GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 22

Textures of Regionally Metamorphosed Rocks

March 27, 2009

REGIONAL METAMORPHISM(DYNAMOTHERM

AL)RELATED TO

CONVERGENT TECTONICS

DEFORMATION AND METAMORPHISM

OROGENESIS (Mountain Building)

Multiple Tectonic Events Multiple Metamorphic Cycles

Each composed of Multiple Each composed of multiple Deformational Events metamorphic reaction eventscaused by reorientation & caused by abrupt changes in intensity of Stresses Pressure and Temperature

NOT ALWAYS 1 to 1 Correlation

a. Compositional layeringb. Preferred orientation of platy

mineralsc. Shape of deformed grainsd. Grain size variatione. Preferred orientation of platy

minerals in a matrix without preferred orientation

f. Preferred orientation of lenticular mineral aggregates

g. Preferred orientation of fracturesh. Combinations of the above

Winter (2001) Figure 23-21. Types of fabric elements that may define a foliation. From Turner and Weiss (1963) and Passchier and Trouw (1996).

Foliation, Layering, Lamination, and Other Planar Fabrics

Deformational foliation is a secondary feature of rocks referring to the planar alignment of elongate minerals resulting from strain imparted to a rock

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 CLEAVAGEMULTI-STAGE DEFORMATION

DEVELOPMENT OF DEFORMATIONAL FOLIATION

IN BEDDED SEDIMENTARY ROCKS

BEDDING – CLEAVAGE INTERSECTIONS

SandySandy(poorly foliated)(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)

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 DEFORMATION AND METAMORPHISM

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. Springer-Verlag.

TIMING OF NEW MINERAL GROWTH

RELATIVE TO DEFORMATION

EVIDENCE FROM INCLUSION-BEARING

PORPHYROBLASTS AND POIKILOBLASTS

Porphyroblast inclusions Porphyroblast inclusions inherit the fabric of the host inherit the fabric of the host

matrixmatrixOrientation - SOrientation - Sii

Si

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 Pre-kinematic crystalscrystals

a.a. Bent crystal with Bent crystal with undulose extinctionundulose extinction

b.b. Foliation wrapped Foliation wrapped around a porphyroblastaround a porphyroblast

c.c. Pressure shadow or Pressure shadow or fringefringe

d.d. Kink bands or foldsKink bands or foldse.e. MicroboudinageMicroboudinagef.f. Deformation twins Deformation twins

Figure 23-34. Typical textures of pre-kinematic crystals. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

Post-kinematic crystalsPost-kinematic crystalsa.a. Helicitic folds Helicitic folds b.b. Randomly oriented crystals Randomly oriented crystals c.c. Polygonal arcs  Polygonal arcs d.d. Chiastolite  Chiastolite e.e.

Late, inclusion-free rim on a poikiloblast (?) Late, inclusion-free rim on a poikiloblast (?) f.f. Random aggregate pseudomorph Random aggregate pseudomorph

Figure 23-35. Typical textures of post-kinematic crystals. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

Syn-kinematic crystalsSyn-kinematic crystals

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.

Spiral Porphyroblasts

OR

OG

EN

Y LE

AD

S T

O

PO

LYM

ETA

MO

RPH

ISM