Inhaltszusammenfassung

Silica-undersaturated, sapphirine-bearing granulites occur in a large number of localities worldwide. Such rocks have historically been under-utilized for estimating P-T evolution histories because of limited experimental work, and a consequent poor understanding of the topology and P-T location of silica-undersaturated mineral equilibria. Here, a calculated P-T projection for sapphirine-bearing, silica-undersaturated metapelitic rock compositions is constructed using THERMOCALC for the FeO-M...Silica-undersaturated, sapphirine-bearing granulites occur in a large number of localities worldwide. Such rocks have historically been under-utilized for estimating P-T evolution histories because of limited experimental work, and a consequent poor understanding of the topology and P-T location of silica-undersaturated mineral equilibria. Here, a calculated P-T projection for sapphirine-bearing, silica-undersaturated metapelitic rock compositions is constructed using THERMOCALC for the FeO-MgO-Al2O3-SiO2 (FMAS) and KFMASH (+K2O + H2O) chemical systems, allowing quantitative analysis of silica-undersaturated mineral assemblages. This study builds on that for KFMASH sapphirine + quartz equilibria [Kelsey et al. (2004) Journal of Metamorphic Geology, vol. 22, pp. 559-578]. FMAS equilibria are significantly displaced in P-T space from silicate melt-bearing KFMASH equilibria. The large number of univariant silica-undersaturated KFMASH equilibria result in a P-T projection that is topologically more complex than could be established on the basis of experiments and/or natural assemblages. Coexisting sapphirine and silicate melt (with or without corundum) occur down to c. 900 degrees C in KFMASH, some 100 degrees C lower than in silica-saturated compositions, and from pressures of c.= 12 kbar. Mineral compositions and composition ranges for the calculated phases are consistent with natural examples. Bulk silica has a significant effect on the stability of sapphirine-bearing assemblages at a given P-T, resulting in a wide variety of possible granulite facies assemblages in silica-undersaturated metapelites. Calculated pseudosections are able to reproduce many naturally occurring silica-undersaturated assemblages, either within a single assemblage field or as the product of a P-T trajectory crossing several fields. With an understanding of the importance of bulk composition on sapphirine stability and textural development, silica-undersaturated assemblages may be utilized in a quantitative manner in the detailed metamorphic investigation of high-grade terranes. » weiterlesen» einklappen

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Klassifikation

DFG Fachgebiet:
3.42 - Geologie und Paläontologie

DDC Sachgruppe:
Geowissenschaften