trace element and sr-nd isotopic constraints on the...

Geochemical Journal, Vol. 20, pp.173 to 189, 1986

Trace element and Sr-Nd isotopic constraints on the compositions of lithospheric primary sources of

Serra Geral continental flood basalts, southern Brazil

S. S. HUGHES', R. A. SCHMITT' and Y. L. WANG" and G. J. WASSERBURG2

Departments of Chemistry and Geology and the Radiation Center, Oregon State University, Corvallis,Oregon 973311, and The Lunatic Asylum, Division of Geological and Planetary Sciences,

California Institute of Technology, Pasadena, California 911252 U.S.A.

(Received December 12, 1985; Accepted April 23, 1986)

Multi-element abundances in twenty-four samples from the Serra Geral continental flood basalt system

(Parana Basin) were determined by INAA to assess the trace element signature of primary magmatric sources. Isotopic Sr and Nd data were obtained on a 24-sample composite and three individual samples of the suite to constrain the influences of older enriched material. Twenty basaltic samples have enriched LILE commensurate with most continental tholeiites and , display fractionated REE patterns similar to those in the Columbia River Group and other flood basalt provinces. The mafic units exhibit low-moderate Ti, variable V, Cr and Ni and relatively uniform Co and Sc . abundances. Four intermediate to silicic units exhibit higher overall incompatible element abundances, strongly fractionated patterns and depletions of compatible elements consistent with derivation by fractionation of basaltic parents. Isotopic data for two basalts and the composite analysis indicate 87Sr/86Sr = 0.7090 to 0.7105 and eNd = -6.6 to -6.9 requiring magma genesis in an evolved LILE-enriched upper mantle or lower crust. A silicic sample has 87Sr/86Sr = 0.7219 and eNd = -9.2 reflecting a stronger crustal influence. Chemical comparisons within the basaltic members allow the delineation of ten least-evolved compositions having a regionally characteristic trace element pattern and two additional samples (of one flow) representing a second, less fractionated pattern. Trace element models for both types predict magma segregation from lower lithospheric sources having relatively uniform enrichments of incompatible elements via regional metasomatism. The most viable scenarios for basalt magma production require either enriched mantle partial-melt liquids, some of which comingle with crustal components, or partial melting of ultramafic, LILE-enriched lower crust. Either process requires a zone of primary magma extraction in a region where initially uniform source enrichments act independently of subsequent contamination, probably in the crust/mantle transition zone.

INTRODUCTION

Source regions of continental flood basalts

(CFBs) and the nature of subcontinental lithosphere are enigmatic in light of equivocal trace

element and isotopic constraints. Trace element

studies of late Precambrian (Keweenawan and

Coppermine River) and Mesozoic-Tertiary

(Karroo, Parana, Siberian Platform, Deccan and Columbia River) CFB provinces (e.g., BVSP,

1981; Balashov and Nesterenko, 1966; Dupuy and Dostal, 1984; Mahoney et al., 1982; Nelson,

1980; Ruegg, 1976; Smith and Schmitt, 1981;

Hughes et al., 1983; Mantovani et al., 1985) have shown that (1) Cr and Ni contents are

often too low for the equilibration of large

volumes of CFB magma with unaltered mantle and (2) abundances of incompatible elements,

e.g. K, Rb, Ba, REE (rare earth elements) and

Th, are variable and high compared to oceanic tholeiite chemistries. As an example, La in

basaltic members of CFB systems ranges from

less than 5 to over 50ppm whereas normal mid

* Permanent address: Chengdu Geological College, Chengdu, Sichuan, P.R.C. ~.

173

174 S. S. Hughes et alt

ocean ridge basalts (Sun et al., 1979) contain

typically 1 to 7 ppm La. The higher incompati

ble element signatures in CFB magmas cannot be

attributed entirely. to fractional crystallization of picritic parents (e.g., Cox. 1980), but reflect

LILE (large ion lithophile element) enrichments

owing to source metasomatism, subsequent

crustal contamination or a combination of

these processes. Generally, CFB lavas display wide ranges in 144Nd/143Nd and 87Sr/86Sr ratios (e .g., DePaolo and Wasserburg, 1979; BVSP, 1981; Carlson et al., 1981 b; Mahoney et al., 1981; Mantovani et al., 1985) which suggest crustal interaction

(Carlson et al., 1981a). Combined Nd, Sr and O isotopic data for Columbia River Basalts

(Nelson, 1983; Carlson et al., 1981 b) and Nd-Sr isotopic evidence from Deccan flows (Mahoney et al., 1982) argue for significant crustal interaction in most CFB provinces. But the cluster of

data around eNd = 0 and (87Sr/8fiSr), = 0.705 for the volume-weighted major proportion of CFB lavas (DePaolo, 1983) supports a primordial mantle source (DePaolo and Wasserburg, 1976,

1979). While CFB lavas portray gross similarities in trace element and isotopic characteristics, the influences of mantle versus crustal

processes are probably unique for an individual province or subsystem. The tectonic association of Serra Geral (SG) lavas of southern Brazil

(Parana Basin) with the Jurassic-Cretaceous breakup of Gondwanaland (Fodor et al., 1985) affords an opportunity to evaluate possible source compositions and crust development

during the transition from continental to oceanic magmatism. Trace element data by Ruegg (1976),

although not including REE analyses, indicate somewhat higher incompatible element abun

dances in Parana rocks relative to most CFB

lavas. Cl chondrite normalized (N) trace element patterns of SG-CFB compositions includ

ing REE were first reported by Hughes et al.

