P-T paths and U-Pb ages of pelitic and semi-pelitic granulites in the Yunkai massif and implication for the tectonic evolution of the Wuyi-Yunkai orogen, South China

Abstract

Understanding the tectono-metamorphic evolution of the Wuyi-Yunkai orogen is crucial to constraining the Early Paleozoic geodynamic history of the South China Block. Here, we present petrological and geochronological characteristics of the pelitic and semi-pelitic granulites from Yunkai massif that occur as lenses or enclaves within orthogneisses and charnockites. These granulites have garnet-cordierite-spinel-sillimanite-bearing and garnet-orthopyroxene-bearing assemblages, respectively, showing typical low-pressure–high-temperature (LP-HT) granulite-facies mineral assemblages. Based on petrological characteristics, P-T calculations and phase equilibrium modelling in the NCKFMASHTO system, three-stage metamorphic evolution can be distinguished: 1) prograde stage (M1) characterized by biotite-plagioclase- or sillimanite-bearing assemblages present as inclusions in garnet or cordierite porphyroblasts, with P-T conditions of 7.0–8.0 kbar/730–780 °C; 2) peak LP-HT stage (M2) featured by garnet-orthopyroxene- or cordierite-bearing assemblages, with P-T conditions of 4.7–5.1 kbar/825–875 °C, suggesting high geothermal gradients (∼50–56 °C/km); and 3) post-peak stage (M3) represented by garnet+quartz corona or spinel-cordierite-bearing intergrowth, with P-T conditions of 4.0–4.3 kbar/650–750 °C. P-T estimates and pseudosection modelling define two consistent clockwise P-T paths involving a pre-peak prominent heating with decompression followed by a post-peak nearly isobaric cooling process. LA-ICP-MS zircon U-Pb dating results reveal that the granulite-facies metamorphism occurred at ∼445 Ma. Combined with previous studies, we proposed that the LP-HT metamorphism in the Yunkai massif might be attributed to the ridge subduction in the Early Paleozoic, which caused asthenosphere upwelling and supplied high heat flux to the lower crust when the slab window opened.