Development of cleavage in lapilli-bearing volcaniclastic rock

Tectono&sic.r,

109 (1984)

DEVELOPMENT

309

309-335

Elsevier Science Publishers

B.V., Amsterdam

- Printed

OF CLEAVAGE

in The Netherlands

IN LAPILLI-BEARING

VOLCANICLASTIC

ROCK

OTHMAR Earth

T. TOBISCH

Sciences Board,

(Received

Applied

Sciences Building,

May 25, 1983; revised version

University

accepted

of California,

Santa

Crur,

CA 95064 (U.S.A.)

June 8, 1984)

ABSTRACT

Tobisch,

O.T., 1984. Development

of cleavage

in lapilli-bearing

volcaniclastic

rock.

Tectonoph.wics,

109:

309-335.

The development known

strain

of cleavage

(Es = 0.14-2.4)

nisms of cleavage

in a suite of lithic and accretionary

has been studied.

formation

have been active during

In both lithic and accretionary directly

by r~rystal~ization

neoblasts

lapilli-bearing

of devitrified

in the matrix with increasing

to cleavage

and

Analysis

at an angle

lapilli-bearing

of these specimens

altered

volcanic

in the matrix appears glass (ash). Preferred

strain results from some combination

to the cleavage

rocks of mecha-

the deformation.

rock, the cleavage

and/or

voicaniciastic

reveals that various

followed

by rotation

to have formed atignment

of neocrystallization

into that plane

of the parallel

as the deformation

progresses. Fragments

in lithic lapilii-bearing

first stage (Es = 0 to - 0.60; dominated

by rigid body rotation

ing the rotation

of lapilli. Recrystallization

are slight at first, but increases

tion becomes

pronounced,

intragranular

movements.

lapilli perimeter,

and is accompanied These movements

and is accompanied

In the accretionary strain

rocks show two stages of rotation

Zi - IlO%,

flattening

process

with strain.

In the second flattening

development.

The

of lapilli is essentially

stage (E, z 0.60), recrystalliza-

allow the lapilli to rotate by migration

deformation.

Lap%

stage described complete,

by inter- and

of the vertex along the

axial ratios.

- Ea - 65%). Reerystaliization

active in the second

is

of the lapilli accompany-

of lapilli facilitated

rocks, lapilli show one continuous

lapilli ductile even in the early stages,of F, > 1.25. rotation

cleavage

and shape change

by a ductile

by increasing

lapilli-bearing

where 2, - 1.25 (f

during

+ ii, = 0 to - 40%; - Es = 0 to - 35%. where E, > Ez 3 E,, elongations)

stage of rotation

up to values of

is active from the start. rendering

rotate

and increase

for lithic lapilli. Once the values of strain

although

the

their axial ratios by the

their axial ratios continue

to increase

exceed with

strain, Primary - 0.70;

crystals

in both

which is accompanied In the second become

rock types also show two stages

+ E, = 0 to - 50%, - g, = 0 to - 40%), the primary stage (E, > O.fO), further

more

deformation.

by some plastic

common, At strains

and recrystallized

and

0040-1951/84/$03.00

rotation

recrystallization

fracturing,

and minor

is statistically becomes

In the first stage (E, = 0 to

undergo

minor,

an important

essentially quartz-mica

the mechanisms response

primary

crystals

tend to become

solution

can be seen in the deformation

smeared

out in the cleavage of primary

B.V.

formation.

just

mentioned

products

to the

of deformed

plane. crystals

of some of the more dense lithic lapilli. It is also relatively

0 1984 Elsevier Science Publishers

all their rotation beard

of the crystal

where Es z 1.25 (+ E, z 110%. - Es > 60%) the metamorphic

The effects of pressure etc.) and in the response

deformation,

of rotation.

crystals

(beard

formation.

well-developed

in a

310

few specimens mechanisms

which

are crystal-rich.

In general,

however,

it plays

a minor

role relative

to the other

described.

INTRODUCTION

The development of cleavage in rock has been a subject of study practically since the inception of geology (e.g., Bakewell, 1815; Sedgwick, 1835) and the literature on the subject

is considerable

(see Siddans,

1972; and Wood,

1974, for reviews). There

has been a great upsurge of detailed work on cleavage in the last two decades, often utilizing advances in technology such as the SEM. These studies have enlarged our views on the diversity

of cleavage morphology

present

in rock (see, e.g., Borrodaile

al., 1982), and in turn have called for models of increased the interaction of various mechanisms.

complexity

et

which involve

This study is concerned with the formation of cleavage in rock which did not have a pre-existing anisotropy of tectonic origin, and in which the cleavage morphology ranges from approximately spaced disjunctive to continuous (Powell, 1979). Most of the work on the formation of this type of cleavage has concentrated on slate (cf. Siddans, 1972; Wood, 1974) a lesser amount has been done on sandstone (e.g., Means, 1975; Gray, 1978), and there have been a few studies on more unusual rock types such as diamictites (Ross and Barnes, 1975) and serpentinite (Williams, 1978). To my knowledge, the only work that discusses the development of cleavage in volcaniclastic rock are the studies done on accretionary lapilli tuff by Oertel (1970, 1971) and Helm and Siddans In

the published

work,

(1971). there

is much

controversy

over

whether

rigid-body

rotation, cleavage

recrystallization, or pressure solution is the dominant mechanism during formation. Several recent studies of slates, for example, have shown that

pressure

solution

is the dominant

active mechanism

(e.g., Beutner,

1978; Woodland.

