Giorna!e di Geologia , sa. 3". "01. 47/1-2. 1985. pp. 253-280. Bologna

On the applicability of the Rb-Sr method toargillaceous sedimentary rocks: some examplesfrom Precambrian sequences of Brazil

U.G. CORDANI, A. THOMAZ-FILHO, B.B . BRITO-NEVES and K. KAWASHITA

V.G. Cordani ami K. Kuwashita . University of S(IO Paulo. institute of Geosciences - P.O. Box 20899 - eEl' 01498 - Sao Paulo. SP.A. Thomaz-Filho . Petrobras Research ami Developmen t Center - Cidadc Universitaria - eE l' 219/0 - Rio de Janeiro. RJ.8 .8 . Brite-Neves - Federal University of Pernambuco - Geology Dept. eEl' 50.000 - Recife. PE.

Riassunto

Ab stract

Di agrammi delle isocrone R b-Sr su campioni interi di argille e shale possono indicare ,nonostante l'inquinarn ento detritico . l'et a approssimata della sedimentazione . Si assumeche Ie particelle fini si me scol an o cornplet arn ente nell' ambiente , producendo una composi­zio ne isotopica uniforme dello Sr nel campione totale . Occorre naturalmente una carnpio­natura adeguata .

Diverse scq uc nze precambriche del Br asil e sono state cos] ana lizzate e hanno fornito etacomprese tra 490 e 1330 Ma (eventi sedimentari e d iagenet ici); la documentazione pilicompleta 1: quell a del Gruppo Bambui (copertura del cratone) . Sono state ana lizzate , oltread altre unit a di co pe rtura, anche terreni di av ampaese di catene o ro geniche e de lle catenestesse. ottenendo sempre significa t ivi allinearn en ti di punti nei diagrammi pur con notevolidispersioni dovute probabilmente a originarie di fferenze nella natura e qu antita del mate­riale clas t ico. II metodo pu o quindi risult ar e importante nella datazione di rocce pelitichenon fossilifere come qu cll c degli scud i pr ecambriani .

P A RO L E OH.-\\"I-:: Geocronologia . mctodo Rb-Sr. scdimc nti pclitici. Precambriano . Brasilc . diagra mmi delle isocronc .

In many cases . in spite o f th e clear pr esen ce o f detritus . Rb-Sr isochron diagrams o f to talrock samples o f sha les ca n indicate the a ppro xima te age of sedimentation. In order toexplain this evidence . a model is assume d in which complete mechanical mixture of th e fineparticles occurs within the depositional environment . producing a uniform isotopic co mposi­tion of strontium in the total-rock samples of a rgillaceo us shale . Thus. if a n ade q uatesamplin g procedure is employ ed . in many cases the age of deposition can be defined withinreasonable tim e limits .

Several Precambrian sed ime nta ry sequences of Brazil were ana lyse d in thi s work . andmost results were reasonably int erpretable in terms of age o f se d ime nta tio n o r early di agen­es is . The Bambui G roup . a cra to n ic cover oc ur ring in th e ce ntra l portion o f th e SaoFr an cisco C rato n . is the o ne pr esenting th e most comple te geochrono log ica l pattern . Rb-Srisoch ron lines fo r to ta l-rock sa mples co llec te d in th e a re as o f Sao Fr an cisco . Itaca rambi .Formiga . Fe lixla nd ia and Pir ap ora (a ll in th e sta te o f Minas Gera is) were o b ta ined with anage ran ge o f SilO to 665 milli on yea rs.

Other sed ime nta ry formati on s showed R b-Sr isochron lin es of total-rock sa mples havinga ppa re nt ages co m pa tib le with th e known geolog ica l history. A mo ng th ese . the followingcan be mentioned:

1) among o the r cratonic co ve rs . the C ha pada Diarnantina Group of the Sao Fr anc iscoCra ton was dated o n its Caboclo Formation (a 1260 Ma d iagen eti c eve nt. and Guari­ba/L encoi s Formati on (a R55 Ma diagenetic eve nt). T he Acari Fo rma tio n o n the Amazo­ni an C ra to n was a lso dated . with a re sult o f 1330 Ma se d ime n ta tio n age:

2) severa l fo re la nd deposits of the fold ed belt s o f th e Bra sili an o Cy cle were a lso datedas foll ows : Es ta ncia G ro up o f th e Sergipe folded belt (490 Ma). Sc po tuba G roup (570 Ma)a nd Di am ant ino Group (660 Ma) of th e Paragu ay fo lded be lt. Ita ja i Grou p (530 and 545Ma) of th e Dom Feli cian o be lt. and J aibaras Group (560 Ma ) of the Borbore ma orogenicp rovince :

3) two units withi n th e la te Precam bri an fo lded be lts we re ana lysed . th e Pa ran oa Groupo f th e Brasili a be lt. with a ppa re nt ages o r anc hi to e pi -metamorp hic events of 600 and 680Ma . and the Coru mba Group of the Pa ra gu ay fo lded be lt. with a metamo rphic age of 490Ma .

In a ll th ese examples. a significa nt a lignme nt of the a na lytica l points was found in th eiso chron dia gr ams . with relevant dispersion indi cating probabl y or igina l differ en ce s in thenature and qu antity o f th e cla stic mat e rial. The autho rs suggest that the Rb -Sr isochronmethod is adequate to es ta b lish th e approxima te tim e o f se d ime nta tio n . and ca n be sp e-

253

u.o. Cordani, A . Thoma z-Filho. B.B. Brito-Neves, K. Kawashita

cially important for non-fossiliferous argillaceous shales, such as those occuring in thePrecambrian shields .

KEY WORDS: Geochrono logy. Rb-Sr meth od . sha les . Precam bian sediments. Brazil. isochron diagrams.

Introduction

In the last two decades , a growing inter­est has been shown by geochronologiststowards problems related to the radiomet­ric dating of sedimentary rocks . At first,the K-Ar and Rb-S r radiometric methodswere used on potassic authigenic minerals,such as glauconites and illites , with the aimof determining the age of sedimentation . Insuch cases, the chemical and isotopic proc­esses are reasonably known, and the ge­ochronological interpretation is very similarto that applied to igneous and metamorphicrocks.

Later, research was started on the ther­modynamic events which occurred duringthe diagenetic evolution of the sediments,using the K-Ar and Rb-Sr radiometric meth­ods on argillaceous minerals. It was veri­fied that isotopic homogenization couldtake place , according to the possibility ofchemical exchanges in the minerals, especiallythose of the montmorillonite group, bothwith the fluids of the depositional mediumand the percolating solutions which remainin the sediments until the late diageneticprocesses. If the chemical systems close toion-exchanges, with the corresponding in­crease in the crystallinity of the minerals,the basic conditions to operate as geologicalchronometers can be fulfilled , in severalcases.

Some implications, less obviously explain­ed , arose in many situations when, despitethe severe restriction imposed by the detri­tus incorporated into the shales , the Rb-Srwhole rock determinations, by isochrondiagrams, yielded results consistent withthose inferred by other geological methods ,in terms of the sedimentation age and/or ofthe incipient metamorphism of the rock(Compston and Pidgeon, 1962; Whitneyand Hurley , 1964; Allsopp and Kolbe ,1965; Faure and Kovach , 1969; Chaudhuriand Brookins , 1969; Bofinger and Gulson ,

254

1970; Mukhopadhyay , 1974). An attempt atexplaining the reasons accounting for theseapparently reliable results was presented byCordani et al . , 1978. These authors exami­ned several formations from the major sed­imentary cratonic basins of Brazil, mostlyfrom core samples supplied by Petrobras ofPaleozoic shales with adequate stratigraphiccontrol. The results were encouraging, andsome criteria for the interpretation of Rb­Sr results were established.

In this paper, originated within the sameresearch group, at the GeochronologicalResearch Center of the USP, a summary ofthe interpretative model is given , togetherwith a series of additional examples , inwhich the applicability of the method is fur­ther demonstrated. The results here presen­ted are mainly obtained in sedimentaryrocks associated to the late Precambrianorogenic belts of the Brasiliano cycle, or incratonic covers , in part equivalent in timeto the formations deposited within or near­by the orogenic belts. These rocks are non­fossiliferous, or their fossil content is gen­erally not adequate for a precise stratigraphiccontrol. Therefore, the results here obtainedcan be considered as good indications of theadequacy of the employed methodology, butnot as inequivocal age markers, because in­dependent paleontological or stratigraphicalcontrols for the apparent age results couldnot be established .

The mechanism of isotopichomogenization in sedimentaryrocks

The model which was suggested by Cor­dani et al. , (1978) envisaged the evidencealready observed in practice, by many pub­lished results, in which the Rb -Sr isochro­nic method exhibited apparent ages in ac-

Precambrian petites. Rb-Sr method

cordance with those of the sedimentationand/or diagenetic and epimetamorphicevents which acted upon the analysed argil­laceous sedimentary rocks.

