Paleomagnetism Of The Early Mesoproterozoic Red Beds In The Northern North China Craton And Its Implications For Paleogeographic And Paleoenvironmental Reconstructions

There are several red beds within the Mesoproterozoic strata in the Yanliao basin in the northern margin of the North China Craton(NCC),among which the Xuanlong-type ironstone of the basal Chuanlinggou Formation(?1.645 Ga)and the purple-red argillaceous dolomite of Yangzhuang Formation(?1.5 Ga)are the most unique.Less researches have aimed to explore the environmental information,location and discuss how location influence the environmental conditions for the precipitation of the Xuanlong-type ironstone and the deposition of the red beds of the Yangzhuang Formation in the Shanxi Province in the perspective of paleogeography although previous studies have learned the redox conditions of the Xuanlong-type ironstone mainly by geochemical methods and have learned the paleo-position of the Yangzhuang Formation in the Jixian area by paleomagnetism;however,there are still controversies on:(1)the paleogeography of the above two mentioned red beds and the origin of the hematite in them;(2)types of magnetic minerals and the machines for the formation of hematite are unknown,adding difficulties to extract paleoenvironmental information;(3)the impact of paleogeography on the formation of early Mesoproterozoic red beds.Therefore,this research plans to use comprehensive research methods,including paleomagnetism,rock magnetism and in-situ analysis technology on the Xuanlong-type ironstone of the Chuanlinggou Formation and the red beds of the Yangzhuang Formation in order to probe spatial and environmental information recorded in them,to further discuss the impact of paleogeography on the formation of red beds and to acquire the environmental responses on the formations of red beds after understanding their magnetic carriers,the transformation between magnetic minerals and the origins of them.In the end,this research can conclude that:(1)Normal and reversed directions of characteristic remanent magnetization could be isolated for samples from the Xuanlong-type ironstone.The mean direction of this primary remanent magnetization after bedding correction was Ds=50.1°,Is=54.4°(?=22.2,?₉₅=5.5°,n=44).The corresponding paleomagnetic pole was at 50.5°N,197.2°E(dm/dp=7.7/5.5,n=44)and a paleo-latitude of 34.9°N was calculated at?1.645 Ga.These directions were able to pass a C-classification reversal test(Mc Fadden and Mc Elhinny,1990)and a fold test at 95%confidence radius(Mc Fadden,1990).This paleomagnetic pole at?1.645 Ga fulfilled a Van der Voo(1990)value Q=6.Thus,this characteristic remanent magnetization was interpreted to be a primary remanent magnetization carried by Al-substituted hematite during the precipitation of the Xuanlong-type ironstone.(2)SEM-EDS analysis and EPMA results revealed that the primary remanent magnetization preserved in the Xuanlong-type ironstone was a chemical remanent magnetization carried by Al-substituted hematite acquired in early diagenesis.After analyzing the possible transformation of ferrihydrite to Al-substituted hematite,this research revealed that the formation of porous and dense laminae indeed reflected two source of water(hydrothermal fluids input and runoff input)that dominated differently in the pore water of sediments during the precipitation of Al-substituted hematite.This process was actually controlled by the relative sea-level changes because the Xuanlong-type ironstone precipitated in the land-ocean transitional zone.(3)SEM-EDS analysis and rock magnetic experiments found very fine detrital hematite in the samples from the Yangzhuang Formation.After thermal demagnetization,normal and reversed directions of characteristic remanent magnetization could be also isolated for samples both from the Shanxi Province and the Jixian area.The mean direction of them were Ds=271.3°,Is=-45.3°(?=11.9,?₉₅=6.9°,n=39)after bedding correction and it was also interpreted as a primary remanent magnetization carried by detrital hematite because this mean direction was similar to previous paleomagnetic results reported in the Jixian area.The mean direction of characteristic remanent magnetization from this research together with data from previous studies were Ds=85.6°,Is=34.9°(?=8.6,?₉₅=6.4°,n=65).The corresponding paleomagnetic pole was at 15.5°N,193.0°E(dm/dp=7.4/4.2,n=65)and a paleo-latitude of 19.2°N was calculated at?1.5 Ga.These characteristic remanent magnetization should be a detrital remanent magnetization.(4)Summary on the paleo-latitude of Paleo-Mesoproterozoic red beds revealed that most Precambrian red beds deposited in mid-latitude or low-latitude area.This research inferred that the enrichment of detrital hematite in Paleo-Mesoproterozoic red beds might be attributed to larger iron flux that transported more detrital hematite into ocean because of the more intense weathering in mid-latitude or low-latitude area.Thus,the deposition of Paleo-Mesoproterozoic red beds were not only controlled by atmospheric oxygen and iron source but also by the paleo-latitude.This research highlighted the influence of paleo-position(e.g.,the land-ocean transitional zone)and paleo-latitude(e.g.,mid-latitude or low-latitude)on the origin of hematite and also on the evolution of paleoenvironment in the perspective of paleomagnetism and rock magnetism.Eventually,paleo-position and paleo-latitude play an easily neglectful but vital role on the evolution of environment.