pimentel et al - 1991 - u-pb zircon geocrhonology of pre cam brian tin-bearing continental-type acid...

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8/2/2019 Pimentel Et Al - 1991 - U-pb Zircon Geocrhonology of Pre Cam Brian Tin-bearing Continental-type Acid Magmatism i… http://slidepdf.com/reader/full/pimentel-et-al-1991-u-pb-zircon-geocrhonology-of-pre-cam-brian-tin-bearing 1/15 Precambrian Research, 52 ( 1991 ) 321-335 321 Elsevier Science Publishers B.V., Amsterdam U-Pb zircon geochronology of Precambrian tin-bearing continental-type acid magmatism in central Brazil M~ircio M. Pimentel a, b, Larry Heaman c, Reinhardt A. Fuck a and Onildo J. Marini a a Departamento de Geologia Geral e Aplicada, Instituto de Geocigncias, Universidade de Brasilia, Brasilia, 70910, Brazil b Department of Earth Sciences, University of Oxford, Parks Road, Oxford, 02(1 3PR, UK c Department of Geology, Royal Ontario Museum, I00 Queens Park, Toronto, Ont.. M5S 2C6, Canada (Received September 19, 1990; revised and accepted February 27, 1991 ) ABSTRACT Pimentel, M.M., Heaman, L., Fuck, R.A. and Marini, O.J., 1991. U-Pb zircon geochronology of Precambrian tin-bearing continental-type acid magmatism in central Brazil. Precambrian Res., 52:321-335. U-Pb geochronological data for alkali-rich granitic and associated rhyolitic rocks from two different tin-bearing sub- provinces of the Goi~is tin province, in central Brazil, reveal two distinct episodes of mid-Proterozoic continental acid magmatism at ca. 1770 Ma and 1600 Ma ago. U-Pb zircon data for the Sucuri and Soledade granites of the eastern Rio Paran~ Sub-Province (RPS) define upper intercept ages of 1767_+ 10 Ma and 1769_+ 2 Ma, respectively. These ages are indistinguishable from a U-Pb age of a rhyolite from the basal portion of the Arai Group ( 1771 _+2 Ma ). These results attest to the contemporaneity of the intru- sion of the granites and the crystallization of the rhyolitic lavas in the Rio Parana Sub-Province. U -Pb isotopic results from zircons from the Serra da Mesa granite in the western Rio Tocantins Sub-Province (RTS) indicate that the granitic igneous activity in that area is ca. 150 Ma younger than that in the RPS. The inheritance pattern observed in the zircons from the Arai rhyolite suggests that the sialic basement in the RPS region may include an early Proterozoic (ca. 2.2 Ga) component. Inheritance of possibly Archaean age was also found in zircons from the Sucuri granite. Lower intercept age of 658 Ma for the Soledade granite zircons indicate Pb loss during the late Proterozoic Brasiliano (Pan-African) thermo-tectonic event (650-550 Ma). Similar associations of tin-bearing, alkali-rich granites with rhyolites and succeeding continental sediments are known in other parts of the Precambrian shield of Brazil. This indicates that extensive areas of this shield experienced extensional tectonics in intracontinental settings between ca. 1700 and 1900 Ma ago. Introduction The Arai Group sediments and rhyolites cover extensive areas in the states of Goi~is and Tocantins in central Brazil. Alkali-rich, tin- bearing granitic rocks, believed to be geneti- cally related to the rhyolites interlayered with the sediments of this group are found in the re- gion. Previous isotopic studies on the rocks of this granite-rhyolite province by the whole- rock Rb-Sr method have consistently revealed open system behaviour of the Rb-Sr isotopic system due to the strong overprint imposed by the late Proterozoic (ca. 600 Ma) Brasiliano/ Pan-African orogenic event (Reis Neto and Cordani, 1984). Accurate age values for these rocks are, therefore, still needed for the recon- struction of the past tectonic history of the To- cantins Province in central Brazil, and for re- gional correlation with similar tin-bearing continental-type provinces in other parts of the Precambrian shield of Brazil. In this study, precise U-Pb zircon ages are reported for the Arai Group rhyolite and for three different as- sociated granitic intrusions. 0301-9268/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.

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Page 1: Pimentel Et Al - 1991 - U-pb Zircon Geocrhonology of Pre Cam Brian Tin-bearing Continental-type Acid Magmatism in Central Brazil_2

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P r e c a m b r i a n R e s e a r c h , 5 2 ( 1 99 1 ) 3 2 1 -3 3 5 3 2 1

E l s e v i e r S c i e n c e P u b l i s h e r s B . V ., A m s t e r d a m

U -P b z ircon geoch rono logy of Precambrian tin-bearingcontinental-type acid magmatism in central Brazi l

M~irc io M. Pim en te l a , b , La r r y He a m a n c , Re i n h a r d t A . Fu c k a a n d O n i l d o J. M a r i n i a

a De par tam ento de Geologia Geral e Aplicada, In sti tuto de Geocigncias, Universidade de Brasilia, Brasilia, 70910, Braz il

b De partm ent o f Earth Sciences, Universi ty o f Oxford , Parks Road, Oxford , 02(1 3PR, UK

c Dep artme nt o f Geology, Roya l Ontario Muse um , I00 Q ueens Park, Toronto , Ont. . M5S 2C6, Canada

(R e ce iv ed S ep tem b er 1 9, 1 9 90 ; r ev i sed an d accep ted F eb ru a ry 2 7 , 1 9 9 1 )

A B S T R A C T

P im e n te l , M . M . , H eam an , L . , F u ck , R . A . an d M ar in i , O . J ., 1 9 9 1. U - P b z i r co n g eo ch ro n o lo g y o f P reca m b r i a n t i n -b ea r in g

c o n t i n e n t a l -t y p e a c i d m a g m a t i s m i n c e n tr a l B r a z il . P r e c a m b r i a n R e s ., 5 2 : 3 2 1 - 3 3 5 .

U -P b g eo ch ro n o lo g ica l d a t a fo r a lk a l i - r i ch g ran i t i c an d a s so c i a t ed rh y o l i t i c ro ck s f ro m tw o d i f f e r en t t i n -b ea r in g su b -

p ro v in ce s o f t h e G o i~ is t i n p ro v in c e , i n cen t r a l B raz i l , r ev ea l tw o d i s t i n c t ep i so d es o f m id -P ro t e ro zo ic co n t in en ta l ac id

m ag m at i sm a t ca . 1 7 7 0 M a an d 1 6 0 0 M a ag o .

U - P b z i r c o n d a ta f o r t h e S uc u r i a n d S o l e d a d e g r a n i te s o f t h e e a s te r n R i o P a r a n~ S u b - P r o v i n c e ( R P S ) d e f i n e u p p e r

in t e r cep t ag es o f 1 7 67 _+ 1 0 M a an d 1 76 9 _+ 2 M a , r e sp ec t iv e ly . T h ese ag es a r e i n d i s t i n g u i sh ab le f ro m a U -P b ag e o f a

rh y o l i t e f ro m th e b asa l p o r t i o n o f t h e A ra i G ro u p ( 1 7 7 1 _+ 2 M a ) . T h ese r e su l t s a t t e s t t o t h e co n te m p o ran e i ty o f t h e i n t ru -

s io n o f t h e g ran i t e s an d th e c ry s t a l l i za t i o n o f t h e rh y o l i t i c lav as i n t h e R io P a ran a S u b -P ro v in ce . U -P b i so to p ic r e su lt s

f r o m z i r c o n s f r o m t h e S e r r a d a M e s a g r a n i t e i n t h e w e s te r n R i o T o c a n t i n s S u b - P r o v i n c e ( R T S ) i n d i c a te t h a t t h e g r a n i t ic

ig n eo u s ac t iv i t y in t h a t a r ea i s ca. 1 5 0 M a y o u n g er t h an th a t i n th e R P S .

T h e in h e r i t an c e p a t t e rn o b se rv ed in t h e z i r co n s f ro m th e A ra i rh y o l i t e su g g est s t h a t t h e s i a l i c b asem en t i n t h e R P S r eg io n

m ay in c lu d e an ea r ly P ro t e ro zo ic ( ca . 2 .2 G a) co m p o n e n t . I n h e r i t an ce o f p o ss ib ly A rch ae an ag e w as a lso fo u n d in z i r co n s

f ro m th e S u cu r i g r an i t e .

L o w e r i n t e r cep t ag e o f 6 5 8 M a fo r t h e S o led ad e g ran i t e z i r co n s i n d i ca t e P b lo s s d u r in g th e l a t e P ro t e ro zo ic B ras i l i an o

( P a n - A f r i c a n ) t h e r m o - t e c t o n i c e v e n t ( 6 5 0 - 5 5 0 M a ) .

S im i l a r a s so c i a t i o n s o f t i n -b ea r in g , a lk a l i - r ich g ran i te s w i th rh y o l i t e s an d su cceed in g co n t in en ta l s ed im e n t s a r e k n o w n

in o th e r p a r t s o f t h e P re cam b r i an sh i e ld o f B raz i l . T h i s i n d i ca t e s t h a t ex t en s iv e a r eas o f t h i s sh i e ld ex p e r i en ced ex t en s io n a l

t ec to n ics i n i n t r aco n t in en ta l s e t t i n g s b e tw een ca . 1 7 0 0 an d 1 9 0 0 M a ag o .

