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Growth, accretion and composition of continental cratons
67 TAIWAN Slfl)IMENrAND CRUSTAL EVOLUTION OF" SE. O41NA CHEN-HONG CHEN, B.M. JAHN, C-Y tAN, T. LEE, and C-H CHEN (Inst.Earth Sci.,Academia Slnica,Tmlpel) Taiwan is an accretionary prism of sediments from Eurasian continent in front of the Luzon Are, which ~ s enlarged by the arc-continent collision. We are conducting an isotopic and geochemics] study of Taiwan's sediments in order to decipher the crustal evolution in southeast China. So far relatively uniform eNd values from -8.4 to -15 were found for sediments from Late Paleozoic to Miocene in age. The crustal residence age relative to the depleted mantle TD, ranges between 1.3 and 1.8 Ga implying that they are recyc:[ed continental material from crust derived from the mantle in middle Proterozoic. There is no obvious trend with respect to depositional ages. This range closely resembles that for suspended matter in the Yangtze River, F u r t h e r m o r e , these values are similar to those for Precambrian/Cambrian basin sediments upstream Yangtze alld to those for pre-Cretaceous granitoids in southeast China Therefore, isotopically these terrains are possible sources for Taiwan sediments. On the other hand, these values are distinct from those observed f o r the Cretaceous g r a n l t o i d s o f s o u t h e a s t C h i n a (eNd = - 2 . 3 t o - 8 . 6 ) and o f Taiwan (~Nd=-l.4 t o - 9 . 0 ) . They a r e t h u s n o t t h e protoliths for Taiwan sediments. Rather, the recycled sediments seem to represent one end member of the mixtures which were melted to form the granitoids. These later granitoids have higher 6Nd and predominantly yolmger TD,{0.71.5Ga). The difference is caused by the admlxlng of fresh mantle input. Most of the sedlments have [IEE patterns resemble that measured in the shelf sediments from the East China Sea outside the mouth of Yangtze River. A minority of the sediments have Ig~EE somewhat lower than the majority which may be caused by different concentrations of heavy minerals.
~ROWTH, ACCRETION AND CRATONS
COMPOSITION
OF
CONTINENTAL
K.C. CONDIE (Geoscience Department, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA) The distribution of U-Pb zircon and Nd isotopic data in igneous rocks from North America and the Baltic Shield ~NAB) indicate that continental growth (extraction from the mantle) and accretion (amalgamation of arcs and microplates) are episodic and roughly coincide at 2.6-2.8, 1.85-2.0, 1.751.80, 1.65-1.75, 1.0-1.2, and 0.25-0.50 Ga. Percentages of growth of NAB with time are as follows: 2.6-3.1 Ga (40%); 1.85-2.0 Ga (16%); 1.75-1.80, 1.65-1.75, 1.0-1.2 Ga (8-10% each); 0.25-0.50 Ga (13%) and <0.2 Ga (5%). Major accretion of NAB occurred at 1.85-2.0 Ga when about 40% of the continent was amalgamated. Accreeional events involving SI0% of NAB are recorded at each of the other growth periods listed above. 20% of NAB had secreted by 2.5 Ga and 55% by the end of the 1.9 Ga event. Archean microplates (except Wyoming) prior to 1.9 Ga may have been part of the same continent. If so, early Proterozoic rifting formed a small ocean basin only in the Trans-Hudson orogen. As indicated by emplacement ages of post-tectonic granites, cratonlzation followed accretion by 100-200 Ma except in the 1.75-1.8 and 1.65-1.75 Ga provinces where up to 400 Ma passed before emplacement of widespread "anorogenic" granites (chiefly at 1.41.5 Ga). Relatively constant values for Eu/Eu*, La/Yb, Th/U, Th/Sc and La/Sc ratios in pelites from cratonic (quartzite-pelite-carbonate) successions of all ages (3.5 Ga to modern) suggest that the average composition of cratons has not changed with time. Previously proposed changes in these ratios in pelites at the Archean-Proterozoic boundary result from comparing greenstone pelltes (>2.5 Ga) with cratonic pelites (<2.5 Ga). Results suggest that early Archean cratons, although probably small, evolved to a chemically mature stage.
