19. PETROLOGY OF BASALT RECOVERED DURING DSDP LEG 39B

R.V. Fodor, J.W. Husler, and K. Keil, Department of Geology and Institute of Meteoritics,University of New Mexico, Albuquerque, New Mexico

ABSTRACTBasaltic basement rock (7.5 m core; 78.1 ±9m.y. old) from the

Brazil Basin, and a basaltic pebble in a Cretaceous chalk from theSào Paulo Plateau were recovered during Leg 39B. Brazil Basinbasalt is typical ocean-ridge tholeiite and is characterized by inter-granular Plagioclase (An««-5i), clinopyroxene (mainly augite), andtitaniferous magnetite, and low bulk K2O content (0.07 wt % in thefreshest sample); relict olivine is also present in the upper, morealtered core samples. Compositional changes attributable toseawater alteration are higher K2O (and possibly Cr) and lower Nicontents when comparing the lowermost (freshest) to uppermost(most altered) core samples.

The basaltic pebble from the Sào Paulo Plateau is extremelyaltered, but high K2O in feldspar, and interstitial alkali feldsparindicate that it may represent oceanic alkalic basalt or alkalic basalttransitional to tholeiite. It was probably derived by slump from theSào Paulo Ridge, which, based on the pebble's composition, may becomposed of former oceanic islands bordering the Sào PauloPlateau.

INTRODUCTION

Basaltic basement rock was recovered from theBrazil Basin, Site 355, during Leg 39B (Figure 1). Thismaterial was located between magnetic anomalies 33and 34 (Pitman et al., 1975) and is the oldest basalticunit cored from the South Atlantic Ocean (78.1 ±9m.y.; McKee and Fodor, this volume). The 7.5 metercore length provides an opportunity to describe petro-logic characteristics with reference to both verticalvariation within the core and to younger oceanic basaltlocated closer to the Mid-Atlantic Ridge. This reportpresents the detailed chemistry and mineralogy ofbasalt samples at four vertical intervals throughout thecore.

Drilling on the Sào Paulo Plateau, Site 356 (Figure 1)(near the Sào Paulo Ridge), yielded a basaltic pebble(~3 cm) from a Cretaceous marly chalk. A petrologicdescription of this material is also presented to helpcharacterize the type of basaltic material present in theSào Paulo plateau region.

ANALYTICAL PROCEDURESWhole-rock analyses of basalt were obtained by the

following techniques: Silica was determinedgravimetrically. Total Fe, AI2O3, MgO, CaO, Na2θ,K2O, Tiθ2, and MnO were determined by atomicabsorption using calibration curves prepared from U.S.Geological Survey standard basalt, BCR-1. Fordetermination of P2O5, an adaptation of the Molyb-denum Blue spectrophotometric method wasemployed. Water was determined by ignition loss, but

the amount of Fe oxidation was taken into account byK2Cr2U7 titration of ferrous iron in both the ignitedand unignited sample. The trace elements Rb, Ba, Sr,Co, Cr, Ni, and V were also determined by atomicabsorption, using larger sample weights than for themajor elements. Mineral compositions were determinedwith an ARL-EMX-SM electron microprobe operatedat 15 keV accelerating voltage and 0.015 to 0.020 µAsample current. Corrections for differential matrixeffects were made according to Bence and Albee (1968).

GENERAL DESCRIPTIONS

Basalt in the Brazil Basin was reached about 450meters below the sea floor in 4900 meters water depth.The contact between basalt and overlying sediment wasnot recovered nor was the basalt drilled extensivelyenough to indicate whether it is a sill or true basement.

The upper 10 cm of the core ranges in color fromdark gray to yellowish-gray and consists almost entirelyof clay minerals produced by seawater alteration. Theremaining portion of the core is medium to dark gray incolor and highly fractured and veined with calcite. Atleast four altered, glassy zones were observed and thesemay represent the tops of separate flows. They aregenerally thin (<l cm) and often mixed with calcite.Local brecciated zones are sometimes associated withthe altered glass. The basalt is essentially aphyric,although Plagioclase laths are visible in certain areas.

