TYPES, CHARACTERISTICS AND GEOLOGIC SIGNIFICANCE OF THE DEVONIAN PYROCLASTIC TEMPESTITES IN EAST AND WEST JUNGGAR, XINJIANG

The Devonian pyroclastic tempestites in East and West Junggar,Xinjiang are a new genetic type of tempestites,with a pyroclastie content ≥ 25% and are formed by both a storm and an explosive volcanic process. The coetaneous eruptive-proximal volcanic tempestite facies (CPT) and coetaneous eruptive-distal volcanic tempestite facies (CDT) are rocognized in the pyroclastic tempestite facies on the basis of the relationship between storm and explosive volcanic process in time and space.
The CPT are the process-facies formed by both a storm (exogenic) and an explosive volcanic (endogenic) process. Generally, the distance between a storm and an explosive volcanic eruption centre is not far (only several to tens of km), and the deposits which constitute CPT consist of both the deposists which are reworked by a storm process and the pyroclastics which are directly provided by an explosive volcanic eruption synchronously. The CPT are mainly found in the Lower Devonian strata in the Baytik Mountain, East Junggar,Xinjiang.
The CDT are characterized by a storm centre far from an explosive volcanic eruption centre (10 to 100 km) and by less pyroclastics provided directly by an explosive volcanic eruption. The CDT are mainly distributed in the Lower Devonian strata in East Junggar and the Upper Devonian strata in the Sarburti Mountain,West Junggar,Xinjiang.
The study indicates that pyroclastic tempestites are the diagnostic event deposit types in the littoral and neritic environments of the volcanic island-arc zones.