PETROLOGY AND GEOCHEMISTRY OF CENOZOIC VOLCANIC ROCKS FROM THE CENTRE OF QIANGTANG,TIBET

The Cenozoic volcanic rocks in Qinghai—Tibetan plateau mainly occur in Qiangtang, Kunlun,Kokshili and eastern Tibet to the north of Bangong\|Nujiang suture. The study area locates in the centre of Qiangtang (N34°00′\|34°50′;E87°30′\|89°00′),where is nearly a blank area of geological work.Thirteen pieces of samples from Hehulin,Luanqingshan,Bandaohu and Donghu show that the K\|Ar ages of the Cenozoic volcanic rocks range from 32 6 to 44 1Ma, which are elder than the Cenozoic volcanic rocks from Kunlun,Kokshili and eastern Tibet.There are two types of occurrence for the volcanic rocks: volcanic lava and sub\|volcanic rock. The lava is dark grey, light purple or dark purple and unconformably covers up on the Jurassic,which interbeds with purplish red tertiary sandstone and pebble\|bearing sandstone and contains a small amount of phenocrysts (andesine,pyroxene,amphibole and biotite). three craters and four volcanic necks have been found in this area. The sub\|volcanic rocks expose as volcanic necks with appearance of isolated peaks,which intrude into Jurassic. The rocks,appearing grey white to light grey, have trachytic texture with the phenocrysts of plagioclase,biotite and amphibole. Besides above rocks,there are some lamprophyre dikes intruding into grey white tertiary lacustrine algal limestone in this area with its length up to 2km and width 3m. The rocks appear dark grey or dark green and contain the phenocrysts of pyroxene,orthoclase,biotite,olivine,and leucitite.