Paper No. 293-8
Presentation Time: 10:20 AM
OPHIOLITES AS FRAGMENTS OF SEAMOUNTS AND OCEANIC PLATEAUX IN THE ALTAIDS AND THEIR CONTRIBUTIONS TO THE PHANEROZOIC CONTINENTAL GROWTH
It is widely accepted that the architecture of continental orogeny, in particular the accretionary orogenesis greatly contributes to the Phanerozoic continental growth. However, the mechanism for juvenile materials added to continental crust is to be undertood. Ophiolites in the Central Asian Orogenic Belt (CAOB) are discussed to address this issue, which record numerous fragments of seamounts and oceanic plateaux in the Paleo-Asian Ocean (PAO). These ophiolitic fragments may have been incorporated into ocean plate stratigraphy and/or arc-forearc complex during the accretion and convergence of three (Mongolia, Kazakhstan, and Tarim-North China) collage systems that were finally rotated into two major oroclines (Tuva-Mongol and Kazakhstan). Some good examples include emplacement of ophiolitic fragments as HP/UHP metamorphosed slices into subduction complexes (Chinese Altai, Tianshan, and many other cases) or long-lived arc edifice (East Junggar), with the protoliths being not only N-MORB ophiolites (sea-floor tholeiite basalts) but also alkaline E-MORB and OIB (seamounts and Oceanic Plateaux). The spectrum of ages for these ophiolitic fragments almost cover all stages of the CAOB from the Neoproterozoic to Early Traissic. Based on analyses of systematical field geology, geochemistry, and geochronology, together with plate tectonic reconstruction, the Neoproterozoic to early Mesozoic evolution of the accretionary margins of the CAOB mimics the modern circum-Pacific Ocean rim. The Central Asian Orogenic belt growth has been produced by the addition of considerable juvenile materials that were extracted from the underlying mantle wedge relatively free from of continental material contamination, which was generated by a global-scale mantle convection cell corresponding to the large low–shear velocity province (LLSVP) in the lowermost mantle, with upwelling zones beneath the South Pacific Ocean separated from another global-scale mantle convection cell corresponding to the LLSVP beneath Africa by a circum-Pacific annulus of cold downwelling mantle.