A paleomagnetic study of about 95 samples from 16 sites sampled in the Early Cretaceous in Luanping basin in Hebei Province was reported. Stepwise thermal demagnetization was used to isolate magnetic components. Most samples have a characteristic direction with a high temperature component above 500°C. The tectonic-corrected data areD = 347.8°,I = 50.4°, α95 = 7.l°, and the corresponding pole position is at 76.1°N, 346.3°E,with dp =6.4°,dm = 3.8°, paleolatitude λ = 31.1°N. This result indicates a counterclockwise post-Cretaceous rotation of 30.7° ±9.8° with respect to the stable Ordos basin in the west of North China Block, and a non-significant northward motion. This rotation could be related to local fault action or structural detachment, or regional NNW-NWWward motion and collision of Kula-Pacific plate with eastern China since the Early Cretaceous.
The Jingtieshan deposit occurs in a Precambrian tectonic-stratigraphic terrane within the Northern Qilian Caledonian Orogen,
and is generally considered as a Superior-type iron formation. The deposit is characterized by Fe-Si-Ba and Cu mineralization
and consists of two types of orebodies, an upper jasper-barite-iron deposit and a lower copper sulfide deposit. The iron orebodies
occur as independent stratigraphic layers concordant within a thick argillaceous succession, and exhibit fine-grained textures
and well-developed sedimentary layering. The ores are predominantly composed of specularite and jasper with lesser amounts
of magnetite, hematite, siderite, and barite. The presence of barite, hematite and jasper as major components shows that the
iron ores were precipitated in a relatively oxidized ocean floor environment. The Cu orebody directly underlies the iron ore
and is hosted by chlorite-sericite-quartz phyllite. The Cu mineralization is composed of pyrite and chalcopyrite and is characterized
by stockwork. The disseminated and stockwork Cu mineralization is metamorphosed and concordant with respect to foliation,
indicating pre-fabric development, i.e. pre-metamorphism, and was probably originally formed by reduced fluids reacting at
the base of and within the oxide iron formation. Geochemical data show that the jasper-barite-iron ores, which resemble Superior-type
iron formations, have a high input of hydrothermal-hydrogeneous elements (SiO2, av.=56%; Fe2O3t, av.=30%; Mn, av.=0.45%; BaO, av.=16.7%) with minimal terrigeneous input (<15% combined Al2O3, TiO2, K2O, MgO, etc.). The δ34S of exhalative barite varies from 28 to 34‰, which is very heavy with respect to other Late Proterozoic sulfate-bearing deposits,
except those of circa 600 Ma in which the sulfides range from 8 to 20‰. The sulfur isotope data indicate that the barite was
formed by the mixing of a Ba-rich hydrothermal fluid with sulfate-rich ambient seawater and that the sulfides ores were most
probably derived from the reduction of seawater sulfate during subsurface reaction with ferrous iron-bearing minerals. These
data are consistent with the jasper-barite-iron deposit forming by hydrothermal exhalative and chemical sedimentary processes
on the floor of an ocean basin, and with the Cu mineralization forming by hydrothermal filling and replacement in base of
and within the iron formation.
Received: 19 March 1997 / Accepted: 14 May 1998 相似文献
“Milanggouwan stratigraphical section” named lately takes down 27 cycles of alternately evolutionary histories of desert depositions
in the Mu Us area with the fluviolacustrine facies and palaeosols since 150 ka B. P. Studies show that the sedimentary form
was induced by the growth and decline and confrontation each other between the winter monsoon and the summer monsoon of East
Asia in the past 150 ka.
Project supported by the National Natural Science Foundation of China (Grant No. 49473192). 相似文献
A paleomagnetic study of about 95 samples from 16 sites sampled in the Early Cretaceous in Luanping basin in Hebei Province was reported. Stepwise thermal demagnetization was used to isolate magnetic components. Most samples have a characteristic direction with a high temperature component above 500°C. The tectonic-corrected data areD = 347.8°,I = 50.4°, α95 = 7.l°, and the corresponding pole position is at 76.1°N, 346.3°E,with dp =6.4°,dm = 3.8°, paleolatitude λ = 31.1°N. This result indicates a counterclockwise post-Cretaceous rotation of 30.7° ±9.8° with respect to the stable Ordos basin in the west of North China Block, and a non-significant northward motion. This rotation could be related to local fault action or structural detachment, or regional NNW-NWWward motion and collision of Kula-Pacific plate with eastern China since the Early Cretaceous. 相似文献