Upper proterozoic volcanic graywackes from northwestern Hoggar (Algeria) — geology and geochemistry

Precambnan Research, 5 (1977) 283--297 283 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

UPPER PROTEROZOIC VOLCANIC GRAYWACKES FROM NORTHWESTERN HOGGAR (ALGERIA) -- GEOLOGY AND GEOCHEMISTRY*

R CABY 1, J. DOSTAL 2 and C DUPUY' ~Centre G~ologique et G~ophystque, U.S.T.L., Place E_ Batadlon, Montpelher Cedex (France)

2Department of Geology, St. Mary's University, Halifax, N.S (Canada) (Received and accepted April 21, 1977)

ABSTRACT Caby, R , Dostal, J and Dupuy, C., 1977 Upper proterozoic volcanic graywackes from northwestern Hoggar (Algeria) -- geology and geochemistry Precambrlan Res , 5 283--297. Late-Proterozoic volcanic graywackes from the NW Hoggar (Algeria) have been investigated from geological and petrological points of view. Thirty-five samples have been analysed for major elements and the following trace elements: Ll, Rb, Sr, Ba, Nl, Co, Cr, V, Zn, Cu These graywackes constitute a thick flysch-like formation deposited in a marine environment They are composed of volcanic (mainly andesltes) and plutonic detritus The chemmal analyses confirm this observation and demonstrate the very immature character of these rocks, The composition of the NW Hoggar graywackes differs chemically from other investigated graywackes in that they have a higher Ca, Na content and lower SiO2, NI content Several geological observahons suggest that the average Hoggar graywacke composition approaches the composition of the W Hoggar Late Precambrian upper crust The graywackes were formed as a result of erosion of a penecontemporaneous calc-alkah volcamc suite, emplaced after widespread mafic intrusions in the Upper Proterozoic shelf deposits

INTRODUCTION Although graywackes are generally absent from the Pan-African belts, the v o l c a n o - d e t r i t a l r o c k s e r i e s ( t h e c l a s s i c a l " P h a r u s i a n " ) a r e w i d e s p r e a d in t h e Hoggar. They were first reported by Lelubre (1952) and Gravelle (1969). Typical volcanic graywackes constitute an up to 6000 m thick flysch-like f o l m a t i o n o f L a t e P r o t e r o z o i c age, t h e " G r e e n S e r i e s " ( C a b y , 1 9 7 0 ) . Because of the wide distribution and the importance of this "Green Series" for t h e g e o l o g i c a l h i s t o r y o f t h e P r e c a m b r i a n t e r r a n e s in t h e H o g g a r , t h e s e r o c k s have been carefully investigated both from a petrological and a chemmal *Contribution C G.G. No 220.

284 point of vmw m an area mapped on a scale of 1:200 000. Thtrty-five samples were analysed for major and trace elements in order to characterize chemically these graywackes and to compare them with similar rocks of different ages from other regions. GEOLOGICAL SETTING Locatton

The "Green Series" is entirely composed of volcanic graywackes and associated volcanic-plutonic rocks and is the youngest sequence affected by the Pan-African orogeny. It forms the highest tectonic level of the Pharusian belt defined in thin zone. Syn-kinematic granites have been dated at 640 m.a. and post-kinematic granites at 580 m.a. by U/Pb on zircons and Rb/Sr on whole rock with ),Rb = 1.39 . 10 -11 yr -1 (All~gre and Caby, 1972; All~gre, pers. communication). The graywacke formation occurs in two distinct structural domains (Fig. 1): (1) The In Zize synclinorium (at least 200 km wide prior to orogenic E--W shortening) which disappears to the north below the Tassendjanet Nappe (Eburnean granites overlain by shelf carbonates of the stromatolite series); (2) the Tanezrouft syncline following the margin of t h e W e s t African Craton. S u b s tra turn

The substratum of the graywacke formation is only exposed in the In Zize synclinorium. In the northern part it is represented by a large lopolith (outcropping over more than 500 k m 2) of gabbrofc and ultra-mafic rocks. This mafic intrusion was emplaced about 800 M a ago (N. Clauer, 1976) in the stromatolite series,a sheld carbonate series deposited between 1100 and 800 M a on the Tassendjanet basement. Farther south, layered meta-ultrabasics (4000 m) with associated diabase dyke swarms also constitute the substratum At the southern margin of the synclinorium, metagabbros and amphibolites

Fig 1. Sketch map of the pan-African belt of central-western Hoggar and Adrar des Iforas. 1. motaasic deposits (including the Cambrian) 2. Upper to Late Proterozoic trough deposits, a) mainly graywackes and associated volcanics; b) semi-pelitic 3. undifferentiated metamorphics and granites with ultramafics in black 4. inliers of ~-- 2000 Ma basement rocks, a)metamDrphics and granites, b) granulite facies rocks 5. major faults.

