Metamorphic Textures

385 Pages·1969·11.884 MB·English

by  Alan Spry (Auth.)

METAMORPHIC TEXTURES
by
ALAN SPRY
Department of Physics^ Monash University, Melbourne, Australia
P E R G A M ON P R E SS
OXFORD · NEW YORK · TORONTO · SYDNEY
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Copyright © 1969 Pergamon Press Ltd.
First edition 1969
Reprinted 1974
Library of Congress Catalog Card No. 68-59126
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Preface
METAMORPHIC petrology is at present undergoing a change in emphasis. For
the past two decades, metamorphic processes have been considered largely
in terms of mineralogical and chemical changes and many petrologists have
been so preoccupied with experimental synthesis and the determination of
the physical conditions under which minerals or mineral assemblages are
stable that the significance of rock textures has been overlooked. There has
been an upsurge of interest in textures in the past few years but no compre­
hensive evaluation of texture-forming processes has been made since Harker's
great work of more than thirty years ago.
It is abundantly clear that a study of both texture and mineral assemblage
is essential to an understanding of metamorphic rocks and that a great deal
of information pertinent to this study is available in the literature on metal­
lurgy, ceramic studies and solid state chemistry or physics.
The purpose of this book is first, to provide definitions, descriptions and
illustrations of metamorphic textures for the senior undergraduate student
and second, to discuss the fundamental processes involved in textural develop­
ment at a level appropriate to the graduate student and practising petrologist.
The fairly extensive (but by no means exhaustive) bibliography attempts to
include references to original definitions, to important examples in geological
literature and to the most relevant papers in adjacent fields.
If this book hsis any underlying theme, it is that an understanding of tex­
tures requires that metamorphism be regarded as a series of structural trans­
formations rather than as chemical reactions. These transformations affect
real, not ideal crystals, and to regard minerals as ideal crystals rather than
imperfect solids containing structural defects is no more satisfactory in meta­
morphic petrology than in metallurgy where many processes are considered
quantitatively in terms of dislocations. An elementary knowledge of the part
that dislocations play in gliding, twinning, nucleation and growth is essential
to the petrologist.
Preparation of this book W2is made possible by the generosity of the
Nuffield Foundation in awarding a Dominions Travelling Fellowship at
Imperial College, London, in 1965, and in providing further funds in 1966
for preparation costs.
viii Preface
I am very grateful to the following friends who criticized various parts of
the manuscript: F. C. Beavis, R. A. Binns, K. A. Crook, D. Flinn, A. W.
Kleeman, R. Kretz, C. B. Raleigh, R. L. Stanton, J. Sutton, and R. H.
Vernon. Excellent photomicrographs provided by S. Amelinckx, R. A. Binns,
J. Christie, A. C. McLaren, P. A. Sabine, W. S. Treffner and R. H. Vernon
are acknowledged in appropriate places. My thanks are due to the following
bodies for permission to use diagrams for which they hold copyright: American
Ceramic Society, American Mineralogist, Butterworths, Geological Society of
America, Geological Survey of Great Britain, Journal of Applied Physics,
McGraw-Hill Book Co., M.I.T. Press, Oxford University Press and John
Wiley & Sons.
CHAPTER 1
Metamorphism and Metamorphic
Processes
METAMORPHISM is the mineralogical and structural (textural) adjustment of
(dominantly) solid rocks to physical and chemical conditions which differ
from those under which the rocks originated. Weathering and similar pro­
cesses are conventionally excluded. The type of metamorphism depends on
the relative values of temperature (T), confining pressure (/*con)> pressure or
chemical activity or fugacity of water (expressed generally as PHJO)* deforma­
tion or directed pressures (P^ir) their variation with time {t). There is
only a limited degree of interdependence of these controls and the meta­
morphic history of a rock is an expression of their mutual interaction with
time (Fyfe, Turner and Verhoogen, 1958; Pitcher and Flinn, 1965).
In the simplest possible terms, the whole field of metamorphism may be
divided into Thermal (Contact) Metamorphism due to heat, Dynamic
Metamorphism due to directed pressures at rather low temperatures, and
Regional (also Plutonic) Metamorphism due to heat plus directed pressures.