At the Sào Paulo Plateau, a sedimentary marly chalkcontaining a basaltic pebble was cored at about 660meters below the sea floor in about 3175 meters waterdepth. The pebble is about 3.5 cm long and 2 cm thick,

513

R.V. FODOR, J.W. HUSLER, K. KEIL

30°N

45°W

Figure 1. Map of the South Atlantic Ocean showing thelocations of Sites 355 and 356, and other sites drilledduring Leg 39.

dark gray in color, and contains phenocrysts of ox-idized olivine (reddish in color), clinopyroxene, andPlagioclase. A distinct weathering rim about 5 mmthick is visible in a cut cross-section. The pebble wasrecovered from a core of Santonian (Cretaceous) marlycalcareous chalk containing black shale and Ino-ceramus fragments. Altered basaltic pebbles (0.2mm to1 cm) were also observed in conglomeratic zones, mud-stones, and graded zones lying 20 to 50 meters belowthe pebble, indicating extensive deposition of coarsematerial during Santonian time, presumably by slumpfrom a nearby ridge (Figure 2).

PETROGRAPHY

Thin-section examination of the Brazil Basin basaltreveals a large range in the degree of alteration; thefreshest samples with closest to original textures arefrom near the base of the core. Texture is intergranularand consists of Plagioclase phenocrysts, micropheno-crysts, and groundmass laths (1 mm to 0.1 mm)intimately associated with microphenocrysts andgroundmass clinopyroxene (0.4 to 0.1 mm) (Figure 3);they sometimes occur in glomerocrysts, or clots (Figure4). In the upper half of the core, most of thegroundmass pyroxene has been altered to brownishclay minerals and highly oxidized remnants of olivineare present as rare phenocrysts. No olivine wasobserved in the lower parts of the core. Present in the

groundmass of the lowermost and freshest core samplesare angular, sometimes skeletal, oxides (~0.05 mm),but they are absent from the more altered upper core.Crystallization of clinopyroxene in part precededPlagioclase as indicated by some microphenocrysts ofpyroxene enclosed by feldspar (Figure 5). Where olivineremnants are present, it may have been the liquidusphase. Plagioclase appears to have been the phase mostresistant to alteration as indicated by the presence of atleast a few remaining laths even in the most intensely al-tered samples.

A thin section of the Sao Paulo Plateau basalticpebble shows an extreme alteration of the originaltexture that probably consisted of intergranularfeldspar and clinopyroxene. Present now arePlagioclase laths, oxides, and clay minerals forming thegroundmass and phenocrysts of Plagioclase,clinopyroxene, and highly oxidized relict olivine(Figure 6). Of the Plagioclase and clinopyroxenephenocrysts, grain margins are altered, but fresh coresremain.

BULK COMPOSITION

Whole-rock analyses and normative mineralogy offour samples from the Brazil Basin basalt core arepresented in Table 1. Areas of the core sampled foranalyses are illustrated in Figure 7; systematic sam-pling, however, was hampered somewhat by fractures,veins, and secondary mineralization present in the core.

Samples throughout the core are remarkably similarin major element concentrations, indicating that theentire core either represents one flow or, more likely,several flows from the same source. There is variation,however, in K2O content, where a very low (0.07 wt %)concentration is present in the lowermost sample (22-5)and values closer to 0.5 wt % are measured higher in thecore. Because samples farthest from the basalt-sediment interface are expected to be least affected byseawater alteration, the low K2O of the bottom sampleis probably closest to the original concentration,whereas higher K2O contents in the upper samplesprobably resulted from seawater alteration (Hart, 1970;Hart et al., 1974). This is in agreement with texturalobservations, but curiously, is not apparent from H2Ocontents (Table 1); the bottom sample has more H2Othan the highly altered upper core samples.

Trace element contents (Table 1) show a generaltrend of increasing Cr and decreasing Ni and Vcontents from the bottom to the top of the core(freshest to most altered); V, however, greatly increasesagain near the top. The change in Ni agrees with datareported by others for the alteration effects in basalts(e.g., Hart et al., 1974), but nothing conclusive isknown about the changes in Cr and V contents. Here,changes in V may be related to weathering oftitaniferous magnetite in the upper samples, as opposedto primary fractionation, whereas there is no apparentreason attributable to weathering for the ~20%increase in Cr.