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interlayered with middle to upper Proterozoic orthoquartzites and other metasedimentary rocks underlie the graywackes (Caby, 1970).

Associated magmat~sm Detailed mapping of the Tassendjanet-Ougda zone (Caby, 1970) has shown that the graywackes overlie, and laterally grade into, volcanic agglomerates and breccias intercalated in a 6000-m thick volcanic sequence of basalts and andesites with rhyo-dacite at the top. It is the only zone where the direct parental source of the volcanic graywackes is evidenced by field observation. These graywackes are intruded by several igneous rocks: in the In Zize synclinorium various subsequently deformed sills and laccoliths appear in the series (complex laccoliths with a large-scale n e t w o r k of calc-alkaline to sodic microgranite, trondjemite, fine-grained diabases to amphibole gabbros, dacites, rhyolites, keratophyres, pretectonic granodiorites to quartz-diorites, etc.). The total a m o u n t of these intrusives is a b o u t 20% of the entire Green Series, b u t locally reaches as much as 80% in zones where graywackes were converted into dark hornfels prior to the Pan-African metamorphism. It is n o t e w o r t h y that very similar rock types are also encountered as pebbles in conglomerates (see below); in the Tanezrouft synclinorium numerous dykes of various volcanic-plutonic acidic rocks occur, with several dykes of albitic trachytes, diabases and spilites. At Adrar and in Beld el Mass no magmatism m known, b u t in the Sebkha el Melah, andesites and basalts were emplaced contemporaneously with graywacke deposition at different levels of the exposed succession (Caby, 1970). FIELD ASPECT AND SEDIMENTARY

FEATURES OF THE GRAYWACKES

A pronounced pale-green to dark-g~een colour characterizes most of the graywackes in the chlorite zone, whereas a dark-grey or dark-blue colour appears in the biotite zone. No systematic variations of rock components (for example quartz increase) could be detected in the exposed successions at the different localities studied. The average graywacke builds m o n o t o n o u s layers 0.1--1 m thick, separated by thin silty, or more shaly bands. Cross-bedding, convolute bedding, graded bedding, load and injection structures are ubiquitous. Disseminated pebbles ate also very frequent. Brittle beds and high-energy current marks very similar in their association to those from typical flysch have been observed in the Bled el Mass. All these features are typical of turbidites (Dzulyn ski and Walton, 1965). A regular tmpure magnesian-silicic limestone, a few metres thick and associated with jaspers, has been mapped in the western part of the In Zize synclinorium. Grading into calcareous graywacke, this peculiar layer may represent a very uniform temporary shallow depositional environment related to a pause in subsidence.

287 Conglomeratm layers and lenses may appear at any level in the series and make up at least 15% of the series in the In Zsze synclinorium. The more frequent conglomerates only contain unsorted plutonic and volcanic clasts, both angular and rounded, up to 1 m 3 in volume. Clasts generally constitute less than 50% of the beds, which pass both laterally and upwards into normal or pebbly graywackes. Rock types forming the pebbles are strikingly similar to the volcanics and plutonic intrusives listed before, in addition to various jaspers, glassy volcanics, tuffs and graywackes. The proportion of different rock types making up the clasts is highly variable. Quartmte pebbles from the substratum occur m places but some conglomerates are only composed of graywacke pebbles, indicating the importance of redeposition phenomena in the series. It must be stressed t h a t except for quartzite pebbles, no older sialm comp o n e n t has been found in the conglomerates. All the plutomc and volcamc rocks forming the clasts were recognlsed as being derived from rocks of the younger magrnatic activity (800 Ma or more recent) which characterizes this zone. We do n o t know whether the pebbles were transported by streams coming from an emerged continent entirely covered by young magmatics and volcanics during rapid erosion, or under littoral conditions along island shores*. The presence of plutonic pebbles, which are even disseminated in brittle beds of the finely laminated silty graywackes of the "pelagic phase", strongly supports the assumption that the pebbles were carried into a deep trough by high-density turbidites ("fluxoturbldites"), as is the case in graywacke deposits (Helwig and Sarpi, 1969). Similar pre-orogenlc development, probably during the same time, has been described by Hughes (1972), from the Avalon Peninsula (Newfoundland) where semi-basra rocks also form the substratum of Paleozoic graywackes m adjacent areas (Helvlg and Sarpi, 1969). The enormous volume of graywacke in Late Proterozoic terranes of the Hoggar shows some similaritms with conditions in Pacific-type geosynchnes (Crook, 1969). It has been suggested by Caby (1970) that the characteristics of this series indmate deposition m an oceanic environment. PETROGRAPHY AND MINERALOGY OF THE LOW-GRADEMETAGRAYWACKE ZONES Four main classes may be defined: (1) quartz-rich graywacke (some K-feldspar rich), (2) sodic feldspathic graywacke, (3) quartz-free, dark basra graywacke, (4) fine-grained silty graywacke with variable amounts of clay components. Heterogeneity and microbreccia textures characterize most of the examined graywackes. C o m m o n graywackes (classes 1 and 2) contain 40--80% of grains (crystal and lithic fragments) ranging in size from 0.5--2 mm and *No evidence for a glacial omgm of these conglomerates has yet been found, but farther south, similar polygenic conglomerates of the Iforas massif may be regarded as tilhtes (J. Fabre, pers communication)