(1983). The patterns indicated wide ranges in LILE, but relatively uniform basaltic composi

tions correlated well with better-known CFB systems. On the basis of Ba, Sr, REE and Sc

signatures, Hughes et al. (1983) proposed that

the Parana mafic magmas represent a series of

partial melts from an enriched plagioclase

peridotite source. ;; Silicic compositions, and their fractionated patterns were further at

tributed to an extensive crustal history involving

fractional crystallization rather than contamina

tion although the possibility of crustal influence

was not totally dismissed. Recently reported trace element abundances in the LPT (low P and Ti) Parana CFB samples by Mantovani et al. (1985) are consistent with worldwide CFB systems and agree with the

patterns reported earlier by Hughes et al. (1983). Mantovani et al., (1985) also report wide ranges in 87Sr/86Sr ratios (0.7074 0.7163) for the LPT samples which are commensurate with ranges in other CFB provinces. Models calculated by Mantovani et al. (198 5) show that the magmas parental to the LPT series originated in a variably LILE enriched and heterogeneous

garnet-free mantle source. They attributed the LPT series to plagioclase dominated fractionation and progressive contamination, thereby confirming the suggestion by Hughes et al.

(1983) that plagioclase was an important residual phase. Additional isotopic data available for SG

CFBs include the average initial 87Sr/86Sr ratio of 0.7057 for six samples by Compston et al.

(1968), and Sr-Nd data on two samples (DePaolo and Wasserburg, 1979) that yield similar 87Sr/86Sr ratios and eNd values of -1.5 and -4.2. These few data are consistent with SG-CFB sources in a LILE-enriched upper mantle and preclude derivation from a depleted mantle unless enrichment occurred subsequent to magma genesis. The 24 Serra Geral CFB samples documented by Fodor et al. (1985) were provided

for trace element and isotopic assessment of

the SG-CFB source region. These samples,

whose trace element patterns were originally

shown in the preliminary report by Hughes et al. (1983), represent four arbitrary geographic

sectors (our designations) in a 300km X 300km

region near Port Alegre (see Fig. 1 in Fodor et

Trace element and Sr-Nd isotopic constraints 175

al., 1985) in the southern portion of this CFB

province. Multi-element data obtained by instrumental neutron activation analysis (INAA) are reported in detail along with Nd-Sr isotopic analyses on selected samples in order to demonstrate the trace element characteristics of the lower lithospheric source at the time of magma segregation and to evaluate further the possibility of crustal involvement in primary magma

genesis. We address these problems in light of (1) the comparison of (low Ti) SG-CFB chemistries with better-known CFB systems and the likely prevalence of regional uniformities which can be related to primary sources of most CFB magmas, (2) initial Nd/Sm ratio in the source as determined by trace element modeling and Nd isotopic systematics, and (3) the feasibility of mathematical models which satisfy trace elements and Nd-Sr isotopic constraints. While

previous arguments for crustal contamination are accepted for silicic CFB compositions, we

question the need for contamination and source heterogeneity in the generation of mafic magmas having uniform trace element signatures.

ANALYTICAL PROCEDURES

Approximately 10 grams of each sample

was processed through a 99.9% pure alumina

plate jaw crusher and rotary mill to insure homogeneous samples and similar counting

geometries. Multi-element abundances were determined using a sequential INAA procedure

similar to that described by Laul and Schmitt

(1973). Aliquants (^'0.5g) of samples and rock standards BCR-1, BHVO-1, GSP-1 and

PCC-l, plus primary standards, were weighed

and irradiated for short-lived radionuclide analyses for 3 minutes at 25 kW in the Oregon

State University TRIGA reactor via a pneumatic

transfer system for a total neutron fluence of 4 X 1013. Each sample and standard was counted

for 400 seconds with a horizontal Ge(Li) gamma-ray detector coupled to a multichannel

spectral analyzer to obtain Ti, Al, Mg, Ca and V

abundances. After 4-8 hours decay, subsequent countings of 1-2 k seconds on similar equipment

yielded analyses for Na, Mn and Dy. Data for long-lived radionuclides were obtained by activa

tion of ^'200mg samples (accurately weighed)

and rock standards (plus a primary REE stan

dard) for 5 hours at 1 MW for a neutron fluence

of 5 X 1016. After 3-6 days decay time, samples

were initially counted 1-2k seconds for K and U,

then 8-10k seconds to determine abundances

of Na, La, Sm, Yb and Lu. Continued sequen

tial countings using longer decay and count

times (several weeks and 20-40k seconds, respec

tively) enabled data acquisition for Fe, Sc, V,

Cr, Co, Ni, Se, Rb, Sr, Cs, Ba, Ce, Nd, Eu, Tb, Zr, Hf, Ta and Th abundances. Isotopic data for

Nd and Sr were determined at the California

Institute of Technology following the proce

dures outlined by DePaolo and Wasserburg

(1976) and Papanastassiou et al. (1977).

