Myron G. Best. Igneous and metamorphic 2003 Blackwell Science

Подождите немного. Документ загружается.

(energy) is required to create surfaces by breaking a

volume of rock or magma, clasts that have the

largest surface-to-volume ratio, namely, ash and

smaller lapilli, can only be created by the most en-

ergetic explosive processes. Blocks are angular

clasts produced by lower energy fragmentation of

solid material, such as pieces crumbled from steep

margins of rigid extrusive lava ﬂows and domes.

Streamlined bombs are blobs of solidifying low-

viscosity maﬁc magma shaped aerodynamically

during ejection from an explosive vent.

2. Composition: Volcaniclasts can be (a) single crystals

and crystal fragments that are typically of ash size;

(b) rock fragments, or lithic clasts, of polygranular

mineral aggregates that are typically lapilli and

block size; (c) fragments of melt that quenched to

glass, or vitroclasts, that are of any size; however,

the term is usually applied to ash-size particles.

3. Heritage: Juvenile clasts, also referred to as cognate

or essential clasts, are derived directly from the

magma involved in the volcanic activity and conse-

quently always consist in large part of glass formed

by rapid quenching of the extruded melt. Crystals

precipitated from the melt prior to extrusion or ex-

plosive eruption of the magma are usually present

in a glassy matrix in larger vitrophyric clasts of

lapilli and block/bomb size. During volcanic explo-

sions, accidental clasts are derived from older rock

torn from the vent walls or swept up from the

ground surface by lava or pyroclastic ﬂows. These

foreign rock clasts can also be referred to as xeno-

liths, or, if the accidental material is in the form of

individual crystals, xenocrysts.

4. Process of fragmentation: These include pyroclastic,

autoclastic, and epiclastic processes. Epiclasts of

a wide range of sizes are created by weathering

and disintegration of volcanic rock—the same

processes that produce sedimentary clasts. Trans-

port and deposition of epiclasts, commonly in

muddy volcanic debris ﬂows, produce clastic de-

posits like the one shown in Figure 7.27.

7.7.1 Pyroclastic Processes

Pyroclastic processes explosively eject and aerially dis-

perse pyroclasts of rock and magma from a volcanic

vent. The terms ejecta and tephra are sometimes used

synonymously for pyroclasts. In the most energetic ex-

plosions, ﬁne ash ascends tens of kilometers above the

vent, where it is entrained in world-circling air cur-

rents before eventually falling to the ground. Ground-

hugging avalanches of ash and lapilli, locally with

blocks, called pyroclastic ﬂows can travel at hurricane-

like speeds more than 100 km from the source. The

wide range of pyroclast size, from ﬁnest ash to huge

Kinetic Paths and Fabric of Magmatic Rocks

167

7.27 Andesitic epiclasts in a volcanic debris ﬂow. Note lack of sort-

ing and stratiﬁcation in the mixture of lithic blocks and lapilli

and crystal-vitric ash. Camera lens cap for scale.

blocks many meters in diameter, reﬂects widely ranging

explosive energies in three pyroclastic processes:

1. Exsolution of volatiles from melt and subsequent

expansion and fragmentation of the bubbly magma

(Figure 4.13)

2. Hydromagmatic interaction between near-surface

magma and explosively vaporized external water

in lakes, ocean, and pore spaces in rock and sedi-

ment

3. Combined exsolution and hydromagmatic inter-

action

The essential attribute of pyroclastic fabric is the

presence of vitroclasts quenched from juvenile melt.

