THE LITHOLOGIC FEATURE AND GENESIS OF THE PORPHYROCLASTIC LAVA IN BAIQI AREA NEIMONGGOL
内蒙古正镶白旗碎斑熔岩岩石学特征及其岩相划分

The main rock type in the Mesozoic volcanic basin in Baiqi area, Neimonggol is a special volcanic rock, the porphyroclastic lava. It is characterized by porphyroclastic texture for phenoerysts, poikilitic texture of pearl margin and vitric-felsitic-microgranular texture for matrix. Its lithologic feature is very similar to the porphyroclastic lava in south-east coastal part of ahina. All field observations indicate that the porphyroclastic lava body in Baiqi area is dome-like and formed on the ground. From outer part to the inner part or from the upper part to deep one, the variation in petrology should be. vitric porphyroclastic lava (margin part), felsitic porphyroclastic lava (transition part) granulitic porphyroclastic lava (central part), and granite porphyry (root part). They are of gradual modification. From vitric porphyroclastic lava to granulitic porphyroclastic lava, the breaking degree of the phenocrysts decreases, The poikilitic texture of pearl margin is well developed in the granulitic porphyroclastic lava. The granite porphyry (root part of the body) is similar to the normal subvolcanic rock. The potash feldspars in the Baiqi Complex body are of lower ordering degree. They are chiefly low sanidine and high orthoclase. It indicates that the rock formed at high temperature. In the same way, the ordering degree of potash feldspar and plagioclass increases from the margin to root of the body. The porphyroclastic lava in Baiqi area belongs to the Pacific calc-alkaline rock series with chemical character of volcanic rocks in orogenic belt.Its original magma is chiefly from the mixed melting of upper mantle and deep crust. The porphyroclastic lava body in Baiqi area is of typical facies model. It consists of flow lava facies (margin)-volcano extrusion facies (central)-subvolcanic facies (root). This is a complex body of "co-existing multiple facies".