Effects of lithology, cataclasis and melting on the composition of fault-generated pseudotachylytes in Lewisian gneiss, Scotland

261

Tectonophysics, 204 (1992) 261-278

Elsevier Science Publishers B.V., Amsterdam

Effects of lithology, cataclasis and melting on the composition of fault-generated pseudotachylytes in Lewisian gneiss, Scotland Robert H. Maddock Department of Geology Royal School of Mines, Imperial College, London SW7 2BP, U.R

(Received January 30,199l; revised version accepted February 6,1992)

ABSTRACT Maddock, R.H., 1992. Effects of lithology, cataclasis and melting on the composition of fault-generated pseudotachylytes in Lewisian gneiss, Scotland. In: J.F. Magloughlin and J.G. Spray (Editors), Frictional Melting Processes and Products in Geological Materials. Tectonophysics, 204 (spec.sect.): 261-278. Petrological studies of fault-generated pseudotachylytes, formed during coseismic slip, need to consider cataclastic processes as well as rapid, closed-system melting and freezing in order to account for the observed similarities and the variations between the chemical compositions of pseudotachylytes and their host-rocks. Whole-rock analyses of pseudotachylytes and their gneissic host-rocks from the Lewisian Complex of northwest Scotland indicate that the composition of the pseudotachylyte is sensitive not only to the bulk host-rock composition but also to its mineralogy and fabric. In anisotropic (e.g., gneissic) rocks, selective cataclastic and melting effects may both contribute to determining the whole-rock composition of pseudotachylytes. Electron microprobe analyses of pseudotachylyte matrices indicate more rapid decomposition and fusion of hydrous ferromagnesian phases, such as biotite and amphibole, and to a lesser extent feldspar relative to quartz. The pseudotachylytes of this study appear to have formed neither by ultracataclasis alone nor by classical equilibrium partial melting or total fusion, but through the interaction of selective cataclastic and melting processes controlled by the lithological layering and mineralogical heterogeneity in their host-rocks. This interpretation may be widely applicable to pseudotachylytes generated by faulting in foliated host-rocks.

Introduction Analyses of pseudotachylytes and their hostrocks have been carried out in this study and in previous studies (e.g., Shand, 1916; Salop, 1949; Dietrichson, 1953; Philpotts, 1964; Ermanovics et al., 1972; Sibson, 1975; Masch et al., 1985; Killick et al., 1988; Magloughlin, 1989; Bossiere, 1991) by bulk whole-rock analysis and by raster (or defocused beam) electron microprobe analysis. Such analyses generally have been directed toward an-

Correspondence to: R.H. Maddock, Geoscience Ltd., Silwood Park, Buckhurst Road, Ascot, SL5 7QW, U.K.

swering three questions pertaining to pseudotachylyte genesis: (1) What is the overall geochemical relationship between pseudotachylytes and their hostrocks? Are pseudotachylytes derived from immediately adjacent rocks, and is there evidence of later alteration of the pseudotachylyte? (2) What is the extent of melting? Is it total or partial, and if partial, by what mechanism(s) is melting accomplished? (3) What does the geochemistry of pseudotachylyte indicate about the mechanical processes involved in its formation? This question has largely been ignored in published studies (but cf. Schwarzman et al., 1983). Published chemical analyses of pseudotachylytes and their host-rocks have led to radically divergent hypotheses in answer to these questions. For example, Bhattacharjee (1963, p.51)

0040-1951/92/%05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved

262

interpreted his analyses as indicating selective fusion of “. . . competent acid gneiss layers rather than incompetent hornblende schist.. . ‘0 whereas Philpotts and Miller (1963, p. 339) inferred from their analyses that “. . . hornblende and biotite . . . have decomposed and been assimilated by the liquid much more readily than.. . quartz and feldspar.. . “. Hypotheses put forward in answer to the second question vary from total fusion (e.g., Masch, 1974; Sibson, 1975; Magloughlin, 19891 to partial melting invalving generation of a near-minimum melt composition at 65%685°C and PH2” (= PTOTALI= 500-700 MPa (SinhaRoy, 1981). In complete contrast, it has also been suggested, on the basis of microstructural observations, that pseudotachylytes form as a result of ultracataclasis with melting playing a very minor part in their genesis (e.g., Wenk, 1978). Whereas whole-rock analyses of pseudotachylyte matrix-plus-clasts can provide information on the overall transformation (involving cataclasis and melting) of host-rock to pseudotachylyte, discrimination of the amount and nature of melting requires analyses of pseudotachylyte matrix without porphyroclasts. This was appreciated during the earliest geochemical studies of pseudotachylytes (e.g., Shand, 1916). Removal of the clasts by hand-picking @hand, 1916), heavy-liquid separation (Philpotts, 19641 and indirectly by subtracting the modal clast fraction from the chemical analysis (Sibson, 1975) have all been used. However, problems arise because of the small clast size and the common occurrence of feldspar both as clasts and as (matrix) crystallites in pseudotachylyte. Electron microprobe raster/defocusedbeam analysis of clast-free pseudotachylyte matrix, pioneered by Ermanovics et al. (1972) seems to be the best technique adopted so far and was used in this study (although microprobe correction procedures are likely to result in poor analyses for light major elements such as Na and Mgl.

pated because of their great diversity 01 chemistries and textures, the contradictions in published hypotheses seem excessive. The purpose of this study is therefore to present new whole-rock and microprobe analyses directed toward answering the questions posed above; and, in particular, to attempt to identify the relative importance of cataclastic and of melting processes in contributing to the compositions of pseudotachylytes. Study area

The pseudotachylytes and host-rocks for this study were collected from the Archaean Lewisian Gneiss complex of the Outer Hebrides and the adjacent northwestern Scottish mainland around Gairloch (Fig. 1). In both areas, the mainly amphibolite facies orthogneiss and paragneisses were last metamorphosed during the Laxfordian event (c. 2300-1600 Ma; Fettes and Mendum, 1987; Park and Tarney, 1987). In the Outer Hebrides, the pseudotachylyte is associated with the longlived Outer Hebrides Fault, a crustal-scale structure which may have originated as long ago as

Purpose of this study Fig. 1. Sketch in northwest

Although a unified theory of pseudotachylyte formation/melting is perhaps not to be antici-

map showing Scotland.

general

location

The major regional

also shown: OHF = Outer

Hebrides

Thrust.

of sample

areas

fault structures

Fault;

