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Origin of the Variscan granitoids from the French Massif Central: A Sr, Nd isotopic study
68 IN
THE DEVELOPMENT OF THE CENTRAL EUROPEAN CONTINENTAL CRUSTSINCETHE EARLYARCHEAN
MARCANTONIO, France, DICKINp A.P. and McNUTT, R.H. (Department of GeologY, HcMaster University, Hamilton, Ontario, Canada L8S 4MI; and 8eeman, L.H., Department of Geology, N.O.M., 100- Queen's Park, Toronto, Ontario, Canada blfS 2C6)
D.GEBAUERI,I.S.WlLLIAMS2,W. COMPSTON 2 and M.GRONENFELDER1 1Laboratory for Isotope Geochemistry and Mass Spectrometry, Swiss Federal Institute of Technology, 8092 Z0rich (Switzerland) 2Research School of Earth Sciences, The Australian National University, Canberra, A.C,T. 2601 (Australia)
A S
FROTEROZOIC ~
CR~JSTAL ' I
I~E~IZXD
The gneiss terrane on Islay, an island south of Mull in Western Scotland, has always been inferred to be part of the Archean Lewisian cowlex~ although no attempt has been made to date it. Five samples were collected from Portnahaven and Port Wemyss on western Islay, which is separated from eastern Islay by the Loch Gruinart fault. In this paper, evidence from four different isotope dating techniques, along with model Nd age data, points toward a mid-Proterozoic age for the Islay gneisses. A single-stage U-Pb zircon analysis, which is 1.4~ discordant, defines a z°vPb/2°ePb age of 1780 bla. The Rb-Sr, Sm-Nd and Pb-Pb isotope systems loosely define "isochrons" ranging from 1720 Ma (Pb-Pb) to 1800 Ma (Sm-Nd) Ma. Furthermore, the Pb-Pb system defines a model m m value of 7.9 which is indicative of primary mantle derivation. Along with the U-Pb zircon determinations~ the mid-Proterozoic depleted mantle Nd model ages (averaging at about 1950 Ms) prove conclusively that the terrane is not a reworking of Archean crustal material. This would be the first case of
recognizable
Proterozoic
Geophysical
correlations
basement in S c o t l a n d , between
the
fslay
t e r r a r i a and t h e a d j a c e n t Grampian b l o c k i n d i c a t e t h a t the P r o t e r o z o i c basement may e x t e n d w e l l e a s t o f I s l a y and hence has g r e a t i m p l i c a t i o n s f o r the c r u s t a l e v o l u t i o n o f Northern B r i t a i n .
Archean components up to 3.84 b.y. old have been detected by ion-microprobe dating of zircon from various sediments and metasediments of the European Hercynides ( Moldanubicum ot NE Bavaria and French Central Massif ). The Early Archean age of 3.84 b.y. was obtained within the euhedral core of a detfital grain from a Moldanubian paragneiss. The same detrital zircon grain reveals at least 3 ages of metamorphic overprints - 2,59 b.y., 1.94 b.y. and 460 m.y. - and at least two cycles of mechanical abrasion during surface recycling in the Early Protarozoic and after the Pan-African cycle. It is thus the crystal with the most complex geological history ever detected. Further Arehean ages for the crystallization of primary magmatic zircons, probably in the course of continental crust formation in the first cycle provenances of the two studied areas , were found at 3.15 b.y., 2.9 b.y., 2.76 b.y., 2.65 b.y. and in a few cases between 2.5 b.y. and 2.6 b.y.. Early Protarozoic crust-forming events between about 1.75 b.y. and 2.15 b.y. play an important role especially for the sediments and metasediments of the Montagne Noire at the southern rim of the French Central massif. Middle and Upper Proterozoic ages cluster around 1.0 b.y. and ca. 600 m.y. for all analysed samples. Consequently, the deposition of the metasedimentary precursors took place after the Pan-African cycle which is in line with the Rb-Sr wholerock systematics of Central European metasediments. The ubiquitous presence of Pan-African detrital zircons in European metasediments is a strong indication that the detrital material was derived from Gondwana. However, from the presence of metamorphic units including eclogite-facies rocks which probably formed at different continental margins and at various times between aboul 500 m.y. and 330 m.y. ago it is probable that various Gondwana-derived micro-continental plates contributed to the different basin fillings. Thus, the continental crust of the European Hercynides probably developed since the Early Archean and grew via many crust adding events to its present state.
