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Major- and trace-element constraints on the petrogenesis of a fault-related pseudotachylyte, western Blue Ridge province, North Carolina — Reply
TECTONOPHYSICS ELSEVIER
Tectonophysics 233 (1994) 148-151
Major- and trace-element constraints on the petrogenesis of a fault-related pseudotachylyte, western Blue Ridge province, North Carolina - Reply Kieran O’Hara Department of Geological Sciences, University of Kentucky, Lexington, KY40506,
USA
(Received October 27,1992; revised version accepted May 18, 1993)
I thank M.G. Adams for pointing out the error in the geologic map used to indicate the location of the psuedotachylytes in my study (O’Hara, 1992, fig. 1). This was caused by a drafting error and it is corrected in Fig. 1. The latitude and longitude of the sample locality given in the original figure are correct, My statement regarding the footwall rocks was not meant to mislead, but merely to point out the nature of the lower plate rocks, without making inferences concerning tectonic imbrication of the footwall on a regional scale. This would have been beyond the scope of the paper. Also my statement that the pseudotachylyte is present in a single exposure was not meant to imply that these rocks were absent elsewhere, but simply that my study was concerned with a single exposure. Regarding the sense of shear on regional fault systems in the area, regional-scale studies of kinematic indicators are the only way to resolve complex fault histories over a large region. My interpretation of a single shear sense indicator at a pseudotachyl~e locality was not intended as a basis for discussion of the tectonics of this complex region. The main purpose was to point out the apparently different shear sense for the cataelastic versus the mylonitic deformation. 0040-1951/94/$07.00
Concerning the main topic of my paper, the petrogenesis of the fault zone rocks, Adams gives a more feldspar-rich modal composition for the Watauga River Gneiss and suggests that because of recrystallization, feldspar was mis-identified as quartz. A more likely expIanation for the difference cited by Adams is the heterogeneous nature of the gneisses, especially in close proximity to the fault zone. If preferential removal of a feldspar component occurred during pseudotachylyte generation, as suggested (O’Hara, 19921, this would explain the quartz-rich nature of my zone zone Al (Watauga River Gneiss) compared to the composition reported by Adams. Electron microprobe analysis presented below (Table 1, zone Cb) of a pseudotachylyte veintet cutting zone Al, indicates a feldspar-enriched composition, supporting this interpretation. Adams presents his own point counting of the clasts in the pseudotachylytes. Because his results indicate a higher proportion of potassium feldspar (44-47%) compared to zone Al reported in my study (20% alkali feldspar), Adams concludes that the corrected pseudotachylyte composition is biased toward K,O enrichment and SiO, depletion. Adams suggests that fluids could be responsible for these chemical changes.
0 1994 Elsevier Science B.V. All rights reserved
SSDI 0040-1951(93)EOO86-A
149
K O’Hara / Tectonophysics 233 (1994) 148-151
Fig. 1. Generalized geologic map showing pseudotachylyte location on the Long Ridge fault (LRF). KR = Valley and Ridge; BR = Blue Ridge; MCW= Mountain City window; GMW = Grandfather Mountain window; SMF = Stone Mountain fault; BZ = Brevard zone; TN = Tennessee; NC = North Carolina. (Modified from Goldberg et al., 1992).