MT = Fdoine

are

COMPOSITION

OF FAULT-GENERATED

PSEUDOTACHYLYTES

IN LEWISIAN

2500 Ma and was certainly active until the early Mesozoic (Sibson, 1977; Smythe et al., 1982; Lailey et al., 1989). The pseudotachylyte probably formed during Caledonian age thrust movements on the low-angle Outer Hebrides Fault (Sibson, 1977; Lailey et al., 1989), although the precise age of pseudotachylyte formation is poorly constrained. In the Gairloch area, the pseudotachylyte is associated with late-Laxfordian age, steep northwest-trending crush belts (Park, 1961; Park and Tarney, 1987). Field relations and petrography The

classification by Sibson (1975) of faultvein/injection-vein relationships appears to be widely applicable to pseudotachylytes generated in foliated rocks (see also Grocott, 1981; Swanson, 1988; Bossiere, 1991). This classification permits a distinction to be made between the protolith from which the pseudotachylyte was generated and the wall rock into which it may be intruded. Thus, in layered rocks (but depending upon the magnitude of fault displacement and transport of associated pseudotachylyte melt) the wall rock will generally correspond to the protolith for a concordant or foliation-parallel fault vein, but an injection vein can easily be intruded (commonly at right-angles to the fault vein) into a completely different wall-rock lithology. In this study, close attention was paid to the field relationships and to the petrography of the pseudotachylyte veins. In all but one of the samples analyzed, the pseudotachylyte clearly formed during a single fault ‘jerk’ (Sibson, 1975) or earthquake. The optical and electron @EM and TEM) petrography of these samples, which is reported elsewhere (Maddock, 1986a,b), demonstrates that the pseudotachylytes ultimately froze from a porphyroclast-loaded melt phase. Brief details of the samples and their localities are given in Appendix 1. Comparative whole-rock geochemistry of pseudotachylytes and their host-rocks During a wider study (Maddock, 1986a) nearly one-hundred rock samples were analyzed by XRF

GNEISS.

SCOTLAND

263

(major, minor and trace elements) in an attempt to answer the questions posed above concerning the genesis of pseudotachylytes. Selected examples from northwest Scotland (and one sample (SA2) from Vredefort) which illustrate the main results are presented here. Details of the analytical techniques together with tabulated analyses are given in Appendix 2. The chemical analyses are presented in the form of ‘spider’ diagrams, using the method adopted for cataclasites by Anderson et al. (1983), which allows for geochemical comparison between pseudotachylytes and their host-rocks. The host-rock samples were prepared for analysis from large samples (typically l-2 kg). The pseudotachylyte samples were much smaller (typically < 100 g) while porphyroclasts smaller than about 5 mm diameter were removed by hand-picking. To minimize the dilution effect due to varying volatile content (between host and vein samples) the analyses were recast to 100% following subtraction of H,O- and “Loss on Ignition” (LOI). Comparisons of H,O+ content, LO1 and Fe,O, (as total iron) content for eighteen pseudotachylyte and host-rock samples suggest that remaining errors due to the dilution effect are insignificant (Maddock, 1986a). A mass balance approach (Gresens, 1967; Kerrich et al., 1980) was also considered but not pursued further for the following reasons: (1) small volume increases during melting are probably balanced by similar magnitude volume decreases during freezing and crystallization (e.g., Maddock, 1983); (2) density measurements of Hebridean gneisses and pseudotachylytes indicate small and non-systematic variations (Sibson, 1977); (3) Rb-Sr isotopic data for one fault-vein/ host-rock pair suggest a closed-system bulk transformation reaction (Maddock, 1986a); and (4) there appears to be no evidence for largescale metasomatism of pseudotachylyte following its formation. The pseudotachylytes and their host-rocks occupy a broad compositional field from granite to ferrobasalt. Prior to this study, the only published comprehensive trace-element analyses of pseudotachylyte/host-rock pairs were those of Wilshire

264

R.H. MAI)DOC‘K

(1971) for Vredefort Dome samples and of Killick et al. (1988) for other South African samples. Systematic trace-element behaviour was not distinguished in either study. Pseudotachylyte fault-vein /single tionships

host-rock rela-

The major/minor-element oxide and traceelement compositions for six concordant faultvein/host-rock pairs are compared in Figure 2. Overall, the analyses demonstrate a remarkable correspondence between the whole-rock composition of the pseudotachylytes and their host-

Si TI AL Fe Mn t-!g Ca Na K

P V Cr Ni Cu Zn Rb Sr Zr Ba Y

lo-' lo'10-'100 ld 100loolooloold 10-210-21~10~1~1~*1~~10-~1u~10~

I1

I I,

rocks, a feature which has been widely noted elsewhere (e.g., Shand, 1916; Philpotts, 1964; Ermanovics et al., 1972). In the absence of any other observations, this similarity in whole-rock composition immediately suggests either total fusion of the host-rock and/or its complete transformation to pseudotachylyte via ultracataclasis or some other mechanism. However, as will be seen below, this is believed to be a simplistic interpretation. The diagrams also illustrate expected major/trace element correlations which are referable to the mineralogy of the host-rock and pseudotachylyte samples. For example, K,O and Rb are positively correlated in samples rich

I 11

11

(1

11

I”

I I I /

Ca Na K P V Cr NI Cu Zn Rb Sr Zr !3aY loo100100IO'lo-~10-~10-~ Iti10~10"10-*10~1~~10

A

Fig. 2. Plots showing chemical compositions of pseudotachylytes and their host-rocks, normalized to a volatile-free basis to minimize the effects of variable hydration. Major and minor elements expressed as wt.% oxides, total iron as Fe,O, and trace elements (V-Y) in ppm. Note logarithmic scale of concentration (y-axis) and the factor by which each concentration is multiplied (x-axis). Solid line is the pseudotachylyte; dashed line is the ‘mafic’ host-rock (< 55 wt.% SiO,); dot-dash line is the ‘felsic’ host-rock (> 55 wt.% SiO*). Tabulated analyses are given in Appendix 2.

COMPOSITION OF FAULT-GENERATED

Si ’liAl FeMnMg

Ca Na K

PSEUDOTACHYLYTES

P V Cr Ni Cu Zn Rb Sr Zr Ba Y

lo'lti1oo lo'loo18 ld lo'lo'1u~l~l~1~llo-'lo-'1~~l~1(T~10“

‘F

I 1 3 I I I I I I I1

265

IN LEWISIAN GNEISS, SCOTLAND

I I I I

in biotite (e.g., G801 and G802 sample series, Fig. 2a); TiO, and V show similar behaviour in samples rich in sphene or iron-titanium oxides. By combining field and petrographic observations with the comparative chemical analyses it is possible to make more subtle interpretations of the chemical relationships between host-rock and pseudotachylyte. The closest compositional similarity exists between concordant fault-vein pseudotachylytes (uniquely identified as such in the field) and homogeneous host-rocks, especially those which are fine-grained and in which any lithological layering is weakly developed. Sample suites meeting these criteria include the G80 series of paragneisses (see Coward et al., 1969) and fault veins (Fig. 2a). Two fault-vein/host-rock pairs collected at an interval of 7.6 m along the same fault vein were analyzed. All four rock