RESULTS

Chemical compositions determined by se

quential INAA are listed in Table I according to the four geographic sectors. Isotopic and

elemental Nd and Sr data are presented in Table

2. Group 1 has been further subdivided on the

basis of relative trace element abundances

(primarily for clarity in presentation of data); although G-648 does not fit readily into any

subgroup, it is assigned to group I c. The low

moderate TiO2 abundances, together with the

LILE abundances, suggest that all of these

samples belong to the LPT series of Mantovani

et al. (1985). The twenty basaltic samples show

moderate ranges in LILE abundances exem

plified by K2O = 0.6-1.7 wt.%, Rb = 14-69 ppm, La = 9-27 ppm, Th = 2-6 ppm and U = 0.3-2.0

ppm while much greater ranges and higher LILE concentrations are evident in the four inter

mediate to rhyolitic samples with K2O = 2.25.4 wt.%, Rb = 73-210 ppm, La = 25-46 ppm,

Th = 8-19ppm and U = 0.4-4.7 ppm. The elements V, Cr and Ni are widely variable in

basaltic units with V = 250-540ppm, Cr =

6-250ppm and Ni = 10-160ppm compatible with wide ranges in other Parana rocks (Ruegg,

1976); whereas less variation is shown by Sc =

176 S. S. Hughes et al.

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Table 2. Isotopic data for Serra Geral samples

Sample Sr (ppm) 87Sr/86Sr Nd (ppm) eNd Sm (ppm) 147Sm/144Nd

RS-65

RS-71

EV-01

Composite-24

161

214

242

207

0.72187±3

0.70899

0.71010 ±3

0.71052 ±5

35.31

22.97

17.76

24.10

-9.22 ±0.37

-6.63 ±0.33

-6.61 ±0.47

-6 .9

7.40

5.32

3.66

5.8

0.127

0.140

0.125

32-44ppm and Co = 39-49ppm. The uniformity

of Sc and Co abundances in basaltic units con

trasts with strong depletions of these elements in more silicious rocks as shown for Co vs. La in

Fig. 1.

Relative abundances of REE plotted in Fig. 2 as La/Sm vs. La/Yb, vary systematically and show enrichment of light REE (LREE) over C l chondritic (minus volatiles) abundances

(Anders and Ebihara, 1982). Also plotted in Fig. 2 are typical LREE-depleted mid-ocean ridge basalts (MORB, Sun et al., 1979) which complement the higher LREE in the continental units yielding a curve which passes through the C 1 value and may possibly pass through the origin. The curve reflects the tendency for La/ Yb ratios to increase faster relative to La/Sm ratios indicating less fractionation of LREE with overall increases in incompatible elements. Figure 2 also portrays minor REE differences between some geographic locales, especially

group I b, although the less-evolved units of most sectors overlap in the ranges La/Sm = 3.34.5 and La/Yb = 4.7-8.0.

Rare earth element abundances in SG sam

ples and other basaltic types, plotted as La vs. Yb and Sm vs. Yb in Fig. 3 show the demarca

tion which is evident between CFB and MORB

(Sun et al., 1979); however, alkalic basaltic compositions from Hawaii (Chen and Frey,

1983; Frey and Clague, 1983), the Rio Grande

Rise (Thompson et al., 1983) suggest chemical overlaps of tholeiitic CFB with alkalic oceanic

basalts of aseismic ridges, especially at higher

total REE contents. At lower REE abundances

SG and other CFB lavas maintain a more de

finitive segregation from the basalts of aseismic

5

E Q

0 0

4

50

40

30

20

10

0

• RS-76

RS ° * eRS-69 •1k0 e G-402 ° a r

• U G-648

GEOGRAPHICGROUP

o fa

• fb a is

o II • III

0 IV

•RS-65

MRS-30

RS-50

3

2

I

0jI

La/Sm

•

Serra Geral

ev ~,

i

i

P CI Chondrites

MORBasalts

e

0 ®'

0 0 0

La/Yb

0 10 20 30 40 50

La (ppm)

Fig. 1. Co vs. La in all Serra Geral CFB units. RS-30, RS-50 and RS-65, silicic members of the suite, have strongly depleted Se as well as Co abundances.

0 2 4 6 8 !0

Fig. 2. Ratios of La/Sm vs. La/Yb in Serra Geral com

positions relative to C1 chondrites (Anders and Ebihara, 1982). The curve reflects a complementary relation between LREE-depleted MORB compositions (Sun et al., 1979) and LREE-enriched Serra Geral primary magmas. Symbols are the same as in Fig. 1.


ridges in the southern Atlantic.