I n t r o d u c t i o n

T h e A r a i G r o u p s e d i m e n t s a n d r h y o l i t e s

c o v e r e x t e n s i v e a r e a s i n t h e st a t e s o f G o i~ is a n d

T o c a n t i n s i n c e n t r a l B r a z i l . A l k a l i - r i c h , t i n -

b e a r i n g g r a n i t i c r o c k s , b e l i e v e d t o b e g e n e t i -

c a l l y r e l a t e d t o t h e r h y o l i t e s i n t e r l a y e r e d w i t h

t h e s e d i m e n t s o f t h i s g r o u p a r e f o u n d i n t h e r e-

g i o n . P r e v i o u s i s o t o p i c s t u d i e s o n t h e r o c k s o f

t h i s g r a n i t e - r h y o l i t e p r o v i n c e b y t h e w h o l e -

r o c k R b - S r m e t h o d h a v e c o n s i s t e n tl y r e v e a le d

o p e n s y s t e m b e h a v i o u r o f t h e R b - S r i s o t o p i c

s y s te m d u e t o t h e s tr o n g o v e r p r in t i m p o s e d b y

t h e la t e P r o t e r o z o i c ( c a . 6 0 0 M a ) B r a s i l i a n o /

P a n - A f r i c a n o r o g e n i c e v e n t ( R e i s N e t o a n d

C o r d a n i , 1 9 8 4 ) . A c c u r a t e a g e v a l u e s fo r t h e s e

r o c k s a r e , t h e r e f o r e , s t il l n e e d e d f o r t h e r e c o n -

s t r u c t i o n o f t h e p a s t t e c t o n i c h i s t o r y o f t h e T o -

c a n t i n s P r o v i n c e i n c e n t r a l B r a zi l, a n d f o r re -

g i o n a l c o r r e l a t i o n w i t h s i m i l a r t i n - b e a r i n g

c o n t i n e n t a l - t y p e p r o v i n c e s i n o t h e r p a r t s o f t h e

P r e c a m b r i a n s h i e l d o f B r a z il . I n th i s s t u d y ,

p r e c i s e U - P b z i r c o n a g e s a r e r e p o r t e d f o r t h e

A r a i G r o u p r h y o l i t e a n d f o r t h r e e d i f f e r e n t a s-

s o c i a t e d g r a n i t i c i n t r u s i o n s .

0 3 0 1 - 9 2 6 8 / 9 1 / $ 0 3 . 5 0 © 1 99 1 E l s e v i e r S c i e n c e P u b l i s h e r s B. V . A l l r i g ht s r e s e r v e d .

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322 M . M . P I M E N T E L E T A L.

Geo logical setting and previousgeochronological studies

T h e m a p i n F i g . 1 d i s p l a y s t h e m a j o r g e o l og -

i c a l e l e m e n t s o f t h e T o c a n t i n s S t r u c t u r a l P r o v -

i n c e ( A l m e i d a e t a l. , 1 9 8 1 ) . T h r e e d i f f e r e n ts u p r a c r u s t a l f o l d b e l t s a r e i d e n t i f i e d i n t h i s

s t r u c tu r a l p r o v i n c e . T h e y a r e e x p o s e d a l o n g t h e

N - S d i r e c ti o n , b e tw e e n t w o m a j o r c r a t o n i c re -

g i o n s : t h e A m a z o n i a n c r a t o n i n t h e w e s t a n d

t h e S 5 o F r a n c i s c o c r a t o n i n t h e e a s t ( F i g . 1 ) .

T h e s u p r a c r u s t a l f o l d b e l t s a r e , f r o m w e s t t o

e a st : ( i ) t h e P a r a g u a i - A r a g u a i a f o l d b e l t w h i c h

i s a B r a s i l i a n o / P a n - A f r i c a n c o n t i n e n t a l m a r -

g i n - li k e s e d i m e n t a r y s u c c e s s i o n d e p o s i t e d a n d

d e f o r m e d a g a i ns t t h e e a s t e r n m a r g i n o f th e

A m a z o n i a n c r a t o n ; ( i i ) t h e U r u a q u b e l t ,

f o r m e d d o m i n a n t l y b y m e t a p e l it i c a n d m e t a p -

s a m m i t i c r o ck s w i t h s o m e m e t a v o l c a n i c in t e r-

c a l a t i o n s ; a n d ( i i i ) t h e B r a s i l i a b e lt , a l a t e P r o -

terozoic sedimentary belt f ormed at the western

margin of the S~o Francisco craton. The supra-

crustal belts were deformed during the late

Proterozoic (Brasiliano orogeny) probably due

to the convergence of the "A maz oni an" and

"Sao Francisco" continents. Encompassed bythe supracrustal fold belts is the so-called Goifis

Mas sif (Fig. 1 ) (Al meid a et al., 1981; Mari ni

et al., 1984). This mass if seems to repres ent a

complex collage of crystalline terranes of Ar-

chaean to late Proterozoic age.

The Arai Group f orms the northea stern por-

tion of the Uruagu belt (Fig. 2). It constitutes

a sedime ntary sequence of contine ntal charac-

ter up to 1200 m thick, which covers extensive

areas in the states of Goifis and Tocantins. The

basal Arraias For mat ion c omprises clastic sed-

imentary rocks (conglomerates, coarse-grained

quartzites and siltstones) deposited in a con-

tinental setting characterized by alluvial fans

65W 45W

~'~ I ~ ~ ~ C Atlantic~ ~ ~ / ~ - - - ~ O c e a n

35S +

' ~ _ L a t e P r o t e r o z o i cm o b i l e b e l t s

~ _ C r a t o n s ( 1 - A m a z o n i a n2 - S a o F r a n c i s c o )

- T o c a n t l n s P r o v i n c e

/ m O 0 a " - d°k b - e

L - ~ _ C ~ I - f

Fig. 1. Area location and main geologicalunits of the Tocantins Province in central Brazil. 1 = Amazonian craton; 2 = SaoFrancisco craton; a = Phanerozoic cover; b = late Proterozoic sedimentary belts: Paraguai-Araguaia in the west and Bras-ilia in the east; c= Uruagu belt; d= Arai Group; e= Goi~is Massif;f= Rio Apa Block. The arrows indicate tectonic vergenceof the metasedimentary belts.

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U-Pb ZIRCONGEOCHRONOLOGYOF PRECAMBRIANTIN-BEARINGCONTINENTAL-TYPEACIDMAGMATISM 323

x :. 6- ) l a t e \

F P ' ° t " ° ' ° ' ° I

:ii~!iiiiiiiii B P G S

::::i:: ~ \

# h G g ~ ~ 8 o ; - /

• ] / o c a n t i ~ ~ -

f ' ~ ' ~ 1 B a h i a

G o i a s . , ~ _

_ 1 ~ ° J B a s i ~ . - . L ~• ' ( M i n a s

: ~ 0 ~ 1 0 k r \ ~ ~ 'S f _ G e r a i s

F i g . 2 . G e o l o g i c a l s k e t c h m a p o f c e n t r a l - e a s t e r n O o i~ is : GE=g r a n i t i c - g n e i s s i c b a s e m e n t ; Mu = m a f i c a n d u l t r a m a f i c c o m -

p l e x e s ( m i d d l e P r o t e r o z o i c ? ) ; Vs= v o l c a n o - s e d i m e n t a r y s e q u e n ce s ( l a t e / m i d d l e P r o t e r o z o i c ? ) . G r a n i t i c p lu t o n s , R T S :

a = S e r r a d o E n c o s t o ; b = S e r r a D o u r a d a ; c = S e r r a d a M e s a ; d = S e r r a B r an c a ; e = P i r a p i t i n g a ; f = S ~ o R o q u e ; g = F l o r e n c i o ;

h = R a i z a m i n h a ; i = S e r r a d a C a n g a i h a . R P S : A = S o l e d ad e ; B = S u c u r i ; C = M a n g a b e i r a ; D = S e r r a d o M e n d e s ; E = P e d r a

B r a n ca ; F = M o c a m b o ; G = B a n h a d o ; H = S ~ o D o m i n g o s .

and lacustrine environments. This sedimen-

tary sequence lies unconformably on micasch-

ists and paragneisses of the Ticunzal Forma-

tion (> 1.77 Ga) or on orthogneisses and

mylonitic rocks of the older granitic-gneissicbasement. Platform sediments of the Parano~i

and Bambui groups of late Proterozoic age un-

conformably overlie the Arai Group. A 50-100

m thick rhyolitic unit is intercalated with the

coarse-grained sediments of the lower portion

of the Arraias Formation.

A number of alkali-rich, A-type granitic in-

trusions are found in the region and form two

tin-bearing sub-provinces which have distinct

field, structural, petrographic and geochemical

characteristics: (i) the Rio Parana Sub-Prov-

ince (RPS) in the east and (ii ) the Rio Tocan-

tins Sub-Province (RTS) ca. 100 km to the

west (Fig. 2 ). Some of these tin-bearing gran-

itic rocks display strong geochemical similari-

ties with the rhyolites at the base of the Arai

Group, suggesting that these extrusive rocks are

coeval with the intrusion of the granites (Bo-

telho, 1984; Marini and Botelho, 1986).

According to Marini et al. (1986) the depo-

sition of the Arai Group sediments, and the

possibly contemporaneous intrusion of the al-

kali-rich granites, took place in an extensional

tectonic setting, comparable to the intraconti-nental rift system with which the typically A-

type anorogenic Nigerian Younger Granites are

associated (Badejoko, 1986).

The gran itic sub-provinces

Eight granitic plutons constitute the RPS

(Rio Paran~ Sub-Province; see Fig. 2). They

intrude into gneissic/mylonitic rocks of the

older granitic-gneissic basement as well as mi-

caschists and paragneisses of the Ticunzal For-

mation. No intrusive relationships have been

found, however, between the granites and the

sediments of the Arai Group (Marini and Bo-

telho, 1986). These granitic intrusions form

approximately circular bodies with diameters

normally smaller than 15 km. Their main in-

trusive phases are biotite granites (amphibole

is rare ). Except for a few small shear zones, the

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324 M. M. P IMENTEL ET AL.

g r a n it e s o f t h e R P S a r e n o t d e f o r m e d a n d e x-

h i b i t w e l l p r e s e r v e d i g n e o u s t e x t u r e s . S u b v o l -

c a n i c t e x t u r e s a r e n o t u n c o m m o n . A n a n o r o -

g e n i c s e t t in g w a s p r o p o s e d f o r th e i n t r u s i o n o f

t h e s e g r a n i t e s ( M a r i n i e t a l ., 1 9 8 6 ) . T i n ( c a s -

s i t e r it e ) m i n e r a l i z a t i o n i s a s s o c i a t e d w i t h p e g -m a t i t e s a n d g r e i s e n s a n d i s u s u a l l y a c c o m -

p a n i e d b y t o p a z , f l u o r it e , b e r y l a n d t o u r m a l i n e .

I n t e n s e l a t e - t o p o s t - m a g m a t i c h y d r o t h e r m a l

a l t e r a t i o n c h a r a c t e r i z e t h e m i n e r a l i z e d a r e a s

( B o t e l h o a n d M a r i n i , 1 9 8 4 ) .