The terms vitroclastic fabric and pyroclastic fabric

are not always synonymous because subordinate crys-

tals and lithic clasts are commonly present in pyro-

clastic material. The shape and size of vitroclasts

(Heiken and Wohletz, 1985) depend on melt compo-

sition and viscosity and the manner of explosive

process. Fragmentation of a highly vesiculated viscous

silicic melt creates ash-size glass shards, many of

which are angular Y shapes and arcuate slivers (Fig-

ure 7.28) that represent broken bubble walls. Un-

exploded silicic pumice, commonly with pipelike

vesicles (Figure 6.27), which can range from ash

to block size, can occur with the shards. Pyroclasts

of low-viscosity basalt and some peralkaline rhyo-

lite magmas are spindle- or ribbon-shaped bombs

(Figure 7.29a). Smaller ash-size particles are smooth

spheres, teardrops, and other aerodynamically stream-

lined shapes, such as basaltic Pele’s tears and hair

(Figure 7.29b). Cinders are lapilli of scoria. Cracked

breadcrust surfaces on blocks and bombs resemble the

crust on French bread, formed by expansion of gases

trapped beneath the solidiﬁed exterior (Figure 7.30).

Hydroclasts, chilled juvenile vitroclasts produced hy-

dromagmatically, are angular, poorly vesiculated ash-

size granules bounded by conchoidal fractures.

In the turbulent ash-and-steam cloud associated

with explosive eruptions, ﬁne moist ash adheres to

some sort of nuclei, such as crystals, building con-

centrically layered, spherical pea-sized accretionary

lapilli.

Crystals are a common part of pyroclastic material.

Some may be accidental xenocrysts, whereas most

are euhedral to anhedral juvenile phenocrysts that

168 Igneous and Metamorphic Petrology

(a)

1 mm

7.28 Vitroclasts produced by explosive disaggregation of bubbly melt. The arcuate and Y-shaped glass shards are broken bubble walls.

(a) and (b) are scanning-electron photomicrographs at two different magniﬁcations. (b) Highly magniﬁed image of a shard in (a) show-

ing minute vesicles in wall of larger vesicle. These two size populations are reminiscent of the two grain sizes in porphyritic aphanitic

rocks. (c) Optical photomicrograph in plane-polarized light of thin section of Bishop ash (erupted from the Long Valley caldera in east-

ern California; see Figure 10.26) collected in southeastern Utah. Note “ﬁbrous” pumice particles (Figure 6.27), which contain minute

stretched vesicles, of same size as Y-shaped shards, testifying to the heterogeneity of bubble size in the erupting melt.

0.2

Bubble-wall

shard

Pumice

(c)

(b)

10 m

crystallized in the magma before eruption. The same

phases occurring as isolated phenocrysts may also

be found in small clotted cumulophyric aggregates.

Pyroclastic deposits also commonly contain irregularly

shaped fragments of quartz and feldspar crystals,

called phenoclasts (Figure 7.31) whose shapes resem-

ble pieces of a jigsaw puzzle. Some contain small in-

clusions of glass. In a decompressing magma exiting a

volcanic vent, volatile-rich melt inclusions entrapped

in crystals at higher pressure expand and rupture their

host crystals, blowing them apart and creating pheno-

clasts (Best and Christiansen, 1997). In slowly ex-

truded lava ﬂows, entrapped melt inclusions have time

to equilibrate with their host crystal and no pheno-

clasts form.

An example of a widespread type of pyroclastic fab-

ric formed by explosive eruption of silicic magma is

shown in Figure 7.32.

7.7.2 Autoclastic Processes

Extrusions of all but the least viscous magma create

block-size autoclasts on the margins of lava ﬂows (see

Figures 10.8 and 10.16). Autoclastic processes are

self-inﬂicting, involve relatively small energy trans-

fers so that fragments are generally short-traveled,

and create essentially monolithologic, poorly sorted,

relatively small-volume deposits of angular juvenile

blocks. In a ﬂowing lava, the cooler, crusted, more

rigid margin breaks up as the hotter more mobile

interior continues to move. Autoclasts may tumble off

the toe of the advancing ﬂow to be overridden in

caterpillar-tread fashion so that the base of the lava

ﬂow becomes autoclastic as well. Talus piles around

Kinetic Paths and Fabric of Magmatic Rocks

169

(a)

(b)

Bombs

04cm

Pele's hair

and tears

2mm

7.29 Basalt vitroclasts streamlined during airborne trajectory of

clots of low-viscosity melt thrown from a lava fountain. (a)

Volcanic bombs are spindle- or ribbon-shaped. (b) Pele’s hair

and Pele’s tears.

