TEPHRASTRATIGRAPHY AND ANALYSIS OF TECTONIC SETTING OF TRIASSIC INTERMEDIATE VOLCANIC STRATA: NANPANJIANG BASIN, SOUTH CHINA

The Nanpanjiang is a deep-marine basin in the southern margin of the Yangtze microcontinent of South China. The Basin contains several shallow-marine carbonate platforms developed in the Triassic. Differential carbonate platform development and subsidence analysis suggests that the Nanpanjiang basin developed into a foreland basin resulting from an arc collision in the southern part of the basin in the Triassic. The tectonics of this area, however, received little previous study. Ash Tuff horizons extend across the basin for ~500 km in both the Permian-Triassic boundary (PTB) and the Lower to Middle Triassic boundary (LMT). Thin-section analysis of the ash tuff horizons revealed the units are pyroclastic composed primarily of carbonate altered rhyolites and rhyodacite vitric ash tuffs (Schmid, 1981). PMT horizons are more abundant and the LMT volcanic horizon is thickest in the southern part of the basin. Petrographic analyses of samples from the LMT volcanic horizon indicate that coeval, geochemically similar lavas that are 150m thick in the southern part of the basin, and may represent volcanic products in the erutpive center, becoming coarse grained pyroclastics northward, into bentonite clay in the northern part of the basin where it is less than 20 cm thick. Immobile trace elements (Zr/TiO2)/(Nb/Y) indicate that the PTB and LMT volcanics are rhyolites and rhyodacites in composition (Winchester 1977). Major element discrimination of tectonic setting indicates a convergent margin in an intra-oceanic or continental volcanic arc; however, the relative mobility of the major elements effects the overall usefulness of this analysis. Immobile trace and REE elements (Th, Ta, Y, Rb, Yb ) also support the interpretation of a convergent arc setting. Trace element fingerprinting of the volcanic horizons indicate basin wide distribution of volcanic units that erupted from a similar magmatic source throughout the Late Permian and Early Triassic. Petrographic and geochemical studies of the volcanic horizons associated with the end-Permian mass extinction and the Early Triassic indicate eruption from southerly volcanic arc that collided with the southern margin of the Yangtze microcontinent in the Triassic.