Important crustal growth in the Phanerozoic: Isotopic evidence of granitoids from east-central Asia

Abstract

The growth of the continental crust is generally believed to have been essentially completed in the Precambrian, and the amount of juvenile crust produced in the Phanerozoic is considered insignificant. Such idea of negligible growth in the Phanerozoic is now challenged by the revelation of very large volume of juvenile crust produced in the period of 500 to 100 Ma in several orogenic belts. While appreciable volumes of juvenile terranes in North America (Canadian Cordillera, Sierra Nevada and Peninsular Range, Appalachians) have been documented based on Nd isotopic data, the mass of new crust formed in the East-Central Asian Orogenic Belt (ECAOB), eastern part of the Altaid Tectonic Collage, appears to be much greater than the above terranes combined. New and published Nd-Sr isotope data indicate that the Phanerozoic granitoids from the southern belt of the ECAOB (Xinjiang-West Mongolia-Inner Mongolia-NE China) as well as from Mongolia and Transbaikalia were generated from sources dominated by a depleted mantle component. These granitoids represent a significant growth of juvenile crust in the Phanerozoic. Although most plutons in this huge orogenic belt belong to the calc-alkaline series, the ECAOB is also characterized by the emplacement of voluminous A-type granites. The origin of these rocks is probably multiple and is still widely debated. However, the isotopic data (Sr-Nd-O) and trace element abundance patterns of A-type granites from the ECAOB clearly indicate their mantle origin. The evolution of the ECAOB and the entire Altaid Collage is most likely related to successive accretion of arc complexes. However, the emplacement of a large volume of post-tectonic A-type granites requires another mechanism-probably through a series of processes including underplating of massive basaltic magma, partial melting of these basic rocks to produce granitic liquids, followed by extensive fractional crystallization. The proportion of juvenile to recycled, as well as that of arc-related to plume-generated, continental crust remains to be evaluated by more systematic dating and isotope tracer studies.