7.30 Breadcrust block of andesite pumice, Unzen volcano, Japan.

Expansion of gas within block after the surface crusted caused

it to crack in the fashion of French bread.

170 Igneous and Metamorphic Petrology

0.5

Phenoclast

Quartz

7.31 Phenoclasts of plagioclase in rhyolite tuff. These grains have shapes resembling jigsaw-puzzle pieces that were broken apart by expan-

sion of volatile-bearing melt inclusions during extrusion of the decompressing magma. Some inclusions that did not burst are visible

within phenoclasts on the left.

(a)

2cm

01mm

(b)

Glass shards

Pumice

Broken crystal

of feldspar

7.32 Pyroclastic fabric in rhyolite lapilli tuff. Sample is from the Bishop pyroclastic ﬂow deposit, eastern California (see Figure 10.26).

(a) Hand sample with lapilli of ﬁbrous pumice and smaller black lithic lapilli in a matrix of ash. (b) Photomicrograph under plane-

polarized light showing ash matrix of glass shards, small pumice, and crystals.

highly viscous silicic domes emerging slowly from a

vent are another type of autoclastic deposit (Figure

7.33).

7.8 FABRICS RELATED TO

CONSOLIDATION OF

VOLCANICLASTS INTO

SOLID ROCK

Ash is consolidated into tuff, an ash and lapilli mixture

into lapilli tuff, and blocks—usually with smaller inter-

vening clasts—into breccia (Figure 2.7). In addition to

the same consolidation processes that lithify sedimen-

tary particles, such as cementation, a vitroclastic body

can become consolidated by other means. For exam-

ple, in appropriate geologic environments, glass is con-

verted into more stable clay and zeolite minerals, which

can bind initially loose vitroclasts together.

Another important lithiﬁcation process that consol-

idates vitroclasts is welding, which takes place if they

are still hot enough to stick together when deposited.

Thus, molten spatter, mostly bombs, from a basaltic

lava fountain can weld together in an agglutinate de-

posit immediately surrounding the vent. Smaller ash-

size vitroclasts dispersed by pyroclastic processes into

the air lose heat and are too cool to weld upon deposi-

tion. However, silicic pyroclastic ﬂow deposits can

be emplaced as thermally retentive, ground-hugging

avalanches of hot ash and lesser lapilli and locally

blocks. Because emplacement temperatures are nearly

that of the erupting magma, vitroclasts in the ﬂow are

still hot and sticky; therefore, as they come to rest, they

can weld together into a welded tuff. Glass shards and

pumice lapilli are ﬂattened by compaction into disks

oriented more or less parallel to the depositional sur-

face, forming eutaxitic fabric (Figure 7.34). Flattening

is more intense toward the base of the deposit as a re-

sult of the increasing weight of the overlying pyroclas-

tic material. Locally, on hill slopes, the welding mass of

hot vitroclasts may creep downhill by secondary, or

rheomorphic, ﬂow, stretching the vitroclasts into linear

shapes or creating folds in the otherwise planar eu-

taxitic foliation.

Deposits of unconsolidated vitroclasts overridden

by hot lava or pyroclastic ﬂows can absorb sufﬁcient

heat that they become sticky and fuse near the contact,

forming fused tuff. This secondary fusion produces a

texture resembling that which develops in a hot pyro-

clastic ﬂow as it cools.

Because silicic pyroclastic ﬂow deposits are hot and

contain entrapped gas at deposition, they can experi-

ence secondary crystallization through vapor-phase

processes and more pervasive devitriﬁcation (Ross and

Smith, 1961). This secondary crystallization can pro-

duce slight bonding of otherwise loose particles. Devit-

riﬁcation can produce spherulitic and felsitic textures

(Figure 7.35) that can obliterate, preserve, or even en-

hance primary vitroclastic fabric.

Vapor-phase crystallization occurs as mineral com-

ponents dissolved in the gas phase nucleate on walls of

any cavity and grow into it.

