Petrology of pseudotachylytes from the Alpine Fault of New Zealand

Tecfonophysics,

173

196 (1991) 173-193

Elsevier Science Publishers

B.V., Amsterdam

Petrology of pseudotachylytes from the Alpine Fault of New Zealand G. Boss&-e Givers&

de Nantes, Loboratoire de Phtrologie et Min&alogie, 2 rue de la HowsinitGe, 44072 Nantes Cedex 03, France (Received

November

23, 1990; revised version

accepted

February

2, 1991)

ABSTRACT Boss&e,

G, 1991. Petrology

of pseudotachylytes

from the Alpine

Fault

of New Zealand.

Tectonophysics, 196: 173-193.

The petrology of pseudotachylytes derived from contrasting host rocks within the Alpine Fault of New Zealand, a well known dextral shear zone, is presented. The structural observations indicate that parallel synthetic shears formed first in a non-dilational system, at low angles to the Alpine Fault. Then, between these shears, fracturing and frictional heating occured according to a new, dilational, Riedel system, accompanied by listric faults. The observations suggest that pseudotachylyte melt is mainly formed in the hstric faults and channelled into neighbouring areas of extension. Pseudotachylyte results from in situ melting and its nature depend strictly on the parent rock mineralogy. The microlitic nature of the pseudotachylites is confirmed, and the variability of the textures explained in terms of degrees of undercooling. The morphology and composition of the minerals in the pseudotachylyte (plagioclase, biotite and amphibole) are documented. They crystallized at a higher temperature and pressure than their parent rock minerals. The melting process is believed to be governed by the mechanical properties of mafic minerals, in a fluid-free environment at the beginning of the melting. Mafic minerals provide water which allows the development of the pseudotachylyte.

Introduction

samples derived from mineralogically different rocks chosen from a well known geodynamic con-

Pseudotachylytes (Shand first coined the term pseudotachylyte in 1916) are widely believed to

text. After a determination of the structural setting, the goal was to study the petrology of the

form by frictional fusion during seismic faulting (Sibson, 1975). The presence of amygdales and sometimes microlites in the groundmass of the

material of the pseudotachylyte in order to answer the following questions: (1) Is the pseudotachylyte mineralogically and/or chemically homogeneous? (2) Is the pseudotachylyte glass? (3) What are the

pseudotachylyte (Maddock et al., 1987) often demonstrates the presence of melt. Nevertheless, it has been suggested, existence of ultrafine

relationships between the parent rock and daughter minerals in the pseudotachylyte, and is there some control of the parent rock mineralogy on the pseudotachylyte composition?

based on the basis of the grained pseudotachylyte de-

void of obvious igneous textures, that pseudotachylyte may also form by cataclastic deformation (Francis, 1972; Wenk, 1976). The absence of glass in pseudotachylyte has also been considered as evidence against frictional melting, and thin veins of mobile microparticles (50-100 Angstrom) have been experimentally produced by friction (Weiss and Wenk, 1983). The purpose of this contribution is to examine the genetic conditions of three pseudotachylyte 0040-1951/91/$03.50

0 1991 - Elsevier

Science Publishers

To control these parameters better, samples were taken along the Alpine Fault (AF) of New Zealand, a well known dextral shear fault zone (Fig. la). No obvious metamorphism has occurred since the formation of these pseudotachylytes. Samples 1 and 3 come from two mineralogically different rocks that are in contact; sample 4 comes from the same area as sample 3 and has a composition that is very close to it. B.V.

. HossII~Kt I

I

)

1 mm

4

HOKITIKA I

9.8 Ma (K-A&

and

Slbson

I

a

Adams (1981

0.03 cs 0.17 Ma (Ftssm Seward

It

ll9BS

t

1

Cl Pseudotachylyte Sibson

occurrences

et al.

(1979)

km

Fig. 1. (a): ‘Ibe Alpine Fault in the South bland of New 2hland: sampIe locations; known 1ocAities of pa~&%&yly4~ oeevra~ces after Sibson et al. (1979), coto the Iow-arkmkity za~e of Evison (1971); radhwck a$~ from Adam (1981) am@!hwtrd and Sibson (1985). (b): Developmu~t of parakl qmthetic R shears at low angles to tbc mylonitic foliation of the Alphe Fault.

pseudotachylyte veina t&t are invirihlc at the surface. The material has been malysud us&g 0.03 mm t&ok powed sa%imU with a CAMEBAX microprobe (Microsonde Chwst, IFRYBLtEF&, contain

Wholerock analyses have not been cartied out because the rocks are finely banded and may

PETROLOGY

OF PSEUDOTACHYLYTES

FROM

THE

ALPINE

FAULT

Brest) operating at an excitation voltage of 1.5 kV and a beam current of 10 nA. Well calibrated mineral standards were used. In addition, polished carbon-coated, thin sections were examined by scanning electron microscopy (SEM), operating in the backscattered electron mode. Microsamples (1-3 mm) of rocks were also carbon- or gold-coated and examined by scanning. Semi-quantitative analyses were obtained by a TRACOR-coupled energy dispersive X-ray analyser on carbon-coated thin sections and microsamples (SMEBA, University of Nantes). As the accuracy and precision of the analyses obtained by this method may be considered lower than those obtained with a microprobe, tests were run on several minerals (garnet, feldspars and micas). The resulting analyses did not differ fundamentally, the chemical compositions varying in the same way and with similar ranges in both methods. All EDS analyses have been performed using a very low beam current (0.1 nA), thus avoiding underestimation of light elements even during long count times (50 s). However, two pollution problems were detected and these had to be overcome in order to obtain top-quality measurements: (1) condensation of oil from the diffusion pump and of products resulting from degassing of the sample occurred on the Be-window of the detector, necessitating the cleaning of the window every week by immersion in freon 113; (2) the detector crystal window (cooled by liquid N,) had a tendency to ice up. As the cleaning of the window necessitates slow heating up to room temperature and then 24 hrs of pumping, this timeconsuming operation (4 days) is only performed every 3 months. Between such cleaning operations, a software correction is necessary. Analyses of biotite and garnet were performed immediately after cleaning on previously analysed minerals, and within 0.001 mm of the previously analysed points. Before each period of analysis, the same standards were re-analysed, allowing one to estimate the increase in thickness of the window. An increase of 0.001 mm/month in the equivalent window thickness was therefore determined and used as a correction in the program. In order better to compare minerals of the parent rock with those of the pseudotachylyte, and

