- Email: [email protected]
New U-Pb Zircon Ages, Geochemistry and Their Significance of the Basement Rocks from the Larsemann Hills, East Antarctica
820
RODINIA, GONDWANA AND ASIA ~
~~
~~
~~
New U-Pb Zircon Ages, Geochemistry and Their Significance of the Basement Rocks from the Larsemann Hills, East Antarctica Yanbin Wangl, Liu Dunyil, I.S. Williams2,Ren Liudong‘ and Tong Laixi3 Institute of Geology, Chinese Academy of Geological Science, Beijing 100 037, China ’ National University of Australia, G.P.0 Boxa, Canberra, A.C.T., 2601, Australia The University of Melbourne, Victoria 3052, Australia In the Larsemann Hills granulite terrane two lithological units, one predominantly orthogneiss and the other predominantly metapelite, have been considered as Archaean basement complex and Mesoproterzoic cover respectively (Carson et al., 1995). New zo7Pb/206Pb single zircon evaporation ages and 4aAr/39Ar age of hornblendes for the mafic-felsic composite orthogneiss in the Larsemann Hills record a widespread Grenville-age event in the period ca.1000-900 Ma. New U-Pb SHRIMP zircon from felsic gneiss and mafic gneiss indicate that the basement rocks of the region are Mesoproterozoic in age. The crystallization ages of metavolcanic rocks yield a range of ca.1100-900 Ma and syn-tectonic Grovness enderbite give ages of ca.1100-1000 Ma. Subsequent metamorphism is recorded at 530 Ma. The latter metamorphic event reached granulite facies. New Sm-Nd data of mafic gneiss indicate that the rock-forming ages of mafic gneiss at 1050rt 120 Ma (Initial 143Nd/144Nd= 0.51127+ .00012). Metasedimentary gneisses contain a population of zircons with ages in the range 1200-900 Ma. Zircon rims have ages in the
range 642-525 Ma. No conclusive evidence for the mafic-felsic composite orthogneiss representing an Archaean basement complex to the metasedimentary sequence of the Larsemann Hills is derived, an assumption based on the lithological similarity with mafic-felsic orthogneiss from the southeastern Rauer Group of the Archaean orthogneiss basement. The thick unit of maficfelsic composite orthogneiss was interpreted as a possible bimodal metavolcanic unit based on geochemical data. The mafic-felsic composite orthogneiss provide important constraints on the timing and extent of Neoproterozoic tectonic events in Rodinia and the critical period of Rodinia’s transformation into Gondwana.
References Carson, C.J., Dirks, P.H.J.M., Hand, M., Sim, J.P. and Wilson, C.J.L. (1995) Compressional and extensional tectonics in low-medium pressure granulites from the Larsemann hills; East Antarctica. Geol. Mag., v. 132, pp. 151.170.
Late Mesozoic Subduction Zone Beneath Southeastern China: Implications for Magmatism and Metallogeny Yang Wang Division of Petrology, China University of Geosciences, Beijing ZOO 083, China, E-mail: [email protected] Southeastern China covers an extensive area from the lower reaches of Yangtze River to the southeast coast of China mainland. Tectonically, the main part of the region belongs to the South China Fold belt, the basement of which is the Cathaysia block, and the northern portion is a part of the Lower Yangtze Platform. The extensive magmatism of late Mesozoic (Yanshanian) as well as the arc shape coastline of SE China implies the existence of a convergent margin between China continent and palaeo-Pacific (Kula) plate. For understanding the relation between magmatism and tectonics, the temporalspatial pattern of late Mesozoic (180-90 Ma) igneous rocks is assessed from nearly 200 isotope age values reported in literature (Jahn et al., 1976; Ye et al., 1986; Li et al., 1989; Martin et al., 1994). Among these, most were obtained from Rb-Sr isochron method and others from zircon U-Pb or 40Ar-39Armethod. The pattern reveals the width of magmatic belts in different tectonic episodes were all less than 500 km, which represents the upper bound of the width of normal magmatic arc (Tatsumi and Eggins,
1995). Three large-scale segments can be identified by the aid of the spatial pattern of Nd-Sr isotope characteristics of granitoids (Hong et al., 1998). The Guangdong segment extends from Nanling Mt. in the north to the coast of Guangdong in the south, and its EW-extending granitic belts migrated from north to south during Jurassic (180-140 Ma). The Fujian segment, the southern end of which overlaps the eastern end of Guangdong segment, covers Jiangxi and Fujian provinces. The NE-striking magmatic belts developed successively from inland to coastline during Jurassic to Cretaceous. The Zhejiang segment extends from the lower reaches of Yangtze River to the coast of Zhejiang province. The magmatism is identified by NE extending volcanic belts, and exhibits seaward migration during late Mesozoic. Besides the seaward migration in each segment, the magmatism of the whole region exhibits the general trend of becoming younger in NE direction. The segmentation of igneous assemblage and related mineralization are obvious (Hong et al., 1998; Tao et al., 1998). S-type granitoids developed dominantly in the Gondwana Research, V. 4, No. 4,2001
