MAGMATISM OF GALC-ALKALINE SERIES IN THE BEISHAN REGION OF GANSU PROVINCE AND ITS RELATION TO PLATE TECTONICS
甘肃北山区的钙碱系列岩浆活动及其与板块构造的关系

According to the recent investigations, the plutonic granitoid batholites of the Late Paleozoic occur widely in the Beishan region, while the Early Paleozoic and the Mesozoic granite intrusions are developed a little. From the Ordovician to Permian periods, persistent magmatic activities resulted in volcanic rocks on different scales. The mixed volcanic formations of effusive and explosive eruptions predominate the northern Beishan region, while the southern part is characterized by explosive formation. The magmatites of various types are generally distributed along the E-W trending folds and faults. Most of the granitoid batholites are complex intrusive massives as a result of superposed magmatism. The magmatites of this region are similar to those of Hercynian orogenic belt in southwest Europe.The Paleozoic epigenetic metamorphic zones of various facies series with linear feature are widespread in the region. The low-medium pressure metamorphic rocks of greenschist facies and epidote-amphibolite facies are outcropped mainly in the northern part. The prehnite-pumpellyite facies of low temperature and katametamorphism are distributed in some areas of the central Beishan. The medium pressure greenschist facies and dynamo-meta-morphic mylonite and cataclasite are widely exposed in the southern part. In the light of isotopic age determination, the high tide of metamorphism took place in the Carboniferous period. The endogenic mineralizations of Fe, Cu, W, Au, Pb, Zn, Nb and Ta in the region are obviously related to calcalkaline magmatic activities.According to the studies of tectonics, the central Beishan occurred as a median massif in the Paleozoic, which is called the Mazongshan Rise. Both the Mazongshan Rise and the Central Tianshan-Mujunkum massif bear a lot of similarities in the time of cratonization and the nature of covers. The southward subduction activities happened two times in the Middle and Late Paleozoic along the northern Beishan. To the south of the Rise, there is a subduction zone and a collision zone underthrusting to the north at the same time.To summarize, the whole Beishan region is part of the Kazakhstanian plate. Its magmatism, metamorphism and endogenic mineralizations are immediately related to subduction activities of plates.