Abstract:

The EarlyMiddle Jurassic Yanan Formation widely distributed in the northwestern margin of Ordos Basin, and the outcrops scattered limitedly with similar lithological features. Many disputes still exist about the source areas of Yanan Formation due to the tremendous geological reformation since the Late Cretaceous. The zircon LaserAblation Inductively Coupled PlasmaMass Spectrometry (LAICPMS) UPb dating was used to determine the ages of the source areas of the Yanan Formation in the region. There are three samples selected from the sandstone of Yanan Formation in two areas, altogether 210 grains were dated, and 193 valid zircon ages were obtained. Most zircons from the samples of Helan Mountain showed circular belt images under the cathodoluminescence microscope and displayed the leftoblique distribution pattern of rare earth elements. Most of those zircons have the Th/U larger than 0.4 indicating the magmatic origin, and a few zircons were from the metamorphic rock in the area. Most zircon UPb dating ages are mainly of the Paleozoic with the major peak in the Permian. The secondary ages are of the Proterozoic. The zircons from Zhuozi Mountain area are with the similar characteristics both in the images under the cathodoluminescence microscope and in the rare earth element distribution pattern. There exists much difference in the zircon ages between the two areas. The old ages in Zhuozi Mountain possess great proportions; 66.7% are of the Late ArcheanProterozoic (mainly the Proterozoic); only 22.7% of the total ages belong to the Permian, and 10.6% are of the EarlyMiddle Triassic. Compared with the ages of surrounding ancient rocks, it is concluded that the main sources of the EarlyMiddle Jurassic in the northwest margin of Ordos Basin were the Proterozoic magmatic and metamorphic rocks in the Alashan PaleoBlock; for the north of Ordos Basin, the main source was the Hercynian granite in the Alashan PaleoBlock. The main provenance in Helan Mountain was the Hercynian magmatic rocks in the Alashan PaleoBlock, and the secondary source was the Proterozoic magmatic and metamorphic rocks(Helan Mountain complex)for its northwest. While the principal source area in the Zhuozi Mountain was Proterozoic magmatic and metamorphic rocks for the northwest and the Hercynian magmatic rocks and EarlyMiddle Triassic magmatic rocks acted as the minor source. There is difference in provenance ages among the two studied areas and the Ciyaobao area. More researches from the sedimentary and petrology need to be done to further describe the provenance attribute.

Key words: Yanan Formation of EarlyMiddle Jurassic, age distribution of detrital zircons, sedimentary provenance, Proterozoic magmatic and metamorphic rocks, Hercynian magmatic rocks