Shock Metamorphism may be regarded as a special type of Dynamic Meta­
morphism in which the confining pressiu-e and temperature may be high but
which is characterized by a very high rate of deformation and very high
directed pressures. Burial Metamorphism may be regarded as being tran­
sitional between diagenesis and metamorphism (either Thermal or Regional).
Metamorphism is essentially a thermal phenomenon and heat is the most
important source of energy allowing mineralogical and textural reconstruc­
tion during metamorphism. Energy is required to overcome initial activa­
tion energies and to allow sufficient thermal vibration for ion movement and
structural change; most metamorphic reactions are endothermic and require
energy. Thermal or contact metamorphism takes place with moderate to high
7, low Pcon> variable ΡΗ,Ο ^^'^ generally zero P^^, The textures (and minera­
logies) of rocks in different aureoles are controlled largely by the relative
rates of heat transport and mineralogical transformation.
1
2 Metamorphic Textures
A rock undergoing metamorphism is subjected to a complex variety of
pressures consisting of the confining pressure, intergranular fluid or gas
pressure and possibly directed tectonic pressures.
The confining (or load) pressure is due to the weight of the overlying
material and is a function of depth and the density of the load. A porous
sandstone (with an open framework) whose pores are filled with ground
water and which is located near the surface will have an intergranular PH^Q
between \ and \ of the ^Ρ η·ο However, with increasing depth the grains are
driven closer together and the intergranular water bears a proportion of the
load, the proportion depending on how quickly the water can be driven out.
As the permeability decreases due to compaction, a state is reached when the
water cannot escape and here P^^Q = Peon 5 this is generally regarded as the
most common metamorphic circumstance but is by no means universal and
in a crystalline rock which is essentially dry, metamorphism takes place with
PHJO < ^con- Where gas is released in considerable quantities PH^O
exceed ^Ρ ^ο ; in fact the gas pressure may be sufficiently in excess to lift the roof,
disrupt the rock or lower the strength by introducing a fluid between the grains.
The pressure in the pores is not only due to water; carbon dioxide will
be present in metamorphosed limestones where Pco, more important than
PHJO- The pore-fluid pressure is the sum of the partial pressures of the
different gases, i.e. Pp^^e = Pu,o + ^co» etc.
Non-directed pressures not only cause mechanical effects such as closer
packing, reduction of pore space and increase in density but are important
also in controlling mineralogical changes and melting. In general, higher
confining pressures favour more closely packed atomic structures and thus
more dense phases. Regional metamorphism of a basalt at low temperatures
gives a greenschist in which sodium is in the feldspar (albite) and iron and
magnesium are contained in ferromagnesian minerals such as chlorite and
actinolite. Under higher pressures sodium may go into a ferromagnesian to
give a glaucophane schist. The behaviour of sodium in eclogites is similarly
pressure-controlled.
However, the effects of directed pressures are almost entirely textural or
structural rather than mineralogical. Harker's suggestion that metamorphic
minerals could be divided into stress and anti-stress minerals and that the for­
mation ΟΪ stress minerals either required or w2is favoured by directed pressures,
and that anti-stress minerals could only be formed in non-stress conditions,
does not appear to have stood the test of time although there is a little
evidence suggesting that directed pressures may control the stability fields
of some minerals. There is considerable evidence that directed stresses act
almost catalytically in assisting metamorphic changes, probably by lowering
activation energies and assisting diffusion; thermodynamically, the directed
pressure may be added to the non-directed pressure in calculating total
Metamorphism and Metamorphic Processes 3
pressure. Directed stresses and movements enable large amounts of energy
to be added quickly to a system and to be localized, e.g. along thrust
zones.
A rock at a certain depth in the crust is subjected to a confining pressure
due to the load of the overlying rocks and if the region is subjected to
unequal stresses, the rock will flow in response. While it is flowing it is sub­
jected to a tectonic overpressure (Rutland, 1965) so that it is, in effect, con­
fined by a greater pressure than that attributable to the load. Overpressures
have been invoked in an attempt to explain discrepancies between the
physical conditions for various kinds of metamorphism implied by experi­
ment, and those indicated by the geological evidence, particularly with
respect to the occurrence of kyanite.