C 1 chondrite-normalized Sr, Ba, REE, Sc,

Hf, Ta and Th patterns of all SG-CFB samples

are shown in Fig. 4 with the REE segments

spaced according to ionic radii. Depletions in

Sr, Eu and Sc, produce negative anomalies in

evolved members of groups lc and 3 which are

coupled to higher total REE, Hf, Ta and Th.

All units display low Sr and many samples,

except in group 4, have REE patterns with small

negative Eu anomalies. Although minor aberra

tions are evident in some SG samples, the

basaltic trace element patterns within each

group (except l b) are essentially parallel and overall pattern variations between groups are

minimal.

50

40

E 30

d -j 20

I0

0

I0

8

CL 6

E

4

2

0

4

• A

++

+_

W

/

w +

A ~~ AAWO 0

O ~ o Cf

w

C.

2J

0

0

Ilk' •,•

0

W

00

00

A Alkalic Hawaiian Basalt

R

(Chen and Frey, 1983; Frey and Clague, 1983)

Rio Grande Rise Basalt

(average of 7, Thompson et al., 1983)

W Walvis Ridge Basalt(Humphris and Thompson, 1983; Liu and Schmitt, 1984)

/_

/

/

+

A

Aw0/ O

A / 00,~ /~ a' Q°O

4+ 4 A + •k`

w /

+ / 108

/

W /W y/y +o O +p

R

O

Z 0

A

A

•

00

W•

O

CO

0

Serra Geral (this study)Col. River Group (Carlson,

1984; BVSP, 1981)0 Siberian Platform (Smith and

Schmitt, 1981; 8alashov and Nesterenko, 1966)

+ Kewe enawan Ref. Suite

(B A MORE

VSP, 1981) (Sun et al., 1979)

0 1 2 3

Yb

4 5

(ppm)

6

Fig. 3. La vs. Yb and Sm vs. Yb for a variety of continental and oceanic basalts. The lower solid lines represent the well-defined boundary between MORB and continental tholeiites, whereas the upper dashed lines represent a less tenable boundary of alkalic compositions.

Figure

minimum

SG basalts

5 illustrates

trace element

and the same


the maximum and from two divisions in the Columbia River (CR)

patterns representing group: (1) Saddle Mountains formation and for reference samples (2) CR members stratigraphically below the

U) W

0 Z 0

U

U NN W -J CL

Q U)

100

50

20

100

50

20

100

50

20

100

50

20

10

5

GROUP

* (a ' Jb

• IC • G648

•II * IV

•III

10

5

2

10

5

2

10

5

2

10

5

2

Sr Ba La Ce Nd SmEuGd Tb Dy Yb Lu Sc Hf To Th

Fig. 4. Sr, Ba, REE, Se, Hf and Th plotted (REE ionic radii) relative to volatile free (minus C and H20) C1 chondritic abundances (Anders and Ebihara, 1982). Major group divisions are geographic sectors reported by Fodor et al. (1985). Scale on right is for Se only.


Saddle Mountains section (BVSP, 1981). The

REE segments in the CR patterns are nearly identical with the CR data in Carlson (1984)

which exhibit increases in total REE and in the ratio of light to heavy REE roughly with

stratigraphic position. Isotopic data for the

CR system (McDougall, 1976; Nelson, 1980;

Carlson, 1981 a) indicate an increasing radiogenic component with higher stratigraphic position

and argue in favor of crustal contributions to

the Saddle Mountain lavas. However, the low

sequences in the CR group have less radiogenic

compositions and are more likely derived with

minimal amounts of crustal interaction. The

patterns in Fig. 5 portray strong geochemical similarities of SG basalts in this study (although

not stratigraphically controlled) to the basalts in the lower CR sequence, an argument in favor

of similar petrogenesis.

Isotopic data (Table 2) for two basaltic samples (EV-01 and RS-71), a rhyodacite (RS65) and a composite analysis of all 24 units indicate high 87Sr/S6Sr ratios (0.709 to 0.722) relative to mantle xenolith minerals (e.g., Menzies and Murthy, 1980a,b; Jagoutz et al., 1979; Stosch et al., 1980). Negative eNd values

(-6.6 to -9.2) are within the mantle mineral realm and argue for a LREE enriched (NdN > SmN) source. The fact that Nd-Sr data for the two basaltic samples are similar to the composite analysis suggests that RS-65 and other silicic derivatives are petrologically distinct from the main sequence of SG-CFB liquids. The displacement of silicic units is also evident in Figs. 1 and 4.

0 W N J

Q: O z

W t a: 0 Z 0

U

U

103

102

10

+

0

!I.

Maximum and Minimum

-mo o.

o--o

Serra

CR8

CR8

Geral

Saddle

below

Patterns

Basalts

Mfns. Fm.

SM Fm.

