THE BASIC CHARACTERISTICS OF CAMBRIAN MARINE VOLCANIC ROCKS IN THE NORTHERN QILIAN MOUNTAINS

The Cambrian marine volcanic rocks in the Northern Qilian Mountains may fall into the south and north belts according to their spatial distribution. With strong volcanic activity the south belt extends discontinuously for over one thousand km. It may be further divided into the east and west sectors. The west sector is mainly characterized by fissure eruption, poor differentation of intermediate-basic volcanic rock facies and occurrence of dominantly iron and manganese deposits. The east sector is characterized by central eruption, good differentiation of intermedate-acid volcanic rock facies and occurrence of dominanily polymetallic mineral deposits. The volcanic rocks are characterized by complex lithology, intense differentiation, relatively profound metamorphism, and high developmeni of schistosity, and they form several volcanic cycles. The rocks, grayish green in colour, are dominantly of the calc-alkaline series and subordinately of the alkali-calcic series, and those of the alkaline series are rare. he spilite keratophyre sequence is well developed in the east. The rocks are rich in sodium, which is related to the enrichment of sodium in the magma itself, but the existence of sodium metasomatism is not ruled out. Most of the volcanic rocks in the area belong to the rocks formed by crustal anatexis in the orogenic belt, but a few are of mantle-derived nature. Volcanic activity began in the Middle Cambrian and was frequent and intense, but it became weak in the terminal Cambrian. Through an analysis of the volcanic rock facies and types of volcanism, it may be inferred from the Middle Cambrian volcanic rocks that there are 11 paleovolcanic craters and 1 eruption centre. Useful minerals related to volcanic rocks are mainly concentrated in the intermediate-acid volcanic rocks in the east that are well differentiated complex in lithology and resulted form central eruption, which form the mafor Baiyin polymetallic mineral deposit.