Veins, fluid, fractals, scale & schist : an investigation of fluid-rock interaction during deformation of the Torlesse terrane, New Zealand
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The Torlesse Terrane of New Zealand is a structurally complex Permian to Jurassic accretionary assemblage, which consists of compositionally uniform quartzofeldspathic greywackes and subordinate argillites. The transition from greywacke to schist is the result of predominantly solution transfer deformation, with crystal plastic mechanisms occurring at highest grades. Differences between primary sedimentological variation and chemical effects of solution transfer enable the protolith of a deformed rock to be estimated, and volume changes as a result of solution transfer to be determined by mass balance analysis.
Volume loss at a hand-specimen scale during deformation has resulted in quartz veins in outcrop. Video image analysis of a superbly exposed section at Lake Hawea defmed vein proportions and distributions using spatial geometry and concepts of fractals. Total vein proportions and number-size distributions vary in response to differences in lithology, increasing metamorphism and deformation. Outcrops of variably interlayered psammites & pelites of pumpellyite-actinolite facies have 7% veins; pelite-dominated and pelite- to psammite-dominated schists at lower greenschist facies have 12-21% veins, with many> 5 cm thick. Well-foliated greenschist facies schists have -23% veins.
Oxygen isotopic signatures of veins and host-rocks outline various equilibria/disequilibria and fluid-rock interaction. In pumpellyite-actinolite facies semi-schists of Area A, veins ((1180==13-15o/oo) and host-rocks ((1180==10o/oo) were generally in disequilibrium, with rocks retaining a remnant detrital signature, and isotopically light fluids ((1180==6-7.5o/oo) buffered by recrystallisation of greywacke matrix. In lower greenschist facies schists od Area C, vein ((1180==17o/oo) rock ((1180==12-17o/oo) and fluid ((1180==ll-13o/oo) compositions are isotopically heavy, and variable rock-vein fractionations indicate greater than outcrop-scale rock-buffered, open-system, conditions. Upper greenschist and amphibolite facies rocks ((1180==I0-12o/oo) and veins ((1180==12-14o/oo) are relatively homogeneous, implying hand-specimen scale rock-buffered conditions and fluid-rock equilibration.
Trace element variations are used to "fingerprint" the origin of fluids. Elevated Cu & Ni contents normalised to greywacke, and lesser Cr, distinguish sedimentological trends. Metavolcanics have equally elevated Cr & Ni. High Cr & Ni anomalies in lower greenschist facies rocks at Lake Hawea are either (1) primary due to a metavolcanic component, or (2) secondary by a fluid equilibrated with metavolcanics. Similarities between Lake Hawea and post-metamorphic precious metal deposits indicate Hawea rocks may have been a source region for shallower level deposits, or else are a diffuse mineralised zone.
Hand-specimen mass balance in different lithologies from Lake Hawea outlines a range in volume change with finer-grained pelitic rocks generally recording volume loss (~ V>-39%), and psammites volume gain (+~V <24% ). Outcrop proportions of selvedges, veins and host-rock indicate mass was generally conserved at an outcrop scale. Average volume changes from 332 hand-specimens over the whole Torlesse Terrane show a general tendency towards volume loss, increasing from semi-schist (LA V =-4. 7%) to schist (LA V =-8.3% ). Highgrade "gneisses" show similar volume loss to schist (L~ V =-9 .1% ). Hand-specimen losses are balanced by, or are less than, proportions of veins in outcrop. Greywacke (LA V =+0.6%) variability provides an estimate of protolith variation and mass balance uncertainty (model ~V s.d.= 10%). Above the 10% level of mass balance uncertainty, variability defines a probability function and a relative measure of the "closed system" scale. Model ~V variability was greater in semi-schistose and schistose rocks (~V s.d.=15% ), than relatively homogeneous "gneisses" (~V s.d.<IO% ).
At shallow crustallevels (prehnite-pumpellyite and pumpellyite-actinolite facies), relatively short source-Sink distances during solution transfer and comparatively small amounts of deformation resulted in mass conservation at a hand-specimen scale (10-3m3). At lower greenschist facies conditions, extensive element mobility during solution transfer "unmixed" the rocks into relatively soluble and insoluble components, and the "closed-system" (net mass conservation) scale that of an outcrop (103m3) or slightly greater. At upper greenschist facies conditions and above, solution transfer mechanisms still operate, but increasing amounts of crystal plastic deformation re-homogenise the rocks, and mass is conserved at a hand-specimen scale. Element mobility during solution transfer deformation of hand-specimens was involved considerably greater amounts of diffusion than advection of material. Despite ubiquitous development of veins, mass was more or less conserved at an outcrop scale, and extremely large fluid/rock ratios were not present. Relatively little amounts of fluid are required for the intense deformation and textural reconstitution in the Torlesse Terrane.