7.9 ANISOTROPIC FABRICS

7.9.1 Descriptive Geometric Aspects

The fabric in some rock bodies looks different and has

different properties in different places in the body. Ob-

viously, this inhomogeneity, or heterogeneity, depends

on the scale of observation. For example, an aphyric,

holocrystalline basalt appears homogeneous on the

scale of a hand sample, but, on a microscopic scale, a

plagioclase in one place in the ﬁeld of view is certainly

not the same as a pyroxene in another; on this scale the

basalt is inhomogeneous. On the scale of the outcrop,

the basalt lava ﬂow may have a glassy vesicular top, un-

like the wholly crystalline, nonvesicular interior, so on

this scale the ﬂow is again inhomogeneous.

Kinetic Paths and Fabric of Magmatic Rocks

171

7.33 Autoclastic talus blocks crumbled from margin of 600-year-

old Glass Creek lava dome, eastern California. Blocks of ﬂow-

layered rhyolite are mingled black obsidian and gray felsitic

rock visible in large block in foreground under black camera-

lens cap.

172 Igneous and Metamorphic Petrology

a) (b)

7.34 Eutaxitic fabric in compacted welded lapilli tuff. Compare with nonwelded lapilli tuff in Figure 7.32. (a) Hand sample showing fresh sur-

face that appears to be featureless obsidian. (U.S. Geological Survey photograph courtesy of Robert L. Smith.) (b) Weathered surface of

the same hand sample as in (a) showing distinctive eutaxitic fabric that is deﬁned by collapsed and ﬂattened pumice lapilli, which have

distinctive frayed, ﬂamelike terminations, hence the designation ﬁamme (Italian, “ﬂame”). The lighter-colored matrix between the lapilli

is composed of compacted welded ash. (U.S. Geological Survey photograph courtesy of Robert L. Smith.) (c) Photomicrograph under

plane-polarized light of a more crystal-rich tuff than the hand sample in (a) and (b) which illustrates the smaller-scale details of the eu-

taxitic fabric of ﬂattened shards and pumice lapilli. The ﬂattened glass shards, which appear white against submicroscopic, ﬁne gray dust

particles, are more tightly compressed around the cumulophyric clot of plagioclase phenocrysts (photo center), which behaved as a rigid

body during compaction of the surrounding soft glass fragments. Also note the collapsed pumice lapillus with its frayed or ﬂamelike ter-

mination. Eutaxitic fabric is produced by gravity-induced compaction and elimination of pore spaces and not by ﬂow; thus, it is not a

type of ﬂow layering.

Biotite

phenocryst

Compressed

glass shard

Plagioclase

phenocryst

Compressed

pumice lapilli

(c)

Kinetic Paths and Fabric of Magmatic Rocks

173

(a)

7.35 Devitriﬁed lapilli tuff from the nonwelded top of the Bishop pyroclastic deposit that was subjected to vapor-phase crystallization. Com-

pare Figure 7.32. (a) Hand sample in which relict pumice lapilli are composed of strings of tiny delicate spherulites replacing the thin

glass walls of the original ﬁbrous pumice. (b) Photomicrograph in plane-polarized light of same rock. In the microcrystalline felsitic ma-

trix most of the original vitroclastic fabric has been erased by the devitriﬁcation, but a few relict shards are still evident. Fibers of alkali

feldspar and cristobalite nucleated on shard walls and grew inward normal to the wall, preserving its outline. Spherulites have replaced

the glass walls of the pumice.