CHEMICAL EVOLUTION OF CANADIAN CONTINENTAL C R U S T SINCE ARCIIEAN A.DIA and C.J.ALLEGRE (Laboratoirc de Gdochimie ct Cosmochimic , IPG , Paris, France) Nd and Pb isotopic compositions and S m , Nd , U , Pb , Li , K , Rb , S t , Ba , Ni and Cr abundances were measured in 25 canadian fine-grained claslic sediments. T h e s e shales from different areas (Abitibi , Labrador Trough , Saint Lawrence river shores and Temiscouata and Rimouski counties ), arc characterized by ages r a n g i n g from Archcan to Sihlriau. Sm-Nd systemalics arc used to ch~actcrize continental crustal genesis through geological time. The chemical composition and evolution o f continental crust was studied by isotopic compositions ( N d , Pb ) and trace element contents vari0.tions. The Nd model ages did not provide any proof of important quantities of crust fommtion through mantle extraction since 1.3 b.y. ago Consequently , it was suggested that the process dominant regulating the crust genesis in this area since this then , is not InalltlC extraction . Tile analysis of Nd isotopic and trace element data led us to suggest , as previously proposed by Taylor, that the chemical composition of contincntal crust mnst have changed between Arcllean //lit] post-Archcan times . The increase o f the range of w l r i a t i o n s may be related to a post-Archean continental crust more diversified in its chemical feahlrcs than thc Archean one.
ARCHEAN, PENOKIAN (1.9 GA) AND GRENVILLE (1.2 GA) CRUSTAL EXTRACTION EVENTS I~CORDED IN TIIE GRENVILLE PROVINCE OF ONTARIO DICKIN r A.P., M c N U T T , R.H. and MARCANTONIO, F. (Department of Geology, McMaster University, Hamilton, Ontario, Canada Lns 4MI)
The Grenville Province is a polygenetic mobile belt, whose 8ceretionary history has been obscured by high grade metamorphism. However, Nd model age mapping of pare- asd ortho-nneisses has allowed us to document crustal growth in the Grenville Province of Ontario. The Archean Foreland of the Superior and Southern Provinces extends into the Grenville 40 and BO km SE of the while the latter boundary merely marks the limit of ca. 1.2 Ga tectonism. A Penokian island arc of ca. 1.9 Ga age, generated by southerly-dipping subduction, i s believed t o have c o l l i d e d w i t h t h e Arehean c r a t o n and became s u t u r e d t o i t a t ca. 1.85 Ga. Province between Grenville Front,
Igneous activity continued intermittently, with apparent reworking of the continental margin and accretion of new crust between 1.75 and 1.45 Ga. This may represent an ensialic arc formed by northerly dipping subduction under Laurentia, since it gave rise to scattered plutons same distance inward from the margin. An "early Grenville" i s l a n d arc represented by the Central Metusedimentary Belt of the Grenville Province was probably generated by southerly dipping subduction at 1.25 Ga. The collision of this terrane with the Peaokian margin caused ca. 1.2 Ga Grenville tectonism documented by o t h e r w o r k e r s . Late Grenville transcurrent motion probably caused the docking of the Frontenac/Adirondack terrane against the Central Metasedimentary Belt, since Penokian ages indicate that this block is a displaced terrane. It is concluded that crustal accretion on t h e SE margin o f L a u r e n t i a s p a n n e d t h e p e r i o d 1 . 9 t o I . I Ga.
~ROWTH, ACCRETION AND CRATONS
COMPOSITION
OF
CONTINENTAL
K.C. CONDIE (Geoscience Department, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA) The distribution of U-Pb zircon and Nd isotopic data in igneous rocks from North America and the Baltic Shield ~NAB) indicate that continental growth (extraction from the mantle) and accretion (amalgamation of arcs and microplates) are episodic and roughly coincide at 2.6-2.8, 1.85-2.0, 1.751.80, 1.65-1.75, 1.0-1.2, and 0.25-0.50 Ga. Percentages of growth of NAB with time are as follows: 2.6-3.1 Ga (40%); 1.85-2.0 Ga (16%); 1.75-1.80, 1.65-1.75, 1.0-1.2 Ga (8-10% each); 0.25-0.50 Ga (13%) and <0.2 Ga (5%). Major accretion of NAB occurred at 1.85-2.0 Ga when about 40% of the continent was amalgamated. Accreeional events involving SI0% of NAB are recorded at each of the other growth periods listed above. 20% of NAB had secreted by 2.5 Ga and 55% by the end of the 1.9 Ga event. Archean microplates (except Wyoming) prior to 1.9 Ga may have been part of the same continent. If so, early Proterozoic rifting formed a small ocean basin only in the Trans-Hudson orogen. As indicated by emplacement ages of post-tectonic granites, cratonlzation followed accretion by 100-200 Ma except in the 1.75-1.8 and 1.65-1.75 Ga provinces where up to 400 Ma passed before emplacement of widespread "anorogenic" granites (chiefly at 1.41.5 Ga). Relatively constant values for Eu/Eu*, La/Yb, Th/U, Th/Sc and La/Sc ratios in pelites from cratonic (quartzite-pelite-carbonate) successions of all ages (3.5 Ga to modern) suggest that the average composition of cratons has not changed with time. Previously proposed changes in these ratios in pelites at the Archean-Proterozoic boundary result from comparing greenstone pelltes (>2.5 Ga) with cratonic pelites (<2.5 Ga). Results suggest that early Archean cratons, although probably small, evolved to a chemically mature stage.