samples contain closely similar absolute abundances of the analyzed oxides and elements. However, small but significant (with respect to analytical precision) differences exist for P,O, Ni and Zn. Such differences almost certainly arise from modal variations in accessory mineral content (apatite, Zn- and Ni-bearing sulphides) heterogeneously distributed on the scale of the analyzed host-rock and pseudotachylyte samples (the analyzed masses of pseudotachylyte and of hostrock typically differed by an order of magnitude). Similar variations in the Zr content of other samples may be ascribed to variations in the modal content of zircon. Where host-rock compositional layering becomes better developed, the chemical relationship between the host-rock and concordant (foliation-parallel) fault veins can become more complex. The thin-section scale geometry of such an example is sketched in Figure 3 and the comparative geochemistry is shown in Figure 2b. The host-rock (GlAH) is a strongly layered paragneiss (Park, 1961) containing quartz (15%), plagioclase (20%), microperthite (13%), amphibole (36%), epidote (ll%), sphene (3%) and magnetite (2%). The layering is defined by the dimensional- and lattice-preferred orientation of the amphibole, weak irregular variations in modal mineralogy and narrow ( < 5 mm wide), quartz-microperthite veinlets (Fig. 3). The K,O, Rb and Ba contents of the pseudotachylyte are markedly greater than the host-rock, whilst the CaO and Sr contents are

__

_

hr

-

, lcm

,

Fig. 3. Sketch drawn from thin section showing geometric relations of pseudotachylyte fault veins in sample GlA. FV = pseudotachylyte fault vein; hr = host-rock; sf = quartzfeldspar veinlet, against which a narrow pseudotachylyte veinlet occurs.

266

K.H. MAI)DO(‘K

markedly less (Fig. 2b). Microprobe analyses of the host-rock and porphyroclast phases (Maddock, 1986a, 1986b) indicate that microperthite is the major K,O-bearing phase (biotite is absent) and that it also contains up to 0.5 wt.% BaO (i.e. a celsian component). These observations, together with the petrographic relationships shown in Figure 3, strongly suggest that the pseudotachylyte fault vein was generated within a layer of host-rock containing a relatively greater proportion of microperthite than the bulk host-rock. The microperthite-rich layer, or its boundary, probably constituted a mechanical anisotropy which localized faulting and pseudotachylyte generation. This localization of faulting will have influenced the whole-rock composition of the pseudotachylyte, irrespective of the relative importance of cataclastic and melting processes in the formation of the pseudotachylyte. In this example (in which melting did occur), a microperthite-quartz veinlet might be expected to have a rather low melting point, particularly if any water

was available, for example from dehydrationmelting of the amphibole (Winkler, 19671. Pseudotachylyte injection-vein /single host-rock relationships

The comparative geochemistry of two injection-vein/host-rock pairs are shown in Figure 2c. Sample series X (a beach boulder) shows a close chemical correspondence between the pseudotachylyte and its host-rock. However, for sample series GQ23, the pseudotachylyte is markedly more ‘mafic’ than the host-rock possibly suggesting that the pseudotachylyte protolith does not correspond to the analyzed host-rock. In a general sense, greater compositional variation between pseudotachylyte injection veins and their host rocks may be expected than for fault veins because of: (1) a greater likelihood that the source rock was of different composition than the host rock (i.e. spatially and hence compositionally unrelated) and (2) inclusion in the pseudotachylyte,

-

.

I

. ..,

.QO17M~ . . ,

_' .

-.

I\

\ \

Fig. 4. Field sketches showing geometric relationships between pseudotachylyte veins and host-rocks whose compositions are plotted in Fig. 5. Lined ornament is quartzofeldspathic gneiss; dotted ornament is amphibolite; black is pseudotachylyte. See Appendix 1 for sample localities.

COMPOSITION OF FAULT-GENERATED

PSEUDOTACHYLYTES

either in solid or melt form, of clasts/porphyroclasts ripped-off from the wallrocks during injection of the pseudotachylyte vein (using an igneous analogy, this would correspond to xenolith inclusion or contamination of the melt respectively).

Pseudotachylyte /multiple host-rock relationships

Where pseudotachylyte is generated at a significant lithological boundary it should be possible to assess the relative contributions to the composition of the pseudotachylyte from the differing lithologies. Four examples are considered here. The field relationships of each vein system,

Ti Al FeMnMgCaNa K P V CrNi CuZnRbSr ZrBa 10'lti10'ld loo16 16 ld 16 10-21ti10-'10"10" 1tilG-210-%

11

1 I I t t I I I I I I I

261

IN LEWISIAN GNEISS. SCOTLAND

critical to the interpretation of the data, are shown in Figure 4. Sample series QD17 possess the best constrained field relationships where a pseudotachylyte fault vein occurs at the faulted contact between quartzofeldspathic and amphibolitic hostrocks (Fig. 4a). The chemical composition of the pseudotachylyte (QD17P) and the adjacent quartzofeldspathic and amphibolitic host-rocks (QD17FH and QD17MH respectively) are shown in Figure 5a. The composition of the pseudotachylyte is ‘enveloped’ by the compositions of the two host-rocks. However, for all the major/ minor-element oxides and the majority of the trace elements, the composition of the pseudo-

Si Ti Al FeMn Mg Ca Na K P V Cr Ni Cu Zn Rb 9 Zr Bc, Y lolld 10-'1@ Id loolooloolo'lo'10-21crw10-'1010-J10%*10

I I I I I I t I,

s

I,,

,

I

I

,

r

I-

f i., i: i 1

Fig. 5. Plots showing normalized chemical compositions of pseudotachylytes and their host-rocks; see caption to Fig. 2 for explanation. For each sample set the pseudotachylyte was generated at the contact between unlike host-rocks; see Fig. 4 for details of field relationships between samples. For pseudotachylyte sample QD7P, the trace elements Cr, Ni, Cu, Zn and Y were not determined due to insufficient sample mass. Tabulated analyses are given in Appendix 2.

