华北地区燕山期岩浆活动热供给的初步数值模拟

大陆地壳和地幔岩石圈形成之后,还可能再次有对流地幔的注入,对原有的陆壳和地幔岩石圈进行改造。华北克拉通于新太古—古元古代形成的陆壳和大陆根,在侏罗纪—白垩纪(J—K)的燕山期遭受重大改造,有对流地幔的一次重要注入。一维的数值模拟表明:(1)在1GPa条件下,1250℃底侵玄武岩对于初始温度400℃的英云闪长岩围岩的加热,并达到使其发生局部熔融作用,其纯的熔融量与纯的底侵玄武岩岩浆结晶的厚度的比值约为0·12;假设仅有25%的熔浆能够分离出来,则能够分离出的熔浆量与底侵玄武岩结晶总量比约为0·03。由此可以推测,如果要产生总量为1km厚的花岗岩的岩基需要33·3km的底侵玄武岩岩浆的全部结晶来提供热通量。而33·3km的底侵玄武岩岩浆可能需要约333km的软流圈物质参与(假设10%的局部熔融,并且全部熔体均能分离出来)。(2)华北克拉通燕山期玄武岩的喷发和地壳浅部辉长岩侵入体的发育,说明底侵的玄武岩岩浆不可能全部固结,从模拟的角度说明,燕山运动需要非常大量的底侵玄武岩岩浆和热通量的注入,才能诱发现今所观察到的燕山期强烈的岩浆活动。文中数值模拟工作对进一步理解“燕山运动”发生的深部背景即来源于软流圈的贡献有一定的意义。

The numerical simulation of heat fluxes of Yanshanian igneous activity, North China

A 1-D numerical simulation of the heat flux in the Yanshanian stage in North China yields the following results.(1) Under 1 GPa pressure, if underplating basaltic magma at 1 250 ℃ heats tonalitic country rocks, and raises its temperature from 400 ℃ to 850 ℃ with localized melting, then the ratio of the thickness of melted country rock to the thickness of crystallized rock of the basaltic magma is approximately 0.12.If it is assumed that only 25% melted magma can be separated from the country rock, the ratio of the amount of separated magma to the total crystallized rock of the underplating basalt is about 0.03.Thus we may infer that if a 1 km-thick-layer of granitic melt is created, then a 33.3 km-thick body of underplating basaltic magma should have been totally crystallized to offer the needed heat flux.Assuming a 10% partial melting of asthenosphere to produce a 33.3 km-thick body of underplating basaltic magma, a thickness of about 333 km of asthenospheric material should be involved. (2) In North China there were considerable basaltic eruption and gabbroic intrusion in the Yanshanian stage.This implies that complete crystallization of the underplated basaltic magma is impossible.Our simulation results show that the intense magma activity in the period of Yanshan movement, as we have observed, could have only been generated with the involvement of a huge amount of underplating basaltic magma and a strong heat flux.The numerical simulation of this paper is of significance in understanding the deep tectonic background of the generation of Yanshan movement.