1982; Wright and Platt, 1982). Other studies have demonstrated that rotation and/or recrystallization are dominant (e.g., Oertel, 1970, 1983; Tullis, 1976; Bell, 1978). There are many interacting factors such as rock type (including composition ratio and size of mineral components), temperature, pressure, availability of fluids, and strain rate that will determine which mechanisms are dominant at any one time during the development of the cleavage (cf. Means, 1977; Knipe, 1981). In this study, I investigate the processes active as cleavage developed in a suite of lapilli-bearing volcaniclastic rocks deformed under medium-grade metamorphism. These rocks are distinct from slates in many ways, especially as they bear fragments which are considerably larger than original phyllosilicates in shales; in fact, common phyllosilicates are essentially absent from undeformed, unmetamorphosed equivalents to these rocks (Moore and Peck, 1962; my own unpublished observations). In the following pages, I first briefly describe the textural and mineralogical nature of comparable undeformed lapilli-bearing volcaniclastic rocks, and then show that

311

during

deformation,

mitant

with recrystallization

rigid-body

been active in some (especially subsidiary NATURE

rotation

and/or

flattening

are the main mechanisms crystal-rich)

specimens,

of the fragments

active. Pressure but generally

concom-

solution

has

it has acted in a

role. OF THE SPECIMENS

The deformed suite has been taken from a thick (largely monoclinal) section of volcaniclastic rock that occurs in the Ritter Range of the central Sierra Nevada, California.

These

rocks have developed

a slaty-type

cleavage

of varying

intensity

which has formed in at least two sequential deformations (Tobisch and Fiske, 1982). The cleavage is usually parallel or subparallel to the bedding. In rocks of lower strain, cleavage is defined by alignment of lapilli, while in rocks of higher strain, mineral alignment and flattened lapilli are the major components which define the cleavage. Locally, the cleavage is crenulated and folded on a small scale. Common metamorphic minerals are white mica, green to brown biotite, green amphibole, plagioclase more calcic than An 20 with varying amounts of epidote, tremolite. calcite and rare almandine garnet. In the field, the rocks have a “slaty-greenschist” look about them, but their mineralogy indicates that they fall into the lower amphibolite facies (Turner 1981). The presence of green, prograde chlorite in a few specimens also suggests that the rocks may have a more complicated metamorphic history than is immediately apparent. A detailed description of the geological setting is given more fully elsewhere (Fiske and Tobisch, 1978; Tobisch and Fiske, 1982). The quantitative strain of the deformed suite has been determined using the technique of Elliott (1970) supplemented by data on primary fabrics (Tobisch et al., 1977). The strain

magnitude

(E,) has been calculated

using

the formula

of Nadai

(1963, p. 68) where:

E,=$((F, -E2)2+(T2-E3)2+(T3-e,)2]“2 where

E, > E2 > e3, E = ln(l+

I, = length before strain. [ -1 parallel

e),

and

e = (I - /0)//o.

E,, E2, and ix, represent

where

the elongations

I = length

after

[ + ] or contractions

to the major axes of the strain ellipsoid (X > Y > Z). and correspond and eJ3, respectively of the finite strain tensor, where: ell, ez2> e,, = ];i;

;;;

and to

!;;I

To facilitate plotting the data on a Hsu diagram (Hossack, 1968) Lode’s parameter (Lode, 1926, p. 932) which is a measure of the symmetry of strain ellipsoid, was calculated using the formula: V=

2E2 - E, - E2 “, - E3

312

Each specimen orthogonal

was slabbed

planes,

were measured obtain

on each plane

the three-dimensional

or not

the cleavage

ellipsoid

of non-rotational because

mineral

parallel

therefore,

ellipsoid

such

and fragment

three

in the rock

the cleavage parallels

plane

of the strain

are considered observed

the (X) direction,

in fold

of measurement

to the principal

approximates

to

suite of specimens,

The planes

to correspond

to

over whether

small for most cases

to the present

in the areas sampled.

lineation

plane

discordances

1982), and are probably

are considered

that

to the (XY)

1967). The large discordances

et al. (1982) are not applicable

no folds are present

each specimen, strain

(Ramsay,

markers

While there is controversy

1976), angular

strain (Ghosh,

of specimens,

of the strain

et al., 1977). These data were combined

lies precisely

1974; Williams,

strain

by Hobbs

(Tobisch

strain ellipsoid.

plane

(cf. Wood,

be small for rotational limbs

on at least two, and in the majority

and the shape and orientation

in

planes of the

the (XY)

plane,

the

and the (Z) direction

is

normal to the cleavage. For reasons already discussed in detail (Tobisch et al., 1977), the value of (2,) quoted for most specimens closely approximates the whole rock strain. In this study, I present additional information on a suite of samples which include a wide range of strain magnitude and fragment/mineral type so that the conclusions would be applicable to lapilli-bearing volcaniclastic rocks in general. The undeformed specimens were taken from environments closely comparable to the deformed specimens, that is, the continental or near-continental arc environments of Oregon, Washington, and Japan (Tobisch et al., 1977). Petrographic study of the fragment

type and composition

to the undeformed ing the amount

suite, a similarity

of rotation

components

in the deformed which is important in the deformed

suite shows strong similarity to establish

when consider-

rocks have undergone.

In studying the development of cleavage, it would be ideal if all the specimens lay on one deformation path. This would assume that rocks showing higher strain have Lithic

Lapilli

Accretionory

Lapllli

+u ‘0.5

7

‘/

.O

‘%

?.O

Fig. 1. Hsu

diagram showing

determined in the deformed

the ranges of intensity

(E\) and symmetry

(P) of the strain

specimens (see Table 1 and text for explanation of symbols).

ellipsoid

as

313

passed

through

assumption

a comparable

stage occupied

may not be always

1967, p. 185; Means,

correct,

the conclusions are considered cause

reached. in terms

PRIMARY

FABRICS

one to make (Ramsay,

to be outlined.

appears

and is not detrimental

are distinct

mineralogically

and

texturally,

IN UNDEFORMED

to to

and

have

ROCK

rock

Mineralogically and texturally, these volcaniclastic rocks are andesite-dacite suite well-known from continental arc environments, of comparable

specimens

have been

1963; Fiske et al., 1963; Fiske and Matsuda, lapilli are porphyritic

specimens

symmetry

to the deformation.