\41 I

/

./

4• 4

L ~I/

DISCUSSION

Trace Element Constraints

The bulk and trace element geochemistries of mafic SG units, as well as REE (Figs. 3 and

5), are virtually indistinguishable from those of most CFB systems despite minor variations

within individual groups. This likeness argues in favor of a unifying geochemical process which

can be generalized to sources of CFB systems on

a worldwide basis although the continental

ft. 5. f patterns (from Fig. 4) in Serra Geral basalts, Columbia River

, rains River data from B VSP, 1981.

Sr Ba La Ce Nd SmEuGdTbDy YbLu Sc Hf Th

Upper and lower boundaries o trace element

basalts stratigraphically below the Saddle Moun

Fm. and CRB Saddle Mountains Fm. Columbia

tectonic settings are not always uniform. The

timely breakup of Gondwanaland certainly

allowed for changes in depth of mantle sources

beneath eruptive zones, but the rifting itself

probably had little impact on the regional geochemistry of pre-established magma sources in

the subcontinental lithosphere.

Descriptions of CFB primary sources is complicated by the requirement of either

picritic parents (Krishnamurthy and Cox, 1977; Cox, 1980) which fractionate to magmas having lower Mg/Fe ratios or melting of an evolved

(Fe-enriched) source. Also, any viable scenario for a CFB origin requires large volumes of chemically uniform magma, high rates of extrusion and, therefore, a regional "magmafer". If the least-evolved units of a CFB system display a characteristic trace-element pattern over a wide area, that pattern should reflect a relatively homogeneous stage of primary magma evolution prior to divergences produced by assimilative mixing or fractional crystallization. Assuming that fractionation of a picritic parent will not appreciably alter incompatible element ratios, the average least-evolved trace-element

pattern is primary and was determined before or during initial magma extraction.


80

60

40

20

10

8

6

4

2

0.3

0.2

0.1

o RS-70 (Rb/K) x /0

RS-7I

p RS-69RS-65 RS-30 RS-50i

p01 O MR-03 ------- *-- p--------------------~a 01

_ .~-0 GI 9

-~ * G-648 M-03Bulk Earth

EV-01 ~'

RS-73 O 0'-02.--~

p0

t7 pRS-2Q,' • •

G-402 La/ Yb

O CI Chondrites

Yb/Sc RS-50

RS-30

00 RS-65

G-641 ~.- J'~ M-03

p,. -O--- - --bMR-03

I 1 I I I I I2 4 6 8 10

Th (ppm)12 14 16 18

Fig. 6. Rb/K, La/Yb and Yb/Sc vs. Th in Serra Geral lavas. Bulk earth composition from DePaolo, 1983; CI chondritic values from Anders and Ebihara (1982). Symbols as in Fig. 1.


The nearly parallel configurations in Fig. 4 suggest a regional trace element uniformity of basaltic units, although G-648 has higher REE abundances and subgroup l b exhibits lower overall abundances (except Se) and lower La/ Yb ratios. The members of group 1 b, especially RS-20 and G-402, exhibit the lowest LILE abundances; however, siderophile and major elements fall within the ranges shown for all basaltic units and do not support a lesser degree of magma evolution. The observation that REE

patterns of lb members, which have lower total REE abundances, are not parallel to other

groups argues for a separate line of descent for subgroup l b and a source region with lower La/Yb ratios and total LILE abundances. Figure 6 illustrates the behaviors of Rb/K,

La/Yb and Yb/Sc ratios relative to the highly incompaticle element Th. The slight increase of Rb/K in evolved members portrays a systematic trend from the assumed bulk earth composition (DePaolo, 1983), but it is offset by G-648, M-03 and members of subgroup 1 a which follow an alternate line of evolution (discussed in greater depth by Fodor et al., 1985). Variations in La/Yb ratios with Th (Fig. 6) are less systematic, although mafic members cluster around La/Yb = 4-6. A positive trend from the

C 1 chondritic value passes through basaltic compositions and subtle differences in REE chemistry between groups are evident. The Yb/ Sc vs. Th data for most basaltic units plot along a line extending from the C 1 chondritic value through the most silicic members of the suite. Offsets of Yb/Sc ratios in M-03, MR-03 and RS65 suggest that diverging evolutionary patterns are initiated below Th = 5 ppm, the cut-off used herein for least-evolved magmas. The clustering in Fig. 6 of most basaltic compositions compared to the scattered points

for silicic and divergent units allows the delineation of samples which best represent the least

evolved trace element signature. A regionally

characteristic . trace-element pattern is established by averaging these basaltic compositions,

but excluding certain samples for the following

reasons: (1) G-648 (Fig. 4) has absolute REE

abundances roughly 50 percent higher than REE in other SG basaltic rocks; (2) subgroup la

(RS-69 and RS-71) has excessive Rb (Fig. 6) and high FeO, V and Th, suggesting a divergence in petrogenesis; and (3) subgroup l b has lower

total LILE abundances and lower La/Yb ratios

(Fig. 6), although bulk chemistry and siderophile trace elements do not substantiate a more primitive stage. The average of 10 SG basaltic compositions

considered to be least-evolved and regionally

represented is depicted in Fig. 7 as SGAV 10.