Comparison of bulk compositions to those ofdredged Mid-Atl antic Ridge basalts shows that theBrazil Basin core is typical mid-ocean ridge basalt

514

PETROLOGY OF BASALT

356 40-3Figure 2. A Cretaceous conglomerite composed partly of basaltic fragments from

the Sao Paulo plateau. This sample is from one of many conglomeratic zonesthat the basaltic pebble (356-38-378) is associated with. Scale is provided bythe typewritten numbers. Photo provided by J. Thiede.

Figure 3. Intergranular texture of Plagioclase and clinopyroxene in the BrazilBasin basalt (22-5). Crossed nicols; scale bar equals 0.5 mm.

(Table 1). Unlike many oceanic tholeiites, however, theBrazil Basin basalts are highly quartz normative (Table1; Figure 8), but this characteristic is largely the resultof the degree of oxidation (i.e., Fe2θ3/FeO). Whennormative mineralogy is calculated after normalizingFe2U3 to 1.5 wt % and omitting CO2, there is acompositional shift into or closer to the field of olivinetholeiites (Figure 8). This is consistent with the relictolivine observed in the uppermost samples. Relativelylow P2O5, Tiθ2, Rb, Ba, and Sr contents alsocharacterize the Brazil basalt as ocean-ridge basalt(Table 1).

The Sao Paulo basalt pebble is highly altered asindicated by exceedingly high H2O (6.5 wt %) and CO2contents (Table 1). This presents difficulty inestablishing the true character of the basalt, whethertholeiitic or alkalic. Some oxide contents, such as lowSiθ2 and high K2O, suggest that the pebble has alkaliccharacteristics, although they are not entirelyconclusive because alteration of ocean-ridge tholeiitecan result in material that closely resembles trueoceanic alkalic-basalt in composition (Hart, 1970). Thehigh K2O content, however, appears too high to haveresulted from an alteration of low-K (tholeiitic)

515

R.V. FODOR, J.W. HUSLER, K. KEIL

Figure 4. A cluster, or glomerocryst, of Plagioclase and clinopyroxene in the BrazilBasin basalt (22-5). Crossed nicols; scale bar equals 0.5 mm.

Figure 5. Clinopyroxene enclosed by Plagioclase in the Brazil Basin basalt (22-2).Crossed nicols; scale bar equals 0.2 mm.

material. For example, Bass et al. (1973) showedanalyses of highly altered basalts with 4.7 and 6.5 wt %H2O that contain only about 0.5 wt % K2O. Also,Matthews (1971) showed that alteration of an oceanicbasalt (7.2 wt % H2O), which is comparable in

alteration to that of the Sào Paulo pebble, increasedK2O by about three times initial K2O content. Incomparison, to account for K2O of 2.5 wt % in thepebble from an original K2O of <0.5 wt % (tholeiitic)would require an increase by about five times.

516

PETROLOGY OF BASALT

al f^'tf ga 5

;

* * s ^ SP

• * V • *ir i ff * %2

JS?) \• • • * * - *

;••*>• ,,.<•v v .

Figure 6. Plagioclase phenocrysts with corroded margins in the highly altered basal-tic pebble from the Sao Paulo plateau. Groundmass consists of clay minerals andPlagioclase laths. Crossed nicols; scale bar equals 1 mm.

Although alteration effects cannot be expected toproduce the same results everywhere, the changes inK2O contents due to alteration observed in other rocksmake it seem unlikely that the basaltic pebble wasoriginally tholeiitic. High Rb, Sr, and Ba in the pebblepossibly also represent true alkalic-basaltconcentrations (Table 1). Although Rb and Baconcentrations increase with seawater alteration, theyare not known to increase more than a few parts permillion under moderate alteration (Hart et al., 1974).

Normative mineralogy has substantial quartz (Table1; Figure 8), but that is largely a function of highFe2θ3/FeO. When FeaOs is normalized to 1.5 wt % andCO2 omitted, the composition is much closer to theplane of undersaturation (Ol-Di of Figure 8).