Lithic lapilli-hearing

acteristics

this

In the paragraphs below, the various aspects of the rocks of lithic-lapilli and accretionary lapilli-bearing rocks. be-

types

differently

While

in Fig. 1. the deformed

path. This range in strain

of the stages of cleavage

the two rock

responded

it is a reasonable

1976, p. 28). As illustrated

do not all lie on one deformation exist independent

by rocks at lower strain.

described

fully

typical of the and the char-

elsewhere

(Fiske,

1964; Peck, 1964; Kay, 1970). Lithic

most commonly glass fragments, to coarsely crystalline, non-vesicular

ranging from cryptocrystalline to vesicular, with lesser amounts

to of

rock fragments, silicified volcanic rock fragments and pumice (Tobisch et al.. 1977, table 1). The degree of alteration of the fragments varies from being essentially

50

a. Lithic

r

5Or

Lapilli

20

IO

OL

.I0

Fig. 2. Plot of angle between

the apparent

surfaces

normal

by Nadai’s sional plotted.

shape

(0)

vs. the shape

long

to bedding.

equation without

index

axis of fragments The shape

for strain implication

I

+-if+

(Nadai. that

.30

.20

.I0

SHAPE

Lapilli

A

OL

.30

.20

b. Accretlonary

INDEX in the undrformrd and

the trace

specimens. of bedding

Angle

index

is the three-dimenslonal

shape

1963,

p,

it is used

68). only

it has evolved

from

in this case a sphere.

(6’) represents

as averaged

Triangle

from

of the fragments to represent

= mean

the angle

two orthogonal

value

as descrihcd three-dimenof specimens

314

absent tion.

to fragments The matrix

altered

to varying

amphibole

which show different originally

and/or

degrees,

relative

ratio of the specimens investigation,

and/or

of quartz, altered),

zeolitic

fine ash) which plagioclase,

alterais now

sometimes

and very fine fragments.

varies from 35 to 90% (Tobisch

et al.,

are well-indurated. the primary

of the three-dimensional to bedding

crystals

(fresh to completely

2). Most samples

In the original in terms

stages of chloritic

of glass (probably

and contains

pyroxene

The fragment/matrix 1977, appendix

consisted

shape

(e/2c* shape, Tobisch

fabric of these specimens of the fragments

and

et al., 1977) and was plotted

plot (F vs. 2a) to show the shape characteristics

of the fabric.

was analyzed

their

orientation on an Elliott

The present

study is

concerned principally with the angle (8) that the apparent long axis of the fragments makes with bedding (or cleavage in deformed rocks) as seen in the slabbed surface, and the data are plotted to bring out this angular relationship. Figure 2a shows the negative slope clearly, and indicates the expected, this is, the more ellipsoidal the fragment, the greater the tendency to show a preferred orientation in the bedding plane (Elliott, 1970). Accretionary The nature

lapilli-bearing and origin

rock of accretionary

Peck (1962). The undeformed

samples

lapilli

have been described

by Moore

used in this study come from either nearby

and or

comparable formations (e.g., Swanson, 1966; Kay, 1970). These rocks are homogeneous, mostly well-indurated, and consist of glass (fine ash) which shows varying amounts of glass shards, and usually displays various stages of devitrification and alteration. In this matrix are set broken crystals of plagioclase. quartz, K-feldspar (?), and rarely lithic lapilh and highly altered pyroxene crystals. Zeolites and chlorite are sometimes present in vugs in lapilli. X-ray analyses on comparable rocks by Moore and Peck (1962). indicated the presence of mordenite. montmorillonite. and clinoptiolite, and these minerals could well be present in the suite studied, although they were not identified

microscopically.

The accretionary lapilli are commonly zoned, containing a core often indistinguishable from the matrix, followed by successive shells of fine ash (cf. Moore and Peck, 1962). The shape and orientation of the lapilli relative to bedding show a similar distribution to those of the lithic lapilli (Fig. 2b), although the angle (8) is consistently smaller for any given value of the shape index (Fig. 2b). This reflects the easier compaction of the accretionary competent lithic lapilli.

lapilli

during

loading

relative

to the more

Primary fabric characteristics It is necessary to define amount of angular rotation

the original fabric of the rock in order to estimate the that the rock components undergo during deformation.

315

Studies

of initial

(1979)

indicate

bedding.

that

Re-analysis

accretionary fabric

fabrics in undeformed

lapilli

(samples

lapilli

of the data

on (8) from Tobisch

(but

of the faces analyzed

showing

moderate,

point

show

considerable

volcaniclastic

variation

rock,

structure

a moderate

relative

to strong

lapilli-bearing

to slight imbrication,

imbrication

to that

imbricate

rock,

three-

the remainder

(Fig. 3). While the available

in original

there is still ample

and Boulter

et al. (1977) indicates

of lithic

show insignificant

and in one case, strong

out that

undeformed

not always)

an imbricate

Fig. 3). Of the samples

quarters data

display

rock by Seymour

accretionary often

l-4,

volcaniclastic

fabric

latitude

can

be expected

in

to make estimates

of

angular rotation in their deformed equivalents which are geoiogically meaningful. For example, one can determine the spatial orientation of the cleavage relative to bedding,

and

hence

can at least constrain

the possible

variations

of the primary

fabric to the strain ellipsoid. In addition, Holst (1982) has analyzed fabrics resulting from superposing various positions of the strain ellipsoid on primary fabrics and considered the errors in strain which may result from varying initial conditions. Since the majority of the lithic lapilli-bearing rocks show insignificant to slight imbrication, it may be possible by careful analysis to detect those specimens showing moderate or greater imbrication and make appropriate adjustments (Seymour and Boulter, 1979). In the present bedding.

group

If one accepts

of deformed

specimens.

the generalization

cleavage

is essentially

that cleavage

approximates

parallel

to

the (XY)

plane of the strain ellipsoid (cf. Wood, 1974; Hobbs et al., 1982), we will have some constraints on the orientation of the strain ellipsoid relative to bedding and the

Beddina

12345678

9

IO

SAMPLE

Fig. 3. Frequency negative

diagram

sides relative

showing

to bedding

an estimate

faces slabbed

normal

of each bar is the number ing, and samples

S-20

to bedding,

of fragments

13

14

of the percent samples

is the difference measured.