OF NEW

ZEALAND

17.5

to avoid artifacts linked to analytical procedures, only the analyses that were performed with the same microprobe or with the same SEM have been considered. The very small size of the minerals of the pseudotachylyte made them impossible to detect under an optical microscope, so the choice of the mineral section to be analysed was made in the backscattered electron mode of the SEM. Nevertheless, the pear-like shape of the analysable volume (usually some 0.001 mm3, but varying with the atomic number), under the electron flux may be greater than the mineral volume itself. Geotogical setting

The Alpine Fault of New Zealand (AF) is a major transcurrent fault that is still active between the Pacific and the Indo-Australian plates. According to Carter and Norris (1976) its transcurrent movement began in the mid-Oligocene (30 Ma) and continued through the late Miocene (10 Ma). A change in the orientation of the slip vector then led to oblique compression that was responsible for the uplift of the Southern Alps. The mylonitic foliation in the fault zone is presently dipping 40-80” to the east, while the stretching lineation plunges in a direction subparallel to the present-day slip vector between the IndoAustralian and Pacific plates. Present-day, oblique convergence at the AF boundary results in dextral strike-slip, vertical uplift and westward overthrusting of schists of the Southern Alps onto the West Coast sequence (Cooper et al. 1987). Samp Ie location

Samples of pseudotachylyte and country rock were collected in a restricted area of the central part of the AF (Fig. la) known for its low seismicity (Evison, 1971). According to Suggate (1963), four mylonitic zones are distinguished from east to west; curly schists, green mylonites, augen mylonites, and cataclasites. As a consequence of the uplift of the Southern Alps, these rock types are exposed together, but they have been formed in environments varying from ductile through ductile-brittle to brittle. The first occurrence of “ hyalomylonite” in the AF was reported by Wal-

176

IIOSS,,~KI

lace (1976).

As shown

pseudotachylytes

by Sibson

et al. (1979).

tachylyte

may be observed

in every zone

Fission

of the AF and every rock type except gouges. The pseudotachylytes except

have not been

for some cataclasis.

age of 9.8 Ma has been obtained

whole-rock

from a pseudo-

ages

Samples the

AF

different

and Sibson.

were collected

(Fig.

1) and

mylonitic

Creek (Adams.

of pseudotachylytes

0.43 & 0.17 Ma (Seward

metamorphosed,

A K-Ar

vein from Harold track

are

contexts.

1981). indicate

1985).

on the western

side of

representative

of the

Samples

1 and 3 are

PrlWrY svnthetlc R-shear

Prlmry

wnthetlc

R-shear

C

Fig. 2. Drawings of two

parallel thin sections from

pscudot.a&ylyte segregations. Intapmation sense of sbcar in tbe volume dcfii

sample 1 pupondkular

in tdms of Riukl

to primary R shears, showing the shape of chc

ehcpr of the -try

of tbc fnults rdatai

to the WadI

ri&t-lawal

by tbc pyimary R sbaus. Doubk arrow bars are parallel to the mylonitic PoIiatiun. +angk~ of intcmal friction, If = listric faults.

PETROLOGY

OF PSEIJDOTACHYLYTES

FROM

THE

ALPINE

FAULT

OF NEW

ZEALAND

177

from the green mylonites, on the left bank of the Wanganui River. They are augen-rich to augen-free rocks, sometimes with a very fine grain size. Although these two samples are nearly in contact in the field, their parent rocks differ slightly in mineralogy. Sample 4 comes from a mylonitic amp~bo~te in Harold Creek Valley, about one mile south of the Wanganui River (Fig. la). Structural setting The genesis of the pseudotachylyte is related to shearing of the AF mylonites. At first, parallel synthetic shears (R) develop at low angles to the mylonitic foliation (Fig. lb). This system is probably activated because no dilation can occur. It is quite similar to that experimentally obtained by Naylor et al. (1986) in models reproducing basement-induced wrench faulting. In our samples, pseudotachylyte may occur in more or less parallel planes, separated by l-10 cm, but most of the pseudota~hylyte developed within the mass between these planes. The main planes lie at low angles (up to 200), to the mylonitic foliation of the AF. Such a geometry is common for pseudotachylytes in previously mylonitized or strongly foliated rocks (Grocott, 1981; Passchier, 1984; Swanson, 1988). The orientation of the shear planes relative to the foliation is explained well in terms of Riedel shear terminology (Riedel, 1929). After the activation of the parallel primary R shears the system may evolve further in the rock mass between the primary shears (Fig. 2). In this volume dilation may be operative, making pseudotachylyte generation possible. The relative orientation of the new fractures was studied in thin sections perpendicular to primary R shears. Fractures are of two types; microfaults and list& faults (Fig. 2). Pseudotachylyte segregations may be associated with palm-tree (or horse-tail) faults (Fig. 3).

A rose diagram (Fig. 4) shows that the relative frequency of the microfaults is consistent with the Riedel model of fracturing (Riedel, 1929). The most frequent fracture types are T, R’ (antithetic) and X. Pseudotachylyte occurs preferentially in

Fig. 3. Drawings from a thin section of palm-tree faults, the main vein being parallel to primary R shears. Stippled areas represent the pseudotachylyte. Scale bar = 0.16 mm.

T-type features which result from extension. They may also occur in the space between R’ and T, R or P and primary R shears; connections between these different locations may be established during pseudotachylyte generation. Listric faults List& faults associated with pseudotachylyte generation may be seen, as curved lines, in Figs. 2 and 3. Listtic faults are best known from sedimentary sequences in which the rheological properties of each stratum do not differ greatly (see, for example, Ramsay and Huber, 1987). The mylonitic banding of the parent rocks of the pseudotachy-

prlmry

R’

T

synthCtlc

Y R4w.r

x

In conclusion, pseudotachylyte genesis occurs in two steps. First. parallel synthetic shears develop at low angles to the AF. Then, in the rock mass between these shear zones, fracturating accompanied by listric faulting takes place according to a new Riedel system. Pseudotachylyte is mainly formed during this period in the listric faults and is channelled by a pumping effect to the extension areas. The primary R shears are pseudotachylyte-poor whereas, in the rock mass between them, pseudotachylyte fills the spaces between R’ and T, R or P and primary R shears.

Optical microsc0p.y

\r

J

Fig. 4. Rose diagram of the faults wxrring in the pinacoid defined by primary R shears (55 measures).

lyte is unlikely to introduce any important rheological contrast. In the listric geometry, the fault surfaces have circular arcuate profiles. Therefore the two walls may glide without any opening along the fault surface. Such a circular rotational motion induces an opening at the rear of the fault. From thin sections (Figs. 2 and 3) list& faults are associated with microfaults of “palm-tree” type, and pseudotachylyte segregation occurs in the convergent part of the faults. In the listric faults, the proportion of refractory minerals such as titanite and zoisite is higher than in the parent rock itself. It is suggested that the list& faults may be fertile zones for the production of the pseudotachylyte (frictional melting zone). Nevertheless, in the microfaults we observe similar mineralogical proportions. It is suggested that the pseudotachylyte forms in the hstric faults and migrates to the rear of the “p&n-tree” and into the extension faults. Although pseudotachylyte seems to have been intruded forcefully between the minerals at the convergmt part of the “palm-tree”, in fact it is likely, from tkc mric observations and considerations, that intrusion was a consequcnc42 of extension.