The textures alone of kyanite-bearing rocks indicate that the tectonic
overpressure explanation is invalid. Kyanite is most conmion as a post-tec­
tonic mineral and the kyanite of the most closely studied metamorphic areas
occurs as an interkinematic mineral which was formed in the static period
between metamorphic phases. Recent (Newton, 1966) experimental work
on the aluminium silicates now makes the laboratory conditions much more
compatible with those predicted by geology and removes the need for over­
pressures.
A correct view of metamorphic history requires an understanding of the
changes in T", Pioad> ^H^O ^dir ^i^h time and metamorphic rocks contain
evidence of variations in all of these controls.
Change of temperature with time is involved in the concepts of progressive,
retrograde and repeated metamorphism. Differences in confining pressure
between regions allow the recognition of diflFerent ''trends", e.g. Miyashiro
(1961) suggested five trends: high pressure (with jadeite and glaucophane),
high pressure intermediate (glaucophane but no jadeite), medium pressure
(the kyanite-sillimanite assemblage, i.e. Barrow type), low pressure inter­
mediate (andalusite plus staurolite), and low pressure (Buchau, andalusite
plus sillimanite). However, the mineralogical assemblages in the rocks of a
single region may indicate different confining pressures at different times.
Harker (1939) divided regional metamorphism into two main trends, the
normal (Barrow) type and a type with "deficient shearing stress". These two
types, commonly referred to as the Barrovian (after Barrow) and Buchau
(after the locality), are now attributed by some authors to diflferences in
confining (non-directed) pressures but there is a certain amount of textural
(not mineralogical) evidence supporting Harker's suggestion that some
regional metamorphic rocks are much less deformed than others and it
would appear that the various trends may be due only partly to differences
in confining pressures and that some differences in amount and rate of
deformation are significant.
4 Metamorphic Textures
Variations in Ρ^,ο to give "dry" or "wet" metamprphism may be pro­
found. Water pressures may be high in the early to middle stages of the first
metamorphism of sediments, may be lowered by compaction and folding,
or temporarily raised by dehydration processes; the subsidiary nature of
retrograde metamorphism suggests that PH^O is low in the late stages.
Increased P^on and Twill both tend to increase ΡΗ,Ο· The metamorphism of
pre-existing crystalline rocks appears to take place at low ΡΗ,Ο and the
granulite facies appears to represent "dry" conditions.
The classical view of regional metamorphism involves simultaneous defor­
mation (constant high P^j,) and rising temperature with constant and equal
Peon and PHJO- However, the history of most regional metamorphic areas
appears to consist of a general rise in temperature with intermittent short
periods of deformation; Ρ^^ο and P^on vary independently.
The concept of "progressive" metamorphism involving continuous recry-
stallization and reaction to form successive mineral assemblages stable at
progressively increasing temperatures must be examined closely. For instance
there is no evidence in many schists that the garnet has been formed by some
such succession of reactions as clay chlorite -> biotite -> garnet, or that a
basic igneous rock has been first broken down to an albite-epidote-actinolite-
chlorite assemblage and then built up to andesine-gamet-hornblende
aggregate. Progressive metamorphism should occur where the rate of reaction
exceeds the rate of heating. Thus it is to be expected in the regional meta­
morphism of wet sediments but not of dry crystallines. It is not to be expected
in all thermal aureoles, especially those containing rocks belonging to the
sanidinite facies. The relations of metamorphic to original textures in many
rocks suggests that transformations have taken place directly from an
indefinite mixed sedimentary material to garnet, sillimanite, etc., or from
individual crystals of igneous olivine, pyroxene and plagioclase to meta­
morphic olivine, pyroxenes and plagioclase.
The Lower Limit of Metamorphism
Rocks (sediments and volcanics) which have been deeply buried but not
appreciably deformed may undergo mineralogical changes at comparatively
low temperatures. These changes belong to a group involving cementation,
lithification, diagenesis and incipient metamorphism and it is a matter of
concern to find a satisfactory definition of the boundary between lithification
and metamorphism. Packham and Crook (1960) suggested that the boun­
dary be made on the basis that processes are diagenetic until the original
fabric is extensively modified. Coombs (1961, p. 213) disagreed v^th this
and emphasized the transitional or progressive nature of the process from
diagenesis to metamorphism. Coombs (1961, p. 322) defined burial meta-
Metamorphism and Metamorphic Processes 5
morphism as "reconstitution without obvious relation to igneous intrusions
and conmionly of regional extent; incipient, extensive or incomplete. The
fabric of silicate rocks is not modified by development of a schistosity.
