Tectonic Setting and Provenance Analysis of Late Paleozoic Sedimentary Rocks in the Ordos Basin

The provenances of Ordos Basin and its surrounding regions, which all comes from the upper crust, are dominated by felsic rocks, which consist of ancient metamorphic rocks, such as metamorphic volcanic rocks and sedimentary rock of Archaeozoic and Proterozoic, and certain amount of granite and alkali basalt. But compositions of source and structural setting of provenances in the north and south are different. Compositions of major and rare earth elements suggest that sedimentary rocks in both north and south show some differences in area and stratum, and the changes of major element, REE and Eu anomaly are in accordance with the variation trend from oceanic island arc, continental island arc, and active continental margin to passive continental margin. Analysis of major elements indicates that the north provenance derived mainly from plate subduction zones and were related to active continental margin and passive continental margin, with minor related to the island arc of passive continental margin, and were related to tectonic setting of active continental margin and passive continental margin until middle-late Paleozoic. REE contrast analysis shows that the source for the northern basin has affinities to Archeozoic and Proterozoic metamorphotic rocks, such as granitic gneiss, diorite gneiss, adamellite, metamorphotic litharenite, phyllite, etc; that for the southern basin was deeply affected by passive continental margin source, characterized by high SiO2 , low Na2O features, and K2O/Na2O1. All these features are consistent to those (high SiO2 and K2O/Na2O1) of rocks of Archean-Proterozoic Taihua Group, Qinling Group and Kuanping Group. Until the end of Late Paleozoic, the provenance was not affected gradually by active continental margin. Beiqinling intermontane basins characterized by rapid accumulation m langes belong to the outer margin of Ordos Basin and have continuous transitional relationship with the basin. Therefore, both show some inherited relation. The Gd content and (Gd/Yb)N ratios of the Late Paleozoic sedimentary rocks in the Ordos basin vary with time changing, and the analysis shows that the northern source area in Taiyuan period was in rapid tectonic activity period, while the southern source area in Shanxi period was fast active period. This is consistent with the regional tectonic evolution, i.e. the uplift of the northern source area was earlier than that in the south.