CHEMICAL EVOLUTION OF CANADIAN CONTINENTAL C R U S T SINCE ARCIIEAN A.DIA and C.J.ALLEGRE (Laboratoirc de Gdochimie ct Cosmochimic , IPG , Paris, France) Nd and Pb isotopic compositions and S m , Nd , U , Pb , Li , K , Rb , S t , Ba , Ni and Cr abundances were measured in 25 canadian fine-grained claslic sediments. T h e s e shales from different areas (Abitibi , Labrador Trough , Saint Lawrence river shores and Temiscouata and Rimouski counties ), arc characterized by ages r a n g i n g from Archcan to Sihlriau. Sm-Nd systemalics arc used to ch~actcrize continental crustal genesis through geological time. The chemical composition and evolution o f continental crust was studied by isotopic compositions ( N d , Pb ) and trace element contents vari0.tions. The Nd model ages did not provide any proof of important quantities of crust fommtion through mantle extraction since 1.3 b.y. ago Consequently , it was suggested that the process dominant regulating the crust genesis in this area since this then , is not InalltlC extraction . Tile analysis of Nd isotopic and trace element data led us to suggest , as previously proposed by Taylor, that the chemical composition of contincntal crust mnst have changed between Arcllean //lit] post-Archcan times . The increase o f the range of w l r i a t i o n s may be related to a post-Archean continental crust more diversified in its chemical feahlrcs than thc Archean one.
ARCHEAN, PENOKIAN (1.9 GA) AND GRENVILLE (1.2 GA) CRUSTAL EXTRACTION EVENTS I~CORDED IN TIIE GRENVILLE PROVINCE OF ONTARIO DICKIN r A.P., M c N U T T , R.H. and MARCANTONIO, F. (Department of Geology, McMaster University, Hamilton, Ontario, Canada Lns 4MI)
The Grenville Province is a polygenetic mobile belt, whose 8ceretionary history has been obscured by high grade metamorphism. However, Nd model age mapping of pare- asd ortho-nneisses has allowed us to document crustal growth in the Grenville Province of Ontario. The Archean Foreland of the Superior and Southern Provinces extends into the Grenville 40 and BO km SE of the while the latter boundary merely marks the limit of ca. 1.2 Ga tectonism. A Penokian island arc of ca. 1.9 Ga age, generated by southerly-dipping subduction, i s believed t o have c o l l i d e d w i t h t h e Arehean c r a t o n and became s u t u r e d t o i t a t ca. 1.85 Ga. Province between Grenville Front,
Igneous activity continued intermittently, with apparent reworking of the continental margin and accretion of new crust between 1.75 and 1.45 Ga. This may represent an ensialic arc formed by northerly dipping subduction under Laurentia, since it gave rise to scattered plutons same distance inward from the margin. An "early Grenville" i s l a n d arc represented by the Central Metusedimentary Belt of the Grenville Province was probably generated by southerly dipping subduction at 1.25 Ga. The collision of this terrane with the Peaokian margin caused ca. 1.2 Ga Grenville tectonism documented by o t h e r w o r k e r s . Late Grenville transcurrent motion probably caused the docking of the Frontenac/Adirondack terrane against the Central Metasedimentary Belt, since Penokian ages indicate that this block is a displaced terrane. It is concluded that crustal accretion on t h e SE margin o f L a u r e n t i a s p a n n e d t h e p e r i o d 1 . 9 t o I . I Ga.