MINERALOGY

Mineral compositions were determined in three ofthe four bulk compositionally analyzed Brazil Basinbasalt samples, representing top, middle, and bottomportions of the core, and for the Sào Paulo basalticpebble.

ClinopyroxeneMicrophenocrysts and groundmass clinopyroxene

are present in each of the sections analyzed of the BrazilBasin basalts, although intense alteration of the topsample, 21-1, left only rare and exceedingly small (~lto 2µm) grains. One phenocryst was also observed in21-1. Average compositions (Table 2) and thecompositional trend (Figure 9) of increasing FeO andMnO attended by a decrease in CaO and Siθ2 fromphenocryst, to microphenocryst, to groundmass are

similar to those of clinopyroxene typically found intholeiitic basalts (Fodor et al., 1975). Throughout thecore, there are no significant compositional differencesin clinopyroxene. Compositional zoning occurs in themicrophenocrysts, and groundmass pyroxene is highlyvariable with respect to Fe and Ca (Figure 9).Groundmass compositions presented for 21-1,however, are subject to some question because ofintense alteration in this sample and difficulty inavoiding overlap onto clay minerals with the electronbeam when analyzing the small grains of remainingpyroxene.

Attending the changes in grain size from phenocrystto groundmass clinopyroxene, there is a substantialincrease in TiOa (Figure 10). Between microphenocrystsand groundmass, AI2O3 content is nearly constant orappears to decrease slightly with increasing Tiθ2, asillustrated by the freshest sample, 22-5 (Figure 10,Table 2) (the high AI2O3 values shown for 21-1 may bedue to particularly high alteration in that sample). Aslight decrease in AI2O3 probably reflects simultaneouscrystallization of groundmass pyroxene andPlagioclase. The increase in Tiθ2 from microphenocrystto groundmass indicates that titaniferous magnetiteprobably crystallized after groundmass pyroxene; thisis also suggested by texture.

Fresh cores of otherwise altered clinopyroxenephenocrysts in the Sào Paulo basalt pebble (Table 2;Figure 9) have compositions (FsnEn49Wθ4θ) commonto pyroxene phenocrysts in tholeiitic basalt. Thiscomposition, however, is also observed for some alkalicbasalts and alkalic basalts transitional to tholeiites(Fodor et al., 1975).

517

R.V. FODOR, J.W. HUSLER, K. KEIL

TABLE 1Bulk Compositions and Molecular Norms of Four Brazil Basin Basalt Samples

(Columns 14; Top to Bottom of Core; Figure 7) and a Basaltic Pebblefrom the S§o Paulo Plateau (Column 5), Western South Atlantic Ocean,Compared to Average Compositions of 33 and 6 Dredged MAR Basalts

(Columns 6, 7; after Melson and Thompson, 1971,and Kay et al., 1970, Respectively)

SiO2

TiO2

A12O3

Fe2O3

FeO

MnOMgOCaONa2OK2OH2O"H2O+

P2O5CO2

Total

qoraban

Di -

Hy

wo

enfsenfsümtapccc

Rb (ppm)BaSr

CoCrNiV

1

48.881.78

15.425.425.650.176.35

10.742.510.570.820.850.170.66

99.99

4.273.46

23.1829.98

7.756.711.06

11.331.782.555.830.37—

1.72

<IO<40100

50231

50390

2

49.261.70

14.755.865.690.166.50

10.752.380.490.961.040.140.38

100.06

5.343.00

22.1229.129.067.911.15

10.661.552.456.340.30-

0.99

<IO<40

100

57238

50240

3

49.281.87

15.154.495.320.196.97

10.602.690.461.501.040.160.57

100.29

3.422.79

24.8428.71

8.247.191.05

12.601.842.684.830.34-

1.48

<IO<40120

72226

80390

4

50.141.88

14.963.786.450.167.00

10.402.700.071.160.810.150.65

100.31

4.590.42

24.8229.187.375.861.51

13.923.592.684.050.32-

1.68

<IO<40112

5718885

440

5

46.062.24

14.106.925.200.056.225.082.812.613.752.660.281.92

99.90

5.4816.6027.1612.09

--

—

18.480.213.367.790.632.985.23

40250320

2913960

260

6

49.211.39

15.812.217.190.168.53

11.142.710.26--

0.15-

98.76

10a

14

130

32297

97292

7

48.551.57

15.20-

10.720.187.55

10.932.610.21-

1.010.15-

98.68

Note: DSDP sample numbers; Column 1, 355-21-1, 102 cm; Column 2, 355-22-2, 44 cm; Column 3, 355-22-3, 12 cm; Column 4, 355-22-5, 120 cm;Column 5, 356-38-8, 78 cm.

aAverage for tholeiitic basalts; after Engel et al. (1965).