Only the general features are given here: a more detailed microscopic description is given in Appendix 1. The veins are generally in sharp contact with the country rock (Figs. 5A and D) but, as previously noted, pseudotachylyte may occur as melt between minerals at the convergence of palm-tree faults. Veins may be as thin as 0.020.03 mm, but may attain a thickness of 3 cm. The periphery of a vein may be darker than its core (Fig. 5E), but this is not a general rule. No amygdales have been observed, but chloriteand/or calcite-filled veins are common. When calcite occurs together with Mg-chlorite, it is preferentially located at the core of the vein. The pseudotachylyte usually truncates the amphiboles sharply (Fig. 5B), in contrast with contacts with plagioclase, which are generally rounded (Fig. 5C). As a rule only quartz and phagioclase are observed as elastic objects in the veins, with the plagioclase rounded and the quartz angular. Typical metamorphic pressure shadows do not occur in the matrix around these clasts; instead, flow structures, very similar to those occuring in lavas (Fig. 5D), marked by tiny opaque minerals and differences in colour, suggest that the material was molten. In crossed polarized light some birefringence may be evident and, using a gypsum plate, common interference colours suggest some preferred orientation of the phyllosilicates that are too small, however, to be discerned clearly under the microscope.

PETROLOGY OF PSEUDOTACHYLYTES

SEM electron microscopy

Petrological observations at the 0.001 mm scale were performed in the backscattered electron image mode, allowing an investigation of the chemical composition. In some microsamples, plagioclase may be almost idiomorphic, while biotite crystals appears to be slightly deformed (Figs. 6A and B). Areas which may appear to be homogeneous under the optical microscope are found to be heterogenedus with a variety of textures (Figs. 6C and D). Minerals that appear white were identified as biotite, while the black areas are plagioclase (see analyses in Table 1). Homogeneity

Microprobe and EDS methods were used to test the homogeneity of the pseudotachylytes, and to determine the influence of the nature of the parent rock on the pseudotachylyte itself. Chemical nature of the whole pseudotachy~te

Microprobe analyses using a defocused beam of 0.003 mm diameter were made on optically homogeneous areas that were supposed to be composed either of glass or very fine grained (cataclastic) material. If the analyses correspond to cataclasites from the minerals of the parent rock, they should plot, on a suitable variation diagram, on a line passing through the point corresponding to the composition of the mother minerals of the parent rock. parent

position from lOO%, which is assumed to represent water content, correlates negatively with silica, soda and lime. The FeO, MgO and K,O occur in biotite, the Na,O and CaO in plagioclase, and because the parent rock is made of biotite + plagioclase + quartz, the results suggest that the pseudotachylyte is a very fine grained mixture of biotite, plagioclase and quartz. To test this hypothesis, backscattered electron photographs were prepared. They show the very heterogeneous nature of the optically homogeneous material (Fig. 6D). It is composed mainly of neocrystals of biotite and plagioclase that are too small to be detected using classical methods of observation. It must be concluded that the microprobe analyses analyses.

of the pseudotachylytes

Biotite-plagioclase-quartz

179

FROM THE ALPINE FAULT OF NEW ZEALAND

rock (sample

1)

Pseudotachylyte from sample 1 has a constant FeO/MgO ratio, as shown in an FeO-MgO diagram (Fig. 7). As the parent rock biotite analyses lie on this line, it is possible that the pseudotachylyte is a mixture of variable amounts of biotite, plagioclase and quartz. Moreover, it is shown in Table 1 that Na,O, CaO (except analysis 102) and SiO, correlate negatively with K,O. In addition, Ti has a systematically higher content when SiO* is greater than 47 wt%. Departure of total com-

were

always

the result

of mixed

Amphibo~itic parent rock (samples 3 and 4)

Because of a more complex mineralogy, a triangular diagram of mole percent (Al-Na-K)(Fe + Mg)-Ca was used to compare the composition of pseudotachylytes with respect to their amphibolitic parent rock. For both samples (Fig. 8A) the analyses tend to plot close to the line linking biotite to plagioclase, even when close to the amphibole composition. This suggests that amphibole was transformed into biotite during the pseudotachylyte formation. When compared with the biotite composition of the parent rock (Fig. 8A), the newly formed biotite is Al-enriched, indicating that the pseudotachylyte formation occured within the biotite stability field. In sample 4, chlorite has been observed and analysed (Fig. XB: solid asterisks) in the pseudotachylyte with the same trend of composition as biotite. Biotite with a similar composition to the parent rock biotite of sample 1 has crystallized. The influence of the amphibole composition is reflected by the dispersion of the analytical points at higher values of the (Al-Na-K). Some high-Al points may represent impure pseudotachylyte plagioclase analyses with a higher An% than the parent rock plagioclase.