T h e R T S ( R i o T o c a n t i n s S u b - P r o v i n c e )

c o n t a i n s a t o t a l o f n i n e g r a n i t i c p l u t o n s w h i c h

a r e g e n e r a ll y b i g g e r t h a n t h o s e o f t h e R P S ( s e e

F i g. 2 ) . T h e y a r e e l o n g a t e d i n t h e N - S d i r e c -

t i o n f o r m i n g d o m e - l i k e s t r u c t u r e s i n t h e s u r -r o u n d i n g s u p r a c r u s t a l r o c k s o f t h e S e r r a d a

M e s a a n d A r a i g ro u p s. T h e s e t w o s e d i m e n t a r y

g r o u p s a r e c o n s i d e r e d b y F u c k a n d M a r i n i

( 1 9 8 1 ) a s c o n t e m p o r a n e o u s ( m i d d l e P r o t e r-

o z o i c ) d e s p i t e d i f f e r e n c e s i n l i t h o l o g i c a l c o n -

t e n t a n d m e t a m o r p h i c g r a d e . T h e m a i n i n t ru -

s i v e p h a s e s o f t h e R T S i n t r u s i o n s a r e b i o t i t e

a n d h o r n b l e n d e g r a n i t e s . T h e y a r e u s u a l l y d e -

f o r m e d a n d r e c r y s ta l l iz e d , e s p e c ia l l y n e a r t h e i r

m a r g i ns . T h e a g e o f d e f o r m a t i o n is u n k n o w n

a n d c o u l d b e a s y o u n g a s 6 00 M a , w h i c h is t h e

a p p r o x i m a t e a g e o f t h e l a t e st t e c t o n i c e v e n t a f -

f e c t i n g t h e r e g i o n ( t h e B r a s i l i a n o / P a n - A f r i c a n

o r o g e n i c e v e n t ) . A c c o r d i n g t o M a r i n i a n d B o -

t e l h o ( 1 9 8 6 ) , t h e g r a n i t i c r o c k s o f t h e R T S

c r y s t a ll i z e d a t d e e p e r c r u s t a l l e v e ls c o m p a r e d

t o t h o s e f r o m t h e R P S . T h e f i e l d r e l a t i o n s h i p s

b e t w e e n t h e g r a n i ti c p l u to n s a n d t h e s u r r o u n d -

i n g m e t a s e d i m e n t s o f th e S e r r a d a M e s a a n d

A r a i g r o u p s a re c o n t r o v e r s i a l . M a r i n i a n d B o -

t e l h o ( 1 9 8 6 ) s t re s s t h a t t h e r e i s n o f i e l d e v i -

d e n c e i n d i c a t i n g t h a t t h e s e g r a n i t e s i n t r u d e t h e

s u p r a c r u s t a l ro c k s a n d i n t e r p r e t t h e m a s m a n -

t l e d d o m e s . O t h e r a u t h o r s ( e .g . M a c a m b i r a ,

1 9 8 3 ) , h o w e v e r , d e s c r i b e i n t r u s i v e r e l a t i o n -

s h ip s . T h e t i n m i n e r a l i z e d a r e a s i n t h e g r a n i t e s

o f th e R P S a r e a s s o c i a t e d w i t h g r e i se n s a n d a l-

b i ti te s . T o p a z , b e r y l, w o l f r a m i t e a n d o t h e r N b -

T a m i n e r a l s a ls o o c c u r i n t h e s e z o n e s .

Previous geochronological s tudies

M o s t p r e v i o u s a t t e m p t s t o d e t e r m i n e e m -

p l a c e m e n t a g e s f o r t h e g r a n i t i c r o c k s f r o m b o t h

R T S a n d R P S b y t h e w h o l e -r o c k R b - S r i so -

c h r o n m e t h o d f a i le d . S ix t y - n i n e R b - S r i s o -t o p i c m e a s u r e m e n t s o n t h e s e r o c k s a r e a v a i l -

a b l e i n t h e l i t e r a t u r e , b u t t h e s e d a t a s h o w

c o n s i d e r a b l e s c a t t e r i n i s o c h r o n d i a g r a m s

( H a s u i a n d A l m e i d a , 1 9 7 0 ; H a s u i e t a l . , 1 9 8 0 ;

M a c a m b i r a , 1 9 8 3 ; R e i s N e t o a n d C o r d a n i ,

1 9 8 4 ) . M o s t o f t h e s c a t t e r h a s b e e n a t t r i b u t e d

t o o p e n s y s t e m b e h a v i o u r d u r i n g t h e l a te P r o -

t e r o z o i c B r a s il i a n o e v e n t c a. 6 0 0 M a a g o ( R e i s

N e t o a n d C o r d a n i , 1 9 8 4 ) . N i n e t e e n R b - S r

i s o to p i c m e a s u r e m e n t s w e r e p e r f o r m e d b y d i f-

f e r e n t a u t h o r s ( H a s u i a n d A l m e i d a , 1 97 0; R e i s

N e r o , 1 9 8 3 ) o n s a m p l e s o f t h e S e r r a d a M e s a

g r a n i t e ( R T S ) . A r e g r e s s io n t h r o u g h t h e s e

p o i n ts y i e l d e d a n " e r r o r c h r o n " w i t h a n a g e o f

1 3 4 0 _ 1 8 0 M a a n d h i g h i n i t i a l 875r/86Sr (ca.

0 . 7 3 4 1 ) . A 5 - p o i n t " e r r o r c h r o n " w a s c o m -

p u t e d f o r t h e S e r ra B r a n c a g r a n i t e ( R T S ) u s -

i n g t h e R b - S r a n a l y t i c a l d a t a o f R e i s N e t o

( 1 9 8 3 ) . T h i s y i e l d e d a n a p p r o x i m a t e a g e o f

1409 M a. A s im i la r age ( 1430 _+ 24 M a; in i t i a l

875r/86Sr o f 0 . 7 8 1 6 ) w a s o b t a i n e d b y a n " e r -r o r c h r o n " d e f i n e d b y f i v e a n a l y s e s f ro m a s u i te

o f s e v e n s a m p l e s o f t h e S e r r a D o u r a d a g r a n i t e

( R T S ) ( R e i s N e t o , 1 9 8 3 ) . I n t h e R P S , s a m -

p l es f r o m b o t h t h e S o l e d a d e a n d P e d r a B r a n c a

g r a n it e s w e r e a n a l y s ed b y t h e R b - S r m e t h o d

(Hasu i e t a l . , 1980 ; S iga , 1981; Re is Nero ,

1 98 3; M a r i n i e t a l ., 1 9 86 ) . A 4 - p o i n t R b - S r i s-

o c h r o n f o r t h e S o l e d a d e g r a n i t e y i e ld s a n a g e

of 1637 _+ 93 a nd ini t ia l 8 7 5 r / 8 6 S r o f c a . 0 . 7 3 3 2

( r e s u l t c a l c u l a t e d w i t h t h e a n a l y t i c a l d a t a f r o m

Hasu i e t a l . , 1980 , S iga , 1981 and Re is Ne to ,

1 98 3 ) . T h e P e d r a B r a n c a g r a n i t e w a s d a t e d b y

M a r i n i e t a l. ( 1 9 8 6 ) a n d t h e w e l l - fi t te d R b - S r

i s oc h ro n ( M S W D = I . 3 ) g iv es t h e re la ti ve ly

p r e c i s e a g e 1 40 5 + 2 1 M a a n d a n a n o m a l o u s l y

l o w i n i t i a l 878r/86Sr r a t i o ( 0 . 7 0 0 4 + 0 . 0 0 0 6 ) .

B a s e d o n t h e g e n e r a l f i e l d , p e t r o g r a p h i c a n d

g e o c h e m i c a l s i m i l a ri t ie s o b s e r v e d f o r t h e s e v-

e r a l g r a n i t e s o f t h e R P S , t h e a u t h o r s a b o v e a s-

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U-Pb ZIRCON GEOCHRONOLOGY OF PRECAMBRIAN TIN-BEARING CONTINENTAL-TYPEACID MAGMATISM 325

s i g n e d a n a g e o f ~ 1 4 0 0 M a f o r t h e g r a n i t i c

p l u t o n i s m a n d a s s o c i a t e d v o l c a n i s m i n t h a t

p a r t i c u l a r a r e a .

S a m p l e s o f t h e S e r r a d a M e s a a n d S e r r a

B r a n c a g r a n i t e s i n t h e R T S w e r e a n a l y s e d b y

t h e w h o l e - r o c k P b - P b i s o c h r o n m e t h o d a n dy i e l d a c o m b i n e d a g e o f 1 6 5 8 _+ 4 4 M a ( P . T a y -

l o r, p e ts . c o m m u n . , 1 9 89 ) .

K - A t m i n e r a l a n d w h o l e - r o c k i s ot o p ic s ys -

t e m s h a v e b e e n v a r i a b l y r e s e t i n r e s p o n s e t o

t h e B r a s il ia n o o r o g e n i c ev e n t . K - A r m i n e r a l

d a t e s r e p o r t e d f o r s e v e r a l r o c k u n i t s i n t h i s re -

g i o n , a s w e l l a s f o r m a n y o t h e r a r e a s i n t h e T o -

c a n t i n s S t r u c t u r a l P r o v i n c e ( R e i s N e t o a n d

C o r d a n i , 1 9 8 4 ) , g i v e l a te P r o t e r o z o i c t o e a rl y

P a l a e o z o i c a g es . B i o t i t e K - A r a g e s o f 5 1 3 a n d

5 5 0 M a w e r e o b t a i n e d f o r g n e i s si c ro c k s o f t h e

b a s e m e n t . K - A r m i n e r a l a g es v a ry i n g b e t w e e n

5 1 3 a n d 5 3 8 M a a r e r e p o r t e d f o r s a m p l e s o f

t h e S e r ra D o u r a d a a n d S e r ra B r a n c a g r a n it e s

( H a s u i a n d A l m e i d a , 1 9 70 ; R e i s N e t o , 1 98 3 ) .

W h o l e - ro c k s a m p l es o f th e A r a i G r o u p r h y o -

l i t e s a r e 5 7 9 M a a n d 6 7 1 M a ( R e i s N e t o a n d

C o r d a n i , 1 9 8 4 ) . T h e s e d a t a s u g g es t t h a t t h e

t h e r m a l e v e n t a t t h e e n d o f th e P r o t e r o z o i c h a s

d i s t u rb e d t h e K - A r s y s te m i n m o s t o f t h e r o c k

u n i t s in th e R T S a n d R P S .