A second compositional average of G-402 and

RS-20 (SUB 1 b, Fig. 7) indicates either LILE

depletion from previous melt extraction or less

LILE enrichment in an alternate source. Our

petrogenetic models of the SG basaltic magmafer assume that (1) the SGAV 10 pattern is

inherent in magmas at the zone of basaltic

magma extraction, regardless of the magmafer

location (crust or mantle), and (2) the processes

that determined the geochemical character of the least-evolved magmas are uniformly opera

tive on a regional scale. Differentiation to more

silicic compositions, accompanied by an increase

in total incompatible elements and more frac

tionated patterns (Figs. 2-4), is dominated by

fractional crystallization of mafic parents

(Hughes et al., 1983; Fodor et al., 1985;

~,100 a)

50

0 t _U 20

U \ 10 Q)

5 0

2

Serra feral CFB Least-evolved Magmas

SGAV 10

SUB Ib -

(RS-20 a G-4021

Fig. p deviations,

, RS-73 1-02

patte less f

primary magmas.

Sr Ba Lo Ce Nd Sm Eu Gd Tb Dy Yb Lu Sc Hf To Th

7. SGA VIO: trace element attern with standard

r, representing least-evolved Serra Geral magis average of RS-16, RS-26, RS-37, RS-60, RS-68, M-01, M-02, and G-215. SUBl b: identical

rns of two samples from one flow which, although representative , reflects a second source or SG


Mantovani et al., 1985) with an allowance for

subordinate effects of crustal assimilation.

Similarities between evolved SG compositions

and post-Archean continental crust (McLennan et al., 1980) would probably cause the effects

of crustal contamination to be cryptic in trace

elements. However, the uniformity of frac

tionated compositions in basalts precludes the

random effects of secondary assimilation and

supports an enriched lower lithospheric source.

Isotopic Constraints

The isotopic data in Table 2 suggest crustal influence on the SG-CFB system in addition to magma equilibration in LILE-enriched regions of the lithosphere. Figure 8 illustrates eNd vs. 87Sr/ 86Sr data of this study plus those for two other

SG basalt samples of DePaolo and Wasserburg

(1979) obtained from a drill core near Santa Catarina (PAR, see Fig. 1, Fodor et al., 1985). The higher 87Sr/86Sr ratio and lower eNd of RS-65 places it near the "upper crust" realm

(DePaolo, 1981a), indicating that RS-65, and

probably other evolved samples contain uppercrust material. The less-evolved units, RS-71 and EV-01, and the composite SG analysis are consistent with a basaltic magmafer in either the lower crust or an enriched upper mantle although equilibration with mantle material is not supported by major elements (MgO C 10%) or transition metal abundances. By contrast, PAR 1 and 2 plot closer to the present day bulkearth composition supporting a less-evolved basaltic magma source. The isotopic differences between these sources are consistent with either variable amounts of older LILE-enriched material assimilated into the liquid or variations in the age of source-enrichment prior to magma extraction.

Assuming little or no difference in source ages, the SG-CFB source. is petrologically positioned between primary mantle and upper crust as shown by combined fractional crystallization and mixing curves in Fig. 7. The curves follow equations given by DePaolo (1981b) using a

primary magma composition of 87Sr/86Sr =

8

4

z o W

-4

-8

-12

Bulk Earth

® PM t PAR-1

o PAR 2 ~

30

Sr (m)

MAGMA 450

CRUST 50

87Sr/86Sr Nd (ppm)

20

45

eNd

II

PRIMARY "

20 RSl71EV-01

/ s

Composite

C /A

0.705

0.760

DSr = 0.96 ONd = 0.14

50A 50

100

-1

-13

-4,1

RS2:1 7o 1. wI'1 oo so

0* 1

1 I 200 I ! I

0.704 Q 708 0.712

87Sr/86Sr0.716 0.720

Fig. 8. eNd vs. 87Sr/86Sr for PAR-1 and PAR-2 (DePaolo and Wasserburg, 1979) and RS-71, EV-01, RS-65 and a 24-sample composite (this study). Curves represent combined effects of crustal assimilation and fractional crystallication using an assumed primary magma and the "E" crustal component of Carlson et al. (1981b). CA Values indicate cumulate to assimilate ratios and labelled points show the percentage of original mass added by assimilation.