An essential feature of the discussion isthe fact that, despite the strontium isotopiccomposition of the different mineral phasescould be very variable, different samples ofwhole rock of a fine grained sedimentaryunit show a strontium isotopic compositionapproximately uniform, at the moment ofsedimentation. Such a premise, if not fulfill­ed, would preclude results such as meaning­ful isochron ages, as referred above . Thus,the problem consists of a search for an ade­quate explanation for this uniform initialisotopic composition of strontium, and thesedimentary processes which could haveproduced the observed pattern.

When dealing with argillaceous sediments(pelites, shales, marls, and related rocks) ,two aspects must be considered: the viabil­ity of chemical exchanges between argil­laceous minerals and the surrounding mate­rials, including small detrital minerals, fluidphases and solutions (favourable to isotopichomogenization) and the greater difficultyof these exchanges between large clastic min­erai grains, such as feldspars and micas,especially important regarding Rb and Srchemical distribution in whole rock sam­ples . If, in the first case, isotopic homogeni­zation is easily explained, by the second thetendency to uniformity of strontium isotop­ic composition must be accounted for by adifferent way. In this context, Cordani etal., (1978) proposed the mechanism of«mechanical dispersion of the detrital mate­rial in the depositional basin, leading to auniform isotopic composition of strontiumfor whole-rock samples a few centimeters insize».

In general, in a depositional environmentfavourable to sedimentation of pelitic mate­rial , quiet and persistent sedimentationconditions prevail , followed by a relativelylong time of residence of the fine grainedparticles together with the fluids of the dep­ositional medium. Grains and mineralsfrom the most diverse sources could , in thismedium, mix homogeneously , whatevermechanism of sedimentation is concerned .In this case, despite of the presence of min­erals containing strontium with differentisotopic compositions, the sediment, as a

255

whole, could present a quite uniform stron­tium isotopic composition, if it is taken intoaccount that samples of total rock areusually collected with sizes in the centime­ters range, including, each one, billions ofparticles of size below 4 fJ-. In addition, sinceRb and Sr distribution is affected intensivelyby the chemical behaviour of these elementswithin the diagenetic environment, anddepends very much upon the local exchan­ges between clays and fluids, the apparentuniformization of the strontium isotopiccomposition does not necessarily imply asimilar effect in the distribution of the totalrubidium and total strontium content in thedifferent whole rock samples of the sedi­ment. Some analyses of recent sediments,as shown by Cordani et al. (1978), presenteda significant dispersion of the Rb /Sr ratios,in samples that otherwise were relatively ho­mogeneous regarding the isotopic composi­tion of its strontium (Dasch, 1969; Biscayeand Dasch , 1971; Hart and Tilton, 1966).

An adequate sampling is essential for thesuccess of the radiometric dating of peliticrocks. It is obvious that , prevailing the des­cribed processes of mechanical mixture ofthe detrital material, the smaller thedistances among the samples collected ,within the same sedimentary layer, thegreater will be the tendency of obtainingwhole-rock samples containing strontium ofan isotopic composition nearly uniform atthe time of sedimentation. The experiencethus far allows the present authors to sug­gest distances of samples-collecting of fewcentimeters in the vertical direction (alwayswithin the same depositional unit), and upto several meters in the horizontal direc­tion , along the same layer. The sampledarea should represent a small fraction com­pared to the much larger depositional envi­ronment of the formation to be dated. Tho­maz-Filho and Lima (1981) proposed ascheme considered adequate to sample col­lection of a sedimentary rock, for Rb-Srdating. (Fig. 1).

The dissimilar behavior of the mineralphases making up the sedimentary rock, asregards the chemical transformations un­dergone by the system, from sedimentationtime, through diagenetic and /or epimet­amorphic conditions , allowed Cordani et al.(1978) to establish some models of isochro­nic diagrams. It is important to note that

8 ~ '" C•

U.C. Cordani, A . Thomo z-Filho. B.B . Brito-Neves, K. Kawashita

those models were based in real data , o­btained on sedimentary rocks submitted tothermodynamic events of different intensi­ties, as controlled by the crystallinity of theillites. (Kubler , 1966). These models repre­sent different situations which concern sedi­mentary rocks, starting with those modifi­cations affecting only the clay fractions, upto those envolving more intense processes,within the field of metamorphism. In thelatter case , it was shown that strontium iso­topic homogenization occurs encompassingthe clay minerals, but also the coarser clas­tic materials , such as feldspars and micas(Fig. 2) .

Examples of Rb-Sr dating inbrazilian sedimentary and/or anchito epimetamorphic rocks

The Rb-Sr whole rock isochron methodin sedimentary formations was employedwith some success in the classical Paleozoicsedimentary basins of Brazil (Kawashita ,1972; Thornaz-Filho , 1976; Thomaz-Filho etal . •1976; and Cordani et al .. 1978). Howev­er. the method increases its importance.and its field of application, when applied tounfossiliferous Precambrian sedimentaryrocks. In this chapter , several examples ofRb-Sr dating of Brazilian Precambrian for­mations will be shown, represented by cra­tonic covers of diverse ages. as well as in­tradeeps, foredeeps and low grade metase­dimentary sequences from fold belts of theLate Proterozoic Brasiliano Cycle. The last­mentioned represents rocks that in someway were submitted to advanced diageneticstages and reached conditions in the begin­nings of metamorphism. Figure 3 brings thelocations of the sedimentary formations stud­ied in this work. in a geotectonic frame­work adapted from Cordani and Brito Ne­ves (1982). In this figure. the tectonic set­ting of the orogenic belts of the BrasilianoCycle. as well as their cratonic counter­parts. are clearly characterized .

The following examples include some re­interpreted data from work already publis­hed in the specialized literature. Becauseof this, and also because the employment ofdifferent values for the radioactive constants,the numerical values here presented are

256

not always in accordance with those in theoriginal papers. All the calculations in thiswork are based on the Rb-87 desintegrationconstant of 1.42 x lO-ll /year.

In the isochron diagrams that follows,some dispersion of the analytical pointsalong the isochron line (or errochron line)can be noted. This fact is accounted for bythe variations that occur as regards the uni­formization of the strontium isotopic com­position, during the process of mechanicalmixing , at the time of the sedimentation.The nature and amount of the coarser clas­tic material seem to be the major factorsbearing on the causes of this dispersion.However, a linear tendency is always clear,in the isochron diagrams, almost always in­dicating an age value consistent with theknown geological evidences and interpreta­tions.

Cratonic covers of various ages

During Mid and Late Proterozoic times,some areas of the current South AmericanPlatform already presented relatively stabletectonic conditions . In the central part ofthe Amazonian Craton. and over most ofthe area of the Sao Francisco Craton,extensive sedimentary and volcanic-sedi­mentary covers were originated in paraplat­form or reactivated platform conditions.Such covers. moderately folded. and nonmetamorphic. or affected by very slight met­amorphism. encompass long stages of sedi­mentation. Being unfossiliferous, isotopicanalysis is the sole possibility of their da­ting.

Normally , complete stratigraphic sec­tions. with clear-cut contact of top andbase for these sequences are lacking. Thethermodynamic conditions. as indicated bymeasurements of the crystallinity of theirillites. were almost always very mild. be­tween diagenesis and slight metamorphism.Some examples of Rb-Sr dating for theseunits are shown below.

Bambui Group - Central-Southern part ofthe Sao Francisco Craton

Among the Brazilian Precambrian sedi­mentary covers, the Bambui Group is in-

Precambrian pe tites. Rb -Sr method

~-Limestonel:=...::j-ShaleSco Franc isco River

'V 'f 'f SURFACE \t ~ Y 'f:c:::r: :r:::::r: .:r:::::c :r:::r: :r:::r: I~ ::r:::::::r :I=r: ::r:::r :r:::r: ~37 :c:::r::c:::r: :r:::::r: :r:=r::. = :r:::r: .:::r::::J: ::r:::::::r ::r=:c :r::::I .::c=c :r:::::r:: :r::::c

"" ......., I '4 "" "'" - "1

= .:r:::::c = = .:r:::::c = = ::r:::::::I:' = = = == = = = = = = = = = = == = = = = = = = = = = == :r:::r: = :c:::r:: ":h = .:r::=c ::r:::::I: :c:c "~ = == = = = = = = = = = = == = = = = = = = = = = =

1211 127 126 125 I - 2 4 '0

= = = = ...ur = = = = :r=. 021= ="204 "20 " 17 " 14 "ll "it- 50- - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - -. I~ - - 119 - - -16 - - - . 13_ - - .10 . - - . !.- - - 4i- - - - - - - - - - - - -- - - - - - - - - - - - - - - -ellA - -e.!..! - - _15 - - .1 2- - -19 - - 16- - -31

- - - - - - - - - - - - - - -= = = = = = = = = = = =IU 12 I I= = :e:::::t :I=t .:r::=c = :e:::::t = = = = :r:::r:

'4 30 meters ..o

10

Outcrop- of the Bambu/ Group- at Sao Fra'ncisco (MG) - Brazil

Fig. 1 - Samp ling example for Rb-Sr datin g of sedimentary rock s (Thomaz-Filho and Lima, 1981) .