An alytical procedures

U-P b zircon analyses

H e a v y m i n e r a l f r a c ti o n s w e r e o b t a i n e d f r o m

c a . 2 0 k g s a m p l e s b y p a n n i n g i n t h e l a b o r a t o -

r ie s o f th e C i a . V a l e d o R i o D o c e , B e l o H o r i -

z o n t e , B r a z i l . T h e s e h e a v y m i n e r a l f r a c t i o n s

w e r e f u r t h e r p r o c e s s e d b y s i e v i n g , m a g n e t i c

s e p a r a t i o n u s i n g a F r a n t z i s o d y n a m i c s e p a r a -

t o r, a n d c o n v e n t i o n a l h e a v y li q u i d ( b r o m o -

f o r m a n d m e t h y l e n e - i o d i d e ) s e p a r a t io n i n t h e

J a c k S a t t e r l y G e o c h r o n o l o g y L a b o r a t o r y ,

R o y a l O n t a r i o M u s e u m , T o r o n t o . Z i r c o n f r ac -

t i o n s w e r e t h e n s e l e c t e d f r o m t h e l e a s t m a g -

n e t i c f r a c t io n s b y h a n d - p i c k i n g i n f i l t e r e d e th y l

a l c o h o l. A c o m b i n a t i o n o f t h e f o l l o w i n g t e c h -

n i q u e s w a s u s e d t o r e d u c e d i s c o r d a n c y : ( i )

h a n d - p i c k i n g o f t h e m o s t t r a n s p a r e n t , l e as t al -

t e r e d a n d c r a c k - fr e e g r a i n s, a n d ( i i ) a i r a b ra -

s i o n u s i n g p y r i t e a s a p o l i s h i n g m e d i u m

( K r o g h , 1 9 8 2 ) . T h e s e l e c te d z i r c o n f ra c t i o n s

w e r e h o m o g e n e o u s i n te r m s o f s iz e ( 1 0 0 - 2 0 0

m e s h ) , c o l o u r ( p i n k ) a n d s h a p e ( e u h e d r a lp r i s m s ) o f th e g r a i n s ( s e e T a b l e 1 ) .

T h e z i r c o n f r a c t i o n s w e r e w a s h e d t w i c e i n

w a r m 4 N H N O 3 ; f i r s t t o r e m o v e a n y r e m a i n -

i n g p y r it e g r a in s a f t e r a b r a s i o n a n d t h e n f o r ca .

4 5 r a i n f i n a l w a s h i n g , f o l l o w e d b y r e p e a t e d

d i s t i l l e d - w a t e r a n d a c e t o n e r i n s e s . F r a c t i o n s

w e r e w e i g h e d o n d i s po s a b le a l u m i n i u m w e i gh -

i n g b o a t s a n d d i s s o l v e d in a m i x t u r e o f H F : 8

N H N O 3 ( 1 5: 1 ) u s i n g P a r r - t y p e T e f l o n b o m b s

a t 2 2 0 ° C . A s m a l l q u a n t i t y o f a m i x e d 2 ° S P b-

2 35 U s p i k e i s o t o p i c t r a c e r ( K r o g h a n d D a v i s ,

1 97 5 ) w a s a d d e d a t t h i s s t a g e . T h e d i s s o l u t i o n

a n d c h e m i c a l e x t ra c t i o n o f U a n d P b f o l lo w e d

g e n e r a l p r o c e d u r e s d e s c r i b e d b y K r o g h ( 1 97 3 ).

P b a n d U w e r e l o a d e d t o g e t h e r a s p h o s -

p h a t e s w i t h s i l ic a ge l o n t o o u t g a s s e d s i n g le r h e -

n i u m f i l a m e n t s a n d t h e i s o t o p i c m e a s u r e -

m e n t s w e r e p e r f o r m e d o n a V G - 3 5 4 m a s s

s p e c t r o m e t e r a t t h e R o y a l O n t a r i o M u s e u m .

D e t a i l s o f t h e m a s s s p e c t r o m e t r i c p r o c e d u r e s

c a n b e f o u n d i n K r o g h a n d T u r e k ( 1 98 2 ) a n dD a v i s e t a l . ( 1 9 8 2 ) .

T h e i s o t o p i c d a t a w e r e c o r r e c t e d f o r f r a c -

t i o n a t i o n v a l u e s o f 0 .1 % p e r a t o m i c m a s s u n i t

( A M U ) . T h e t o t a l U a n d P b b l a n k s i n t h e

R O M l a b o r a t o r y a t th e t i m e o f t h e a n a ly s e s

w e r e 1 p g a n d 2 p g , re s p e c ti v e ly . T h e c o m p o -

s i t i o n s f o r t h e i n i t i a l c o m m o n P b c o r r e c t i o n

w e r e d e r i v e d f r o m t h e S t a c e y a n d K r a m e r s

( 1 9 7 5 ) g r o w t h c u r v e s . F o r t h e b l a n k c o r r e c -

t i o n ,

2°spbfl°7pb:2°6pb:2°4pb = 37 .6:1 5.6 :18 .3:1 . Er-

r o r s q u o t e d a r e a t t h e 2 a l e v e l a n d w e r e c o m -

p u t e d i n d i v i d u a l ly . R e g r e s s i o n l in e s w e r e c o m -

p u t e d u s i n g t h e m e t h o d o f D a v i s ( 1 9 82 ) , a n d

t h e q u o t e d e r r o r s i n a ge a re a t t h e 9 5 % c o n f i -

d e n c e l e v e l . S c a t t e r o f t h e a n a l y t i c a l p o i n t s i s

m e a s u r e d b y a p a r a m e t e r c a l le d p r o b a b i l i t y o f

f it . W e l l f i t t e d r e g r e s s i o n s h a v e a p e r c e n t p r o b -

a b i l i t y o f f i t g r e a t e r t h a n 1 0% . D e c a y c o n s t a n t

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U - P b Z I R C O N G E O C H R O N O L O G Y O F P R E CA M B R 1 AN T I N -B E A R I N G C O N T IN E N T A L -T Y P E A C ID M A G M A T IS M 327

v a l u es u s e d a n d t h e i s o t o p i c c o m p o s i t i o n o f

u r a n i u m a r e th o s e r e c o m m e n d e d b y S t e ig e r

an d J~iger ( 1977 ) .

Re su l t s

m i n a t i o n o f p re c i se e m p l a c e m e n t ag es . In t w o

cases ( ana lyses 3 and 9 in T ab le 1 ) , c rys t a l s

w i t h s m a l l c r a c k s w e r e i n c l u d e d in o r d e r t o i n -

v e s t i g a t e t h e d i s c o r d a n c y p a t t e r n o f t h e r e -

spec t ive su i t es .

U-P b zircon results Ar ai Group rhyol i te

T h e U - P b d a t a f o r t h r e e g r a n it e s am p l e s a n d

o n e r h y o l i t e a re l i s t e d in T a b l e 1 . S a m p l e s w e r e

c o l l e c t e d f r o m s i n g le o u t c r o p s . T w o o f t h e

g r a n i t e s a m p l e s a re f r o m t h e R P S ( S u c u r i a n d

S o l e d a d e i n t r u s i o n s ) a n d t h e t h i r d , th e S e r r a

d a M e s a g r a n i t e , is f r o m t h e R T S ( F i g . 2 ) .

I n p r e v i o u s U - P b i s o to p i c s t u d ie s o f B ra z il -

i a n P r e c a m b r i a n r o c k s ( e . g . M a c h a d o e t a l . ,

1 9 8 8, 1 9 8 9 a , b ) i t b e c a m e e v i d e n t t h a t z i r c o n s

s u f f e r e d s e v e r e l e a d - l o s s a n d a r e d i s c o r d a n t

e v e n a f t e r a b r a s i o n . T h i s i s p o s s i b l y d u e t o a

c o m b i n a t i o n o f: ( i ) t h e s u p e r i m p o s i t i o n o f

s e v e r a l s u c c e s s i v e P r e c a m b r i a n t h e r m o - t e c -

t o n i c e v e n ts , a n d ( i i ) i n t e n s e r e c e n t w e a t h e r-

i n g . T h e s a m p l e s s e l e c t e d f o r th i s s t u d y c o n -

t a i n e d p l e n t y o f g o o d q u a l i t y ( t r a n s p a r e n t a n d

c r a c k -f r e e) z i r c o n s w h i c h w e r e a b r a d e d d o w n

t o s m a l l s p h e r e s in o r d e r t o m i n i m i z e d is c o r-

d a n c y . T h i s s t r a t e g y m a d e p o s s i b l e t h e d e t e r -

T w o p o p u l a t i o n s o f z ir c o n w i t h d i f f e r en t

c o l o u r ( c o l o u r l e s s a n d p i n k ) a r e p r e s e n t in th e

rhyo l i t i c rocks . A n a lyses 1 an d 2 (T ab le 1 )

r e p r e s e n t s i n g l e g r a i n r e s u l ts o n o n e d a r k - p i n k

a n d o n e c o l o u r l e ss c r y st a l, r e s p e c t i v e ly . T h e y

h a v e n e a r l y i d e n t i c a l v a lu e s f o r th e P b i s o t o p i c

r a t io s a n d m o d e l 2 °7 p b /2 ° 6 pb a ge s ( 1 7 8 0 M a

a n d 1 7 82 M a ) , s u g g e s t in g t h a t t h e t w o p o p u -

l a t ions a re cogene t i c . T h e ana ly t i ca l da t a po in t s

p l o t s l ig h t ly a b o v e t h e c o n c o r d i a c u r v e ( c a .

0 . 3 % a n d 1 .0 % d i s c o r d a n t , r e s p e c t i v e l y ) ( F i g .

3 ) . A n a l y s i s 3 ( s a m p l e G . 8 8 . 1 . 2 B ) w a s o b -

t a i n e d o n a f r a c t i o n i n w h i c h s o m e c r a c k e d

gra ins w ere~presen t. T h i s an a lys i s is m uc h m ore

d i s c o r d a n t ( 1 7 . 3 % ) t h a n t h e o t h e r fo u r.

D e s p i t e t h e f a ct th a t n o o b v i o u s c o r e - o v e r -

g r o w t h r e l a t i o n s h i p s w e r e o b s e r v e d i n t h i s z i r-

c o n p o p u l a t i o n , t h e U - P b i s o to p i c d a t a f o r

f r a c t i o n s G . 8 8 . 1 . 1 B a n d G . 8 8 . 1 . 1 C ( a n a l y s e s

A R A I R H YO L I T E

1 9 9 o / /

Y¼/ / / 2°7pb/235U

~ o 2 4 5 M a

I I I I I I3.5 4.0 4.5 5.0 5.5 6.0

Fi g. 3 . U - P b c o n c o r d i a d i a g r a m f o r z ir c o n s f r o m t h e A r a i G r o u p r h y o li te .

