Metamorphosed alkaline intrusions and dyke complexes within the Pan-African belt of western Hoggar (Algeria): Geology and geochemistry

Precam brian Research, 10 (1979) 1--20

1

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

METAMORPHOSED ALKALINE INTRUSIONS AND DYKE COMPLEXES WITHIN THE PAN-AFRICAN BELT OF WESTERN HOGGAR (ALGERIA): GEOLOGY AND GEOCHEMISTRY

J. DOSTAL', R. CABY 2 and C. DUPUY 2 'Department of Geology, Saint Mary's University, Halifax, Nova Scotia (Canada) 2Centre Gdologique et G$ophysique, U.S.T.L., 34060 Montpellier Cedex (France)

(Received November 14, 1978 ; revision accepted April 18, 1979)

ABSTRACT

Dostal, J., Caby, R. and Dupuy, C., 1979. Metamorphosed alkaline intrusions and dyke complexes within the Pan-African Belt of western Hoggar (Algeria): geology and geochemistry. Precambrian Res., 10: 1--20. Five successive generations of alkaline, subalkaline and peralkaline felsic rocks have been r e c o g n i z e d in a Pan-African belt of northwest Hoggar, central Sahara. Granites and

syenites of the first generation (~ 1 350 Ma old) form intracontinental alkaline layered intrusions and were affected by deformation and metamorphic phases of the Pan-African Orogeny (650--580 Ma ago). The other four, syntectonic and post-tectonic generations were emplaced as dyke complexes along or within abyssal faults and include both silicaoversaturated and undersaturated felsic rocks probably of upper-mantle origin. They may represent deeply eroded paleorift systems comparable to modern African rifts.

INTRODUCTION Alkaline intrusive c o m p l e x e s o f various ages have b e e n r e c o g n i z e d in the Pan-African belt o f w e s t e r n Hoggar, central Sahara (Caby, 1 9 7 0 , 1973). These r o c k s m a k e u p an i m p o r t a n t p a r t o f the Middle (?) t o U p p e r P r o t e r o z o i c terrains o f t h e western s e g m e n t o f the Pan-African belt and o u t c r o p o v e r a p p r o x i m a t e l y 4 0 0 0 k m 2. Five successive generations (I--V) o f alkaline, subalkaline and peralkaline igneous r o c k s can be distinguished o n the basis o f field g e o m e t r y , m e t a m o r p h i c and t e c t o n i c c o n t r o l s , and the available g e o c h r o n o l o g i c a l d a t a (All~gre and Caby, 1972). T h e Pan-African O r o g e n y has b e e n d a t e d in this area b e t w e e n 6 5 0 Ma f o r the s y n t e c t o n i c granites and 580 Ma f o r the p o s t - t e c t o n i c granites (All~gre and Caby, 1972). This p a p e r deals with the o c c u r r e n c e , p e t r o g r a p h y and g e o c h e m i s t r y o f these c o m p l e x e s with special emphasis o n the pre- and s y n t e c t o n i c r o c k s (generations I, II and III) which have a bearing o n the p a l e o g e o d y n a m i c r e c o n s t r u c t i o n o f this Pan-African s e g m e n t ( B e r t r a n d and Caby, 1 9 7 8 ) .

GEOLOGY

AND PETROGRAPHY

Pretectonic intrusives o f Middle Proterozoic age: generation I

The rocks of this generation crop out within the deep-structural levels of the Pan-African belt, which forms N--S trending blocks transected and laterally displaced by major strike-slip faults {Fig. 1). In the TideridjaouineEgatalis region, these rocks are in most places orthogneisses layered to banded conformably with the metasedimentary country rocks (quartzites, metapelites, and rare marbles). Metamorphism in the area is predominantly to amphibolite facies gradually increasing from the kyanite-chloritoid zone in the south, northwards to the sillimanite zone (--muscovite) and in a few places even to granulite-facies grade. In the Tideridjaouine area (Fig. 1) a volcano-plutonic association is still recognizable and forms sills and laccoliths. The sills, which are composed of thinly-banded subalkaline rhyolite were intruded by laccoliths of fluoritebearing alkaline microgranite with granophyric texture, albitic biotite granite and rare syenite. The metasedimentary country rocks have been described as the probable lateral equivalents of the shelf-type stromatolite series (Caby, 1970), the depositional age of which is between 1,150 and 800 Ma (All~gre and Caby, 1972; Bertrand and Caby, 1978). However, the Rb/Sr dating of the volcano-plutonic association gives an age of about 1350 Ma (C. All~gre, personal communication, 1976) while the U/Pb age of zircon from the same samples is ca. 1750 Ma {J. Lancelot, personal communication, 1976). With increasing grade of metamorphism, the intrusive rocks in places laterally grade into layered and banded orthogneisses. In these gneisses, a magmatic layering, which is sometimes rhythmic, is well preserved up to the sillimanite zone. The layering is defined by very rapid variations of rocktypes, colour index, grain size, and content of opaque and accessory minerals. In areas with the highest grade of metamorphism, defined by local occurrence of hypersthene in pelitic gneisses and their large scale migmatization, the orthogneisses have a sub-isotropic fabric and grade into autochthonous alkaline granitoids. Most of the rocks of the first generation are fine-grained, composed essentially of quartz, K-feldspar and albite with minor amounts of biotite, amphibole and/or pyroxene and opaques. Accessory minerals are sphene, apatite, fluoroapatite, allanite, calcite, fluorite, zircon, rare Zr-silicates and undetermined metamict minerals. Amphibole usually has the optical properties of ferrohastingsite and pyroxene those of hedenbergite. However, sodic amphibole (riebeckite-arfvedsonite group), aegirine-augite and aegirine have also been observed in some samples. A few syenite gneisses, with anomalously high K/Na ratios, probably due to K-metasomatism, have also been collected. The general parallelism between the magmatic layering and the bedding of

