K‐Ar and ⁴⁰Ar/³⁹Ar ages on Mesozoic volcanic rocks from the Lower Yangtze Volcanic Zone, southeastern China

Abstract

Volcanic rocks, mainly of intermediate composition, occupy several basins within the rift zone along which the Yangtze River flows in its lower reaches. Potassium‐argon (K‐Ar) age measurements on minerals and whole rock samples from lavas and syenitic intrusives in the Lujiang‐Tzungyang volcanic basin range from 131 to 123 m.y., and biotites from two lavas in the Nanjing‐Wuhu basin have measured ages of 127 and 130 m.y. Incremental heating experiments by the ⁴⁰Ar/³⁹Ar method on biotite from two volcanic rocks, one from each basin, yield simple age spectra with plateau ages of 129 to 130 m.y. These data provide evidence that the two biotites have remained undisturbed since crystallisation. The combined results show that volcanism was contemporaneous within the two basins in the Early Cretaceous. Vol‐canism in the Yangtze Volcanic Zone is thought to be related to adjustments within the Eurasian plate as a consequence of collision between the earlier Pacific (Kula) plate and the Eurasian plate.