Feldspar

Phenocrysts, microphenocrysts, and groundmassfeldspar in the Brazil Basin basalt (Table 3; Figure 11)range in composition from calcic bytownite to sodiclabradorite and are exceedingly low in K2O contents(<O.ll wt %). Compositional changes from phenocrystto groundmass include an increase in Na2θ, K2O, FeO,and SiCh and a decrease in AI2O3, whereas MgOremains nearly constant. No high-K rims on large

feldspar grains nor interstitial material were observed.All of these features are characteristic of feldspar intholeiitic basalts (Keil et al., 1972).

Fresh cores in phenocrysts of the Sào Paulo basaltpebble are also bytownite in composition, but higher inK2O relative to feldspar in the Brazil Basin basalt(Table 3). The groundmass feldspar is labradorite incomposition and higher in K2O relative to labradoritein the Brazil Basin basalt. Most notably, there isinterstitial material in the groundmass very high in K2O

518

PETROLOGY OF BASALT

SITE 355 Ne*

core 21

sect. 1

core 22sect. 1

core 22sect. 2

core 22sect. 3

core 22sect. 4

core 22sect. 5

SED.

A3 -

V ^* UV A

r

A -,

' y t

VOID

< >l

> 7

* >7 r

Λ y

VOID

1 *

» V '

> v>T Λ

i -?

< -»

V A

r j

V

VOID

7 A

1 m

Figure 7. Schematic diagram showing the sampling areas(arrows) in the Brazil Basin basalt core.

content, some reaching AmAbioOrβs. The presence of2.1 wt % Or end member in the groundmass laths andthe high-K interstitial material are characteristic foralkalic basalt (Keil et al., 1972). Whether or not theenrichment in K in the feldspar is innate or a result ofthe intense alteration is essential to interpreting the truecharacter of the rock. Seawater alteration increases theK content of basaltic material by forming clay minerals(Melson and Thompson, 1973) and K-feldspar cores inPlagioclase, but to have K enter Plagioclase withoutobvious alteration (optically, the Plagioclase coresappear fresh) and to form interstitial material that isstoichiometrically high-K feldspar seems highlyunlikely. Although replacement of Plagioclase bystoichiometric orthoclase has occurred in deep-seabasalts (Matthews, 1971; Bass et al., 1973; Steward etal., 1973), no incipient or partial alteration, such as

01 Hy

Figure 8. Normative mineralogy plotted on a diopside (Di)-hypersthene (Hy)-olivine (Ol)-quartz (QJ-nepheline (Ne)diagram. Closed circles represent norms listed in Table 1and open circles represent norms recalculated after CO2

is omitted and Fe2O3 normalized to 1.5 wt%.

only a small (<l.O wt %) increase in K2O throughoutPlagioclase, has been reported. Therefore, the alkaliccharacteristics of the feldspar and interstitial materialare thought to be real.

The coexistence of pyroxene having tholeiiticaffinities and feldspar that is typically alkalic haspreviously been reported for a Hawaiian tholeiite thatwas classified by Keil et al. (1972) and Fodor et al.(1975) as transitional between tholeiitic and alkalicbasalt. On the basis of pyroxene and feldsparcompositions, the Sào Paulo pebble may be a basalttransitional between tholeiitic and alkalic basalt, buthighly altered by seawater.

OxidesThe only oxide present in the Brazil Basin basalt is

titaniferous magnetite in the lowermost portion of thecore (22-5). Its composition (Table 4) is similar to thatof titaniferous magnetite in other tholeiitic basalts, suchas of Hawaii (Fodor et al., 1972).

