 | P-T-t Paths |
See http://plate-tectonic.narod.ru/metpetrographylinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html
A. Definition
-Pressure-Temperature-time path: the sequence of temperatures and pressures a rock is subjected to during a metamorphic cycle:
-Burial
-Subduction,
-Heating/pressure
-Uplift/erosion
B. Example of rocks in subducting oceanic lithosphere
-Young hot slab increases rapidly in T at first, slower in P
-Old cold slab increases in both at a relatively constant rate
-This has implications for metamorphic reactions in slab and generation of arc magmas
B. Example of ideal metamorphic terrane
-Sequence of zones records increasing T and P
-Not exactly a geotherm, because different zones may have formed at different times.
-Sequence is more properly called a metamorphic field gradient.
C. P-T-t paths vs. metamorphic field gradient
-Does a high-grade rock (granulite) develop the successive sets of minerals described by the medium-P metamorphic facies series?
-Temporal vs. spatial sequences
II. Metamorphic P-T-t paths
A. Two ways to approach the problem
-Backward: deduce T and P conditions from rocks using:
-Experimental phase relations
-Compositions of co-existing minerals
-Zonation of mineral grains
-Forward: calculate T and P conditions based on processes such as subduction or crustal thickening
B. Schematic paths
-Path a: typical path for regional metamorphism:
-Crustal thickening increases P rapidly.
-Thickening raises the geotherm, but T rises much more slowly than P.
-Rocks reach maximum depth while still heating up.
-Erosion starts to reduce depth, and P, while T continues to increase.
-T reaches a maximum.
-This is called a clockwise P-T-t path.
-Path b: possible path for contact metamorphism:
-Steady decrease in P corresponds to uplift and erosion.
-T increases sharply, then decreases, without affecting P.
-Path c: counterclockwise path - often found in high-grade terranes:
-interpreted as a result of intrusion of mafic magma into the middle crust.
-T and P both increase below the intrusion.
-Because mafic magma is dense, little uplift and erosion occur => nearly isobaric cooling
B. Implications
-T and P don''t increase in tandem or at the same rate throughout metamorphism.
-Tmax and Pmax don''t coincide in time; in clockwise paths, Pmax occurs earlier; the equilibrium mineral assemblage seen today records Tmax at some lower P.
-Variations in clockwise paths may result in oddities such as Ky>Sil with decreasing T
-In general, the metamorphic field gradient does NOT represent a "fossil" geotherm. .
-Blueschist metamorphism may not be uncommon or restricted to subduction zones, but it may be overprinted by retrograde reactions in amphibolite or greenschist facies.