PETROLOGY

OF ~~U~TACHYLYT~S

FROM

THE ALPINE

FAULT

OF NEW

ZEALAND

181

Fig. 6. SEM micrographs (scale bars = 0.001 mm) from sample 1, showing (A) idiomorphic plagioclase and (B) biotite crystals. (C, D) Backscattered electron images showing, in white, different kinds of biotite crystallization (C = sample 4; D = sample 3).

~~~eraio~ of the ~~e~dotffc~y~tes No deformation of the veins and dykelets of pseudotachylyte may be seen in outcrops or hand

specimens. There is no ~crost~cture or paragenetic evidence that the pseudotachylytes underwent any metamorphism. They are neither finegrained cataclasites nor homogeneous glass, and it

Fig. 5. Optical aspects of pseudotachylyte. (A) Thin section of sample 1, showing a type of occurrence of the pseudotachylyte; the mylonitic banding is NW-SE; scale bar = 4 mm. (B) Sharp cut of an amphibole crystal by a pseudotachylyte vem (sample 3); pseudotachylyte is biack and contains white rounded clasts of quartz and plagioclase; scale bar = 0.06 mm. (C) Plagiociase rounded contact with the pseudotachylyte in black (cf. Fig. 1B); scale bar = 0.06 mm. (D, E, F) Optical aspects of pseudotachylyte in another thin section of sample 3: (D) relationships of the veins (scale bar = 3 mm); (E) part of (D) showing flow-like structures and zoning (scale bar = 0.24 mm); (F) detail from (E) showing coated clasts of quartz and optically homogeneous areas (scale bar = 0.08 mm). In (D) the rectangle indicates the location of(E) and, in (E), the location of(F).

94.46

94.24

95.61 95.51

47.18 22.92 1.49 7.42 0.13 3.53 4.56 4.61 3.67

102

96.86

63.75 21.48 0.15 0.45 0.01 0.20 3.55 6.57 0.70

119

Sample 3

96.30

49.27 19.56 0.18 7.54 0.04 7.40 1.27 4.94 6.1

128

96.28

46.37 18.69 262 10.35 0.23 5.81 3.53 3.81 4.81

126

96.82

95.70

!xs4

54.53 20.84 1.75 3.94 0.14 2.04 3.93 6.77 1.76

48.86 21.83 2.07 7.26 0.17 4.02 4.14 5.02 3.45 96.88

22.77 0.77 3.63 0.00 2.04 S.20 6.07 1.86

130

125

127

samples

93.66

36.77 20.44 0.17 13.82 0.16 17.23 2.09 1.95 0.93

133

Sample 4

99.39

56.44 18.48 1.58 6.57 0.62 7.46 4.58 3.53 0.13

118

97.15

53.97 21.65 0.18 4.97 0.08 5.21 5.96 4.83 0.30

150

97.79

56.60 23.21 0.15 2.67 0.05 2.69 5.47 6.51 0.44

1431

99.6X

64.74 19.65 0.00 1.85 0.15 1.68 7.21 3.81 0.52

131

the original numbering of the analyses.

* The samples have been classified from left to right in order of increasing SiOa content to show that in samples 1 and 3 it correlates (1) negatively with the Fe5, MgO and K,O contents (it corresponds to a decrease in biotite content), and (2) positively with CaO (increase in plagioclase content). In the pseudotachylyte veins and patches no chemical changes were observed between the core and the rim. A test has been made by EDS analyses on 0.0015 mm2 homogeneous zones of sample 1. This has shown that the chemical changes that may be observed in the veins or patches are only related to a different modal percentage between new plagioclase and new biotite. Numbers on line two correspond to

Total

39.01 22.37 1.55 14.28 0.11 8.36 0.65 0.00 9.28

37.28 21.93 0.98 15.66 0.14 8.11 0.47 0.07 9.60

34.65

18.97 3.92 17.99 0.26 8.53 0.64 O.&Z 9.48

(106)

98

116

ShnpIe 1

TABLE 1 Representative microprobe analyses of optically homogeneous materials in three pseudotachylyte

PETROLOGY

OF PSEUDOTACHYLYTES

FROM

THE

ALPINE

FAULT

OF NEW

can

FeO biotite in parent-rock

t 18

-

16

-

14

-

183

ZEALAND

be inferred,

mainly

electron

photographs

observed

crystallized

(Fig.

-

10

-

Biotite.

0

Sample

in Table biotite-fm

0.

-

0

biotite

1:. , , . , 0

in sample

parent

rock

(sample

analyses

are shown (Fig. 9) the

value

1 biotite

is slightly

2

4

diagram

homogeneous

8

IO

biotite

analysis

analyses

with hornblende,

with

region (South

by the

the AF

See text for explanation

but is richer in

the

(Portugal) Carolina).

analyses

pseudotachylyte

or of the

of biotite

amphibolite-grade metamorphic 1984) using an Al”’ us Mg/(Mg

vein (circles)

is indicated

to biotite

the phlogopite-eastonite-siderophyllite the biotite compositions vary similarly, to the data compiled by Speer (1984) to

Compared

of

lower in pseudotachy-

rock. Compared

has a value close to that of

those of the Aregos Clouds Creek pluton WI0

microprobe

parts of a pseudotachylyte

1. The host-rock square.

6

of electron

Al”

associated

Mg. On diagram, according

t

Fig. 7. FeO-MgO

the minerals

2. On an Al’” us fm diagram

from granitdids,

6

optically

6), that

1)

lyte than in the parent

:

the backscattered

from a melt.

Plagioclase-biotite-quartz

12

8

from

rocks

biotites

from

(Guidotti,

+ Fe) diagram, have the same

(Al-Na-K)

(Al-Na-K)

B

moles percent

A

SAMPLE 4

0

plagioclase

0

amphibole

0

in parent-rock

biotite

4

“gLs.5”

+

chlorite

I

in pseuktachylyfe 1

I\

(Fe+MQ)

( Fe+Mg )

Fig. 8. (Al-Na-K)-Ca-(Fe

+ Mg) moles percent

diagrams

lyte material

of electron from samples

microprobe

analyses

3(A) and 4(B).

of optically

homogeneous

pseudotachy-

184

composition as biotite from mus~o~te-bea~~g or muscovite-free rocks derived from pelites or from other rocks with coexisting Al-rich phases of amphibolite- to granulite-grade rocks. In the AF case the parent rock is also Al-rich, and the high Al content of biotite may be explained by the whole-rock chemical control on the biotite composition generated during pseudotachylyte formation. The Ti content of the pseudotachylyte biotite is lower than the Ti content of the parent rock biotite (average of six analyses = 0.342), ranging between 0.15 and 0.25 (average 0.289 over 11 analyses), but is similar to that of biotite from greensc~st-jade metapelites and semi-pelites (Guidotti, 1984). Biotite composition is also depleted in Fe and Mg relative to its source material. In a pseudotachylyte from a paragneiss of similar composition, Maddock (1986) found that biotite from pseudotachylyte contained less TiO, but more SiO, and Al’” than the host biotite.

~~ugi~~~use.

Plagioclase analyses are given in Table 2. A positive correlation is observed between the Fe, Mg and K contents. Since crystals are smaller than the analyser beam, interferences with adjacent minerals, mainly biotite, occur. The plagioclases of the parent rock have an An content of l&20% but lower values (8%, even 1%;)may be observed related to the transformation of plagioclase into epidote and more sodic plagioclase in variable proportion. The An content of the pseudotachylyte plagioclase may be as high as 24-26%, richer than in the parent rock plagioclase. It is sometimes idiomorphic, as observed in the SEM (Fig. 6A). In addition, even more calcic plagioclase is formed in association with rutile (see below). Opaque minerals. In sample 1 the pseudotachylyte contains rutile coated by ilmenite, in association with An65 plagioclase. Ilmenite crystals contain about 4% MnO (0.175 Ti in the