MODEL ill

MODEL II (Diagenesis stage )Note : argillaceous rocks wit h a low

content of coarser detritus" .... ' b........-

c <A--·e~~

~~_A~.~ r •~

~'- Rb 1 7

Sr 16

MODELN I Anchi/ epimetamorphismstage)

Rb 87'-- - - -------------s;-..

~Sr 1 6

MODEL I ( Diagenesis stage )Note : argillaceous rocks containing signif icant

amount of coarser detr itus

Rb 17'----------------- Sr"

L- ~

Sr "I Diagenesis / anchi­

metamorphism stage)Sr 17

S;U

Isochron line for cloy f raction ( f ine fract ion - FF < 2 fl)-age of diagenesis and/or anchimetamorphism.Isochron line for total rock (TR) - sedimentation age

o Tota l rockn Cloy fraction (fine fraction )8 Acid- leached mater ial8 Residue of acid leaching

Fig. 2 - Rb-Sr isochron diagram models fo r argi llaceous sedimentary rocks (Co rdani et al .. 1978).

257

V.G. Cardoni. A . Thom az-Filho, B.B . Brito-Neves, K. Kawashita

Adopt ed from Cardoni and Brito Neve s ( 8 2)

ACARi

ALTO

PARANOA'

--<. [ G[ N D ---------------,

I - $QtfTH A"" ~lLEL.ATF~

CRATONIC AREA S AND SMAL L ER CRATONIC FRAG ME NTSOF THE B RASILIANO C YCL E

[:::;::1 1A~ AZQt.l ~A '" I ~o FAhClSCO 3 RIO DE L A PLATA». .... .. SAO LUIS 5 LUIS Alva

MO BIL E BElTS OF THE BRASIL/AN D CYCL e WI TH INTE RIOR ~AS:Srv[5

~' I ' _~_.H 6 TOCANTI NS PR(NINCE

I;: -..,. ': 7 8OR BOH EIol ' PROVINCE- , '4 8 Al Eif.IRA f S') AND [)(lP'O FELIClAHO IS " 18£l .rs

9 U"'OIl/IOEO BRASILlAN O ~lT S {51[ RRAS PA"",PEA,,,-AS I

ta-~CHAIN AN D MA IN RE MO BILIZ ED PREC AMBRIAN

rt":"iifl o } AYAPE e I RIO SA MBA/ IolAGOALE""

~ t JRtO "'A RA N~ d }A REOUlPA- OJ l CO

. l AWAS CALI[ NTE S

.IlL -SUS-ANDEAN BA SINS

't-.mj LL AP«>S(U, BENt I B), CHACOI C) .,B e p . PA... PAS IP )

Fig. 3 - Lo cation of the sedimentary formati on s studied in this work , with the indic ation o f the are as of sampling.

doubtly the mo st studied from the geochro­nological point of view . Ov erl ying a greatpart of the Sao Francisco Craton, where itis very slightly folded , the Bambui sedi­ments grade int o the marginal fold ed beltsof the Brasiliano Cycle. In th ese geot ecton­ic units , already beyond the boundaries ofth e Sao Francisco Craton, metasedimentsstratigraphically equivalent to the BambuiGroup are fold ed and regionally metamor­ph osed at gre en schist to amphibolite facies.

The isochron diagram s shown below re­fer to th e centra l portion of th e Sao Franci­sco Craton , where the Bambui Group com­prises mainly flat lying limestones , siltstoneand shales.

In 1976, Thornaz-Filho collected in thevicinity of Sao Fra ncisco, MG , in a singleand fairl y large outcrop , sa mples of shaleand limestone occuring in alternate layers(Fig . 1). Rb-Sr results , on whole rock sam-

258

pies, were obtained by Thomaz-Filho andBonhomme (1979). The same data (Table1) were re-interpreted by Thornaz-Filhoand Lima (1981) , whose diagram is herereported as Fig. 4. Th e scatter of the an­alytical points in the diagram is large, buta clear tendency for alignment is indicated,and the apparent Rb-Sr age is 665± 16 Ma,with initial (Sr 87/Sr 86

) ; of 0,7073 . These val­ues are somewhat lower than that figurescalcul ated by Thornaz-Filho and Bonham­me in the previou s paper (695± 12 Ma , and(Sr87/Sr86);= 0 .7077) , and the inte rpretationgiven is a minimum age for the sedimenta­tion of the Bambui Group , but probablyclose to the real value. This is supported byth e low initial ratio , compatible with Sr iso­topic equilibrium between the clay mineralsand sea water , in the deposition al environ­ment.

Parenti Couto et al. , (1982) exhibit the

Precambrian petites, Rb-Sr method

Tab. 1 - Rb-Sr Analytical Results of Whole Rock Samples of the Bam bui Group - Sao Francisco A rea - MinasGe rais (T homaz-Filho and Lima . 191\1) .

CE NPESROCK Rb Sr Sr 87 86 Rb87 ISr 86Sj\)!PLE I Srl ab. no . TYPE (ppm) (ppm)

14 ATB-l Limestone 9,4 117 4,4 0,7080!0,000 6 0, 02:!:0,01

22 ATB-8 Shale 161 ,2 57,0 0,7873!0,00 30 8 ,2 5! 0 ,23

23 ATB -9 " 156,3 55 , 1 0, 780 4+0, 000 8 8 ,27 ! 0 ,23

25 ATB-ll " 139,4 58, 4 0,7 737 ! 0, 0007 6 ,9 5~0 ,2 0

26 AT B- 12 " 162 , 7 53 , 1 0, 7955!0,0 013 8 ,94 ! 0 ,25

27 ATB-13 " 167 ,9 51 ,4 0,7839!0 ,O069 9,52 !0,27

28 ATB -14 " 145 ,7 53, 8 0,7712!0 ,OO12 7,89!0 ,22

29 ATB - 15 " 148 ,8 63, 0 0,77 68!0, OO07 6,88 ! 0 ,19

30 ATB-17 " 130,9 57 ,6 0 , 7740 !0 ,OO18 6 ,62! 0 ,19

31 ATB -1 8 " 15 7 ,6 68 ,7 0 , 763 5!0 ,O03 9 6,68! 0 ,19

32 ATB-18A " 11 7 , 1 64 , 7 0,7 585!0, 0011 5,26!0 ,15

33 AT B-19 " 14 7 , 3 49 ,5 0 ,7924! O, OO1 5 8 ,68! 0 ,25

34 ATB-19A " 150 ,4 51,5 0 ,7924!0 ,0 023 8,52!0 ,24

38 ATB-2 5 11 6 ,8 12 5,5 0,72 77! 0 , 0016 2,70!0 ,08

42 ATB- 29 Limest one 18,1 107 4,7 0, 70 95!O ,O0 25 O, 05!0,0 1

87 AT B-3 2 Shale 14 6,2 63, 6 0,77 41! 0, 00 31 6 ,70 ! 0 , 19

45 AT B- 33 " 14 5 , 6 70,1 0, 765 0! 0 ,0 013 6 ,O4! O, 17

48 ATB-37 Limes t one 22 , 9 1141, 2 O, 70 47!O, OO 25 O, O6!O ,0 1

results of Rb-Sr ana lyses in samples of silt­stone and sha le be longing to the Pirapor a,Tres Mari as and Paraopeb a Format ions, themai n unit s that make up the stra tigraphicsequence of the Bambui Group in the Itaca­rambi, Formiga, Fe lixlandia and Piraporaareas, in Minas Gerais.

For the younger Pirapor a Formation , theage of 577+32 million years (with an initia lSr87/Sr86 ratio=0.7120) was established,from analyses carried out in siltstones pre­senting no evidence of metamorphism (Ta­ble 2 and Fig. 5) . For the Tres Marias For­mation, stratigraphically inte rmediate be­tween the two other formations, the age

259

obtained was 620±38 million yea rs (with aninitial Sr87/Sr86 ratio=O.7125), for sam plesof arkosic siltstone (Table 3 and Fig . 6).The Paraopeba Formation, the oldes t one,and comprising also silts tones , yielded anage of 638± 12 million years, with a initialSr87/Sr86 ratio of 0.7109 (Table 4 and Fig.7).