2hH

K.H. MADDO(‘K

tachylyte is much closer to that of the amphibolite than the quartzofeldspathic host-rock. Using the normalized oxide concentrations, the reaction x% QD17MH +y% QD17FH = 100% QD17P was modelled according to the least squares generalized mixing model of LeMaitre (1979). A best-fit solution yielded the result: 71.76% QD17MH (amphibolite) + 28.24% QD17FH (quartzofeldspathic

gneiss)

= QD17P (pseudotachylyte)

TABLE 1 Generalized mixing model results for sample series QD17 2 SiO, TiO, AI@, FeA MnO MgQ CaO Na,O KzQ pa5

The ‘fit’ of this calculation, quantified by the residual sum of the squares of the differences between the input and the calculated oxide values, is very good (Table 1). Using these calculated mixing proportions, the trace element concentrations were modelled-also resulting in a very good fit (Table 1). This mixing model indicates that a substantially greater proportion of the amphibolitic host-rock than the quartzofeldspathic host-rock contributed to the formation of the pseudotachylyte. Thus it appears that in layered rocks, lithology exerts a strong control on the whole-rock composition of pseudotachylyte. This behaviour is believed to be simply a larger-scale analogue of the effect of mineralogical layering described above for sample GlA. The field relations of three other suites of host-rocks and pseudotachylytes are illustrated in Figure 4b-d. Their chemical compositions are shown in Figure 5 and least-squares mixing model calculations are given in Table 2. The fit of the pseudotachylyte compositions to the compositional envelopes defined by their host-rocks is poorer than for sample series QD17. Similarly, the goodness of fit statistics for the least-squares mixing calculations are poorer. However, in these three suites, the concentrations of the major, minor and trace elements in the pseudotachylyte also correspond more closely to the more mafic host-rocks. The poorer fit of the mixing models for these three suites, as compared to sample series QD17, is probably due to a combination of factors including: (1) the poorer field constraints on the vein/host-rock relationships; and (2) the fact that samples QDlP, QD2P, QD7P, GQ4P, GQ12P, GQ13P and GQ16P are injection veins

55.57 1.47 13.57 12.69 0.19 3.80 7.65 3.71 1.15 0.20

55.40 1.42 13.96 12.79 0.20 3.25 7.50 4.11 1.18 0.18

0.69

X2

V Cr Ni cu Zn Rb Sr Y Zr Nb Ba X2

0.17 0.05 0.39 0.10 0.01 0.55 0.15 0.40 0.03 0.02

261 67 42 45 113 21 275 35 141 15 189

265 53 35 45 124 36 270 29 141 12 194

4 14 0

11 15 6 0 3

702

1: Best-fit least-squares solution to mixing model (oxides only) QD17MH + QD17FH = QD17P; trace-element data derived from oxide mixing model. 2: XRF whole-rock analysis of QD17P (normalized to 100% following deduction of LOI and H,O- ). 3: Differences ( Icolumn 1 -column 2 I); x2 = residual sum of squares of differences.

and, therefore, are more likely to differ in composition from their host-rocks, as discussed above. In this latter context it is perhaps significant that the smallest residual statistic is yielded by a reaction involving a fault vein (GQ17P-Table 2). The chemical analyses and model calculations for these four pseudotachylyte/host-rock suites indicate that where pseudotachylyte is generated between two lithologies, one richer and one poorer in hydrated ferromagnesian minerals (amphibole and/or biotite), the former appears to contribute in larger amount to the pseudotachylyte whole-rock (matrix-plus-clasts) composition. Because of the large differences in chemical composition (i.e., a function of modal mineralogy)

COMPOSITION OF FAULT-GENERATED

PSEUDOTACHYLYTES

269

IN LEWISIAN GNEISS, SCOTLAND

between the mafic and felsic host-rocks in the four examples described, major-element oxides as well as trace elements are sensitive to this selective mixing effect. Such selective mixing should, all other factors being equal, exert a control on the composition and/or the amount of pseudotachylyte melt that is produced (see discussion in Magloughlin, 1989). The behaviour seen in these four samples is in contrast to that seen in sample series GlA (Figs. 2b and 31, since in that sample the pseudotachylyte was generated from a layer in the host-rock with more microperthite than the bulk host-rock. However, this may simply be a scale effect. A note of caution is appropriate here. Pseudotachylytes and host-rocks from the QD sample series (Fig. 5a) contain a small amount of chlorite veining (typically < 0.05 mm width). Any preferential chloritization of the pseudotachylyte will clearly lead to a weighting of its chemical composition to a more ‘mafic’ composition. However, the amount of chlorite veining involved does not appear to be sufficient to invalidate the interpretations made above. The GQ sample series are essentially free from chlorite veining.

@GIllA

QD1lH*

.X

PDZP

l QDlOH

.GQ23 /

1.0 0.5 Host H20+Iwt%)

1'3

Fig. 6. Plot of H,O+ content in pseudotachylyte versus H,O+ content in host-rock. Tie-lines join host-rock values where pseudotachylyte is generated at the contact between two unlike host-rocks.

Volatile content of pseudotachylyte In order to further characterize the geochemical variations and similarities between host-rock and pseudotachylyte, the H20+ and CO, contents of eight host-rock/pseudotachylyte vein pairs were determined by C-H-N analysis.

The comparative H,O+ contents are plotted in Figure 6. The pseudotachylytes are generally more hydrated than their host-rocks with a maxi-

TABLE 2 Results of mixing calculations according to the least-squares mixing model of LeMaitre (1979). See Fig. 4 for the location of samples and Fig. 5 (and Appendix 2) for their chemical analyses. ‘mafic’ host

+

‘felsic’ host

=

pseudotachylyte

sum squares differences

68.41% 81.05% 62.80% 53.89% 60.73% 68.85% 80.44% 50.77% 47.08%

+ + + + + + + + +

31.59% 18.95% 37.20% 46.11% 39.27% 31.15% 19.56% 49.23% 52.92%

= = = = = = = = =

QD7P QD12P QDllP GQ17P GQ16FP GQ16IP GQ4P GQl2P GQ13P

13.88 30.33 10.20 5.06 15.96 14.68 10.51 20.22 18.36

QDlOH QDlOH QDlOH GQ22H GQ22H GQ22H GQ9H GQ9H GQ9H

QDllH QDllH QDllH GQ2lH GQ21H GQ21H GQlOH GQlOH GQlOH

K.H. MADDOCK

.QDlOH

0

Host

0.5

the host-rock prior to pseudotachylyte formation; (2) preferential incorporation of hydrated hostrock mineral phases during pseudotachylyte formation; or (3) hydration of the pseudotachylyte at some time following its formation. In these samples, there is no convincing evidence to suggest that localized hydration occurred during any prepseudotachylyte deformation (cf. Magloughlin, 1992). Preferential incorporation of hydrated host-rock mineral phases during pseudotachylyte formation would be consistent with the chemical data presented above. However, the chlorite veins found in some pseudotachylyte samples would also result in minor hydration.

co2 (wt.%)

Fig. 7. Plot of CO, content in pseudotachylyte versus CO, content in host-rock.Tie-lines join host-rock values where pseudotachylyte is generated at the contact between two unlike host-rocks.