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Fig. 1. Generalized geological map of northwestern Hoggar showing the distribution of pretectonic igneous rocks. Legend: 1, basic and ultrabasic rocks (ca. 800 Ma old); 2, subalkaline to alkaline rocks; 3, undifferentiated Upper Proterozoic rocks; 4, Upper Proterozoic andesites (a), and graywackes (b); 5, molassic deposits ("S~rie Pourpr~e"). Boldface numerals I--V refer to the successive generations of the alkaline rocks.

the metasedimentary country rocks suggests that the igneous complexes were originally thick sills and laccoliths with a distinct mineral layering similar to that of the fluorite-bearing granitic and peralkaline intrusions of southwest Greenland (Harry and Emeleus, 1960; S0renson, 1969). The mineral layering may be due to intermittent crystallization and/or deposition caused by convective currents of a rather fluid magma (Kanaris-Sotiriov, 1974) the viscosity of which is anomalously low due to a high content of volatiles (Sorenson, 1969). The emplacement of the igneous complexes of this generation may have been controlled by a N--S trending early fault-zone later reactivated during the Pan-African phases of deformation. The rocks of this generation have been collected from two areas: Egatalis (IA) and Iileouine (IB) regions, both belonging to the kyanite or sillimanite zones. Alkaline metamorphic d y k e complex -- the root o f a paleorift system: generation H

The rocks of the second generation belong to a metamorphic dyke complex which outcrops over more than 500 km 2 along the westernmost part of the Pan-African belt. These dykes include both oversaturated and undersaturated rocks, mostly rhyolites, felsites and albitic trachytes, granites, syenites and nepheline syenites. The rocks were affected to varying degrees by mylonitization and recrystallization. They form dyke-on-dyke sets up to 100% which were intruded along N--S trending zones into an earlier metabasic dyke complex composed of recrystallized fine-grained gabbros and diorites {Fig. 2). The fine-grained acid rocks, especially the trachytes and phonolites, have their volcanic structure still preserved, while other rocks are typical tectonites with a porphyroclastic to mosaic-porphyroclastic structure. The magmatic mineral assemblages of the dykes are partly preserved (volcanic rocks with almost complete recrystallization of the albite-rich matrix) or completely preserved (coarse-grained syenites). They are composed of K-feldspar and albite with a variable amount of quartz in silica-oversaturated rocks and of nepheline in undersaturated ones. The ferromagnesian minerals are amphibole of ferrohastingsite type and iron-rich biotite, except in some peralkaline rocks containing riebeckite and aegirine. Accessories include opaques, apatite, allanite, epidote, and calcite. The syenites and trachytes are always rich in fluorite and zircon. Fig. 2. Metamorphic dyke complex along the margin of the Pan-African belt of western Hoggar (Generation II). 1, early metabasic dykes: amphibole quartz dolerites, microdiorites and amphibole trachyandesites; 2, folded acidic dykes, mainly felsitic and volcanic rhyolites; 3, hyperalkaline granite; 4, heterogeneous mainly mylonitic syenites and banded syenitic gneisses with some coarse-grained syenite relicts; 5, nepheline syenite and gneisses; 6, nepheline pegmatoids; 7, mesocratic diorite; 8, migmatite syenite; 9, undifferentiated orthogneisses; 10, granodiorite and calc-alkaline gneisses; 11, serpentinites; 12, marble lenses (metacarbonatites?). Location shown on Fig. 1.

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The dyke complex can be regarded as pretectonic with respect to the late N--S trending folds and shear zones characteristic of the late Pan-African of Hoggar (Bertrand and Caby, 1978). It seems that these intrusions in a N--S trending zone of weakness represent the r o o t of either a premetamorphic or synmetamorphic paleorift system. The same also applies to the dykes of generation III.

