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Archaean gneiss complex of Eastern Hebei province, North China
149 NEW RB-SR WHOLE ROCK ~ATES FOR SO,~2 FOSTOROGENIC GRANITES (S.L.) FROM THE SHAMVAHARARE GREENSTONE BELT, Z I ~ A B % ~
M.L. V l N Y U a n d
J.D. K R $ ~ R S
( Geology Department University of Zimbabwe P 0 Box MP167 Mt. Pleasant Harare)
Recent whole rock dates for some postoroEenic granites (s.l.) from the Shamva-Harare Greenstone Belt in the north-central part of Zimbabwe indicate Proterozoic ages of intrusions. This data suggests continuation of Arehaean style igneous activity in the Zimbabwe craton during the early Proterozoio era. This is important in that some of the granitic (s.l.) in%ru~ien~ ~re ~ssoeialed with ~ i ~ e n e t i e auriferous mineralization which is currently thought to he connected to late Archaean metamorphic event. The above data suggests that felsic igneous activity and hence the mineralization straddles the ArchaeanProterozoie boundary (at circa 2.5Ga in Zimbabwe). Some of the stocks have high 87Sr/86~r initial ratios c.f. Concession stock at70899 ± .00136 suggesting remelting of earlier formed crust t or derivation from a source removed from a homoKeneous mantle for a long time. Precise U-Pb zircon dating and geochemistry will allow a more realistic model of events at the Archaean Froterozoic Boundary in Zimbabwe.
ABCHAEAN GNEISS COMPLEX OF EASTERN HEBEI PROVINCE, NORTH CHINA WANGKAIYII , BRAINF. WINDLEY2 and JANE D. SILLS2 (1 I n s t i t u t e o f Geology, Academia S i n i c a PO Box 634, Beijing. 2 Department of Geology, Leicester U n i v e r s i t y , L e i c e s t e r LE1 7RH UK) The Qianax g r a n i t e - g n e i s s
dome i n c l u d e s t h r e e gneiss
complexes : the Caozhuang, the Shuichang and Dantazi, separated by shear zones or faults. The Caozhuang complex consists of supracrustal rocks and orthogneisses. The supracrustal suite comprise amphibolite, felsic gneiss, metapelite, marble, quartzite, calc-silicate and BIF. They are highly deformed, show evidence of polymetamorphism and were intruded and broken up into discrete bodies by several periods of orthogneisses. The amphibolites yielded a 3.5 Ga Sm-Nd isochron age. More recently, a similar Sm-Nd isochron age of 3.56 + 0.08 Ga for four supracrustal gneisses has been reported and interpreted ast the time of deposition. The orthogneisses in this area were mainly generated in three periods : the earliest Huangbaiyu gneiss at ca. 3.5 Ga, the coarse 9ranodiorite at 2.67 Ga, and the late granite gneiss at ca. 2.5 Ga. The Caozhuang amphibolites and nearly contemporaneouos Huangbaiyu gneisses are considered as an early Archaean bimodal sulte. The Shuichang supracrustal suite (Shuichang Group) includes greywacke, metapelite and BIF, all of them have undergone granulite facies metamorphism and occur in isoclinal synforms within a polyphase complex of leucogranites, charnockites and microcline granites. The age of the supracrustals are not known; but they pre-date the charnockites (2.65 Ga) as judged from their field relationships. The leucogranites may represent products of early partial melting with a fluid component enriched in H?O, whereas the formation of the charnockites is r~lated to the late stage of partial melting under the condition with increasing temperature and
decreasing H20/CO2 r a t i o
i n the f l u i d s .
COHERENT AND INCOHERENT LIGHT ON OLD ROCKS DEREK YORK (Department of Physics, University of Toronto, Toronto, Canada) In unravelling the earth's physico-chemical Archaean history it is important to know more than simply the crystallization or recrystallization ages of minerals. The full thermal histories must be worked out. It is argued that to achieve this it is necessary to take full advantage of the remarkable flexibility of the 4°Ar/=gAr analytical approach. While the argon method is sensitive to mild thermal events, it also has the power via hornblendes to see back through s o m e p a t t e r n s of metamorphic veils. Furthermore, with the development of the continuous laser as a step-heating tool, a wholly new light is being shed on the Archaean. Illustrations are provided using recent results obtained with this approach by a number of workers in Toronto. Data for the Barberton Mountain (Swaziland)and Red Lake (Canada) greenstone belts will be shown. The remarkable tenacity of hornblende will be demonstrated via comparisons with U-Pb ages on zircons. It will be shown how ages of ~3.45 b.y. can be obtained from detailed step-analyses of Barberton komatiites. Data for several Barberton intrusives will be presented. A cooling curve just derived for the Red Lake greenstone belt will be contrasted with a Grenville curve. Results obtained with and without the continuous laser will be presented. IC will be seen that the use of the continuous laser enables thermal histories to be extracted from millimetre-sized (or less) single grains. Evidence is also presented which suggests that argon isotopes may be used as a tracer of the convective motions set up in fluids in the crust by dike emplacement. GEOGHRQNOLOGIG~L AND ND ISOTOPIC EVIDENCES FOR THREE M61N EPISODES OF CONTINENT6L C R U S T G R O W T H IN C H I N 6 B . - Q . ZIi~, S . - K . (Inst. G e o c h e m .
