吉林省白头山火山岩的岩石学研究

白头山火山位于吉林省东部,主要由一套粗面岩—钠闪碱流岩组成。岩石中长石斑晶的成分表现出逐步富Or的趋势。单斜辉石早期向富Fe,而晚期表现为向富NaFe³⁺Si₂O₆的方向演化。矿物成分变化规律的研究揭示了白头山火山岩与长白山玄武岩之间的成因联系。岩石主要元素化学成分在过碱性酸性岩系统的Q—Ab—Or相图及SiO₂—Al₂O₂—Na₂O+K₂O图上的投影,表明了白头山火山岩的演化主要是受碱性长石的分离结晶所控制,但在从粗面岩到石英粗面岩的演化过程中涉及少量斜长石组分的分离结晶。用最小二乘法对白头山火山岩的成因及其演化过程进行了定量模拟。所得结果与岩相学和岩石化学的分析结果相吻合,证实了白头山火山岩是由长白山玄武岩分离结晶形成的成因假说。根据热力学计算和过碱性岩石的高温高压实验,推测岩浆形成温度分别为1074—928℃,形成压力约在2-3kb。

PETROLOGY OF BAITOUSHAN VOLCANIC ROCKS IN JILIN PROVINCE

Baitoushan volcanic rocks, overlying statigraphically on Changbaishan basalts, include trachyte, quartz trachyte, comendite etc.. Most of the rocks occur as strongly porphyritic ones. Composition of the alkali-feldspar that is the most plentiful member of the phenocrysts show a trend from Ca-, Na-rich to K-rich. A general iron-rich trend followed by enrichment of NaFe3+SizO6 component later occurs in clinopyroxenes. These two trends reveal the close genetic relationship between Baitoushan volcanic rocks and Changbaishan basalts. Projections of chemical analysis of the rocks on the oversaturated peralkaline system Q -Ab-Or diagram and SiO2-Al2O3ONa2O + K2O diagram indicate that the evolution of Baitoushan volcanic rock would mainly controlled by the fractional crystalli zation of alkali-feldspar. In addition, the evolution from trachyte to quartz trachyte might involve the fractionation of a few of plagioclases. The process of the origin and evolution are quantitatively modelled using the least square method. The results of the calculation, which are consistent with the evidences of petrography and petrochemistry, support the dedaction that Baitoushan volcanic rocks are derived from the magmas related to Changbaishan basalts through fractional crystallization. The thermodynamic calculation shows that the tempera tures of the magmas range from 1074 to 928?. The pressure has been estimated to be roughly 2-3kb based on the related experiments and petrographical evidences.