Syntectonic peralkaline dyke complex: generation III The rocks of the third generation make up a dyke complex which covers an area of more than 120 km 2 along the western strike-slip fault zone on the margins of the In Ouzzal granulitic block (Fig. 3). Caby (1973) has suggested a sinistral lateral m o v e m e n t as great as 350 km and a vertical displacement of a b o u t 25--30 km along this abyssal fault zone of Hoggar. The dyke complex was probably emplaced in a dilation zone related to the curved fault plane. Rb/Sr dating yields an age of a b o u t 600 Ma (AllAgre and Caby, 1972). The dykes are composed of leucocratic granitic rocks. They have a sigmoidal shape with thicknesses varying from a few dm to a b o u t 10 m and lengths of up to several km. The dykes were emplaced into a basic dyke complex composed of 100% dyke-on-dyke of various gabbros. The structural observations (Caby, 1973) imply that the alkaline dykes were intruded during the main m o v e m e n t along the fault zone and after the culmination of metamorphism of Barrovian type which is characterized in this area by the presence of kyanite. Deformation was, however, still active long after the intrusion of the dykes, and most of the rocks are typical gneisses with a porphyroclastic to mylonitic structure, similar to those described b y Floor (1974). The magmatic mineral associations mostly recrystallized under lower greenschist conditions, allowing metamorphic growths of sodic pyroxene and amphibole in the acid rocks and blue-green amphibole and clinozoisite in the basic rocks. Exsolution of albite in the magmatic perthite is frequent, as well as granoblastic recrystallization of low-temperature albite together with microcline and quartz. The rocks from the thicker dykes are only weakly recrystallized and the granophyric structures are preserved, whereas the fine-grained rocks may represent both ultramylonites and aphyric rhyolites to pantellerites. Amphibole is mostly of the riebeckite-arfvedsonite group and pyroxene is aegirine-augite or iron-rich aegirine. The abundance of Fig. 3. Syntectonic peralkaline metamorphic dyke complex along the In Ouzzal granulite block (generation III). 1, main strike-slip fault; 2, laccoliths of layered olivine and amphibole gabbros (late tectonic); 3, basic dykes (mainly gabbros); 4, + foliated peralkaline granite; 5, dykes of alkaline-peralkaline granites, microgranites and lavas grading eastwards into mylonites; 6, amphibole metagabbros and diabases (country rocks of 4 and 5); 7, injection zones (mainly pegmatites); 8, banded gneisses; 9, metatectic orthogneisses; 10, orthogneisses of generation I with amphibolite bands intruding; 11, PanAfrican metasediments: 1, kyanite quartzites; b, schists; c, marbles and dolomites; d, metabasic sills. Location shown on Fig. 1.

10 accessory minerals (sphene, apatite, fluorite, zircon, aUanite, opaques, and undetermined Zr-silicates) is highly variable and may reach up to 20% in rare dark schlieren and some metasomatic rocks with Zr contents of up to 20,000 ppm.

Pretectonic to syntectonic peralkaline granites o f central HoggarTimgaouine area: generation I V This generation includes granites from an elongated and boudinaged body, which was intensely sheared with horizontal stretching. The rocks are typical peralkaline granites with perthite porphyroclasts and subordinate iron-rich biotite, riebeckite and rare aegirine. The massif is probably related to acid metavolcanics of late Proterozoic age and may represent a source of uranium for the nearby deposit of Abankor.

Post-tectonic peralkaline granites and lavas o f Cambrian age: generation V The rocks of this generation belong to an elongated massif (5 km × 1 kin) in the Taoudrart area of Hoggar. It is composed of a N--S trending dyke-ondyke complex of various types of undeformed peralkaline granites and microgranites, 1--5 m thick, with rare dykes of fine-grained gabbros. The massif lies within another major strike.slip fault zone with an estimated sinistral displacement of about 150 km and a vertical movement of 25--30 km (Caby, 1970). It is spatially associated with flows and ignimbrites of rhyolitic and subordinate pantelleritic composition, which overlie molassic sediments of the "S6rie Pourpr~e", about 530 Ma old (Caby, 1973). The peralkaline granites are coarse-grained, leucocratic and contain perthite as the only feldspar, quartz and minor amounts of riebeckite, aegirine and stflpnomelane. The accessory minerals are apatite and zircon. ANALYTICAL NOTES Seventy-three samples have been analyzed for major and trace elements (Li, Rb, Ba, Sr, Zr, Nb, La and Ce). The major elements and Li, Rb, Ba and Sr have been determined by atomic absorption while Zr, Nb, La and Ce were analyzed by X-ray fluorescence. The precision of the trace element data is better than 10%. The chemical analyses and locations of the samples are available on request. GEOCHEMISTRY

Major elemen ts According to chemical composition, age and tectonic setting, the rocks have been divided into two groups. Group A includes rocks of the first

11 generation (> 1000 Ma old) which are mainly subalkaline and silica-oversaturated and outcrop within deep-seated structural levels. Group B comprises rocks of the remaining four generations with ages of a b o u t 530--700 Ma. They occur at shallower structural levels of the Pan-African belt and are either alkaline or peralkaline. Based upon the classification of Wright (1969), rocks of group A are suball'aline and occasionally alkaline. Their "agpaitic index" (mol. Na20 + K20/ A1203) varies from 0.83 to 1.0. The rocks are low in FeOtot, MgO and CaO and show a large variation in SiO2 content. They can be further subdivided into syenites with a b o u t 63% SiO2 and predominant granites with SiO2 ranging from 70 to 76%. Rocks with SiO2 intermediate between the two rocktypes are sparse. Syenites, which are slightly quartz-normative, have a distinctly higher content of A1203 and K20 than granites. The average compositions of the syenites and granites are given in Table I, which also shows that there is no significant difference between the composition of granites from both areas sampled (Egatalis and Iileouine). The rocks of group A contain more than 90% of normative felsic constituents. When plotted in the Q--Ab--Or ternary diagram (Fig. 4) the normative composition of the granitic rocks fall within Tuttle and Bowen's (1958) granitic field, close to the ternary minimum. In the Or--Ab--An diagram (Fig. 5) granites cluster around the low-temperature trough of Kleeman (1965) within the 2% contour of Tuttle and Bowen (1958) for "normal granites". Group B includes acid rocks of the remaining four generations, and syenitic rocks of the second generation. All granitic (and rhyolitic) rocks have a similar major-element composition and according to Wright (1969) they are peralkaline and alkaline. Their "agpaitic index" varies from 0.93 to 1.0, from 0.95 to 1.16, from 0.98 to 1.0 and from 0.99 to 1.11 in generations II--V, respectively. There is no obvious separate grouping of alkaline and peralkaline granites and as Teng and Strong (1976) have argued, the boundary between them appears to be an arbitrary one. The rocks have low contents of FeOtot, MgO and CaO. In comparison with granites of group A, they have a higher c o n t e n t of SiO2 and are lower in K20, CaO, MgO, TiO 2 and P205. In accordance with MacDonald's (1974a) classification, the granites are of commenditic affinities. In the normative Q--Ab--Or projection (Fig. 4), granites of group B cluster around the ternary minima, between the cotectic curves at 0.5 and 2.5 kbar. As the granites are mildly peralkaline, their position in this projection is only slightly affected by the excess of alkalies relative to A1203 (Bailey and Schairer, 1964). The frequent occurrence of fluorite in these rocks suggests that the pressure of 2.5 kbar probably represents an upper limit since the presence of HF in a granitic melt leads to the expansion of the quartz field (Wyllie and Tuttle, 1961). The relatively low PH20 of crystallization is also consistent with a close association of granites and volcanic rocks. The normative composition of the granites fall into the thermal trough of the Or--An--Ab diagram (Fig. 5).