RODINIA, GONDWANA AND ASIA ~
~~
~~
~~
New U-Pb Zircon Ages, Geochemistry and Their Significance of the Basement Rocks from the Larsemann Hills, East Antarctica Yanbin Wangl, Liu Dunyil, I.S. Williams2,Ren Liudong‘ and Tong Laixi3 Institute of Geology, Chinese Academy of Geological Science, Beijing 100 037, China ’ National University of Australia, G.P.0 Boxa, Canberra, A.C.T., 2601, Australia The University of Melbourne, Victoria 3052, Australia In the Larsemann Hills granulite terrane two lithological units, one predominantly orthogneiss and the other predominantly metapelite, have been considered as Archaean basement complex and Mesoproterzoic cover respectively (Carson et al., 1995). New zo7Pb/206Pb single zircon evaporation ages and 4aAr/39Ar age of hornblendes for the mafic-felsic composite orthogneiss in the Larsemann Hills record a widespread Grenville-age event in the period ca.1000-900 Ma. New U-Pb SHRIMP zircon from felsic gneiss and mafic gneiss indicate that the basement rocks of the region are Mesoproterozoic in age. The crystallization ages of metavolcanic rocks yield a range of ca.1100-900 Ma and syn-tectonic Grovness enderbite give ages of ca.1100-1000 Ma. Subsequent metamorphism is recorded at 530 Ma. The latter metamorphic event reached granulite facies. New Sm-Nd data of mafic gneiss indicate that the rock-forming ages of mafic gneiss at 1050rt 120 Ma (Initial 143Nd/144Nd= 0.51127+ .00012). Metasedimentary gneisses contain a population of zircons with ages in the range 1200-900 Ma. Zircon rims have ages in the
range 642-525 Ma. No conclusive evidence for the mafic-felsic composite orthogneiss representing an Archaean basement complex to the metasedimentary sequence of the Larsemann Hills is derived, an assumption based on the lithological similarity with mafic-felsic orthogneiss from the southeastern Rauer Group of the Archaean orthogneiss basement. The thick unit of maficfelsic composite orthogneiss was interpreted as a possible bimodal metavolcanic unit based on geochemical data. The mafic-felsic composite orthogneiss provide important constraints on the timing and extent of Neoproterozoic tectonic events in Rodinia and the critical period of Rodinia’s transformation into Gondwana.
References Carson, C.J., Dirks, P.H.J.M., Hand, M., Sim, J.P. and Wilson, C.J.L. (1995) Compressional and extensional tectonics in low-medium pressure granulites from the Larsemann hills; East Antarctica. Geol. Mag., v. 132, pp. 151.170.
Late Mesozoic Subduction Zone Beneath Southeastern China: Implications for Magmatism and Metallogeny Yang Wang Division of Petrology, China University of Geosciences, Beijing ZOO 083, China, E-mail: [email protected] Southeastern China covers an extensive area from the lower reaches of Yangtze River to the southeast coast of China mainland. Tectonically, the main part of the region belongs to the South China Fold belt, the basement of which is the Cathaysia block, and the northern portion is a part of the Lower Yangtze Platform. The extensive magmatism of late Mesozoic (Yanshanian) as well as the arc shape coastline of SE China implies the existence of a convergent margin between China continent and palaeo-Pacific (Kula) plate. For understanding the relation between magmatism and tectonics, the temporalspatial pattern of late Mesozoic (180-90 Ma) igneous rocks is assessed from nearly 200 isotope age values reported in literature (Jahn et al., 1976; Ye et al., 1986; Li et al., 1989; Martin et al., 1994). Among these, most were obtained from Rb-Sr isochron method and others from zircon U-Pb or 40Ar-39Armethod. The pattern reveals the width of magmatic belts in different tectonic episodes were all less than 500 km, which represents the upper bound of the width of normal magmatic arc (Tatsumi and Eggins,
1995). Three large-scale segments can be identified by the aid of the spatial pattern of Nd-Sr isotope characteristics of granitoids (Hong et al., 1998). The Guangdong segment extends from Nanling Mt. in the north to the coast of Guangdong in the south, and its EW-extending granitic belts migrated from north to south during Jurassic (180-140 Ma). The Fujian segment, the southern end of which overlaps the eastern end of Guangdong segment, covers Jiangxi and Fujian provinces. The NE-striking magmatic belts developed successively from inland to coastline during Jurassic to Cretaceous. The Zhejiang segment extends from the lower reaches of Yangtze River to the coast of Zhejiang province. The magmatism is identified by NE extending volcanic belts, and exhibits seaward migration during late Mesozoic. Besides the seaward migration in each segment, the magmatism of the whole region exhibits the general trend of becoming younger in NE direction. The segmentation of igneous assemblage and related mineralization are obvious (Hong et al., 1998; Tao et al., 1998). S-type granitoids developed dominantly in the Gondwana Research, V. 4, No. 4,2001