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32 8 M.M. PIMENTEL ET AL.

4 a n d 5 ) r e v e a l t h e p r e s e n c e o f a n i n h e r i t e d

c o m p o n e n t . T h e a p p r o x i m a t e a g e o f t h is i n -

h e r i t e d c o m p o n e n t c a n b e a s s e s s e d b y c o n -

s t r u c t in g a c h o r d t h r o u g h t h e s e tw o p o i n t s a n d

t h e t w o s i n g l e g r a i n a n a l y s e s ( F i g . 3 ) . T h i s

p r o c e d u r e y i e l d s a n u p p e r i n t e r c e p t a g e o f c a .2 1 6 0 M a a n d a l o w e r in t e r c e p t o f a p p r o x i -

m a t e l y 1 7 7 4 M a .

T h e s e d a t a i n d i c a t e a c r y s t a l l i z a t i o n a g e o f

1771 _+ 2 M a fo r thes e rh yo l i t i c rocks . Th is ag e

p r o v i d e s a n i m p o r t a n t c o n s t r a i n t o n t h e d e -

p o s i t i o n a l a g e o f t h e A r a i G r o u p a n d c o n t r a -

d i c t s p r e v i o u s s u g g e s t i o n s t h a t t h i s g r o u p i s

c o n s i d e r a b l y y o u n g e r ( c a . 14 0 0 M a ) ( M a r i n i

a n d B o t e l h o , 1 9 8 6 ) .

Sole dade gra n ite

T h i s g r a n i t i c p l u t o n i s a n e l l i p t i c a l ( 7 .5 X 4 .0

k m ) s u b v o l c a n ic i n tr u s i o n e m p l a c e d i n t o

b a s e m e n t g n e i s s e s a n d m y l o n i t e s . L o c a l l y ,

g n e i s s i c x e n o l i t h s u p t o 1 m a c r o s s a r e f o u n d .

T h e g r a n i t e a t t h e s a m p l e s i t e i s u n d e f o r m e d ,

m a s s i v e a n d c o n s i s ts o f b i o t i t e , b l u e i sh q u a r t z

a n d f e l d s p a r s . G r a n o p h y r i c t e x t u r e s a r e w e l l

d e v e l o p e d ( B o t e l h o e t al ., 1 9 8 6 ) .

F o u r U - P b i so t op i c m e a s u r e m e n t s w e r e

p e r f o r m e d o n z i r c o n f r a c t i o n s f r o m t h i s s u b -

v o l c a n i c g r a n i t i c b o d y . Z i r c o n s s e p a r a t e d f r o m

t h i s i n t r u s i o n w e r e o f p a r t i c u l a r l y g o o d q u a l -

i ty , b e i n g i n g e n e r a l v e r y t r a n s p a r e n t a n d f r ee

o f c r a ck s o r a l t e r e d p a r t s. T h e z i r c o n f r a c t i o n s

a n a l y s e d i n c l u d e d s t u b b y , e u h e d r a l p r i s m a t i cc r y s ta l s o f li g h t p i n k c o l o u r . T h r e e o f th e f o u r

a n a l y s e s a r e l e s s t h a n 1 % d i s c o r d a n t ( s e e F ig .

4 ) , w h i l e t h e f o u r t h , i n w h i c h c r a c k e d g r a i n s

w e r e i n c l u d e d , i s 1 2 % d i s c o r d a n t . T h e a n a l -

y s es d e f i n e d a d i s c o r d i a l i n e ( 2 8 % p r o b a b i l i t y

o f f it ) w i t h a n u p p e r i n t e r c e p t a g e o f 1 76 9 + 2

M a . T h i s p r e c i s e U - P b z i r c o n a g e i s i n t e r -

p r e t e d a s t h e t im e o f g r a n it e e m p l a c e m e n t . N o

o b v i o u s i n h e r i t a n c e w a s d e t e c t e d i n t h e f r a c -

t i o n s i n v e s t i g a te d . T h e l o w e r i n t e r c e p t a g e o f

c a . 6 5 8 M a , a l t h o u g h i t h a s a la r g e u n c e r t a i n t y ,

s u g g e s ts a n e p i s o d e o f P b - l o s s d u r i n g t h e B r a s -

i l ia n o t h e r m a l e p i s o d e . T h i s i s c o n s i s t e n t w i t h

t h e K - A r c o o l i n g a g es o f 5 1 3 M a t o 6 71 M a

r e p o r t e d f o r s e v e r a l r o c k u n i t s i n t h e r e g i o n

( H a s u i a n d A l m e i d a , 1 9 70 ; R e i s N e t o , 1 9 8 3 ) .

T h e U - P b a g e r e s ul t o b t a i n e d h e r e f o r t h e

S o l e d a d e g r a n i t e i s i n d i s t in g u i s h a b l e f r o m t h a t

f o r t h e A r a i r h y o l i t e s , s u g g e s ti n g t h a t t h e s e t w o

u n i t s a r e c o n t e m p o r a n e o u s a n d p o s s i b l y

c o g e n e t i c .

.3 1

.2 9

.2 7

.2 5

3 . 5 4 . 0 4 . 5

F ig . 4. U - P b c o n c o r d i a d i a g r a m f o r zi r c o n s f r o m t h e S o l e d a d e g r a n i te .

S O L E D A D E G R A N I TE

E F1 7 6 9 +/- 2 Ma 1750

5 .0

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U-Pb ZIRCON GEOCHRONOLOGY OF PRECAMBRIAN TIN-BEARING CONTINENTAL-TYPE ACID M AGMATISM 329

T h e U - P b z i r c o n ag e o f 1 76 9 + 2 M a i s c o n -

s i d e r a b l y o l d e r t h a n t h e 4 - p o i n t R b - S r i so -

c h r o n a ge ( 1 6 3 7 + 9 3 M a ) c a l c u la t e d u s in g

p r e v i o u s l y p u b l i s h e d a n a l y t i c a l d a ta .

Sucuri grani te

T h e S u c u r i s u b v o l c a n i c g r a n i t e i s a s m a l l

( 4 .5 × 1 .5 k m ) e l l ip t i c al b o d y m a d e o f a m e -

d i u m - g r a i n e d , g r e y b i o t i t e g r a n i t e . I t i s c h a r -

a c t e r i s t i c a l l y c u t b y s m a l l s h e a r z o n e s ( B o -

t e l h o e t al ., 1 9 8 6 ) . S a m p l e G . 8 8 . 3 r e p r e s e n t s a

h o m o g e n e o u s b i o t i te g r a ni te .

U - P b i s o t o p i c a n a l y s e s o f z ir c o n f r a c t io n s

f r o m t h e S u c u r i g r a n i t e s h o w t h e p r e s e n c e o f a

s i gn i fi c an t a m o u n t o f a n i n h e r i t e d c o m p o n e n t

( F i g . 5 ) . I n c r e a s i n g U c o n t e n t s i n t h e z i r c o n s

a r e a c c o m p a n i e d b y a g e n e r a l i n c r e a s e i n

2°Tpb/2°6pb ages ( see Tab le 1 ) . Th is fe a tu re i s

c o n t r a r y t o t h e n o r m a l r e l a t i o n s e e n i n c o g e -

n e t i c z i r c o n p o p u l a t i o n s a n d c a n b e e x p l a i n e d

b y th e p r e s e n c e o f a n i n h e r i t e d c o m p o n e n t

h a v i n g h i g h e r U c o n t e n t s c o m p a r e d t o t h e

y o u n g e r c o m p o n e n t . A b e s t- f it r e g r e s s io n l i n e

( 2 0 % p r o b a b i l i t y o f f i t ) c a l c u l a t e d f o r t h e

p o i n t s w i t h i n h e r i t a n c e s u g g e s t s a n A r c h a e a n

a g e ( c a. 3 .3 G a ) f o r t h e o l d c o m p o n e n t . A

l o w e r i n t e r c e p t a ge o f 1 7 61 w a s o b t a i n e d f r o m

t h i s r e g r e s s i o n .

O n e c o n c o r d a n t ( 0 .3 % d i s c o r d a n t ) a n a ly s is

w a s o b t a i n e d ( a n a l y s i s 1 0 i n T a b l e 1 ) . R e g r e s -

s i o n t h r o u g h p o i n t s 11 a n d 1 3 w o u l d g i v e a n

u n r e a l i s t i c n e g a t iv e v a l u e f o r t h e l o w e r i n t e r-c e p t a g e , s u g g e s t i n g t h a t o n e o r b o t h h a v e a

c o m p l e x P b i s o t o p i c c o m p o s i t i o n . F o r e x a m -

p l e , i t i s p o s s i b l e t h a t f r a c t i o n s G . 8 8 . 3 . 2 B

( a n a l y s is 1 3 ) m a y a l s o h a v e a m i n o r i n h e r i t e d

c o m p o n e n t c o m p o u n d e d b y a d d i ti o n a l re c e nt

l e a d - l o s s . T h e r e f o r e , t h e b e s t e s t i m a t e f o r t h e

crys ta l l i za t io n age o f th i s g ra n i te i s 1767 _+ 10

M a w h i c h i s t h e 2 ° 7 p b F ° 6 p b a g e c o m p u t e d f o r

t h e c o n c o r d a n t a n a l y si s . T h e a s s i g n e d e r ro r o f

_+ 1 0 M a t a k e s i n t o c o n s i d e r a t i o n a l l r e a s o n a -

b l e p r o j e c ti o n s t o t h e c o n c o r d a n t p o i n t .

Serra da M esa grani te

T h i s g r a n i t i c b o d y f o r m s t h e c o r e o f a l ar g e

b r a c h y a n t i c l i n e s t r u c t u r e t r e n d i n g N E - S W

( F i g. 2 ) . I t is e n t i re l y s u r r o u n d e d b y m e t a s e -

d i m e n t s o f t h e S e r ra d a M e s a G r o u p . M a r i n i

a n d B o t e l h o ( 1 9 8 6 ) a r g u e t h a t t h e a ge r e la -

t i o n s h i p b e t w e e n t h e g r a n i te a n d t h e e n c l o s i n g

s u p r a c r u s t a l r o c k s o f t h e S e r r a d a M e s a G r o u p

S U C U R I G R A N I T E

: ~ ~ 8 5 0 ~

.33 ~ 1 8 3 0 J

"~ 1767 + /- 10 Ma 1810

.32 ~ 1790

1 7 ~ ~ 3 0 3 2 8 7 Ms

1 7 5 0 ~ / ~ ~ ' - 1 1

.31 t 7 3 ~ ~ 1 3 ~ - 1 0

1690 / /.30 r~ / 207__..235..

