Provenance and Paleogeography of the Mesozoic Southern Ordos Basin, North Central China: Implications of U-Pb detrital Zircon Geochronology
Department of Geology and Geophysics University of Wyoming 1000 E. University Ave. Laramie, WY 82072
telephone: 307-766-2245 [email protected]
Two deformation belts, the Qinling orogenic belt to the south and the Western Liupanshan thrust belt to the west, including the Qilian-Qaidam terrane, control the evolution of the southern Ordos Basin during the Mesozoic. U-Pb detrital zircon geochronology is used to identify provenance, reconstruct paleogeography and document relative timing of basin margin deformation.Four samples of the Triassic Yanchang Formation were collected from the southern Ordos Basin (one from the south and three from the southwest). In total 447 detrital zircon grains were analyzed using LA-ICP. Three major age populations, 240–490 Ma, 1.8-2.0 Ga, and 2.2-2.8 Ga, characterize the detrital zircon grains of the Yanchang Formation. The two oldest age groups match ages of basement rocks found in the underlying North China block. The younger ages can be subdivided into three distinctive groups: 240-300, 300-400 and 400-490 Ma. The youngest group matches ages exposed to the west in the Qilian-Qaidam terrane, whereas two older groups indicate a southern Qinling orogenic belt source. In all samples detrital zircon from the western source overwhelms the southern source, regardless of proximity to either basin margin. This result suggests that the Qinling orogenic belt, even during deformation, was never the dominant source to the southern Ordos Basin. Furthermore, stratigraphic changes in provenance within the Yanchang Formation indicate that: 1) deformation in the Qinling and Western Liupanshan belts began in Middle Triassic time in this area; and 2) the southern source area barely contributed sediment by Late Triassic time.
AAPG Search and Discovery Article #90070 © 2007 AAPG Foundation Grants in Aid