12

TABLE I Average major and trace-element compositions of the studied rocks Generation: n: Norm. Q SIO2(%) AI20 ~ Fe20 ~ Fee MnO MgO CaO Na20

IA

IB

2

4,1 63.1 17.0 2.0 1.3 0.09 0.41 1.6 4.0

18

(3,0-6.3) (62.6-63.6) (16,5--17,5) (1.8--2.2) (1.3) (0.07---0.10) (0.38-0.43) (1.3--1.9) (3.7--4.2)

K~O

8.5 (8.3--8.7)

Tie 2 P2Os

0.59 (0.58--0.60) 0.12 ( 0 . 0 8 - 0 . 1 5 )

H~O

0.49(0.40--0.60)

28.0 71.9 12.7 2.0 1,2 0.08 0.36 1.3 3.9

3 316

148 1315 111 232 583 51

n:

CaO Na;O K~O TiO~ P2Os H20 I: Li (ppm) Rb 3r Ba La Ce Zr Hb

20.7 72.8 12.6 1.6 1.1 0.06 0.41 1.1 3.8

5.1 (3.8--5.9)

0.07 ( 0 . 0 2 - 0 . 1 2 )

0.6 (0.3--1.2)

(2--4) (304--329) (144--152) (1265--1365) (89--133) (206--258) (559-607)

12 180 132 1013 80 170 556 45

0.5 (0.3--0.7) 99.6

(1--46) (129--323) (64--302) (540-2000) (20---156) (39--252) (76--1062) (12--79)

8 233 116 855 87 193 414 44

16 599 35 90 15 38 713 106

(3--20) (166--273) (51--198) (500-1150) (37--137) (146---258) (309--471) (38--46)

II

2

8

5

4

5.2 (1.1--15.6) 22.4 57.8 23.0 0.1 1.3 0.07 0.04 0,87 9,3 5.3 0.05 0.01 1.3 99.2

(26.1--34.3) (70.8--75,9) (12.0--13,0) (1.1--2.7) (0.7--1.9) (0.03-0.09) (0.18-0.80) (0.3--1.7) (3.2--4.6)

0.49 (0.36--0.58) 0.09 ( 0 . 0 7 - 0 . 1 2 )

99.3

Generation:

Norm.@ Nonn.Neph. 8i0=(%) A|20 ~ F%0) Fee MnO MgO

(19.2--27.8) (69.1--75.1) (12.2--13.9) (0.5--4.0) (0.5--2.4) (0.02-0.15) (0.16--0.76) (0.4-2.5) (3.2--5.3)

4.9 (3.8--5.7) 0.30 (0.10--0.58)

99.4 Li (ppm) Rb Sr Ba La Ce Zr Nb

5

(19.0--25.8) (57.5--58.2) (22.8--23.2) (0.1) (1.2--1.3) (0.07) (0.02--0.06) (0,82-0.93) (9,0-9,6) (5.1--5.5) (0.04--0.06) (0.01) (1.0-1.7) (15--17) (571--627) (5--64) (82--98) (11--20) (27--49) (512--914) (62--150)

2.0 62.2 18.1 1.2 2.4 0.15 0.12 1.5 5.8 7.0 0.09 0.06 0.7 99.3 2 276 50 137 17 43 333 32

(60.7--63.5) (60.7-63.5) (17.7--19.0) (0.5---1.9) (1.9--3,7) (0.11--0.21) (0.0?-0.26) (1.2--2.0) (5.4--7.0) (5.9--7.5) (0.04-0.16) (0.04-0.08) (0.4--1.2) (1--5) (212.-444) (5---158) (11--330) (9--29) (24--65) (184"-489) (22--62)

66.2 16.4 1.8 I.I 0.09 0.10 0,66

(63.6--66.8) (14,5-17.3) (1.3--3.1) (0.4--1.8) (0.07--0.12) (0.02-0.31) (0,27--1.30) 6,9 (5,0--7.7) 5.3 (5.0--5.9) 0.14 ( 0 . 0 6 - 0 . 4 5 ) 0.05 ( 0 . 0 2 - 0 . 0 7 ) 0.5 (0.2--0,7) 99.2

1 464 8 140 47 110 594 81

(1--5) (58--639) (5-20) (54---450) (38--52) (94--129) (200--1440) (22--234)

36.6 (35.2--39.5) 76.5 11.3 1.9 0.8 0.05 0.07 0,47 3.8 4,5 0.17 0.04 0.3 99,9 2 227 16 202 36 98 466 35