288 set in a finer-grained matrix. The angular shape of fragments is only partly primary, because corrosion features resulting from the reaction between fragments and matrix have frequently been observed. Like in most graywackes, the fine-grained matrix represents the recrystallised (metamorphic) portion of finer detrital elements (Dzulynslo and Walton, 1965) which are mostly of volcanic origin (Brenchley, 1969). Up to 30% of clastic quartz occurs in rocks of class 1. It varies in size from < 0.1--5 mm. All the coarse quartz grains examined have a typical grey-blue colour and exhibit well-preserved crystal faces indicating their volcanic origin. Intergrowths of perthite and/or albite (granophync texture) with the quartz are also frequent. Smaller-sized quartz grains are angular or acicular. Albite is the most frequent feldspar. It may occur as detrital grains (albltiC granophyres, trondjemite fragments} but is mostly of secondary origin, as shown by the abundance of small epidote inclusions, which outlines an old zonation of a primary basic plagioclase. Typical andesite grains also contain low-temperature albite instead of Ca-rich plagioclase phenocrysts or microlites. This phenomenon has been observed even in zeolite-facies rocks; this process may be due to sea-water reaction with calcic plagioclase (Martini, 1968). Quartz-free graywackes (class 3) are rich in andesitm, basaltic or diabase fragments. In low-grade rocks associated with the Tassendjanet-Ougda volcanics, unaltered andesite fragments and phenocrysts (clinopyroxene, amphibole, rare biotite, calcic plagioclase) may be preserved. But these rocks are very sensitive to low-grade recrystallisation and are generally converted into dark prasinites (chlorite, actinolite, albite, calcite, epidote) towards the south Laminated silty graywackes (class 4) contain quartz shards, both detrital and newly-formed tiny flakes of white mica, chlorite, calcite and/or epidote. They possibly represent the "pelagic phase" deposited during quiet periods devoid of clastic sedimentation. PROGRESSIVE METAMORPHISM

In the In Zize synclinonum, progressive metamorphism can be observed from the upper greenschmt facies in the north to lower amphibolite facies in the south. Common minerals found in the greenschist facies rocks are: chlorite, clinozoisite and/or pistacite, leucoxene, actinolite, tremolite, calcite, albite, illite or white mica and opaques. Pumpellyite has been determined optically. Piedmontite and spassartine have been observed occasionally in andesite fragments and in a pink Fe-rich shaly layer, together with Fe and Mn oxides. The replacement of calcite by epidote already occurs in the carbonate-rich graywackes of the Sebkha et Melah, where prehnite and zeolites are frequent in andesitic detritus. In the biotite zone, detrital textures are usually destroyed and the rocks are converted into various green biotite schists. The macroscopic aspect of the rocks and their sedimentary features can, however, be recognised up to lower amphibolite facies (especially the polygenic conglomerates). Lower