Volume loss at a hand-specimen scale during deformation has resulted in quartz veins in outcrop. Video image analysis of a superbly exposed section at Lake Hawea defmed vein proportions and distributions using spatial geometry and concepts of fractals. Total vein proportions and number-size distributions vary in response to differences in lithology, increasing metamorphism and deformation. Outcrops of variably interlayered psammites & pelites of pumpellyite-actinolite facies have 7% veins; pelite-dominated and pelite- to psammite-dominated schists at lower greenschist facies have 12-21% veins, with many> 5 cm thick. Well-foliated greenschist facies schists have -23% veins.
Oxygen isotopic signatures of veins and host-rocks outline various equilibria/disequilibria and fluid-rock interaction. In pumpellyite-actinolite facies semi-schists of Area A, veins ((1180==13-15o/oo) and host-rocks ((1180==10o/oo) were generally in disequilibrium, with rocks retaining a remnant detrital signature, and isotopically light fluids ((1180==6-7.5o/oo) buffered by recrystallisation of greywacke matrix. In lower greenschist facies schists od Area C, vein ((1180==17o/oo) rock ((1180==12-17o/oo) and fluid ((1180==ll-13o/oo) compositions are isotopically heavy, and variable rock-vein fractionations indicate greater than outcrop-scale rock-buffered, open-system, conditions. Upper greenschist and amphibolite facies rocks ((1180==I0-12o/oo) and veins ((1180==12-14o/oo) are relatively homogeneous, implying hand-specimen scale rock-buffered conditions and fluid-rock equilibration.
Trace element variations are used to "fingerprint" the origin of fluids. Elevated Cu & Ni contents normalised to greywacke, and lesser Cr, distinguish sedimentological trends. Metavolcanics have equally elevated Cr & Ni. High Cr & Ni anomalies in lower greenschist facies rocks at Lake Hawea are either (1) primary due to a metavolcanic component, or (2) secondary by a fluid equilibrated with metavolcanics. Similarities between Lake Hawea and post-metamorphic precious metal deposits indicate Hawea rocks may have been a source region for shallower level deposits, or else are a diffuse mineralised zone.
Hand-specimen mass balance in different lithologies from Lake Hawea outlines a range in volume change with finer-grained pelitic rocks generally recording volume loss (~ V>-39%), and psammites volume gain (+~V <24% ). Outcrop proportions of selvedges, veins and host-rock indicate mass was generally conserved at an outcrop scale. Average volume changes from 332 hand-specimens over the whole Torlesse Terrane show a general tendency towards volume loss, increasing from semi-schist (LA V =-4. 7%) to schist (LA V =-8.3% ). Highgrade "gneisses" show similar volume loss to schist (L~ V =-9 .1% ). Hand-specimen losses are balanced by, or are less than, proportions of veins in outcrop. Greywacke (LA V =+0.6%) variability provides an estimate of protolith variation and mass balance uncertainty (model ~V s.d.= 10%). Above the 10% level of mass balance uncertainty, variability defines a probability function and a relative measure of the "closed system" scale. Model ~V variability was greater in semi-schistose and schistose rocks (~V s.d.=15% ), than relatively homogeneous "gneisses" (~V s.d.<IO% ).
At shallow crustallevels (prehnite-pumpellyite and pumpellyite-actinolite facies), relatively short source-Sink distances during solution transfer and comparatively small amounts of deformation resulted in mass conservation at a hand-specimen scale (10-3m3). At lower greenschist facies conditions, extensive element mobility during solution transfer "unmixed" the rocks into relatively soluble and insoluble components, and the "closed-system" (net mass conservation) scale that of an outcrop (103m3) or slightly greater. At upper greenschist facies conditions and above, solution transfer mechanisms still operate, but increasing amounts of crystal plastic deformation re-homogenise the rocks, and mass is conserved at a hand-specimen scale. Element mobility during solution transfer deformation of hand-specimens was involved considerably greater amounts of diffusion than advection of material. Despite ubiquitous development of veins, mass was more or less conserved at an outcrop scale, and extremely large fluid/rock ratios were not present. Relatively little amounts of fluid are required for the intense deformation and textural reconstitution in the Torlesse Terrane.
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1 v. (various pagings): ill. (some col.), maps ; 30 cm.
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1993Cox
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Citation
Cox, Simon C. (Simon Christopher), 1964-, “Veins, fluid, fractals, scale & schist : an investigation of fluid-rock interaction during deformation of the Torlesse terrane, New Zealand ,” Otago Geology Theses, accessed August 11, 2026, https://theses.otagogeology.org.nz/items/show/276.