摘要
岩浆底侵在大陆地壳的形成和演化过程中起着非常重要的作用。本文基于二维热传导方程模拟不同规模的地壳底侵产生的热-流变学效应,以及幔源岩浆温度和含水量对底侵厚度的影响;并以现有的岩石地球化学分析、深部地球物理探测结果为约束,模拟了峨眉山大火成岩省内带幔源岩浆底侵对应的地表热流随时间演化,探讨了形成幔源岩浆底侵的潜温和初始熔融的深度制约。结果显示:1)幔源岩浆底侵引起的热扰动的耗散时间取决于岩浆底侵的初始厚度。以幔源岩浆侵入温度为1300℃,20km厚的地壳底侵为例,热扰动完全耗散需经历约150Myr;而5km厚的地壳底侵,只需经历50Myr热扰动已基本耗散殆尽。2)在初始阶段,岩浆底侵会造成岩石圈强度的显著降低;随着热耗散的进行,岩石圈强度会逐渐恢复;在热扰动耗散殆尽之后,岩石圈强度反倒比底侵前的岩石圈强度更大。这表明岩浆底侵不但可以导致地壳增厚,还会最终导致岩石圈的强化。3)温度对地壳底侵厚度的影响比含水量的影响要大得多。将我们的模型应用于峨眉山大火成岩省,结果表明内带地壳底侵的热耗散需持续上百个百万年,岩浆潜温超过1500℃,初始熔融深度超过200km。
Magmatic underplating plays a critical role in the growth and evolution of the continental crust. Here we employ 2-D numerical simulation to investigate the thermal-rheological effects of magmatic underplating and the influences of magmatic temperature and water content on crustal thickening due to magmatic underplating. Our models are constrained with the available petrologicalgeochemical and geophysical data from the Emeishan Large Igneous Province( ELIP). We compare the model predictions with the temporal evolution of the surface heat flow in the ELIP,and estimate the potential temperature and initial melting depth of the magmatic underplating beneath the inner zone of the ELIP. The results show that: 1) The dissipation of the thermal perturbation caused by magmatic underplating depends on the initial thickness of magmatic intrusion. For example,it requires ~ 150 Myr to dissipate the heat inputted by a large magmatic intrusion( 1300℃ and 20-km thick),while this time scale reduces to ~ 50 Myr for a small-size magmatic intrusion( 1300℃ and 5-km thick). 2) The magmatic underplating causes the lithospheric bulk strength to decrease significantly at the early stage. Then it restores gradually as the thermal decay proceeds. When the lithosphere returns to thermal equilibrium state again,its strength becomes even stronger than its pre-intrusion value. This indicates that magmatic underplating not only can contribute to crustal thickening,but also can eventually strengthen the lithosphere. 3) Our models also demonstrate that the initial magmatic temperature plays a more influential role than the magmatic water content in crustal thickening. The application of our models to the ELIP suggests that the crustal underplating,as revealed by recent geophysical studies,is likely caused by a large magmatic intrusion originated from a magmatic chamber with potential temperature over 1500℃ seated at a depth deeper than 200 km,and it's heat would take more than one hundred Million years to dissipate.