(75.3--77.3) (10,4--12.3) (0.4--2.4) (0.5-1.1) (0.02--0.07) (0.02--0,16) (0.30--0.65) (3.0--4.4) (3,7--5.7) (0.04-0.21) (0.02--0.05) (0,1--0.4) (1--5) (113--527) (10--28) (154--437) (8--51) (18--135) (75--671) (26--47)

13

Generation: III n:

IV

15

3 8

Norm. Q

34.5

(31.1--38,7)

33.5

SiO 2 (%) AI~O3 Fe~O s FeO MnO MgO CaO Na~O K~O TiO 2 P205 H~O ]~

76.3 11.4 1.5 0.7 0.09 0.16 0.34 4.3 4.3 0.19 0.03 0.3 99.7

(73.0--77.8) (10.9--12.6) (1.0--2.2) (0,6--1.4) (0.03--0.18) (0.06--0.25) (0.05--0.95) (3.6--5.2) (3.5--4.9) (0.02----0.31) (0.02--0.06) (0.2----0.6)

76.5 12.0 0.8 0.9 0.04 0.02 0.23 4.3 4.5 0.12 0,02 0.3 99.7

Li (PPm) Rb Sr Ba La Ce Zr Nb

8 62 24 298 32 79 338 14

(1--30) (35--107) (5--103) (120--810) (8--66) (20--138) (80--1129) (4--40)

32 219 5 59 39 94 367 35

33.9 75.8 11.3 1.2 1.2 0.03 0.10 0.36 4.3 4.2 0.15 0.02 0.5 99.4

(30.6--35.7) (75.0--77.4) (10.9--12.6) (0.7--0.9) (o.8--1.0) (0.03--O.05)

(0.01---0.02) (0.14--0.35) (4.2---4.4) (4.3--4.8) (0.10---0.14) (0.02--0.03) (0.3---0.4) (3--58) (196--246) (5) (47--79) (28--57) (76--125) (311--413) (33--37)

164 38 92 562 37

(31.9--34.9) (74.5--76.5) (11.0--11.3) (1.1--1.5) (1.1--1.4) (0.03) (0.04--0.15) (0.21---0.52) (4.2--4.4) (4.0--4.5) (0.11--0.19) (0.02--0.03) (0.4--0.8)

(110--205) (25--54) (77--121) (465--623) (31--41)

n = number of sar~ples; ( ) = variation interval. Q

0,5

Ab

Or

Fig. 4. N o r m a t i v e c o m p o s i t i o n s (wt. %) o f t h e a v e r a g e s o f t h e d i f f e r e n t r o c k g e n e r a t i o n s a n d o f several s e l e c t e d s a m p l e s in r e l a t i o n t o t h e Q - - A b - - O r t e r n a r y p r o j e c t i o n . C o t e c t i c c u r v e s a n d m i n i m a f o r v a r i o u s p r e s s u r e s are s h o w n a f t e r T u t t l e a n d B o w e n ( 1 9 5 8 ) . T h e irregular b o u n d a r y is t h e 1% c o n t o u r o f T u t t l e a n d B o w e n ( 1 9 5 8 ) f o r e x t r u s i v e r o c k s . F u l l stars: a v e r a g e s o f g r o u p s 2D, 3, 4 a n d 5; full circles: s a m p l e s f r o m g r o u p 1; e m p t y circles: s a m p l e s f r o m g r o u p 2.

14

Or

Fig. 5. Normative compositions (wt. %) of the averages of the different rock generations and of several selected samples in relation to the An--Ab--Or ternary projection. The solid lines are the boundaries of the low temperature trough while the dashed lines show uncertainty due to the possibility of analytical error (Kleeman, 1965). The irregular boundary is the 2% contour of Tuttle and Bowen (1958) for granitic rocks that contain more than 80% normative Ab+Or+Q. Symbols as on Fig. 4.

The rocks of the second generation also include three types of syenites (Table I): quartz-normative syenites, mildly undersaturated syenites with 1--6% of normative nepheline and nepheline syenites with 19--26% of normative nepheline. Quartz-normative syenites vary from nearly silicasaturated with only < 1% of normative quartz to quartz syenites with 15% of normative Q. They have low contents of CaO, MgO and FeOtot, although higher than those of granites. In comparison with granites, they are also significantly higher in A1203 and alkalies. In the Q--Ab--Or diagram, syenites plot near the feldspars' join at Or3~-3s. The minimum or eutectic in a pure feldspar join, as determined experimentally, varies from Or3s at 1 kbar PH O to Or~9 at 10 kbar (Luth et al., 1964; Kleeman, 1965; Morse, 1968). A l ~ o u g h quartz-normative felsic rocks show a distinctly bimodal distribution, there seem to be several transitional rocks between syenites and granites. Silica-undersaturated syenites are higher in A1203, CaO and alkalies than oversaturated syenites and granites. Fig. 6 shows the normative composition of these rocks in relation to the Q--Ne--Ks ternary diagram. Nepheline syenites fall close to the nepheline syenite minimum, while normative nepheline-poor syenites are close to the thermal valley although somewhat towards the Or corner from the alkali feldspar minima. This displacement, however, is frequent among syenitic rocks and is probably produced by the confluence of other phases (Morse, 1968). The composition of syenitic rocks from Gardar province, Greenland (Morse, 1968) is shown in Fig. 6 for comparison, along with Nockolds' (1954) average compositions of phonolites, alkali trachytes and peralkaline rhyolites.

