Application of P-T-t Paths

See http://plate-tectonic.narod.ru/metpetrographylinks.html
http://plate-tectonic.narod.ru/metpt1photoalbum.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html


I. Example from Winter

-Zoned garnet poikiloblast
-Garnet contains Pl inclusions
-Also, separate Pl porphyroblast
-Not shown: zoned Bt as inclusions in Grt and as porphyroblasts

B. Calculations

-Use GASP to get pressures from co-existing Grt + Pl
-Use Grt-Bt for T.Plot on PT diagram and connect points from core (older) outward to rim (younger)

-Clockwise path
-Little change in T, P decreases
-Isothermal decompression => rapid uplift and erosion after an episode of crustal thickening

II. Fall Mountain NH

A. General

-Structure is 2 nappes separated by a thrust fault
-Upper plate: high-grade schists and gneisses; retrograded and rehydrated in amphibolite facies
-Lower plate: peak conditions in middle amphibolite facies

B. Upper plate

-Pseudomorphs of Sil after And => early part of P-T-t paths passed through And field
-Peak conditions:: Grt + Bt + Sil + Q + Pl + Kfs; 700-750 oC; 3-4.5 kbar
-See garnet profiles: garnets were unzoned at peak ; then rims affected by Fe-Mg exchange
-See P-T-t paths : lack of change in Grs in garnet interior => retrograde P-T path began with change along Grs = 0 isopleth (Path A/Along this path, Alm decreases and Sps increases); close to rim, Alm and Grs increase, Sps decreases i- path B
-Further information: matrix mineral assemblage: 500 oC, 5 kbar - retrograde path must intersect these conditions = parallelogram; an content of Pl; result is path C
-Thus upper plate rocks experienced initial cooling from peak conditions along path A, then isothermal compression up to path C, then nearly isobaric cooling along path C.

C. Lower plate

-Pseudomorphs of Ms after And
-T/P conditions: 470-560 oC and 2.5-7 kbar: wide spread because of variable degree of equilibration; no evidence of high-grade metamorphism, as in upper plate rocks.

-Mineral assemblage in equilibrium with rim: Grt + Bt + Chl + Q + Pl + Ms
-Mineral assemblage in equilibrium with core: Grt + Bt + And + Pl + Ms + Q

- P-T-t paths from geothermobarometry on this garnet : paths show isobaric heating, then isothermal compression

D. Thermo-tectonic model for Fall Mtn from upper and lower plates

-Early period of high-T, low-P metamorphism => contact met: note that plates are separated by the Bellows Falls pluton.
-P-T paths for both show isothermal compression of 2-3 kbar - interpreted as result of thrusting that superimposed these 2 suites
-Isobaric cooling of upper plate & slight cooling of lower plate after thrusting => hot upper plate was emplaced above cool lower plate, which in turn was emplaced above still cooler rocks.

III. A thermal model of P-T paths

A. Regional metamorphism

-Presumed to be due to crustal thickening associated with continental collision. Reasoning:

-Metamorphic terranes may give P > 5 kbar. Rocks now at surface where crust is of normal thickness (35 km) => at time of metamorphism, crust > 53 km.
-Modern collisional orogen of Himalayas/Tibet has crust 55-70 km

-Continental collision is a complex process, but 1-D models of thickening provide insight.

B. Crustal thickening by thrust faultingf

-Thrusting is instantaneous. Reasonable because tectonic rates of heat emplacement much faster than rates of heat conduction
-Thermal structure is allowed to "relax"
-Uplift and erosion restores normal thickness crust

-Assumptions:

-Pre-collision thermal structure is ignored
-Burial part of P-T-t path is neglected - model begins with rocks at maximum depth
-Only vertical heat transfer is permitted
C. Model

-Based on differential equations of heat transfer: solved numerically by a finite difference method; find temperatures at specific depth increments and specific time increments.
-Must specify initial and boundary conditions: before faulting, geotherm depends on thickness and radioactivity of heat-producing layer and heat flow into the base of this layer ; after faulting, geotherm repeats above and below faults; these temperatures are taken as the initial conditions for the model; boundary conditions (Surface T is assumed to be 0 oC at all time increments/T at lowest depth increment is calculated from the value of the temperature directly above it).
-In general, for a single time, T at each depth = function of T''s above & below at the previous time increment and at the current time: plug in starting values; calculate T''s; plug calculated values into equations and repeat calculation; until new values match old ones.
-See thermal evolution of model, geotherms in Ma. No erosion in this model.

E. P-T-t paths

-Add erosion of 1mm/yr: base of heat-producing layer rises through time.
-See geotherms evolve through time
-Consider a rock that is initially at 40 km depth: T = 100 Oc; after 10 Ma, rock is at 30 km, T = 350; etc, connect to make path
-calculated path: P-T values: rock starting at 40 km: isobaric heating, isothermal decompression, then both Tand P decrease, rock starting at 30 km: continuous cooling.
- model modified so that erosion doesn''t start until 20 Ma after faulting: 40-km rock heats isobarically, then heats during decompression, then cools with continued decompression
-30-km rock cools isobarically, then heats slightly / isothermal with decompression, then cooling with continued decompression.