18. PETROGRAPHY AND K-Ar DATING OF THE MAZAGAN GRANODIORITE, DEEP SEADRILLING PROJECT LEG 79, HOLES 544A AND 547B1

H. Kreuzer, P. Müller, and G. Wissmann, Bundesanstalt fur Geowissenschaften und Rohstoffeand

T. Reinecke, Mineralogisch-Petrographisches Institut der Technischen Universitàt Braunschweig2

ABSTRACT

Gneissic granodiorite was recovered by drilling at the base of the Mazagan escarpment, 100 km west of the Casa-blanca, Morocco, at 4000 m water depth. Coarse, predeformative muscovite yielded dates of -515 Ma, fine-grainedmuscovite of -455 Ma, biotite -360 and 335 Ma, and feldspar -315 Ma. These dates are tentatively correlated withthe microscopic results. We assume a minimum age of middle Cambrian for the granodiorite, an Ordovician deforma-tion and mylonitization, and a Late Carboniferous overprint under upper greenschist facies conditions.

INTRODUCTION

At the base of the slope leading up to the MazaganPlateau, 100 km west off Casablanca, Morocco, pistoncores (Vema 30) and dredge samples (Valdivia 79) havetraced a mylonitic "granite." A geoseismic section showsan up-faulted wedge with a thin sedimentary cover (Wiss-mann and von Rad, 1979, fig. 5; Winterer and Hinz,this volume). K-Ar dates on muscovite from one pistoncore and one dredge sample (Table 2) were discordantand considerably older than mineral dates from the Mo-roccan mainland, which recorded mainly late Paleozoicevents (Michard and Pique, 1980). We hoped that theDSDP core material would be suitable for a Rb-Sr whole-rock isochron, but core recovery was poor and the Rb-Srratio of the granodiorite was low. Therefore, only K-Armineral dating was attempted on this new material.

PETROGRAPHYWe investigated ten samples by thin sections (Vema

30-225 and 30-227; Valdivia 79-96 KD/1 to KD/4; DSDP544A-27-1, 19-28 cm and 37-42 cm, 544A-28-1, 5-13cm, and 547B-H3, CC, 18-22 cm). The sampling loca-tions are shown in Figure 1. All samples represent thesame granodioritic gneiss (or gneissic granodiorite). Theyconsist of rather coarse plagioclase, potassium feldspar,quartz, and some biotite and muscovite of minor grain-size within a very fine-grained mylonitic ground mass.The feldspar grains are bent and fractured. Plagioclaseis clouded by masses of very fine-grained epidote andcontains traces of secondary greenish biotite. The turbidpotassium feldspar is perthitic. Under crossed nicols,the coarse patches of quartz reveal severe fracturing andrecrystallization. Medium-grained biotite of dark red-dish-brown color and medium-grained muscovite aggre-

1 Hinz, K., Winterer, E. L., et al., Init. Repts. DSDP, 79:.Washington (U.S. Govt. Print-ing Office).

2 Addresses: (Kreuzer, Müller, Wissmann) Bundesanstalt für Geowissenschaften undRohstoffe, Postfach 510153, D-3000, Hannover 51, Federal Republic of Germany; (Reinecke)Mineralogisch-Petrographisches Institut der Technischen Universitàt Braunschweig, Postfach3329, D-3300, Braunschweig, Federal Republic of Germany.

gates are bent and sheared. Biotite is surrounded bysome fine-grained intergrowths of muscovite and hema-tite and by an intergrowth of greenish secondary biotitewith quartz. Accessories (apatite, zircon, and brownishepidote with allanite cores) are concentrated in the pri-mary mica aggregates (see Plate 1).

The mylonitic groundmass consists of quartz, somefeldspar, sericitic muscovite, greenish secondary biotite,and considerable amounts of an epidote mineral. Theepidote mineral shows a faint brown color and has ahigh birefringence.