In the three diagrams (Figs. 5, 6 and 7),the number of points is low, an in additionthe y are not perfectly aligned. Neverthe­less, the apparent ages are reasonable , inthe sense that they do not contradict theestablished stratjJgraphic sequence, and theobtained Sr87/Sr initial ratios, of the or-

--

V.G. Cordani, A . Thom az-Filho, B.B . Brito-Nev es, K. Kawashira

5r·875r-86

0 .7 90

AT~'1AlB . I,A 0

00A 8 - 19

ATB-e

0 .770

ATB- 'o

ATB -15

AT B-IbYB- I I AT8 . 14

/ 0ATB· 36

d AT8-1 8

4T B-:;/ 0

AT8 ·13o

0 .750

0 .730

T' 665 ± 16 rn.o.

Ri=O,7073 ± 0 ,0011

ATB ·290.710 /

. ATB - I

~OATB'37

oi6

Ito

Rb-87

5r-86

Fig. 4 - Rb-Sr isochron diagram for samples of the Bambui Group-Sao Francisco Area - MG (Thornaz-Filho andLima , 1981) .

Tab. 2 - Rb-Sr Analytical Results of Whole Rock Samples of the Bambui Group - Pirapora Formation (ParentiCouto et al.. 1982).

LAB ROCK Sr 87/Sr 86 Rb87/Sr 86N9 SAMPLE TYPE RB (P?M) SR (PPM)SPR

3889 AY-3 Siltstone 75 ,3 66 , S 0,7392::: 0 ,0009 3,27~0,0 7

3.8 90 AY -4 " ]59 ,6 57 ,6 0,7827::0 ,0017 8,05:0 ,19

4040 AY-6 " 180,6 44,4 0,8046:::0,0017 11, 81:0,27

3892 AY -10 " 55 , S 87 , S 0,7268:0,0008 1 ,84::0 ,04

260

Precambrian pelites, Rb-Sr method

51"-87~

0 .800

07~0

T=577±32m .o.

Ri=O,7120± 0,0019

Rb ·875r-86

12'06

0700-+------,--------,------,----__.-- ---, .......-_

o

Fig. 5 - Rb-Sr isochron diagram for samples of the Bambui Group-Tres Marias Formation (Parenti Couto et al .•1982).

der of 0.710-0.713 are realistic, in terms ofsedimentary sequences with a significantclastic component.

Parenti Couto et al., (1982), pointing outthe absence of metamorphism in the anal­ysed samples, ascribe the obtained ages tothe time of sedimentation of each forma­tion . One should note, however, that onlytotal rock samples were analysed, and thelack of data concerning the isotopic behav­ior of the argillaceous fraction, in each casedoes not allow to discard the hypothesisof relating these ages to diagenetic eventsoccurred after sedimentation. However, thereasonably low initial ratio, as well as theexistence of isochrones of argillaceous frac­tions (below 2[1-), with much younger ages,in different samples of the Bambui Groupfrom the same area (Bonhomme, 1976) areindications which make probable the inter­pretation favoured by Parenti Couto et al.,(1982), and endorsed by the present au­thors.

Chapada Diamantina Group - North­Eastern part oj the Sao Francisco Craton

The Caboclo Formation represents asiltic-argillaceous unit of the middle part ofthe Chapada Diamantina Group, outcrop­ping in central Bahia State, in the north-

eastern part of the Sao Francisco Craton .Brito Neves et al., (1979) analysed argilla­ceous siltstones (total rock and fine fractionsamples) of this formation (Table 5) collec­ted along the classical transverse profilebetween Mirassera and America Dourada.

The Rb-Sr isochron diagram of Fig. 8 in­cluded the analytical points, and the refer­ence isochron drawn yielded an apparentage of 1264±48 million years, with an ini­tial Sr87/Sr86 ratio of 0.7383. Since the totalrock samples and their fine fractions arereasonably aligned (Model IV in Figure 2),Brito Neves et al . , (1979) interpreted theobtained age as the time in which thermo­dynamic processes favoured the isotopic re­homogenization of strontium, in the field ofanchimetamorphism as indicated by someanalyses of crystallinity of the illites. Thehigh initial ratio seems to endorse this in­terpretation, and the obtained value, Mid­Proterozoic, shall be considered as a relia­ble minimum age for the sedimentation ofthe Caboclo Formation.

The Guariba/Lencois Formation , alsoanalysed by Brito Neves et al. , (1979), com­prises argillaceous siltstones and argillites thatoccur in the intermediate section of theMorro do Chapeu Formation , in the sameChapada Diamantina Group, but strati­graphically located above the Caboclo

261

UiG. Cordani, A . Thoma z-Filho, B.B. Brite-Neves, K. Kawashita

Tab. 3 - Rb -Sr Analytical Results of Whole Rock Samples of the Bambui Group - Tres Marias Formation (ParentiCouto 1'( al. 19~Q) .

LAB ROCKSr 87/Sr 86 Rb87/Sr 86N9 SN-IPLE TIPE Rb (ppm) Sr (ppm)

SPR

38 93 BX-3 S i lts t o ne 170,6 102,2 0,7563~0,0009 4,86~0,10

3 894 BX-8 II 137,9 3 85,2 0,7 221~0,001l 1,04~0,02

3-895 BX-9 II 153,5 133,7 0,7399~0,0012 3.33~0.07

389 6 BX-10 II 146,4 142,9 0.7 393~0,0008 2.98~0.06

Tab. 4 - Rb-Sr An alytical Result s of Whole Rock Samples of the Bambui Group - Paraopeba Form ation (ParentiCouto et a/ . . 19R2).

LAB ROCKN9 SAMPLE TYPE Rb (ppm) Sr (ppm) Sr 87/Sr 86 Rb

87/Sr 86SPR

3958 CY-1 Siltstone 132,4 53,3 0,7764~0.0014 7,24~0,14

3959 CY-4 II 48,6 182,1 0, 7l74~0 ,0017 0,77~0,02-

3960 CY-5 II 146,6 53,1 0,7844~0,0016 8,05~0,16

3961 CY-6 II 57,8 61,3 0,7373~0,O022 2.7tl~0,OS

Formation.The isochron diagram , drawn for four

samples of total rocks plus their respectivefine fractions below 2[J. (FF) , defined an ageof 856± 17 million years, with an initialSr~17/Sr86 ratio of 0.7222 (Table 6 and Fig.9). The analytical points are well aligned,and Brito Neves et al. , (1979) related theisochron line to an event in the field of an­chimetamorphism accounting for the isoto­pic re-homogenization of strontium. Thevalue of about 860 Ma shall be considered aminimum age for the time of sedimentationof the Guariba/Lencois Formation, andmost probably shall also be considered amaximum age value for the deposition ofthe Bambui Group in this part of the SaoFrancisco Craton , whose sediments (SalitreFormation) overlie the Chapada Diarnanti­na Group, without a significant and observ­able unconformity .

262

Acari Formation - Central part of theAmazonian Craton

The analyses performed in samples ofshales from the Acari Formation , a Prot­erozoic sedimentary cover of the AmazonianCraton, collected in cores from the 2-CA-l­AM well , drilled by Petrobras in the Mid­dle Amazon Basin, yielded an isochronicage of 1330±32 million years, with an ini­tial Sr87/Sr86 ratio of 0.7129 (Table 7 andFig . 10). Most probably, this age is relatedto a tectonic event of significant intensity,producing anchimetamorphic conditions inthe studied sequence. Fine fractions werenot analysed , and the analytical points inFig . 10 are not ade~uate to control thevalue of the initial Sr8 /Sr8fi ratio. Neverthe­less, the alignment is remarkable , and theapparent age value of about 1330 Ma,Middle Proterozoic , can be considered as areliable minimum age for the sedimentationof the Acari Formation .

Precambrian petites. Rb-Sr method

Sr- 87Sr- 86

0 .7!10

0 .730

T = 620 1" :38 m. o.

Ri = 0,7125 :t 0,0017

0 .7 10

Rb-B7

Sr - 86

4

Fig. 6 - Rb-Sr isoch ron diagram for sa mples of the Bambui Group-Paraopeba Formation (Parent i Couto et al ..1982).

5r-87Sr - 86

0 . 7 80

0 .760

0 .7 40

0720

C "~/C"l/o

T = 6:3B! 12 m.o .

Ri : 0,7109! 0,0009

Rb-B 7

Sr -86o 70 0 + -,- -,..-- ,.- -.-- ...,.... _

o 4 6 8 10

Fig. 7 - Rb-Sr isochron diagram for samples of thc Bambui Group-Paraopcba Formation (Pa renti Couto et al .•1982).

263

U.C . Cart/ani. A . Thoma z-Filh o, B.B . Brito-Neves, K. Kawashita

Tah . 5 - Rb-Sr Analytical Results of Samples of the Caboclo Formation (B rito Neves et al .. 1979) .