/ ~ P b / - ~ U

~ t o 6 50 MaI I I [ I

4.2 4.4 4.6 4.8 5.0

F ig . 5 . U - P b c o n c o r d i a d i a g r a m f o r z i r c o n s f r o m t h e S u c u r i g r a n i te .

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3 3 0 M . M . P I M E N T E L E T A L .

c a n n o t b e r e l ia b l y e s ta b l i s h e d i n t h e f i el d b e-

c a u s e t h e r e is n o e v i d e n c e t h a t t h e s u p r a c r u s t a l

r o c k s h a v e b e e n c u t b y t h e g r a n i t i c r o c k s . I n -

c r e a s i n g c a t a c l a s t i c d e f o r m a t i o n f r o m t h e

c e n t r e t o t h e m a r g i n o f t h e p l u t o n s u g ge s ts th a t

i t h a s b e e n t e c t o n i c a l l y e m p l a c e d i n t o i t s p r es -e n t s e t t i n g b y d i a p i r i c u p r i s e a f t e r o r i g i n a l

c r y s t a ll i z a ti o n . H o w e v e r , r e c e n t d e t a i l e d f i e ld

w o r k h a s i d e n t i f i e d e v i d e n c e o f a n i n t r u s i v e

r e l a t i o n s h i p b e t w e e n t h e g r a n i t e a n d t h e s u r -

r o u n d i n g s u p r a c r u s t a l u n i t s .

Z i r c o n f r a c t i o n s a n a l y s e d c o n s i s t o f l o n g

p r i s m a t i c c r y s t a l s w i t h l e n g t h : w i d t h r a t i o s o f

c a . 5 : 1 . T h e y a r e p i n k a n d m a r k e d l y r i c h i n

f l u id i n c l u s io n s . D e s p i t e a b u n d a n t f l u id i n c lu -

s i o n s , n o a n o m a l o u s i n c r e a s e i n t h e c o m m o n

P b c o n t e n t w a s o b s e r v ed . N o c o r e - o v e rg r o w t h

r e l a t io n s h i p s w e r e s e e n .

T h e t h r e e z i r c o n f r a c t i o n s i n v e s t i g a t e d a r e

n o t c o l l i n e a r ( F i g . 6 ) a n d d o n o t f i t a s i n g l e

r e g r e s s i o n l in e . T h e y a r e b e t w e e n c a . 1 .2 % a n d

4 . 0% d i s c o r d a n t , d e s p i t e i n t e n s e a b r a s i o n . T h e

s c a t t e r e d d a t a p a t t e r n p o s s i b l y r e f l e c t s t h e s u -

p e r i m p o s i t i o n o f m u l t i p l e P b l o ss s ta g es . M a x -

i m u m a n d m i n i m u m u p p e r i n t er c e pt a ge va l-

u e s , h o w e v e r , c a n b e i n f e r r e d f r o m p r o j e c t i o n s

1 a n d 2 , re s p e c t i v e ly , in F i g . 6. T h e m a x i m u m

e s t i m a t e o f th e a g e ( 1 6 1 4 M a ) i s g i v e n b y l in e

1 , c o n s t r u c t e d t o p a s s t h r o u g h p o i n t s 1 6 a n d

1 4. T h i s g i v e s a l o w e r i n t e r c e p t a g e o f c a . 1 1 2 6

M a . T h i s m a x i m u m e s t im a t e f o r t h e a g e a g re e s,

w i t h i n e r r o r , w i t h a P b - P b w h o l e - r o c k i so -

c h r o n a g e o f 1 6 5 8 _ 4 4 M a o b t a i n e d f o r sa m -p l e s o f t h e S e r r a d a M e s a a n d S e r r a B r a n c a

g r a n i t i c i n t r u s i o n s ( P . T a y l o r, p er s . c o m m u n . ,

1 9 8 9 ) . A m i n i m u m a g e o f 1 5 7 4 M a c a n b e ca l-

c u l a t e d b y c o n n e c t i n g p o i n ts 1 4 a n d 1 5 t o t h e

o r i g i n ( l i n e 2 ) .

E v e n c o n s i d e r i n g t h e l a r g e u n c e r t a i n ty i n t h e

w h o l e - ro c k R b - S r " e r r o r c h r o n " a ge ( 1 3 4 0 _

1 79 M a ) c o m p u t e d f o r n i n e t e e n s a m p l e s o f t h is

gran i t i c body , th i s age va lue i s s t i l l s ign i f i -

c a n t ly l o w e r t h a n t h e U - P b z i rc o n a ge .A l t h o u g h a p r e c is e U - P b z i r c o n a ge w a s n o t

o b t a i n e d , t h e d a t a p r e s e n t e d h e r e i n d i c a t e t h a t

t h e e m p l a c e m e n t o f t h e S e r r a d a M e s a g r a n it e

t o o k p l a c e a t l e a s t c a . 1 5 0 M a a f t e r t h e i n t r u -

s i o n o f t h e g r a n i t i c r o ck s o f t h e R P S a n d t h e

c r y s t a l l i z a t i o n o f th e A r a i r h y o l i t e s . T h i s c o n -

t r a d i c t s p r e v i o u s s u g g e s t i o n s b y M a r i n i a n d

B o t e l h o ( 1 9 8 6 ) a n d M a r i n i e t a l. ( 1 9 8 6 ) t h a t

t h e g r a n i t i c m a g m a t i c a c t i v i t y i n t h e R T S i s

o l d e r t h a n t h a t i n th e R P S , p r e - d a t i n g t h e b e -

SERRA DA MES A GRANITE

Z) 1630

.285 ~ 1 6 1 4 ~ 1 6 1 4a

.280 ~ 1 5 7 4 M a ~

/ /

, ~ / ~ o 0 M a , , , ,

3.4 3.5 3.6 3.7 3.8 3.9

Fig. 6. U-P b concordia diagram for z i rcons from the Serra da Me sa grani te.

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U-Pb ZIRCON GEOCHRONOLOGY OF PRECAMBRIAN TIN-BEARING CONTINENTAL-TYPEACID MAGMATISM 331

g i n n i n g o f t h e s e d i m e n t a t i o n o f th e A r a i

G r o u p .

Discussion--geological implications

T h e r e s u l t s p r e s e n t e d a b o v e s h e d n e w l i g h to n t h e t i m i n g o f m a g m a t i c e v e n t s i n t h e G oi~ is

t i n p r o v i n c e . T h e U - P b z i r c o n a g es f o r t h e S u -

c u r i a n d S o l e d a d e g r a n i t e s f r o m t h e R P S s h o w

t h a t t h e y a r e a p p r o x i m a t e l y c o e v a l w i t h t h e

i n i ti a t io n o f t h e A r a i G r o u p s e d i m e n t a t i o n a t

a p p r o x i m a t e l y 1 7 70 M a a g o . I t c o n f i r m s t h e

i n t e r p r e ta t i o n o f M a r i n i a n d B o t e lh o ( 1 9 8 6 )

t h a t t h i s g r a n i t ic m a g m a t i s m i s t y p i c a ll y a n o -

r o g e n i c ( A - t y p e ) a n d w a s f o r m e d a t s h a l l o w

c r u s t al l e v e ls i n a n e x t e n s i o n a l , i n t r a c o n t i n e n -

t al e n v i r o n m e n t . T h e g e o c h e m i c a l c h ar a c te r -

i st ic s o f t h e g r a n i t e s a n d r h y o l i te s a r e c o m p a t -

i b le w i t h d e r i v a t i o n o f t h e o r i g in a l m e l t b y r e -

m e l t i n g o f p re - e x i st i n g l o w e r s i a li c c ru s t ( M a r -

i n i a n d B o t e l h o , 1 9 8 6 ) . I n f a c t , t h e U - P b z i r -

c o n d a t a d e m o n s t r a t e t h a t o l d e r c r u s t in e x ce s s

o f 2 . 2 G a i s l i k e l y t o b e p r e s e n t i n t h e r e g i o n .

I n h e r i t a n c e f o u n d i n z i r c o n s o f t h e A r a i r h y o -

l it e i n d i c a t e t h e p r e s e n c e o f e ar l y P r o t e r o z o i c

( T r a n s a m a z o n i a n ) z i r c o n s i n t h e s i a l i c b a s e -

m e n t i n th e R P S r e gi o n.T h e U - P b d a t a f o r t h e S e r ra d a M e s a g r a n it e

i n d i c a t e t h a t t h i s g r a n i t e c r y s t a l l i z e d a b o u t

1 6 0 0 M a a g o , a n d i s , t h e r e f o r e , c o n s i d e r a b l y

y o u n g e r th a n t h e g r a n it e s o f t h e R P S a n d t h e

A r a i G r o u p r h y o l i t e ( ~ 1 7 7 0 M a ) . T h e d a t a

s u g g e s t t h a t a t l e a s t t h i s p l u t o n , a n d p o s s i b l y

o t h e r g r a n i t i c i n t r u s i o n s i n t h e R T S , a r e i n t r u -

s iv e i n to t h e s u r r o u n d i n g m e t a s e d i m e n t s o f th e

S e r r a d a M e s a a n d A r a i g r o u p s . T h i s i s c o n s is -

t e n t w i t h r e c e n t d e t a i l e d f i e l d s t u d i e s w h i c h

i d e n t i f i e d i n t r u s i v e r e l a t i o n s h i p s b e t w e e n t h e

S e r r a d a M e s a g r a n i t e a n d t h e e n c l o s i n g s u p r a -

c r u s t a l r o c k s .

T h e l o w e r i n t e r c e p t ag e o f ~ 6 5 8 M a f o r t h e

S o l e d a d e g r a n i t e s u g g e s t s a n e p i s o d e o f l e a d -

l os s d u r i n g t h e B r a s i l i an o e v e n t a n d c o u l d p o s -

s ib l y d a t e t h e d e v e l o p m e n t o f th e s m a l l s h e a r

z o n e s f o u n d i n t h i s g r a n i t e . T h i s P b - l o s s a g e is

c o n s i s t e n t w i t h th e o b s e r v a t i o n o f R e i s N e t o

a n d C o r d a n i ( 1 9 8 4 ) t h a t th e s e g r a n i t i c /r h y o l -

i t i c r o c k s i n t h a t p a r t o f G o i ~ i s h a v e b e e n i n -

v o l v e d i n t h e r e g i o n a l B r a s i l ia n o t e c t o n o - t h e r -

m a l e v e n t a s r e v e a l e d b y t h e r e s e t ti n g o f K - A r

a n d R b - S r i s o t o p i c s y s t e m s .

