CONSTRAINTS FROM METASEDIMENTS AND GNEISSES IN THE THE QILIAN FOLD BELT FOR THE POSITION OF THE NORTH CHINA BLOCK IN THE LATE PROTEROZOIC AND EARLY PALEOZOIC

The Qilian fold belt located along the southwestern margin of the North China block comprises continental margin and oceanic basin strata belonging to the Trans-Hudsonian, Grenville, and Caledonian orogenic events. The evolution of the belt may be deciphered from dating and provenance determination of the metasedimentary and gneissic units. Slates found with MORB in the Xinglongshan Formation in the Central Belt yield a Late Proterozoic Rb-Sr age. Geochemical discrimination diagrams and Sm-Nd model ages indicate derivation from Grenville gneissic basement to the south. Similar crust may also have contributed to sediment units along the southern margin of the North Qilian terrane which define a Rb-Sr age of 434 Ma. However, high silica and Th contents suggest an additional source equated with supracrustal assemblages of the North China block. The Ta-Den Creek section of the northern North Qilian belt represents the youngest Caledonian basin investigated, with metasediments defining an age of 412 Ma and having compositions showing a more significant contribution from ocean floor and arc material. The sedimentary record is thus one of provenance shifting northwards with time. Similarities in Rb-Sr age, Sr-initial ratio and Sm-Nd model ages between the Qing-Shui sediments and assemblages in Antarctica suggest a developing link between North China and Gondwana through the Early Paleozoic. The Central Qilian belt may thus represent fragments of rifted continental basement dispersed through the western Iapetus Ocean basin, similar to the tectonic structure of the modern western Pacific. The change to supracrustal sources in the Ordovician marks the approach of the North China block, with Silurian compositions reflecting obduction of ophiolitic assemblages on final closure of basins.