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.Metamorphic Isograds (Figures 19-1; 19-2 on the handout)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.
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
- really an isograd surface: the line on the map is formed by the intersection of the erosional plane with the isograd surface.
- presumed to represent an equivalent metamorphic grade along it's entire surface.
- appearance of key metamorphic index minerals mark a mineralogic reaction driven by changes in Pressure, Temperature, and Fluid Pressure (PH2O, PCO2, etc.)
Chlorite zone ® Biotite zone ® Garnet zone ® Staurolite zone ® Kyanite zone ® Sillimanite zone (® direction of increasing metamorphic grade)Metamorphic Facies (Eskola, 1915) (Figures 19-3; Table 19-1 on the handout)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 + H2OKAlSi3O10(OH)2 + SiO2 ® Al2SiO5 + KAlSi3O8 + H2O
The number of minerals that occur abundantly in metamorphic rocks is limited:Metamorphic Facies Series (Figures 19-4; 19-8 on the handout)Metamorphic rocks are equilibrium chemical systems (and not random accumulations of minerals that form at different stages in the development of the rock)
- within individual rocks
- across the spectrum of rock compositions
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)
A sequence of metamorphic rocks of increasing metamorphic grade which appear adjacent to one another within an individual metamorphic terrane.P-T-t pathways (the stuff the metamorphic petrologist's dreams are made of.....) (Figures 19-9; 19-10 on the handout)
- Represents a plane through a former P-T- fluid "space" now exposed by erosion.
Best referred to as a "Metamorphic Field Gradient (MFG)"
- Does not represent a former geothermal gradient, nor does it represent the actual P-T path of individual rocks.
- used as a graphical means to indicate whether high Ts were achieved @ shallow or deep crustal levels in a particular terrane.
Modeling, based on geochemical and geophysical principles, of the actual pathways rocks travelled in the Earth's lithosphere!Normal Burial Metamorphism (Fig. 19-9)
Burial Stage
Heating Stage- rocks are buried faster, by rapid sedimentation, than they can heat up (rocks are poor conductors)
- compression/compaction dominates over heating
- Subsidence rates and deposition rates decrease as the basin matures
- rocks heat up as they try to achieve thermal equilibrium, tend to overshoot the normal geothermal gradient.
Uplift Stage
- rocks begin to rise to achieve isostatic equilibrium. This may be accelerated by a decrease in density as the rocks heat up.
- decompression proceeds faster than thermal equilibration and rocks overshoot the normal geothermal gradient.
Unroofing Stage
- erosion strips material from the rising pile, promoting continued decompression, and transporting anomalously hot rocks to shallow crustal levels.
- uplift rates wane as the crust approaches isostatic equilibrium, cooling begins to predominate over uplift. Eventually that rock is returned to the surface.