TABLE 2 Representative analyses of pscudotaehylyte minerak from sample 1 (quartz-plagioelase-biotite) Primary minerals

Pseudotwhylyte minerals

biotite

biotite

plagioelase

141

142

*

plagwase

144

105

106

108

1

12

4

79

122

123

SiO,

40.63

40.12

39.89

65.42

65.73

67.89

38.30

37.02

37.16

63.00

60.30

64.65

A’ 203 TiO,

19.02

19.09

21.07

21.13

21.48

19.34

19.87

19.70

19.12

22.68

22.74

21.66

2.21

2.22

2.26

1.24

1.40

1.81

F&i

20.01

19.55

18.28

17.00

16.74

17.06

WO Ci30 Na,O

8.49

8.84

8.68

K2O

9.71

10.2s

8.98

9.38

9.41

3.80

3.22

1.80

1.17

0.44

0.65

5.62

6.59

5.42

9.72

9.64

11.04

0.46

0.00

0.15

8.76

8.21

8.34

8.71

9.62

9.31

9.90

Si

5.812

5.754

5.663

2.880

2.885

2.974

5.676

5.588

5.602

2.791

2.744

2.850

AS”

2.188

2.246

2.337

0.120

0.11s

0.026

2.324

2.412

2.398

0.209

0.256

0.190

Al”’

1.019

0.981

1.188

0.976

0.9%

0.972

1.146

1.092

0.999

0.97s

0.963

0.934

Ti

0.238

0.239

0.241

0.139

0.159

0.205

Fe

2.394

2.345

2.170

2.106

2.113

2.151

Mg

1.810

1.890

1.837

1.985

2.110

2.114

Ca

0.179

O.lSl

0.084

0.185

0.070

0.10s

0.267

0.321

0.256

Na

0.830

0.820

0.938

0.132

0.000

0.044

0.752

0.724

0.713

1.646

1.852

1.790

15.339

15.956

15.408

4.PP4

5.009

4.943

K TotaJ

1.772

1 .a75

1.793

15.233

15.331

15.229

4.986

4.969

4.995

* Numbers on line three correspond to the origin& numbering of-the analyses.

PETROLOGY

OF PSEUDOTACHYLYTES

FROM

THE ALPINE

FAULT

OF NEW

185

ZEALAND

according to Liou et al., 1974). This reflects the progressive decrease of T as the amphibole recrystallizes during shearing before pseudotachylyte generation. The most important observation is that the pse~dotach~~te encloses secondary amphiboie. In contrast with the primary ferro-hornblende which evolves at increasing Si’“, the pseudotachylyte hornblende evolves at lowering Si’” content, towards the ferro-tschermakite hornblende member, with a low K and Ti content. If we consider, following Hammarstrom and Zen (1986) that the Al” content of hornblende is a pressure indicator for the crystallization of plutonic rocks of appropriate bulk composition and mineral assemblage, then, the pseudotachylyte amphiboles have crystallized under higher pressure than those from the parent rock. A decrease in Si cont.ent and an increase in Al’ with increasing T have been ex-

unit cell) and 52% TiO, (2.000 Ti in the unit cell). Such a replacement of Fe’+ by Mn in manganiferous ilmenite may indicate a high oxygen fugacity. However, no significant differences were noticed between the parent rock and the pseudotachylyte ilmenite. Amphibolitic parent rock (samples 3 and 4) In sample 3 (Table 3), it is a Amphibole. ferro-hornblende which evolves towards ferroactinalitic hornblende and then ferro-actinolite (Fig. lo), whereas in sample 4 (Table 4), the parent rock amphibole is a Mg-hornblende. The replacement of the early ferro-hornblende by ferro-actinolitic hornblende and ferro-actinolite (Table 4 and Fig. 10) is characteristic of the transition between amphibolite and greenschist facies (i.e. temperature between 550° and 450 o C

siderophyllite

eastonite

2. SUfJ

*

0

0

2.NJu

0

0 0

0

m l

2.300

D

00 2.200

8

l

.

0 : 8

0 1

&

2.100

‘8 0’

n

,

1

2.000

Fig. 9. Al” versus Fe/(Fe + Mg) diagram of the host-rock (solid symbols) and pseudotachylyte (open symbols). Circles show biotite, and squares show samples 1 and 3. In both samples the compositional variations occur in the same sense, corresponding to a lowering of the fm ratio.

15.343

1.846

4.786

15.431

-

2.195

2.567

0.163

4.986

0.830

0.179

0.977

1.119

2.880

9.72

3.80

21.13

4.995

0.938

0.084

0.973

0.026

2.214

11.04

1.80

19.34

14.988

2.027

1.751

2.381

0.829

o.aOs

7.195

13.06

0.12

19.65

9.57

161

10.17

14.910

1.929

1.797

2.36s

0.819

0.640

7.360

14.880

1.966

2.163

2.079

0.672

0.433

7.567

12.86

a.36 12.48

17.42

6.57

53.03

WV

t9.60

8.58

51.02

* Numbers on line three correspond to the original numbering of the analysis.

Total

K

-

2.044

Ma

Na

2.470

Fe

-

0.209

Ti

-

0.774

Al”’

ca

2.121

2,031

0.719

5.879

5.969

9.70

AP

10.06

KP

1.50

si

nd

Nap

10.2fl

9.53

nd

20.53

Fdf

CnO

21.26

1.93

16.69

16.54

A&D,

TiOz

49.66

(1511

(loaf 67.89

WI

40.72

41.48

65.42

(125)

SiO,

(128)

amphibole

plrtgioctase

biotite

Primary minerals

8.238

2.087

0.028

0.780

2.160

3sa2

24.67

0.24

11.81

23.21

40.29

(160)

epidote

Representative andyses of primary and pseudotachylyte minerals from sampfe 3 (amphibolitef *

TABLE 3

15.362

1,779

2.206

2.4a7

0.216

0.673

2.051

5.949

9.69

10.29

20.67

2.00

16.07

41.35

014)

biotite

15.373

1.830

2.160

2.553

0.234

0.615

2.000

6.000

9.94

10.04

20.98

2.16

15.37

41.57

(lla)

4.922

0.6M

0.304

O.%l

0.149

2.851

7.67

6.43

21.35

64.61

021)

0.713

0.256

0.975

0.150

2.850

a.34

5.42

21.66

64.65

(123)

4.944 ___.

plagiociase

Pseudotachylyte minerals

15.057

(117)

15.021

0.267

I .a21

1,852

2.092

0.199

0.789

0.962

7.038

1.45

11.77

0.61

17.33

1.84

IO.29

48.75

_l.--l.-__._-~_-

0.223

1.891

1.525

2.350

0.102

0.966

1.061

6.938

1.20

12.12

7.03

19.30

0.93

11.82

47.66

016)

aaphibole

PETROLOGY

OF PSEUDOTACHYLYTES

FROM

THE ALPINE

FAULT

OF NEW

187

ZEALAND

actinoliLe

,

0.50

n

aclinolilic

hom-

Fig.

10. Mg/(Mg

pseudotachylyte

+ Fe) versus amphibole).

Si diagram

for amphiboles

The arrow shows the evolution

metamorphic

evolution

in sample

of the AF at a low structural

square = host-rock

amphibole

during

shearing,

level well before the pseudotachylyte

perimentally observed (Spear, 1981); however, the same relationships may be observed at 700 o C and under increasing pressure. TABLE

3 (solid

of the primary

amphibole;

open

square =

which may be related

Laird and Albee (1981) have shown that in a metamorphic area amphibole ranges in composition from actinolite in the biotite zone through

4

Representative

analyses

Primary

of primary

and pseudotachylyte

minerals

amphibole 135

137

minerals

from sample 4 (amphibolite)

Pseudotachylyte plagioclase 27

11

*

minerals

plagioclase

140

biotite

chlorite

