Detrital zircon U-Pb ages of the Nanwuli metamorphic rocks in the Changle-Nan’ao fault zone, Fujian and their geological significance

The Nanwuli metamorphic rocks, located in the northern part of the Changle-Nan’ao fault zone of Fujian Province, consist mainly of sillimanite mica-quartz schist and sillimanite biotite quartz schist and overlay gneissic granite. Petrological characteristics indicate that the Nanwuli metamorphic rocks sourced from terrigenous clastic rocks. LA-ICP-MS zircon U-Pb age spectrum shows two peaks: the first peak is 189.6±5.7 Ma with a ²⁰⁶Pb/²³⁸U age range of 199～182 Ma, and the second is 1 899±4 Ma with a ²⁰⁷Pb/²⁰⁶Pb age range of 1 922～1 831 Ma. The former zircons(Early Jurassic zircons)are euhedral crystals with obvious oscillatory zonal texture of magmatic zircons and have Th/U ratios of 0.4～0.96 and distinct REE differentiation. As to the latter(Paleoproterozoic zircons), most of them show better psephicity with no obvious oscillatory zonal texture, have lower Th/U ratios(0.03～0.3)and weak REE differentiation, revealing that they derived from metamorphic rocks. A few Paleoproterozoic magmatic zircons have higher Th/U ratios(>0.3)and obvious REE differentiation, indicating that they might derive from old igneous rocks in the Cathaysia Block. The age of 189.6±5.7 Ma represents the sedimentary time(Early Jurassic)of the Nanwuli metamorphic rocks with its protolith equivalent to the early Jurassic Qingyingshan Formation.