Each

and each bar represents

measured

lithic lapilli-bearing

12

-

I5

I6

17

I8

19

20

NUMBER

in the undeformed

calculated by Sk = A Nr,” /N, x 100, where A!& values. and N, the total number of fragments orthogonal

II

Trace

skewness analyzed.

rock.

towards skewness

in number sample

of positive

has been

positive

or

has been vs. negative

measured

on two

data from one face; the value on top

on that face. Samples

volcaniclastic

of (0) Percent

1-4 are accretionary

lapiili-hear-

P

0.39 0.42

0.53

a.53

0.59

0.78

0.78

523 612

532

164

630

614

519

0.19

0‘68

0.65

0.45

-0.13

0.89

0.64 0.19

- 0.43 0.48

0.46

0.58

0.63

0.72

0.76

557

528

522

55

549

I”

-’

.I-

-*

‘-

_

and symmetry

-0.X6

0.54

0.69

0.12

0.X-t

OS3

crystals

2.40

I .40

1.40

1.28

0.94

0.80

1.0

1.0

1.0

2.4

3.5

3.9

relative to matrix:

.I

-’

--

-

-

^’

surfxr or in thin-section

of fragments

0.26

0.74

0.71

0.28

0.26

OA4

its seen on a slabbed

(9,) ix the percent

make with the cleavage

F/M

703

697

x1

10

562

544

suite *

respectively:

31.0

30.0

34.u

36.0

30.0

3x.5

rocks; deformed

of the strain ethpsold,

5.5

7.8

7.2

6.1

5.7

12.0

volcaniclastic

to the angle 8’ that the long axes of lapilii and primary

* E, and Y are the magnitude

0.33

61-l

-lapiili uolcaniefasrrc rocks

0.21 0.35

401 28

A tcretionary

0.35

674

-_ Lithic I kprpilliucdeaniclasti~ rocks DW-1 0.14 1.0

i;,

Sample

No.

in lapilli-bearing

1

Results of measurements

l’Am.E

faer text).

lap 8’ and xl1 8’ refer

29.0

28.0

31.0

31.0

29.0

30.0

317

primary show

fabric.

In Fig. 2a, the plots from 15 samples

a mean

lapilli-bearing

value

in angle

(8)

of 34’,

while

of lithic lapilli-bearing four

samples

rocks

of accretionary

rocks (Fig. 2b) show a mean value of 23”. These mean angles will be

used to estimate

the mean

given the additional CHARACTERISTICS

angular

rotations

data presented

of the fragments

in the following

OF THE DEFORMED

during

deformation

sections.

ROCKS

Most of the original mineralogy in these rocks has been destroyed by deformation and metamorphism, although vestiges and often well-preserved primary volcanic texture and crystals are still identifiable. There is an increasing degree of textural reconstitution shown

of the rock

by the mineral

with

progressive

assemblages,

strain.

however,

The

is essentially

grade

of metamorphism

constant

specimens regardless of the intensity of strain. Observations on the matrix in thin-sections show an increasing

for the suite of preferred

tion of minerals (mostly micas) developing in rocks with increasing neoblasts formed from what was probably a devitrified and/or Because estimate

These glass.

of the heterogeneity of the fabric on a microscopic scale, it is difficult the percentage of neoblasts that formed parallel to cleavage as compared

those which formed

at an angle and were later rotated

of mica in the matrix preferred showing Oertel,

orienta-

strain. altered

of weakly deformed

samples,

into the cleavage.

however,

sometimes

orientation suggesting that at least some aligned mica greater strain and higher preferred orientation underwent 1970,

1983).

In addition,

not

much

evidence

to to

Neoblasts show little

in specimens rotation (cf.

was observed

that

would

support pressure solution as being a widespread mechanism in the formation of matrix cleavage. From the evidence at hand, therefore, it is assumed that recrystallization * and rotation are the dominant processes of cleavage formation in the matrix, with pressure solution active in some (especially crystal-rich) samples. In view of the above, I will discuss cleavage development of the behavior crystals,

making

of lithic lap&, reference

priate.

Various

LITHIC

LAPILLI-BEARING

In the lithic angular

rotation

quantities

accretionary

measured

lapilli-bearing

1983). as well as by neom~neralization from breakdown

specimens

plagioclase/quartz only where approare given in Table

fragments

the formation

is preserved during

a considerable

of neoblasts

(e.g. volcanic

syntectonic

where the neoblasts

of feldspar).

show

the first stage of deformation.

is used herein to include

to clay which is then recrystallized

mica forming

in the deformed

rocks,

phase in which the sheet silicate framework processes

and primary

active in the matrix

1.

