BoUG-USP, Publ.Esp., 10:27-34, 1991
PROTEROZOIC DYKES OF SALVADOR AND ILHÉUS-oLIVENÇA AREAS : A GEOLOGICAL-GEOCHEMICAL COMPARISON
C.Moraes-Brito 1
G.Bellienf P. Comin-Chiaramonti3
A.J.Melfr E.M.Piccirill04
The mafic dykes of Salvador anel the dyke swarm of IIhéus-Olivença present very similar geologlca/, geochronological, paleomagnetlc, petrographlca/, anel geochemical characteristics.
Such slmUarlty suggests' that both areas may have been subjected In Late Proterozolc times to similar crustal extenslon processes besides other processes, followed by basaJtic magmatism. These dykes are located on the eastem edge of the São Francisco Craton Intrudlng Ea"y Proterozoic granulitic rocks of the Atlantlc Coast Mobile Belt (PEDREIRA et ai., 1976; MASCARENHAS, 1979; Fig. 1).
In Salvador two dyke assemblages are observed : the flrst one, with individual dyke thlcknesses arounel 2 m, is metamorphosed (orthoamphibolites), trenels E-W, anel is associated with metagranitolds. The seconel assemblage Is unmetamorphosed, trenellng N-S, anel the dykes are up to 50 m thlck, although thlcknesses less than 5 m predominate (MORAES-BRITO et ai., 1989). The latter assemblage Is not widespread. The IIhéus-Olivença dyke swarm trenels E-W anel is unmetamorphosed with predomlnant thlcknesses, of less than 5m (TANNER DE OUVEIRA et ai., 1989).
Geochronologica/ data
(W
Ar /39 Ar methad, RENNE et aI.1990) yielded ages of 1,000 anel 1,100 Ma for the emplacement of the unmetamorphosed dykes of Salvador and llhéus-11nat1tuto Aa1ronOmlco e Geoflsico, Universidade de São Paulo, Caixa Postal 30627, 01051 São Paulo, SP, Brazil.
2Dipartlmento di Mineralogia e Petroiogia, Universitá di Padova, Corso Garibaldi 37, 35100 Padova, ltaly.
31at1tuto di Mineralogia e Petrografia e Geochimica, Universitá di Palermo, Via Archiraffi, 90100 Palermo, ltaly.
41at1tuto di Mineralogia e Petrografia, Universitá di Trieste, Piazzale Europa 1,34100 Trieste, ltaIy.
,. • 1.1 I j ;; ;; ;; ;; " ;; 7t'? i ~ J) ,120 13° 15° 400 38° o 100 200K",
lEGENO Cenozoic Covers ] Mesozoic SedimentCU'y bosins ( Recôncovo (11 ond Almodo (21
Wá:01
Eorly Proterozoic ond Archeon ferroins1+++ 1
Migmotitic Metomorphic Complex FoulfP~--;I Gronulific Complex 1-Jequlé Compl •• II-Atlontic Coas, mobile- belt Block limif
~ ,..-... _~ ..J Structuro I lineoment Agure 1 -Simplifled geological map of the Salvador and Ilhéus-Olivença regions (modified from PEDREIRA et ai .• 1976).
Ollvença, respectlvely. Paleomagnetlc data on the dykes from both areas show slmllarltles suggestlng a slmUar age ot emplacementfor both occurrences (O'AGREUA FILHO et ai., 1989).
The orthoamphlbolltes ot Salvador are flne-gralned anel composed ot aneleslne, homblenele, red blotlte, opaque minerais, anel seldornly pyroxene. The accessory minerais are zlrcon anel abundant apatite. The texture Is predomlnantly granonematoblastJc, but prlmary magmatlc textures are stll preserved.
The unmetamorphosed dykes ot Salvador anel IIhéus-Ollvença are composed ot
labradorlte (Anoo-eo>, auglte, anel opaque minerais; some contaIn hypersthene anel olhers, plgeonlte.
OIMne seldornly appears anel Is often altered to Iddlngslte anel opaque minerais. T extures are glassy (In dykes 1888 than 5 cm thlck; FARIAS & CONCEIÇÃO, 1985), porphyrltlc wIth aphanltlc grounelmass, or medlum-gralned aphyrlc.
Major and trace element concentratlons show that the Salvador anel IIhéus- Ollvença dykes are similar. Table 1 shows that the average values for the Tl02-bearing dykes are lower than 3%. Nevertheless, dykes wlth TI02 greater than 3% occur In the IIhéus-Ollvença area. When compared to rocks from known tectonlc envlronrnents, the studled dykes -show great slmllarlty to continental rlft basalts.
