Element mobility associated with syn‐metamorphic shear zones near Scotchport, NW Mayo, Ireland

Winchester, J. A.; Max, M. D.

doi:10.1111/j.1525-1314.1984.tb00281.x

Cited by 38 publications

(14 citation statements)

References 27 publications

(24 reference statements)

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“…Beach (1976), Winchester & Max (1984), and Vocke et al (1987) argue for little (<lo%) to n o volume change during shearing, in contrast to the c. 60% volume losses proposed by O'Hara (1988) and in this study. In each of the nearly isovolumetric shear zones, Si and Ti concentrations remain nearly constant and Fe concentrations decrease from the wallrock to the centre of the zone.…”

Section: Discussioncontrasting

confidence: 82%

Fluid channelling during ductile shearing: transformation of granodiorite into aluminous schist in the Tauern Window, Eastern Alps

Selverstone¹,

Morteani²,

Staude³

1991

Journal Metamorphic Geology

187

118

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Ductile shearing in the core of the Tauern Window, Austria, transformed metagranodiorite into Si-undersaturated garnet-chlorite-staurolite schist at a depth of c. 35-40 km during the Alpine orogeny. Four distinct zones have been recognized extending from the wallrock into the centre of the shear zone: Zone I-unaltered metagranodiorite with subordinate amphibolite; Zone 11-biotite-white mica-garnet schist; Zone 111-biotite-phengite schist; Zone IV-quartz-absent, garnet-chlorite-staurolite schist with garnets up to 10 cm across. Whole-rock analyses show a dramatic decrease in SiO, from >65 wt% in Zone I to <35 wt% in Zone IV; Ca, Na, and Sr also decrease across the shear zone, whereas Al, Ti, Fe, Mg, P, Cr, Ni, Zn, and Rb all increase towards Zone IV. Mass-balance calculations indicate that shearing was accompanied by up to 60% volume loss near the centre of the shear zone. Comparison of the Tauern Window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in Mg, Fe, and Ti is typical for volume-loss shear zones, but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour.In the samples studied here, volume loss appears to have resulted from channellized fluid flow during shearing, producing time-integrated fluid fluxes of 210s cm3 cm-, in Zone IV. This large volume of, fluid may have originated, in part, from dehydration of flysch camed beneath the metagranodiorites during Eocene movement on the North Penninic subduction zone. Development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating.

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Section: Discussioncontrasting

confidence: 82%

Fluid channelling during ductile shearing: transformation of granodiorite into aluminous schist in the Tauern Window, Eastern Alps

Selverstone¹,

Morteani²,

Staude³

1991

Journal Metamorphic Geology

187

118

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“…Pressure solution associated with shearing commonly gives rise to a redistribution of major and minor elements (Beach and Tarney 1978;Winchester and Max 1984;Plimer 1980). The removed elements are considered to have been deposited in the nearby mineralized granite which is relatively enriched in these elements.…”

Section: Lithological and Stratigraphic Influences On Mineralizationmentioning

confidence: 99%

“…Element mobility in shear zones is important during metamorphism (Beach and Tarney 1978;Winchester and Max 1984) and is to be suspected where mineralization is developed in particular deformation zones such as the ECDZ. Selective removal and redeposition of given elements should result in definable depletion and enrichment zones.…”

Section: The Role Of Shearing In Mineralizationmentioning

confidence: 99%

Deformational and stratigraphic influences on mineralization in SE Ireland

McArdle

Kennan

1992

Mineral. Deposita

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Abstract. The evidence that has accumulated to date to support the thesis that deformational and stratigraphic controls are reflected in the occurrence and siting of metal deposits in SE Ireland is evaluated. In the case of an important tungsten deposit, stratigraphic influences can be seen in the geochemical and mineralogical development of wallrocks prior to crystallization of granite-hosted mineralization. Tourmalinites, confined to a definable stratigraphic interval, are associated with pegmatite-hosted lithium deposits. Though the stratigraphic control is important, the tungsten and lithium deposits are, in addition, confined to the trace of a major deformation zone.A number of recent studies address the question of the origin of various metal deposits associated with tourmalinite on the margin of the ca. 400 Ma Leinster Granite in SE Ireland (Kennan