ROCKS

that occurs during

* The term recrystallization

lapilli.

to the processes

in these rocks mostly in terms

amount

of

two-thirds

that form from a similar

glass altering

metamorphism

form directly

Nearly

by primary

volcanic

to white mica; cf. Oertel.

from an unlike phase (e.g. white

of the values for (8) in the undeformed

specimens

are above 30’ (Fig. 4). If we take made with the f XV)

the mean value (& = 34O) as the angle that the average fragment plane

of the strain

ellipsoid

more than three-quarters strain

reached

extension

at the start of the deformation,

(26”) of the total rotation

a value of ES- 0.50-0.60

( + E,) of 35-40%

(-

then it is apparent

31”) was accomplished

that before

(Fig. 4). This value of E, is comparable

in and contraction

( - Ej) of 30-35%

normal

to

to cleavage,

given a mean value of symmetry (v) for the suite of about + 0.5 (cf. Fig. 1). The evidence for the mechanism of the rotation is shown in several specimens. those that show only weak deformation by the partial

alignment

This is especially

(cf. Table

of fragments;

true in specimens Lithic

textural

reconstitution

Of

is largely defined

is not well-developed.

with a high fragment/matrix

ratio, although

less

Laoilli

.60

.30

l), the cleavage

.90

1.2

1.5

Es Fig. 4. Plot of angle (8) or (8’) for lithic lapilli vs. the strain

Angle (l?) is described of fragments vertical

in the caption

and the trace of cleavage

bar for each specimen

while the filled square indicates

on the YZ and X2 planes

gives the average

represents

magnitude

the average

that only two planes were measured

value between

the two faces. Sample

Horizontal

bar shows strain error determined

measured,

and

undeformed

specimens,

filled triangle

in the deformed

specimens.

long axis Ends of

with no vertical

bar

for strain, one of them being the XY plane where (~9’) could

three

were

in each specimen.

the apparent

(0’) on the YZ (high value) and XZ (low value) planes

not be determined. planes

(Es) determined

of Fig. 2, and angle (19’) is the angle between

the relation

i,,

for those specimens

= EXr + ErZ could be calculated. = mean value of undeformed specimens.

in which strain on Open

circles =

so for those with lower ratios. In the latter specimens, orientation

of mica neoblasts,

in the matrix

but still significant

shows little or no preferred

show formation

of neoblastic

tion which either partially

quartz

the matrix can show preferred

areas exist in the rock where mica

orientation.

Some specimens,

or green amphibole

or completely

surround

lapilli

lacking

however,

preferred

orienta-

(Fig. 5a,b). Such textures

can be created where closely packed fragments are bodily rotating, impinging upon each other over different surface areas, thereby forming zones of high contact (high pressure) and low or no contact (‘low pressure) in the process. Localiy. fragments may temporarily “float” (at least in two dimensions)~ while surrounding fragments are part of the load bearing framework of the rock. The resulting low pressure areas as well as the interstices between fragments (Fig. 5~) are likely sites for deposition of new material without preferred orientation. This textural

evidence

suggests that the fragments

tion by rigid body rotation,

stage of cleavage development Evidence for pressure solution, crystal-rich

samples

have responded

and that this mechanism

to the deforma-

is predominant

during

the first

in these rocks, that is, up to ca. Z, - 0.50-0.60. aside from solution channels found in uncommon

or the deposition

of quartz

described

above

apparent. tion and

In addition, neoblast formation is often spottily developed. pressure solution, therefore, have not contributed much

definition

of the cleavage

fabric itself during

(Fig.

5)

is not

Recrystallizato the actual

this stage in its development.

The transition from first to second stage of cleavage development is not sharply defined, but two observations are noteworthy: (a) there is a break in slope of data that the amount of angular points shown in Fig. 4 at about E, - 0.60, indicating rotation of fragments per increment of strain is considerably reduced beyond that point, and (b) textural evidence for rigid body rotation as described earlier is lacking. The second stage of cleavage formation, that is Es > 0.60, is characterized by two main

features.

The first of these is the increasing

shown by the progressive green to brown termolite-actinolite neoblasts strains

appear

biotite

showing

to nucleate

preferred

of neoblasts

orientation.

This is

of white mica and

In the more calcareous

alignment

first at lower strains

As the recrystallization

tends to coarsen

role of recrystallization.

in the occurrence

also tend to show preferred

in the fragments.

phic texture

increase

as strain

in the matrix (and strain)

and show a developing

increases.

rocks. These

and then at higher

increase,

the metamor-

degree of preferred

alignment

of its components. Neoblasts of white mica generally show the higher degree of preferred orientation, while green or brown biotite show it to a lesser degree. Locally biotite can occur with poor preferred orientation, or in clots with quartz and/or epidote with essentially no preferred orientation, indicating the heterogeneous nature of the deformation on the microscopic scale in some specimens. The second feature is the change in shape of fragments. With increasing recrystaliization and strain, fragment axial ratios show a marked increase. While in some thin-sections one can observe a few fragments with quartz- or quartz-mica fiiled

320

fractures

oriented

increase many

at a high angle

in axial ratios lapilli.

volcanic

tiny neoblasts

quartz

recrystallization ments.

and

to the long axis of the fragment,

takes place without of quartz

plagioclase

and crystal

These observations

can

plasticity

and feldspar show

(?) form mosaics.

undulatory

have facilitated

involving

intergranular

and

apparent

angle

gradually

decreases

suggested

by Elliott (1970, p. 2227).

the long

intragranular

in most cases,

deformation.

extinction.

while larger that

of the frag-

shape by a flattening

movements.

axes of the fragments

Within

indicating

the deformation

suggest that the lapilli are changing

process

between

any sign of brittle

and

In addition, cleavage

the

(angle

8’)

is most likely an apparent (rather than a solid) body rotation with the vertex appearing as a “ ‘bump’, taking up successive material points around the perimeter” of the fragment with increasing strain such as

Fig. 5. Photomicrographs

(L) completely

surrounded

of bar is 1 mm. Crossed

(Fig. 4). This rotation

showing

textural

by neoblasts polars.

b. Lapilli

characteristics

of quartz

of lithic lapilli-bearing

rocks. a. Lithic lapillus

(q). Sample DH-1. See text for interpretation.

(L) partially

or wholly surrounded

by unoriented

Length

neoblasts

of

(A). Sample DH-1. Length of bar 1 mm. Uncrossed polars. c. Low pressure zone between lapilli (15) filled with neoblasts of quartz (4). Sample 674. Length of bar is 1 mm. Uncrossed green

polara.

amphibole

322

Lastly, pressure

solution

cleavage

development

difficult

to distinguish

and opaque Figs.

residue

11 and

mechanism

appears

between

extremely

crystal-rich, flattened,

which could be left in presumed

12~). In any

described

ACCRETIONARY

to be an active mechanism

in some, especially

event,

this mechanism

in the second stage of

samples.