9

16

17

11

125

SiO,

42.89

46.50

62.96

61.80

55.79

57.94

44.20

45.32

30.77

28.14

A’ 20s TiO,

13.26

9.94

22.57

24.68

25.90

25.39

16.44

15.74

18.15

16.70

0.34

0.22

2.85

2.45

0.19

0.10

Fe0

15.40

14.28

16.25

15.28

15.78

15.01

MgO CaO

11.87

13.55

9.71

10.77

22.23

23.05

10.88

11.07

3.57

5.40

0.79

9.19

1.69

1.33

0.29

0.41

Na,O

1.72

1.39

9.99

8.89

5.76

5.54

2.06

1.27

0.16

0.11

K,O

0.48

0.28

6.68

6.97

0.80

0.40

Si

6.393

6.814

2.809

2.720

2.599

2.606

6.160

6.307

6.088

5.766

Al’”

1.607

1.186

0.191

0.280

0.401

0.394

1.840

1.693

1.912

2.234

Al”’

0.722

0.530

0.994

1.000

0.972

0.952

0.860

0.888

2.321

1.800

Ti

0.038

0.024

0.299

0.257

0.029

0.015

Fe

1.919

1.750

1.894

1.779

2.611

2.572

Mg

2.638

2.959

2.016

2.234

6.555

7.650 0.090

Ca

1.738

1.737

0.171

0.254

0.423

0.443

0.252

0.198

0.062

Na

0.497

0.397

0.864

0.759

0.502

0.483

0.557

0.343

0.062

0.044

K

0.091

0.052

1.187

1.238

0.208

0.104

15.643

15.449

15.065

14.937

19.848

20.275

Total * Numbers

on line three correspond

5.029

5.013

to the original

4.897

numbering

to the

generation.

4.878

of the analyses.

IRX

hornblende in the garnet zone, to tschermakite in the staurolite kyanite zone. Nevertheless, the variation diagrams established in areas studied comprehensively by these workers have shown that tschermak substitutions dominate during lowpressure facies series metamorphism. In sample 3 (Table 3) the primary plagioclases are transformed to epidote and more sodic plagioclase in variable proportions, and their An content ranges from nearly pure albite (An 1.2%), when it contains epidote, to oligoclase (An 17.7%). The average value is 15.5%. In the pseudotachylyte the An content is higher, and three analyses indicate 31.6%, 38% and 26%. Even if the analyses are not from pure phases in every case, the newly formed plagioclase is clearly richer, in An. In sample 4 (Table 4), the plagioclase crystallites were large enough to allow six good microprobe analyses. The An content ranges from 31.6% to 47.6% (mean 43%). This confirms the higher An content of plagioclase in the pseudotachylyte compared to the parent rock plagioclase. The plagioclases may also contain iron in some amounts as observed in artificially generated friction melt (Spray, 1988). Piagioche.

around 2.5 wt%. The An content, calculate dorectly from mixed analyses of very fine grained biotite + plagioclase, has an average value 01 36.5%, which is in agreement with the An content of the plagioclase crystallites. The pseudotachylyte of sample 4 contains Mgchlorite (Table 4). If this mineral is considered to be metamorphic, it can be compared with analyses compiled by Laird (1988) to examine the chlorite composition as a function of metamorphic grade. Analyses of the chlorite from the pseudotachylyte indicate P-T conditions similar to that of the upper half of the greenschist facies. Because the Ti content of the neo-biotite indicates a higher temperature origin, possibly above the stability limit of chlorite, the chlorite may be of secondary origin. The mineralogical composition of the parent rock is very similar to the sample frictionally melted by Spray (1988). In these experiments the almost clast-free melt contains gas vesicles, two clinopyroxenes and an unusually high-Fe0 anorthite related to the high crystallization temperature. In sample 4, the plagioclase is also relatively Fe-rich, but its Fe content may also be erroneous since the small size of the crystals renders the analyses somewhat suspect. minerals and accessories. The parent rock of sample 3 contains some pistacite, with 0.2-0.7% MgO and 11.20% FeO. Sometimes the crystals are zoned showing, from core to rim, an Fe increase and a negative correlation between Fe and Al. The pseudotachylyte of sample 3 does not contain epidote, but pyrite and ilmenite are present. The pseudotachylyte of sample 4 contains neocrystals of an Mn-rich ilmenite, indicating a high fox (11.66 wt% MnO for 47.5 wt% TiOz) and magnetite and pyrite which are almost idiomorphic. including negative crystals (Figs. 11A and B). It also contains gypsum crystals which seem to be primary (Fig. 1lC). In conclusion: (1) the composition of the pseudotachylyte depends on the mineraIogical nature of the parent rock; (2) biotites have contrasting compositions depending upon the parent rockthey have a higher Al’” and a lower Ti content in a plagioclase-biotite-quartz parent rock and higher Ti- and lower AI”‘-contents in amphibolitic Opaque

In sample 3 (Table 3 and Fig. 8), the Biotite. fm value is slightly lower in pseudotachylyte biotite than in the parent rock biotite, whereas Ti is higher and Al” lower. This result, quite different to those obtained in sample 1, suggests that a different substitution mechanism may have been operative. The parent rock of sample 4 is biotite-free, and therefore pseudotachylyte biotite crystallizes from the Mg-hornblende. It has a slightly lower value of Mg/(Mg + Fe) than the amphibole but, in contrast, this ratio is very high in the newly formed chlorite. Two kinds of analyses have been performed on this sample, using a microprobe and EDS (Tracer system). Microprobe analyses were made on dendritic minerals (Table 4), but the analyses may be not representative of pure phases because the size of the minerals is too small and results may be polluted by plagioclase. The TiO, content of the pseudotachylyte biotite is high,

PETROLOGY

OF PSEUDOTACHYLYTES

FROM

THE

ALPINE

FAULT

parent rock; (3) plagioclases are richer in An in the pseudotachylyte than in the parent rock; (4) in an amphibolite sample, a new hornblende more tschermakitic than the parent rock one crystallizes while in a biotite-free amphibolite a new biotite crystallizes; (5) newly formed minerals indicate higher fo, higher temperatures, and probably higher pressure than in the parent rock. The textures and mineral compositions confirm that pseudotachylytes are neither homogeneous glasses nor ultrafine mixtures, but are the result of a crystallization from a melt. Discussion

and conclusions

From the structural point of view the pseudotachylites are related to a reworking of the AF. An initial shearing produces the R shear at low angles to the mylonitic foliation. Between these shears, a secondary Riedel system may develop. The pseudotachylyte generation seems to be linked to listric faults, and to the associations of ~crofaults and listric faults in palm-tree structures. The melt would be extruded by a pumping effect linked to the development of extension zones and channelled into previously formed microfaults. The pseudotachylyte shape is mostly triangular in rela-

