大兴安岭北段中生代火山岩形成的动力学环境

THE DYNAMIC BACKGROUND OF MESOZOIC VOLCANIC ACTIVITY IN NORTHERN PART OF DAXING'ANLING MOUNTAINS

对大兴安岭北段中生代火山岩新的岩石化学、微量元素、构造等方面的分析表明，该地区中生代火山岩形成于明显的挤压环境．再结合区域构造、区域岩浆作用、稳定同位素、古地磁特征等的分析，得出以下认识：该区中生代火山岩来源较深，具幔源特征；岩浆总体形成在挤压的环境中，是“减压”－“剪切”作用的产物，而不是裂谷环境中的产物。

The Mesozoic volcanic activity in northern Daxing'anling Mts (NDM) is very violent,in which a lot of geologists are interested The volcanic rocks,formed mainly in Late Jurassic to Early Cretaceous(150～100 Ma),are divided into the Tamulangou formation(J 3 t ),Jixiangfeng formation(J 3 j ),Shangkuli formation(K 1 s ) and Yilikede formation(K 1 y ) The lg τ lg σ diagram of Rittmann reflects the volcanic rocks of NDM to be formed in compressional environment In K 2O Na 2O diagram,the K 2O content of volcanic rocks in NDM is 1 2%～6 5%,Na 2O 3 0%～4 7%;showing the content of K 2O in a relatively wide range,while the Na 2O relatively stable The characteristics of K 2O and Na 2O are just like those in active continental margins,confirming the volcanic rocks' compressional forming environment The intermediate basic volcanic rocks(SiO 2≈51%～56%) of NDM are distributed within orogenic region in FeO  MgO Al 2O 3 diagram,which also suggests the forming environment should be compressional background The trace element characteristics of volcanic rocks of NDM reflect that a lot of crust source materials have added in the volcanic rocks,and indicate that the volcanic rocks were formed in a compressional background Stable isotope data show a deep source characteristics of the volcanic rocks The stress analysis suggests that the stress field background of volcanic active period in Ergun is different from that in continental rift and median ridge Palaeomagnetic features reflect that with the ending of the collision between Siberian plate and North China plate in Late Paleozoic Era,the together ward adjustment of the two plates was not finished soon,and the process of adjustment was just convergence,other than extenison The geochemical characteristics of intrusive rocks show that eastern China was strongly comproessed from Middle Jurassic to the early middle stage of Late Jurassic(180～150 Ma) Tectonic analysis reflects the strong compression of NWW(NW) orientation to be gradually reduced in eastern China from the period of 180～150 Ma to 150～100 Ma During 180～150 Ma,a lot of NNE(NE) striking overthrusts or nappes with mineral lineations striking NWW(NW) were formed in eastern China,showing that eastern China was in strongly compressional background,and the primary stress was nearly NWW(NW) oriented. The overthrusting action was ended in the age of about 150 Ma Therefore,the Mesozoic volcanic rocks of NDM were formed in the compressional background with a “reduced pressure”and “shearing”process That is why there are so many crust source materials added in the Mesozoic volcanic rocks