However.

opaque-rich pressure

pumice

solution

is subsidiary

it is often fragments

channels

(see

to the two main

above.

LAPILLI-BEARING

ROCKS

The behavior of the accretionary lapilli-bearing rocks during cleavage development is considerably different from that just described. This is mainly because the

Fig. 6. Photomicrographs a. Accretionary much

finer grain

showing variations matrix.

size. Length

only barely

ends of lapilli.

textural

similarity

characteristics between

of several samples

core of lapilli and matrix,

of bar is 2 mm. Uncrossed

distinguishable

Length

in character Length

showing

lapilli showing

polars.

rims that are in part gradational

of bar is 2 mm. Uncrossed

polars.

of the rims, which often show coarser

of bar is 2 mm. Uncrossed

polars.

Sample

Sample

bearing

accretionary

lapilli.

and a multilayered

rim of

544. b. Accretionary

with the matrix. 703. c. Accretionary

grain size and different

Sample 549.

Arrows

lapilli point

to

lapilli showing

composition

than the

324

ductility

contrast

pearance

of the accretionary

undeformed

between

equivalents,

lapilli

and

matrix

some noteworthy show

While

variations a distinct

exist. In several mineralogical

the ap-

akin to their and

samples,

from the cores of the lapilli and the matrix (Fig. 6a,c), while in others the rim, core, and matrix is slight and very gradational

the rims of the lapilli are essentially

absent,

grain

for

difference

between

lapilli

negligible.

rock is generally

the

a few specimens,

of the

is often

in the deformed

example, distinction

rims

lapilli

size

(Fig, 6b). In

being defined

only by

slight color changes. These differences suggest that not all lapilli may have responded by exactly the same mechanisms during deformation. There are not many obvious clues to help decipher the mechanisms active during cleavage formation in these rocks, but microscopic evidence available from this suite as well as other studies suggests that rotation and recrystallization are predominant (cf. Oertel. 1970, 1971, 1983; Helm and Siddans. 1971). The accretionary lapilli which show differences between rim (or total lapilli) and matrix (Fig. 6c) may have had sufficient ductility contrast to have undergone However, it would appear that most lapilli underwent

some rigid body rotation. an apparent rotation akin to

that described for lithic lapilli where with increasing strain, the vertex of the lapilli migrates like a “bump” around the perimeter, effecting an angular change (8’) of the long axis relative

to cleavage

(cf. Elliott,

1970). For such behavior

to take place. the

rocks must have been in a relatively ductile state throughout deformation. This is likely to have been the case considering the initial condition of their undeformed equivalents described earlier, since it is probable that even prior to deformation, secondary processes had caused devitrification and alteration of the glass which made up most of the rock (cf. Best, 1982, p. 70). Even slight increases in temperature and pressure in such fine-grained homogeneous from the start of deformation onward, thereby deformation.

rocks could initiate recrystallization rendering the rock capable of ductile

Of two samples showing weakest deformation (Table 1). the least strained shows slight alignment of groups of exceedingly small (optically unidentifiable) neoblasts within

the lapilli. The matrix

however, relatively indicative

(rich in neoblasts

of quartz-feldspar

strain, neoblasts of white mica and to a lesser degree abundance and show an increasing degree of alignment gradually

and minor

mica),

shows scant signs of preferred orientation. The other specimen shows clear alignment of tiny neoblasts of white mica in both lapilli and matrix. of recrystallization taking place during deformation. With increasing

change

shape and their long axes approach

biotite gradually increase in (Figs. 6 and 8) while lapilli parallelism

with the cleavage

(Figs. 6 and 7). In the sample showing greatest strain (E, = 2.4) abundant neoblastic white mica seen in sections parallel to the (XZ) plane shows extremely high preferred orientation and coarsening of grain size compared to rocks with lower strain (cf. Figs. 6 and 8). The angular changes that the accretionary lapilli go through during the progressive deformation are somewhat less than those of the lithic lapilli, but are still

325

ACCRETIONARY

LAPILLI

Al H

20

15

i

IO

4-f ~

T-r

I .50

Fig. 7. Plot of angle specimen.

Symbols

1.0

I 2.0

1.5

(8) or (!I’) for accretionary

lapihi

vs. the strain

magnitude

,

I, 2.5

determined

in the

same as in Fig. 4.

Fig. 8. Photomicrograph

of matrix

in highly deformed

Note grain size of mica (m) which is somewhat the high angle the long axes of the plagioclase ( - E1) value of 84% parallel

and recrystallized

greater

accretionary

(P) makes with the cleavage,

to the Z axis. Samples

lapilh-bearing

rock.

than that shown in rocks of lower strain. Also note

703. Length

even though

this sample has a

of bar is 0.2 mm. Crossed

polars.

3’6

substantial

(Fig. 7). Insufficient

of the total amount 7, however. 15” rotation,

primary

of rotation

indicate

lapilli

that lapilli

and probably

data does not warrant

underwent

during

in the most deformed

substantially

making

deformation. specimens

more considering

data (Fig. 7). The break in slope at ca. E5- 1.25 (equivalent

an estimate Points

in Fig.

underwent

at least

the spread in initial

fabric

to + E, - 105-l 10% and

indicates that essentially no statistical rotation of lapilli takes place -E1 - 60-65s) at greater values of strain, although their axial ratios continue to increase, INFLUENCE

OF STRAIN SYMMETRY

ON FRAGMENT

ALIGNMENT

In previous pages, the degree of alignment of both lithic and accretionary lapilli has been wholly attributed to the magnitude of strain (E,) undergone in these rocks. While this parameter is clearly the principal factor involved, some data indicate that strain symmetry (v) may also be a contributing factor. In Table 1, some pairs of specimens 164/532, relatively

show comparable

values

of (2,) but differing

values

of (v) (e.g. pairs

13/681, 168/669). In any given pair, fragments in the sample with a high component of constriction symmetry show smaller average angles (lap