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“…Many examples of such zones acting as fluid conduits have been described (Winchester and Max, 1984;Smalley et al, 1988;Dipple et al, 1990;Gates and Speer, 1991). Removal or addition of alkalis and Sr, potentially aided by recrystallization, neomineralization, and mineral dissolution, has the potential to change greatly the Rb/Sr of the protolith relative to the unaltered rock.…”

Section: Fluids and Contacts/fault Zonesmentioning

confidence: 99%

“…Removal or addition of alkalis and Sr, potentially aided by recrystallization, neomineralization, and mineral dissolution, has the potential to change greatly the Rb/Sr of the protolith relative to the unaltered rock. Indeed, Rb (or K) gain and Sr (or Ca) loss appear to be likely at least in certain lithologies based on field and experimental studies (Orville, 1963;Winchester and Max, 1984;Smalley et al, 1988;Gates and Speer, 1991;Hamamoto et al, 1994). Sr isotopic exchange between the fluid and the protolith can also lead to initial slopes of the pseudotachylyte-host rock isochron.…”

Section: Fluids and Contacts/fault Zonesmentioning

confidence: 99%

An evaluation of Rb-Sr dating of pseudotachylyte: Structural-chemical models and the role of fluids.

Magloughlin

2003

Geochem. J.

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Rb-Sr geochronometry has been used in several studies in an effort to date pseudotachylyte and therefore the fault hosting the pseudotachylyte and the tectonism that produced the faulting. The usual procedure has been the "thin-slab" technique, which in metamorphic rocks involves construction of an isochron using whole rock analyses of adjacent but compositionally different slices of rock, and is based on the assumption of isotopic equilibrium between the slices. In attempts to date pseudotachylyte, analyses have been obtained using a pseudotachylyte vein and the immediately adjacent host rock. Such studies commonly yield: 1) apparent ages with high errors owing to small a small range of Rb/Sr values or to "scatterchron" data; 2) host rocks not plotting on the same isochron or scatterchron defined by the pseudotachylyte; 3) apparent dates that are inconsistent with other estimates of the time of faulting; or 4) apparent ages that are not independently constrained. These studies commonly fail to record the type of pseudotachylyte vein utilized; injection veins are commonly larger and more obvious targets for collecting but are likely to yield geologically meaningless dates.A Rb-Sr geochronometric study of pseudotachylyte from the North Cascade Mountains, Washington, reveals: 1) an apparent age older than the likely age of the pseudotachylyte; 2) pseudotachylyte and host rock chemical and isotopic characteristics indicating that the pseudotachylyte was not the result of bulk melting of normal schist; and 3) metasomatism prior to pseudotachylyte formation likely affected both the protolith and what is now the host rock.In light of previous studies, the new data, and related information regarding fault-related metasomatism, three closed system models and one open system model for pseudotachylyte formation are proposed. These models make specific predictions regarding apparent whole rock-pseudotachylyte dates, microstructural features, and the relationship between the whole-rock pseudotachylyte date and the whole rock-biotite RbSr dates in the same rock. The models suggests pseudotachylyte-host rock dating is unlikely to produce real ages of pseudotachylyte formation. Worse, because many pseudotachylytes apparently form during cooling and uplift, the technique will instead yield a seemingly reasonable but geologically meaningless date. Certain approaches to the problem, including a microstructural analysis coupled with a whole rockbiotite age on the same rock, may yield an actual age of pseudotachylyte formation, but the assumptions made heretofore about the behavior of the Rb-Sr system at the time of pseudotachylyte formation are questionable.

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Element mobility associated with syn‐metamorphic shear zones near Scotchport, NW Mayo, Ireland

Cited by 38 publications

References 27 publications

Fluid channelling during ductile shearing: transformation of granodiorite into aluminous schist in the Tauern Window, Eastern Alps

Fluid channelling during ductile shearing: transformation of granodiorite into aluminous schist in the Tauern Window, Eastern Alps

Deformational and stratigraphic influences on mineralization in SE Ireland

An evaluation of Rb-Sr dating of pseudotachylyte: Structural-chemical models and the role of fluids.

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