Transcript PwrPt

GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 22
Textures of Regionally Metamorphosed Rocks
March 27, 2009
REGIONAL
METAMORPHISM
(DYNAMOTHERMAL)
RELATED TO
CONVERGENT
TECTONICS
DEFORMATION
AND
METAMORPHISM
OROGENESIS (Mountain Building)
Multiple Tectonic Events
Each composed of Multiple
Deformational Events
caused by reorientation &
intensity of Stresses
Multiple Metamorphic Cycles
NOT ALWAYS
1 to 1
Correlation
Each composed of multiple
metamorphic reaction events
caused by abrupt changes in
Pressure and Temperature
Foliation, Layering, Lamination, and Other Planar Fabrics
a. Compositional layering
b. Preferred orientation of platy
minerals
c. Shape of deformed grains
d. Grain size variation
e. Preferred orientation of platy
minerals in a matrix without
preferred orientation
f. Preferred orientation of lenticular
mineral aggregates
g. Preferred orientation of fractures
h. Combinations of the above
Deformational foliation is a secondary
feature of rocks referring to the planar
alignment of elongate minerals resulting
from strain imparted to a rock
Winter (2001) Figure 23-21. Types of fabric elements that may define a foliation. From
Turner and Weiss (1963) and Passchier and Trouw (1996).
CLASSIFICATION OF DEFORMATIONAL FOLIATION
CLEAVAGE AND SCHISTOSITY
Figure 23-22. A morphological (non-genetic)
classification of foliations. After Powell (1979)
Tectonophys., 58, 21-34; Borradaile et al. (1982)
Atlas of Deformational and Metamorphic Rock
Fabrics. Springer-Verlag; and Passchier and
Trouw (1996) Microtectonics. Springer-Verlag.
DEVELOPMENT OF DEFORMATIONAL
FOLIATION
Proposed mechanisms for the
development of foliation
a. Mechanical rotation.
b. Preferred growth normal to
compression.
c. Grains with advantageous
orientation grow whereas
those with poor orientation do
not (or dissolve).
d. Minerals change shape by
ductile deformation.
e. Pressure solution.
f. A combination of a and e.
g. Constrained growth between
platy minerals.
h. Mimetic growth following an
existing foliation.
Winter (2001) Figure 23-27. Proposed mechanisms for the development of foliations. After
Passchier and Trouw (1996) Microtectonics. Springer-Verlag.
DEVELOPMENT OF
DEFORMATIONAL FOLIATION
Winter (2001) Figure 23-28. Development of foliation by simple shear and pure shear
(flattening). After Passchier and Trouw (1996) Microtectonics. Springer-Verlag.
CRENULATION CLEAVAGE
MULTI-STAGE DEFORMATION
DEVELOPMENT OF DEFORMATIONAL FOLIATION
IN BEDDED SEDIMENTARY ROCKS
BEDDING – CLEAVAGE INTERSECTIONS
Sandy
(poorly foliated)
Clayey
(well foliated)
TIMING OF DEFORMATION AND
METAMORPHISM
Successive dynamothermal events and microstructures
are numbered:
Metamorphic Events – M1, M2, M3, ...
Deformational Events – D1, D2, D3, ...
Foliation Orientations – S0, S1, S2, S3, ... (S0- primary feature)
Lineation Orientations – L0, L1, L2, L3,...(L0- primary feature)
TIMING OF DEFORMATION AND
METAMORPHISM
Winter (2001) Figure 23-42. (left) Asymmetric crenulation cleavage (S2) developed over S1
cleavage. S2 is folded, as can be seen in the dark sub-vertical S2 bands. Field width ~ 2 mm.
Right: sequential analysis of the development of the textures. From Passchier and Trouw
(1996) Microtectonics. Springer-Verlag.
TIMING OF NEW MINERAL GROWTH
RELATIVE TO DEFORMATION
EVIDENCE FROM INCLUSIONBEARING PORPHYROBLASTS
AND POIKILOBLASTS
Porphyroblast inclusions inherit
the fabric of the host matrix
Orientation - Si
Si
Winter (2001) Figure 23-33. Illustration of an Al2SiO5 poikiloblast that
consumes more muscovite than quartz, thus inheriting quartz (and
opaque) inclusions. The nature of the quartz inclusions can be related
directly to individual bedding substructures. Note that some quartz is
consumed by the reaction, and that quartz grains are invariably
rounded. From Passchier and Trouw (1996) Microtectonics. SpringerVerlag.
TIMING OF NEW MINERAL GROWTH
RELATIVE TO DEFORMATION
Post-kinematic: Si is
identical to and
continuous with Se
(external foliation)
Pre-kinematic: Porphyroblasts
are post-S2. Si is inherited from
an earlier deformation. Se is
compressed about the
porphyroblast in (c) and a
pressure shadow develops.
Syn-kinematic: Rotational
porphyroblasts in which Si is
continuous with Se suggesting
that deformation did not
outlast porphyroblast growth.
Pre-kinematic
crystals
a.
b.
c.
d.
e.
f.
Bent crystal with
undulose extinction
Foliation wrapped
around a porphyroblast
Pressure shadow or
fringe
Kink bands or folds
Microboudinage
Deformation twins
Figure 23-34. Typical textures of
pre-kinematic crystals. From Spry
(1969) Metamorphic Textures.
Pergamon. Oxford.
Post-kinematic crystals
a.
Helicitic folds b. Randomly oriented crystals c. Polygonal arcs d. Chiastolite e.
Late, inclusion-free rim on a poikiloblast (?) f. Random aggregate pseudomorph
Figure 23-35.
Typical
textures of
post-kinematic
crystals. From
Spry (1969)
Metamorphic
Textures.
Pergamon.
Oxford.
Syn-kinematic crystals
Spiral Porphyroblasts
Winter (2001) Figure 23-38. Traditional interpretation of spiral
Si train in which a porphyroblast is rotated by shear as it grows.
From Spry (1969) Metamorphic Textures. Pergamon. Oxford.
OROGENY LEADS TO
POLYMETAMORPHISM