My re-examination of the original data for pseudotachylyte zones B and C indicates that only O-0.6% lithic fragments consist solely of feldspar and only 0.2-3.4% of quartz. The remainder (96.6-100%) are quartz-rich mylonitic in which feldspar (and phyllosilicates) are only a minor component. Therefore, the bulk of the pseudotachylyte lithic fragments are quartz-rich, consistent with the modal and chemical composition of zone Al. Correcting for the presence of these quartz-rich lithic fragments using the composition of zone Al as the source, will deplete the bulk composition in SiO, and enrich it in K,O as indicated by the mormative data in table 1 of O’Hara (1992). The samples examined by Adams appear therefore not to be representative of the samples analyzed chemically in my study. On the basis of point counting, Adams also questions my assumption that the clasts have a composition similar to the inferred source rock (zone Al). Apart from the problem of non-representative samples, the procedure of identifying the source rock on the basis of proportion of clast types in the pseudotachylyte is unsound. This is because, if the pseudotachylyte formed by preferential enrichment of a feldspar-rich component (either by melting or by some other process), the
proportion of visible feldspar clasts will reflect the extent or efficiency of pseudotachylyte generation and not the composition of the source. In this regard it is emphasized that, in my study, zone Al was chosen as the likely source not on the basis of the proportion of alkali feldspar lithic fragments but because: (1) zone Al is gradational in handspecimen with the pseudotachylyte; (2) the pseudotachylyte lithic fragments have similar mylonitic microtextures to zone Al; and (3) the REE patterns and other trace-element signatures of the pseudotachylytes are similar to zone Al, but distinctly different from other likely sources. Because the source gneisses are heterogeneous and the proportion of different clast types in the pseudotachylytes is variable, it is important that point counting is done on the same samples that are chemically analyzed, as was the case in my study. Sample heterogeneity would explain the large differences in clast proportions observed by Adams. The problem of lithic fragments in the matrix of pseudotachylytes producing a biased bulkchemical composition is a well known one and is best overcome using electron microprobe analyTable 1 Analyses of pseudotachylyte zones Zone C a (sd) n=7 SiO, TiO, Al,O, KzG NazO MgG Fe0 +
63.44 (1.89) 0.83 (0.55) 16.37 (1.10) 13.87 (1.21) 0.64 (0.11) 0.58 (0.07) 2.72 (1.25)
Total
98.43
Zone C a.b Zone D a (sd) n=l n=5 63.42 0.15 17.48 14.87 0.61 0.62 3.48 100.73
58.08 (4.32) 1.13 (0.55) 12.65 (1.23) 10.41 (1.05) 1.18 (0.21) 1.27 (0.23) 16.01 (4.16) 100.63
Zone AIC 88.40 0.36 5.00 2.91 0.56 0.44 1.18 100.20
a Analysis by mocroprobe. Wavelength-dispersive electron microprobe analyses were performed using a defocusse beam with an accelerating voltage of 15 kV and a beam current of 15 nA. Standards used were USNM rhyolite and basalt glass. Defocussed beams areas were approximately 100 square microns. b Microprobe analysis of zone C micro-veinlet intruding cataelastic zone Al. ’ Whole-rock XRF analyses of inferred pseudotachylyte source rock from O’Hara (1992). + Total iron expressed as Fe0 sd = standard deviation.
150
K O’Hata/ Tec~o~op~~~~cs 233 (1994) 148-151
sis. Microprobe analyses of the matrix in pseudotachylyte zones C and D are presented in Table 1. The bulk-rock analysis of the inferred source (O’Hara, 1992, table 1, zone Al) is also shown for comparison. Because zone D contains the fewest included clasts (O’Hara, 1992, p. 2821, it should be more representative of the uncontaminated pseudotachylyte composition. The microprobe analyses of zone D clearly show that the pseudotachylyte matrix is strongly depleted in SiO, and enriched in K,O relative to zone Al (Table 1). Zone C shows similar enrichments and depletions, confirming the trends presented in O’Hara (1992). ~though the CIPW-normative compositions of zones C and D (using the microprobe data) plot closer to orthoclase compared to bulk-rock analyses (O’Hara, 1992, fig. 3), the conclusion of O’Hara (1992) that the pseudotachylytes are enriched in an alkali feldspar component is reaffirmed. Adams points out the “depletion” of CaO and Na,O in the pseudotachylytes relative to zone A2, and suggests fluid flow maybe a cause for these changes. As pointed out above and also in the original paper (O’Hara, 1992, pp. 283 and 286-287), zone Al is the preferred source of the pseudotachylyte, not zone A2. Therefore, comparing the chemical compositions of these two zones is not relevant to the origin of the pseudotachylytes. Many faults and shear zones in the Blue Ridge province have experienced substantial metasomatic changes due to fluid-rock interaction but they display an intense overprint by retrograde assemblages, typically evidenced by secondary growth of phyllosilicates after feldspar (e.g., O’Hara, 1990). The absence of an alteration mineral assemblage in the pseudotachylytes examined, despite their enrichment in a feldspar component indicated by the microprobe data, suggests that fluid-rock interaction is not a likely explanation for the chemical differences. Adams states “Without constraints on the nature of fluids in the Long Ridge fault zone, the possibility that fluids affected the chemical system of the pseudotachylytes cannot be discarded as a viable hypothesis”. In this case surely the burden of
proof lies with Adams to demostrate an important role for fluids in the origin of these rocks. The above arguments are not to suggest that a melt origin has been proved for these pseudotachylytes but only that melting can successfully explain the available major-, minor- and traceelement and petrographic data, as well as the microprobe data presented herein, and that invoking fluid-rock interaction as a cause of the chemical changes, without supporting evidence, is speculative. An alternative origin for these rocks is suggested by an experimental study (Yund et al., 1990) in which an amorphous material was produced in granite at room temperature in rotary sliding experiments. The fine-grained material has an alkali feldspar-enriched composition but shows no evidence for melting. Yund et al. (1990) suggest a process in which feldspar, because of its good cleavages, undergoes preferential comminution over quartz. Fluids are not implicated in the process, and the gouge material appears to be highly reactive, suggesting that, if a similar product occurs in natural fault zones, it will likely undergo alteration. The feldspar-enriched composition of the fine-grained material nevertheless suggests that cataclasis under dry conditions can result in chemical dfferentiation and produce material that is amorphous to 120 kV electrons. In contrast to the pseudotachyi~es under discussion, the experimentally produced material shows no evidence for mobility in the form of injection veins or wetting of grain boundaries. In addition, the enrichments of the pseudotachyl~e in elements such as Fe relative to the source rock (Table 1) is difficult to account for in terms of co~inution of alkali feldspar alone. Additional experiments and natural studies of ultrafinegrained fault-zone rocks are required before the current debate of a cataclastic versus a melt origin for these rocks is resolved.