LAB ROCK Rb S1' S1'87/ S1'86 Rb87/S1'86Nt? SN1PLE TYPE MAT ERIAL (p p m) (ppm)SPR

1511 CAB-l ar g .FF 29 ,6 120 ,3 0 ,7500 :0 ,0013 0,71 5: 0,0 2sil t.

1744 CAB-l " TR 28 , 7 49 ,2 0 , 7687:0, 0010 1,70 !0,03

151 2 CAB-2 " FF 27 , 0 83 ,5 0 ,75 00! 0 , 009 5 0,94 ! 0 , 03-1745 CAB-2 " TR 23, 0 32 , 7 0,7741=0,00 13 2 ,0 4 ! 0, 04

15 15 CAB-7 " FF 116 ,9 32 ,70 0 ,9 110 ! 0 ,0 010 10,5 5 : 0,301

1916 CAB- 7 " TR 80 ,2 17 ,90 0 ,9724 :0,0013 14,1 7 ::0 ,38

1516 CAB -8 " FF 11 7,7 55, 80 0 , 84 59::0, 00 13 6 , 19 : 0, 176

191 7 CAB- 8 " TR 52 ,9 17 ,30 0 , 901 4:0,00 09 9 ,02 !0,1 8

151 7 CAB-9 " FF 92 , 1 64 ,40 0, 8230:0, 0010 4, 19 !0 ,1l9

1761 CAB -9 " TR 49, 6 24 ,00 0 , 8587! 0 , 0017 6 , 07 ! 0 , 12

0 - Tol a l roc k

n - Cla y fraclion (FF <2fl )

T • 1264148 m.o .

R/: 0,7383 t 0,002~

CAB-lI / .cV_

/

P-CAB ••

CAB'lI

CAB -CA&-l .

o

CAB-e

0'1I

O,e

1.00

Sr-875r-86

Rb-87

Sr - 86o 10

Fig. 8 - Rb -Sr isoch ron diagram for samples of thc Cahoclo For mat ion (Brito Neves et III. , 1( 79) .

264

Precambrian petites, Rb-Sr method

Tab . 6 - R b-Sr Analytical Resul ts of Whole Rock Samples of the G uar iba/Lc ncois Form ation (Brito Neves et al.,1979).

LAB ROCKSr 87/ Sr 86 Rb87/S r 86N9 SJ\,\IPLE TYP E MATERIAL Rb (ppm) Sr (ppm)

SPR

1501 GUAB-1 arg .silt FF 263 , 5 40 9,4 0,7450:!:0,0 001 1, 87:!:0,004

17 57 GUAB - 1 " TR 20 6 , 7 262,7 0,7503:!:0,0 01 0 2 , 24 :!: 0 ,0 5

1502 GUAB -2 " FF 147,1 498,3 0, 7320:!:0,001 8 0, 86:!:0,02

1753 GUAB -2 " TR 108 , 9 161, 4 0 ,745(,:!:0 ,0014 1 ,96:!:0 ,04

1513 GUAB -3 " FF 262 , 7 367, 1 0, 7490:!: 0, 0011 2 ,08 :!:0 , 04

1759 GUAB -3 " TR 199 ,8 212 ,1 0,7 557:!:0,0 01l 2 ,74:!:0 ,05

1514 GUAB- 4 " FF 225 , 0 115, 2 0, 7920:!:0,0010 5 ,70 :!:0 , 01

1760 GUAB-4 " TR 150,8 128 , 5 0,7 628:!:0,0010 3 ,4 1:!: 0 ,0 7

o - Tota l rock

(1 - Cloy fraction ( FF < 2 u )

T : 856 .t 17 m .o.

Ri : 0,7222 :t O,OOO!5

Rb - 8 75r - 86

0.7 ~0

0 .770

0 . 7

0 .79

Sr-87

Sr-56

2 3

Fig. 9 - Rb-Sr isochron diagram for samples of the Guariba I Lencois Formation (Brito Neves et al.. I!Ii! /).

265CJ_ . _,AS - U SP

8181.1 0TECA -

U.C. Cordani, A . Thomaz-Filho, B.B. Brito-Neves, K. Kawashita

Tab. 7 - Rb-Sr Analytical Results of Whole Rock Samples of the Acari Formation.

SAM- ROCK Sr 87;Sr 86 Rb87;Sr 86PLE SAMPLE TYPE Rb (ppm) Sr (ppm)Nt?

1 AT-CA-995 Shale 130,0 56,3 0,8406:0,0007 6,770:0,189

2 I " ]03 ,2 56,9 0,8147:0,0011 5,305:0 ,148IAT- CA- 996

3 AT-CA-105e " 127,4 41,0 0,8884:0,0008 9,153:0 ,253

4 AT-CA-105 " 141,0 51,S 0,8663:0 ,0013 8, 048: 0,229

Sr- 87Sr- 86

O.'~O

o.e

0.7 ~

/'~"~o AT'CA "0~3

T: 1330 ± 32 m.o.

Ri : 0,7129 ±0,0031

Rb -87Sr - B6

0700-t----r---.---,-----.---.,---..----.,..---r------r----r--~-2 6 9 10 I I

Fig. !O - Rb-Sr isochron diagram for samples of the Acari Formation .

In tr amontane and/o r fore land basi nsof the Brasiliano Cycle

The intramo ntane and the foreland ba­sins of the Brasiliano orogeny are sedimen­tation basins located near the borders ofthe cratonic nuclei (S. Francisco , Amazo­nian, Luis Alves) with the folded systems ofthe Braziliano Cycle . The litho-stratigraph­ic filling of these bas ins may be penecon-

266

tempora neous and can be corre lated withsome of the sequences of the marginal fold­ing syste ms surrounding the cratons.

In genera l, these post-orogenic units pres­ent thermodynamic cha racteristics alreadyin the field of anchi-epimetamorphism; andtheir apparent ages are among the youngestevent re lated to the Brasiliano orogenicCycle, in each of the considered regions .

Precambrian pelites, Rb-Sr method

Estancia Gro up - Sao Francisco Craton /Borborema Province

This Group comprises unfossiliferous sed­ime nts occur ing in Ser gipe and BahiaSta tes of Brazil, and consider ed mola ssic asregards to the Brazilian Cycle , This unit islocat ed overl ying the northeastern cornerof the Sao Franc isco Cra to n, in a regionadjacent to the Sergipean folded belt , thesouthernmos t orogenic un it which belongsto the Borbor em a Province ,

Brito Neves et al. (1977) analysed fiveargillaceous siltsto nes collected in theLagarto area, State of Sergipe, as total rocksyste ms (T R) an d argillaceous fractions(FF<2p.). T he illite crysta llinity index (Ku­bier, 1966) indic ated that the Estancia For­mat ion was subjecte d to conditions withinthe field of anchi to epimetamorphism .

The points wer e aligned along an iso­chron line that defined the age of 489±8millio n yea rs, with an initia l Sr87/Sr86 ratioof 0.710 1. (Table 8 and Fig . 11).

The poi nts of the total rock samples, aswell as the fine fractions, plotted close tothe best fit line , indicating the conditionsfor Model IV of Figure 2. The age value ,close to 490 Ma , shall be considered a met­amorphic age, and a minim um value for theage of sed imentation. However , becauseof the relati vely low value for the initialSr87/Sr86 ra tio , close to 0.710, it can also beconsidered significant regarding sedimenta­tion time. bot h events (deposition andstrong diagenesis /epimetamorphism) havingoccurred probably in a short time interval.

Alto Paraguay Group/A mazonian Craton /Paraguay-A raguaia Belt.

The A lto Paraguay Group is located inthe fore land basin of the same name , in thesouthern border of the Amazonian Craton.Almeida (1964) identified th ree litho-strati­graphic un its (Raizarna , Sepotuba and Dia­mantino Formations) , moderately folded,with decreasing deformations and thicknes­ses from the orogenic belt to the cratonicnucleus . They ar e made up by detrital sed­iments. with sparse remains of volcanicmaterial; and they are considered to bepost-orogenic foreland basi n deposits in res­pect to the Paraguay-Araguaia folded be lt,which belongs to the Brasiliano orogenic

267

Cycle.Cordan i et al. (1977) analysed four sam­

ples of tot al rock sha les of the Sep otubaFormation , collected in the same outcrop.Th e Rb-Sr isochro n diagram indicated anage of 569± 20 million years (with an initialSr87/Sr86 ratio of 0,7109), with an exce llentalignment of the analytical points. (Ta ble 9and Fig. 12).

The illite crystallin ity index (Kubler,1966) indicated an anchime tamorphic stageof the rock and thus the indicated age shallbe considered as belonging to this event ,subsequent to sedimentation of the sedi­mentary unit.

The Diamantino Formation occurs in thecentral-eastern part of Mato Grosso State,Lithologically, it comprises interbeddedsha les, siltsto nes and sandstones, and re pre­sents the younger for ma tio n of the A ltoParaguay Group, in the sout hern border ofthe Amazonian Craton. Its lower contact isconcordant and trasitional to the shales ofthe Sepotuba Formation.