In the Sào Paulo basalt pebble, titaniferousmagnetite and ilmenite coexist, both as individualgrains and in the same grain (Table 4). The ulvöspinelcontent of the magnetite is lower than that of magnetitein the Brazil Basin basalt. Equilibration temperatureand oxygen fugacities estimated from the coexisting Fe-Ti oxides (Buddington and Lindsley, 1964) are 925°Cand I0"12 atm, respectively.

PERTINENT OBSERVATIONS AND SUMMARYAside from small compositional differences in major

elements that can be attributed to seawater alteration(increasing K, Fe+3), bulk and mineral compositionsare consistent throughout the 7.5-meter core of BrazilBasin basalt. Absence of significant chemicalfractionation suggests that the core represents onebasaltic source. The basalt is characterized byintergranular Plagioclase and clinopyroxene, and in the

519

R.V. FODOR, J.W. HUSLER, K. KEIL

TABLE 2Average Compositions of Clinopyroxene Phenocrysts (ph), Microphenocrysts (m),

and Groundmass (g) in Basalt from the Brazil Basin (21-1; 22-2; 22-5) and in aBasaltic Pebble from the Sao Paulo Plateau (28-3), Western South Atlantic Ocean.

Compositions are Averages of 20 to 50 Spot Analyses (in wt %)

ph

21-1

m g m

22-2

g m

22-5

g

38-3

ph

SiO2

TiO2

52.30.39

51.60.82

48.81.8

52.20.80

49.51.6

51.40.97

49.71.3

52.00.58

A12O3

Cr203

FeOMnO

MgOCaONa2O

Total

3.90.904.60.17

17.919.40.30

99.86

4.30.476.6

0.2017.418.00.32

99.71

6.10.07

10.90.31

14.116.60.40

99.08

Number of Ions on the Basis of 6 (0)

SiAlivAlyiTiCrFe

MnMg

CaNa

XSum

1.9030.0970.0700.0110.0260.1400.0050.9710.7560.021

2.0004.000

1.8900.1100.0760.0230.0140.2020.0060.9500.7060.023

2.0004.000

Molecular End Members

FsEnWo

7.5

52.040.5

10.951.138.0

1.8330.1670.1030.0510.0020.3420.0100.7890.6680.029

1.9943.994

19.043.937.1

4.10.376.90.21

16.818.10.26

99.74

1.9110.0890.0880.0220.0110.2110.0070.9170.7100.018

1.9843.984

11.549.938.6

4.4<.Ol

14.70.37

12 A15.70.35

99.02

1.8860.1140.0840.046

-

0.4680.0120.7040.6410.026

1.9813.981

25.838.835.4

4.20.347.7

0.2216.817.80.31

99.74

1.8910.1090.0730.0270.0100.2370.0070.9210.7010.022

1.9983.998

12.749.637.7

3.60.03

14.30.37

13.916.1

0.30

98.60

1.8840.1160.0450.0370.0010.4530.0120.7850.6540.022

2.0094.009

24.041.534.5

2.40.297.00.22

16.819.40.19

98.88

1.9320.0680.0370.0160.0090.2170.0070.9300.7720.014

2.0024.002

11.348.540.2

Di Wo

[phenocrysts •

S microphenocrysts o

{groundmass

356-38-3 (pebble)

En FsFigure 9. Pyroxene electron-microprobe spot analyses plotted in terms of mole percent enstatite (En; Mg2Si206), ferrosilite

(Fs; Fe2Si206), and wollastonite (Wo; Ca2Si206). Di=diopside (CaMgSi2O6) and Hd=hedenbergite (CaFeSi2O6). Brazil Basinbasalt is indicated by Site 335 (top to bottom of core; 21-1; 22-2; 22-5) and the Sao Paulo pebble by Site 356.

uppermost portions, by minor olivine (relict).Restriction of olivine to the top may indicate aninverted magma chamber where an olivine-enrichedbottom portion was the last material to be extruded on

the sea floor; magma-chamber zoning, however, is notapparent from the bulk compositions. Whether or notfu~ kn.nit « o c\\\ o r true basement cannot be

of the relatively shallowthe basalt is a silldetermined because

520

PETROLOGY OF BASALT

eviO

i - 2

α>oα>α.