6’) with the cleavage. While this may not be true for every case, there is a tendency for strain ellipsoids with a significant component of constriction to produce greater extension in one direction, and thereby ments for any given value of strain (F,). RESPONSE OF PRlMARY

CRYSTALS

produce

greater

DURING CLEAVAGE

angular

rotation

of frag-

FORMATION

For the most part, deformation of primary crystals in this suite of volcaniclastic rocks did not contribute significantly to the development of the cleavage. At high strains, however, the crystals begin to contribute to cleavage morphology. The primary crystals surviving metamorphism in size and shape are quartz, plagioclase and rarely K-feldspar. These minerals have responded to the deformation by a number of mechanisms, namely rigid body rotation, plastic deformation (shown by undulatory

extinction),

pressure

solution,

havior of the primary neously.

crystals

RIGID-BODY

OF PRIMARY

ROTATION

in both

recrystallization, kinds

and fracturing.

of rock will be considered

The besimulta-

CRYSTALS

Measurements of the angle that the apparent long axis of primary crystals make with bedding (8) in the undeformed samples averages about 40 (Fig. 9). In the deformed equivalents, the overall small amount of rotation is well-demonstrated, and indicates that the primary crystals in these rocks have undergone much less rotation (ea. lo”, see Fig. 9) than the lithic and accretionary lapilli {see Figs. 6 and 8). The distribution of points in Fig. 9 suggests that the rotation of primary crystals

Gr

PRIMARY

CRYSTALS

I Anglea 8

,

01

I

I

I

.50

Fig. 9. Plot of angle (0) or (0’) for primary specimens.

Open

circles = primary

filled squares = primary

I

I

1.0

crystals

crystals

crystals

vs. the strain

in undeformed lithic

in their deformed equivalents.

Angle Fig. 10. Histograms respectively. distribution

of angle

a relatively

and are roughly

crystals

showing

relative

an increasing

with increasing

proportion

symmetrically

et al.. 1977. appendix

of primary disposed

to bedding tendency

strain.

b-d.

it.

8’ and deformed

2). showing Deformed

for long axes of primary

maintain

go0

’

Even with values of (-

crystals about

trace.

rock.

8’

o”

J-3 (Tobisch

m the

lapilli-bearing

are same as in Fig. 4.

Angle

specimen

large

(E,) determined

accretionary

(0) and (0’) shown by primary crystals in undeformed

to the cleavage

cleavage.

magnitude

’ Angle

8’

of long axes of primary

however,

2.5

o”

a. Undeformed

parallel

’

8

672, 631. and 168, respectively) themselves

and

I

I

2.0

Other symbols

_cjoo Angle

I

I

1.5

rocks.

a relatively specimens crystals

even (Nos.

to align

Es) as high as 6R% (d).

a significantly

high angle

to the

takes place largely rotation

during

was accomplished

(equivalent

the first stage of cleavage by the time the strain

rocks during

such that most

reached

to +F, - 50% and - F3 - 40%). These values of strain

than those shown by the lithic lapilli-bearing development

development,

magnitude

ca. Z, - 0.70

are slightly

higher

the first stage of cleavage

(Fig. 4).

There is only a slight tendency

for the apparent

long axes of crystals

to be aligned

parallel to bedding in the undeformed rocks (Fig. 10). In the deformed rocks, with cleavage forming parallel or subparallel to bedding, there is a slight to moderate realignment of crystal long axes toward cleavage as the rock reaches high strain (Figs. lob-d). In a number of samples, the fabric in the matrix is very well-devrloped, yet some primary crystals show only minor amounts of apparent rotation toward the cleavage even at higher strain (Figs. 8 and 11). This is geometrically comparable

to the behavior

Fig. 1 I. Photomicrograph f P) maintaining

of “old

of highly strained

a high angle to the cleavage

micas”

in slates (Beutner.

lithic lapilli-bearing (inclined

indicate

residue left from solution developed

active in the matrix

in most samples.

primary

20” to right) even though

value of 62% (sample 679). Dark lines f R) may represent or poorly

rock showing

1978: Mancktelow,

highly

flattened

in this specimen.

Length of bar is 2 mm. Crossed

pumice;

although polars.

plagioclase

crystals

the rock shows an alternatively.

such evidence

( - Fl) it may

is lacking

329

1979; Weber, retain

1981). as well as quartz

crystals

in siltstone

(Roy, 1978) which often

high angles to the cleavage.

Ghosh

and Ramberg

(1976) have investigated

the reorientation

of rigid inclusions

in a viscous matrix. They found that the degree of alignment of rigid inclusions parallel to the “cleavage” is a function of their original orientation, their axial ratio, and the relative higher

rates of pure shear to simple

axial ratios

of inclusions

shear tend to align the inclusions inclusions

remain

oriented

(e.g. crystals)

shear during along

the deformation.

While

with a given value of simple

at small angles to the “cleavage”,

at a high angle to the “cleavage”

suitably

oriented

even after considerable

deformation (cf. Ghosh and Ramberg. 1976, figs. 23-28). It is not possible to know the relative rates of pure shear to simple shear that may have affected the volcaniclastic rocks on a microscopic scale. However, the orientation of primary crystals in the undeformed volcaniclastic rocks studied show a

Fig.

12. Photomicrographs

undergone crystals neoblastic

by primary of plagioclase quartz

epidote-chlorite

of textures plagioclase

(P) showing

in the deformed

with increasing some undulatory

strain.

rocks Polars

extinction

illustrating

in Fig. 5a,b (sample

the fractures

is surrounded DH-1).

transformation

in all examples.

and fracturing,

(4). The small lapilli (L) below the plagioclase (CC), much like that illustrated

the gradual

are crossed

Length

a. Large filling with

by quartz

(4)

of bar is 1 mm.

and

Fig. 12 (cont.). quartz/mica

b. Relatively

(WI). Length

of bar

partition

made

pumice.

