G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 22 Textures of Regionally Metamorphosed Rocks March 27, 2009

R EGIONAL M ETAMORPHISM (D YNAMOTHERMAL ) R ELATED TO C ONVERGENT T ECTONICS

D EFORMATION AND M ETAMORPHISM OROGENESIS (Mountain Building) Multiple Tectonic Events Multiple Metamorphic Cycles Each composed of Multiple Each composed of multiple Deformational Events metamorphic reaction events caused by reorientation & caused by abrupt changes in intensity of Stresses Pressure and Temperature NOT ALWAYS 1 to 1 Correlation

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 Winter (2001) Figure 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

C LASSIFICATION OF D EFORMATIONAL F OLIATION C LEAVAGE AND S CHISTOSITY Figure 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.

D EVELOPMENT OF D EFORMATIONAL F OLIATION 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 Proposed mechanisms for the development of foliations. After Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

D EVELOPMENT OF D EFORMATIONAL F OLIATION Winter (2001) Figure Development of foliation by simple shear and pure shear (flattening). After Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

C RENULATION C LEAVAGE M ULTI - STAGE D EFORMATION

D EVELOPMENT OF D EFORMATIONAL F OLIATION IN B EDDED S EDIMENTARY R OCKS

B EDDING – C LEAVAGE I NTERSECTIONSSandy (poorly foliated) Clayey (well foliated)

T IMING OF D EFORMATION AND M ETAMORPHISM Successive dynamothermal events and microstructures are numbered: Metamorphic Events – M 1, M 2, M 3,... Deformational Events – D 1, D 2, D 3,... Foliation Orientations – S 0, S 1, S 2, S 3,... (S 0 - primary feature) Lineation Orientations – L 0, L 1, L 2, L 3,... (L 0 - primary feature)

Winter (2001) Figure (left) Asymmetric crenulation cleavage (S 2 ) developed over S 1 cleavage. S 2 is folded, as can be seen in the dark sub-vertical S 2 bands. Field width ~ 2 mm. Right: sequential analysis of the development of the textures. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag. T IMING OF D EFORMATION AND M ETAMORPHISM

Winter (2001) Figure Illustration of an Al 2 SiO 5 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. T IMING OF N EW M INERAL G ROWTH R ELATIVE TO D EFORMATION E VIDENCE FROM I NCLUSION - B EARING P ORPHYROBLASTS AND P OIKILOBLASTS Porphyroblast inclusions inherit the fabric of the host matrix Orientation - S i SiSi

T IMING OF N EW M INERAL G ROWTH R ELATIVE TO D EFORMATION Post-kinematic: S i is identical to and continuous with S e (external foliation) Pre-kinematic: Porphyroblasts are post-S 2. S i is inherited from an earlier deformation. S e is compressed about the porphyroblast in (c) and a pressure shadow develops. Syn-kinematic: Rotational porphyroblasts in which S i is continuous with S e suggesting that deformation did not outlast porphyroblast growth.

Pre-kinematic crystals a.Bent crystal with undulose extinction b.Foliation wrapped around a porphyroblast c.Pressure shadow or fringe d.Kink bands or folds e.Microboudinage f.Deformation twins Figure 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 Typical textures of post-kinematic crystals. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

Syn-kinematic crystals Winter (2001) Figure Traditional interpretation of spiral S i train in which a porphyroblast is rotated by shear as it grows. From Spry (1969) Metamorphic Textures. Pergamon. Oxford. Spiral Porphyroblasts

OROGENY LEADS TO POLYMETAMORPHISM