I

phenocrysts •

microphenocrysts o

groundmαss

355-21-1o o u

o o

o o c

pOO ^

••rt

355-22-2 355-22-5

• • β

Weight percent AI2O3

Figure 10. Plot of TiO2 versus A12O3 for clinopyroxene electron-microprobe spot analyses of the Brazil Basin basalt (top tobottom of core, 21-1; 22-2; 22-5). High A12O3 in pyroxene of Sample 21-1 may partly be due to alteration of most pyroxeneto clay minerals in that sample.

penetration depth and no recovery of upper contact;the age of 78.1 ±9 m.y. (McKee and Fodor, thisvolume), however, indicates that it is probablybasement. Mineral compositions and textural

observations show that Mg-rich clinopyroxenecrystallization was attended by, and partly succeeded,calcic Plagioclase. G r o u n d m a s s - p y r o x e n ecrystallization was attended by intermediate

TABLE 3Average Compositions of Feldspar Phenocrysts (ph), Microphenocrysts (m), and Groundmass Laths (g)

in Basalt from the Brazil Basin (21-l;22-2;22-5) and in a Basaltic Pebble from the Sao Paulo Plateau (38-3),Western South Atlantic Ocean. Compositions are Averages of 20 to 50 Spot Analyses (in wt%)

SiO2

AI2O3FeO

MgO

CaO

Na2OK 2 O

Total

ph

49.432.10.730.43

16.22.40.04

101.30

21-1

m

50.430.7

0.850.47

14.83.2

0.06

100.48

g

53.029.4

1.0

0.51

13.03.2

0.11

100.22

Number of Ions on the Basis of 32 (0)

Si

Al

Fe

Mg

Ca

Na

K

Z

X

Sum

8.9566.8590.1110.1163.1470.8440.009

15.8154.227

20.042

9.1916.5980.1300.1282.8911.1310.014

15.7894.294

20.083

Weight Percent End Members

An

Ab

Or

79.720.1

0.2

72.826.8

0.4

9.6006.2760.1510.1382.5231.1240.025

15.8763.961

19.837

69.929.4

0.7

22-2

m

50.030.70.830.44

15.13.0

0.07

100.14

9.1566.6250.1270.1202.9621.0500.016

15.7814.290

20.056

74.425.2

0.4

g

53.228.5

1.3

0.41

11.84.8

0.09

100.10

9.6826.1130.1980.1112.3011.6940.021

15.7954.325

20.120

58.740.8

0.5

ph

49.131.6

0.770.38

16.32.5

0.04

100.69

8.9696.8030.1180.1033.1900.8850.009

15.7724.305

20.077

79.120.7

0.2

22-5

m

51.530.0

0.940.45

13.33.9

0.06

100.15

9.3876.4440.1430.1222.5971.3780.014

15.8314.254

20.085

66.433.2

0.4

g

53.828.7

1.3

0.43

11.15.3

0.09

100.72

9.7186.1100.1960.1162.1481.8560.021

16.5424.337

20.165

54.844.7

0.5

ph

50.131.90.590.23

15.02.8

0.14

100.76

9.0946.8240.0900.0622.9170.9850.032

15.9184.086

20.004

75.224.00.8

38-3

g

53.229.7

0.950.15

12.14.2

0.34

100.64

9.6096.3220.1430.0402.3411.4710.078

15.9314.073

20.004

61.536.4

2.1

Hi-Ka

65.518.3

0.77

0.090.921.9

12.8

100.28

11.9693.9410.1180.0250.1800.6732.984

15.9103.980

19.890

4.7

16.778.6

Groundmass interstitial feldspar.

521

R.V. FODOR, J.W. HUSLER, K. KEIL

An

phenocrysts •

microphenocrysts o

groundmαss

interstitial material

90 or

Figure 11. Ternary feldspar diagrams where individual feldspar electron-micro probe spotanalyses are plotted in terms of weight percent anothite (An; CaA^^Og), albite (Ab;NaAlSi3O8), and orthoclase (Or; KAlSi3O8). Brazil Basin basalt is indicated by Site355 (top to bottom of core, 21-1; 22-2; 22-5) and the Sao Paulo pebble by Site 356.

