Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Porphyroblastic texture

Relatively large single crystals, which formed by metamorphic growth in a more fine-grained matrix, are known as porphyroblasts (from the Greek word blastos meaning growth). Porphyroblasts are a valuable source of information on local tectonic and metamorphic evolution. Inclusion patterns in porphyroblasts can mimic the structure in the rock at the time of their growth and allow a reconstruction of the relative timing of mineral growth, reflecting metamorphic conditions, and deformation. As such they can play a key role in the determination of pressure-temperature-deformation-time (P-T-D-t) paths experienced by metamorphic rocks. Apart from their inclusion patterns, porphyroblasts may also record the metamorphic evolution from core to rim; either a growth zoning may be present, or inclusions of certain minerals may show P-T conditions different from the matrix.

Classification of Porphyroblast-Matrix Relations

Porphyroblasts with inclusion patterns contain information on the nature of early deformation and metamorphic events, and on the relative age of mineral growth and deformation. Zwart (1960, 1962) elaborated a scheme based on the idea that crystals may be older, younger or of the same age as specific deformation phases. Zwart's scheme has long been used for basic reference, but, as pointed out by Vernon (1978), several criteria are ambiguous, and relations have to be studied with great care. Porphyroblasts can be classified as pre-, syn-, inter- and post tectonic (some texts use kinematic instead of tectonic), to describe the time relation between porphy-roblast growth and one or two specific phases of deformation, normally represent-ed by a foliation or by folding in the matrix (Fig.1).

porphyroblast2020(2).png

Fig.1: Schematic representation of pre-, inter-, syn-, and post-tectonic porphyroblast growth. The upper part of the diagram refers to deformation resulting in a single foliation or deformation of an earlier foliation without folding; the lower part considers deformation resulting in crenulation of an older foliation. Pretectonic porphyroblasts (a and b) show strong foliation deflection and randomly oriented inclusions. Intertectonic porphyroblasts (c and d) grew passively over a fabric in absence of deformation, and protect the resulting inclusion pattern from later deformation. Inclusion patterns are usually straight but more complex situations (c3) are also possible. Syntectonic porphyroblasts (e and f) have grown during a phase of deformation. Inclusion patterns are usually curved and continuous with the fabric outside the porphyroblast, and show evidence of having been modified during porphyroblast growth. The distinction of syn- and intertectonic porphyroblasts is usually difficult since transitions occur and differences are subtle (c1 and e1; c2 and e3; c3 and f1). Post-tectonic porphyroblasts (g and h) have grown after cessation of deformation. The inclusion pattern is identical to and continuous with the external fabric. No strain shadows, strain caps or deflection of foliation occur. Symbols: D_n < P Growth interval of mineral P post-tectonic with respect to D_n; P < D₁ Growth interval of mineral P pretectonic with respect to D₁; D_n ⊃ P Growth interval of mineral P syntectonic with respect to D_n; D_n ≤ P Growth interval of mineral P syn- to post-tectonic with respect to D_n; D_n < P < D_n+1 Growth interval of mineral P intertectonic between D_n and D_n+1; D_n < P ≤ D_n+1 Growth interval of mineral P post D_n and pre to syn D_n+1. From Passchier (2005).

• Pretectonic Porphyroblast: Pretectonic porphyroblasts are rarely described and seem to be uncommon in areas affected by regional metamorphism, except possibly in low-pressure/high-temperature metamorphism. If present, inclusions in pretec-tonic porphyroblasts are randomly oriented. It is incorrect, however, to interpret any crystal with random inclusions as pretectonic; in high-grade rocks, early folia-tions may be destroyed by grain growth, and subsequent porphyroblast growth may give rise to apparently pretectonic structures. Pretectonic porphyroblasts may be surrounded by a matrix with polyphase deformation.

• Intertectonic Porphyroblast: The term intertectonic is used for porphyroblasts that have grown over a secondary foliation and are surrounded by a matrix affected by a later deformation phase that did not leave any record in the porphyroblast.

• Syntectonic Porphyroblast: Syntectonic porphyroblasts have grown during a single phase of deformation D_n and are the most frequently encountered type of porphy-roblasts in nature. Inclusion patterns are generally curved in syntectonic porphy-roblasts, and random or straight in pre- and intertectonic porphyroblasts.

• Post-Tectonic Porphyroblast: This group is easy to define by the absence of deflec-tion of external foliation, strain shadows, undulose extinction or other evidence of deformation, which is common to pre- syn- and intertectonic porphyroblasts. If in-clusions are present, internal foliation is continuous with external foliation, even if folded.

