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摘要
Granulites have attracted much attention as a window of probing the lower crust tectonic evolution. Since 1990s one of the most important advances in the study of granulite is quantitatively modeling the melting reactions, melt compositions, melt loss and its influence on mineral assemblages in high-grade rocks. Based on a series of P-T projections, compatibility diagrams and qualitative P-T pseudosections for fixed bulk-rock compositions in the simple systems involving KASH, NKASH and KFMASH, and available experimental results, discussions were presented in this paper in terms of the relations among melting reactions, mineral assemblages, bulk-rock compositions and P-T conditions for the meta-pelitic and greywacke rocks under HT-UHT conditions. The fluid-saturated solidus reactions for most meta-pelites/greywackes can be modeled by the fluid involving reactions in the NKASH system, which can produce melt of < 3mol% without fluid infiltration. The P-T conditions of muscovite dehydration melting reactions vary in different systems. For example, muscovite melting reactions calculated in the systems NKASH and KFMASH, which are in good accordance with the relevant experimental results, constrain respectively the lower and upper temperature limits of muscovite breakdown. The muscovite breakdown temperature may increase as increasing its Fe, Mg and Ti contents, or the anorthite content in coexisting plagioclase. Muscovite melting can produce melt of similar to 10mol% in metapelites. In the system KFMASH biotite dehydration melting involves four univariant reactions, with calculated temperature conditions somewhat lower than the experimental results. These biotite melting reactions are continuous in the system NCKFMASH, covering a temperature range of similar to 100 degrees C in metapelites and 30 similar to 50 degrees C in metagreywackes. The biotite breaking temperature may increase as increasing its Mg-#; and the upper temperature limit is greatly influenced by the involvement of Ti. Biotite melting can produce melt of 30mol% similar to 40mol% in metapelites. Phase modelling in the system KFMASH suggests that the diagnostic UHT assemblages with orthopyroxene + sillimanite, or sapphirine can easily appear in Mg-rich metapelites, and most metapelites of normal composition can only have the assemblage of garnet + sillimanite even under UHT conditions of 1000 degrees C, which can transform into sapphirine + spinel under much higher temperatures. The fluid-saturated solidus reactions, and muscovite and biotite breaking reactions can be used to define the boundaries between metamorphic facies. For instance, the fluid- and muscovite-involving melting reactions and the subsolidus muscovite breakdown reaction in the system NKASH can well define the boundary between low- and high-amphibolite facies under medium and low pressures. The experimentally constrained initial melting and disappearance reactions of biotite in metapelites can be used, respectively, to define the boundaries between high-amphibolite and (normal) granulite facies, and between (normal) granulite and UHT granulite facies. Consequently, in the context of mineral assemblages in metapelites, the (normal) granulite facies may cover a temperature range from the start of biotite melting to its disappearance, and the UHT granulite facies can be defined to the temperature conditions above biotite disappearance.
- 出版日期2016-6
- 单位北京大学