mum absolute increase in H,O + content of 1 wt.%; the maximum relative increase is by a factor of three. For sample series GlA, the chemical and petrographic observations discussed above suggest that the actual protolith may be less hydrated than the analyzed sample. The high H,O+ content of sample GQlAP (2.5 wt.%) correlates well in a qualitative sense with the high chlorite content of this pseudotachylyte. Apart from this example, no significant correlation could be made, from this admittedly small data set, between pseudotachylyte petrography and H,O+ content (Maddock, 1986a). The CO, contents, measured in the same host-rock/vein pairs, are plotted in Figure 7. Both the host-rocks and the pseudotachylytes contain very little CO,, but the figure indicates that the latter tend to contain slightly more CO, than the former. The CO, may be largely contained in small amounts of carbonate visible as an alteration or cavity-filling phase in some samples (e.g., Maddock et al., 1987). For seven out of the eight sample pairs analyzed, the assemblage pseudotachylyte matrix plus clasts is slightly more hydrated (and carbonated) than the host-rock protolith. This small increase in volatile content could result from one, or a combination of factors, including: (1) hydration of

Composition of pseudotachylyte mined by electron microprobe

matrix deter-

The previous section has demonstrated that for pseudotachylytes generated in gneissic rocks, the mineralogical and lithological layering (defining the strength anisotropy which localizes faulting) exerts an influence on the whole-rock (pseudotachylyte-matrix-plus-clasts) composition of the pseudotachylyte. It is suggested that this may also exert an influence on the composition of the pseudotachylyte matrix which can very broadly be assumed to represent the melt composition. To examine this suggestion, electron microprobe (EMP) analyses have been made of pseudotachylyte matrices. Details of the analytical techniques and tabulated analyses are given in Appendix 3. These analyses were restricted to pseudotachylyte matrices with grain sizes less than about 5 microns in an attempt to minimize analytical artefacts resulting from the physics of electron beam-specimen interaction. This means that a somewhat biased sample, of fine-grained, phyllosilicate- and pyroxene-spherulitic pseudotachylytes (Maddock, 1986b), has been considered only. For the purposes of graphical representation, the analyses have been recast into their normative constituents (with Fe,O,/FeO ratios of 0.35 and 0.25 assigned to the pseudotachylyte and the host-rock, respectively). Analyses of pseudotachylyte whole-rock (matrix-plus-clasts) determined by XRF, and pseudotachylyte matrix determined by EMP for the same

COMPOSITION OF FAULT-GENERATED

PSEUDOTACHYLYTES

IN LEWISIAN GNEISS, SCOTLAND

o-host-rock

Fig. 8. Qz-Ab-Or (recalculated normative wt.%) plot showing relationship between pseudotachylyte whole-rock (matrixplus-clasts) composition determined by XRF (open symbols) and pseudotachylyte matrix composition determined by EMP (closed symbols). Tie lines join whole-rock to one or more mineralogically/ texturally distinct matrix component. Data from samples G802, GQ33, GlA, SU802B, X and R2. Tabulated analyses are given in Appendices 2 and 3.

sample are shown in Figure 8. For five of the seven samples, the whole-rock (XRF) composition is significantly more quartz-normative than the matrix (EMP) composition, demonstrating the ‘weighting’ effect of quartz porphyroclasts on the whole-rock analyses. This same effect can be seen petrographically in terms of modal quartz content, by comparing point count analyses of hostrock and pseudotachylyte (e.g., sample series G80, Table 3). In Figure 9, host-rock (XRF) and pseudotachylyte (EMP) compositions are compared for four analyses from this study and two from the literature. Again the majority of samples indicate that the pseudotachylyte matrix is poorer in TABLE 3 Modal composition (vol. %) of sample series G80

Quartz Plagioclase Biotite Garnet Opaque Pseudotachylyte matrix

1

2

3

4

5

6

23 39 28 9 1

19 5 0 0 0

22 8 0 0 0

21 8 0 0 0

20 10 0 0 0

15 5 0 0 0

0

76

70

71

70

80

1: G801H = host-rock 2: G801P = pseudotachylyte 3: G802P = pseudotachylyte 4: G803P = pseudotachylyte 5: G804P = pseudotachylyte 6: G805P = pseudotachylyte.

(XRFI

Ab

Fig. 9. Qz-Ab-Or (recalculated normative wt.%) plot showing the relationship between host-rock composition determined by XRF (open symbols) and pseudotachylyte matrix composition determined by EMP (closed symbols). Tie lines join host-rock to one or more texturally/mineralogically distinct matrix component. Data from samples G802, GQ33, GlA, X, and published data of Masch (1973) and Ermanovics et al. (1972). Tabulated analyses from this study are given in Appendices 2 and 3.

normative quartz than the host-rock. The data shown in both figures are consistent with feldspar and biotite (when present) preferentially being incorporated into the pseudotachylyte melt relative to quartz. Discussion Accepting that pseudotachylyte formation almost certainly involves both frictional fusion (e.g., Sibson, 1975) and cataclasis (e.g., Wenk, 1978) (although the end-product is clearly a melt-rock), modem accounts of seismic faulting (e.g., Scholz, 1990; Swanson, 1992-this issue) suggest that four main processes may influence the composition of pseudotachylytes: (1) fracture-mechanical and cataclastic processes occurring during rupture propagation and fault slip; (2) ‘initial’ melting processes; (3) assimilation of porphyroclastic material by the initially-formed melt; and (4) freezing and crystallization of the melt. These are not considered to be a set of rigidly defined stages in the formation of pseudotachylyte, but an approximate sequence of processes

272

which may overlap in time and space along a fault during and after slip (Swanson, 1992). Petrographic and chemical data have been presented (sample GlA excepted) which strongly suggest that during pseudotachylyte melt formation the major host-rock phases are consumed preferentially in the following order: biotite and amphibole more rapidly than feldspars, and feldspars more rapidly than quartz. This is clearly shown, for example, by comparing the modal analyses of porphyroclastic and host-rock mineral phases in sample series G80 (Table 3). Two features of this proposed sequence of mineral consumption are significant; first, it corresponds to the dry l-atmosphere melting and/or breakdown points of these minerals (Clarke, 1966); secondly, it is suggested that this sequence may also reflect an increasing resistance to comminution during cataclastic deformation. The preferential incorporation of hydrated ferromagnesian minerals into pseudotachylyte suggested by the geochemical data presented above complements the results of a geochemical study of cataclastic rocks along the San Gabriel Fault, California, by Anderson et al. (1983). These workers found that along faults separating two different lithologies, “ . . . originally micaceous, foliated or physically more heterogeneous rock units may contribute a disproportionally large amount to the resultant intrafault material” (Anderson et al., p. 233). For pseudotachylyte faulting, this idea is further supported by the observation that biotite and amphibole tend to be preferentially ‘smeared’ out along sub-barren pseudotachylyte microfaults. Biotite shows similar behaviour in high-temperature/high strain rate triaxial deformation experiments on granite (Stesky, 19781, although in such experiments it probably deforms plastically. A high degree of resistance to comminution of quartz relative to other minerals during pseudotachylyte formation has also been proposed by Philpotts (1964), Allen (19791 and Schwarzman et al. (1983). Unfortunately, there appears to be little published quantitative data concerning the relative rates of comminution of minerals during crushing or shearing of silicate rocks. However, frictional melting experiments designed to simulate pseudotachylyte generation