Several total rock samples of shale wereanalysed , and the obtained age resulted660±60 million years, with an init ialSr87/Sr86 ratio of 0.7082 (Table 10 and Fig .13). The scatter of the analytical poin ts inthe isochron diagram is considerable andthe age value shall be considered as an ap­proximation of the sedi men tation age forthe Diamantino Formation. Taken togeth­er, the examples of the Sepotuba andDiamantino formations may serve as agood indication for the age and evolution ofthe Alto Paraguay Group . Th e total rocksof the Diamantino sha les may be related tothe sedimentation event (around 600 MA ,or slightl y older) , and the anc himetamor­phic samples of the Sepotuba Formationpossibly indicate the age of the diagen et­ic/metamorphic event, at about 570 Ma.

ltajai Group - Luis Alves Craton / DomFeliciano Belt

Th e Itajai Group occurs in the northeas­tern part of the Santa Catarina State , nearBlum enau , in the northern border of theLuis Alves Craton . Lithologically it comprisessiltstones, sandstones , conglomerates,as well as frequent tuffaceous levels. Aciddikes and other shallow intrusions cutthrough the sediments. The illite crystalli-

u.o. Cordani, A . Thomaz·Filho, B.B . Brito-Neves, K. Kawashita

nity indexes indicate a thermodinamic evo­lution already in the anchi to epimetamor­phism stage.

Macedo et al., (1982) analysed severalsamples from two outcrops located in thevicinity of Blumenau and Apiuna , approxi­mately 20 km apart. The samples are verylow grade slates , which were analysed intotal rock (TR) and argillaceous fraction(FF<2[J-), in the geochronology laboratoriesos Strasbourg (France) and the Sao PauloUniversity . For the Blumenau (VelhaGrande) samples, the apparent age obtain­ed was 529± 12 million years , with an ini­tial Sr87/Sr86 ratio of 0.7173 (T able 11 andFig. 14). For the Apiuna samples, the agewas 545±21 million years, with an initialSr87/Sr86 ratio of 0.7173 (Table 12 and Fig.15).

Both ages, with concordant values, with­in experimental errors, correspond to te­ctonic events within the epimetamorphicfield, most probably related to one of thelast tectonic phases of the Brasiliano Cycle.In both diagrams, there is some scatter ofthe analytical points, but the indicated va­lues, calculated through isochron lineswhich include the points of the fine frac­tions, are concordant and shall be consid­ered as geologically significant. Taking intoaccount the anchimetamorphic character ofthe analysed samples , once more the inter­pretation shall be made employing modelIV of Fig. 2, and an age of about 535 MAshall be attributed to a deformational eventaffecting the Itajarf Group.

Jaibaras Group - Borborema Province

The Jaibaras Group corresponds to anintramontane molassic sequence which oc­cupies grabens and semi-grabens within theMedic Coreau Folding System, in the north­western part of Ceara State.

Radiometric dating of this unit , perfor­med by Garcez Novais et al . , (1979), wascarried out on samples of the Pacuja Forma­tion, collected in the vicinity of the cities ofSobral and Granja. Lithologically, the for­mation comprises, in that area , fine silticand argillaceous sandsto nes.

In spite of the coarse character of theanalysed material, which makes this casevery unfavourable for Rb isochron dating, asignificant isochron line could be obtained,

268

including points representative of the finefractions, total rocks, as well as residualmaterials to acid leaching (Table 13).

Fig. 16 is a re-interpretation of the dataobtained by Garcez Novais et al. (1979).The isochron line with the apparent age of561±24 Ma , and (Sr87/Sr86)i=O .7101 wastraced, close to the analytical points ofmost samples. The scatter shall be attribu­ted to initial differences in the Sr87/Sr86 ra­tios , and this fact is well observed by tra­cing the internal isochron through the poin­ts of sample PAC-19. In this case, the agevalue is not significantly different from 560Ma , but the initial ratio is lower. In anycase, the formation is anchirnetamorphic,and the observed age is related to one ofthe last deformational phases of the Brasi­liano Cycle , in Ceara State.

Marginal folded belts of theBrasiliano Cycle

The marginal fold systems occur aroundthe syn-Brazilian cratonic cores. They arecharacterized by litho-stratigraphic fillingstypical of simple transgressive-regressiveunits, where a pelitic carbonaceous se­quence of pre-inversion stands out (typicalmiogeosynclines) as the huskiest.

Some dating in rocks of these holomorphi­cally folded belts represent events of a low­degree thermodynamic transformation .

Paranoa Group - Vazante and Paracatuareas (Minas Gerais) 1 Brasilia Folded Belt

The samples collected in the mentionedareas correspond to lithotypes strongly fold­ed and slightly metamorphosed up to thegreenschist facies that occur in the foldedbelts near the borders of the Sao FranciscoCraton, and that can be characterized aspart of the marginal folding system.

Amaral and Kawashita (1967) dated pelit­ic samples collected in the Vazante-MGarea, containing illite as the prevailing min­eral, showing evidences of having reachedanchimetamorphic stages.

The obtained age, 598±23 million ~ears,

taking into account the high initial Sr 7/Sr86

ratio (0.8225), has to be related to the ther­modynamic events that affected the' rock

Precam brian petites, Rb-Sr method

Tab. 8 - Rb-Sr An alytical Results of Wh ole Rock Samples of the Estancia Group - Laga rto Formation (Brito'''CH'' ct 1//. . 1(77) .

LAB ROCK Sr 87/Sr 86 Rb87/ Sr 86Nt? SAMPLE TYPE ~lATER IAL Rb(ppm) Sr (ppm)SP R

1396 EST-SA Si lt - FF 446,5 50, 6 0, 8968~0 , 0016 26 , 0 ~ 0 , 5 1s to ne

18 11 EST-SA 11 TR 2 74 , 8 6 8 , 9 0, 7 9 2 2 ~ o, o o 0 5 1l ,6 ~ 0 ,2 3

1 81 2 EST- 5B 11 TR 459 , 9 47,4 0, 90 9 8~ 0, 0 0 1 2 28 ,6~0 , 5 6

1398 EST-5C 11 FF 166,9 101,7 0, 7411~0 , 0 010 4 ,8~ 0 , l o

1813 EST- 5C 11 TR 96,2 87,2 0, 7 327~0,00 1 S 3 ,2~0, 0 6

13 81 EST-5D 11 FF 37 9 , 1 51,5 0, 8578~0,00 2 o 2 1,6~0 ,4 3

1 814 EST-5D 11 TR 2 52 , 5 74,8 0,7 786!0,0014 9, 8!o ,2 o

1 400 EST-5E 11 FF 372,5 55,3 0, 845 7~0,002o 1 9 , 8! 0, 39

HO 1 EST-5G Sand- FF 142,5 72,9 0,7468~0,002o 5,7~0,1lstone

1816 EST-5G 11 TR 57,6 92,4 0, 7241~0,0009 1, 8~0,04

5r -875r -86

1.000

0 .900

oeoo~5A

/50,~.c-

ES T 5G

/U

5A

o Tot aI ro ck

0. Clay f raction (FF < 2 J.l )T = 489:t 8m.c.

Ri= 0,7101 ± 0,0010

Rb-875r -86

0 .7 00 t-----,----,----.----,-----,------,------,------,------,------,------,-----,-10 20 30 50 60

Fig. 11 - Rb-Sr isochron d iag ra m lo r samples 01 the Estancia G ro up-Lagarto Formation (Brito Neves et et.. 1977) .

269

U.C. Cordani, A , Thoma z-Filho, B.B. Brito-Neves, K. Kawashita

Tab. 9 - Rb-Sr A na lytica l Resul ts of Whole Rock Sa mples of the Sepo tuba Form ation (Co rda ni et al .• 1977 ).

LAB ROCKN9 SAMPLE TYP E Rb (ppm) Sr(ppm) Sr 87/Sr 86 Rb87/Sr 86SPR

1668 CB-62A-RT Sha le 168 ,5 56, 6 0, 78 13 ~ 0 , 0021 8,68~ 0 ,24

2159 CS-88-RT Siltstone 55,0 127,0 0, 72 1 2 ~ 0 , 0024 1 ,2 6~0 , 04

2160 CS-9 1 " (j 9 , 0 107,0 0 , 72 38!0 ,0010 1, 60!0 , 05

2 16 1 CS -96 " 66 , 0 109,0 0 ,725 Z!0 ,0007 1 ,76!0 ,OS

Sr- 87Sr- 86

o

0 .a<?0

0 .71C8-62 A

o

T : ~69 t 20 rn.a .Ri: 0 ,7109 t 0,0001

Rb-87Sr -86

0 .70 01-/-- - - - - - - ,-- - - - - - .....,.-- - - - - - - ,..-- - - - - - ---.- - - - - -

10

Fig . / 2 - Rb-Sr isochro n d iagra m for samples of the Sep otuba Formation (Co rda ni et al .. 1977 ).