TABLE 4Average Compositions of Titaniferous Magnetite and Ilmenitein Basalt from the Brazil Basin (22-5) and in a Basaltic Pebble

from the Sao Paulo Plateau (38-3) Western South Atlantic Ocean.Compositions are Averages of 30 to 50 Spot Analyses (in wt %)

SiO 2

TiO 2

A1 2O 3

C r 2 O 3

V 2 O 3

FeOMnOMgOCaOZnO

Sum

Recalculated

FeOF e2°3

Total

Usp (mol %)Hem

22-5

mt

0.9522.4

3.20.030.27

67.70.740.880.710.29

97.17

48.721.1

99.27

66.6-

mt

1.117.3

1.70.100.57

71.50.260.910.370.02

93.83

43.830.8

96.93b

51.4-

38-3

ilm

0.7747.8

1.10.060.27

45.20.882.40.160.15

98.79

38.47.5

99.49

-8.1

(Range)

(46.1-51.9)(0.75-1.5)

(37.246.1)

(1.4-3.1)

ilma

1.047.5

1.60.010.18

45.20.572.60.310.09

99.06

38.27.7

99.96

-8.4

Coexisting with magnetite.

Total may be low due to more Fe being Fe ̂ + than calculatedstoichiometrically.

Plagioclase, but preceded titaniferous magnetite. Bulkcompositions border between quartz and olivinetholeiites. The compositions show that ocean-ridgebasalt was essentially the same in upper Cretaceoustime, shortly after initial rifting of the South AtlanticOcean, as many basalts that were only recently eruptedat the Mid-Atlantic Ridge (Table 1). Compositionally,such material represents extensive melting of mantlematerial at relatively shallow depth (e.g., Kay et al.,1970). Consistent compositions over at least 80 m.y.indicate no major temperature changes beneath theMid-Atlantic Ridge.

The Sao Paulo Plateau basaltic pebble (recoveredfrom Cretaceous sediments) has been extensivelyaltered by seawater. Concentrations of certainelements, however, such as K, Rb, Ba, and Sr, areindicative of initial alkalic-basalt characteristics,somewhat modified by alterations, and indicate that thepebble was not typical ocean-ridge basalt. Thissuggestion is supported by the relatively high K inPlagioclase and the interstitial alkali feldspar, which arecharacteristic of alkalic basalt, or alkalic basalttransitional to tholeiite.

The large size (3.5 cm) of the pebble precludes thepossibility of long transport (i.e., continental source)and suggests that its source was local. In addition,substantial amounts of basaltic pebbles inconglomerites (Figure 2) associated with mudstonesand graded zones underlying the pebble indicateextensive deposition of coarse material from a localsource during Santonian time. The most likely source

522

PETROLOGY OF BASALT

for such slump deposits is the nearby Sào Paulo Ridge,of east-west trending feature that bounds the Sào PauloPlateau on the south (see, for example, Leyden et al.,1971).

The implications, then, are that the Sào Paulo Ridge(the presumed source of the pebble) is not composed oftypical ocean crust (tholeiitic basalt), at least in part. Itmore closely resembles oceanic alkalic basalt (ortransitional basalt), which almost invariably isassociated with seamounts and volcanic islands (e.g.,Engel et al., 1965) and commonly represents post-tholeiitic activity. This in turn, suggests that the east-west-trending Sào Paulo Ridge bordering the SàoPaulo Plateau may be composed of a continuous orsemicontinuous chain of volcanos that developedduring early Cretaceous time along the east-westfracture zone that bounds the Rio Grande Rise on thenorth, or by a "hot spot" on the Mid-Atlantic Ridge.

NOTE ADDED IN PROOF

The following analyses have been provided by Dr.Roman Schmidt for Samples 355-22-5, 120 cm, usingINAA techniques:

Se (ppm)HfLaCeSmEuTbYbLu

43.93.43.5

14.04.01.61.14.00.59

ACKNOWLEDGMENT

This work was supported in part by the NationalAeronautics and Space Administration, Grants NGL 32-004-063 and NGL 32-004-064 (K. Keil, Principal Investigator).

REFERENCES

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