0.705 and eNd = -1. The assimilated composition is assumed to be similar to the "E" com

ponent used by Carlson et al., (1981a) with 87Sr/86Sr = 0.760 and eNd = -13, but with an absolute Nd abundance (45 ppm) more com

patible with silicic SG rocks. The evolutionary lines represent specific ratios of cumulate lost to assimilate added (C:A) and the numbered

points mark the percentage of original mass added by assimilation. The position of RS-65 on the C:A = 2:1 curve implies an evolutionary trend marked by more than 80 percent crustal assimilation with twice that amount (of original mass) being removed as cumulate. Basaltic SGCFB units lie above the 2:1 line and simulate wsystem controlled by 30-50% assimilation of the the crustal component along with larger amounts of crystallization. However, 30-50% upper-crust addition to regionally uniform

basaltic magmas seems fortuitous unless a shallow homogenization process can be demonstrated. Alternatively, the positions of basaltic SG

units in Fig. 8 represent magma generation in an older enriched lithospheric source, possibly in the mantle/crust transition zone. The differences between regional Parana magmafers become those of location and age of LILE enrichment, assuming similar petrogenetic behavior and somewhat variable isotopic signatures. This discretion is supported by similar or lower Sm/Nd ratios in the two PAR samples

(Sm/Nd = 0.17 and 0.22, DePaolo and Wasserburg, 1979) compared to that of the SGAV 10 composition (Sm/Nd = 0.21). It is likely that source enrichments for PAR 1 (Fig. 8) occurred within a relatively short time before magma genesis.

TRACE ELEMENT MODELING

Models of SG-CFB source regions follow

three inherent premises. First, absolute trace

element abundances can only be approximated

due to ranges in partition coefficients for most

minerals (e.g., Fujimaki et al., 1984; Irving,

1978), whereas relative behaviors are more

accurately predicted. Second, uncertainties in the relative amounts of partial melting and fractional crystallization for large magmatic volumes force a combination of mineral/melt calculations into a single, presumably equilibrium, process. Residual minerals in source models may be either unmelted solids at the original point of extraction or subsequent

precipitates. Third, the most viable model should predict a flat chondrite-relative pattern for the most compatible elements, HREE, Sc and Hf which will remain relatively unfractionated during source enrichments or depletions. Assuming that the SG lavas are not derived from an initially depleted reservoir, enrichments determined by modeling represent sources prior to magma segregation. Predicted sources for SGAV 10 and SUB 1 b,

shown in Fig. 9 were calculated using the equi

librium non-modal melting equation of Shaw

(1970) to simulate the combined effects of anatexis and crystallization. Partition coef

ficients (D-values, Appendix) are those ap

plicable to tholeiitic basalt systems and D-values for Th are assumed zero. The strong depletions in Sr compared with minor negative Eu anom

alies support higher and lower relative D-values

for Sr and Eu, respectively, of plagioclase equi

librated at elevated fo 2 and lower temperatures

(Drake and Weill, 1975). Melting proportions correspond approximately with observed mass

modal proportions of minerals (excluding late

stage Fe-Ti oxides) in tholeiitic basalts although

the modeled sources depend more on residual

proportions. The preferred model for SGAV10 (Fig. 9)

assumes 10 percent melting of an initial plagioclase peridotite source and a residual mineralogy of of 60 opx 10 cpx 1 p129. The greater relative amounts of residual olivine and plagioclase may be related to subsequent crystallization of these

phases and do not necessarily indicate the initially solid proportions at greater depths. Enrichments in Sr, Ba, LREE and Th are consistent with melting of metasomatized ultramafic lithosphere having an initial (Sm/Nd)N ratio of 0.65. Lower degrees of melting, even for a less


reasonable source mineralogy required to minimize inflections in the REE-Sc-Hf pattern, require similar or lower (Sm/Nd)N ratios as shown for F = 5 percent melting.

The SUB 1 b composition yields a viable

source at 15 percent melting (Fig. 9) which is

similar to the SGAV 10 source in HREE, Sc and

HE Lower incompatible element abundances

100

10

QJ

N v

E

__

O

I00

v

10

I

F=10 %

0I

SOURCE 55 MELT 10

RESIDUUM 60

P1?x 10

to 10

CPX 5

40I

PI. 30 40 29

F=5%

SOURCE 78

MELT 10 RESIDUUM 81

3

25 2

1

25 0

\

I8 40 17

10

5

SGAV10

/

i

-J

I

I If

F=15%

01

SOURCE 77

MELT 20 RESIDUUM 87

5 20

3 20

0

P 1

15 40

11

F=10%

15

IO

SOURCE 83 4 2 11

MELT 20 20 20 40 RESIDUUM 90 2 0 8

SUB t b

, II - -------- ----------------------r i

-J

Sr 8a La Ce Nd Sm Eu Gd Tb Dy Yb Lu

Fig. 9. Trace element models using non-modal batch melting equations (Shaw, SGA VIO and SUB1 b Serra Geral magmas. D-values given in Appendix.

