深熔过程中熔体成分与锆石行为模拟计算

发生深熔作用是高级变质作用的一个重要特征。深熔过程中产生的熔体可为淡色花岗岩提供潜在的源区;深熔过程中锆石的行为直接影响对变质锆石记年地质意义的理解。在含Zr体系下的相平衡模拟显示泥质成分深熔过程中产生熔体的成分在P-T空间中规律变化。温度升高时熔体Zr/Si值、Zr、FeO、MgO以及CaO等含量明显增加,压力较高时K2O含量也随温度升高而明显增加。Na2O含量随温度升高而降低,但随压力升高而增加。压力升高时Al/Si值显著升高。温度较高时Na/(Na+K)等值线较陡,减压熔融过程不会显著改变熔体Na/(Na+K)值,而升温减压过程以及近等压升温过程都会明显降低熔体Na/(Na+K)值。中压时随温度升高熔体Fe/(Fe+Mg)值缓慢升高,而石榴石的生长发育会迅速降低熔体Fe/(Fe+Mg)值。不同温压条件下对应的固相线熔融、白云母脱水熔融以及黑云母脱水熔融形成的熔体成分具有明显差异。对比模拟熔体成分在P-T空间的演化,喜马拉雅地区电气石淡色花岗岩对应熔体的形成温压条件应低于二云母淡色花岗岩,同类型淡色花岗岩之间在形成条件上也可能存在一定差异,并经历了差异性演化过程。含Zr体系下的相平衡关系显示进变过程是消耗锆石的过程,因而在进变过程中变质锆石难以生长,发生深熔作用的岩石中的变质锆石主要在退变过程中形成并记录退变质年龄。熔体丢失相关模拟显示不同温度阶段发生熔体丢失对锆石稳定性的影响不同。温度较低时Zr含量较少的熔体丢失会扩大持续进变过程中锆石的稳定范围,而温度较高时富Zr熔体的丢失会降低持续进变过程中锆石的稳定温度。类似于分离熔融作用的过程最利于残留相中剩余锆石在持续进变过程中的保存。

Modelling of melt composition and zircon behaviour during anatexis

Anatexis is a common process accompanying high grade metamorphism. The melt produced by anatexis can be potential source for leucogranites; and the behavior of zircon during anatexis is crucial for understanding the geological meaning of metamorphic zircon. Phase equilibrium modelling for pelitic composition in the Zr-bearing system indicates that the modelled melt composition varies regularly in the P-T space. The Zr/Si ratio, Zr, FeO, MgO and CaO contents of the melt increase and the Na₂O content decreases with increases in temperature. At high temperature the K₂O content rises as the temperature increases. The Al/Si ratio and the Na₂O content increase with increasing of the pressure. Decompression melting occurs at high temperature where the contours of the Na/(Na+K) ratio are steep will not lead to significant changes in the Na/(Na+K) ratio of the melt. But the processes involving decompression-heating and isobaric heating will remarkably decrease the Na/(Na+K) ratio of the melt. At medium pressure, the Fe/(Fe+Mg) ratio of the melt rises slowly with increases in temperature before the growth of garnet, which then effectively decreases the Fe/(Fe+Mg) ratio. The melt by solidus melting, muscovite and biotite dehydration melting has different compositions. The variation trend of the modelled melt compositions in the P-T space can help interpret the origin of leucogranites. The modelled relationships between zircon and major silicate mineral phases and melt indicate that prograde metamorphism is basically a zircon consuming process, and metamorphic zircon should mainly form during cooling and record the time of retrograde metamorphism. Loss of melt with less Zr content at low temperature can expand the zircon stability during prograde metamorphism. On the contrary, the melt loss at high temperature can decrease the zircon stability. A process similar to the fractional melting could most effectively expand the stability of zircon in the residuum.