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SHRIMP zircon ages, Hf-in-zircon isotopic compositions and whole rock geochemistry were analyzed on basement metamorphic rocks from drill cores collected from the northeastern margin of the Ordos Basin. Geochemical data from four metasedimentary rocks show large variations in major element compositions, but have similar REE patterns and trace element compositions, with ΣREE = 161.80 × 10−6~341.82 × 10−6, δEu = 0.26~0.63 and LaN/YbN = 3.44~25.38. SHRIMP zircon U-Pb dating of granitic gneiss yielded the magmatic zircon ages between 1856 ± 14 Ma and 2188 ± 11 Ma, with the upper intercept age of 2229 ± 88 Ma. The cores have εHf(t) values of −5.46 to +6.57, and Hf model ages vary from 2869 Ma to 2300 Ma. The analyses of metamorphic overgrowths on zircon grains yield an average metamorphic age of 1865 ± 17 Ma (MSWD = 3.5). The zircon cores have εHf(t) values of −3.98 to 1.95, and Hf model ages vary from 2782 to 2416 Ma. Combined with data from earlier studies, we draw the conclusion that the metamorphic rocks in the borehole were formed during middle Paleoproterozoic time and were involved in a major late Paleoproterozoic tectono-thermal event which might be attributable to the collision between the Western and Eastern Block along the TNCO (the Trans-North China Orogen). The basement rocks are petrologically, geochronologically and geochemically comparable with rocks from the Khondalite series. The fact that the material properties of basement rocks beneath the northeastern Ordos Basin are different from those of the Western Block indicates that the extent and exact boundaries of the Ordos Block need to be refined.

The Qarhan Salt Lake area is the Quaternary depocenter of the Qaidam Basin, and carries thick lacustrine sediments, as well as rich potassium and magnesium salt deposits. The abundant resources and thick sediments in this lake provide an ideal place for the study of biogas formation and preservation, salt lake evolution, and the uplift of the Tibetan Plateau. In this study, we attempt to construct a paleomagnetic and 230Th age model and to obtain information on tectonic activity and salt lake evolution through detailed studies on a 1300-m-long drill core (15DZK01) from the northwestern margin of the Qarhan Salt Lake area (Dongling Lake). Based on gypsum 230Th dating, the age of the uppermost clastic deposit was calculated to be around 0.052 Ma. The polarity sequence consist of 13 pairs of normal and reversed zones, which can be correlated with subchrons C2r.1r-C1n of the geomagnetic polarity timescale (GPTS 2012) (from ∼2.070 Ma to ∼0.052 Ma). Sedimentary characteristics indicate that Dongling Lake witnessed freshwater environment between ∼2.070 Ma and 1.546 Ma. During this period, the sedimentary record reflects primarily lakeshore, shallow-water and swamp environments, representing favourable conditions for the formation of hydrocarbon source rocks. Between 1.546 Ma and ∼0.052 Ma, the Dongling Lake was in sulphate deposition stage, which contrasts with the central Qarhan Salt Lake area, where this stage did not occur in the meantime. During this stage, Dongling Lake was in a shallow saltwater lake environment, but several periods of reduced salinity occurred during this stage. During the late Pleistocene at ∼0.052 Ma, the Dongling Lake experienced uplift due to tectonic activity, and saltwater migrated through the Sanhu Fault to the central Qarhan Salt Lake area, resulting in the absence of halite deposition stage. The residual saline water was concentrated into magnesium-rich brine due to the lack of freshwater, and few potassium salt deposits occur in the Dongling Lake area. Keywords: Qaidam Basin, Magnetostratigraphy, 230Th dating, Salt lake evolution, Tectonic activity