Spherulite in

devitrified

pumice

Phenocryst

Relict

shards

Devitrified

matrix

(b)

01mm

The fabric in some rock bodies looks different and

has different properties in different directions in the

body. This directional, or anisotropic, fabric can also

depend on the scale of observation, as will become ob-

vious. Bedded sedimentary rocks have anisotropic fab-

ric, whereas most—but not all—bodies of granite are

isotropic in that the fabric is random, appearing the

same in any direction. Anisotropic fabric can be planar,

like a stack of blank sheets of paper; or it can be linear,

like a clutched handful of pencils; or it can be com-

bined planar and linear, like the pages with printed

lines in this textbook. Rocks with planar fabric are fo-

liated, or possess foliation; the eutaxitic welded tuffs

just described are foliated rocks. Rocks with linear fab-

ric are lineated, or possess lineation. If both lineation

and foliation are present in a rock, the lineation almost

always lies within the foliation, like the lines of print

running across the pages of this book. It is, therefore,

necessary to examine the foliation surface directly for a

lineation. Some of the ways in which anisotropic fabric

is expressed in rocks are shown schematically in Figure

7.36.

An important attribute of foliation and lineation is

that they are penetrative properties; that means the di-

rectional aspect is repeated throughout the rock and

can be found in any part of it. If the “rock” blocks in

Figure 7.36 were broken into many pieces, each piece

would still show the anisotropic fabric, within some

limits of size and number of pieces. Thus, widely

spaced joints or dikes in a granitic body do not consti-

tute a foliation.

A common point of confusion is to conclude that

any linear aspect that is visible on a random rock sur-

face manifests an intrinsic, penetrative lineation within

the rock volume. However, if other surfaces of the rock

oblique to the ﬁrst also reveal the same linear aspect

and all of these lie parallel to one another (with due al-

lowance for warped and folded foliations), then the in-

ternal fabric within the rock is a foliation. The linear as-

pect seen on the rock surfaces are simply line traces of

the foliation where it intersects the surfaces, like the

lines on the sides of this textbook, which are the edges

of the planar pages in it. (The fabric of the rock may

also be lineated, but this pattern must be veriﬁed by

close examination of the foliation surface itself.) Such

apparent lineations, that is, traces of foliation on

obliquely intersecting exposure surfaces, and true pen-

etrative foliation and lineation can be distinguished by

careful examination of Figure 7.36. Careful examina-

tion of a rock is required to discern the true nature of

the anisotropic fabric, whether it is planar, linear, or a

combination of the two. It generally helps to examine

the rock on all scales—outcrop, hand sample, and thin

section.

It may be noted in the lower left of Figure 7.36 that

the sides of the layered block have contrasting mineral

bands, deﬁning a banding that is the two-dimensional

trace of the layers on the random oblique surfaces.

However, the term layers, deﬁning a layering, more

accurately describes the actual penetrative three-

dimensional fabric element within the rock than do the

less appropriate terms bands and banding.

Expressions of Foliation. Figure 7.36 shows that folia-

tion can be expressed or deﬁned in different ways:

1. A common type of foliation is the layering just dis-

cussed. In sedimentary rocks and some volcaniclas-

174 Igneous and Metamorphic Petrology

Foliation

Lineation

LINEAR OR

LINEATED

PLANAR OR

FOLIATED

ANISOTROPIC

FABRICS

Oriented inequant crystals

Schlieren (aggregates of mafic minerals)

Layering

COMBINED

LINEAR AND PLANAR

7.36 Schematic examples of anisotropic fabric in rocks.

Kinetic Paths and Fabric of Magmatic Rocks

175

7.37 Sequence of curved and lenticular composition layers in more felsic granodiorite, Alta, Utah. This is a type of anisotropic fabric.

tic deposits, layering is called bedding, or stratiﬁca-

tion. Layering is expressed by planar contrasts and

heterogeneities in texture (grain and pore size

and/or shape) or mineral modal proportions. Vari-

ous names have been applied to layering that is de-

ﬁned by contrasts in modal proportions of minerals

(e.g., Figure 7.37), but the nongenetic term compo-

sitional layering is appropriate.

2. Another expression of foliation is inequant min-

eral grains, whose longest dimensions are preferen-

tially oriented within parallel planes. Oriented platy

micas are a common expression of foliation. Tabu-

lar grains, such as feldspars, can have their two

longest dimensions preferentially oriented in planar

fashion, whereas columnar amphiboles can have

their one longest dimension so oriented. Within

that foliation plane, the longest dimensions of

feldspars and amphiboles are randomly oriented if

there is no accompanying lineation or are oriented

in linear fashion to express a combined planar-

linear fabric. Planar preferred orientation of tabu-

lar feldspars in phaneritic rocks, or igneous lami-

nation, is shown in Figures 7.16b and 7.38. A

similar fabric in an aphanitic aggregate is trachytic.

