地学前缘 ›› 2010, Vol. 17 ›› Issue (1): 11-23.

• 论文 • 上一篇    下一篇

长白山火山岩浆柱岩浆上升作用过程

  

  1. 中国地震局 地质研究所, 北京 100029
  • 出版日期:2010-01-28 发布日期:2010-01-28
  • 作者简介:魏海泉(1958—),男,硕士,研究员,岩石学专业,主要从事火山学研究。Email: weihq2002@sohu.com
  • 基金资助:

    中国地震局地震行业科研专项(200782701);科学技术部科技攻关项目“火山预警项目”(2002DIA200920)

Magma upmoving process within the magma prism beneath the Changbaishan volcanoes.

 WEI  Hai-Quan   

  1. Institute of Geology, China Earthquake Administration, Beijing 100029, China
  • Online:2010-01-28 Published:2010-01-28

摘要:

长白山火山岩浆柱是一个在长白山区地下总体呈串珠状排列的向东南倾斜的层状富岩浆集合体,岩浆柱宽度宽者300~500 km,窄者30~50 km,深度延伸可达上千km。在这个岩浆柱内,热物质聚集与挥发份富集可以发生部分熔融而形成不同成分与密度的岩浆,岩浆聚集上升至某个深度时的停滞聚集又可形成水平向扩展的岩浆房,压力作用下岩浆房内岩浆演化出密度较轻的岩浆则可进一步上升直至喷出地表。天池火山的母岩浆粗面玄武岩来自地幔岩浆库,由其演化形成的碱型系列粗面岩类和碱流岩类岩石则来自地壳岩浆房。拉斑玄武岩系列的偏酸性岩石来源的地壳岩浆房与碱型系列碱流岩来源的地壳岩浆房深度位置也不相同。天池火山造盾玄武岩TiO2含量和SiO2含量之间反相关关系不能单纯用岩浆房分异结晶来解释,TiO2含量较高的样品代表了源区地幔的较低熔融程度的熔体,而低程度熔融的岩浆来源于更深的位置。玄武质岩浆“熔融结束”的深度随时间的增加而增加的过程控制了岩浆形成深度随时间的增加而增加并且岩浆形成速率随时间的增加而降低的规律。天池火山碱流质岩浆房千年大喷发时岩浆超压极大值Δpmax=625 MPa,层状岩浆房半径35 km,喷出岩浆层厚700 m,喷出岩浆体积30 km3;粗面质喷发的岩浆房超压极大值Δpmax=15 MPa以上。天池火山千年大喷发时临界喷发熔体黏度μcritm>27×1010  Pa·s-1,碱流质岩浆是从一个粗面质岩浆母体经几万年的结晶分异时间演化得来的。气象站寄生火山活动喷发前临界熔体黏度μcritm=12×1011 Pa·s-1,这极高的熔体黏度与喷发物中含有大量晶体与气泡相吻合。千年大喷发级别的大规模喷发周期上万年,远大于小规模喷发几百年以内的时间周期。天池火山作用造盾阶段因为玄武岩都直接喷出了地表,多数传导与扩散的岩浆热都没有用于加热深地壳,所以早期加热效率不高。在1~16 Ma之后造锥阶段在深地壳内形成残余的部分熔融带并阻止了玄武岩的喷发,系统的热效率变得很高,残余熔体生产率也就得到了加速。全新世造伊格尼姆岩喷发阶段大量的演化的碱流质残余熔体因重力不稳定而侵入上地壳内,并且形成大得足以引起造破火山口喷发的岩浆房。

关键词: 岩浆柱, 熔融结束深度, 岩浆房尺寸与形状, 岩墙注入, 岩浆超压

Abstract:

The magma prism beneath the Changbaishan volcanoes is a layered magmarich collection, which dips to the SE and stretches deep into hundreds of kilometers, with the width varying from 3050 km to 300500 m. The accumulation of thermal and volatile material within the prism brought about the partial melting which caused the production of magmas with different composition and density. As the magma rose up to a certain depth, the stagnation of magma made it horizontally dispersed and formed chambers. The less dense magma evolved in the chamber reached the surface and made eruptions as the chamber pressure increased. Trachybasalts, the primitive magma of the Tianchi lava shield, were derived from the mantle reservoir and the trachytes and comendites of alkalic series evolved from trachybasalts were derived from the crustal chamber. There existed a difference between the depth of the crustal chamber that evolved tholeiite series of acidic rocks and the depth of the crustal chamber that evolved comendites of alkalic series. The negative relationship between TiO2 and SiO2 contents of the shieldforming basalts of the Tianchi volcano could not be explained solely by magma chamber processes, such as fractional crystallization. The higher TiO2 content samples might represent the melt with lower degree of melting from the source mantle and the magma with lower degree of melting was produced from a deeper depth. The depth of end of melting of basaltic magma will increase with time, and the mechanism of which also controlled the phenomena that the magma generating depth increases with time and the magma generating rate decreases with time. Before the Millennium Eruption, a maximum overpressure in the comendite chamber reached to Δpmax=625 MPa. From the layered chamber with radium of 35 km, the magma erupted and the erupted magma occupied a space of 30 km3 with the thickness of 700 m. The trachyte chamber once got an overpressure as high as Δpmax=15 MPa. The viscosity of critical erupting melt before the Millennium Eruption was over 27×1010 Pa·s-1. The viscosity of critical melt before the Qixiangzhan parasitic eruption was about 12×1011 Pa·s-1, which is consistent with high contents of crystals and bubbles in the magma. An average interval between the giant eruptions such as the Millennium Eruption was about thousands of years, which was much longer than the intervals of less explosive parasitic eruptions of hundreds of years. The heating of deep crust was inefficient in early time because much of the basalts erupted out directly and much of the heat from magma was not used to heat the wall rocks. Once a zone of residual partial melt developed in the deep crust and started to prevent the basalt from eruption, the system became much more heating efficiently and the production of residual melt was accelerated. Large amounts of evolved residual comenditic melt could be produced in the Holocene, and then became unstable and intruded into the upper crust to form magma chambers large enough to result in the calderaforming explosive eruptions.

Key words: magma prism, endingdepth of melting, scale and shape of chamber, dyke injection, overpressure

中图分类号: