Isograds; Metamorphic Facies; Pressure-Temperature-Time Evolution of Metamorphic rocks (P-T-t pathways)

Metamorphic Facies

Metamorphic mineral assemblages are a function of both metamorphic grade (P-T conditions) and orginal bulk composition.

Therefore: two rocks with the same bulk composition, subjected to the same P-T conditions (i.e., metamorphic grade) in different metamorphic terranes should produce the same metamorphic mineral assemblage.

This implies rocks rigorously obey the laws of physical chemistry and thermodynamics.
 

Metamorphic Isograds (Figures 19-1; 19-2 on the handout)
A metamorphic isograd is a line drawn on a map representing the first appearrance of a metamorphic mineral in a rock sample at the Earth's surface.
Example: Prograde Metamorphism of Mudrocks
Chlorite zone ® Biotite zone ® Garnet zone ® Staurolite zone ® Kyanite zone ® Sillimanite zone  (® direction of increasing metamorphic grade)

Each zone marks the appearance of a mineral not present in the previous assemblage as the result of chemical reaction.

Muscovite + Quartz ® Kyanite + Alkali Feldspar + H2O

KAlSi3O10(OH)2 + SiO2 ® Al2SiO5 + KAlSi3O8 + H2O

Metamorphic Facies (Eskola, 1915) (Figures 19-3; Table 19-1 on the handout)
The number of minerals that occur abundantly in metamorphic rocks is limited: Metamorphic rocks are equilibrium chemical systems (and not random accumulations of minerals that form at different stages in the development of the rock)

Essentially identical metamorphic mineral assemblages occur in similar rock compositions, subjected to the same P-T conditions (i.e., metamorphic grade) world wide.

"In any rock of a metamorphic formation which has arrived at a chemical equilibrium through metamorphism at constant Temperature and Pressure conditions the mineral composition is controlled only by the chemical composition"  Eskola (1915)
Metamorphic Facies Series (Figures 19-4; 19-8 on the handout)
A sequence of metamorphic rocks of increasing metamorphic grade which appear adjacent to one another within an individual metamorphic terrane. Best referred to as a "Metamorphic Field Gradient (MFG)"
P-T-t pathways (the stuff the metamorphic petrologist's dreams are made of.....) (Figures 19-9; 19-10 on the handout)
Modeling, based on geochemical and geophysical principles, of the actual pathways rocks travelled in the Earth's lithosphere!

Normal Burial Metamorphism (Fig. 19-9)