PERMIAN-TRIASSIC (260-220 MA) CRUSTAL GROWTH OF EASTERN CENTRAL ASIAN OROGENIC BELT AS REVEALED BY DETRITAL ZIRCON STUDIES

Present global compilations of ages and isotopic data suggest insignificant crustal growth after 450 Ma. Previous Nd isotopic studies of whole rocks from the Central Asian Orogenic Belt suggest large volumes of juvenile crustal additions in the Phanerozoic. To test this, we studied detrital zircons from the Xinglonggou Formation of western Liaoning at the northern margin of the North China craton, which was deposited after the collision between the Siberian Plate and the North China craton to form this part of the Central Asian Orogenic Belt. Zircons from the Xinglonggou sandstones are characterized by two major groups of U-Pb ages (2.6-2.4 Ga and 260-220 Ma) except for three grains (1.9-1.6 Ga). The 2.6 to 2.4 Ga zircons have positive epsilon(Hf) (t) values up to coeval depleted mantle value, which suggest juvenile crustal addition. The results are consistent with existing data for the North China craton (NCC). epsilon(Hf) (t) values of 260 to 220 Ma zircons range widely from -15.4 to 13.3. While zircons with the negative epsilon(Hf) (t) values are similar to igneous zircons from intrusive rocks of the North China craton and indicate recycling of ancient continental crustal materials, those with the positive values and young model ages point to a significant period of crustal generation with the source provenance from the eastern Central Asian Orogenic Belt (CAOB) to the north. Mixing of detritus from both the North China craton and the eastern Central Asian Orogenic Belt suggests the ca. 250 Ma closure of the Paleo-Asian ocean and collision between the North China craton and the Siberian Plate along the eastern Solonker zone. The youngest zircons constrain uplift of the eastern Central Asian Orogenic Belt to be no older than 208 Ma. Thus, the 260 to 220 Ma crustal growth is related to the subduction of the Paleo-Asian ocean Plate along the Solonker suture, which records the termination of the Central Asian Orogenic Belt. The Solonker suture occupies an area of 700 km long and 60 km wide and extends from Solonker via Sonid Yuoqi to Linxi in Inner Mongolia and further west and northeast. This implies extensive juvenile crustal additions during this period. It may change our present views of Phanerozoic crustal growth.