Elemental and Nd-Sr isotopic geochemistry of granitoids from the West Junggar foldbelt (NW China), with implications for Phanerozoic continental growth

We report elemental and Nd-Sr isotopic data for two coeval postcollisional magmatic suites (∼300 Ma), the Miaogou and Karamay suites, from West Junggar foldbelt (NW China), aiming to determine their source regions and implications for continental growth. The Miaogou rocks, monzonitic to granitic in composition, show low Mg# (<0.30) and depletion of HFSE, and are highly depleted in isotopic compositions, with ISr ranging from 0.7035 to 0.7045, eNd(300 Ma) from + 8.4 to + 6.6 and young Nd model ages (0.37–0.70 Ga) that coincide with the ages of the ophiolites in the area. These features suggest that the Miaogou rocks were derived essentially by partial melting of a juvenile basaltic lower crust formed in the early to middle Palaeozoic. The Karamay suite contains a variety of rock types from gabbroic diorite to granite. These rocks show enrichment of LREE and LILE and depletion of HFSE, and have chemical composition (e.g., Mg# = 0.63–0.41) rather different from the Miaogou suite, though isotopically they are indistinguishable. This suggests that the parental magma of the Karamay suite originated from melting of a young lithospheric mantle that had previously been metasomatized during the Palaeozoic subduction. The West Junggar magmatic rocks represent production of juvenile continental crust in the Phanerozoic, triggered by upwelling of asthenophere in an extensional regime. The basement beneath the area is dominated by “trapped” Palaeozoic arc series and oceanic crust.

[1]  Alexander Yakubchuk,et al.  Architecture and mineral deposit settings of the Altaid orogenic collage: a revised model , 2004 .

[2]  R. Coleman,et al.  Isotopic studies bearing on the tectonics of the West Junggar Region, Xinjiang, China , 1989 .

[3]  S. Wilde,et al.  Phanerozoic crustal growth: U–Pb and Sr–Nd isotopic evidence from the granites in northeastern China , 2000 .

[4]  P. Molnar,et al.  Convective instability of a thickened boundary layer and its relevance for the thermal evolution of , 1981 .

[5]  J. Valley,et al.  Oxygen and neodymium isotope evidence for recycling of juvenile crust in northeast China , 2002 .

[6]  N. Rogers,et al.  Calc‐alkaline magmatism, lithospheric thinning and extension in the Basin and Range , 1995 .

[7]  N. Rogers,et al.  The Generation of Potassic Lavas from the Eastern Virunga Province, Rwanda , 1998 .

[8]  A. Saunders,et al.  Magmatism in the Ocean Basins , 1989 .

[9]  B. Jahn,et al.  Crustal evolution of southeastern China: Nd and Sr isotopic evidence , 1998 .

[10]  A. Hofmann,et al.  Mantle plumes and episodic crustal growth , 1994, Nature.

[11]  O. Oncken,et al.  Crustal-scale balanced cross sections through the Variscan fold belt, Germany: the central EGT-segment , 1991 .

[12]  D. Othman,et al.  Nd–Sr isotopic relationship in granitoid rocks and continental crust development: a chemical approach to orogenesis , 1980, Nature.

[13]  S. Samson,et al.  The Canadian Cordillera as a modern analogue of Proterozoic crustal growth , 1991 .

[14]  D. DePaolo.  A neodymium and strontium isotopic study of the Mesozoic calc‐alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California , 1981 .

[15]  A. M. Larin,et al.  Isotope provinces, mechanisms of generation and sources of the continental crust in the Central Asian mobile belt: geological and isotopic evidence , 2004 .

[16]  B. Jahn,et al.  Crystal fractionation in the petrogenesis of an alkali monzodiorite–syenite series: the Oshurkovo plutonic sheeted complex, Transbaikalia, Russia , 2002 .

[17]  A. Hofmann,et al.  Precambrian crustal components, plutonic associations, plate environment of the Hercynian Fold Belt of central Europe: Indications from a Nd and Sr isotopic study , 1988 .

[18]  G. Schubert,et al.  The continental crustal age distribution: Methods of determining mantle separation ages from Sm‐Nd isotopic data and application to the southwestern United States , 1991 .

[19]  B. Jahn,et al.  Nd and Sr isotopic compositions and REE abundances of cretaceous MORB (Holes 417D and 418A, Legs 51, 52 and 53) , 1980 .

