News from the North American Proterozoic

71 M~kJuR AND T d A C E ELEM]~NT AJ ]L:C LAfIq<;J I~, C.~[3dSL.;,'~i'/~S 'iF B i l l i i A a-'.Sll~,!'L:~(,;if'.[K,., SOUTH l,;blA M.~.MALUi{, d.it~}k.i,bi{k, a'.id [.i.i{l.[L~(~[±~{ ~ n t of Geology, Univerz~ity of blysore, l.i~nasagango%ri, ]lysore-57) OC6 India) R e p r e s e n t a t i v e carbon:{te rock s,-'tmples from B h i m a b~sin (late P r o t e r o z o i e Cambrian) have been used to ew{luage t:~e m i n e r a l o g i c a l a s s o c i a t i o n of trace and major ~lements. Roc':s are eryptoerysta~ iine in lhatute, i~ence an.dysed by X~D. The dolomite 9nd ca[citic limestones are disttn:7]isi~ed by the s t a i n i n g techntnue, but it [5 observed that the d o l o m i t i c n~t,,re is r~ntchy. The e~rbon~tc ~ h-.ve 0ominnntly qunrtz, cn~cite, ~lumine, mr~rnr, n i ' J m 9nO [ran fol lowed by other elements. The e s t i m a t i o n i-~ m'~,le by , i s t , ~ IL-7~,I &'~S i n s t r u m r u t , The t r ~ c e e! emeqts recorded ~ r e f b , Zn, ']u, ']r, Ba, Co, Cr znd i~b. The distributt ,n n.~ttern )f :~qi~r ~sd tr,ce ~Icments is d i:~eus: cd.

PROTEROZOIC CRUSTAL GROWTH: UNDERPLATING AND MAGMATISM M.T.McCULLOCH, L.P. BLACK and R.W. PAGE. (Research School of Earth Sciences, Aust. National University) Underplating, as a mechanism for the growth and development of continental crust has generally been envoked when there is an absence of evidence for growth via lateral accretion at plate margins or subduction zones. Direct evidence for underplating and its role in crustal growth is obscure. Here we report Sm-Nd isotopic data which indicates that large volumes of underplated material can exist for periods of >1800Myr. The underplated material is subject to reactivation by subsequent magmatic events, which in turn form new underplates. In the Mt lsa, Pine Creek and Georgetown inliers, volcanic and platonic rocks of at least three magmatic episodes (1570, 1760 and 1860Myr) have e.Nd values in the relatively narrow range of from -2.0 to -4.0. This is mterpretted as protolith formation in the early Proterozoic and is part of what appears to be a worldwide magmatic event. Silurian granites from the Georgetown inlier have eNd values of --16 which are consistent with derivation from the same p~otolith, thus indicating the existence of the underplated protolith for periods >1800Myr. The close relationship between magmatism and underplating is illustrated by the positive end values (+2 to+3.5) of androgenic granites from Mt Isa, which are fff/erpretted as partial melts of underplates formed during the 1570-1870Myr magmatic events. This second younger underplate is also represented in the Permo-Carboniferous granites of the Georgetown inlier which have eNd values o f ~ - 8 .

IO 5 0

~,.~

-20 -25

F.McDERMOTT and C.J. HAWKESWORTH (Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K.). Sr and Nd isotope data for f'me-grained elastic sediments indicate that Sr model ages are younger than Nd model ages, due in large part to the intracrustal fraetionation of Rb/Sr relative to Sm/Nd. Model age ratios (TNdDMfI~SrDM) are typically in the range 1 to 3, and in a model of continuous intracrustal reworking we infer that Rb/Sr ratios have increased at average rates of 1.2 - 2.2 per Ga. Most A.rchaean free-grained elastic sediments have Rb/Sr ratios less than 1, whereas post-Arehaean elastic sediments typically have Rb/Sr ratios of 0.7 - 10. A_rehaean elastic sediments typically have Nd model ages which are similar to their stratigraphie ages, and their lower Rb/Sr ratios are therefore attributed to the relatively short residence times of Archaean sediment sources in the reworked upper emst. The rates of Rb/Sr fractionation inferred from the isotope data (1.2 - 2.2 per Ga) are consistent with the observed increases in the Rb/Sr ratios of clastie sediments since the Archaean, and they imply that the rate of increase of 87Sr/86Sr (by decay of 87Rb) in upper crustal rocks has increased from the Archaean to the present. This is also consistent with the restricted and relatively low depositional 87Sr/a6Sr ratios (< 0.725) exhibited by many late-Proterozoic fine-grained elastic sediments. Exchange of crustal Sr with mantle derived Sr via subduction of hydrothermaUy altered basalts results in a loss of crustal S7Sr to the mantle. However, taking account of the increase in Rb/Sr in the upper ernst with time reduces the total amount of 87Sr generated in the upper crust over earth history, and hence that available for recycling into the upper mantle, by about 50% compared with that estimated ~Treviously. This model further implies that the depositional Sr]86Sr ratios of fine grained clastic sediments primarily reflect the average residence time of a sediment's source material in the reworked upper crust, and predicts that many S-type granites with initial 87Sr/86Sr ratios > 0.710 should have Nd model ages in excess of 1.5 Ga. NEWS FROM THE NORTH AMERICAN PROTEROZOIC

M.A. HAMILTON, K.E. OLSON, D.WEIS*, AND S.A. MORSE. ( G e o l o g y / G e o g r . , UMass, A m h e r s t MA 0 1 0 0 3 USA and * U n i v . L i b r e de B r u x e l l e s ) . 1> E n r i c h e d N d - i s o t o p e s i g n a t u r e s o f 6 anorthosites and g a b b r o s o f n o r t h e r n L a b rador, earlier ascribed to crustal contamination, show s y s t e m a t i c b e h a v i o r i n e - f diagrams of epsilon but n o t i n l i n e w i t h any known c o m b i n a t i o n o f d e p l e t e d m a n t l e (DM> and o b s e r v e d t 3 . 6 G a ) crust. All are consistent with source enr i c h m e n t f r o m DM a t 2 . 3 + / - 0 . 1 Ga, f o l l o w e d by g r o w t h o f <-e) t o e m p l a c e m e n t ages 1 . 2 - 1 . 5 Ga. Rocks 2 . 3 G a o l d i n c l u d e u n d e f o r m e d g r a n i t e s and M u g f o r d s u p r a c r u s tals. The t e r r a n e l i e s a c r o s s 2 - 4 s t r u c t u r a l p r o v i n c e s o v e r an a r e a ~125K km2 , and r e p r e s e n t s a m a j o r m a n t l e e n r i c h m e n t , p e r h a p s e x t e n s i v e w i t h Penokean e v e n t s i n the mid-continent. (2) M a r i e r o c k s ( m e t a basalts, metagabbros) of the Adirondack H i g h l a n d s i n New Y o r k , ~ 1 . 3 Ga o l d , r e t a i n i g n e o u s c h e m x s t r y and d e f l n e a p r o v i n c e o f h i g h A I - F e b a s a l t l l k e some A r c h e a n Fe b a s a l t s and p a r e n t s t o l a t e r a n o r t h o s i t e s . F o r most e l e m e n t p a i r s , the points fall on the trend for liquids of the Kiglapalt Intrusion, 2000 km away i n L a b r a d o r . A common s o u r c e , p r o c e s s , and f r a c t l o n a t i o n hlstory are implied. (3> Pb i s o t o p e s o f

~:iglapalt component

-;0 -IS

I N T R A C R U S T A L Rb/Sr F R A C T I O N A T I O N AND THE IMPLICATIONS FOR 87Sr/S6Sr E V O L U T I O N IN THE UPPER CRUST

m

MAGMATISM

TIME (Myr)

i

i

i

500

1000

1500

i 2000

f e l d s p a r s are low in r a d i o g e n i c (MU, ~7.2) and are tied to

r a d i o g e n i c r o c k s on l i n e s i m p o s s i b l y o l d (~3.2 Gav. 1 . 3 Ga) t h a t may p r e c l u d e crustal contamination. <4) C o n v e c t i v e partial m e l t i n g may h a v e i n v o l v e d b o t h e n r i c h e d a s t h e n o s p h e r l c ( p l a g , S t , LREE) and depleted llthospheric