Detrital Zircon U-Pb Geochronology And Tectono-Paleogeographic Implications Of Tangwangling Conglomerate Deposits In The Southern Margin Of Ordos Basin

The southern margin of the Ordos (Block or) Basin (S-OB) is commonly regarded as the west part of the southern margin of North China Block (S-NCB), involved in the Phanerozoic multiple orogeny of the North Qinling Belt (NQB) Tangwangling conglomerate, outcropped in the S-OB, has long been controversial on sources and ages, which is important not only for identifying its coeval tectono-palaeogeography, but also for understanding the correlation of the NCB and North Qinling Terrane (NQT) enveloped in the NQB. Here, LA-ICPMS zircon U-Pb dating of 3-interlayered sandstone samples from Tangwangling conglomerate deposits is carried out to qualify the conglomerate deposit ages and sources. More than 396 analyzed zircons of all the tested samples show rounded or oval shapes with CL imaging oscillatory zones and Th/U values of 0. 23 similar to 1. 51, suggesting a magmatic origin. The concordant ages of all the analysed zircons display a wide spectrum with the age range of 2696 similar to 744Ma, which is almost similar to the age pattern of each samples. The largest population of zircons exhibits ages of 2120 similar to 1618Ma (n = 268) with a dominant peak at 1. 8Ga, and the subordinate populations display ages of 2531 similar to 2364Ma (n = 25), 1230 similar to 940 Ma (n = 29) and 905 similar to 744Ma (n =10), corresponding to the peak ages of 2. 44Ga, 1. 09Ga and 0. 81Ga, respectively. This age spectrum indicates that the detritus of the Tangwangling conglomerate were mainly sourced from the Early Precambrian high-grade metamorphic bedrocks of the NCB and subordinately from the Late Precambrian metamorphic complex and igneous rocks of the NQB, in which the youngest peak age of 0. 81Ga with the youngest single detrital zircon age of 744 +/- 8Ma suggests the conglomerate deposit age to be no more than 810 similar to 744Ma, approaching to the Late Neoproterozoic or Sinian Period. This dating result is similar to that of the Sinian Luoquan and Zhengmuguang formations exposed in the southwest Ordos Basin, but different from or lack of the dominant peak-aged 454Ma zircons of the Ordovician Pingliang Formation outcropped in the S-OB. Furthermore, all the analysed sand-mudstone and shale samples from Tangwangling conglomerate deposits are mainly located at the passive continental margin area in the tectonic setting discrimination charts of major and trace elements. While, the redox sensitive parameters of trace and rare elements of the analysed mudstone samples mainly indicate a volatile neritic-littoral sedimentary environment. All above data integrated with sedimentary facies reveal that Tangwangling conglomerate deposits, sourced bidirectionally from the NCB and NQB, were developed limitedly in a rift passive continental mar g in alon g the S-OB of the S-NCB durin g the Late Neoproterozoic Sinian Period. It can be therefore supported that the continental mar g in of the S-NCB amal g amated with the NQT as a whole experienced a rift extensional sedimentary process durin g the Sinian Period, which is most probably close to the Rodinia break-up event after the Grenvillian collision especially followin g the post-collisional upliftin g process (ca. 1. 09 similar to 0. 81Ga).