Petrology and ~(40)Ar/~(39)Ar Geochronology of the Lawsonite-Bearing Blueschist and Eclogite from the Qingshuigou Blueschist Belt in North Qilian Mountains in NW China and Their Tectonic Implication

This study reports the discovery of lawsonite, which was for the first time identified in the Qingshuigou high-grade blueschist belt in the North Qilian Mountains and was also discovered in accompanying eclogite. It occurs as enclaves in quartz grains. There are no euhedra crystals observed, with size ranging from 9.5 to 4 μm. Using internally consistent mineral thermodynamic detabase of Holland and Powell (1998), and Domino/Theriak calculation software by de Capitani and Brown (1987), we calculated metamorphic p-T conditions of the HP/LT metamorphic belt: 2.24~2.39 GPa and 495~519℃ for the lawsonite-bearing eclogite in Qingshuigou; 1.35~2.15 GPa and 530~600℃ for the epidote blueschistin Baijingsi; 2.20~2.35 GPa and 520~545℃ for the epidote blueschist in Qingshuigou; 1.92~2.08 GPa and T=544~576℃ for the lawsonite-bearing blueschist in Arou coal mine. Results show that the Qingshuigou high-degree blueschist experienced metamorphic transformation from lawsonite blueschist to epodite blueschist, and has the similar p-T condition to that of lawsonite-bearing eclogite, indicating that they were the product of high temperature and low pressure metamorphism triggered by the northward subduction of the ancient Qilian Mountain Ocean. The high quality laser 40Ar/39Ar dating of the epidote blueschist yields the metamorphic ages of 447±1.7~447±5 Ma and 453±2~454±2 Ma. The age of 392±12~400±3 Ma can be interpreted as retrograde metamorphism age for low temperature eclogite. Combined with the lithological, geochemical and isotopic data above, Early Paleozoic tectonic evolution process of North Qilian orogeny was further discussed.