THE AMBIENT OROGENESIS OF THE NORTH CHINA CRATON FROM MIDDLE ORDOVICIAN TO LATE CARBONIFEROUS CONSTRAINT BY DEBRIS ZIRCON ANALYSIS

The lower member of the Carboniferous Benxi Formation in the North China Craton (NCC), unconformably overlies the middle Ordovician Majiagou carbonates (~460 Ma), is weathered remnant formed during a period about 150 Ma. The bauxite ores in the Benxi formation are sampled from north to south in the NCC, and the Shrimp U-Pb ages of the detrital zircons exacted from ores are tested to discuss the evolution of the surrounding orogens. The zircons varied in a large age range with two obvious cluster peaks, ~450 Ma, and ~320 Ma. The peak ~450 Ma, the most obvious peak in most samples, responds to orogenesis in both the south orogeny of Central Asian Orogenic Belt (S-CAOB) to the north of the NCC and that in the North Qinling orogeny (NQO) to the south. Via comparison of detailed age spectrc between the bauxite samples and the geological bodies in the NQO and Bainaimiao arc, the zircons with ages around ~450 Ma are inferred to come from the NQO. Therefore, it is suggested that a continent-continent collision may happen between the South Qinling Block and the NQO attached to NCC after the Shangdan ocean closed at ~400 Ma, inducing an overall north-dipping (modern orientation) paleographic terrain of the NCC. On the contrary, to the north, the Bainaimiao arc in the S-CAOB is considered to be an oceanic arc, the development of a back arc basin hindered the southwards transportation of weathered remnant. The 320 Ma peak, which is obvious in the north samples yet feeble in the south samples, is consistent with the magmatism and uplift of the north margin of NCC (N-NCC) caused by the southwards subduction of the ancient Asian ocean. It indicates that the north part of NCC is tilted southwards after 320 Ma, as is resulted from the bulge behind the ocean subduction zone. Constraint by the orogenesis on the south and north sides, the NCC experienced an overall south-dipped before 320 Ma that is adjusted by a north-dipped in the north part, evolving into an analogue walled basin.