In the R1-R2 dlagram (Flg. 2), the dykes plot In the transltlonal, tholelltlc, latl- anel anelesltlc-basalt fleld. Thelr tholeiltlc character is clear1y demonstrated In the AFM dlagram (Flg. 3). The llhéus-Ollvença dykes present a slight enrichment In FeOt in relation to MgO anel the slkall elernents.
For the Salvador orthoamphibolltes the results saem to lnellcate dramatlc changes in thelr original compositlon. The most remarkable modlflcatlon involves their content in SI02, FeOt' ~O,
Cr, Sr, Ba, Zr anel La (Table 1), which would expIaln the plot ot these rocks in the intermedlate fIeId (latlte, lati-anelesite anel anelesite) ot Figure 2 anel mostly in the calc-alkaline fleld ot Figure 3.
Field observations show that anatexls affected the crystalllne basement rocks (acld anel baslc granulltes anel mlgmatltic gnelsses) anel probably led to lhe forrnatlon ot granltolds. The orthoamphlbolltes were siso affected by such processes, which would account for the changes in their originai composltlon as is also evldent from certaln geochemlcal slmUarltles between the granltolds and the orthoamphlbolltes.
Flnally, It can be Inferred that durlng the Proterozolc, when South Amerlca and AfrIca comprlsed a single block, two major events of Intraplate continental magmatism occurred, which are recorded by these dykes: the first one, represented by the orthoamphlbolites, probably 1,800-1,900 Ma oId (as suggested by fleld data) anel the seconel, represented by 1,000-1,100 Ma-old unmetamorphosed dykes of probable common origin from Salvador anel llhéus-Ollvença.
29
Ci + 12500
N
+
~ ~
500
QUARTZ TRAQUITE
PICRITE
c
- -
Salvado,
c Or toompMbolites o BOloltl
• 8010Itl
RHYODACITE
RHYOLITE
~--- ALCAU-RHYOUTE
O~---~---~---~---
1500 2000 2500 RI
2+ 3+
RI14Si-IIINo+K)-2IFe+Fe+Ti)
Rgure 2 -Data from the Salvador and IIhéus-Olivença dykes plotted on the diagram of De La ROCHE et aJo (1980) . . modified by BEWENI et aJo (1981).
F(FeO)t
o CA
Salvador
o Orto omph i bolites o Bosolts
Ilhéus - Olivença
• Bosoltl
A~---~---~---~M(MQO)
(No20+~0) ~O
Rgure 3 - AFM diagram showing the distribution of the Salvador and IIhéus-Olivença dykes. calc-alkaJine (CA) and tholeiitic (T) fi.lds. after IRVlNE & BARAGAR (1971).
31
Table 1 -Average composltlons of dykes and country rocks from Salvador and IIhéus-Ollvença anel continental rIft basalts. References: 1 -Thls work; 2 -TANNER DE OLIVEIRA (1989); 3 -CONDIE (1982). ORTHOAMPHIBOUTES UNMETAMORPHOSED UNMETAMORPHOSED BASALTSOF MET AGRANITOIDS OFSALVADOR1 DYKESOF DYKESOF CONTINENTAl RIFTS3 OF SAlVADOR 1 SALVADOR 1 ILH~US-OUVENÇA2 SI02 56.5 49.0 49.0 50.3 66.4 TI02 2.0 2.5 2.0 2.2 1.0 A1203 14.5 14.5 14.5 14.3 14.6 FeOT 8.0 13.0 13.8 13.5 4.3 MgO 4.2 5.7 5.6 5.9 1.3 Cao 5.0 9.0 10.0 9.7 2.0 N~O 2.8 2.6 2.8 2.5 3.0 ~O 3.8 0.7 0.6 0.66 5.8 Cr 88 140 113 160 12.7 NI 50 80 79 85 11.3 Rb 110 22 16 31 175 Sr 540 265 237 350 284 Ba 1500 450 288 170 1335 Zr 310 130 151 200 506 La n 24 17.5 33 97 Ce 115 45 48 98 158
ACKNOWLEDGEMENTS
The authors are grateful to A.J.POOreira (CPRM) and N.Ussami (IAGjUSP) for revlslon of the Engllsh version of the manuscrlpt; to M.A.F.Tanner de Oliveira (IGjUFBA) for assistance In the fleld.
REFERENCES
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Contribution 81, IUGS, Subcommision on the Systematics of Igneous Rocks. 19p.
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