270

Precambrian petites. Rb-Sr method

Tab. /0 - Rb -Sr Analytical Results of Whole Rock Samples of the Diam antino Formation.

LA DSr 87/Sr 86 Rb

87/Sr 86N9 SN-IP LE ROCK TYPE Rb(ppm) Sr (ppm)SPR

6164 7 9 -~1B- 1 2 7 Sh a l e 105,1 117,5 0,7301:0,0010 2,594:0,073

6165 7 9-~m-131 " 152,3 99,4 0,7535:0,0013 4,454:0,1 25

6166 79-MB- 132 " 116,1 104,8 0,7421~0,0014 3 , 217: 0, 091

6167 79-~m-133 " 164,3 100,9 0,757 6:0, 0007 4,736:0, 133

6168 79-MB- 136 " 131,2 104,0 0,741 8:0,0010 3,66 3: 0 , 103

616 9 79-MB-139 " 171,5 94,1 0,7627:0,001 2 5 ,303~0 , 1 4 9

6172 79-MB- 1l9 " 149,0 48,8 0, 781 5~0 ,0010 8,9 01: 0,2 50

6173 79-~m-120 " 115,8 68,3 0,7515~0,001l 4,928:0,139

6174 79-MB-I21 " 169,6 48,0 0, 8075~0,0015 10,327:0,289

617 5 79-~1B-128 " 153,6 83 , 1 0,7532~0,0014 5,373:0,151

0 .82

0 7 8 0

0 .740

79-M8 ·139

7~"Bn3 0o 79 -"8 '128

79~J331 0 0~ 79 ·MB ·120

o79 -"B -136

T = 660:!: 60 m.aR = O,70B2.±O,0040

Rb-B7Sr -B6

'060 700+ - - -,--- - --,- - - .-- - .-- - -,--- - -,- - --.- - - .--- - ,--- - --,-- - -,-- - -

Fig. / 3 - Rb-Sr isochron diagram for samples of the Diaman tino For mation.

271

V .G. Corda IIi. A . Thomaz-Filho, B.B. Brito-Neves, K. Kawashita

Tab. IJ - Rb-Sr An alytical Result s o f Slates of the Itajaf Group, Velh a Gra nde Area -SC (Macedo et af.. 1982) .

LA BSr 87/Sr 86 Rb87/S r 86

N9 SAMPLE MATERIAL Rb (ppm) Sr (ppm)SPR

4055 78-HB-201 TR 210 , 2 103,5 0, 7621~0 ,0014 5, 91:0,12

4232 202 TR 20 7 , 5 110,1 0,7576~0,0018 5,48~0,11

423 3 203 FF :, 35 , 0 92,7 0,79670:':0,00016 10,471:0,31

203 TR 219,3 106,5 0,7615:':0,0015 5,99:0,12

4234 20 4 TR 20 5 , 5 111,9 0,7557:0,0019 5,34:0,11

42 35 205 FF 326,8 86,9 0,79982:0,00030 10, 902:0, 33

205 TR 193,80 113,1 0,7570:0,0013 4,98:0,10

4236 206 TR 200,7 116,6 0,7547~0,0010 5,00:0,10

4237 207 TR 217,5 106,3 0,7605:0,0021 5,95:0,12

4057 208 TR 197,4 113,4 0,7554~0,0019 5,06:0,10

4238 209 TR 202,1 121,4 0,7546:0,0013 4,84~0,10

40 58 210 FF 336,9 85,3 0,80369:0,00016 11,446:0,34

210 TR 211,6 108,9 0,7603:0,0016 5,66:0,11

5,-875,-86

0800

07BO

07

01

o Total rock

a Cloy fraction (FF<2fl)

T: ~29.± 12 m.n.

Ri: O,7173! 0,0011

Rb-875,-86

II'090 .700+---~--,----"----,.------r---..,-----r---...---.----r----,r---­

o

Fig. 14 - Rb-Sr isochro n diagram for sa mples of the Itajaf Group. Velh a G ra nde A rea -SC (Mace do et lit .. 11) :-;2).

272

Precambr ian petites, Rb-Sr method

Tab. 12 - Rb-Sr An alytical Results on Slates of the Itajai Group - Apiuna Area - SE (Macedo et al., 1982).

LABSr 87/Sr 86 Rb87/Sr 86N9 SAMPLE MATERIAL Rb(ppm) Sr (ppm)

SPR

4059 78-MB- 212 FF 320,9 56,1 0,84508~0,00029 16,577~0,49

212 TR 197,7 69,S 0,7830 ~O ,0011 8, 30~0 ,16

4239 213 TR 192,4 70,8 0,7797 ~O ,0021 7, 92~0,16

4240 214 TR 197,8 71,7 0,7 8 0 2~0, 0 01 3 8 , 04 ~ 0 , 1 6

4060 215 TR 191, 4 73,7 O,7 816~0,0015 7, 57~O ,15

406 1 .216 FF 331,5 53, 3 0,8553~0,0003 18,040~0 ,54

216 TR 224,8 67,3 0,7949~0,0033 9,75~0,19

4241 217 FF 316,7 58,7 0,83848~0,00043 15, 650~ 0 , 47

21 7 TR 180,2 81,8 O,7644~O,OOll 6,41~O,13

4242 218 TR 207,7 72,2 O,7804~O,OOll 8,3 9~O,17

4243 219 TR 194,0 77,0 O,7730~O,OO09 7 ,3 4'!: 0 , 15

4062 220 TR 178,6 90,S 0,7638~O,O022 5,75~O,ll

o Tot al rock

[1 Clay Fr actio n (FF<2f..r)T :545+21m .o.

RI : 0,7173 ± 0,0026

0 8

07~

0 .8

5, -875,-86

Rb-875 , -86

o 10 12 16 18 20

Fig. 15 - Rb-Sr isochron diagram for samples of the Itajai Group-Apiuna Area-SC (Macedo et al., 1982).

273

V.G. Cordani . A . Thomaz-Filho , B.B . Brito-Neves , K. Kawashita

Tab. /3 - Rb-Sr Analytical Results of Samples of the Jaibaras Group - Pacuja Formation (Garcez Novais et al.,1979) .

LAB ROCK Sr S7/Sr S6 RbS7/Sr S6:\0 SAMP LE TYPE MATERIAL Rb (ppm) Sr(ppm)SPR

3277 PAC- IS Fi ne TR 68,S 116, 6 0,721 9:!:0,000s 1, 700:!:0,003sand-st one

342L PAC-IS " FF 134,1 115,7 0,7369:!:0,0016 3,37 :!:0,01

-SS2 :> PAC-IS 11 RES 65,S 111,1 0,7231~0,001s 1,71 :!:0,03

31 61 PAC-1 6 si l t . TR I S,S 71,4 0 , 71 37:!:0 ,001 6 0,64 :!:0,01sa nd .

34 23 PAC-1 6 11 FF 23,1 SO,S 0, Sl S6:!:0,000S 0,S3 :!:0,02

3162 PAC-1 9 si lt - TR 14 3,3 10 6, 9 0, 736S:!:C , 0014 3,S9 :!:O,O Ssto ne

34 24 PAC-19 11 FF 160,0 SS,S 0,7719:!:0,0019 7,93 :!:0,16

3S22 PAC-19 11 RES 14S,4 9S, 2 0,74 33:!:0,001s 4,39 :!:0,09

3S00 PAC-2O s i 1 t . TR 67 ,S 12S,2 0 ,724S:!:0,0013 1,53 :!:0,03sand.

380 3 PAC-2O " FF 157,3 107,4 0,7474:!:0,0010 4,26 :!:O,OS

380 J. PAC-21 " TR 61,10 121,9 0, 72s 3:!:0 ,0010 1,45 :!:0,03

3804 PAC-21 11 FF 162,3 108,6 0,74s9:!:O,OO08 4 ,34 :!:O,09

380 2 PAC-2 2 " TR S4,9 174,1 0, 721 7:!:0 ,0009 1,41 :!:0,03

3805 PAC-22 11 FF 211,6 127, 3 0,746s:!:0 ,0008 4,83 :!:0,10

3572 PAC-23 " TR 109,40 165,3 0, 7237:!:0 , 0014 1,92 :!:0,04

357 4 PAC-23 " FF 227,90 9S,6 0 ,76 80:!:0,OO13 . Ii ; 73 :!:0,13

35 73 PAC-24 " TR 69,40 137,6 O,7234:!:O,OO17 1 ,46 :!:0,0335 75 PAC-24 " FF 170,00 95,7 0,7508:!: ? 5,17 :!:O,10

during th e evo lution of th e Br asilian o Oro­genic Cycle (Table 14 and Fig. 17), andmay correspond to th e main regional meta­morphism whi ch affecte d th e BrasiliaFolded Belt.