K.H. MADDOCK

have led to the suggestion that the mechanical properties of minerals may control melting (Spray 1987, 1988). This theme was expanded by Magloughlin (1989) who suggested that the mechanical properties of individual minerals first control the composition of (ultra-) cataclasite and subsequently of pseudotachylyte, which forms by melting of the cataclasite matrix. Magloughlin’s model appears to be entirely compatible with the geochemical and petrographic data presented above. The importance of biotite (when present in the host-rock) during initial melting of pseudotachylytes (and of landslide-generated frictionites) has been stressed by Scott and Drever (1953), Allen (1979) and Bossiere (1991) and these authors discuss some of the effects outlined above. In contrast, Philpotts (1964) suggested that corundum-normative pseudotachylytes might result from assimilation of biotite or chlorite porphyroclasts into an already-formed melt. In the present study, seven out of eight Co-normative microprobe analyses were from pseudotachylytes whose host-rocks contained 15 modal% or more biotite. However, the above discussion points to the importance of biotite during initial melting rather than during later assimilation. Initial melting may be expected to affect first the finest grain size fraction having a high surface area/volume ratio and a high density of defect and damage structures. There is abundant experimental evidence that these parameters exert a kinetic control on initial melting in both hydrostatic and non-hydrostatic partial melting experiments (e.g., van der Molen and Paterson, 1979). On the basis of the chemical and petrographic data given above, it is argued that the finest grain size fraction will contain abundant biotite or amphibole, depending on the host-rock mineralogy. Frictional heating sufficient to cause melting requires very low pore fluid pressures (Sibson, 1973) and, in gneisses and other low-porosity/low-permeability rocks, this means that the total water content must be small. In this situation it appears likely that initial melting is largely controlled by the dehydration-breakdown of hydrous ferromagnesian minerals. This behaviour previously has been shown to occur during the assimilation and partial melting of lithic porphyroclasts in two of

COMPOSITION

OF FAULT-GENERATED

PSEUDOTACHYLYTES

IN LEWISIAN

these pseudotachylytes (Maddock, 1986b). This may affect the course of melting in a number of ways: (1) Release of water by dehydration will lower the melting points of other minerals. (2) Released water will lower the viscosity of the melt assisting heat transfer (i.e., the rate of heating of unmelted domains) by convection and advection, and also injection of the melt away from the fault plane which is necessary to maintain frictional contact and continued melt production (Swanson, 1992). (3) Incorporation into the melt of networkmodifying cations such as Fe2+, Mg+, Ca2+, Na+ and K+ will also tend to reduce the viscosity of the melt. (4) The reaction products of dehydration breakdown will be extremely fine grained, enhancing their susceptibility to melting (A.J. Brearley, pers. commun., 1985). (5) In the case of the dehydration breakdown of biotite, an orthoclase component (KAlSi,O,) should be released into the melt tending to reduce the ‘liquidus’ temperature (Winkler, 1967). The rare occurrence of mixtures of pseudotachylyte and cataclasite/ultracataclasite, in the samples of this study, suggests that (?initial) melting is pervasive except perhaps at the cooler margins of some fault veins (but cf. Magloughlin, 1992). Following initial melting, larger and more refractory porphyroclasts (mainly quartz and feldspar according to the above model) will begin to be corroded and assimilated by the melt. Good evidence exists for this, but the degree of assimilation of clasts and the extent to which it affects the ultimate composition of the pseudotachylyte melt is difficult to assess. This will be largely dependent upon: (1) the initial composition and temperature of the melt; (2) the porphyroclast mineralogy, grain size and deformation state; and (3) the time-integrated amount of heat available. Irrespective of all other factors, the net result of porphyroclast assimilation can only be to direct the melt composition toward that which would result from total fusion. However, is clear from Figure 9 that this is rarely if ever achieved, probably because of fast cooling rates, moderate melt viscosities and the rapid crystal nucleation that

GNEISS,

SCOTLAND

213

occurs in pseudotachylyte melts ‘seeded’ with abundant porphyroclast nuclei (Maddock, 1983). Published, generalized hypotheses which suggest that pseudotachylyte melt forms in gneissic rocks by equilibrium partial melting of a lowmelting-point granitic fraction, or by total melting, are rejected here. This is because the chemical and petrographic data reported here and elsewhere point to the rapid preferential breakdown (by cataclastic and melting processes) of hydrated ferromagnesian minerals relative to felsic minerals during pseudotachylyte formation. Acknowledgements The majority of this work was undertaken during the tenure of a NERC research studentship (1979-1982) at Imperial College which is gratefully acknowledged. The supervision of J. Nolan and R.H. Sibson is also gratefully acknowledged. J. Glasser kindly performed the C-H-N analyses at Imperial College. John Spray provided the vital impetus for completing a manuscript and his incisive comments, together with those of John Grocott, Kieran O’Hara, Jerry Magloughlin, A.R. Philpotts and two anonymous reviewers assisted my thinking. References Allen, A.R., 1979. Mechanism of frictional fusion in fault zones. J. Struct. Geol., 1: 231-243. Anderson, J.L., Osborne, R.H. and Palmer, D.F., 1983. Cataelastic rocks of the San Gabriel fault-an expression of deformation at deeper levels in the San Andreas Fault zone. Tectonophysics, 98: 209-251. Bhattacharjee, C.C., 1963. The late structural and petrological history of the Lewisian rocks of the Meal1 Deise area, N of Gairloch, Ross-shire. Trans. Geol. Sot. Glasgow, 25: 31-60. Bossiere, G., 1991. Petrology of pseudotachylytes from the Alpine Fault of New Zealand. Tectonophysics, 196: 173193. Clarke, S.P., 1966. Handbook of physical constants. Mem. Geol. Sot. Am. 97. Coward, M.P., Francis, P.W, Graham, R.H., Myers, J.S. and Watson, J., 1969. Remnants of an early metasedimentary assemblage in the Lewisian Complex of the Outer Hebrides. Proc. Geol. Assoc., 80: 387-408. Dietrichson, B., 1953. Pseudotachylit fra de kaledonske Skyresoner i Jotunheimens forgarder, Gudbrandsdalen, og

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COMPOSITION

OF FAULT-GENERATED

PSEUDOTACHYLYTES

IN LEWISIAN

Spray, J.G., 1988. Generation and crystallization of an amphibolite shear melt: an investigation using radial friction welding apparatus. Contrib. Mineral. Petrol., 99: 464-475. Stesky, R.M., 1978. Mechanism of high temperature frictional sliding in Westerly Granite. Can. J. Earth Sci., 15: 361-375. Swanson, M.T., 1988. Pseudotachylyte-bearing strike-slip duplex structures in the Fort Foster Brittle Zone, S. Maine. J. Struct. Geol., 10: 813-828. Swanson, M.T., 1992. Fault structure, wear mechanisms and rupture processes in pseudotachylyte’ generation. In: J.F.

