PETROGRAPHY OF VOLCANICLASTIC SEDIMENTS FROM THE QIJIAGOU FORMATION (LATE CARBONIFEROUS), XINJIANG- UYGAR AUTONOMOUS REGION, CHINA

Lane et al (1996) described a camerate-rich Late Carboniferous crinoid fauna preserved in a volcaniclastic conglomerate from the Tian Shan in Xinjiang- Uygar Autonomous Region, China. The fauna is important because it is one of only four reasonably diverse Paleozoic echinoderm faunas known from China and because the fauna was found in a tectonically active terrane.

The Late Carboniferous sedimentary sequence in the Tian Shan of Xinjiang records the tectonic amalgamation of northwestern China. The sequence exposed in Taoshigo Valley reflects kilometers of volcanogenic sediments shed off active volcanic island arcs in the southern Junggar Basin as the oceanic basin separating the Tarim, Turpan, and Junggar blocks neared closure.

The crinoid fauna is preserved in graded conglomeratic sandstones that overly a carbonate mound. The conglomerates contain clasts up to 3 meters in dimension, although most clasts range in size from 0.5 to 1.5 cm. Major pebble lithologies are as follows: (1) aphanitic basalt, (2) amygdaloidal basalt: scoriaceous, amygdules infilled with carbonate, chlorite, epidote, original phenocrysts of plagioclase and magnetite, (3) porphyritic basalt with oriented plagioclase needles, (4) andesites, (5) rhyolite: porphyritic with devitrified groundmass and (6) rhyolite: devitrified volcanic tuff with a groundmass of devitrified glass, some have xenoliths of rhyolitic composition, flow banding present in one sample. The clasts are angular to sub-rounded. Basalt pebbles tend to be smaller and more angular than the rhyolite pebbles perhaps suggesting separate sources for the pebbles. Many samples have a groundmass of chlorite and sericite that we interpret as altered volcanic ash. The entire sequence has been subjected to greenschist facies metamorphism.

The volcaniclasic sediments form a series of graded sequences that are interpreted as subaqueous debris flows in an active arc setting. The echinoderms were living in ephemeral carbonate environments in an active volcanic island arc setting likely similar to the modern day Japan or the Philippines.