Characteristics of the crystalline basement beneath the Ordos Basin:Constraint from aeromagnetic data

Aeromagnetic anomaly zonation of the Ordos Basin and adjacent areas was obtained by processing high-precision and large-scale aeromagnetic anomalies with an approach of reduction to the pole upward continuation. Comparative study on aeromagnetic and seismic tomography suggests that aeromagnetic anomalies in this area are influenced by both the magnetic property of the rock and the burial depth of the Precambrian crystalline basement. Basement depth might be the fundamental control factor for aeromagnetic anomalies because the positive and negative anomalies on the reduction to the pole- upward-continuation anomaly maps roughly coincide with the uplifts and depressions of the crystalline basement in the basin. The results, together with the latest understanding of basement faults, SHRIMP U-Pb zircon dating of metamorphic rock and granite, drilling data, detrital zircon ages, and gravity data interpretation, suggest that the Ordos block is not an entirety of Archean.

Potash–Forming Regularity of the Ordovician Marine Northern Shaanxi Salt Basin: Insight from Review and Prospect of the Deep Geology of the Ordos Basin

The Ordovician Northern Shaanxi Salt Basin （ONSSB）, located in the east-central Ordos Basin, western North China Craton （NCC）, is one of the largest marine salt basins yet discovered in China. A huge amount of halite deposited in the Mid-Ordovician Majiagou Formation, and potash- containing indication and local thin layer of potash seam were discovered in O2m^65 （6th submember, 5th member of the Majiagou Formation）. This makes ONSSB a rare Ordovician potash-containing basin in the world, and brings new hope for prospecting marine solid potash in this basin. However, several primary scientific problems, such as the coupling relationship between ONSSB and the continent nucleus, how the high-precision basement fold controls the ONSSB, and how the basement faults and relief control ONSSB, are still unclear due to the limitations of the knowledge about the basement of the Ordos Basin. This has become a barrier for understanding the potash-forming regularity in the continental nucleus （CN） area in marine salt basin in China. Up to now, the material accumulation has provided ripe conditions for the answers to these questions. Latest zircon U-Pb ages for the basement samples beneath the Ordos Basin reveal that there exists a continental nucleus （Yi-Meng CN） beneath the northern Ordos Basin. And this brings light into the fact that the ONSSB lies not overlying on the Yi- Meng CN but to south Yi-Meng CN. Both do not have superimposed relationship in space. And borehole penetrating into the basement reached Palaeoproterozoic meta-sedimentary rocks, which suggests the ONSSB is situated in the accretion belt of Yi-Meng CN during geological history. Basement relief beneath the ONSSB area revealed by seismic tomography and aeromagnetic anomaly confirms the existence of basement uplift and faults, which provides tectonic setting for sedimentary center migration of the ONSSB. Comparative research with various data sources indicates that the expanding strata in the ONSSB adopted the shape of the basement folds. We found that the orientations of the potash sags showed high correlation with those of several basement and sedimentary cover faults in the ONSSB. The secondary depressions are also controlled by the faults. Comparative research between all the global salt basins and continental nuclei distribution suggests that distribution of the former is controlled by the latter, and almost all the salt basins developed in or at the margin of the continental nucleus area. The nature of the tectonie basement exerts a key controlling effect on potash basin formation. And on this basis we analyzed in detail the geological conditions of salt-forming and potash-forming in the ONSSB.

Precambrian crustal evolution in Rangrim Massif,Korean Peninsula

Petrological,chronological and geotectonic geological analysis of Precambrian metamorphic rock in Korean Peninsula shows that the remnants(>3.4 Ga) of continental nuclei crust were formed in the Paleoarchean in the Rangrim Massif.In the massif,the main formation ages of continental crust range from 3.2 Ga to 2.5 Ga,its important growing period was 2.8-2.5 Ga.The subsequent expansion period of the Rangrim Massif was 2.4-2.2 Ga.The division events occurred in 1.85 Ga and in the Late Paleoproterozoic--Early Mesopro-terozoic,respectively.Since then the massif was relatively stable.However,the last division of the Rangrim Massif occurred at 793 Ma.

Mesoproterozoic to Paleozoic Nd isotope stratigraphy of the South China continental nucleus and its geological significance

The Yangtze continental nucleus in South China is situated in the Huangling-Shennongjia area in western Hubei Province.It comprises the early Precambrian crystalline basement and generally successive Proterozoic to Phanerozoic sedimentary covers.This paper reports a comprehensive Nd isotope study of fine-grained sedimentary rocks from the regional Mesoproterozoic to early Paleozoic strata.The Nd model ages display a V-shaped variation from ～2.5-2.8 Ga during the late Mesoproterozoic through ～1.5-1.7 Ga during the Neoproterozoic Nanhua Period to ～1.8-2.1 Ga during the Paleozoic Ordovician,with corresponding Nd (t) ranges of 11 to 14,1.1 to 5.3,and 7.9 to 9.9,respectively.This evolutionary trend broadly resembles those documented in the sedimentary strata along the southeastern margin of the Yangtze Craton and within the Jiangnan orogenic belt,indicating that the whole South China block underwent a mantle-input related tectono-magmatic event at ～0.8 Ga or earlier.However,the Huangling-Shennongjia area is discriminated from the other two regions by highly variable model ages (～1.5-2.7 Ga) and Nd (t) values (1.38-12.0) of the early Mesoproterozoic strata.Combined with the Archean model ages of the late Mesoproterozoic strata,it is suggested that the Mesoproterozoic sedimentary provenance of the Yangtze continental nucleus was distinct from that of the southeastern Yangtze Craton,likely indicative of an intervening aulacogen (or rift zone) or oceanic basin.In addition,the high comparability in Neoproterozoic to Paleozoic Nd isotope stratigraphy between the Yangtze Craton and the Jiangnan orogenic belt infers that the whole South China block had shared the same sedimentary basin and provenance since the Yangtze-Cathaysia welding at ～0.9 Ga.Thus,it is proposed that the Yangtze Craton had comprised a collage of micro-continents during the pre-Neoproterozoic until the Rodinia assemblage when the primitive South China block was formed.