From the Neoproterozoic mafic rock to the Silurian high-grade metamorphic rock: evidence from zircon U-Pb geochronological, bulk-rock geochemical and mineral EPMA studies of Longyou garnet amphibolite in SE China

The Longyou garnet amphibolite is exposed on the southeast of the Neoproterozoic Jiangshan-Shaoxing fault zone of South China. Some recent studies propose that the Longyou garnet amphibolite was retrograded from an Early Paleozoic collision-related eclogite, which is inconsistent with previous numerous studies. Here we carry out a systematic study including field investigation, petrography, mineral chemistry, zircon geochronology and bulk rock geochemistry to ascertain its genesis. Our results indicate that a small amount of plagioclase inclusions found in garnet and clinopyroxene are anorthite, while plagioclase that largely distribute in the outer part of garnet and clinopyroxene porphyroblasts are andesine and labradorite (with An values of 44-66). No sodium-rich plagioclase (albite or oligoclase) which is indicative of omphacite symplectite has been found. The zircon CL images and Th/U ratios indicate that, apart from some newborn-metamorphic small zircons, a number of large zircon crystals exhibit obvious core-rim structures. Zircon U-Pb dating results indicate that the zircon cores yield concordant weighted average age of 879±11Ma and the zircon metamorphic rims yield weighted average age of 446±5Ma. Furthermore, the bulk rock geochemical features of garnet amphibolite suggest that its protolith is basic igneous rock (the SiO₂ content within 46.8-48.2%), and its trace elements exhibit E-MORB geochemical affinity. This study also constrains the P-T pseudosection for Longyou garnet amphibolite (LGA), which revealing that the LGA at least experienced three stages of metamorphism in Silurian: the prograde metamorphism stage with P-T condition of 0.67-0.75GPa/685-719°C (M1), the peak-metamorphism with P-T condition of 0.9-1.1GPa/760-778°C (M2), and the retrograde metamorphism stage with P-T condition of ∼0.68GPa/∼767°C (M3). In combination with regional geological setting, this study constrains the formation process for the LGA.