HISTORY OF A CONTINENT FROM A SAMPLE OF RIVER SAND S.L. GOLDSTEIN and N.T. ARNDT (Max-Planck-lnstitut fiir Chemic, Postfach 3060, 6500 Mainz, FRG) ORIGIN OF]HE VARISCAN GRANITOIDSFROM]HE FRENCH MASSIF CENTRAL: A St, Nd ISOTOPIC STUDY C. PIN and J.L. DUTHOI,/ (UA 10 CNRS et Universit~ Blaise Pascal, Clermont-Ferrand, France) The French Massif Central is characterized by a great abundance and variety of granitoids emplaced during Variscan times (360-290 M.a.) The main conclusions of a combined St, Nd isotopic study of these rocks are : ,T_,Ndiand T_,Srido not fully reflect the various petrographic types, although lower Y_,Ndivalues (~. -8,5) commonly found among the younger leucogranites from the western part of the massif. In these rocks Sri varys widely (0.705 to 0.717). Intermediate Y.Ndi ~-6) correlate with aluminous monzogranites and higher values (-3 to -6) with calc-alkaline granites. • In the E. Massif Central TNdi values are quite homogeneous at the regional scale (~.-6 to -7) . In contrast, the W Massif Central displays larger variations in .T_,Ndi.These correlate with the petrographic characteristics and the ages of the plutons. From these results, extreme models involving a D.M., an old Archean or an early Proterozolo source may be rouled out. However, a variety of geologically dissimilar situations could be consistent with the isotope data. These alternative models are discussed. •
Outstanding problems of continental evolution include: (1) was continental growth continuous or episodic, and (2) how did the growth rate vary through geological time? Age distributions of detfital grains in sediments from major rivers can help to solve these problems, because such sediments are derived from large areas and comprise material from all the tectonic provinces that have contributed to the sediment• Therefore, they provide a record of major crust building episodes• To obtain such information, we measured U-Pb ages of zircons from Orinoco River sand with the SHRIMP ion-probe at ANU. Crustal age provinces in the Orinoco basin range from Archean to Cretaceous. The sample contains a small but discrete -2.8 Ga zircon population, and a larger 2.0-2.1 Ga population. These ages correspond to known granite-forming episodes within the Imataca and Trans-Amazonian orogenies in the Guyana Shield and the Birrimian in West Africa. Zircons with ages between 1500 and 180 Ma do not form discrete groups but are distributed along Concordia, perhaps with a gap between 900 and 400 Ma. The Sm-Nd "crustal residence" age of Orinoco fine particulate is - 1 . 9 Ga, typical of recent sediments with large source areas, e•g. major rivers, wind blown dusts, loess. The data have important implications for the evolution of the Guyana Shield, and continent growth in general. (1) The scarcity of Archean grains indicates that Archean rocks form only a minor proportion of the Guyana Shield. (2) A major portion of the Guyana Shield formed 2•0-2.1 Ga ago. (3) Major crust forming events of the North Atlantic continents, at -2.7 and -1.8 Ga, are not represented, showing that the Guyana Shield has been isolated from the northern continents throughout its evolution. (4) The 2. l Ga event falls within a - 9 0 0 Ma age gap observed in the North Atlantic continents and Australia, demonstrating that this was not a period of global orogenic quiescence. (5) The Archean and early Proterozoic events lasted only _<100 Ma and are separated by longer inactive periods. This pattern contrasts with the more continuous activity recorded by grains younger than 1.5 Ga, implying different pre- and post-l.5 Ga tectonic regimes.