Sc

1970)

Hf

for

Th

primary sources of

186 S. S. Hughes et alt

indicate a less enriched or evolved source having a (Sm/Nd)N ratio of 0.85. Similar relative enrichments are portrayed by the modeled source at 10 percent melting. Models for both magma compositions from Fig. 7 indicate LREE-fractionated sources with (Sm/Nd)N ratios similar to the observed ratios. Essentially no fractionations of Ba and LREE are obtained due to their low bulk D-values in the residual mineralogy. It is likely that PAR-1 and PAR-2

(DePaolo and Wasserburg, 1979; Liu and Schmitt, 1984) compositions also yield source

(Sm/Nd)N ratios that are similar to their observed ratios. Model-dependent sources for PAR-1 and PAR-2 (not shown) should have minimum (Sm/Nd)N ratios of 0.52 and 0.67, respectively, which suggest slightly greater source enrichments despite the more primitive isotopic signatures.

PRIMARY MAGMA SOURCES

The apparent disparity between source composition and the observed eNd values argues in favor of source enrichments to produce the observed trace element patterns as well as source variability between subprovinces in the SG-CFB system. Added crustal components are operative in altering some isotopic signatures, but do not significantly alter the overall trace element

pattern. Slightly different trace element patterns in sources required by SGAV 10 and SUB 1 b derive from regional differences in the subcontinental mantle not unlike those which have been proposed for the Columbia River Basalt group (Carlson, 1984). Isotopic differences, apart from the pronounced effects in silicic compositions, may be imparted with variable ages of enriched lithospheric sources or variable proportions of crustal components added to isotopically uniform primary magma. In light of similar Sm/Nd source ratios required for most SG-CFB magmas, variations in source age appear to be the more viable hypothesis. Assimilation of a crustal component to alter isotopic ratios requires that the contaminant have a trace element signature close to that in

the primary melt.

At least two geologically reasonable scenarios are possible: (1) primary melts from enriched lithospheric mantle comingled and homogenized with underplated crustal com

ponents of variable ages, probably in stratigraphic succession; and (2) primary magmas were derived by partial melting of enriched lithosphere at variable time-stratigraphic horizons. Both hypotheses allow the existence of regional homogeneous primary magma subsystems and require the placement of CFB magmafers within the crust/mantle transition at levels permissible for regionally uniform partial melting. In either scenario trace element enrichments are obtained in the zone of primary magma production.

CONCLUSIONS

Basaltic low-moderate Ti members of the Serra Geral CFB system display characteristic trace element signatures that reflect a regional constancy of magmatic processes. Trace element models require at least two separate sources enriched in incompatible elements via metasomatic processes. Isotopic data indicate variable contributions of crust or evolved upper mantle in some basaltic magmas, a more substantial crustal component in silicic units, and support models of magma genesis in LILEenriched sources. The basaltic magmafer for a

given subprovince in the SG-CFB system may be obtained by either of two mechanisms: (1)

primary melts from enriched mantle comingle with assimilated lower crustal components of a

particular age or (2) partial melting of ultramafic LILE-enriched lower lithosphere occurs within a specific time-stratigraphic horizon. Similarity of these two scenarios requires the

placement of SG-CFB magmafers in the mantle/ crust transition zone allowing proportioned contributions from variable-evolved LILE-enriched material. The similarity of modeled SG-CFB source

chemistries, especially incompatible trace ele

ment ratios, with better known CFB systems


reflects the processes of primary source enrich

ment on a regional scale and not the random

effects of subsequent contamination by crustal

components. Evolution of the SG-CFB source region, despite the breakup of Gondwanaland,

represents a possibly uniform and continuous

global process of LILE enrichment in the continental lower lithosphere.

Acknowledgements-We thank A. Roisenberg for providing the suite of Serra Geral samples and T. V. Anderson, W. T. Carpenter and D. Pratt for assistance with neutron activations in the Oregon State University TRIGA reactor. Helpful comments and criticisms by R. V. Fodor greatly improved the manuscript. Financial support was provided by NASA grant NAG9-63 to R. A. Schmitt and grant NAG9-49 to G. J. Wasserburg.

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APPENDIX

Distribution coefficients used in trace element models

Olivine Orthopyroxene Clinopyroxene Plagioclase

Sr

Ba

La

Ce

Nd

Sm

Eu

Gd

Th

Dy

Yb

Lu

Se

Hf

Th

0.009

0.009

0.0056

0.0056

0.0058

0.0078

0.0086

0.010*

0.011

0.012

0.042

0.053

0.37

0.022

0.0

0.017

0.013

0.022

0.024

0.033

0.054

0.054

0.091

0.11*

0.15

0.34

0.42

1.4

0.051

0.0

0.12

0.026

0.105

0.125

0.287

0.477

0.562

0.595 0.61 *

0.622

0.601

0.560

3.8

0.121

0.0

2.7**

0.23

0.21

0.12

0.081

0.067

0.10**

0.063

0.059*

0.055

0.059*

0.060

0.044*

0.01

0.0

* Values which are interpolated or estimated from several sources. ** Estimated values for D

Sr and DEu in plagioclase at higher f02 values(see text).

Sources of data:

Sr, Ba: Philpotts and Schnetzler (1970), Arth (1976). REE, Hf: Fujimaki et al. (1984), Arth (1976). Sc: Lindstrom (1976), McKay and Weill (1977).

trace element and sr-nd isotopic constraints on the...

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