Identification of multiple magnetizations of the Ediacaran strata in South China.

A suspected Silurian remagnetization of the Ediacaran strata of South China was proposed decades ago by many researchers, but, there has been no systematic study of its causes and mechanI_sms. In thI_s study, we investI_gate the multiphase remagnetization processes that affected the Ediacaran strata and the possible mechanI_sms of these remagnetization events.We conducted detailed palaeomagnetic, rock magnetic and scanning electron microscope (SEM) studies of samples from the Ediacaran strata in the Jiulongwan (JLWE, JLWS), Qinglinkou (QLK) and Sanxiarenjia (SXRJ) sections in the Three Gorges Area, South China. After removal of a recent vI_scous remanent magnetization below150°C, an intermediate temperature component (ITC; D_g = 27.6°, I_g = 45.3°, N = 12 sites, k_g = 184.3, α₉₅ = 3.2° for JLWE; D_g =22°, I_g =45.3°,N=11 sites, k_g =789.2, α₉₅ =1.6° for JLWS; andD_g =25.5°, I_g =52.5°, N = 6 sites, k_g = 533.4, α₉₅ = 2.9° for SXRJ) was removed below 300°C which coincides with the Jurassic results from South China, suggesting a pervasive Jurassic remagnetization. In addition, a hI_gh temperature component (HTC; Ds = 84.8°, I_s = 19.2°, N = 9 sites, ks = 35.5, α₉₅ =8.8° for JLWE; Ds =74.1°, I_s =49.4°, N=7 sites, ks =218.9, α₉₅ =4.1° for JLWS; and Ds =89.5°, I_s =30.7°, N=8 sites, ks =129.2, α₉₅ =4.9° for SXRJ) was I_solated between 300 and 480-540°C. Rock magnetic and SEM studies suggest that the ITC and HTC are carried by pyrrhotite and magnetite, respectively. SEM observations also demonstrate the occurrence of massive authI_genic magnetite in cavities or cracks, mineralogical changes from pyrite to Fe oxides, and the reaction between gypsum and Fe oxides. Based on similarities to the Silurian poles of South China, together with the SEM observations, we suggest that the HTC from the JLWE and SXRJ sections I_s a Silurian age remagnetization. The oxidation of iron sulphides and thermochemical sulphate reduction induced by the multiple generations of oil and gas in the Ediacaran and Cambrian strata are suggested as the main mechanI_sm for remagnetization. Despite the pervasive Silurian remagnetization of the Ediacaran strata, most of the HTC from the thick-bedded dolostone of Doushantuo Formation Member 3 at the JLWS section appears to carry a primary remanence, because its pole differs from other poles of South China and the results pass both the fold and reversal tests. The relatively low-geothermic conditions and the absence of both hydrocarbon and smectite/illite explain why most results from the Doushantuo Member 3 of JLWS section were not affected by the Silurian remagnetization. ThI_s new Ediacaran pole supersedes the previous suspected remagnetized poles, which can be used to constrain the palaeoposition of South China both in Rodinia and Gondwana. [ABSTRACT FROM AUTHOR]