[20]  Bin Chen,et al.  Two contrasting paleozoic magmatic belts in northern Inner Mongolia, China: petrogenesis and tectonic implications , 2000 .

[21]  R. Walker,et al.  Osmium isotopic constraints on the age of lithospheric mantle beneath northeastern China , 2003 .

[22]  F. Albarède.  The growth of continental crust , 1998 .

[23]  C. Hawkesworth,et al.  Is average continental crust generated at subduction zones , 1988 .

[24]  R. Coleman.  Continental growth of northwest China , 1989 .

[25]  K. Condie.  EPISODIC CONTINENTAL GROWTH AND SUPERCONTINENTS : A MANTLE AVALANCHE CONNECTION? , 1998 .

[26]  Yuan-lin Sun,et al.  Depleted-mantle source for the Ulungur River A-type granites from North Xinjiang, China: geochemistry and Nd–Sr isotopic evidence, and implications for Phanerozoic crustal growth , 1997 .

[27]  A. Şengör,et al.  Evolution of the Altaid tectonic collage and Palaeozoic crustal growth in Eurasia , 1993, Nature.

[28]  Bin Chen,et al.  Genesis of post-collisional granitoids and basement nature of the Junggar Terrane, NW China: Nd–Sr isotope and trace element evidence , 2004 .

[29]  D. Darbyshire,et al.  Nd and Sr isotope constraints on the origin of the Cornubian batholith, SW England , 1994, Journal of the Geological Society.

[30]  Y. Feng,et al.  Tectonic evolution of the West Junggar Region, Xinjiang, China , 1989 .

[31]  Huang Huaizeng,et al.  The Permian alkaline granites in Central Inner Mongolia and their geodynamic significance , 1994 .

[32]  A. Masuda,et al.  Fine structures of mutually normalized rare-earth patterns of chondrites , 1973 .

[33]  G. Eby.  The A-type granitoids: A review of their occurrence and chemical characteristics and speculations on their petrogenesis , 1990 .

[34]  G. Qiao.  NORMALIZATION OF ISOTOPIC DILUTION ANALYSIS——A NEW PROGRAM FOR ISOTOPE MASS SPECTROMETRIC ANALYSIS , 1988 .

[35]  X. Pichon,et al.  Deep basins of the Black Sea and Caspian Sea as remnants of Mesozoic back-arc basins , 1986 .

[36]  E. Middlemost.  Naming materials in the magma/igneous rock system , 1994 .

[37]  M. Sandiford,et al.  Some geodynamic and compositional constraints on postorogenic magmatism , 1992 .

[38]  E. Watson,et al.  Dehydration melting of metabasalt at 8-32 kbar : Implications for continental growth and crust-mantle recycling , 1995 .

[39]  S. Graham,et al.  Junggar basin, northwest China: trapped Late Paleozoic ocean , 1990 .

[40]  R. Rudnick.  Growing from below , 1990, Nature.

[41]  D. Karig.  Origin and development of marginal basins in the western Pacific , 1971 .

[42]  Bin Chen,et al.  Granitoids of the Central Asian Orogenic Belt and continental growth in the Phanerozoic , 2000, Earth and Environmental Science Transactions of the Royal Society of Edinburgh.

[43]  B. Lehmann,et al.  Paleozoic crustal growth and metallogeny of Central Asia: evidence from magmatic-hydrothermal ore systems of Central Kazakhstan , 2000 .

[44]  D. Pearson.  The age of continental roots , 1999 .

[45]  I. Bindeman,et al.  A Stable Isotope Study of Anorogenic Magmatism in East Central Asia , 1996 .

[46]  W. Collins.  Evaluation of petrogenetic models for Lachlan Fold Belt granitoids: Implications for crustal architecture and tectonic models , 1998 .

[47]  W. McDonough,et al.  Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes , 1989, Geological Society, London, Special Publications.

[48]  Guoxin Zhang,et al.  Crustal evolution and Phanerozoic crustal growth in northern Xinjiang: Nd isotopic evidence. Part I. Isotopic characterization of basement rocks , 2000 .

[49]  Bin Chen,et al.  Geochemical and isotopic studies of the sedimentary and granitic rocks of the Altai orogen of northwest China and their tectonic implications , 2002, Geological Magazine.