Two contrasting paleozoic magmatic belts in northern Inner Mongolia, China: petrogenesis and tectonic implications

Abstract The Solonker zone, extending from Solonker to Sonidzuoqi and Xilinhot in northern Inner Mongolia, China, has been considered as the suture between the Manchurides and Altaids. Two magmatic belts have been recognized along the suture zone: an older, variably deformed arc-related magmatic belt represented by the Baolidao magma suite, and a younger, collisional granite belt, represented by the Halatu plutons. The Baolidao suite is composed of gabbroic diorite, quartz diorite, tonalite and granodiorite. Hornblende is present in all rock types. The Halatu plutons consist mainly of two-mica adamellite and subordinate granodiorite and leucogranite. Hornblende is absent in all rock types. The bulk of Baolidao rocks were emplaced at ca. 310 Ma (zircon 206Pb/238U age). They are metaluminous to weakly peraluminous and possess high Na2O/K2O ratios from 2 to 10. They have no Eu anomalies in the REE patterns but show strong positive Sr anomalies in the primitive-mantle-normalized spidergrams. In contrast, the Halatu granites were formed at ≈230 Ma based on a whole-rock Rb–Sr isochron. They are weakly metaluminous to peraluminous and have rather uniform chemical compositions. In comparison with the Baolidao magma suite, they have much lower contents of FeO, MgO, CaO, TiO2, Cr, V and Sr, but are richer in SiO2, K2O and Rb. They also exhibit distinct negative Sr, P, Ti and Nb anomalies in the spidergrams and significant negative Eu anomalies in the REE patterns, a feature typical of crustal melts. The most primitive rock type of the Baolidao suite (gabbroic diorite) has ISr=0.7052 and ϵNd(T)=+2.4, whereas the other rock types show small variations in both ISr (0.7056–0.7059) and ϵNd(T) (+0.7 to −0.1). Geochemical modelling suggests that the gabbroic diorite was derived from a metasomatized upper mantle and the other rock types were generated from the gabbroic diorite magma by fractional crystallization coupled with assimilation of older crustal material (AFC), and followed by hornblende-dominated fractionation in a closed system. The Halatu granites exhibit a range of ISr (0.7048–0.7063) and ϵNd(T) (+1.0 to −2.2). They were derived from sources dominated by young mantle-derived rocks with variable amounts of ancient continental crust. The largely juvenile isotopic signature for both the Baolidao suite and Halatu granites indicates a significant addition of new continental crust in northern Inner Mongolia.

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

[2]  D. DePaolo,et al.  Magma genesis in the New Britain island-arc: Constraints from Nd and Sr isotopes and trace-element patterns , 1979 .

[3]  J. Kirschvink,et al.  Zircon U-Pb ages for the Early Cambrian time-scale , 1992, Journal of the Geological Society.

[4]  B. Chappell,et al.  I- and S-type granites in the Lachlan Fold Belt , 1992, Earth and Environmental Science Transactions of the Royal Society of Edinburgh.

[5]  C. Hawkesworth,et al.  Magma genesis in the lesser Antilles island arc , 1980 .

[6]  H. Martin.  Adakitic magmas: modern analogues of Archaean granitoids , 1999 .

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

[8]  K. Tang.  Tectonic development of Paleozoic foldbelts at the north margin of the Sino‐Korean Craton , 1990 .

[9]  B. Chappell,et al.  Granitoid types and their distribution in the Lachlan Fold Belt, southeastern Australia , 1983 .

[10]  S. Kay,et al.  Evidence in Cerro Pampa Volcanic Rocks for Slab-Melting Prior to Ridge-Trench Collision in Southern South America , 1993, The Journal of Geology.

[11]  W. S. Pitcher.  Magmatism at a Plate Edge: The Peruvian Andes , 1988 .

[12]  W. Sawka.  REE and trace element variations in accessory minerals and hornblende from the strongly zoned McMurry Meadows Pluton, California , 1988, Earth and Environmental Science Transactions of the Royal Society of Edinburgh.

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

[14]  A. Tindle,et al.  Geochemical characteristics of collision-zone magmatism , 1986, Geological Society, London, Special Publications.

[15]  F. McDermott,et al.  Element fluxes associated with subduction related magmatism , 1991, Philosophical Transactions of the Royal Society of London. Series A: Physical and Engineering Sciences.

