Retrograde Schists of the Amphibolite Facies at broken hill, New South Wales

Vernon, R. H.; Ransom, D. M.

doi:10.1080/00167617108728766

Cited by 58 publications

(11 citation statements)

References 14 publications

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“…Superimposed upon these zones are medium-pressure/medium-to low-pressure retrograde assemblages which formed both regionally (Corbett & Phillips 1981) and within discrete retrograde schist zones (Vernon & Ransom 1971 ;Plimer 1975). Isotope determinations indicate a primary age for metavolcanic rocks of 1820 _+ 60 Ma (Shaw 1968), an age for the culmination of prograde metamorphism of 1660 _+ 16 Ma (Shaw 1968;Reynolds 1971 ;Harrison & McDougall 1981) and a period of retrograde metamorphism from c. 1605 Ma to at least 1500 Ma (Plimer 1976;Harrison & Mc-Dougall 1981).…”

Section: Geological Settingmentioning

confidence: 99%

The Geochemistry of the Lower Proterozoic Willyama Complex Volcanics, Broken Hill Block, New South Wales

James¹,

Pearce²,

Oliver³

1987

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S U M M A RY: The major and trace element geochemistry of the early to middle Proterozoic meta-igneous rocks of the Willyama Complex in the Broken Hill Block indicates that the basic rocks originated by partial melting of a depleted mantle source, followed by fractional crystallization along a trend of strong iron enrichment coupled with some crustal assimilation. At least some of the intermediate-acid rocks were derived from fractionation of these basic parents, but major crustal involvement may be necessary to explain the large quantities of felsic volcanics in the lower part of the Complex. The volcanic rocks in the Broken Hill mine area show an unusually high degree of primary iron enrichment (up to 25 wt % total iron as Fe203) and a lesser degree of bimodality than elsewhere. By analogy with Recent ferrobasalt provinces, this extreme iron enrichment may indicate an origin in a propagating rift where small, high-level magma chambers separated from the magma source. If so, the consequent high geothermal gradient and high fracture permeability may have provided the necessary conditions for genesis of the Broken Hill ore deposit.

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Section: Geological Settingmentioning

confidence: 99%

The Geochemistry of the Lower Proterozoic Willyama Complex Volcanics, Broken Hill Block, New South Wales

James¹,

Pearce²,

Oliver³

1987

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“…The weight of published work thus suggests that dislocation flow processes dominate in shear zones, but this is somewhat misleading, because most of the studies have concentrated on zones in essentially monomineralic rocks, or on the outer (lower strain) parts of zones where dislocation flow dominates even in many polyphase rocks. Many polyphase rocks in retrograde shear zones are characterized, at least in their more highly strained portions, by unstrained, relatively equidimensional grains, low-energy grain shapes, uniform grain sizes, predominant contacts between unlike phases and commonly weak quartz c-axis preferred orientations (Ransom, 1969(Ransom, , 1971Vernon & Ransom, 1971;Bell & Etheridge, 1973. In other words, they are similar to progrude rocks of similar grade and composition.…”

Section: Retrograde Metamorphismmentioning

confidence: 99%

“…In other words, they are similar to progrude rocks of similar grade and composition. In some shear zones, there is little or no evidence of significant straining due to dislocation processes (Vernon & Ransom, 1971), and even in zones in which dislocation flow dominates in the outer (low strain) parts, the central regions may have deformed substantially by mass transfer processes (Bell & Etheridge, 1973;Ransom, 1969;Kerrich, Beckinsale & Durham, 1977;Etheridge & Cooper, 198 1 ).…”

Section: Retrograde Metamorphismmentioning

confidence: 99%

“…Where extensive retrogression does occur, it is generally restricted to zones of deformation and/or obvious hydrothermal activity (Turner, 1981 ;Bridgwater, Escher & Watterson, 1973;Beach, 1976;Vernon, 1976). Shear (mylonite) zones are commonly loci of retrogressive mineral reactions that appear to have been enhanced by deformation (Vernon & Ransom, 1971;Bell & Etheridge, 1973;Beach, 1976Beach, , 1980Beach & Fyfe, 1972;Kerrich, Beckinsale & Durham, 1977;Etheridge & Cooper, 1981).…”

Section: Retrograde Metamorphismmentioning

confidence: 99%

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The role of the fluid phase during regional metamorphism and deformation

Etheridge

Wall

Vernon

1983

Journal Metamorphic Geology

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510

218

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Abstract. Evidence from rock microstructures, mass transfer and isotopic exchange indicates that substantial quantities of aqueous fluids are involved in low-and medium-grade regional metamorphism. Similar conclusions are drawn from many retrograde environments, whereas high-grade metamorphic fluids may be melt dominated. The mobile fluids play essential roles in metamorphic reactions, mass transport and deformation processes. These processes are linked by the mechanical consequences of metamorphic fluid pressures (Pr) generally being greater than or equal to the minimum principal compressive stress. Under such conditions metamorphic porosity comprises grain boundary tubules and bubbles together with continuously generated (and healed) microfractures. Deformation results in significant interconnected porosity and hence enhanced permeability. Lithologically and structurally controlled permeability variations may cause effective fluid channelling.Simple Rayleigh-Darcy modelling of a uniformly permeable, crustal slab shows that convective instability of metamorphic fluid is expected at the permeabilities suggested for the high Pf metamorphic conditions. Complex, largescale convective cells operating in overpressured, but capped systems may provide a satisfactory explanation for the large fluid/rock ratios and extensive mass transport demonstrated for many low-and medium-grade metamorphic environments. Such large-scale fluid circulation may have important consequences for heat transfer in and the thermal evolution of metamorphic belts.

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“…1). These metamorphic rocks have undergone ubiquitous minor retrogres sion and are transgressed by retrograde shear zones of lower amphibolite and greenschist grade (VERNON, 1969;VERNON and RANSOM, 1971). These retrograde shear zones may follow very primitive linear structures in the area.…”

Section: Geologymentioning

confidence: 99%

A metamorphogenic alteration zone around the stratiform Broken Hill ore deposits, Australia.

Plimer

1975

Geochem. J.

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Abstract-A symmetrical metamorphic alteration zone around the Broken Hill orebodies derives from retrogressive metamorphism. This zone is at least 300m wide and is characterized by a great increase in Rb/Sr and the replacement of earlier high grade metamorphic minerals by sericite. The alteration observed probably results from fluid ascent along permeable sulphide-silicate competency boundaries at the waning stages of granulite facies metamorphism.

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Retrograde Schists of the Amphibolite Facies at broken hill, New South Wales

Cited by 58 publications

References 14 publications

The Geochemistry of the Lower Proterozoic Willyama Complex Volcanics, Broken Hill Block, New South Wales

The Geochemistry of the Lower Proterozoic Willyama Complex Volcanics, Broken Hill Block, New South Wales

The role of the fluid phase during regional metamorphism and deformation

A metamorphogenic alteration zone around the stratiform Broken Hill ore deposits, Australia.

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