Acknowledgements I thank Jim McHugh for polished thin sections and Neel Chatterjee for help with the microprobe analyses. Partial support for the microprobe facil-
K O’Hara / Tectonophysics 233 (1994) 148-151
at the University of Kentucky is provided by NSF grant EAR 92-19691 and the University of Kentucky Office of Research and Graduate Studies. ity
References Goldberg, S.A., Butler, J.R., Trupe, C.H. and Adams, M.G., 1992. The Blue Ridge thrust complex northwest of Grandfather Mountain window, North Carolina and Tennessee.
151
Field Trip #13, Geol. Sot. Am., SE Section, WinstonSalem 1992 meeting, NC., pp. 213-233. O’Hara, K.D., 1990. State of strain in mylonites from the Western Blue Ridge province, southern Appalachians: the role of volume loss. J. Struct. Geol., 12: 419-430. G’Hara, K.D., 1992. Major- and trace-element constraints on the petrogenesis of a fault-related pseudotachylyte, western Blue Ridge province, North Carolina. Tectonophysics, 204: 279-288. Yund, R.A., Blanpied, M.L., Tullis, T.E. and Weeks, J.D., 1990. Amorphous material in high strain experimental fault gouges. f. Geophys. Res., 95: l&589-15,602.
Tectonophysics 233 (1994) 148-151
Major- and trace-element constraints on the petrogenesis of a fault-related pseudotachylyte, western Blue Ridge province, North Carolina - Reply Kieran O’Hara Department of Geological Sciences, University of Kentucky, Lexington, KY40506,
USA
(Received October 27,1992; revised version accepted May 18, 1993)
I thank M.G. Adams for pointing out the error in the geologic map used to indicate the location of the psuedotachylytes in my study (O’Hara, 1992, fig. 1). This was caused by a drafting error and it is corrected in Fig. 1. The latitude and longitude of the sample locality given in the original figure are correct, My statement regarding the footwall rocks was not meant to mislead, but merely to point out the nature of the lower plate rocks, without making inferences concerning tectonic imbrication of the footwall on a regional scale. This would have been beyond the scope of the paper. Also my statement that the pseudotachylyte is present in a single exposure was not meant to imply that these rocks were absent elsewhere, but simply that my study was concerned with a single exposure. Regarding the sense of shear on regional fault systems in the area, regional-scale studies of kinematic indicators are the only way to resolve complex fault histories over a large region. My interpretation of a single shear sense indicator at a pseudotachyl~e locality was not intended as a basis for discussion of the tectonics of this complex region. The main purpose was to point out the apparently different shear sense for the cataelastic versus the mylonitic deformation. 0040-1951/94/$07.00
Concerning the main topic of my paper, the petrogenesis of the fault zone rocks, Adams gives a more feldspar-rich modal composition for the Watauga River Gneiss and suggests that because of recrystallization, feldspar was mis-identified as quartz. A more likely expIanation for the difference cited by Adams is the heterogeneous nature of the gneisses, especially in close proximity to the fault zone. If preferential removal of a feldspar component occurred during pseudotachylyte generation, as suggested (O’Hara, 19921, this would explain the quartz-rich nature of my zone zone Al (Watauga River Gneiss) compared to the composition reported by Adams. Electron microprobe analysis presented below (Table 1, zone Cb) of a pseudotachylyte veintet cutting zone Al, indicates a feldspar-enriched composition, supporting this interpretation. Adams presents his own point counting of the clasts in the pseudotachylytes. Because his results indicate a higher proportion of potassium feldspar (44-47%) compared to zone Al reported in my study (20% alkali feldspar), Adams concludes that the corrected pseudotachylyte composition is biased toward K,O enrichment and SiO, depletion. Adams suggests that fluids could be responsible for these chemical changes.