"The metamorphism appears to follow burial and is not accompanied by
significant penetrative movements. Diagenetic processes in the most re­
stricted use of that term, that is, occurring essentially at the temperature of
deposition, are excluded. Burial metamorphism has been observed to produce
mineral assemblages conventionally ascribed to the zeolite facies, the green-
schist facies and perhaps the glaucophane schist facies. Many slates and
slightly sheared greywackes such as those of the Ch. 1. subzone of Otago mark
transitions from the products of burial metamorphism to those of regional
metamorphism."
Coombs (1954, 1960) stressed the transitions from undeformed burial
metamorphic rocks to deformed regional metamorphics but it would appear
thatthe Glaucophane Schist and Greenschist Facies rocks should be regarded
as genetically distinct from the Zeolitic Facies of burial metamorphism.
The study of the Zeolite Facies has been almost entirely mineralogical and
the textures have been largely ignored. Future work may well show that
Coombs' transitions from the Zeolitic Facies (without schistosity) through the
Prehnite-Pumpellyite Metagreywacke Facies (with or without schistosity) to
the Greenschist or Glaucophane Schist Facies (with schistosity) can be sub­
divided on a textural basis.
The retention of abundant original grains and textures and also of complex
mineral assemblages suggests that equilibrium, both chemical and textural,
has not been achieved generally, although it may have been achieved in
limited regions of matrix or cement. The textures are dominated by palimp­
sest (igneous or sedimentary) features but have been little studied. Reference
may be made to the papers of Coombs et al, (1959), Crook (1960,1961, 1963),
Coombs (1954, 1960, 1961) and Packham and Crook (1960).
METAMORPHIG TEXTURES
The texture of a rock involves the size of the component crystals (both
absolute and relative to each other), their shape, distribution and orientation.
Textures can be divided into two main categories:
Intergranular (between grains); concerned with grain boundaries, the size
and shape of crystals, preferred orientations, compositional layering etc.
Intragranular (within grains); concerned with zones, twins, kinks, sub-grain
structure, exsolution intergrowths, inclusions, etc.
The origins of textures are most easily understood when regarded as fimc-
tions of three variables: crystallization, deformation and time. The process
6 Metamorphic Textures
of crystallization includes the recrystallization of existing minerals and the
crystallization of new minerals. Iniismuch as crystallization can generally
be regarded as a positive process in which minerals are "built up" and con­
verted to a lower energy condition, deformation is a negative process in which
existing crystals are strained, broken up and converted to a higher energy
condition. The time factor is extremely important because both crystalliza­
tion and deformation are slow processes and textures must be interpreted as
much in terms of kinetics and interrupted processes as in terms of thermo­
dynamics and an equilibrium arrangement. Minerals and mineral textures
may persist metastably through a number of metamorphic episodes.
The texture of a metamorphic rock may be subdivided into three possible
elements:
(1) Relict: original pre-metamorphic features which have not been
obliterated by the metamorphism.
(2) Typomorphic: the characteristic texture produced by the metamor­
phism.
(3) Superimposed: alteration or modification textures due to later events
which are not part of the metamorphism proper.
Metamorphic rocks which have undergone deformation and crystallization
may contain evidence of various chronological relationships, e.g. crystalliza­
tion of a given mineral may be said to be:
(1) Pre-tectonic, if it took place before deformation.
(2) Syntectonic, if it took place during deformation.
(3) Post-tectonic, if it took place after deformation.
The three main kinds of typomorphic textures, in descriptive terms, are:
(1) Granular (granoblastic).
(2) Foliated.
(3) Porphyroclastic (cataclastic).
The term "granoblastic" is used in its normal sense and "foliated" means
possessing a fissility or dimensional preferred orientation, not a compositional
layering. The genetic term "cataclastic" is generally used rather than the
descriptive "porphyroclastic" or "mortar texture" and it is used here in the
normal sense of mechanically crushed or dynamically metamorphosed. The
term "cataclasis" is generally taken to indicate mechanical fragmentation
without any recrystallization; it should be realized that such an ideal process
is rare geologically and that most rocks accepted as cataclastic have evidence
of considerable recrystallization in their fine-grained matrices.