15 Si 0 2

Na A I S i O 4

KAISiO 4

Fig. 6. Normative compositions (wt. %) of the averages of the different rock generations and of several selected samples in relation to the Q--Ne--Ks diagram at 1 kbar PH.o (after Hamilton and MacKenzie, 1965). The irregular boundary delineates the field of syenitic rocks from Greenland (Morse, 1968). Empty stars with circles represent the average phonolite, alkaline trachytes and peralkaline rhyolite (Nockolds, 1954). Other symbols are as on Fig. 4. Trace elements The two groups of rocks also differ in their concentrations of Ba, St, La and Ce. In rocks of a given SiO2 content, the abundances of these elements are 2--3 times higher in group A. In the rocks of group A, Ba shows a positive correlation with Sr and they both tend to decrease with the increase of SiO2. Assuming t h a t these elements were n o t significantly affected by alteration, such trends suggest plagioclase fractionation. Syenitic rocks have a higher c o n t e n t of La, Ce and Rb than granites. Rb and K positively correlate with the K / R b ratio varying from 173 to 277. There is no obvious difference in K/Rb ratios between syenites and granites. The c o n t e n t of Li in the rocks of group A is usually very low (3--8 ppm) with the exception of some granitic rocks high in SiO2, which have - 20 ppm Li. The contents of trace elements in the rocks of group A are similar to those of Paleozoic alkaline granites from New England (Buma et al., 1971) and of Precambrian alkaline granites from SW Africa (Clifford and Rooke, 1969). The rocks of group B have a low c o n t e n t of Ba and Sr (< 300 ppm and < 55 ppm, respectively) which appears to be typical of highly-fractionated

16 alkaline magmas (MacDonald and Edge, 1970) including peralkaline rocks (Ewart et al., 1968; Villari, 1974; Noble and Parker, 1974). The rocks are also low in La, Ce and Li; their abundances are distinctly lower than those of crustal rocks. The concentrations of Rb vary from 62 to 485 ppm with syenitic rocks having higher contents than granites. Rb abundances in the rocks of group B are comparable to other intrusive alkaline and peralkaline suites such as Kfign~t (62--375 ppm, Upton, 1960) and Ilimaussag {150-623 ppm, Hamilton, 1964) from Greenland. However, Rb does not show a clear correlation with K; K/Rb ratios range from 50 to 800. The granites of the third generation have the highest ratios, similar to those reported by Appleyard (1974) for syenitic gneisses from eastern Ontario and northern Norway. The low K/Rb ratios observed particularly in rocks of the second generation suggest a post-magmatic modification (Bowden and Turner, 1974). Although most rocks are distinctly high in Zr and Nb (Table I), there is no obvious difference in their abundances between the individual types of rocks. Seven analyzed samples were omitted from the above discussion and from the averages given in Table I. These samples have anomalous concentrations of alkalies including anomalous Na20/K20 ratios. The high content of Na in some of these rocks (up to 11%) is also accompanied by high abundances of Zr Cup to 6100 ppm). The distribution of the elements in the rocks cannot be readily explained in terms of fractionation of the observed major rock-forming minerals. It seems that the original content of some elements, particularly alkalies, was probably modified by metasomatic processes (cf., MacDonald, 1969). PETROGENESIS

The positions of most of the rocks in the low-temperature regions of the experimental residua system and the similarities of their chemical composition to that of well-documented magmatic series such as Gardar from Greenland, White Mountains from New England or volcanic rocks of the SW Pacific (Ewart et al., 1968; Baker, 1975; Smith et al., 1977) and of Panteleria (Villari, 1974) suggest that crystal-liquid equilibria played a dominant role in the genesis of these rocks and in most cases, their composition was not significantly affected by subsequent metamorphism. The available data, therefore, can put some constraints on the petrogenesis of the rocks from Western Hoggar, in particular, on the relationship between different rock-types and the origin of the rocks of individual generations. Although the quartz-normative rocks of the second gener.~tion have a bimodal distribution, there are some intermediate rocks which lie in the thermal trough of the Q-Ab-Or projection and closely follow the trends of differentiation observed, e.g., in the White Mountains plutonic-volcanic series (Kamer, 1968) or the Younger ~ i t e s of northern Nigeria (Jacobson et al., 1958). This gradual increase in normative quartz towards the silica