289 amphibolite facies and mobilisation occur close to granodmritic and diontic syn-kinematm intrusmns. SAMPLING AND ANALYTICAL METHOD Thirty-five samples were analysed of which eight samples were collected in the Adrar and Sebkha el Melah mliers; and the others were collected in an area west of Ouallen and in the northern part of the In Zlze synclinorium. The samples were chosen to be representative of the abundance and type of rocks and they belong to the zeohte and chlorite zones; only five samples were collected m upper amphibohte facies. Major elements (except SiO2 and P2Os) and trace elements were determined by atomic absorptmn spectrometry. Silica and phosphorus were analysed by colonmetry. The prec:sion and accuracy of all analysed elements are estimated to be less than 5%. The trace-element values obtained for the U.S. geological standard sample Wl (Table I) are in good agreement with the data of Flanagan (1973). MAJOR-ELEMENT GEOCHEMISTRY The graywackes show relatively large variations in chemical composition, with $102 content ranging f r o m approximately 53--73% (Table I). Sample 8 is an exception (due to the high content of CaCO3 the silica is only 42%). Some other elements, especially Ca and K, have even larger relative variations (Table II, column 1). As graywackes are m e t a m o r p h o s e d to variable degrees, it m a y perhaps be suggested that some chemical variations are due to metamorphism. However, from the examination of the major-element distribution no obvious difference is apparent between samples representing variable metamorphic grade. If we expect a release of CO2 dunng high-grade metamorphism from the carbonate matrix, it may be concluded that metamorphism was approximately isochemmal although the relatively small number of samples and their compositional variation do n o t allow a more quantitative evaluation of this effect. The t y p e of lsochemical metamorphism was suggested by Coleman (1965) and Condie (1967), especially in the low and medium grades of metamorph:sm. The comparison of graywackes with calc-alkaline rocks which occur m the surrounding areas and with boulders from interlayered conglomerates shows that these rocks have comparable compositions and similarly large chemical variations (Figs. 2 and 3). However, some graywackes are enriched in Ca and/ or Na (Fig. 2). These graywackes are generally slightly higher in A1203 in comparison with volcanics from the adjacent area and with some samples of igneous rocks (Fig. 4) and this feature may indmate the contribution of clay minerals. The high c o n t e n t of Al~O3 in a few samples from silty layers of the "pelagic phase" is also indmated by the presence of normative corundum,

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Li Rb Sr Ba V Cr Co Ni Cu Zn

24 10 385 325 340 55 34 170 114 107

Trace e l e m e n t s ( p p m )

53.1 16.7 10.7

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1

18 45 455 685 121 234 22 142 44 67

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Ref. o f samples

51 38 362 450 40 17 n.d. 9 10 184

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0.1

67.4 14.7 5.5

3

L.I. Loss o n Ignition, n.d. : n o t d e t e r m i n e d

9 32 380 1220 110 35 n.d. 26 14 75

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0.1

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4

C o m p o s i t a o n s o f s o m e graywackes f r o m NW Hoggar, wt.%

TABLE I

40 46 379 820 110 79 n.d 24 10 97

99.3

63.0 14.4 6.0 0.1 2.8 2.6 4.1 1.5 0.8 0.2 3.8

5

14 46 290 1040 61 35 n d. 17 33 64

100.5

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6

17 65 480 880 105 104 19 290 26 84

99.9

61.7 17.2 6.1 0.1 2.5 3.7 3.1 2.1 0.8 01 2.5

7

19 3 300 330 269 32 27 16 61 83

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8

6 4 157 300 69 27 8 14 8 55

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12 21 195 180 258 121 46 77 113 86

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Typtcal common graywackes (1) semi-basic metagraywackes with mmrobreccla structure---greenschlst facms. (2) pebbly metagraywacke with mainly epidote-rich, cherty fragments and graywacke pebbles Very representative of the serms, In Zize synclinorium. (3) typical fine-grained banded graywacke of Adrar. Quartz, albitised plagioclase fragments and numerous andesite grains (zeolitechlorite matrix < 10 %) (4) quartz-rich, coarse-grained graywacke of Ouallen, rich in acidic sodic plutonic fragments (5) pink fine-grained graywacke of the Sebkha el Melah. (6) very recrystalhsed acldm graywacke Fine-grained matrix constitutes more than 60 % of the rock. Northern part of In Zize synclinorium Trace elements values obtained on the U S. geological standard Wl (10).

Epldote-rzch hzgh-grade metagraywacke (9) epidote-hornblende-andesme quartz-rich gneiss. Relicts of coarse-grained acidic plutomc pebbles are recognisable in hand specimen. Lower amphibohte facies In Zize synclinorium.

