La-ICP-MS U–Pb and 40Ar/39Ar geochronology of the sheared metamorphic rocks in the Wuyishan: Constraints on the timing of Early Paleozoic and Early Mesozoic tectono-thermal events in SE China

Mylonite and ultramylonite have been well documented in Pre-Devonian meta-volcanic and meta-sedimentary rocks in the Wuyishan, SE China. Field observations and thin section analyses show dextral strike-slip shearing. Deformation temperature is estimated in 400 °C–500 °C and the pressure between 7 kbar and 10 kbar. La-ICP-MS U–Pb zircon and 40Ar/39Ar step-heating biotite–muscovite ages are used to constrain the timing of the dextral shear zone and its tectonothermal history. La-ICP-MS U–Pb dating of zircons from three gneissic granitic samples provides magmatic crystallization ages between 441 and 431 Ma, and dating of the growth rims of zircons from one ultramylonite sample provides metamorphic age of 433.8 ± 6.8 Ma. These dates provide additional constraint on the timing of the Early Paleozoic tectono-thermal event in the Wuyishan region. 40Ar/39Ar step-heating dating of biotite and muscovite separated from two mylonite samples yields ages of 235.3 ± 2.8 Ma and 238.5 ± 2.8 Ma, respectively; and an undeformed granitic dyke intruded into the shear zone was dated, 229.8 ± 2.2 Ma, by La-ICP-MS U–Pb zircon method. These dates reveal that metamorphic rocks in the Wuyishan suffered from dextral shearing at the time span between 230 Ma and 239 Ma, during the Triassic Indosinian tectono-thermal event.

Research Highlights
► Documentation of sheared rocks in the Wuyishan, SE China.
► La-ICP-MS U-Pb dating of zircons from syn-metamorphic rocks and syn-deformational granites confirms the timing of the Early Paleozoic tectono-thermal event.
► 40Ar/39Ar step-heating dating of mica from mylonites indicates Triassic dextral shearing.