Premium
Calculated phase equilibria in K 2 O‐FeO‐MgO‐Al 2 O 3 ‐SiO 2 ‐H 2 O for silica‐undersaturated sapphirine‐bearing mineral assemblages
Author(s) -
KELSEY D. E.,
WHITE R. W.,
POWELL R.
Publication year - 2005
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.2005.00573.x
Subject(s) - granulite , quartz , geology , silicate , mineral , mineralogy , facies , geochemistry , thermodynamics , crystallography , chemistry , materials science , physics , metallurgy , paleontology , organic chemistry , structural basin
Abstract 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‐Al 2 O 3 ‐SiO 2 (FMAS) and KFMASH (+K 2 O + H 2 O) 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 °C in KFMASH, some 100 °C lower than in silica‐saturated compositions, and from pressures of c. ≤1 to ≥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.