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Appendix 1: Sample details

G801H-2H G801P-5P GQlAH GQlAP GQlOH GQ9H GQ4P GQ12P GQ21H GQ22H GQ16P GQ17P GQ23H GQ23P GQ24H GQ24P GQ33H GQ33P QDlOH QDllH QDZP QD7P QD16H QDlSP QD17FH QD17MH QD17P QLlH QLlP SU802B R2 X/H X/P GlAH GlAP

quartz-plagioclase-biotite-garnet paragneiss; SE Grimsay (NF877547) pyroxene-spherulitic, concordant fault vein cutting G801H amphibolite gneiss; roadside cutting N Grimsay (NF867571) chlorite-spherulitic fault vein cutting GQlAH quartz-rich amphibolite gneiss; roadside cutting, N Grimsay (NF867571) amphibolite gneiss; roadside cutting, N Grimsay (NF867571) injection vein generated at contact between GQlOH and GQ9H (see Fig. 4) injection vein generated at contact between GQlOH and GQ9H (see Fig. 4) quartz-rich amphibolite gneiss; roadside cutting, N Grimsay (NF867571) amphibolite gneiss; roadside cutting, N Grimsay (NF867571) injection vein generated at contact between GQ21H and GQ22H (see Fig. 4) fault vein generated at contact between GQ21H and GQ22H (see Fig. 4) quartz-rich amphibolite gneiss; roadside cutting, N Grimsay (NF867571) injection vein cutting GQ23H amphibolite gneiss; roadside cutting N Grimsay (NF867571) injection vein cutting GQ24H garnet-amphibolite gneiss; roadside cutting, N Grimsay (NF869569). injection vein cutting GQ33H amphibolite gneiss; Druidibeg Quarry, S Uist (NF766355) quartz-rich amphibolite gneiss; Druidibeg Quarry, S Uist (NF766355) injection vein generated at contact between QDlOH and QDllH (see Fig. 4) injection vein generated at contact between QDlOH and QDllH (see Fig. 4) amphibolite gneiss; Druidibeg Quarry, S Uist (NF766355) fault yein cutting QD16H quartz-rich amphibolite gneiss; Druidibeg Quarry, S Uist (NF766355) amphibolite gneiss; Druidibeg Quarry, S Uist (NF766355) fault vein generated at contact between QD17FH and QD17MH amphibolite gneiss; Askernish Quarry, S Uist (NF753218) fault vein cutting QLlH phyllosilicate-spherulitic injection vein generated in felsic gneisses; Rubha Ardvule, S Uist (NF711299) plagioclase-microlitic injection vein generated in felsic gneisses; Rubha Ardvule, S Uist (NF712269) quartzofeldspathic gneiss; beach boulder from Rubha Ardvule, S Uist (NF711301) phyllosilicate-spherulitic injection vein cutting X/H amphibole-plagioclase-microperthite-epidote paragneiss; roadside cutting, Gairloch (NG831781). Collected by R.H. Sibson fault vein cutting GlAH

276

R.H. MADDOCK

Appendix 2: XRF analyses of pseudotachylytes

and their host-rocks

Analyses were performed using a Phillips 1212 automated XRF wavelength-dispersive spectrometer. For major and minor elements fused glass discs were prepared; pressed powder briquettes were used for trace elements. H,O- was determined from sample weight loss after drying for 8 hours at 110°C. LO1 was determined from weight change after ignition at 950°C for l/2 hour. International rock standards were used to prepare calibration lines. Full details of the techniques are given by Parker (1979; 1982). Details of the analytical precision determined for these analyses are given in Maddock (1986a). G801H

G801P

G802H

G802P

GQ9H

GQlOH

GQ4P

GQl2P

GQ2lH

GQ22H so.34

SiOz

60.64

59.64

60.06

60.67

47.38

67.01

51.39

55.60

66.16

TiO,

0.57

0.63

0.63

0.60

1.28

0.26

1.41

0.74

0.34

1.11

AI&h

16.63

17.42

16.53

16.92

13.61

15.76

14.56

18.14

16.99

13.57

FeA*

7.31

7.27

7.19

6.94

14.81

3.41

14.64

8.31

2.76

13.33

MI-IO

0.13

0.12

0.12

0.10

0.19

0.06

0.25

0.11

0.11

0.25

MgQ

3.89

4.13

4.36

4.21

6.70

2.oi

4.28

3.22

1.77

5.88

CaO

3.37

3.44

3.44

3.94

11.25

4.99

8.28

7.16

3.82

10.40

Na,O

3.23

3.67

3.82

3.32

2.76

5.17

2.86

5.39

5.94

3.23

KzQ

2.33

2.68

2.43

2.20

0.65

0.47

0.78

0.67

1.16

0.64

PA

0.16

0.12

0.27

0.15

0.15

0.17

0.19

0.27

0.22

0.20

H,O-

0.07

0.07

0.05

0.15

0.06

0.04

0.06

0.04

0.07

0.06

LO1

0.60

0.81

0.66

0.94

0.30

0.20

0.27

0.18

0.25

0.31

Total

98.93

100.30

99.56

100.14

99.14

99.55

98.97

99.83

99.59

99.32

V

162

144

153

131

365

60

326

145

59

320

Cr

291

251

280

251

99

24

29

55

45

99

Ni

72

109

134

106

75

17

44

47

31

65

CU

37

46

38

46

127

46

125

151

9

99

Zn

68

99

112

93

110

32

131

106

0

53

Rb

102

125

101

102

5

13

19

3

57

7

Sr

301

311

332

319

167

575

231

590

645

183

Y

23

21

14

20

36

11

27

12

2

30

Zr

124

141

120

143

81

161

88

72

0

301

Ba

511

523

583

393

27

89

184

174

497

61

ND = not determined; Fe,O:

= total iron expressed as Fe,O,.