OF NEW

ZEALAND

189

tion to the post-generation filling of the space between the microfaults, which intersect with an “Y” shape as a result of the Riedel shearing. From the petrological point of view, the origin of the pseudotachylyte studied is undoubtedly the result of a melting, as previously quoted by Wallace (1976) for hy~omylo~te in the AF and, more generally, for pseudotachyl~es (Maddock, 1983). Pseudotachylyte was a melt, as attested by: (1) the skeletal and euhedral crystals of Fig. 6, which typically result from crystallization in a melt; (2) the minerals in the pseudotachylyte, which are different in composition from those of the parent rocks. In the pseudotachylyte the plagioclase is richer in An, indicating a higher ~~stalli~tion temperature; the biotite composition is different from the parent rock and the hornblende is more tschermakitic. However, the variation in composition of the pseudotachylyte minerals depends on the primary mineralogy of the host rock. In two of the three samples (one is biotite-free) new biotite from the pseudotachylyte has a lower fm value than the parent rock biotite. The values of Ti in biotite are a function of its paragenesis. In sample 1 (biotite-plagioclase-quartz) pseudotachylyte biotite has a lower Ti content than the parent rock, in sample 3 (biotite-amphibolite) Ti

Fig. 11. Drawings made from backscattered electron image photographs from sample 4. The shape of the crystals and their inclusions indicate a neocrystallization. (A) skeletal unzoned pyrite crystal in a complex matrix made of plagioclase, biotite and quartz. (B) Skeletal unzoned magnetite crystal in the same matrix as in (A). (C) A gypsum crystal (in white), which appears to be the result of crystallization in the pseudotachylyte. The fine stippled area indicated “plagioclase” actually corresponds to an association of biotite and plagioclase crystals in the same manner as in Figs. 2D and E.

is higher, and Al’” lower in the pseudotachylyte, and in sample 4, devoid of primary biotite, secondary biotite has a TiO, content as high as 2.5 wt%. In metamorphic rocks the Ti content of biotite is strongly influenced by the Mg/(Mg + Fe) ratio (Guidotti, 1984): a decrease in Ti is needed to equilibrate the increase of Si’” for charge balance reasons. From experiments (see, for example, Arima and Edgar, 1981) it is known that the Ti solubility in phlogopite increases with the increasing fo, and temperature and, possibly, with decreasing pressure at constant fo,_ Ti also decreases with increasing pressure (Robert, 1976). Since the Ti value decreased in the sample 1 it is suggested that it is a pressure-dependent phenomenon. In that case the fm ratio is not changed, but Fe, Ti and Mg decrease, whereas Al is higher. A low fm ratio characterizes low-pres~re-high-temperature rne~rno~~c rocks (Boss&e, 1980). Usually the Ti content of igneous rock biotite is greater than in metamorphic rocks (Velde, 1969, in Robert, 1981). Nevertheless, in the present case pressures during pseudotachylyte formation were probably higher than pressures during the host rock formation, as suggested by the nature of the new amphibole crystalIizing in the pseudot~hylyte. For sample 3, where Ti is higher and Al’” lower, a different substitution mechanism may have been operative. It has been experimentally observed that the Ti content is higher, in runs at 850 * C and 10 kbar, than the original biotite (Le Breton and Thompson, 1988). As the Ti component increases with increasing temperature it may be concluded that biotite from the pseudotachylyte of sample 3 crystallized at a higher temperature than its parent rock biotite. The nature of the pseudotachylyte minerals indicates crystallization in a high-pressure environment under a high fo,, and at a higher temperature than the parent rock; nevert.heIe.ss, these conditions were strictly restricted to the zones of melting. Cooling in pseudotachylytes is thought to be fast because the veins are thin. Hence, as noted by Macaudiere et al. (1985) for pseudotachylytes from meta-anorthosites, large degrees of undercooling could occur, as indicated by spheruhtic textures.

Spherulites of feldspars have been experimentally obtained by Naney and Swanson (1980) at undertoolings ranging from AT = 100” C to AT = 350 OC, with a gradual morphological change of the spherulites. A comparison between the observed textures (for example, in the biotiteplagioclase assemblage) and experimental results (Lofgren, 1983) shows that textural differences in the various assemblages are probably dependent on the rate of cooling and AT. The rate of cooling is also dependent on the stress-induced temperature, and is a function of the frictional heating in the volume, defined by the primary R shears, and in immediately adjacent rocks. The temperatures implied by the depth of formation and the geothermal gradient are the same for all the rocks of the system. The AT between the rock mass where pseudotachylyte occurs and the country rock may explain why the crystallization textures of the p~udo~chylytes may differ from one outcrop to another. The occurrence of idiomorphic minerals is probably a consequence of the fact that the clasts, whatever their size, favour the nucleation process, and this reduces the role of undercooling. Naney and Swanson (1980) observed that crystals nucleate more successfully in Fe- and Mg-bearing systems than in mafic-free systems. In the absence, or due to the rarity of, nuclei, textures may be complex, with an intergrowth of fan-spherulites and dendrites. The melting process is quite different from the classical anatexis phenomena because biotite or hornblende seems to be the first host rock minerals to melt, and in every case the clasts are quartz or plagioclase (Fig. 6C). This was also observed in pseudotachylyte derived from meta-anorthosites (Macaudiere et al. 1985). In rocks melted at atmospheric pressure, observed at archaeological sites (Youngblood et al. (1978) and unpublished observations from Boss&e), biotite and amphibole are the first minerals to melt, and the residuai minerals are quartz and feldspars. This observation may well be explained by the fact that, in the absence of a fluid phase, the degree of melting is directly proportional to the amount of hydrous minerals and the temperature at which, or over which, the dehydration-melting occurs, at any particular pressure (Le Breton and Thompson, 1988).

PETROLOGY

