Early Paleozoic Accretionary Orogenesis on Western Margin of Baoshan Block

On the western margin of the Baoshan block, a set of tectonic mélange can be seen along the western Lushui-Luxi tectonic zone. It is characterized by matrices consisting of strongly deformed and low-grade metamorphic clastic rocks bearing graptolite of Sinian-Paleozoic Pumanshao Group and Gongyanghe Group and volcanic flysch turbidite, and structural lens consisting of siliceous, graywacke, part limestone, conglomerate, basalt and granite. The ages of these rocks range from Sinian to Paleozoic and get younger from east to west, which shows the characters of retrogressive accretion. In the early stage, the structural styles are characterized by imbricate structures formed by thrust faults and homoclinal-overturned folds, and nearly N-S-trending shear in the late stage. The characteristics of basalt-andesite and rhylite are of arc volcanic rocks, and the basalt shows characteristics of continental intraplate volcanic rocks. Both of the two types of volcanic rocks maybe account for the tectonic setting of island arc and back-arc extension. Cambrian and Ordovician granite can be divided into the western belt and eastern belt, and the ages of the latter are earlier than that of the former. Those granites are part of magmatic arc of the western margin of the Baoshan block. Therefore, this tectonic mélange records the geological events such as extinction to the subduction of the oceanic crust and magmatic sedimentation-volcanic-magmatic-tectonic events during the formation process of accretionary wedge, and also records the information of the Early Paleozoic accretionary orogenesis on the western margin of the Baoshan block. Meanwhile, it suggests the existence of an ocean basin during Sinian-Paleozoic in the Lushui-Luxi tectonic zone.