Except for the fine-grained secondary muscovite, bi-otite, and epidote, the only mineral that was not affect-ed by deformation is a clear microcline accompanyingor replacing turbid potassium feldspar to varying degreesin the ten thin sections. The mineral assemblage of latesyntectonic to posttectonic origin includes microcline,albite, plagioclase, muscovite, secondary biotite, and epi-dote. This is an indication of a prograde metamorphism ofupper greenschist facies conditions. Temperature wascertainly high enough to reset the K-Ar dates of primarybiotite but probably not high enough to affect the K-Ardate of the primary (coarse) muscovite.

The chemical analysis of the composite Sample 544A-27-1, 37-42 cm + 544A-28-1, 5-13 cm (Table 1A) issimilar to that of Sample 544A-26-1, 32-36 cm given bySachtleben and Schmincke (this volume). The calculatedplutonic modes (Müller, 1982) of our sample (Table 1C)and of the five samples analyzed by Sachtleben and Sch-mincke all plot into the granodioritic field of the Q-A-Pdiagram of Streckeisen (1973) (Fig. 2).

The second analysis in Table 1 is of mica-depletedmaterial from Sample 544A-27-1, 19-28 cm. The majordeviation from the whole-rock analysis is a higher con-tent of CaO and Na2O and a lower content of Fe2O3,MgO and K2O. The CIPW norm of this feldspar-quartzconcentrate gives normative contents of wollastoniteand diopside instead of corundum {Table IB). In orderto verify the nature of the cloudy material in plagio-clase, an epinorm was computed (Table 1C). Cloudingby sericitic mica can be excluded due to the absence of

543

H. KREUZER, P. MULLER, G. WISSMANN, T. REINECKE

33° 50' -

33° 40' -

33°30'9° 30' 9° 20' 9° 10'

Figure 1. Bathymetric map of the Mazagan escarpment. Granodiorite mylonite was recovered by Valdivia 79-96 dredge course (arrow), Vema 30 pis-ton cores (crosses), and by DSDP Holes 544A and 547B (circles). Source of seabeam bathymetry: Auzende et al., this volume.

appropriate normative corundum. The epinorm of thefeldspar-quartz concentrate yields 10% of epidote andsome biotite (0.3%) (expressed in Niggli-Barth's 1-cat-ion norm) (Shaw, 1969). The anorthite content of the re-maining plagioclase is very low (about 2%). Cloudingof plagioclase by epidote and accompanying traces of sec-ondary biotite could be ascertained microscopically. Thelow anorthite content of the plagioclases could not beverified by microscopic methods due to severe clouding.

K-Ar DATING

Sample Preparation and Analytical Procedures

A piston core sample of 24 g and a dredge sample of8 g were carefully disaggregated step by step in a presserand mica flakes were separated with needle and tweez-ers. The DSDP sample of 130 g was crushed, ground ina cylinder mill, and micas and feldspar were separatedby a Frantz isodynamic magnetic separator. Potassiumwas determined by flame photometry and argon by con-ventional total fusion mass spectrometric isotope-dilu-tion analysis (Seidel et al., 1982). The IUGS recommendedconstants were used (Steiger and Jàger, 1977). The resultsare given in Table 2.

Discussion

The mineral dates of the three samples of Mazagan gra-nodiorite are highly discordant. They range from about500 Ma to 300 Ma (Fig. 3). This range is far more thancan be explained by analytical and splitting uncertain-ties and is clearly of geological significance. However, adetailed interpretation of discordant results on only threesamples, which moreover were unusually small, must beregarded with caution. We tentatively correlate the min-eral dates with the sequence of crystallizations and de-formation deduced from the microscopic investigation ofthe thin sections. The petrographic evidence leads us toassume three events (1) an intrusion, relics of which arethe coarse-grained, deformed muscovites; (2) a defor-mation, which caused the mylonitization and partial re-crystallization, especially of fine-grained muscovite; and(3) a post-deformative partial recrystallization, which isindicated by the epidote minerals and some clear K-feld-spars, but also could be related to the event that causedthe deformation.