OF PSEUDOTACHYLYTES

FROM

THE

ALPINE

FAULT

This suggests that the frictional melting was at first an anhydrous, _/kid-free process, as suggested by the absence of vesicles. Rapid heating due to friction beginning

of melting

dissolution

of the

1989)

may induce

of plagioclase albite

which, afterwards,

component

oxidizing

gives way to K-feldspar K-feldspar

observed

by von Platten

component

melts with quartz

kali plagioclase. The high fo, reaction during pseudotachylyte tested by the Mn-rich ilmenites.

agent. to melt, under

(1965). Biotite

+ spine1 + H,O,

the interface sandstone boulders by shear stress

melting

drop

takes

place

seconds,

or less, the duration

between

fracturing

The dissipation

and

period

during

present

rocks is limited

at

between high-carbon steel teeth and during mechanical excavation of back shovel and front loader. The and velocity was estimated at 175

MPa at 1 m s-’ for the back shovel, and 100 MPa at 2 m s-l for the front loader, and mean surface

by the

Melt pro-

lubricates

to the dissipation

the fault

of shear stresses.

that the pseudotachylytes

in the presently

low-seismicity

and al-

observed

of heat. by conduc-

of the phenomenon.

it is noted

some

of an earthquake)

fault movement

and contributes However,

(in

of the heat produced

tion into the neighbouring duced

rapidly

the production

the time of their generation.

needed for this generation is at-

has been

stress

AF. This means

competency contrast of minerals, as well as their hydrous or anhydrous nature, plays an important role in the inititiation of melting. bulk

total

and the

The absence of amphibole clasts, and the smooth or angular edges of the plagioclase and quartz clasts respectively, are indications that the

Localized

191

ZEALAND

very short

(Johannes,

acts like a fluxing

in the reaction

conditions

the

by selective

This first melt may then allow the biotite as for example

also

OF NEW

are

zone of the

that the zone was more seismic at

Acknowledgements This work was supported

by projects

funded

by

the Centre National de la Recherche Scientifique: through “ATP Geodynarnique, INAG. 1980”, for the field study and sampling, and through the DBT INSU 88-3855 program for the analytic work (contribution CNRS-INSU-DBT “Fluides, mineraux et cinetique”). benefitted Geology

no. 245, theme The field works

from the help of the Department of the University of Canterbury

of at

Christchurch (New Zealand) and, in particular, from D. Shelley who, moreover, improved the English.

temperatures of 1400-1700 o C were achieved (Spray, 1989). In the case under study the survival of the more refractory phases such as titanite indicates that temperatures did not exceed 1400 o C. The maximum temperature, buffered by

The analytical procedures were adjusted with the collaboration of A. Barreau, electronics engineer in charge of the apparatus. J.-L. Bouchez, A. Nicolas and J.-L. Vigneresse made valuable comments and suggestions for refining the manuscript.

the melting point of the most abundant host phase, in this case amphibole or biotite, is such that the

Appendix 1: microscopic description of the samples

maximum temperature probably does not exceed 1000” C. Such a temperature may be attained because all of the deformation is concentrated in a restricted volume defined by the parallel R shears, where frictional heating caused by shear deformation intervenes. Moreover, the conduction of heat is severely limited during the short period of the pseudotachylyte formation. A crustal earthquake may occur in a seismic zone when the frictional resistance is overcome and slip is initiated. Almost all of the deformation is concentrated in a restricted volume, in this case the volume defined by the primary R shears. The

Parenr rock number I This consists mylonitic

mainly

foliation

of biotite,

is underlined

crystals,

colourless

to light

formed,

associated

with

elongate-lensoid

0.25-0.75 Plagioclases,

either circular

to slightly

Quartz

occurs

in tiny rounded

zoisite

crystals

fractures

cut

Pseudotachylyte

brown

may

opaque

veins

slightly and

Chloritization

up to 2/l (0.03-0.05

the plagioclase cross-cut

any

(0.30

is observed

x

Some

calcite-filled

the other

pre-existing

are

0.15 mm).

mm) crystals.

Sometimes and

dewith

that are parallel

up to 0.35 mm in diameter,

elongate

its

mm) biotite

minerals

mm thick bands

also be present.

across

and quartz

and generally

rounded

white mica flakes.

into discontinuous, to the foliation.

plagioclase

by tiny (0.03-0.3

crystals. structures;

192

where a thin film is formed it is blackish and isotropic. In the other part of the vein it is brownish and heterogeneous in colour, slightly pleochroic, and displays flow structure (Figs. 58 and C), surrounding rounded plagioclase clasts. Parent rock number 3

This is in contact with number 1. A vein of pseudotachylyte, made of rounded single crystals or polycrystalline clasts of plagioclase in a dark isotropic matrix, infills a fracture. Where veins are thin a homogeneous birefringence colour suggests that this represents the first product formed during pseudotachylyte generation. Several veins indicate successive generations. As for sample 1, lensoid plagioclase crystals have a variable An content in relation to the amount of included zoisite crystals. Elongated lenses of amphibole present a brownish core, including very fine inclusions (of unknown composition) surrounded by a halo of light green, slightly blue amphibole. The primary amphiboles are Mg-enriched Fehornblende (Leake, 1978). The lensoid crystals are included in a fine-grained matrix (0.07 X0.02 mm) made of plagioclase, biotite, blue-green amphibole, titanite, clinozoisite, zoisite and quartz. The pseudotachyl~e veins may be cross-cut by fractures infilled with chlorite-calcite crystals. Parent rock number 4

This is biotite-free and consists of elongate green amphiboles (0.10 mm x 0.07 mm) with a slightly brown core. SmaIler amphiboles (0.035 mm) may be included in plagioclase (An = 26%). aitemating with quartz ribbons (0.10 mm). Zoisite crystals, calcite in veins and other isolated crystals have also been observed. The pseudotachylyte vein is about 3 cm wide and made of rounded clasts occurring in the pseudotachylyte s.s., which also contains small (0.015 mm) newly crystallized plagioclases, crystallites and numerous dusty opaque minerals. The foliation presents evidence of slight folding linked to cataclastic episode.

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