Accretionary orogens of the central Asian orogenic belt and growth of the continental crust from Neoproteozoic to recent: Nd isotopic evidence

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Jahn, Bor-Ming
Holding Date 23 September 2008

The Central Asian Orogenic Belt (CAOB) or the Altaid Tectonic Collage is a prime example of accretionary orogeny evolved from the Neoproterozoic to Mesozoic. A common feature of the individual orogens in Central Asia is the complex but recurrent arrangement of accretionary prism and magmatic arcs, interspersed with massifs of Precambrian terranes and slivers of oceanic crust. Early Paleozoic UHP and HP metamorphism occurred in Kazakhstan (Kokchetav) and the Gorny Altai, respectively. In addition, all the orogens are intruded by massive granitoids of early Paleozoic to late Mesozoic ages. Overall, the CAOB grew southward from the Siberian Craton. The final closure of the Paleo-Asian Ocean probably took place in the late Permian or Permo-Triassic time when the North China Craton collided with the CAOB. Nd isotope tracer studies in the last decade revealed that massive juvenile crust was generated in the CAOB. While this being true, continued isotope analyses also showed that in some terranes Precambrian crust has also played an important role in the generation of the apparent "juvenile" crustal mass. Juvenile crust was likely produced in two ways: by lateral accretion of arc assemblages and syntectonic granitoids, and vertical accretion of non-orogenic granitoids and their volcanic equivalents. The vertical accretion was probably achieved by underplating of mantle-derived basic rocks, followed by partial melting of these rocks and later intracrustal differentiation. However, ocean closure and arc accretion are still considered to be the dominant process of crustal formation in the CAOB. Available Nd isotope data indicate that the proportion of the mantle component in post-orogenic granitoids is greater than that in syntectonic granitoids. In the gigantic granitoid belt in northern Mongolia and Transbaikalia, three stages of alkaline granitoids were intruded from late Permian to Cretaceous. Among them, two stages are found to have negative εNd(T) values (-1 to -5), which are unusual in the CAOB. However, several pieces of evidence suggest that they were produced from enriched mantle-derived sources, rather than due to crustal contamination during petrogenesis. The scenario of crustal growth in the CAOB will be compared with that in an older accretionary orogen, the Neoproterozoic Arabian-Nubian Shield, and a younger orogen, the Japanese Islands. In conclusion, accretionary orogens are generally characterized by abundant distribution of juvenile crust, and represent the major sites of continental growth (and mineralization, not discussed) in the history of the Earth. However, the isotopic evidence also shows that pre-existing Precambrian crust has played different roles in the generation of crustal mass in individual orogens or terranes. The proportion of the juvenile or mantle component in accretionary orogens is highest in the Arabian-Nubian Shield, followed by the Central Asian Orogenic Belt and lowest in the Japanese Islands.