Par enti Couto et al . , ( 1982) analysedsamples of th e Par an oa Group , collecte d inthe Paracatu-M G ar ea . Th e samples exhib­ited clear ev ide nces of havin g undergonerelatively int en se th ermod ynamic eve nts,being silts to nes sho wing obvious tectoni ccleavage . Fo r these sa mples th e age of68 1± 10 million years was obta ine d (with aninit ial Sr~n/S r86 ra tio of 0.7255), int erpretedas a co mplete isot opi c re -homogeni zation

274

which took place during the met am orphism(Table 15 and Fig. 18) .

Corum ba Group - Tam engo Formation(Mato Grosso) / Paraguay- Aragua ia FoldedBeh .

The Tamengo Formation makes up theupper part of th e Corurnba Group , whichocc urs in th e western a rea of Mat o Grossodo Sui State. It co mprises limest on es withint erbedded layers of cla stic sedime nts,wh ere ferriferous siltsto nes fro m th e Par a­guay -Araguaia Folded System pred ominate .

Fugita (1979) ana lysed 4 sa mples of silt-

Precamb rian petites. Rb-Sr method

5r-875r-86

0 .111

0 .7

o

o Total rock

Q Clay fraction{FF< zu )o Acid leaching residueT: 561 ! 24 m . o.

Ri: O,7101!: 0,0010

Rb-87Sr -86

10,842o

o 700j1----,.- - --,.- ---.---,------,-----.---.----;------,-----r---

Fig. 16 - Rb-Sr isochron diagram for samples of tbe Jaibar as Group-Pacuja Formation (Ga rcez Novais <'I al . , 1979).

stones , in total rock samples, and obtaineda Rb-Sr isochron age of 489 ±29 millionyea rs, with an initi al Sr~o/Sr~o ratio of0,7144 (Table 16 and Fig. 19) . This agesha ll correspond to that of late thermo­dynamic events within the Brasiliano orogen­ic Cycle, which affected the rock lon gafte r its sedime ntation.

Conclusions

The examples described in thi s paper ,even if they can not be unequivocally com­proved by independent evidences, seem toenhance the potentiality of the Rb-Sr whol erock method as applied to pelitic rocks.Most of them were re lated to seque nces ofthe Brasilian o Cycle (ages be twee n 700 and450 Ma) , and the age va lues res ulting fro mthe interpre ta tion of the isochron diagram ssee med to be always ve ry reasona ble, inte rms of the known geological sett ing, theobserved stra tigraphic column , and the pre-

diet ed seque nce of geologic events, in eac hof the sampled regions . The other result s,obtained in old er formations , such as thecratonic cove rs of the Sao Francisco andAmazonian craton s, were also very reason ­able, and there ar e no means to considerthem as subjec ted to any suspicion.

Many of the studied samples were of pel it­ic rock s which had already suffered anch i­or epi-me tamorphic conditio ns. In all th esecases, th e isochron diagrams exhibitedalignments of gene ra lly good qu ality, indic­ating th at a fairl y complete Sr isotopic ho­mogenization was reached within the mate­rial, despit e the presence of even largeam ounts of sma ll clastic grains of differentmin eral s ph ases, which must have participa­ted of the gene ra l chemica l exc ha nges. Thi senha nces the validity of Model IV , as pre­sented by Cordani et al. (1978) and repro­du ced in this paper as Figure 2.

O ne other fea ture which shall be mention­ed is the fair ly low values for the Sr initialrati os, obtai ned in the isochro n diagram s,gene ra lly be twee n 0.708 and 0.720. Th ey

275

V.G. Cordani, A . Thomaz-Filho , B.B . Brito-Neves, K. Kawashita

Tab. 14 - Rb-Sr An alytical Result s of Whole Rock Samples of the Paranoa Group - Vazant e Are a - Minas Gerais(A maral and Kawashita . 19(7) .

LAB ROCKN9 SAMPLE Sr 87/Sr 86 Rb87Sr 86SPR TYPE Rb( ppr.l) Sr (ppm)

83 A-4 Sha le 11 9,7 37,8 0,8760~0,0100 9,32~0,18

74 I A- I II 138,9 18,2 1,0249~0,0040 ~2,79~0,44

95 A- 5 II 95 ,9 4,6 1,3900~0,0200 ~4, 43~1,21

97 A-6 II 167,0 7,1 1,4400!0,0300 73,02~1,36

96 A-7 II 167,0 4,9 1,6600!0,0500 107,95!1, 97

76 A-3 II 144,9 3,0 2,1700!0,0200 160,02~0,80

Tab. 15 - Rb -Sr Analytical Results of Whole Rock Samples of the Paranoa G roup - Paracat u Ar ea (Pare nti Co utoct al . . 19R2).

LABSr 87/Sr 86 Rb 87/Sr 86

N9 SAMPLE ROCK TYPE Rb (ppm ) Sr (ppm)SPR

3897 DX-2 Si lt s t one 97 , 2 130, 9 0,7 45 8!0,0012 2 , 16~0 , 0 4

3898 DX- 3 II 144,5 67,8 0, 7859! 0,0010 6,22 :':0 , 12

3899 DX-4 II 55,8 189,0 0,7 341:':0,0011 0, 86:':0,02

3900 DX-6 " 103,2 79,0 0,7627:':0,0009 3,80!0,08

276

Precambrian pelites, Rb-Sr method

Sr- 87Sr- 86

2.0

1. ~00

1000

~o 100 I~O

T = 598± 23m .a .

Ri=0.8225 ± 0 ,0117

~Sr - 86

Fig. 17 - Rb- Sr isochro n diagram for samples of the Paranoa Group-Vazante Area-MG (A ma ral and Kawashita,1967).

i!=§!Sr-e6

0 .770

0 .7 ~0

07

-:00'-3

T = 681 ± 10m.a.

Ri = 0,7255 ±0,0005

~Sr - 86

0 7100+------,-----..,-----,..-----,-----....,..----..,-----,-----,o 4 6 7 I

Fig. / 8 - Rb-Sr isochron diagram for samples of the Paranoa Group-Paracatu Area (Parenti Cou to et al ., 1982).

277- USp

B

V.G. Core/alii. A . Thomaz-Filho . B.B. Brito-Neves. K. Kawashita

Tab . 16 - Rb-Sr A na lytica l Results of Wh ole Rock Samples of the Co rurnba Group - T am en go Formation (Fugita ,1979) .

LABSr 87/ Sr 86 Rb87/Sr 86N9 SN-IPLE ROCK TYPE Rb(pp m) Sr Ippm )

SPR

3825 UP-69-lAG Silts tone 100,4 69 , 4 0, 7429: 0,0008 4 , 20:0,08

3826 UP- 70-lAG " 79,2 99,1 0,731 5: 0 ,0011 2 ,30:0,05

38"27 UP-79-lAG " 128, 8 67 ,2 °,7523: 0,0011 5,5 7:0,11

383 1 IUP- 72-lAG " 160,9 68 ,7 0,7 629: 0,000 7 6,79:0,15

i

5r- 875r - 86

O .B ~ O

o BO

o r so

.>:~ up·n lAG

...........- T= 4 8 9 + 2 9m o ......,........,0 u P- 79 lAG -

............- Ri = 0,71 44.j; 0 ,0020....--0~ UP -69 l AG

~UP ' 70 r A GRb - 8 7Sr - 8 6

o 700-t-- - - - - - - - - - - - - ---r- - - - - - - - - - - - - - ,-- - - - - - - -' 0

Fig. 19 - Rb-Sr isochron diag ram for samples of the Co rumba G ro up-Tarne ngo Formati on ( Fugi ta , 1979) .

ar e ex actl y in the ran ge to be expected.in sedimentary ro cks in which th e clasticcomponent is a lways relevant. In addition ,se ve ra l cases be low 0.7 1. and approachingth e iso to pic va lues of the marine strontium.in geolog ic tim e , a re good indica tions thatsevere chemica l cha nges did not tak e plac e .in the rock systems. thus stre nghte ning th epot entia l validity of the int erpretati on s. in

278

terms of the ca lculated ages. The onlyexample in whic h the initial Sr~;?/SrX6 rati ois substantia lly high (the Paranoa formati onat Vazante , with 0.82) represents a case inwhich substa ntial che mica l chan ges associa­ted to a co mplete Sr hom ogeni zat ion occur­red lon g aft er the origina l dep osition of th eformation .

In conclusion , the Rb-Sr method . when

Precam brian petites, Rb-Sr mel hod

applied to pelitic sedimentary rock s, see msto be capable to yield significant result s interms of the original age of deposition , orof relevant events occurred during the evo ­lution of the sedimentary rock . In this case,the modifying conditions must have beenintense enough to produce a Sr isoto pic ho­mogenization, a fact which seems always tooccur, in shal es or related rocks when af­fected by anchi or epi met amorphism.

It must be men tioned that in many of thesedimentary unit s here reported , the basiccrit eria of sampling, developed recently

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