Through the study of the geological conditions of potash deposits in China from recent years, a new understanding of potash theories has arisen that appropriate Chinese geological features. Important progress and substantial breakthroughs have been gained in the direction and management of potash prospecting: (1) Important breakthroughs in continental potassium prospecting: The “Quaternary gravel type deep potassium rich brine metallogenic model in western Qaidam” ensures Quaternary deep potassium rich brine prospecting will grow new KCl resources by 350 Mt, providing a resource guarantee for meeting the Chinese demand for sylvite. (2) The Marine facies potash prospecting shows good prospects: the determination of the new type of Triassic polyhalite potash ore deposits in Sichuan provide an important scientific basis for the establishment of exploration planning and the selection of exploration target areas for polyhalite minerals in the Sichuan Basin; The “two-storey potash deposits model” in southwestern Yunnan has been confirmed, which indicates prospects for the exploration of potash in the deeper Marine facies in southwestern Yunnan are likely to be successful. The discovery of a high concentration of rich bromite salt and potash salt in the Paleogene of the Kuqa depression and the southwestern Tarim region provides strong support for the likelihood large-scale potash deposits exist in these regions.

The thick Cenozoic sediments in the Qaidam Basin contain considerable information about the uplift of Tibetan Plateau, climate change and salt deposits. In this paper, a palaeomagnetic study was performed on the 1400-m-long drilling core 15YZK01 from the Yiliping playa in the northwestern Qaidam Basin to obtain a timescale documenting this information. The polarity sequence consisted of 17 pairs of normal and reversed zones and can be correlated well with subchrons C2Ar-C1n of the geomagnetic polarity timescale GPTS 2012 (from ca. 4.1 to 0.002 Ma). Three periods of high sedimentation rates occurred at ca. 4.1–3.207 Ma, 2.581–1.945 Ma, and 0.781–0.002 Ma. These 3 periods correspond to 4 episodes of tectonic activity on the Tibetan Plateau: the Qingzang Movement (3.6 Ma), the B phase of the Qingzang Movement (2.6 Ma), the Kunlun-Huanghe Movement (1.1–0.7 Ma) and the Gonghe Movement (0.15 Ma). The dips of the formations in the drilling core indicate that the Yiliping area was uplifted at ca. 3.236 Ma. After this uplift, the sedimentation rate sharply decreased from 39.1 cm/ka to 19.2 cm/ka. In the Pliocene, the Yiliping area mainly featured a freshwater environment, and the earliest gypsum sediment formed at ca. 3.530 Ma after the Qingzang Movement. After the B phase of the Qingzang Movement, the Yiliping area alternated between freshwater and semi-saline lake environments, and 20 layers of gypsum-bearing strata began to appear since 2.103 Ma. After the Kunlun-Huanghe Movement, the Yiliping area alternated between semi-saline and saline lake environments, and salt deposits began to form at 0.733 Ma or 0.746 Ma in the middle Pleistocene. Four salt-forming periods in the Yiliping area can be correlated with Quaternary glaciations on the Tibetan Plateau: the S1 deposit corresponds to the Wangkun Glaciation; the S3 and S4 deposits and the mirabilite-bearing stratum between them correspond to the Penultimate Glaciation; and S5 began to form in the Last Glaciation. The calculated formation age of S2 has to be considered with caution, and it is very close to the timing of the Wangkun Glaciation. A reduction in the water supply of the salt lakes during the Quaternary glaciations was an important driving factor in the formation of salt deposits in the Qaidam Basin, and the Quaternary glaciations were closely related to the tectonic activity of the Tibetan Plateau.

The Mengyejing potash deposit is the only solid chloride type potash deposit in our country. There has been divergence about the source material of this deposit. Some popular sayings in the old days were the Monophyletic Theory, the Marine Source Theory, and the Terrestrial Source Theory. With the further exploration of acknowledge in recent years, many geological scholars hold the point of the Polygenesis Theory. This thesis will put forward new metallogenic modes through the regional geological research and salt structure analysis, combining with its geochemical characteristics. That is, the potash layer is formed by the extrusion in the deep and plastic flows to the surface.