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Transcript of Pimentel Et Al - 1991 - U-pb Zircon Geocrhonology of Pre Cam Brian Tin-bearing Continental-type Acid...
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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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