[16]  B. Barreiro,et al.  Andean magmatism : chemical and isotopic constraints , 1984 .

[17]  A. Tindle,et al.  Trace Element Discrimination Diagrams for the Tectonic Interpretation of Granitic Rocks , 1984 .

[18]  H. Martin.  Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas , 1986 .

[19]  S. Taylor,et al.  Chemical characteristics of island-arc basalts: Implications for mantle sources , 1980 .

[20]  W. White,et al.  HfNdSr isotopes and incompatible element abundances in island arcs: implications for magma origins and crust-mantle evolution , 1984 .

[21]  C. Deniel,et al.  Isotopic study of the Manaslu granite (Himalaya, Nepal): inferences on the age and source of Himalayan leucogranites , 1987 .

[22]  W. Compston,et al.  U‐Pb geochronology of zircons from lunar breccia 73217 using a sensitive high mass‐resolution ion microprobe , 1984 .

[23]  N. Petford,et al.  Generation of sodium-rich magmas from newly underplated basaltic crust , 1993, Nature.

[24]  D. Hamilton,et al.  Phase relations in the system NaAlSi3O8-KAlSi3O8-CaAl2Si2O8-SiO2 at 1 kilobar water vapour pressure , 1969 .

[25]  C. Hawkesworth,et al.  A geochemical traverse across the North Chilean Andes: evidence for crust generation from the mantle wedge , 1985 .

[26]  Y. Tatsumi,et al.  Chemical characteristics of fluid phase released from a subducted lithosphere and origin of arc magmas: Evidence from high-pressure experiments and natural rocks , 1986 .

[27]  H. Bellon,et al.  High field strength element enrichment of Pliocene-Pleistocene island arc basalts, Zamboanga Peninsula, Western Mindanao (Philippines) , 1996 .

[28]  A. Glikson,et al.  REE geochemistry and isotopic data of Archean silicic volcanics and granitoids from the Pilbara Block, Western Australia: implications for the early crustal evolution , 1981 .

[29]  S. R. Taylor.  The application of trace element data to problems in petrology , 1965 .

[30]  T. Harrison,et al.  The tectonic evolution of Asia , 1996 .

[31]  J. D. de Boer,et al.  The geochemistry of young volcanism throughout western Panama and southeastern Costa Rica: an overview , 1992, Journal of the Geological Society.

[32]  A. Cocherie,et al.  Geochemical investigations of the origin of the Manaslu leucogranite (Himalaya, Nepal) , 1982 .

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

[34]  D. DePaolo.  Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization , 1981 .

[35]  A. Sengor.  Paleotectonics of Asia : fragments of a synthesis. , 1996 .

[36]  D. Millward,et al.  The petrology and geochemistry of Cretaceous to Recent volcanism in Colombia: the magmatic history of an accretionary plate margin , 1984, Journal of the Geological Society.

[37]  B. Fryer,et al.  Garnet, tourmaline and muscovite-bearing leucogranites, gneisses and migmatites of the Higher Himalayas from Zanskar, Kulu, Lahoul and Kashmir , 1986, Geological Society, London, Special Publications.

[38]  K. Hsu,et al.  Geologic evolution of the Neimonides: a working hypothesis , 1991 .

[39]  J. Tarney,et al.  Geochemical Characteristics of Magmatism in the Southern Andes (45–46° S) , 1984 .

[40]  J. Joron,et al.  Geochemistry of basalts from central and southern New Hebrides arc: implication for their source rock composition , 1982 .

[41]  M. Perfit,et al.  143Nd/144Nd,87Sr/86Sr and trace element constraints on the petrogenesis of Aleutian island arc magmas , 1981 .

[42]  J. Gill.  Orogenic Andesites and Plate Tectonics , 1981 .

[43]  M. Drummond,et al.  A model for Trondhjemite‐Tonalite‐Dacite Genesis and crustal growth via slab melting: Archean to modern comparisons , 1990 .

[44]  William Walden Rubey,et al.  The Geotectonic development of California , 1981 .

[45]  M. Atherton,et al.  The chemical variation and evolution of the super-units of the segmented Coastal Batholith , 1985 .

[46]  Xueya Liu,et al.  Paleoplate tectonics between Cathaysia and Angaraland in inner Mongolia of China , 1986 .