3. Yet another expression of foliation in granitic rocks

is schlieren (singular, schliere). These are oriented

wispy, diffuse concentrations of maﬁc minerals in a

more leucocratic matrix (Figure 7.39) that may be

planar disks, planar-linear blades, or linear pencil-

like shapes. They can originate in different ways;

some may be ﬂattened or otherwise deformed maﬁc

inclusions (Section 7.10).

7.9.2 Origin

It is one thing to discern and describe whether a fabric

is isotropic or anisotropic, what its nature is, and how

it is expressed, as in the preceding paragraphs. But

deciding exactly how a particular anisotropic fabric

in a rock body originated can be challenging and con-

troversial. Many different forms of layering in mag-

matic rocks have many possible origins (Naslund and

McBirney, 1996). A particular type of anisotropic fab-

ric may be polygenetic. For example, igneous lamina-

tion could result from the following:

1. Tabular plagioclases crystallizing and settling to the

ﬂoor of a magma chamber, much like large snow-

ﬂakes settling to the ground

2. Compaction of a random array of tablets in a semi-

crystalline “mush” resulting from the weight of

more crystals accumulating on top

3. Planar ﬂow of the magma, orienting the tabular pla-

gioclases; a process that would be expected also to

create a linear preferred orientation of the tabular

crystals within the foliation, unlike the ﬁrst two

processes, in which the crystals would be randomly

oriented within the igneous lamination.

Flow Orientation and Flow Layering. Magmas convect-

ing in staging chambers within the crust, moving

through overlying conduits, and ﬂowing onto the sur-

face of the Earth create anisotropic ﬂow fabrics.

Magma ﬂow occurs by movement of viscous melt

between rigid suspended crystals. Flow velocity is

slowest along ﬁxed boundaries of a moving magma

body but can increase inward, creating a gradient in

ﬂow velocity (see Figure 8.13). The pattern of velocity

gradients and how internal ﬂow markers interact with

it result in different forms of anisotropic fabric (Figure

7.40). Suspended rigid crystals, volatile-ﬂuid bubbles,

and distinctive mineral aggregates embedded in

magma can serve as ﬂow markers.

Consider, ﬁrst, ﬂow orientation of inequant grains

and grain aggregates. If inequant grains of columnar

shape are caught in ﬂowing magma that has a veloc-

ity gradient, the columns rotate into a stable orien-

tation parallel to the velocity vectors, as logs do in

a river, forming a lineated fabric (Figure 7.40a). In

the same velocity-gradient ﬂow regime, tabular feld-

spars rotate into a stable orientation that imparts

both foliation (igneous lamination) and lineation to

the magmatic body (Figure 7.40b). Other anisotropic

fabrics are possible for different combinations of

ﬂow pattern and markers, either in the form of in-

dependent isolated crystals or as aggregated concen-

trations of crystals. If a ﬂowing, homogeneous melt

contains no markers, then no anisotropic ﬂow orien-

tation can result. Likewise, if the ﬂowing magma has

no velocity gradients—that is, if ﬂow is uniform—or if

the magma is stationary, then again, no anisotropic

ﬂow fabric can develop, regardless of the nature of the

contained ﬂow markers. Turbulent, random ﬂow of

magma containing markers can also create isotropic

fabric.

176 Igneous and Metamorphic Petrology

7.38 Weak igneous lamination in granodiorite, Kern Mountains,

Nevada. This is a type of planar anisotropic fabric.

7.39 Schlieren, a type of planar anisotropic fabric, are concentra-

tions of maﬁc minerals in a more felsic granitic host rock.

Schlieren may be planar, linear, or planar-linear. Note pocket

knife for scale.