I t w a s o b s e r v e d t h a t , i n t h e c a s e s o f t h e S e r r ad a M e s a a n d S o l e d a d e g ra n i t es , w h o l e - r o c k

R b - S r i s o t o p i c s y s t e m a t i c s y i e l d e d a g e s t h a t

a r e c o n s id e r a b l y y o u n g e r th a n t h e U - P b z ir -

c o n a g e s . T h e R b - S r r e s u l t s f o r m o s t g r a n i t i c

p l u t o n s i n b o t h s u b - p r o v i n c e s a l i g n a b o u t

r e g r e s s i o n l i n e s c o r r e s p o n d i n g t o a g e s o f c a .

1 4 00 M a a n d m a y b e i n d i c a t i n g re s e tt i n g o f th e

R b - S r i s o t o p i c s y s t e m . T h i s m i g h t h a v e b e e n

c a u s e d b y r e - d i s t r i b u t i o n o f 875r a c c o m p a n y -

i n g h y d r o t h e r m a l a l t e r a t i o n o r m e t a m o r -

p h i s m , w i t h S r b e i n g f lu s h e d o u t o f t h e w h o l e -r o c k i so t o pi c s y st em s b y h y d r o t h e r m a l / m e t a -

m o r p h i c f lu i d s.

T h e n e w g e o c h r o n o l o g i c a l r e s u lt s h a v e m a -

j o r g e o lo g i c al im p l i c a t i o n s i n t e r m s o f r e g io n a l

c o r r e l a t i o n o f t h e s e u n i t s i n G o i~ is s ta t e w i t h

s i m i l a r r o c k a s s o c i at i o n s i n o t h e r p a r t s o f t h e

B r a z i l i a n s h i e l d . C o n t i n e n t a l - t y p e a c i d m a g -

m a t i s m a s s o c i a t e d w i t h e x t e n s i v e s e d i m e n t a r y

s u c c es s io n s o f c o n t i n e n t a l c h a r a c t e r a r e k n o w n

f u r t h e r to t h e e a st , in B a h i a a n d M i n a s G e r a i ss t at e s a n d i n e x t e n s i v e a r e a s o f t h e A m a z o n i a n

c r a t o n , w h e r e v e r y la r g e t i n d e p o s i t s a r e k n o w n

( e .g . M a c a m b i r a e t al ., 1 9 8 7 ) ( s e e F ig . 7 ) .

I n M i n a s G e r a i s s t a te , a U - P b z i r c o n ag e o f

c a . 1 7 7 0 M a , c o m p u t e d f o r t w o z i r c o n f r ac -

t i o n s is r e p o r t e d b y N e v e s e t a l. ( 1 9 7 9 ) f o r

c o n t i n e n t a l - t y p e r h y o l i t i c r o c k s ( C o n c e i 9 a o d o

M a t o D e n t r o r h y o l i t e ) f r o m t h e b a s e o f t h e E s -

p i n h a ~ o S u p e r g r o u p , w h i c h i s a d o m i n a n t l y

d a s t i c s e d i m e n t a r y s u c c e s s io n e x t e n d i n g i n t h e

N - S d i r e c ti o n f r o m t h e s t at e o f M i n a s G e r a i s

t o t h e s o u t h o f P i a u i s t a te ( s e e F ig . 7 ) . F u r t h e r

i s o t o p i c w o r k c a r r i e d o u t o n t h e r h y o l i t e s fr o m

t h e C o n c e i g 5 o d o M a t o D e n t r o a r e a r e v e a l e d a

y o u n g e r U - P b z i r c o n a g e o f 1 7 1 1 + 48 M a ( M a -

c h a d o e t a l . , 1 9 8 9 b ) . Z i r c o n f r a c t i o n s f r o m

r h y o l i t e s f r o m t h e s a m e S u p e r g r o u p i n t h e

S e r r o a n d D e s e m b a r g a d o r O t o n i r e g i o n s , f u r -

t h e r t o t h e s o u t h , g a v e u p p e r i n t e r c e p t a g es o f

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3 5 S - { -

F i g . 7 . D i s t r i b u t i o n o f t h e m a i n e a rl y - t o m i d - P r o t e r o z o i c

g r a n i t e - r h y o l i t e a s s e m b l a g e s i n t h e B r a z i l i a n s h i e l d , r e -

f e r r e d t o i n t h e t e x t . A r e a s i n b l a c k r e p r e s e n t t h e d i s t r i -

b u t i o n o f t h e s u p r a c r u s t a l s e q u e n c e s; A = C e n t r a l A m a -

z o n i a n P r o v in c e ; B = A r a i G r o u p ; C = E s p i n h a q o

S u p e r g r o u p ; a = P h a n e r o z o i c s e d i m e n t a r y b a s i n s ; b = L a t e

P r o t e r o z o i c o r o g e n i c a r e a s ; 1 = A m a z o n i a n c r a t o n ; 2 = S ~ o

F r a n c i s c o c r a t o n .

1 71 5 ± 2 M a a n d 1 7 52 + 2 M a , r e s p e c t i v e l y

( M a c h a d o e t a l. , 1 9 8 9 b ) . I n th e P a r a m i r i m r e-g i o n ( B a h i a s t a t e ) , r h y o l i t e s b e l o n g i n g t o t h e

R i o d o s R e m 6 d i o s G r o u p ( b a s e o f t h e E s p in -

h a g o S u p e r g r o u p ) h a v e t i n - b e a r i n g g r e i s e n s

a n d a r e a s s o c i a t e d w i t h e v o l v e d g r a n i t e s h a v -

i n g a U - P b z i r c o n a g e o f 1724___ 5 M a ( T u r p i n

e t a l . , 1 9 8 8 ) . T h e r e i s , t h e r e f o r e , a r e l a t i v e l y

g o o d t e m p o r a l c o r r e s p o n d e n c e b e t w e e n t h e

c o n t i n e n t a l - t y p e a c i d m a g m a t i c e v e n t a s s o c i -

a t e d w i t h t h e A r a i G r o u p i n G o if is a n d t h a t a s-

s o c i a t e d w i t h t h e E s p i n h a g o S u p e r g r o u p i n

B a h i a a n d M i n a s G e r a i s .

V e r y f e w U - P b z i r c o n d a t a a r e a v a i l a b l e f o r

t h e g r a n i t i c / r h y o l i t ic p r o v i n c e s o f t h e A m a -

z o n i a n r e g io n . T h e c h r o n o l o g y o f m a g m a t i c

e v e n t s i n t h a t a r e a i s m a i n l y b a s e d o n w h o l e -

r o c k R b - S r i s o c h r o n r e s u l t s . N e v e r t h e l e s s , a

g e n e r a l c o r r e l a t i o n b e t w e e n t h e A - t y p e g r a n i -

t ic m a g m a t i s m i n G o if is a n d i n t h e A m a z o n i a n

c r a t o n c a n b e e n v i s a g e d , a s a l r e a d y s u g g e s t e d

b y D a l l ' A g n o l e t a l . ( 1 9 8 7 ) . D a l l ' A g n o l e t a l .

i d e n t i f y o n e m a i n m i d - P r o t e r o z o i c A - t y p e

g r a n i t i c / r h y o l i t i c a s s e m b l a g e i n t h e C e n t r a l

A m a z o n i a n P r o v i n c e , i n th a t c r a t o n i c a re a . T h e

g r a n i t es f r o m t h a t a r e a h a v e R b - S r a g es v a r y -

i n g f r o m 1 8 00 t o 1 4 00 M a . U - P b z i r c o n d a t aa r e r e p o r t e d f o r t h e S e r r a d o s C a r a j f i s g r a n i t e

i n t h e s o u t h e a s t e r n p a r t o f t h e p r o v i n c e . T h e y

y i e l d e d a n u p p e r i n t e r c e p t a g e o f 1 8 2 0 ___4 9 M a

( W i r t h e t a l. , 1 9 8 6 ) . O l d e r U - P b z i r c o n ag e s

( f r o m 1 8 74 to 1 88 5 M a ) a r e r e p o r t e d f o r t h r e e

a n o r o g e n i c g r a n it e s i n t h e C a r a jf is a r e a ( M a -

c h a d o e t a l. , 1 9 8 8 ) .

T h e m a g m a t i c e v e n t t h a t p r o d u c e d v o l u m i -

n o u s a l k a l i - r i c h t i n - b e a r i n g g r a n i t e s a n d a s s o -

c i a t e d r h y o l i t e s a t ,-, 1 8 0 0 - 1 7 0 0 M a i n t h e

A m a z o n i a n r e g io n , w h i c h is c o n t e m p o r a n e o u s

w i t h s i m i l a r e v e n t s i n G o i~ is , B a h i a a n d M i n a s

G e r a i s , i s k n o w n a s t h e A k a w a i a n e p i s o d e ( Is -

s l er a n d L i m a , 1 9 8 7 ) . T h i c k r h y o l i t ic b e d s i n -

t e r c a l a t e d w i t h c o a r s e - g r a i n e d c l a s t i c s e d i -

m e n t s t h a t c o v e r l a rg e a r ea s o f t h e C e n t r a l

A m a z o n i a n P r o v i n c e h av e y i el d e d R b - S r a g es

i n t h e r a n ge 1 9 0 0 - 1 8 0 0 M a ( U a t u m a v o l ca n -

i c s ) ( C u n h a e t a l ., 1 98 1; S a n t o s , 1 9 8 4 ) .