THE DEVELOPMENT OF THE CENTRAL EUROPEAN CONTINENTAL CRUSTSINCETHE EARLYARCHEAN
MARCANTONIO, France, DICKINp A.P. and McNUTT, R.H. (Department of GeologY, HcMaster University, Hamilton, Ontario, Canada L8S 4MI; and 8eeman, L.H., Department of Geology, N.O.M., 100- Queen's Park, Toronto, Ontario, Canada blfS 2C6)
D.GEBAUERI,I.S.WlLLIAMS2,W. COMPSTON 2 and M.GRONENFELDER1 1Laboratory for Isotope Geochemistry and Mass Spectrometry, Swiss Federal Institute of Technology, 8092 Z0rich (Switzerland) 2Research School of Earth Sciences, The Australian National University, Canberra, A.C,T. 2601 (Australia)
A S
FROTEROZOIC ~
CR~JSTAL ' I
I~E~IZXD
The gneiss terrane on Islay, an island south of Mull in Western Scotland, has always been inferred to be part of the Archean Lewisian cowlex~ although no attempt has been made to date it. Five samples were collected from Portnahaven and Port Wemyss on western Islay, which is separated from eastern Islay by the Loch Gruinart fault. In this paper, evidence from four different isotope dating techniques, along with model Nd age data, points toward a mid-Proterozoic age for the Islay gneisses. A single-stage U-Pb zircon analysis, which is 1.4~ discordant, defines a z°vPb/2°ePb age of 1780 bla. The Rb-Sr, Sm-Nd and Pb-Pb isotope systems loosely define "isochrons" ranging from 1720 Ma (Pb-Pb) to 1800 Ma (Sm-Nd) Ma. Furthermore, the Pb-Pb system defines a model m m value of 7.9 which is indicative of primary mantle derivation. Along with the U-Pb zircon determinations~ the mid-Proterozoic depleted mantle Nd model ages (averaging at about 1950 Ms) prove conclusively that the terrane is not a reworking of Archean crustal material. This would be the first case of
recognizable
Proterozoic
Geophysical
correlations
basement in S c o t l a n d , between
the
fslay
t e r r a r i a and t h e a d j a c e n t Grampian b l o c k i n d i c a t e t h a t the P r o t e r o z o i c basement may e x t e n d w e l l e a s t o f I s l a y and hence has g r e a t i m p l i c a t i o n s f o r the c r u s t a l e v o l u t i o n o f Northern B r i t a i n .
Archean components up to 3.84 b.y. old have been detected by ion-microprobe dating of zircon from various sediments and metasediments of the European Hercynides ( Moldanubicum ot NE Bavaria and French Central Massif ). The Early Archean age of 3.84 b.y. was obtained within the euhedral core of a detfital grain from a Moldanubian paragneiss. The same detrital zircon grain reveals at least 3 ages of metamorphic overprints - 2,59 b.y., 1.94 b.y. and 460 m.y. - and at least two cycles of mechanical abrasion during surface recycling in the Early Protarozoic and after the Pan-African cycle. It is thus the crystal with the most complex geological history ever detected. Further Arehean ages for the crystallization of primary magmatic zircons, probably in the course of continental crust formation in the first cycle provenances of the two studied areas , were found at 3.15 b.y., 2.9 b.y., 2.76 b.y., 2.65 b.y. and in a few cases between 2.5 b.y. and 2.6 b.y.. Early Protarozoic crust-forming events between about 1.75 b.y. and 2.15 b.y. play an important role especially for the sediments and metasediments of the Montagne Noire at the southern rim of the French Central massif. Middle and Upper Proterozoic ages cluster around 1.0 b.y. and ca. 600 m.y. for all analysed samples. Consequently, the deposition of the metasedimentary precursors took place after the Pan-African cycle which is in line with the Rb-Sr wholerock systematics of Central European metasediments. The ubiquitous presence of Pan-African detrital zircons in European metasediments is a strong indication that the detrital material was derived from Gondwana. However, from the presence of metamorphic units including eclogite-facies rocks which probably formed at different continental margins and at various times between aboul 500 m.y. and 330 m.y. ago it is probable that various Gondwana-derived micro-continental plates contributed to the different basin fillings. Thus, the continental crust of the European Hercynides probably developed since the Early Archean and grew via many crust adding events to its present state.
