Palaeomagnetism of Early Triassic limestones from the Huanan Block, South China: no evidence for separation between the Huanan and Yangtze blocks during the Early Mesozoic

There has been extensive debate on the timing of the collision between the Huanan and Yangtze blocks ever since Hsu and co-workers (Hsu et al. 1987, 1 988) proposed a Mesozoic collision model for South China tectonics. We repo rt new rock and palaeomagnetic data from an Early Triassic limestone format ion from the Huanan Block that help to constrain its tectonic history. The samples are from two localities [Jiupi (JT), and Huangben (LT)] in Lianzhou County, northern Guangdong province. Only thermal demagnetization is effective in decomposing a multicomponent r emanent magnetization. The majority of samples show three components: a low -temperature component (A), an intermediate-temperature component (B), and a high-temperature characteristic remanent magnetization (ChRM). The A comp onent has a steep inclination and is probably drilling-induced. The B compo nent clusters around the present geomagnetic field direction and is a Brunh es-age viscous remanence. Fold test results suggest that the JT ChRM is pre -tectonic, and that the LT ChRM is a syn- or pre-folding remagnetization. S amples from the two localities show quite similar acquisition and thermal d emagnetization of the isothermal remanent magnetization (IRM), and suggest that magnetite carries the ChRM component. However, partial anhysteretic re manent magnetization (pARM) spectra, anisotropy of anhysteretic remanence ( AAR), rock fabric, and morphology of magnetic extracts are quite different for the two localities. The AAR of JT samples shows low anisotropy (average 2 per cent), and reflects a composite compaction and strain fabric. The AA R of LT samples shows a high degree of anisotropy (average 10 per cent), an d is clearly consistent with rock deformation fabrics. The magnetic extract s of LT samples are dominated by spherical, botryoidal Ti-poor magnetite, w hich is probably precipitated from tectonic fluids during deformation, whil e the JT magnetic extracts are dominated by rod-shaped Ti-poor magnetite, p robably derived from the erosion of igneous rocks, although minor amounts o f botryoidal magnetite and rod-like magnetite particles can be seen in JT a nd LT extracts, respectively. The rock fabric and magnetic fabric data, and SEM observations of magnetic extracts corroborate the results of the fold test. The best clustered LT Ch RM direction is D = 294 degrees, I = -12 degrees (alpha(95) = 12 degrees); its pole position is 187 degrees E, 19 degrees S (dp = 6 degrees, dm = 12 d egrees). The assumption of a reversed polarity for the JT ChRM direction (D = 198 degrees, I = -20 degrees, alpha(95) = 9 degrees) implies that the Hu anan Block was in the northern hemisphere during the Early Triassic, and it s pole position, 239 degrees E, 68 degrees N (dp = 5 degrees, dm = 10 degre es) is close to Early Triassic pole positions from the Yangtze Block. A nor mal polarity for the JT-site ChRM would require tectonic movements inconsis tent with the local geology. Therefore, the Huanan and Yangtze blocks were not separated in the Early Triassic. The rotation of the LT locality may ha ve been caused by the collision of the southeast China coastal terranes and /or subduction of the Pacific plate in the Late Mesozoic.