Calczte rich graywacke (8) mlcrobreccla with calcite matrix Llthm fragments are p o r p h y n t m andesites and basalts invaded by pistacite and chlorite

Al-rlch silty graywacke (7) pink silty graywacke of the "pelagic phase" with torerocycles outhned by opaques, In Z~ze synclinorium

to

292

TABLE II Average chemical composition (x_+ s), wt %

n SiO 2 AI~O3 Fe:C~3 MnO MgO CaO Na20 K20 TiO 2 P205

1 35

2 32

3 9

62.9+6.5 14.9±1.3 5.9±2 2 0.13+0.06 2.6+ 1.4 4 5+3.3 3.8+1 1 1.7-+1.3 0.8-+0 3

58.8-+1 4 16.0±0.9 6.3±1.1 0.09+0 02 3.3 ± 1.3 5.1+1.0 3.8+-0.6 2.9±0.6 1.0±0.2

58.5±1.3 15.8+0.9 6.2+1.1 0.09+0.01 4 4± 1.9 6 5±1 5 4 0±1.7 1 0+0.2 0.9-+0.2

0.2-+0.06

0 3+0.1

4 8

0.3-+0.1

5 8

64.7+1 7 15.6+0.6 4.6±1 0 0 05±0.01 1.3± 0.5 2 5±0.7 4.3±0.4 4 1+0 2 0 8±0.2 0.3±0.05

75.2±2.2 12 2+1 0 2.5+0.9 0.07~0 04 0.4+- 0.4 1 0-+0 6 4 4+1A 3.1~1 5 0.3±0.2 0 07~-0 04

Trace elements (ppm) Li Rb Sr Ba V Cr Co Ni Cu Zn

22±13 41±30 383±228 743±490 119±73 64±61 21+10 49±66 31±26 83+27

20±5 82±21 595±112 1018±179 155±29 70±92 23±7 36±32 43±20 69±19

11±1 28±11 734~214 721±210 151±25 188±167 27±6 91±58 76±38 63±8

19±7 126±8 462±85 1455±320 94±57 17±12 12±6 8±6 16±9 51±7

5~5 54±46 140±79 549±345 27±21 12~18 < 5 7t6 11~4 93+84

1, 2, 3, 4, 5,

graywackes of NW Hoggar calc-alkali andesite from the surrounding country alterated calc-alkali andesites from the surrounding country dacite from the surrounding country acidic plutonics intruding graywackes and similar rocks found as pebbles in conglomerates (columns 2, 3 and 4. transition elements determined on half the number of samples, unpublished data). n. number of samples

whereas other silty graywackes have a normal composition similar to no. 2 or n o . 5 o f T a b l e I. T h e a v e r a g e c o m p o s i t i o n o f g r a y w a c k e s ( T a b l e II) f r o m n o r t h w e s t e r n H o g g a r is c o m p a r a b l e t o o t h e r P r e c a m b r i a n g r a y w a c k e s ( P e t t i j o h n , 1 9 6 3 ; C o n d i e , 1 9 6 7 ; C o n d i e e t al., 1 9 7 0 ) . A l t h o u g h t h e f o r m e r is s o m e w h a t l o w e r i n 8iO2 a n d h i g h e r i n C a O a n d N a 2 0 . F o r t h e l a t t e r t w o elem e n t s t h e a v e r a g e o f t h e i n v e s t i g a t e d g r a y w a c k e s is s i m i l a r t o D e v o n i a n volcanic graywackes from Australia (Chappetl, 1968).

293

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Fig. 3 Total Fe as F%O3--MgO--TiO~ composition diagram showing the compositional range of NW Hoggar graywackes (solid circles) and andesitic rocks (dashed lines) Also shown as squares some of the average graywackes compiled by CondIe (196"/)

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Fig. 4. MgO--CaO--AI203 composition diagram showing the composittonal range of NW H o g g ~ graywackes (solid circles) and ~md~itic rocks (dashed lines) Also shown as squares some of the average graywackes compiled by Condie (1967). The continuous hnes delineate the igneous-rock domains.