COMPOSITION

OF FAULT-GENERATED

PSEUDOTACHYLYTES

IN LEWISIAN

GNEISS,

211

SCOTLAND

Appendix 2 (continued) GQ16P

GQ23H

GQ17P

GQ23P

QDlOH

QDllH

QD2P

QD7P

QD16H

QDlSP

55.11

48.83

49.27

SiO,

53.46

57.09

60.21

51.42

48.39

68.95

53.03

TiO,

0.82

0.75

0.76

1.34

0.96

0.31

1.08

1.25

0.89

0.86

AI2o3

11.51

17.08

17.54

15.32

13.31

15.08

13.46

13.46

14.16

13.83

Fe,O,*

9.73

8.64

6.07

13.94

13.81

2.91

3.31

12.96

13.15

12.85

MnO

0.11

0.12

0.12

0.23

0.23

0.09

0.19

0.20

0.26

0.25

M&J CaO

3.60

3.27

2.18

4.16

6.04

0.81

2.47

3.12

7.30

6.35

7.44

7.00

6.79

8.27

10.86

3.87

11.93

7.35

11.50

10.91

Na,O

4.99

4.71

5.10

3.40

3.42

6.62

0.95

3.06

2.98

3.13

K2O

0.65

0.52

0.44

0.84

1.02

0.85

0.17

0.73

1.07

1.04

p205

0.37

0.20

0.18

0.19

0.11

0.14

0.12

0.15

0.09

0.07

H,O-

0.19

0.05

0.02

0.05

0.08

0.06

0.16

0.14

0.06

0.10

LO1

0.36

0.17

0.27

0.94

0.72

0.82

1.91

1.72

0.46

0.81

99.29

99.60

99.68

99.30

98.95

100.51

98.78

99.25

100.75

99.47

Total V

162

130

111

302

302

35

242

235

285

283

Cr

43

75

26

31

157

21

71

ND

136

114

Ni

63

46

35

46

83

13

46

ND

86

77

CU

147

46

72

148

132

74

68

ND

37

45

Zn

121

126

51

125

101

29

123

ND

92

105

Rb

5

2

3

22

16

36

2

18

8

6

Sr

528

501

550

280

145

378

982

363

238

255

17

16

11

25

29

3

27

ND

21

19

Zr

109

175

27

82

57

132

102

124

71

40

Ba

143

132

116

212

114

251

57

299

105

109

Y

ND = not determined; Fe,O,* = total iron expressed as Fe,O,,

QD17FI-I

QD17MH

QD17P

GlAH

GlAP

X/H

X/P

SU802B

R2

SiO

70.51

49.17

54.97

54.35

54.05

68.62

67.00

64.23

TiO:

0.26

1.93

1.41

1.34

1.50

0.40

0.47

0.65

0.69

15.53

12.68

13.85

13.29

13.25

15.21

15.91

15.05

16.47

Fe,O,*

2.09

16.75

12.69

14.16

14.39

3.85

4.26

5.82

6.89

MnO

0.05

0.24

0.20

0.20

0.12

0.08

0.07

0.10

0.09

MgG CaO

0.84

4.94

3.22

3.15

2.83

1.12

1.22

1.02

1.64

3.36

9.28

7.44

7.02

3.48

3.78

3.69

4.26

4.30

Na,O

5.24

3.07

4.08

2.56

3.28

4.47

5.16

3.39

4.49

IW

1.14

1.14

1.17

1.81

4.58

1.60

1.59

2.56

2.89

p205

0.12

0.23

0.18

0.19

0.21

0.12

0.17

0.27

0.26

H,O-

0.04

0.05

0.00

0.06

0.27

0.12

0.05

0.19

0.04

LOI

0.32,

0.27

0.45

1.04

1.36

0.42

0.62

1.25

0.41

99.56

99.75

99.66

99.17

99.32

99.79

100.21

98.79

99.98

403

Total

61.81

V

26

353

265

283

291

56

60

ND

99

Cr

25

83

53

10

7

25

22

ND

14

Ni

12

54

35

32

32

15

16

ND

11

cu

31

50

45

34

47

16

14

ND

20

Zn

37

142

98

68

70

ND

117

44

143 13

124

Rb

36

23

90

43

59

ND

113

Sr

573

159

270

433

211

412

412

ND

450

2

48

29

29

30

5

4

ND

9

Zr

177

127

141

138

158

812

73

ND

197

Ba

339

130

194

890

2446

625

669

ND

855

Y

ND = not detemuned; Fe,O,*

= total iron expressed as Fe,O,.

278

Appendix 3: Electron microprobe

analyses of pseudotachylyte

matrices

Electron microprobe analyses were made on C-coated, polished thin sections in a Cambridge Instruments Microscan V electron microprobe. The machine is fitted with a Link Systems energy-dispersive X-ray analyser and was operated at 15 kV accelerating voltage and a beam current of 80-120 PA. Well-characterized mineral standards were analyzed at the same time as the samples and in the same mode i.e. defocussed or raster. Defocussed beam and raster beam areas of 50-100 square microns were typically used; standard deviations (n - 1) of replicate analyses are shown bracketed. Microprobe analytical precision data for the elements determined are given in Maddock (1986b).

SiOz TiO *1& Fe0 * MnO MgO CaO Na,O K,O Total

1

2

3

4

5

6

7

55.64 (0.51) 0.63 (0.06) 19.08 (0.37) 7.47 (0.53) 0.20 (0.04) 4.57 (0.36) 4.76 (0.12) 4.24 (0.17) 1.69 (0.14) 98.27

57.31 0.76 16.00 7.00 ND 5.43 2.65 2.64 4.15 96.34

38.11 (2.11) 2.49 (0.42) 13.49 (1.65) 17.97 (1.83) 0.24 (0.10) 10.30 (0.61) 4.63 (1.18) 0.31 (0.21) 3.13 (0.34) 90.67

53.16 (1.51) 2.01 (0.23) 15.95 (0.75) 12.50 (2.47) ND 0.34 (0.30) 1.87 (0.10) 1.10 (0.48) 11.75 (0.56) 98.68

57.13 (2.28) 1.74 (0.20) 18.00 (0.64) 6.98 (1.13) 0.19 (0.31) 2.69 (0.74) 5.53 (0.86) 5.43 to.471 1.94 (0.43) 99.63

60.68 (1.15) 0.27 (0.08) 18.24 (0.66) 5.32 (1.04) ND I .Ol (0.36) 3.06 (0.53) 8.43 (0.48) 0.79 (0.31) 97.80

58.82 (1.32) 0.76 (0.12) 17.09 (0.63) 5.43 (0.15) ND 1.87 (0.14) 5.71 (0.23) 6.85 (0.52) 0.56 (0.11) 97.09

ND = not detected; Fe0 * = total iron expressed as FeO; 1. G802P 4 raster analyses of component A. 2. G802P 3 raster anlyses of component B. 3. GlA 7 defocussed spot analyses of interspherulite matrix. 4. GlA 5 defocussed spot analyses of spherulitic haloes surrounding potassium feldspar porphyroclasts. 5. R2 10 raster analyses of fan spherulites. 6. SU802P 5 rastef analyses of dark brown phase. 7. SU802B 5 raster analyses of marginal dark brown phase.