FAN AND C.-X. MAD A c a d e m i a Sini,=a)
The reliable geochronological ,data from ~rchean TTG rocks with various metamorphic d e g r e e s f r o m N- and N E O h i n a m a i n l y f o c u s e d on 2.8 ~nd 2.5 Ga T h e e v e n t of 2.8 Ga w a s c o n f i r m e d at A n s h a n of L i a o n i n g ~ H e n g s h a n in S h a n x i ~ Oianxi and Fuping in H e b e i , T a i h u a in Sh~an::i and H u o q ~ u ~n ~ n h u i . R o c L s with 2.5 Ga o c c u r r e d in e v e r y 6 r e h e a n t e r rain~suLh ab H u a d i a n a n d Q i n g y u a n in H a - O h i na ,ai anx i , Z h a n g j iakou, W u t a i ,Ta ishan and Dengleng in N - C h i n a . T h e residual z i r c o n s f r o m P d ] e o z o i c granites in S - C h i n a a l s o y i e l d e d U - P b d i s c o r d i a age of 2 . 5 G a . T h e igneous activties w i t h a g e s of 1 , 6 - 2 . 0 Ga v a s t l y d i s t r i b u t e in a]] N- and S - C h i n a . T h e Nd isot o p i c ,data f r o m c a . 1 0 0 s a m p l e s of P h a nerozoi,- g r a n i t e s l~e in a r a n g e of 0.5114-0.5127.Based on p a r a m e t e r s of Sm-Nd systematics in m a n t l e and c r u s t , which were established throuqh Sm-Nd determination of E e n o z o i : vo]canlc5 a n d PrecamL, r i ~ n c r u s t a l r o c k s and c a l ,:glation o f r e c y c l i n g model o f 4 - s y s t e m s , t h e two s t a g e N d - m o d e l ages of depleted mantle were calculated. The histogram of d a t a e v i d e n t l y ~ h o w two s h a r p p e a k s of 2.5 ~n,J 2 . 8 Ga ~nd a w i d e ~ge ir,t e r v a i uf 1 . 6 - 2 . 1 Ga,whi,=h are basi,zally ,-onsistent w i t h t h o s e ,:a1,:ulated f r o m P[:, i s o t o p e s of c o n t i nent of China.Therefore~the ~-ontlnent~I ,_rust~i in E a s t GhLi,a w e r e m a , r l l y g r o w n d u r i n g t h r e e eplso,Jes of T , 3 , 2 . 5
~n,:J L . 6 - . 2 . 0
Ga.
M.L. V l N Y U a n d
J.D. K R $ ~ R S
( Geology Department University of Zimbabwe P 0 Box MP167 Mt. Pleasant Harare)
Recent whole rock dates for some postoroEenic granites (s.l.) from the Shamva-Harare Greenstone Belt in the north-central part of Zimbabwe indicate Proterozoic ages of intrusions. This data suggests continuation of Arehaean style igneous activity in the Zimbabwe craton during the early Proterozoio era. This is important in that some of the granitic (s.l.) in%ru~ien~ ~re ~ssoeialed with ~ i ~ e n e t i e auriferous mineralization which is currently thought to he connected to late Archaean metamorphic event. The above data suggests that felsic igneous activity and hence the mineralization straddles the ArchaeanProterozoie boundary (at circa 2.5Ga in Zimbabwe). Some of the stocks have high 87Sr/86~r initial ratios c.f. Concession stock at70899 ± .00136 suggesting remelting of earlier formed crust t or derivation from a source removed from a homoKeneous mantle for a long time. Precise U-Pb zircon dating and geochemistry will allow a more realistic model of events at the Archaean Froterozoic Boundary in Zimbabwe.
ABCHAEAN GNEISS COMPLEX OF EASTERN HEBEI PROVINCE, NORTH CHINA WANGKAIYII , BRAINF. WINDLEY2 and JANE D. SILLS2 (1 I n s t i t u t e o f Geology, Academia S i n i c a PO Box 634, Beijing. 2 Department of Geology, Leicester U n i v e r s i t y , L e i c e s t e r LE1 7RH UK) The Qianax g r a n i t e - g n e i s s
dome i n c l u d e s t h r e e gneiss
complexes : the Caozhuang, the Shuichang and Dantazi, separated by shear zones or faults. The Caozhuang complex consists of supracrustal rocks and orthogneisses. The supracrustal suite comprise amphibolite, felsic gneiss, metapelite, marble, quartzite, calc-silicate and BIF. They are highly deformed, show evidence of polymetamorphism and were intruded and broken up into discrete bodies by several periods of orthogneisses. The amphibolites yielded a 3.5 Ga Sm-Nd isochron age. More recently, a similar Sm-Nd isochron age of 3.56 + 0.08 Ga for four supracrustal gneisses has been reported and interpreted ast the time of deposition. The orthogneisses in this area were mainly generated in three periods : the earliest Huangbaiyu gneiss at ca. 3.5 Ga, the coarse 9ranodiorite at 2.67 Ga, and the late granite gneiss at ca. 2.5 Ga. The Caozhuang amphibolites and nearly contemporaneouos Huangbaiyu gneisses are considered as an early Archaean bimodal sulte. The Shuichang supracrustal suite (Shuichang Group) includes greywacke, metapelite and BIF, all of them have undergone granulite facies metamorphism and occur in isoclinal synforms within a polyphase complex of leucogranites, charnockites and microcline granites. The age of the supracrustals are not known; but they pre-date the charnockites (2.65 Ga) as judged from their field relationships. The leucogranites may represent products of early partial melting with a fluid component enriched in H?O, whereas the formation of the charnockites is r~lated to the late stage of partial melting under the condition with increasing temperature and
decreasing H20/CO2 r a t i o
i n the f l u i d s .