Y o u n g e r R b - S r a g es ( 1 6 5 0 - 1 6 0 0 M a ) a r e a l so

r e p o r t e d f o r t h e T e l e s P i r e s a c i d v o l c a n i c s( T a s s i n a r i e t a l ., 1 9 7 8 ) . A c c o r d i n g t o

D a l l ' A g n o l e t a l . ( 1 9 8 7 ) t h e r e a r e s e v e r a l o c -

c u r r e n c e s o f u n d a t e d a c i d p y r o c l a s t ic r o c k s i n -

t e r c a l a t e d i n t h e s e d i m e n t a r y c o v e r s , i n d i c a t -

i n g v o l c a n i c e p i s o d e s o f v a r i e d a g e s i n t h e

C e n t r a l A m a z o n i a n P r o v i n c e . T h e r e f o r e , d e -

s p i te t h e f a c t t h a t t h e c h r o n o l o g y o f e v e n t s i n

t h e A m a z o n i a n r e g i o n i s n o t v e r y w e l l c o n -

s t r a i n e d , t h e r e i s s o m e e v i d e n c e i n d i c a t i n g

g e n e r a l c o n t e r r i p o r a n e i t y b e t w e e n t h e c o n t i -

n e n t a l m a g m a t i c e v e n t s i n G o if is , M i n a s G e r -

a i s a n d B a h i a , i n c e n t r a l B r a z i l, a n d a t l e a st p a r t

o f t h e A - t y p e a c i d m a g m a t i c e p i s o d e i n t h e

C e n t r a l A m a z o n i a n P r o v i n c e . T h e m a j o r d i f -

f e r e n c e b e t w e e n t h e g r a n i t e / r h y o l i t e a s s e m -

b l a g e s i n t h i s p r o v i n c e a n d t h o s e i n c e n t r a l /

e a s t e r n B r a z i l , i s t h a t t h e l a t t e r h a v e b e e n i n -

v o l v e d i n a l a t e r t e c t o n i c e p i s o d e , t h e B r a s i l i -

a n o / P a n - A f r i c a n o r o g e n i c e v e n t ( ca . 6 0 0 M a

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U-Pb ZIRCON GEOCHRONOLOGY OF PRECAMBRIAN TIN-BEARING CONTINENTAL-TYPEACID MAGMATISM 333

a g o ) , w h i l e t h e P r e c a m b r i a n t e r ra n e s i n t h e

c e n t ra l p o r ti o n o f t h e A m a z o n i a n c r a t o n h a v e

b e e n s t ab l e s i n c e e a r l y - P r o t e r o z o i c t i m e s .

T h e m a g m a t i c e v e n t s d i s c u s s e d h e r e a r e

b r o a d l y c o n t e m p o r a n e o u s w i t h t h e e p i s o d e s

t h a t g a v e r i se t o t h e e x t e n s i v e g r a n i t e - r h y o l i t ep r o v i n c e s b e t w e e n c a . 1 8 0 0 a n d 1 3 40 M a a g o

i n t h e m i d c o n t i n e n t r eg i o n o f N o r t h A m e r i c a

( V a n S c h m u s a n d B i c k f o r d , 1 98 1; V a n S c h m u s

a n d H i n z e , 1 9 8 5; B i c k f o r d e t a l. , 1 9 8 6 ; V a n

B r e e m e n a n d D a v i d s o n , 1 9 8 8) a n d i n o t h e r

p a r ts o f t h e N o r t h e r n H e m i s p h e r e s u c h a s

s o u t h G r e e n l a n d ( G u l s o n a n d K r o g h , 1 9 7 5 )

a n d t h e e as t E u r o p e a n p l a t fo r m , U S S R ( S a lo p ,

1 9 7 7 ) w h e r e a n o r o g e n i c g r a n i t i c r o c k s d a t e d

b e t w e e n 1 6 00 a n d 1 7 50 M a a r e p re s e n t .

A c c o r d i n g t o V a n B r e e m e n a n d D a v i d s o n

( 1 98 8 ) , t h e a g e s o f t h e g r a n i t e - r h y o l i t e a s s o -

c i a ti o n s i n t h e m i d c o n t i n e n t r e g i o n o f N o r t h

A m e r i c a f a l l i n t o t w o m a j o r g r o u p i n g s . T h e

o l d e r ( 1 8 0 0 - 1 6 0 0 M a ) is b r o a d l y c o n t e m p o r -

a n e o u s w i t h t h e g r a n i t e - r h y o l i t e a s s o c i a t i o n

d a t e d i n G oi~ is , M i n a s G e r a i s a n d B a h i a s t a t e s

i n c e n t r a l - e a s t e r n B r a z i l , a n d p o s s i b l y w i t h

t h o s e f o r m e d d u r i n g t h e e a r l i e r p h a s e s o f A -

t y p e f e l s i c m a g m a t i s m i n t h e A m a z o n i a n r e -

g i o n ( t h e A k a w a i a n e p i s o d e ) .I n t h e B r a z i l ia n s h i e l d , t h e c a u s e s o f s u c h e p -

i s o d e s a r e s t il l n o t v e r y w e l l u n d e r s t o o d . I n t r a -

c o n t i n e n t a l , e n t i r e l y a n o r o g e n i c s e tt i ng s , w h e r e

r i f t in g o f t h e c o r f t i n e n t a l c r u s t a n d a s s o c i a t e d

a c i d m a g m a t i s m i s r e l a t e d t o a m a n t l e h o t s p o t ,

h a v e b e e n s u g g e s t e d b y M a r i n i e t a l. ( 1 9 86 ) t o

e x p l a i n t h e g e n e s i s o f t h e g r a n i t e - r h y o l i t e

p r o v i n c e i n G o i ~ i s . D a l l ' A g n o l e t a l . ( 1 9 8 7 )

s u g g e s t t h a t u n e x p o s e d m a f i c m a g m a c o u l d

h a v e b e e n t h e h e a t s o u r c e f o r t h e e x t e n s i v e

c r u st a l r e m e l t in g e v e n t i n t h e A m a z o n i a n c r a-

t o n d u r i n g m i d - P r o t e r o z o i c t i m e s . H o w e v e r ,

t h e g e n e r a l l a c k o f g e o c h r o n o l o g i c a l w o r k i n

b o t h r e g i o n s d o e s n o t a l l o w d e f i n i t i v e c o n c l u -

s i o n s . I n f a c t , e x t e n s i v e p r o v i n c e s o f fe l s ic

m a g m a t i c r o c k s i n c o n t i n e n t a l s e t t i n g s c a n b e

s u b d u c t i o n - r e l a t e d . O n e e x a m p l e i s t h e c a .

1 00 0 k m w i d e " i g n i m b r i t e f l a r e- u p " t e r r a in o f

t h e W e s t e r n U S C o r d i l l e r a , e x t e n d i n g f r o m t h e

S n a k e R i v e r P l a i n t o c e n t r a l M e x i c o . S u c h a

s e t ti n g , a c c o r d i n g t o V a n B r e e m e n a n d D a v -

i d s o n ( 1 9 8 8 ) , c o u l d re p r e s e n t a r e c e n t a n a -

l o gu e o f t h e m i d - P r o t e r o z o i c g r a n i t e - r h y o l i t e

p r o v i n c e s o f t h e m i d c o n t i n e n t r e g io n o f N o r t h

A m e r i c a .

Con c l u s i o n s

T h e g e o c h r o n o l o g i c a l d a t a o b t a i n e d r e v e a l

t h e e x i s t e n c e o f a n i n t r a -p l a t e , c o n t i n e n t a l - t y p e

a c i d m a g m a t i c e p i s o d e a f f e c t i n g t h e e a s t e r n

p o r t i o n o f t h e T o c a n t i n s S t r u c t u r a l P r o v i n c e i n

c e n t r a l B r a z il a t c a . 1 7 7 0 M a a g o . T h i s e v e n t is

r e p r e s e n t e d b y t h e t i n - b e a r i n g , A - t y p e g r a n i t ic

p l u t o ns o f t h e R i o P a r a n ~ S u b - P r o v i n c e a n d

b y t h e r h y o l i t e s o f t h e b a s e o f t h e A r a i G r o u p .

A c o n s i d e r a b ly y o u n g e r U - P b z i r co n a g e

( 1 5 7 4 - 1 6 1 4 M a ) w a s o b t a i n e d f o r o n e g r a n i -

t i c p l u t o n o f t h e R i o T o c a n t i n s S u b - P r o v i n c e

( S e r r a d a M e s a g r a n i t e ) . I t re v e a l s a d i f f e r e n t

p h a s e o f i n t r u s i o n o f t in - g r a n i t e s a n d r a is e s t h e

p o s s i b i l i t y t h a t t h e o t h e r g r a n i t e s i n t h e R T S

a r e o f a p p r o x i m a t e l y t h e s a m e a g e , a s t h e y a r e

s i m i l a r w i t h r e s p e c t t o t h e i r f i e l d , s t r u c t u r a l ,

p e t r o g r a p h i c a n d g e o c h e m i c a l c h a r a c t e r i s ti c sa n d t o t h e s ty l e o f t h e t i n m i n e r a l i z a t i o n ( M a r -

i n i a n d B o t e l h o , 1 9 8 6 ) .

T h e d a t a p r e s e n t e d h e r e d e m o n s t r a t e t h a t t h e

g r a n i t e - r h y o l i t e p r o v i n c e i n c e n t r a l - e a s t e r n

G o i ~ i s , c o v e r i n g l a r g e a r e a s i n b e t w e e n t h e

A m a z o n i a n a n d S ~ o F r a n c i s c o c r a t o n s ( F i g .

7 ) , i s b r o a d l y c o n t e m p o r a n e o u s w i t h s i m i l a r

r o c k a s s o c i a t i o n s e x p o s e d i n t h o s e c r a t o n i c

r e g i o n s .

I n c o n c l u s io n , a n e n o r m o u s v o l u m e o f al -

k a l i - r i c h , c o n t i n e n t a l - t y p e a c i d m a g m a s w a s

p r o d u c e d i n s e v e r a l a r e a s o f t h e B r a z i l i a n P r e -

c a m b r i a n s h i e l d a t c a . 1 8 0 0 - 1 6 0 0 M a a g o, a n d

a l so a t l a t e r t im e s , i n t h e c a s e o f t h e A m a z o n -

i a n r e g i o n . T h e p a r e n t a l m a g m a s m a y h a v e

b e e n d e r i v e d f r o m t h e r e m e l t i n g o f p r e -e x i st -

i n g e a r l y P r o t e r o z o i c o r A r c h a e a n c o n t i n e n t a l

c r u s t, a s s u g g e st e d b y t h e i n h e r i t a n c e p a t t e r n s

i n t h e z i r c o n s f r o m t h e R P S i n G o i~ is .

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3 3 4 M.M. PIM ENTEL ET AL

Acknowledgements

The authors would like to thank the finan-

cial support provided by CIDA (Canadian In-

ternational Development Agency) for the U-

Pb analysis in the Royal Ontario Museum. Wealso thank Stephen Moorbath and Paul Taylor

for reviewing early versions of the manuscript.

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U-Pb ZIRCON GEOCHRONOLOGY OF PRECAMBRIAN TIN-BEARING CONTINENTAL-TYPEACID MAGMATISM 3 3 5

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