TRACE-ELEMENT GEOCHEMISTRY

Li, R b, Sr, Ba The concentration of these trace elements varies widely but does not show any systematic variation related to the major elements, except for potassium. There is a positive correlation between K and Rb with a K/Rb ratio ranging from 250--450. Rb also has a correlation with Ba. These correlations may reflect a magmatic trend of their source material. According to the relationship between the Ca/Sr ratio and the Ca content (Fig. 5), the graywackes can be divided into two groups; the group with the highest Ca/Sr ratio also has a higher Ba/Sr ratio. Although major and most trace elements do not show any obvious change with the metamorphic grade, Li and Rb are generally enriched in the rocks of zeolite facies (about twice). Rb, Sr and Ba content is similar to that of other graywackes (data reported for different countries by Condie, 1967 and Condie et al., 1970). In fact, Sr and Ba abundances are very clc~e to the values reported for Early Precambrian graywackes from South Africa (Condie et al., 1970) whilst Ba is closely comparable to Silurian graywackes from northern California (Condie

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Fig. 5. Ca/Sr vs Ca diagram showing the compositional range of NW Hoggar graywackes (solid circles), acidic plutonm intruding the graywackes or found as pebbles in the conglomerates (open cLrcles) and andesitlc rocks (dashed lines)

and Snansleng, 1971). The Tassendjanet graywackes have higher K/Rb, Ba/ Rb and lower Rb/Sr ratios in comparison with other graywackes. It is of interest that these ratios, together with the abundances of Sr, Ba, Rb, and Li, are similar to those of calc-alkaline volcanic rocks (Table II).

V, Cr, Ca, Ni, Cu, Zn The c o n t e n t of the transition elements is variable (Tables I and II) and does n o t show any change with metamorphic grade. Ni and Cr show the highest, Co and Zn the lowest variations as in calc-alkali rocks. Some samples are enriched in Cr and NI with a c o n t e n t comparable with t h a t of basalts. The graywackes display a good positive correlation between V--Cr--Ni--Ti--Fe. Contrary to igneous rocks, the correlation between Ni and Fe is more distinct than t h a t between Ni and Mg. Compared to other graywackes, the NW Hoggar rocks are generally lower in Ni with an average content similar to t h a t of andesitic rocks from an adjacent locality but also close to that of Gazelle formations in northern California (Condle and Snansieng, 1971). INTERPRETATION

Field evidence, petrography, and the major and trace-element compositions indicate that the NW Hoggar graywackes were derived to a large degree from calc-alkaline rocks. However, the presence of these volcanic rocks cann o t explain all the observed facts. For instance Fig. 5 shows t h a t two groups of graywacke samples may be distinguished on the basis of Ca/Sr ratios and

296

Ca content. Those with high Ca/Sr ratios obviously imply some influence of carbonates. The high Na c o n t e n t may be related to the effect of the volcanic and plutonic rocks, which are the main constituents, but in some cases {e.g. column 3, Table I) the enrichment is so great that it appears to be more likely related to an early albitisation by sea water during diagenesis or low-grade metamorphism (Martini, 1968; Blatt et al., 1972). The increase of N1 and Cr in some rocks is certainly influenced by the opaque-mineral distribution in the sample. This observation is supported by the positive correlation between Ti--V--Cr--Ni--Fe and by the abundance of iron oxide minerals CONCLUSION

Graywackes of northwestern Hoggar differ from many other graywacke deposits by having an extremely immature character. This fact is underlined by the large variation m chemical composition, accompamed by a distinct evolution of the A1203/Na20 ratio, as suggested by Pettijohn (1957). This immature character implies that volcanism, plutonism and graywacke deposition were penecontemporaneous; intrusion of various calc-alkaline plutonic rocks was immediately followed by vertical uplift and rapid erosion. The parental source of NW Hoggar graywackes is indicated by comparison with the analysed volcanic rocks of the same area and, furthermore by field evidence in the Tassendjanet area. The parent material consists essentially of calc-alkaline rocks (mostly andesites) and, to a lesser degree, of some more acidic plutonic rocks. The negligible volume of rock components from typical pre-graywacke sialic basement is very striking. These graywackes show some chemical similarities with other graywackes, although the former are somewhat higher in Ca, Na and lower in Ni. The chemical composition of our Late Precambrian rocks confirms the absence of secular variations, since the Archaean, as noticed by Condie (1967). In actual fact, the shght chemical differences among graywackes of various areas perhaps reflect the different parent-rock composition. We stress here that the average composition of the analysed graywackes probably approaches the average composition of the Late Precambrian upper crust of the western Hoggar. This crust can be regarded as accreted material of calc-alkaline character emplaced some 650--800 Ma ago between the margin of the West African Craton and the sialic crust of the central Hoggar, and it is situated on the future site of the Pan-African fold belt. This crust is less sialic than that of other shield areas. These observations are in agreement with the scarcity of old sialic basement in this part of the Precambrian fold belt of the western Hoggar. From these conclusions we may deduce the possible existence of a Late Proterozoic ocean (Caby, 1970; Leblanc, 1976) and a possible Island-arc environment.

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