustal melting processes in the pm-l&-African crust of northwestern Somalia are broadly contempo- raneous with subduction-related oceanic magmatism which, farther north, created extensive island arcs and marginal basins. Clearly, there must be a dynamic link between these two processes. Interest- ingly, a granitoid gneiss near Hima in eastern Ethio- pia provided a zircon a,ge of 84&8 Ma (Teklay et aL, 1993) and may reflect the same anatectic event as recorded in the Mora complex.

iii) Dioritic and syenitic intrusive rocks, related to the emplacement of the Gabbro-Syenite belt, were intruded between 778&8 and 81&7 Ma ago (So 1, So 7). The authors speculate that these rocks are related to extensive magmatic underplating in northwestern Somalia, possibly during an episode of crustal thinning, and again dynamically related to the subduction-accretion process in the ANS farther north. The uprising gabbro-syenite bodies may have heated the crust sufficiently for mligmatization and anatexis to

occur, as documented by the zircon ages of 761*4 Ma from a granitoid gneiss (S 12A/80) and 808*6 Ma for a leucosome granite vein (So 8) in the Qabri Bahar complex. Therefore Ms metamorphism, which is described as a greenschist facies event by Sassi et al. (1989), reached anatectic conditions in the surround- ings of the major gabbro bodies.

Previous Rb-Sr whole rock dating of syenites from the Agabar and Daba Shabeli gabbro-syenite massifs (Sassi et al., 1989) resulted in considerable scatter of the analytical data and errorchron ‘ages’ (695*91 and 694&31 Ma with MSWD=82 and 9.4, respectively), which the above authors related to post-emplacement isotopic disturbance. The Harar granite in eastern Ethiopia yielded a mean Yl’b/*Vb age of 78157 Ma (Teklay et al., 1993) and again seems to reflect the same igneous event as seen in northwestern Somalia.

iv) Further extensive granitoid magmatism (G3) occurred at about 720 Ma ago and was followed by intense deformation and low-grade metamorphism (see Sassi et al., 1989, pp12-13), as revealed by granite- gneiss sample So 10. The above age is consistent with field relationships and a four point Rb-Sr whole-rock errorchron of the Gebiley metagranitoid GJ, suggesting an age of 697klOl Ma (MSWD=33, *‘Sr/%r initial

12 A. KRCiNER and F. P. SASSI

775 120 f

Age in Ma

800 825 850 I I m

Mean age: 842k4 Ma

So 14, Mora Gnelss, ,.. L-4 granltic leucosome

Grain 1, 75 ratios

Grain 2, 52 ratios

Grain 3, 38 ratios

Grain 4, 30 ratios

Igneous zircons

0.0 8

Age in Ma

1400 1450 40 -

Grain 5, 57 ratios

Mean age: 1422f5 MI

lttl Xenocrvst

0.089

(2°7PbPo6Pb)* (2°7PbPosPb)*

(a) (b)

Figure 9. Histograms showing the distribution of radiogenic F’b isotope ratios derived from the evaporation of single zircons from granitic leucosome sample So 14, Mora complex. (a) Spectra for 4 grains, integrated from 193 ratios and interpreted to approximate the age of magmatic emplacement. (b) Spectrum for a xenocrystic grain, integrated from 57 ratios. Mean age; are giver; with stand&d &or.

ratio=O.7l06M.0142; Sassi et al., 1989). Felsic volcanism of broadly the same age is also

known from the ANS in Saudi Arabia, the Red Sea Hills of Sudan and western Ethiopia. It suggests that magmatic events recorded in the juvenile domains of the ANS have their equivalents in the intracrustal magmatism in Somalia.

CONCLUSIONS

The zircon age data presented in this paper are consistent with the hypotheses of Sassi and Ibrahim (1981), Kroner ef a2. (1989) and Sassi et al. (1989) in that the NSB is not a juvenile Pan-African terrain but a composite basement. Its eastern part consists of juvenile terranes (the Inda Ad and Mait complexes), whereas the western part consists of pre-Pan-African crust, bracketed in age by zircon xenocrysts between -1400 Ma and -1820 Ma. This old crustal segment was affected by igneous and metamorphic processes at -840 Ma, -800-760 Ma and -720 Ma, respectively. The Pan-African sequence of events was controlled by dynamic and thermal processes related to crustal thinning and took place at the same time in which, farther north in the Arabian-Nubian Shield, subduc-

tion-related magmatism led to the formation of in- traoceanic island arcs and marginal basins.

Pre-Pan-African, Palaeoproterozoic to Archaean rocks, similar in age to those making up the western part of the NSB, also occur in eastern Ethiopia and, as sketched in Fig. 10, the two regions may be parts of the same continental terrane, sandwiched in between juvenile domains farther west in Ethiopia and farther east in northern Somalia. However, the possibility that older crust also underlies the Inda Ad complex cannot be excluded as the age data of Lenoir et al., (1994) may suggest. It is likely that the ancient crustal domain extends northwards into the Arabian Penin- sula since Whitehouse et al. (1993) have described grey gneisses from the region southeast of Sana’a in Yemen with Nd model ages of 2.7-2.9 Ga. The crystalline basement of the Bur area in southern Somalia also belongs to this partially rejuvenated, pm-Pan- African terrane, considering that Pan-African granitoids (Carmignani et al., 1987; Lenoir et al., 1994) in- trude repeatedly folded and metamorphosed high- grade rocks (granulites, migmatites, amphibolitefacies sequences), which also include a banded iron formation (D’Amico ef al., 1981; Bakos and Sassi, 1994; Haider, 1989).

I 36’ ia0

+ 16’

I,+ . . n 12'

. . . .

. . . . . . .

,, . . . . *. . . . . 11. . , . . . . /I

_. . - .

, . . , , . . .

aa

+

state bolmdary

40” 440 46’

0"

Figure 10. !Sketcb map showing he pre-Pan-Afrkan, Axchaean to Protenxoic terrain, including the eastem Ethioph and the western part of Nchem!hnalibasement,sandwichedbetweenthejuv&domairrs ofwestemEthiopiatothewestandiheMaitplusIndaAdcom- plexes to the east. Based on Beydcnm (WC)), Pohl (MB), Shackleton (l988), Teklay etul. (1993) and the Geological Map of Somalia (19%).

Acknowledgements the international conference ‘GEOSOM ‘87’, and A.

A. Krliner acknowledges the use of the mass spectrometer facilities ;at the Max-Planck-Institut fiir Chemiti in Mainz. Five of the six samples analysed for this study were colkxted during a field trip following

Kraner thanks the Deutsche Forschungsgemeinschaft (DFG) for a travel grant. F. P. Sassi acknowledges support by the Italian Ministry for University and Re- search (MURST) and the Italian Council for Research (CNR). The authors also thank Prof. D. Visoti who

13

14 A. KRONER and F. I’. SASS1

kindly supplied the rock sample S 12A/80. The re- views of J. Lenoir, R. J. Stern and B. F. Windley greatly improved the manuscript.

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Geological map

Geological Map of Somalia; 1:l 500 000. 1994. Com- piled by Abbate, E., Sagri, M. and Sassi, F. P., Somali National University, Mogadishu, Somalia.

evolution of the northern somali basement: new constraints from zircon ages

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