THE SOUTHWESTERN QINLING SHAN OF CENTRAL CHINA: A LATE PALEOZOIC SUBDUCTION/ACCRETIONARY WEDGE SYSTEM AND CONTINENTAL ARC/FOREARC CONSTRUCTED UPON THE NORTH-FACING DEVONIAN PASSIVE MARGIN OF THE SOUTH CHINA BLOCK

Recently completed fieldwork in the southwestern Qinling Shan, central China, indicates that published tectonic models of the area are incorrect. Based upon our field observations, we propose a new model for the evolution of the area. In the northern part of the field area, the Baishuijiang Group contains blueschists, greenschists, metavolcanic rocks, serpentinite, chert, gabbros, blocks of crinoid-bearing recrystallized marble, and finely laminated black shale/slates. Faults within the Baishuijiang Group are south-southeastwardly dipping and deformation increases to the south. We interpret the Baishuijiang Group to be an accretionary wedge complex that accumulated above a southward dipping subduction zone along the northern passive margin of the South China Block (SCB). South-southeast of the Baishuijiang Group is the Sanhekou Group which contains thick sequences of marble and highly sheared schist, with lesser amounts of quartzite. It also contains small blocks of mafic/ultramafic rocks. The contact between the Baishuijiang and Sanhekou Groups is a south-southeast dipping thrust fault. Further to the west within the Sanhekou Group where metamorphism is weaker, Devonian coral fossils have been reported. We interpret the Sanhekou Group to be part of the Devonian, north-facing, passive margin of the South China Plate which was deformed during Late Paleozoic subduction. Further to the south are the Hengdan Group and the Bikou Group. These units have been variably interpreted as a Precambrian microcontinent, an island arc/forearc/trench system, or a continental rift basin by previous workers. Basal Hengdan clasts include granite, marble, and quartzite, as well as volcanic and chert clasts. However, clasts higher in the section are all of volcanic origin. The Bikou Group consists of metavolcaniclastic rocks, and metamorphosed ash and intermediate metaplutonic rocks. We interpret the Bikou Group to be a continental arc that is built upon the Devonian passive margin of the SCB and the Hengdan Group to be a forearc to the Bikou which initially received detritus from the passive margin and the SCB, but later received material only from the Bikou arc.