0 1994 Elsevier Science B.V. All rights reserved
SSDI 0040-1951(93)EOO86-A
149
K O’Hara / Tectonophysics 233 (1994) 148-151
Fig. 1. Generalized geologic map showing pseudotachylyte location on the Long Ridge fault (LRF). KR = Valley and Ridge; BR = Blue Ridge; MCW= Mountain City window; GMW = Grandfather Mountain window; SMF = Stone Mountain fault; BZ = Brevard zone; TN = Tennessee; NC = North Carolina. (Modified from Goldberg et al., 1992).
My re-examination of the original data for pseudotachylyte zones B and C indicates that only O-0.6% lithic fragments consist solely of feldspar and only 0.2-3.4% of quartz. The remainder (96.6-100%) are quartz-rich mylonitic in which feldspar (and phyllosilicates) are only a minor component. Therefore, the bulk of the pseudotachylyte lithic fragments are quartz-rich, consistent with the modal and chemical composition of zone Al. Correcting for the presence of these quartz-rich lithic fragments using the composition of zone Al as the source, will deplete the bulk composition in SiO, and enrich it in K,O as indicated by the mormative data in table 1 of O’Hara (1992). The samples examined by Adams appear therefore not to be representative of the samples analyzed chemically in my study. On the basis of point counting, Adams also questions my assumption that the clasts have a composition similar to the inferred source rock (zone Al). Apart from the problem of non-representative samples, the procedure of identifying the source rock on the basis of proportion of clast types in the pseudotachylyte is unsound. This is because, if the pseudotachylyte formed by preferential enrichment of a feldspar-rich component (either by melting or by some other process), the
proportion of visible feldspar clasts will reflect the extent or efficiency of pseudotachylyte generation and not the composition of the source. In this regard it is emphasized that, in my study, zone Al was chosen as the likely source not on the basis of the proportion of alkali feldspar lithic fragments but because: (1) zone Al is gradational in handspecimen with the pseudotachylyte; (2) the pseudotachylyte lithic fragments have similar mylonitic microtextures to zone Al; and (3) the REE patterns and other trace-element signatures of the pseudotachylytes are similar to zone Al, but distinctly different from other likely sources. Because the source gneisses are heterogeneous and the proportion of different clast types in the pseudotachylytes is variable, it is important that point counting is done on the same samples that are chemically analyzed, as was the case in my study. Sample heterogeneity would explain the large differences in clast proportions observed by Adams. The problem of lithic fragments in the matrix of pseudotachylytes producing a biased bulkchemical composition is a well known one and is best overcome using electron microprobe analyTable 1 Analyses of pseudotachylyte zones Zone C a (sd) n=7 SiO, TiO, Al,O, KzG NazO MgG Fe0 +
63.44 (1.89) 0.83 (0.55) 16.37 (1.10) 13.87 (1.21) 0.64 (0.11) 0.58 (0.07) 2.72 (1.25)
Total
98.43
Zone C a.b Zone D a (sd) n=l n=5 63.42 0.15 17.48 14.87 0.61 0.62 3.48 100.73
58.08 (4.32) 1.13 (0.55) 12.65 (1.23) 10.41 (1.05) 1.18 (0.21) 1.27 (0.23) 16.01 (4.16) 100.63
Zone AIC 88.40 0.36 5.00 2.91 0.56 0.44 1.18 100.20
a Analysis by mocroprobe. Wavelength-dispersive electron microprobe analyses were performed using a defocusse beam with an accelerating voltage of 15 kV and a beam current of 15 nA. Standards used were USNM rhyolite and basalt glass. Defocussed beams areas were approximately 100 square microns. b Microprobe analysis of zone C micro-veinlet intruding cataelastic zone Al. ’ Whole-rock XRF analyses of inferred pseudotachylyte source rock from O’Hara (1992). + Total iron expressed as Fe0 sd = standard deviation.
