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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…
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 .
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.
8/2/2019 Pimentel Et Al - 1991 - U-pb Zircon Geocrhonology of Pre Cam Brian Tin-bearing Continental-type Acid Magmatism i…
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
8/2/2019 Pimentel Et Al - 1991 - U-pb Zircon Geocrhonology of Pre Cam Brian Tin-bearing Continental-type Acid Magmatism i…
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-
8/2/2019 Pimentel Et Al - 1991 - U-pb Zircon Geocrhonology of Pre Cam Brian Tin-bearing Continental-type Acid Magmatism i…
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
8/2/2019 Pimentel Et Al - 1991 - U-pb Zircon Geocrhonology of Pre Cam Brian Tin-bearing Continental-type Acid Magmatism i…
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.
8/2/2019 Pimentel Et Al - 1991 - U-pb Zircon Geocrhonology of Pre Cam Brian Tin-bearing Continental-type Acid Magmatism i…
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 .
8/2/2019 Pimentel Et Al - 1991 - U-pb Zircon Geocrhonology of Pre Cam Brian Tin-bearing Continental-type Acid Magmatism i…
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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