The two coarse fractions of muscovite yielded similardates of 519 ± 21 and 515 ± 8 Ma, respectively. Weknow that muscovites seldom have high concentrations

544

Table 1. Composition of the Maza-gan granodiorite.

PETROGRAPHY AND K-Ar DATING OF THE MAZAGAN GRANODIORITE

Q

A. X-ray fluorescence analysis

Rock

Major elements (wt.%)

SiO2

TiO2

A12O3

Fe 2 O 3

MnOMgOCaONa 2OK 2 O

P2O5LOI

Sum

73.180.10

14.821.100.030.261.633.993.400.030.63

99.17

Trace elements (ppm)

BaCeCoCrCuMoNbNiRbSrThVWYZπZr

116473138

135

105

93547

725

133788

(0.21%)

F-qtz

73.200.02

14.020.270.010.092.694.522.930.021.50

99.27

854

965

59011

76

1848

(0.08%)

B. C L Λ W norm (wt.%)

QCOrAbAnWoDiHyAp11Zr

32.241.70

20.2033.84

8.34

2.360.070.190.01

C. Calculated mode(equivalence % a )

QuartzOrthoclaseAlbiteAnorthiteMuscoviteBiotiteEpidoteApatiteIlmeniteMagnetiteZircon

31.4714.9236.52

8.465.932.20

0.060.140.280.01

30.46

17.3738.24

9.301.081.30

0.050.040.01

29.4517.5141.52

0.71

0.3310.400.040.03

0.01

Note: Li-tetraborate glass disks, PhilipsPW 1450-spectrometer; analysis by J.Lodziak and D. Requard. Rock =composite Sample DSDP 544A-27-1,37-42 cm + 544A-28-1, 5-13 cm. F-qtz= feldspar-quartz concentrate, mica-depleted DSDP 544A-27-1, 19-28 cm.

a Niggli-BaruVs 1-cation norm. For rock,the plutonic norm is given, for f-qtz, theepinorm.

Figure 2. Q-A-P diagram of the Mazagan granodiorite of DSDP Hole544A. Dots, labeled with core numbers, are from analyses by Sacht-leben and Schmincke (this volume). Circle = composite sampleCores 27 + 28; triangle = feldspar-quartz concentrate from Core27. Calculation of the plutonic norm according to Müller, 1982.

of excess argon. We can therefore assume that the dateof about 515 Ma is geologically meaningful and that theintrusion of the Mazagan granodiorite is, at least, ofmiddle Cambrian age according to "the Phanerozoictime scale in 1981" (Odin et al., 1982).

The coarse grain-size fraction of muscovites from theDSDP sample has an unusually high concentration ofatmospheric argon. It is more than twice the amountobserved in the fine fraction of muscovite, in biotite, orin feldspar, all of which are within the usually observedranges. Therefore, the high concentration in the coarsefraction of muscovite cannot be ascribed to a contamina-tion by these minerals but is characteristic for the coarsemuscovite of DSDP core Sample 544A-27-1, 19-28 cm.The finer fractions of muscovite yielded dates around455 Ma, a value significantly less than that for thecoarser fractions. Contamination by abraded parts ofthe coarser muscovite is judged to be insignificant be-cause the fine fraction of muscovite from the DSDPsample shows no unusually elevated atmospheric argoncontent.

Two simple hypotheses explain the different dates forcoarse and fine muscovite:

1. The middle Ordovician date (time scale from Odinet al., 1982) is real and probably is the time of deforma-tion and mylonitization.

2. The muscovite dates are mixed ages resulting frompartial losses of radiogenic argon, higher in the finerthan in the coarser grain-size fractions. The initial ageof the Mazagan granodiorite would be older than 515Ma, and the rejuvenation would be either Late Devoni-an/Early Carboniferous (i.e., Acadian) as tentatively sug-gested from the biotite dates or Late Carboniferous asindicated by the feldspar dates.We prefer the first explanation because of the internalconsistency of the dates on the coarse, premylonitic mus-

545

H. KREUZER, P. MULLER, G. WISSMANN, T. REINECKE

Table 2. K and Ar determinations, mean values, and K-Ar dates of the Mazagangranodiorite.

