Review Section     

《International Geology Review》2012,54(3):371-388

ABSTRACT

The bulk composition of silicic igneous rocks hovers directly over the ‘minimum melt’ located on the NaAlSi₃O₈–KAlSi₃O₈–SiO₂ ternary. It is universally accepted that these rocks are igneous, reflecting the thermodynamic equilibrium between minerals and melt. However, there is a contradiction between the use of this phase diagram and current models of differentiation: fractional crystallization or partial melting (or any mechanical separation process) implies granites were melts extracted from a quartz-bearing residue.

Here, I propose that a thermodynamically controlled process, wet thermal migration, provides a more consistent explanation for producing silicic rocks. This process, when coupled with slow incremental emplacement of sills, allows magmatic differentiation to take place without mechanical separation and produces minimum-melt compositions from input magmas not saturated in quartz. Examination of phase equilibria in the SiO₂–Al₂O₃–Na₂O–K₂O–H₂O system, as well as assessment of thermodynamic models, provides a template for understanding the wet thermal migration process (diffusion-based differentiation of crystal-mush in a temperature gradient). Phase equilibria in SiO₂–Na₂O–K₂O–Al₂O₃ indicate a cotectic surface between quartz and alkali feldspar slopes down-temperature as melt peralkalinity increases. Experiments show quartz and two feldspars coexist with a single water-rich (>40 wt.% H₂O) melt at 400°C and 0.1 GPa. Modelling suggests that development of water-rich melt at the hot end of a temperature gradient drives the process. Given the propensity of alkalis to rapidly diffuse down-temperature gradients and form these melts, small amounts (<5%) of interconnected melt form, leading to the differentiation of partially molten materials by wet thermal migration.

Because the quartz-feldspar cotectic ranges from 650°C to 330°C, granites can reflect formation by a process that never involves the existence of rhyolitic melt. If this is correct, the process has implications for understanding the formation of plutons and batholiths and, therefore, the continental crust.  相似文献   

21世纪最初十年变质岩石学研究进展   总被引：4，自引：0，他引：4  

魏春景 《矿物岩石地球化学通报》2012,31(5):415-427

21世纪以来变质岩石学发展迅猛.对大型俯冲带和造山带综合数值模拟研究,所得到的变质作用p-T-t与以往一维热模拟结果很不相同;利用内部一致性热力学数据库,进行变质相平衡的定量研究,改变了人们对变质反应和相平衡关系的理解,开辟了定量研究变质作用的新阶段;超高压变质作用的深入研究,发现了更多超高压变质作用的标志及地体,指示陆壳俯冲深度可能达300～350 km,并对地壳岩石深俯冲的机理及变质作用演化进行了进一步探讨;对麻粒岩尤其是高压和超高温麻粒岩的研究,进一步了解了麻粒岩相条件下的深熔作用与熔体演化机理,为认识早前寒武纪的板块作用与造山过程提供了新的窗口;利用多种方法对俯冲带变质流体的研究,为深刻认识俯冲带的岩浆作用及地幔演化提供了更为广阔的视野.  相似文献