17 apex (Fig. 4), along with the close spatial and temporal associations of syenitic and granitic rocks, suggests that granites could be produced by fractional crystallization of a magma of syenitic composition. The close resemblance of granites of the second generation and other alkaline and peralkaline granites of group B in major and trace-element composition and in mineralogy indicates that all these rocks have a similar origin. Their very low contents of Ba and Sr also show that extensive feldspar fractionation played a role in their genesis. Furthermore, there is some indirect evidence for the derivation of the granitic rocks of group B from a syenite/trachyte magma. The major- and trace-element composition of the silicic rocks is closely comparable to that of oversaturated peralkaline volcanic rocks, for which an origin by fractional crystallization of a more basic, probably trachytic, magma is generally accepted (cf., MacDonald, 1974b). Such a process is also consistent with experimental studies which show that pantellerites and commendites may be derived by fractional crystallization of a trachytic magma (Carmichael and MacKenzie, 1963}. It is of interest that the rocks of the second and third generations are closely associated with the abundant mafic rocks and on the basis of the field relationship, Caby (1973) has suggested a genetic link between them. Arguments, similar to those for quartz-normative syenites and granites, can be invoked for the derivation of nepheline syenites from mildly nepheline-normative syenites by fractional crystallization. The field association of phonolites and trachytes with a composition similar to that of undersaturated rocks from western Hoggar is well documented and studies on these rocks (Nash et al., 1969; Baker, 1969) indicate that phonolites may be derived from trachytes. The feasibility of such a mechanism has also been confirmed by experimental studies (Hamilton and MacKenzie, 1965). The relation between oversaturated and undersaturated rocks of the second generation, which are apparently closely related in time and space, is not very clear. However, the association of oversaturated and undersaturated rocks in both plutonic and volcanic suites and even in their metamorphic equivalents (alkaline gneisses) are known and frequently considered to be comagmatic (cf., MacDonald, 1974b). It is usually suggested that a syenitic! trachytic magma was parental to both undersaturated and oversaturated trends, leading to the formation of alkaline granite/rhyolite in one instance and to nepheline syenite/phonolite in another. Several processes which may produce such a differentiation of syenitic/trachytic magmas were recently summarized by MacDonald (1974b). The available data for the Hoggar rocks, however, cannot evaluate these mechanisms nor can they negate a derivation of both types from separate mafic magmas of variable degrees of silica saturation. The low contents of Sr and Ba in the Hoggar syenites suggest that these rocks already underwent rather extensive feldspar fractionation from more primitive magmas. Considering the similarity of the chemical composition of the Hoggar rocks to that of well-studied volcanic rocks, (e.g., Smith et al., 1977), it seems that parental magmas of the rocks of group B are of upper-mantle origin.

18 The characteristic feature of the rocks of group A is the still well-preserved original magmatic layering. In this respect, group A resembles the Precambrian layered intrusions of southern Greenland of similar age, which also include syenites and granites (Harry and Emeleus, 1960). It may indicate, along with close spatial association, that the granites and syenites of group A can be the products of magmatic differentiation. Derivation of granites from syenites is also consistent with the negative correlation of Ba and Sr with SiO2, which suggests feldspar fractionation. However, the content of trace elements (La, Ce, Ba, Sr), which is 2--3 times higher in these rocks than in those of group B, implies that the composition of the parental magma differed from that of group B. Constraints from experimental studies (cf., Bailey and Schairer, 1966; Morse, 1968; MacDonald, 1974b) suggest that such a parental magma was of upper mantle origin, although it could have been affected by crustal contamination (Chapman, 1968; Upton, 1974). CONCLUSIONS Five successive generations of alkaline, subalkaline and peralkaline felsic igneous rocks recognized in a Pan-African belt of northwestern Hoggar, central Sahara, belong to at least two different geodynamic settings. These differences are also reflected in the chemical composition of the rocks. The rocks of the first generation form alkaline layered intrusions similar to those of Precambrian age from southern Greenland. They are typical intracontinental rocks probably derived from highly differentiated magmas. The rocks of the four other generations (particularly generations II and III) are from felsic dyke complexes, closely associated with earlier basic dykes which were emplaced along abyssal faults. Although felsic and basic rocks may be genetically related, intermediate rocks are missing. Felsic dyke complexes, probably derived from magmas of upper mantle origin, may represent deeply eroded paleorift systems comparable to modern African rifts. REFERENCES All~gre, C.J. and Caby, F., 1972. Chronologie absolue du Pr~cambrien de l'Ahaggar occidental. C.R. Acad. Sci. Paris, 275, D: 2095--2098. Appleyard, E.C., 1974. Syn-orogenic igneous alkaline rocks of eastern Ontario and northern Norway. Lithos, 7: 147--169. Bailey, D.K. and Schalrer, J.F., 1964. Feldspar-liquid equilibriain peralkaline liquids. The orthoclase effect. A m . J. Sci., 262: 1198--1206. Bailey, D.K. and Schalrer, J.F., 1966. The system Na20--AI203--F%O3--SiO ~ at 1 atmosphere and the petrogenesis of alkaline rocks. J. Petrol., 7 : 114--170. Baker, I., 1969. Petrology of the volcanic rocks of Saint Helena Island, South Atlantic. Geol. Soc. A m . Bull.,80: 1283--1310. Baker, P.E., 1975. Peralkaline acid volcanic rocks of oceanic islands. Bull. Volcanol.,

38: 735--754. Bertrand, J.M.L. and Caby, R., 1978. Geodynamic evolution of the Pan-African orogenic belt: a new interpretation of the Hoggar shield. Geol. Runschau, 67: 357--388.

