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