COHERENT AND INCOHERENT LIGHT ON OLD ROCKS DEREK YORK (Department of Physics, University of Toronto, Toronto, Canada) In unravelling the earth's physico-chemical Archaean history it is important to know more than simply the crystallization or recrystallization ages of minerals. The full thermal histories must be worked out. It is argued that to achieve this it is necessary to take full advantage of the remarkable flexibility of the 4°Ar/=gAr analytical approach. While the argon method is sensitive to mild thermal events, it also has the power via hornblendes to see back through s o m e p a t t e r n s of metamorphic veils. Furthermore, with the development of the continuous laser as a step-heating tool, a wholly new light is being shed on the Archaean. Illustrations are provided using recent results obtained with this approach by a number of workers in Toronto. Data for the Barberton Mountain (Swaziland)and Red Lake (Canada) greenstone belts will be shown. The remarkable tenacity of hornblende will be demonstrated via comparisons with U-Pb ages on zircons. It will be shown how ages of ~3.45 b.y. can be obtained from detailed step-analyses of Barberton komatiites. Data for several Barberton intrusives will be presented. A cooling curve just derived for the Red Lake greenstone belt will be contrasted with a Grenville curve. Results obtained with and without the continuous laser will be presented. IC will be seen that the use of the continuous laser enables thermal histories to be extracted from millimetre-sized (or less) single grains. Evidence is also presented which suggests that argon isotopes may be used as a tracer of the convective motions set up in fluids in the crust by dike emplacement. GEOGHRQNOLOGIG~L AND ND ISOTOPIC EVIDENCES FOR THREE M61N EPISODES OF CONTINENT6L C R U S T G R O W T H IN C H I N 6 B . - Q . ZIi~, S . - K . (Inst. G e o c h e m .
FAN AND C.-X. MAD A c a d e m i a Sini,=a)
The reliable geochronological ,data from ~rchean TTG rocks with various metamorphic d e g r e e s f r o m N- and N E O h i n a m a i n l y f o c u s e d on 2.8 ~nd 2.5 Ga T h e e v e n t of 2.8 Ga w a s c o n f i r m e d at A n s h a n of L i a o n i n g ~ H e n g s h a n in S h a n x i ~ Oianxi and Fuping in H e b e i , T a i h u a in Sh~an::i and H u o q ~ u ~n ~ n h u i . R o c L s with 2.5 Ga o c c u r r e d in e v e r y 6 r e h e a n t e r rain~suLh ab H u a d i a n a n d Q i n g y u a n in H a - O h i na ,ai anx i , Z h a n g j iakou, W u t a i ,Ta ishan and Dengleng in N - C h i n a . T h e residual z i r c o n s f r o m P d ] e o z o i c granites in S - C h i n a a l s o y i e l d e d U - P b d i s c o r d i a age of 2 . 5 G a . T h e igneous activties w i t h a g e s of 1 , 6 - 2 . 0 Ga v a s t l y d i s t r i b u t e in a]] N- and S - C h i n a . T h e Nd isot o p i c ,data f r o m c a . 1 0 0 s a m p l e s of P h a nerozoi,- g r a n i t e s l~e in a r a n g e of 0.5114-0.5127.Based on p a r a m e t e r s of Sm-Nd systematics in m a n t l e and c r u s t , which were established throuqh Sm-Nd determination of E e n o z o i : vo]canlc5 a n d PrecamL, r i ~ n c r u s t a l r o c k s and c a l ,:glation o f r e c y c l i n g model o f 4 - s y s t e m s , t h e two s t a g e N d - m o d e l ages of depleted mantle were calculated. The histogram of d a t a e v i d e n t l y ~ h o w two s h a r p p e a k s of 2.5 ~n,J 2 . 8 Ga ~nd a w i d e ~ge ir,t e r v a i uf 1 . 6 - 2 . 1 Ga,whi,=h are basi,zally ,-onsistent w i t h t h o s e ,:a1,:ulated f r o m P[:, i s o t o p e s of c o n t i nent of China.Therefore~the ~-ontlnent~I ,_rust~i in E a s t GhLi,a w e r e m a , r l l y g r o w n d u r i n g t h r e e eplso,Jes of T , 3 , 2 . 5
~n,:J L . 6 - . 2 . 0
Ga.