HISTORY OF A CONTINENT FROM A SAMPLE OF RIVER SAND S.L. GOLDSTEIN and N.T. ARNDT (Max-Planck-lnstitut fiir Chemic, Postfach 3060, 6500 Mainz, FRG) ORIGIN OF]HE VARISCAN GRANITOIDSFROM]HE FRENCH MASSIF CENTRAL: A St, Nd ISOTOPIC STUDY C. PIN and J.L. DUTHOI,/ (UA 10 CNRS et Universit~ Blaise Pascal, Clermont-Ferrand, France) The French Massif Central is characterized by a great abundance and variety of granitoids emplaced during Variscan times (360-290 M.a.) The main conclusions of a combined St, Nd isotopic study of these rocks are : ,T_,Ndiand T_,Srido not fully reflect the various petrographic types, although lower Y_,Ndivalues (~. -8,5) commonly found among the younger leucogranites from the western part of the massif. In these rocks Sri varys widely (0.705 to 0.717). Intermediate Y.Ndi ~-6) correlate with aluminous monzogranites and higher values (-3 to -6) with calc-alkaline granites. • In the E. Massif Central TNdi values are quite homogeneous at the regional scale (~.-6 to -7) . In contrast, the W Massif Central displays larger variations in .T_,Ndi.These correlate with the petrographic characteristics and the ages of the plutons. From these results, extreme models involving a D.M., an old Archean or an early Proterozolo source may be rouled out. However, a variety of geologically dissimilar situations could be consistent with the isotope data. These alternative models are discussed. •
Outstanding problems of continental evolution include: (1) was continental growth continuous or episodic, and (2) how did the growth rate vary through geological time? Age distributions of detfital grains in sediments from major rivers can help to solve these problems, because such sediments are derived from large areas and comprise material from all the tectonic provinces that have contributed to the sediment• Therefore, they provide a record of major crust building episodes• To obtain such information, we measured U-Pb ages of zircons from Orinoco River sand with the SHRIMP ion-probe at ANU. Crustal age provinces in the Orinoco basin range from Archean to Cretaceous. The sample contains a small but discrete -2.8 Ga zircon population, and a larger 2.0-2.1 Ga population. These ages correspond to known granite-forming episodes within the Imataca and Trans-Amazonian orogenies in the Guyana Shield and the Birrimian in West Africa. Zircons with ages between 1500 and 180 Ma do not form discrete groups but are distributed along Concordia, perhaps with a gap between 900 and 400 Ma. The Sm-Nd "crustal residence" age of Orinoco fine particulate is - 1 . 9 Ga, typical of recent sediments with large source areas, e•g. major rivers, wind blown dusts, loess. The data have important implications for the evolution of the Guyana Shield, and continent growth in general. (1) The scarcity of Archean grains indicates that Archean rocks form only a minor proportion of the Guyana Shield. (2) A major portion of the Guyana Shield formed 2•0-2.1 Ga ago. (3) Major crust forming events of the North Atlantic continents, at -2.7 and -1.8 Ga, are not represented, showing that the Guyana Shield has been isolated from the northern continents throughout its evolution. (4) The 2. l Ga event falls within a - 9 0 0 Ma age gap observed in the North Atlantic continents and Australia, demonstrating that this was not a period of global orogenic quiescence. (5) The Archean and early Proterozoic events lasted only _<100 Ma and are separated by longer inactive periods. This pattern contrasts with the more continuous activity recorded by grains younger than 1.5 Ga, implying different pre- and post-l.5 Ga tectonic regimes.