IS THE BIKOU TERRANE OF THE SOUTHWEST QINLING MOUNTAINS, CENTRAL CHINA, THE RESULT OF LATE PROTEROZOIC SUBDUCTION ALONG THE NORTH MARGIN OF THE YANGTZE PLATE?

The age and tectonic setting of the Bikou terrane of the southwestern Qinling Mountains, Central China, has long been controversial. The Bikou terrane is comprised of a metamorphic assemblage of volcanic rock and marine sediments, known as the Bikou Group, and a metamorphosed volcaniclastic sedimentary basin known as the Hengdan Group, located adjacent to the north margin of the Yangtze Plate. Previous work has variously attributed the age of the terrane as Early Proterozoic to Late Paleozoic, and the tectonic setting as continental arc, island arc, or continental rift. New SHRIMP U-Pb zircon dating of various volcanic rocks of the Bikou Group performed at the Chinese Academy of Geological Sciences in Beijing has revealed that arc magmatism was active within the Bikou arc from at least ca. 840 to 770 Ma. Similarly, U-Pb microprobe dating of zircons collected from epiclastic tuff layers in the Hengdan Group was performed at the Stanford/USGS SHRIMP-RG laboratory in Palo Alto, California. The results yielded a detrital age range of ca. 830 to 700 Ma. The Hengdan Group is comprised of an approximate 10 to 15 km thick succession of volcaniclastic sand-rich turbidites that form a prograding and coarsening-upward sequence with upward increasing input of epiclastic tuffs. New paleocurrent data collected from the Hengdan Group confirms northward (i.e., derived from the direction of the Bikou arc) sediment transport. The long-lived volcanism, interbedded marine sediment within the Bikou arc, and previously published geochemistry indicating enrichment of high field-strength elements suggests a continental subduction arc setting. Similarly, the upward-coarsening sequence of volcaniclastic turbidites within the Hengdan Group is best described as a forearc basin. The tectonic setting of many similar-aged volcano-sedimentary complexes along the north and west margins of the Yangtze Plate remains controversial, with some interpreting their existence as evidence of a mantle plume preceding Rodinia break-up at ca. 750 Ma. However, a number of recent publications indicate that these same volcano-sedimentary complexes comprised a continental subduction arc from ca. 850 to 750 Ma. This study lends support to the latter hypothesis, namely that the north and west margins of the Yangtze Plate comprised an active continental arc during the Late Proterozoic.