Material(size-fraction[µm] in

parentheses)

K-Ardate(Ma)

Charge(mg)

Piston Core Sample Vema 30-225

Rock,depletedin mica(400-200)

Coarsemuscovite(hand-picked,not pure)

3198

51921

1350

28.6

23.3

Dredge Sample Valdivia 79-96 KD/4

Rock,depletedin mica(400-200)

Finemuscovite(hand-pickednot pure)

31510

44427

425

5.1

5.0

Argona

Radiogenic(nl/g)STP

36.17(20)

36.1745

201.0(1.2)

205.5(l•l)

203.24.5

33.54(18)

33.5480

150.4(1.5)

143.3(1.4)

1477

DSDP Leg 79, Hole 544A, Core 27, Section 1, 19-28 cm

Feldspar 75%+ quartz,(200-100)

Biotite 80%;muscovite 10%,(200)

Biotite 80-90%+ quartz+ feldspar,(200-100)

Muscovite 80%+ feldspar+ quartz(200)

Muscovite 80-90%+ feldspar+ quartz,(200-100)

3143

369

3353

5168

45913

104

104

9.6

71

68

17

16

35

32

34.02(24)

33.83(20)

33.9219

109.2(7)

99.43(55)

99.40(54)

99.4238

181.0(1.2)

184.2(l•l)

182.63.2

160.6(9)

161.0(9)

160.87

Atmospheric(nl/g)STP

2.24(5)

1.46(47)

1.453(49)

3.5(1.8)

0.4(1.4)

23

ú

1.82(19)

1.94(12)

1.8811

1.5(l•l)

1.88(21)

1.90(19)

1.8914

5.7(9)

4.0(8)

4.91.7

2.13(38)

2.25(40)

2.1927

Potassium

Charge

(mg)

150

150*>

11

11

150

150

10

106

104

11

34

35

16

18

89

91

K(wt .%)

2.702(16)

2.631(15)

2.66770

8.51(45)

8.87(45)

8.6936

2.538(15)

2.477(15)

2.50760

7.517(40)

7.5236

2.555(15)

2.533(15)

2.55424

6.854(36)

6.966(37)

6.915(40)

6.94053

7.858(41)

7.875(41)

7.86630

8.040(42)

7.797(41)

7.92' 25

Note: Estimate of uncertainty at the 95% level of confidence is given in italics. Uncertaintiesin parentheses are merely analytical, not including sample inhomogeneity. The latter iscovered by the difference of duplicate determinations. Above the rule are analyticalresults; below are average values—the adopted values and calculated K-Ar data, re-spectively. IUGS-recommended constants used (Steiger and Jàger, 1977):

λç + λe = 0.581 1 0 - ^ a - 1 , ^Ar/^Ar = 295.5,\ß = 4.962 l O - i O a ~ 1 , 40K/K = 0.01167 atomic %

For results on standard minerals, see Odin, 1982.* Nanoliter/g at standard temperature and pressure.° From 500 mg ground material.c From 425 mg ground material.d This amount constituted one quarter of Section 1.

546

PETROGRAPHY AND K-Ar DATING OF THE MAZAGAN GRANODIORITE

200

150

Coarse muscovite (deformed) •

K (wt.%)

Figure 3. Diagram of radiogenic argon versus potassium for mineralconcentrates from Mazagan granodiorite. There is no correlationwith a positive intercept at the argon axis, i.e., no indication forexcess radiogenic argon from this plot. Reference isochrons aregiven for the means values of coarse muscovites (minimum age ofthe intrusion), for the fine muscovites (probably time of deforma-tion), for the biotites (mixed ages), and for the feldspars (indica-tion for a Late Carboniferous weak overprint). VE = coarse mus-covite from the Vema piston core, VA = fine muscovite from theValdivia dredge sample. The other two muscovites, the biotites,and one of the three feldspar concentrates are from Hole 544A,Core 27, Sec. 1, 19-28 cm.

covites as well as that of the dates on the fine, recrystal-lized muscovites, each from two samples from differentlocations.

