ON THE PATTERNS OF METAMORPH1SM AND GEOLOGICAL EVOLUTION IN THE QI-LIAN MOBILE BELT
Lu Liangzhao, Ye Huiwen and Chi Hongxing
The Early Palaeozoic Qi-Lian mobile belt is a rift and sag belt,overlying the Early Proierozoic crystalline basement. During the Early Proterozoic era, Qi-Lian and the neighbouring massif were within a single mobile belt, which were filled by thick terrigeneous clastic sediments with some carbonates and volcanics. The first cratonization of these regions took place by the end of the Lower Proterozoic age, accompanied by extensive regional dynamo thermal metamorphism of intermediate pressure(prograd metamorphism from green schist facies to amphibolite facies).In Mid to Late Proterozoic period(1, 700-600 m.y .), some NNW trending subsidence belt, such as the Central Qi-Lian, developed along a weak zone, which were filled by a thicker shallow-water shelf seqence(mainly mature clastic sediments and dolomites). Volcanics is rare.These features implicate that the upper crust of this region began to mobilize again at this stage, but it is not a independent geosyncline period.Towards the end of the Late Proterozoic, the Central Qi-Lian uprised through a vertical movement.On the contrary,the North and the South Qi-Lian began to rift and sag since the Middle Cambrian and Ordovacian respectively. The south side of the former was the central part of the rift-a deep sea trough, characterized by dominate pillow lava, radiolarian chert, Ultramafic intrusion belt and local glaucophane-green schist facies belt of intermediate-high pressure facies series.The north subbelt of North Qi-Lian and the South Qi-Lian correspond to miogeosyncline with thick flysch formation.At the end of Silurian, the whole seqence Pt2 to Silurian underwent a intensive deformation and regional dynamic metamorphism or monotoneous low green schist facies through Caledonian orogeny.But the prograd metamorphism may appear locally owing to heat flux rising from the depth, and in addition,the anatexis of Lower Proterozoic basement produced granitic magma, which was emplaced to the high-level and gave rise to the granitoid intrusions about 390-360 m.y.ago.The Qi-Lian mobile belt finished its history, and became a stable terrin since the Devonian.