RECOGNITION OF A LATE PALEOZOIC ARC/FOREARC SYSTEM DEVELOPED ON THE NORTH MARGIN OF THE SOUTH CHINA PLATE, SOUTHWESTERN QINLING MOUNTAINS, CHINA

Located within the southwestern Qinling Mountains of central China, the Hengdan Group and Bikou Group together form a metamorphosed terrane of volcaniclastic sediment and associated volcanic rock. Recent theories have interpreted this terrane as a micro-continent of Late Proterozoic age, which developed in a continental rift or forearc setting. However, the Hengdan/Bikou terrane is bordered on the north/northwest by what we have interpreted as a south dipping subduction complex of Late Paleozoic age, which developed on the north margin of the South China Plate. A detailed section was measured through the Hengdan Group near the town of Luotang in Gansu Province, where metamorphism is generally of low phyllite grade. A second section was also measured to the southwest near the town of Wenxie, where metamorphism and deformation are somewhat greater. A steeply southward dipping volcaniclastic turbidite succession was encountered, which generally coarsens upward. The lower portion of the Hengdan Group is typically comprised of pelagic sediment, distal turbidites, and some successions of conglomeratic debris flows containing sub-rounded clasts of both volcanic and continental provenance. The middle portions of the Hengdan Group are comprised mainly of channel sands and thick sequences of overbank deposits. Paleoflow orientations taken from crossbedded sands indicate transport to the north. The upper portions of the Hengdan Group are composed of thick tabular sands, coarse channel sands and conglomerate with volcanic provenance. Thin to moderately thick tuffaceous beds become increasingly common. To the southeast, the metamorphism increases to lower schist grade. The Bikou Group lies immediately to the southeast, and has been interpreted as the arc component of the system. The Bikou Group is composed of schist grade volcaniclastic siltstone, sandstone and conglomerate, interbedded with thick sequences of tuff and altered volcanic rock. Metamorphosed diorite, serpentinite and gabbro crop out within the central Bikou Group.