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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)
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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 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
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 • Mechanical rotation. • Preferred growth normal to compression. • Grains with advantageous orientation grow whereas those with poor orientation do not (or dissolve). • Minerals change shape by ductile deformation. • Pressure solution. • A combination of a and e. • Constrained growth between platy minerals. • 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.
Development of Deformational Foliationin 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 DeformationEvidence from Inclusion-Bearing 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. Springer-Verlag.
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 • 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 • Helicitic folds b. Randomly oriented crystals c. Polygonal arcs d.Chiastolitee. 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.
