Characteristic Of The Eclogitic Metasedimentary Rocks In North Qilian Mountains And Its Geodynamic Implications

In the HP/LT belt of North Qilian Mountains, eclogitic metamorphism is always recorded in the basaltic rocks and eclogites were previously regarded as the product of the sea floor subduction. However, present study reveals that elcogitic minereal assemblages are not only recorded in the basaltic rocks, but also in the metasedimentary rocks, which are the host rock of eclogites.In this paper, we present new Petrological, geochemical and age data for metasedimentary rocks from Baijingsi area, North Qilian Mountains, in which typical early-Paleozoic ophiolitic sequences and HP/LT metamorphic rocks have been previously recognized. The studied metasedimentary rocks contain eclogitic facies assemblages reflecting PT conditions of 450-520℃and 1.9-2.3 GPa, consistent with those of adjacent eclogites.Geochemical data indicate that the precursor rocks of the metasedimentary rocks were immature sediments formed in continental margin or island arc environments. Zircons from 4 eclogitic metasedimentary samples were separated for U-Pb and Lu-Hf dating. The data demonstrate detrital zircon ages concentrate in 480-560 Ma and around ca.1800 Ma, suggesting that they derived from the mixture of Paleoproterozoic continent basement and Late Neoproterozoic-early Paleozoic oceanic materials, which were probably accreted into active continental margin during subduction. These data implies that subduction erosion of the active continental margin occurred, as these sediments, formed in the upper plate (fore-arc basin or accretionary prism), were transported into the subduction zone to 60-70 km depth prior to their exhumation.