The dates on the two grain-size fractions of biotitefrom the DSDP sample are analytically different fromone another. By taking into account contaminations,i.e., muscovite with an older date in the coarse fraction,and feldspar with a younger date in the fine fraction,the dates of the pure biotites would be closer together.But they remain discordant with values of about 360 ±10 and 336 ± 4 Ma, respectively. The concentrate offeldspar from the DSDP sample as well as the whole-rock fractions, depleted in micas, from the two othersamples appear to be similar in the mineral phase thatdominates the K-Ar system. We can assume that thedates are essentially governed by the microcline. Thedates of 314 ± 3, 319 ± 8, and 315 ± 10 Ma are regard-ed as concordant. However, perthitic microcline is known

to be an unreliable chronometer with variable losses ofradiogenic argon. Neither biotite nor feldspar allow adefinitive dating. However, both point to a post-Devoni-an (i.e., Variscan), probably a Late Carboniferous, reju-venation which we ascribe to the late metamorphic in-fluences of greenschist facies conditions as indicated byepidote and the recrystallization of feldspar.

Comparisons with Adjacent Areas

At Cape Mazagan (El Jadida), 100 km southeast ofthe outcrops of the offshore Mazagan granodiorite, thereare outcrops of rhyolite overlying presumably early Cam-brian sedimentary rocks. This is evidence that magmaticrocks, at least as old as Cambrian, are present in the vi-cinity onshore (Gigout, 1965). To the south of Cape Maza-gan, in the Doukkala Basin, a Silurian-Devonian suc-cession overlies tectonized Cambro-Ordovician schistsdemonstrating the existence of an Ordovician deforma-tive event (Barbu, 1977). An event of roughly the sameage is also recorded to the north: In the Rabat-Tifliszone, the thick sequence of flysch-like sediments of theBou-Regreg formation of presumably Cambrian agewas folded and metamorphosed before Late Ordovician.The event is followed by the Rabat-Tiflis granite whichis 430 Ma according to a Rb-Sr isochron of one musco-vite, one biotite, and eight whole-rock samples (Chariotet al., 1973, recalculated; Pique, 1982).

Schenk (1980) pointed out that the lower part of theMeguma formation of Nova Scotia is similar in litholo-gy, style of sedimentation, and age as the MoroccanBou-Regreg formation, and Wissmann et al. (1982) spec-ulated that the Mazagan granodiorite could belong tothe source region of the Meguma formation. But thereare facts which do not fit with these hypotheses. For ex-ample, there is no trace of an Ordovician metamorphicevent in the Meguma formation. Furthermore, the De-vonian event that affected the Meguma of Nova Scotiais only faintly indicated in Morocco by redbeds.

ACKNOWLEDGMENTS

We are indebted to E. L. Winterer and C. J. D. Adams for critical-ly reading the manuscript and improving our English. W. B. F. Ryanprovided the two Vema piston cores for a previous study (Wissmannand von Rad, 1979). H. Klappert, M. Metz, and L. Thieswald per-formed the argon analyses and H. Klappert and G. Stövesand the po-tassium determinations.

REFERENCES

Barbu, A., 1977. Le concept de zone pétrolière potentielle dans l'ex-ploration du bassin des Doukkala (Maroc occidental). Min. Géol.,42:49-57.

Chariot, R., Rhalib, M. and Tisserant, D., 1973. Etude géochronolo-gique préliminaire des granites de la region de Rabat-Tiflet (Marococcidentale). Notes Service Géologique Maroc, 33(no. 249):55-57.