Sample

plagioclase Length

large primary

beards (h). Feldspar is 0.5 mm.

up of neoblasts

is starting Sample of quartz

crystal

of perthitic

to recrystallize

672. c. Plagioclase

and beards

feldspar

(4). Dark stringers

13. Length of bar is 0.5 mm. d. Extensive

(P). the partitions

consisting

(p/) showing the development

feldspar

as shown by the presence

(P) undergoing

(d)

probably

boudinage.

breaking

of neoblasts

of neoblasts

of quartz

represent

boudinage. highly

the

flattened

up and recrystallization

and mica (qm).

of

of mica

Sample

of 310.

of bar is 1 mm.

relatively high degree of randomness (Figs. 9 and 10a). This geometry, along with a component of simple shear would provide the conditions necessary for suitably oriented crystals to undergo substantial rotation as the cleavage formed, while leaving crystals in orientations less propitious for large rotation at a high angle to the cleavage at the end of the deformation. The above mechanism is distinctly different from those called upon to explain crystals at a high angle to the cleavage in slates where pressure solution has been active (e.g. Beutner, 1978) or in limestones and mylonitic gneisses, where the obliquely oriented crystals are newly recrystallized (Schmid et al., 1981; Simpson 1983).

332

OTHER MECHANISMS

OF CRYSTAL

In the lithic lapilli-bearing lower strains by plastic

during

rock, there is ample

evidence

of crystal

the first stage of cleavage development,

deformation

of larger crystals

DEFORMATION

as indicated

in crystal-rich

by undulatory

samples

E, < 0.70) pressure solution begins quartz/mica forming in pressure

primary

extinction,

plasticity.

crystals

At

deform

with some fracturing

(Fig. 12a). At somewhat

higher strains

(but

to affect certain crystals as shown by beards of shadows of the crystals (Fig. 12b). A small

percentage display signs of recrystallization ment of quartz/mica partitions. Boudinage

or fracturing with concomitant developof primary crystals is rare. When strain

beards become more comexceeds E\ -0.70(+& -50%. -E3 - 40%), quartz/mica mon, plagioclase is more frequently recrystallized. and tends to show breakup and boudinage (Fig. 12~). In excess of Es - 1.1-1.2 (+E, - 100%. -Ej - 60%) the primary crystals can contribute to the cleavage as the above mechanisms tend to distribute

the crystals

12d). As might

and their metamorphic

be expected,

parallel

to the (XZ)

toward

a constriction.

products

this characteristic

plane of the strain ellipsoid,

into the cleavage

is more pronounced especially

plane (Fig.

in the sections

when the symmetry

tends

CONCLUSIONS

Volcaniclastic rocks commonly go through an involved pre-deformation history. and the resulting primary textures and compositions show much variation. Subsequent deformation and metamorphism of these often heterogeneous rocks involves a variety of deformation mechanisms. Such complexities make precise correlation between the amount of strain and mechanisms of cleavage formation difficult. In the present suite, the predominant mechanisms active show some lithologic control. In rock-bearing lithic lapilli, rigid-body rotation (and perhaps some neoblasts of mica in the matrix) up to strain values of about E, - 0.50-0.70 the first stage, recrystallization rich in primary

crystals.

0.60-0.70) recrystallization tion of lapilli is no longer

- 35-50%,

-F3 - 30-40%).

is minor but it starts to become significant

of the first stage is approached. rocks

(+e,

of lapilli and primary crystals is the predominant mechanism

Effects

of pressure

In the second

stage

solution

are minor

of cleavage

Early in as the end except

formation

in

(2, >

becomes the predominant mechanism. Rigid-body rotaactive, but the lapilli accomplish the remainder of their

rotation by a ductile flattening process facilitated by inter- and intragranular movements. The primary crystals appear to undergo no further rotation (statistically), but instead show fracturing, boudinage, and recrystallization, and can become smeared out in the cleavage planes at very high strains. Pressure solution continues to contribute to the cleavage formation to a subsidiary degree except in crystal-rich samples, where it is more significant. The accretionary lapilli-bearing rocks, do not show the stages of cleavage develop-

333

ment that are evident

in the lithic lapiili-bearing

mineratogy

glass)

(altered

recrystallization rendering

with

it capable

Recrystallization at an angle dominant

of this rock

even

slight

to the cleavage

cleavage

increases

of deforming

of the originally forming

rock. The homogeneous,

is considered

to be easily

in temperature

ductilly

and

followed

pressure,

to

thereby

at very early stages in the deformation,

glassy rock to form neoblasts by rotation

mechanisms

fine-grained susceptible

into

of mica parallel

that planar

active throughout

its strain

to or

surface

are the

history.

Accre-

tionary lapilli also progressively change shape and orientation by ductile flattening as mentioned above, while primary crystals behave in much the same way as they do in lithic ~apilli-bearing rock. By ca. Es - 1.25 (+E, - 110%. -T, - 65%). no further rotation of lapilli takes place, although the axial ratios of the lapilli do continue to increase with strain. Pressure solution is of minor importance except in crystal-rich rocks. The somewhat complicated nature of cleavage development in this suite of rocks is probably typical of heterogeneous rock assemblages. Such complexity suggests the need for detailed investigations of cleavage mechanisms in all rock types, nature and development of cleavage is to be thoroughly understood.

if the

ACKNOWLEDGEMENTS

Illustration preparation was financially assisted by a Faculty Research Grant from the University of California, Santa Cruz, for which I am grateful. Tim Byrne initially

made

which prodded

some astute

microscopic

observations

me to start on the present

on a few of these specimens

investigation.

Comments

and anonymous reviewers greatly aided in clarifying the original are due to Suzanne Harris for typing the manuscript.

by Mel Friedman manuscript.

Thanks

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