藏北新生代火山岩主要造岩矿物特征及温压条件

MAJOR CHARACTERISTICS AND TEMPERATURE-PRESSURE CONDITIONS OF THE MINERALS IN CENOZOIC VOLCANIC ROCKS IN THE NORTHERN TIBETAN PLATEAU

藏北新生代火山岩在空间上呈带状分布 ,时代上从南向北由老逐渐变新 ,表现为其带状由南向北迁移。岩石系列分属于碱性玄武岩系列和钾玄岩系列 ,主要造岩矿物为橄榄石类、辉石类和长石类等 ,它们都有类似的演化特点 ,同一岩带中岩浆总体由东向西演化。运用相应的方法 ,计算出藏北新生代火山岩中不同矿物的结晶温度为 90 0～ 12 0 0℃ ,形成压力为 1 1～ 3 4GPa ,从而确定了岩石的近似来源深度 ,这对青藏高原隆升及深部动力学机制的研究有着重要意义。结合对脉动性的岩浆活动事件的主体时代的分析 ,认为 6 0～ 30Ma期间岩石圈持续增厚 ,表现为岩浆起源深度不断增大 ;30Ma以来 ,岩石圈厚度近于恒定 ,表明随地壳的持续增厚 ,岩石圈地幔发生了减薄作用。

The Cenozoic volcanic rocks extend on strap in the northern Tibetan Plateau,and their ages become younger from south zone to north zone. The volcanic rocks belong to the alkaline basalt series and kali basalt series,and essential minerals hosted by them are olivine, pyroxene and feldspar and so on,and are of the similar evolutionary characteristic. Because the crystal temperatures(900-1 200℃) and the different pressures (1 1-3 4 GPa) of the diverse minerals or rocks were figured out by different means, the approximate original depth of the rocks is estimated, which is of great significance to search after the Tibetan Plateau's swelling and deep dynamics mechanism. The lithosphere became thicker and thicker between 60Ma and 30 Ma, but it's thickness is almost invariable since 30 Ma, which shows the mantle thinning occurred with the continue thickening of the crust.