1
Pressure-temperature-time paths (P-T-t)
Collisional belts: crustal thickening by thrust faulting
Tectonic thickening
Pressure increase first, followed by temperature
increase- clockwise P-T path
Extensional belts- rifting: temperature increase first,
followed by, uplift and erosion- counterclockwise path
Time: mineral ages- give time at temperature of closure
to daughter product
Diffusion: D = Do exp (-Q/RT)
Closure or blocking temperatures
Rb-Sr biotite – 300oC; Rb-Sr Muscovite 500oC
Ar/Ar biotite 300oC; Ar/Ar hornblende 500oC
U-Pb zircon/monazite : 700-800oC
Apatite fission tracks: 150oC
1
2
Rheology of crust
Brittle (strong) upper crust- earthquakes nucleate
Ductile (weak) lower crust (quartz-feldspar)
Strong upper mantle (olivine) at Moho- gets weaker at
depth
Fluids (water; CO2)
Upper crust permeable- fracture pathways;
Quartz/calcite veins common
Hydrothermal ore (Pb-Zinc) deposits
High permeability (>10-17 m2)
Fluid pressure is hydrostaticMiddle to lower crust- low permeability (10-20 - 10-18 m2)
Fluid pressure (if present) is lithostatic
Makes rock more brittle/weakerMohr failure envelope- veins
Prograde metamorphism : Dehydration reactions:
Liberation of water with increasing temp.
Muscovite = Al2 O3 +feldspar + H2O
2
3
Greenschist facies (wet)– amphibolite facies- granulite
facies (dry).
Electrical conductivity- sometimes high in middle crust:
1) high salinity pore water
2) Metamorphic dehydration water
3) Graphite- highly conductive
4) Partial melting - magma
Lower crust dry: granulite facies
(CO2-rich fluid inclusions)
Composition of crust
Seismic velocities
Upper mantle/lower crust: 8 km/sec:
Mixtures of olivine, pyroxenes +- garnet
Dunite, eclogite, harzburgite, garnet lherzolite
Mean composition: ultramafic
Lower crust: mafic metamorphic rocks 6.0 – 7.5 km/s
Meta-diorite, mafic granulite, amphibolite; meta-basalt
Mean composition: diorite-basalt
3
4
Upper crust: 5- 6 km/sec. Sedimentary rocks, schists,
granite gneiss. Less mafic.
Mean composition = granodiorite
Lower crustal seismic reflections
1) deep fluids in porous layers???
2) Metamorphic ductile layering- Shear zones
3) Lithological layering
Shear zones commonly traced up to surface exposures
Fine grained sediments
Shales have very uniform REE compositionsEnriched in incompatible elements
Average of upper continentasl crust
4
5
Crustal xenoliths
Lower crust overall mafic composition
Consistent with poisson’s ratio and seismic velocities
5