Palaeointensity studies of a late Permian lava sucession from Guizhou Province, south China: implications for post-Kiaman dipole field behaviour

Abstract

Palaeomagnetic results are presented from a volcanic sequence in Zhijin County, Guizhou Province, Southern China. The lavas and associated volcanic breccias comprising the sequence represent a southern extension of the Emeishan volcanic province. Biostratigraphic dating of interbedded limestone units and stratigraphic constraints indicate that the section formed during the late Permian (~2636-255 Ma), and is thus somewhat older than the Emei stratotype section in Sichuan Province, and close in age to reported estimates of the termination of the Permo-Carboniferous (Kiaman) reverse superchron. Rock magnetic analyses and reflected light microscopy indicate that the magnetic mineralogy of the lava units is dominated by fresh, primary magnetites containing a significant fraction of single-domain grains. Thermal demagnetization behaviour of the breccia units is poor, but most lava samples have one or two components of remanence above 250°C. The normal polarity characteristic remanence held by the lavas implies a post-Kiaman age for this sucession and suggests that the termination of the Kiaman occurred prior to 263 Ma, supporting recently published estimates. According to standard criteria, Thellier palaeointensity results from the lavas are of good quality and reveal that the dipole field strength was comparatively low shortly after the termination of the superchron. 80% of samples record relative VDM values in the range 42-52% of the present-day value, supporting recent studies of mid-Kiaman field intensity. This suggets that a low-energy dipole existed at least between 300 and 255 Ma and does not appear to have been confined to the stable reverse polarity interval.