Gigout, M., 1965. Carte géologique de la Meseta entre Settat et Maza-gan (El Jadida) (Doukkala et Chaola occidentale)—notice explica-tive. Editions du Service Géologique du Maroc: Rabat (Royaumedu Maroc, Direction des Mines et de la Géologie, Division de laGéologie).

Michard, A., and Pique, A., 1980. The Variscan belt in Morocco:Structure and developmental model. In Wones, D. R. (Ed.), TheCaledonides in the USA, Proc. IGCP project 27, Blacksburg, Va.,Sept., 1979. Univ. Va. Geol. Mem., 2:317-322.

Müller, P., 1982. Von der CIPW-Norm ausgehende Berechnung vonMineralbestànden magmatischer Gesteine in Analogie zu der Mo-

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H. KREUZER, P. MULLER, G. WISSMANN, T. REINECKE

dalzusammensetzung plutonischer and vulkanischer Gesteine. Geol.Jahrb., D 55:3-41.

Odin, G. S., 1982. Interlaboratory standards for dating purposes. InOdin, G. S. (Ed.), Numerical Dating in Stratigraphy: Chichester(John Wiley), pp. 123-150.

Odin, G. S., Curry, D., Gale, N. H., and Kennedy, W. J., 1982. ThePhanerozoic time scale in 1981. In Odin, G. S. (Ed.), NumericalDating in Stratigraphy: Chichester (John Wiley), pp. 957-960.

Pique, A., 1982. La zone de Rabat-Tiflet (Meseta marocaine septen-trionale). Sa place dans 1'ensemble des noyaux paléozoiques de laMéditerranée occidentale. C. R. Seances Acad. Sci. Paris, 295, sér.11:263-266.

Schenk, P. E., 1980. Paleogeographic implications of the MegumaGroup, Nova Scotia,—a chip of Africa? In Wones, D. R. (Ed.),The Caledonides in the USA, Proc. IGCP project 27, Blacksburg,Va., Sept., 1979. Univ. Va. Geol. Mem., 2:27-30.

Shaw, D. M., 1969. Evaluation of data. In Wedepohl, K. H. (Ed.),Handbook of Geochemistry I: Berlin (Springer Verlag), pp.324-375.

Seidel, E., Kreuzer, H., and Harre, W, 1982. A late Oligocene/earlyMiocene high pressure belt in the external Hellenides. Geol. Jahrb.,E 23:165-206.

Steiger, R. H., and Jàger, E., 1977. Subcommission on Geology: Con-vention on the use of decay constants in geo- and cosmochronolo-gy. Earth Planet. Sci. Lett., 36:359-362.

Streckeisen, A., 1973. Classification and nomenclature of plutonicrocks, recommendations by the I.U.G.S. subcommission on thesystematics of igneous rocks. N. Jahrb. Mineral., Abh., 4:149-164.

Wissmann, G., Müller, P., Kreuzer, H., Harre, W, and Reinecke, T.,1982. Mazagan granite (off Morocco): the source of Meguma gray-wacke (N.S.)? 42. Jahrestagung Deutsche geophysikalische Gesell-schaft, Hannover, March 30-April 4, 91. (Abstract)

Wissmann, G., and von Rad, U., 1979. Seismic structure, continentalbasement, and Mesozoic sediments from the Mazagan Plateau offMorocco. Meteor Forschungs-Ergebnisse C 31:1-20.

Date of Initial Receipt: January 1, 1983Date of Acceptance: July 27, 1983

548

PETROGRAPHY AND K-Ar DATING OF THE MAZAGAN GRANODIORITE

micPlate 1. Photographs of thin sections DS 27726 of Sample 544A-27-1, 19-28 cm. Scale = 0.5 mm. Upper 4 figures, deformed premylonitic miner-

als: pi, turbid plagioclase; q, quartz with mylonitic rims; bi, biotite with crossing secondary muscovite; mu, bent muscovite. Lower figure: mic,postdeformative microcline.

549