150
K O’Hata/ Tec~o~op~~~~cs 233 (1994) 148-151
sis. Microprobe analyses of the matrix in pseudotachylyte zones C and D are presented in Table 1. The bulk-rock analysis of the inferred source (O’Hara, 1992, table 1, zone Al) is also shown for comparison. Because zone D contains the fewest included clasts (O’Hara, 1992, p. 2821, it should be more representative of the uncontaminated pseudotachylyte composition. The microprobe analyses of zone D clearly show that the pseudotachylyte matrix is strongly depleted in SiO, and enriched in K,O relative to zone Al (Table 1). Zone C shows similar enrichments and depletions, confirming the trends presented in O’Hara (1992). ~though the CIPW-normative compositions of zones C and D (using the microprobe data) plot closer to orthoclase compared to bulk-rock analyses (O’Hara, 1992, fig. 3), the conclusion of O’Hara (1992) that the pseudotachylytes are enriched in an alkali feldspar component is reaffirmed. Adams points out the “depletion” of CaO and Na,O in the pseudotachylytes relative to zone A2, and suggests fluid flow maybe a cause for these changes. As pointed out above and also in the original paper (O’Hara, 1992, pp. 283 and 286-287), zone Al is the preferred source of the pseudotachylyte, not zone A2. Therefore, comparing the chemical compositions of these two zones is not relevant to the origin of the pseudotachylytes. Many faults and shear zones in the Blue Ridge province have experienced substantial metasomatic changes due to fluid-rock interaction but they display an intense overprint by retrograde assemblages, typically evidenced by secondary growth of phyllosilicates after feldspar (e.g., O’Hara, 1990). The absence of an alteration mineral assemblage in the pseudotachylytes examined, despite their enrichment in a feldspar component indicated by the microprobe data, suggests that fluid-rock interaction is not a likely explanation for the chemical differences. Adams states “Without constraints on the nature of fluids in the Long Ridge fault zone, the possibility that fluids affected the chemical system of the pseudotachylytes cannot be discarded as a viable hypothesis”. In this case surely the burden of
proof lies with Adams to demostrate an important role for fluids in the origin of these rocks. The above arguments are not to suggest that a melt origin has been proved for these pseudotachylytes but only that melting can successfully explain the available major-, minor- and traceelement and petrographic data, as well as the microprobe data presented herein, and that invoking fluid-rock interaction as a cause of the chemical changes, without supporting evidence, is speculative. An alternative origin for these rocks is suggested by an experimental study (Yund et al., 1990) in which an amorphous material was produced in granite at room temperature in rotary sliding experiments. The fine-grained material has an alkali feldspar-enriched composition but shows no evidence for melting. Yund et al. (1990) suggest a process in which feldspar, because of its good cleavages, undergoes preferential comminution over quartz. Fluids are not implicated in the process, and the gouge material appears to be highly reactive, suggesting that, if a similar product occurs in natural fault zones, it will likely undergo alteration. The feldspar-enriched composition of the fine-grained material nevertheless suggests that cataclasis under dry conditions can result in chemical dfferentiation and produce material that is amorphous to 120 kV electrons. In contrast to the pseudotachyi~es under discussion, the experimentally produced material shows no evidence for mobility in the form of injection veins or wetting of grain boundaries. In addition, the enrichments of the pseudotachyl~e in elements such as Fe relative to the source rock (Table 1) is difficult to account for in terms of co~inution of alkali feldspar alone. Additional experiments and natural studies of ultrafinegrained fault-zone rocks are required before the current debate of a cataclastic versus a melt origin for these rocks is resolved.
Acknowledgements I thank Jim McHugh for polished thin sections and Neel Chatterjee for help with the microprobe analyses. Partial support for the microprobe facil-
K O’Hara / Tectonophysics 233 (1994) 148-151
at the University of Kentucky is provided by NSF grant EAR 92-19691 and the University of Kentucky Office of Research and Graduate Studies. ity
References Goldberg, S.A., Butler, J.R., Trupe, C.H. and Adams, M.G., 1992. The Blue Ridge thrust complex northwest of Grandfather Mountain window, North Carolina and Tennessee.
151
Field Trip #13, Geol. Sot. Am., SE Section, WinstonSalem 1992 meeting, NC., pp. 213-233. O’Hara, K.D., 1990. State of strain in mylonites from the Western Blue Ridge province, southern Appalachians: the role of volume loss. J. Struct. Geol., 12: 419-430. G’Hara, K.D., 1992. Major- and trace-element constraints on the petrogenesis of a fault-related pseudotachylyte, western Blue Ridge province, North Carolina. Tectonophysics, 204: 279-288. Yund, R.A., Blanpied, M.L., Tullis, T.E. and Weeks, J.D., 1990. Amorphous material in high strain experimental fault gouges. f. Geophys. Res., 95: l&589-15,602.