19

Bowden, P. and Turner, O.C., 1974. Peralkaline and associated ring-complexes in the Nigeria-Niger province, West Africa. In: H. S~renson (Editor), The Alkaline Rocks. Wiley, N e w York, N.Y., pp. 330--351. Buma, G., Frey, F.A. and Wones, D.R., 1971. N e w England granites: trace element evidence regarding their origin and differentiation.Contr. Mineral. Petrol., 31: 300--320. Caby, R., 1970. La Chafne Pharusienne dans le Nord-Ouest de l'Ahaggar (Sahara Central, Alg~rie); Sa Place dans l'Orogen~se du Pr~cambrien Sup~rieur en Afrique. Thesis, Univ. Montpellier. Caby, R., 1973. Alkaline and peralkaline rocks in Western Hoggar. 7e Coll. Geol. Afr., Florence, Tray. Lab. Geol. Fac. Sci. Marseille, 1975, s~r. B, no. 11, p. 31. Carmichael, I.S.E. and MacKenzie, W.S., 1963. Feldspar-liquid equilibriain pantellerites: an experimental study. A m . J. Sci., 261: 282--296. Chapman, C.A., 1968. A comparison of the Maine coastal plutons and the magmatic central complexes of N e w Hampshire. In: E. Zen (Editor), Studies of Appalachian Geology: Northern and Maritimes. Wiley, N e w York, N.Y., pp. 385--396. Clifford, T.N. and Rooke, J.M., 1969. Petrochemistry and age of the Franzfontein granitic rocks of northern South-West Africa. Geochim. Cosmochim. Acta, 32: 1303--1315. Ewart, A., Taylor, S.R. and Capp, A.C., 1968. Geochemistry of the pantelleritesof Mayor Island, N e w Zealand. Contr. Mineral. Petrol., 17: 116--140. Floor, P., 1974. Alkaline gneisses. In: H. S~renson (Editor), The Alkaline Rocks. Wiley, N e w York, N.Y., pp. 124--142. Hamilton, E.I., 1964. The geochemistry of the northern part of the Ilimaussaq intrusion, S.W. Greenland. Bull. Gronlands Geol. Unders., 42:104 pp. Hamilton, D.L. and MacKenzie, W.S., 1965. Phase equilibrium studies in the system NaAISiO4--KAISiO4--SiO2--H20. Min. Mag., 34: 214--231. Harry, W.T. and Emeleus, E.H., 1960. Mineral layering in some granite intrusions of S.W. Greenland. Intern. Geol. Congr. 21st sess.,Norden, pp. 172--181. Jacobson, R.R.E., Macleod, W.N. and Black, R., 1958. Ring-complexes in the Younger Granite province of Northern Nigeria. M e m . Geol. Soc. Lond., 1 : 72 pp. Kanaris-Sotiriov, R., 1974. Fine-scale layering in igneous intrusions: a possible mechanism for a non-depositional origin. Geol. Mag., 111: 157--162. Karner, F.R., 1968. Compositional variation in the Tunk Lake granite pluton, Southeastern Maine. Geol. Soc. Am. Bull., 79: 193--222. Kleeman, A.W., 1965. The origin of granitic magmas. J. Geol. Soc. Aust., 12: 35--52. Luth, W.C., Jahns, R.H. and Tuttle, O.F., 1964. The granitic system at pressure of 4 to 10 kilobars. J. Geophys. Res., 69: 759--773. MacDonald, R., 1969. The petrology of alkaline dykes from the Tugtut6q area, South Greenland. Bull. Geol. Soc. Denmark, 19: 257--282. MacDonald, R., 1974a. Nomenclature and petrochemistry of the peralkaline oversaturated extrusive rocks. Bull. Volcanol., 38: 498--516. MacDonald, R., 1974b. The role of fractional crystallizationin the formation of the alkaline rocks. In: H. Sorenson (Editor), The Alkaline Rocks. Wiley, N e w York, N.Y., pp. 442--458. MacDonald, R. and Edge, R.A., 1970. Trace element distribution in alkaline dykes from the TugtutSq region, South Greenland. Bull. Geol. Soc. Denmark, 20: 38--58. Morse, S.A., 1968. Syenites. Carnegie Inst. Wash. Yb., 67: 112--120. Nash, W.P., Carmichael, I.S.E. and Johnson, R.W., 1969. The mineralogy and petrology of Mount Suswa, Kenya. J. Petrol., 10: 409--439. Noble, D.C. and Parker, D.F., 1974. Peralkaline silicicvolcanic rocks of the Western United States. Bull. Volcanol., 38: 803--827. Nockolds, S.R., 1954. Average chemical composition of some igneous rocks. Geol. Soc. Am. Bull., 65: 1 0 0 7 - - 1 0 3 2 .

20 Smith, I.E.M., ChappeU, B.W., Ward, G.K. and Freeman, R.S., 1977. Peralkaline rhyotites associated with andesitic arcs of the southwest Pacific. Earth Planet. Sci. Lett., 37 : 230--236. S~renson, H., 1969. Rhythmic igneous layering in peralkaline intrusions. An euay review on Ilimaummq (Greenland) and Lovozero (Kola, U.S.S.R.). Lithos, 2: 261--283. Teng, H.C. and Strong, D.F., 1976. Geology and geochemistry of the St. Lawrence peralkaline granite and mmociated fluorite deposits, southeast Newfoundland. Can. J. Earth Sci., 13: 1374--1385. Turtle, O.F. and Bowen, N.L., 1958. Origin of granite in the light of experimental studies in the system NaAISisO,--KA1SisOs--SiO~--H=O. Mem. Geol. Soc. Am., 74:153 pp. Upton, B.G.J., 1960. The alkaline igneous complex of KQngn~t Field, South Greenland. Bull. Gr~nlands Geol. Unders., 27, 145 pp. Upton, B.G.J., 1974. The alkaline province of south-west Greenland. In: H. Screnson (Editor), The Alkaline Rocks. Wiley, New York, N.Y., pp. 221--238. Villari, L., 1974. The Island of PanteUeria. Bull. Volcanol., 38: 680--724. Wright, J.B., 1969. A simple alkalinity ratio and its application to questions of nonorogenic granite genesis. Geol. Mag., 106: 370--384. Wyllie, P.J. and Tuttle, O.F., 1961. Experimental investigation of silicate systems containing two volatile components. Part II. Am. J. Sci., 259: 128--143.