Rotated objects

• Snowball structures: Porphyroblasts do not deform with the rest of the rock but roll as rigid grains during ductile shearing of the matrix. As they grow, they enclose adjacent minerals from the matrix, which results in an internal foliation or helical trail of inclusions representing segments of matrix foliation overgrown by, and included in, the porphyroblast. Typical examples are snowball porphyroblasts that have a spirally oriented internal foliation (Fig.2). The relationship between the orientation of the internal foliation and the external matrix foliation further defines the sense of rotation of the blasts and thereby the sense of shear in the rock.

porphyroblast2020(1).png

Fig.2: Helical trails of inclusions in a granet porphyroblast rotated by 180Â°. From Jean-Pierre Burg.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Garnet crystals with internal foliation. Moine Schists, Glenborrodale, Ardnamurchan (Scotland). PPL image, 2x (Field of view = 7mm)	Garnet crystal with internal foliation. Moine Schists, Glenborrodale, Ardnamurchan (Scotland). PPL image, 2x (Field of view = 7mm)	Garnet crystal with internal foliation. Moine Schists, Glenborrodale, Ardnamurchan (Scotland). PPL image, 2x (Field of view = 7mm)
Garnet crystal with internal foliation. Moine Schists, Glenborrodale, Ardnamurchan (Scotland). PPL image, 2x (Field of view = 7mm)	Garnet crystal with internal foliation. Moine Schists, Glenborrodale, Ardnamurchan (Scotland). PPL image, 2x (Field of view = 7mm)	Garnet crystals with internal foliation. Moine Schists, Glenborrodale, Ardnamurchan (Scotland). PPL image, 2x (Field of view = 7mm)
Garnet crystals with internal foliation. Piani di praglia (piota river), Liguria, Italy. Immgine a N//, 2x (lato lungo = 7mm)	Garnet crystals with internal foliation. Piani di praglia (piota river), Liguria, Italy. Immgine a N//, 1x (lato lungo = 9mm)	Garnet crystals with internal foliation. Piani di praglia (piota river), Liguria, Italy. Immgine a N//, 2x (lato lungo = 7mm)
Garnet porphyroblast, partially altered by chlorite. Italian Alps. PPL image, 2x (Field of view = 7mm)	Garnet porphyroblast, partially altered by chlorite. Italian Alps. XPL image, 2x (Field of view = 7mm)	Garnet Porphyroblast altered by chlorite. Italian Alps. PPL image, 2x (Field of view = 7mm)
Garnet Porphyroblast altered by chlorite. Italian Alps. XPL image, 2x (Field of view = 7mm)	Garnet Porphyroblast with quartz inclusions. Italian Alps. PPL image, 2x (Field of view = 7mm)	Garnet Porphyroblast with quartz inclusions. Italian Alps. XPL image, 2x (Field of view = 7mm)
Garnet porphyroblast, partially altered by chlorite. Italian Alps. PPL image, 2x (Field of view = 7mm)	Garnet porphyroblast, partially altered by chlorite. Italian Alps. XPL image, 2x (Field of view = 7mm)	Garnet porphyroblast, bordered by muscovite. Italian Alps. PPL image, 2x (Field of view = 7mm)
Garnet porphyroblast, bordered by muscovite. Italian Alps. XPL image, 2x (Field of view = 7mm)	Garnet porphyroblast, bordered by muscovite. Italian Alps. PPL image, 2x (Field of view = 7mm)	Garnet porphyroblast, bordered by muscovite. Italian Alps. XPL image, 2x (Field of view = 7mm)
Garnet porphyroblast, bordered by muscovite. Italian Alps. PPL image, 2x (Field of view = 7mm)	Garnet porphyroblast, bordered by muscovite. Italian Alps. XPL image, 2x (Field of view = 7mm)	Helicitic (spiral shaped) inclusions in a garnet. PPL image, 2x (Field of view = 7mm)
Helicitic (spiral shaped) inclusions in a garnet. XPL image, 2x (Field of view = 7mm)	Helicitic (spiral shaped) inclusions in a garnet. PPL image, 2x (Field of view = 7mm)	Helicitic (spiral shaped) inclusions in a garnet. XPL image, 2x (Field of view = 7mm)
Helicitic (spiral shaped) inclusions in a garnet. PPL image, 2x (Field of view = 7mm)	Helicitic (spiral shaped) inclusions in a garnet. PPL image, 2x (Field of view = 7mm)	Helicitic (spiral shaped) inclusions in a garnet. XPL image, 2x (Field of view = 7mm)
Snowballs garnets. XPL image, 10x (Field of view = 2mm)	Snowballs garnets. PPL image, 10x (Field of view = 2mm)	Snowballs garnets. XPL image, 10x (Field of view = 2mm)
Snowballs garnet. XPL image, 10x (Field of view = 2mm)	Snowballs garnet. PPL image, 10x (Field of view = 2mm)	Snowballs garnet. XPL image, 10x (Field of view = 2mm)
Snowballs garnet. PPL image, 10x (Field of view = 2mm)	Snowballs garnet. XPL image, 10x (Field of view = 2mm)	Snowballs garnet. PPL image, 10x (Field of view = 2mm)
Snowballs garnet. XPL image, 10x (Field of view = 2mm)	Snowballs garnet. PPL image, 10x (Field of view = 2mm)	Snowballs garnet. XPL image, 10x (Field of view = 2mm)
Snowballs garnet. PPL image, 10x (Field of view = 2mm)	Snowballs garnet. XPL image, 10x (Field of view = 2mm)	Snowballs garnet. XPL image, 10x (Field of view = 2mm)
Syntectonic albite from Sardinia (Italy). PPL image, 10x (Field of view = 2mm)	Syntectonic albite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Syntectonic albite from Sardinia (Italy). PPL image, 10x (Field of view = 2mm)
Syntectonic albite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Syntectonic albite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Syntectonic albite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)
Syntectonic albite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Syntectonic albite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Syntectonic albite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)
Syntectonic albite from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)