Metapelites Flashcards
(12 cards)
What is a tie line flip?
A rxn line on a compatibility diagram that flips indicating minerals that were stable before and no longer stable together anymore
What type of silicates are most dominant in pelitic rocks?
Al-K rich phyllosilicates
How are pelites chemically distinguished?
Pelites have high Al2O3 and K2O, and low CaO, indicating dominance of muscovite and quartz over a wide range of metamorphism
Why are metapelites sensitive indicators of metamorphic conditions?
Metapelites undergo large mineralogical changes with pressure and temperature, and the limited P-T stability of their minerals provides tight constraints on metamorphic P-T conditions
How does the stability of white mica change with increasing metamorphic grade?
White mica becomes less phengitic (less Fe & Mg substituting for Al) as grade increases, especially after biotite appears
How is chloritoid introduced into pelites?
Chl + Prl = Cld (+Qtz + H2O)
What reaction forms chloritoid in the Mg-free KFASH system?
Fe-Chl (+Ms) = Fe-Bt + Fe-Cld (+Qtz + H2O)
Why does the Fe-end-member typically define the low-temperature limit in Fe-Mg reactions?
Mg2+ is smaller and forms stronger bonds than Fe2+, so Fe-end-member minerals usually break down at lower temperatures
What is a terminal (out) reaction in metamorphism?
A reaction that completely removes a mineral from the assemblage, marking an “out” isograd
What is the difference between discontinuous and continuous reactions?
Discontinuous (univariant): Occur at a single P-T point, involve a fixed set of minerals (e.g., tie-line flip or triangle reaction).
Continuous: Occur over a P-T interval, involve compositional changes within solid solutions (e.g., Fe-Mg exchange)
How can garnet be in a system but not present in the rock?
When garnet undergoes a tie line flip and results in garnet being present where chloritoid was, which only happens in Fe-rich pelites
