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David R.m. Pattison - One of the best experts on this subject based on the ideXlab platform.
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metamorphic devolatilization of basalts across the greenschist Amphibolite Facies transition zone insights from isograd mapping petrography and thermodynamic modelling
Lithos, 2019Co-Authors: Paul G Starr, David R.m. PattisonAbstract:ABSTRACT The greenschist-Amphibolite Facies transition zone within metabasites is of great importance due to the common occurrence of this transition within greenstone belts, and the proposition that fluids released across the transition may be involved in orogenic gold deposit formation. In this work, the nature of devolatilization reactions occurring across the greenschist-Amphibolite Facies transition zone is assessed from petrological examination and thermodynamic modelling of an exceptionally exposed part of the Flin Flon Greenstone Belt (Manitoba/Saskatchewan). The sequence at Flin Flon comprises an intact metamorphic sequence of ~10 km length spanning the greenschist and lower Amphibolite Facies, the latter of which can be subdivided into three zones (from S to N): the hornblende-actinolite zone, the hornblende-actinolite-oligoclase zone, and the hornblende-oligoclase zone, demarcated by the hornblende-in, oligoclase-in and actinolite-out isograds. The crossing of the hornblende-in isograd is associated with small amounts of hornblende growth, relatively little change in the proportions of chlorite, and negligible fluid loss. By contrast, significant changes in the modal mineralogy occur across the 1500 m wide hornblende-actinolite-oligoclase zone, with the breakdown of approximately 75% of the chlorite and the loss of approximately 1-2 wt% H2O. There is no textural or modal evidence for significant loss or gain of carbonate or sulphide minerals going through this interval. Petrological estimates of devolatilization across the greenschist-Amphibolite Facies transition zone are compared with predictions from thermodynamic modelling. Although there are differences between the two, both indicate that the fluid loss is strongly tied to variation in bulk composition, the latter predominantly the result of pre-metamorphic alteration processes. High-Mg and high-Ca basalts, representative of pillow rim and core material, undergo an average of 1.8 wt% (modelling estimate: 2.2 wt%) and 1.1 wt% H2O loss (modelling estimate: 1.8 wt%), respectively, across the hornblende-actinolite-oligoclase zone. T-XCO2 modelling predicts that the XCO2 content of fluid produced from the hornblende- and oligoclase-producing reactions is low ( 0.2) above the greenschist-Amphibolite Facies transition zone within samples containing high carbonate contents (>5%). However, in contrast to the modelling, the majority of chlorite (>75%) breaks down at the main greenschist-Amphibolite Facies transition zone in the Flin Flon sequence, and most samples have low carbonate contents (1.5% average), limiting the volumes of fluids with higher XCO2 compositions that can be generated at higher grades. Thus, whilst the greenschist-Amphibolite Facies transition zone at Flin Flon is the site of significant devolatilization (1-2 wt%) over a small interval of P-T space, is does not appear to have been accompanied by significant carbonate and sulphide breakdown, and the fluids generated across this interval were CO2-poor. If the Flin Flon sequence is representative of other metamorphosed greenstone belts, it may be that metabasites metamorphosed across the greenschist-Amphibolite Facies transition zone do not, in general, release the CO2– and gold hydrosulphide-bearing fluids characteristic of orogenic gold deposits.
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equilibrium and disequilibrium processes across the greenschist Amphibolite transition zone in metabasites
Contributions to Mineralogy and Petrology, 2019Co-Authors: Paul G Starr, David R.m. PattisonAbstract:Documentation of textures, modes and compositions of minerals in metamorphosed basalts across two greenschist-to-lower Amphibolite Facies sequences (Flin Flon, Manitoba and Rossland, British Columbia) was combined with a compilation of the literature data to assess equilibrium and disequilibrium processes across this important transition zone. At Flin Flon and Rossland, the greenschist–Amphibolite Facies transition occurs over a narrow spatial interval marked by hornblende-in, oligoclase-in and actinolite-out isograds. The data suggest the existence of stable miscibility gaps separating coexisting actinolite and hornblende, and coexisting albite and oligoclase, in the lowermost Amphibolite Facies. However, actinolite and hornblende show a divergence in compositions going upgrade across the oligoclase-in isograd, suggestive of disequilibrium between the new, progressively more aluminous hornblende and metastably persisting actinolite. Likewise, coexisting albite and oligoclase compositions show no evidence for converging compositions at higher temperatures, suggesting they do not remain in equilibrium across a miscibility gap. Compositional gaps within epidote phases are attributed to disequilibrium preservation of lower grade epidote compositions to higher grade conditions, rather than the existence of a miscibility gap at greenschist and Amphibolite Facies conditions. Recognition of equilibrium and disequilibrium relations highlights the difficulty of using natural compositional datasets to extract information on miscibility gaps and more generally to extract a–X relationships. The greenschist-to-Amphibolite Facies transition is controlled principally by the consumption of chlorite, which primarily occurs upon crossing the oligoclase-in isograd, concomitant with conversion of actinolite to hornblende and albite to oligoclase. The result is a focused pulse of devolatilization over a small spatial and thermal interval.
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low pressure regional Amphibolite Facies to granulite Facies metamorphism of the paleoproterozoic thompson nickel belt manitoba
Canadian Journal of Earth Sciences, 2012Co-Authors: Chris G Coueslan, David R.m. PattisonAbstract:The Thompson Nickel Belt is a ca. 35 km × 400 km northeast-trending segment of the northwest margin of the Archean Superior craton in Manitoba, bounded to the west by the Paleoproterozoic Reindeer Zone. The belt was metamorphosed and deformed during the Trans-Hudson orogeny (ca. 1.9–1.7 Ga). Mineral assemblages in metamorphosed pelite, aluminous greywacke, mafic igneous rock, iron formation, and ferruginous wacke define regional metamorphic domains, separated by mineral isograds, that are subparallel to the strike of the belt and to regional-scale D3 structures. An elongate, ca. 5 km × 73 km, central zone of middle Amphibolite-Facies rocks is characterized by the following: muscovite-bearing mineral assemblages in pelites containing combinations of staurolite, andalusite, and sillimanite; muscovite-free, staurolite + cordierite + garnet-bearing mineral assemblages in greywackes; hornblende-bearing mineral assemblages in mafic metaigneous rocks; and grunerite-bearing mineral assemblages in iron formation. ...
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regional low pressure Amphibolite Facies metamorphism at the pipe ii mine thompson nickel belt manitoba and comparison of metamorphic isograds in metapelites and meta iron formations
Canadian Mineralogist, 2011Co-Authors: Chris G Coueslan, David R.m. Pattison, Douglas K TinkhamAbstract:The Ospwagan Group supracrustal sequence of the Thompson Nickel Belt, Manitoba, includes semipelitic and pelitic schist, iron formation, and mafic volcanic rocks that were deformed and metamorphosed during the Trans-Hudson orogeny ( ca. 1.8 Ga). North–south-trending metamorphic isograds at the Pipe II mine in the central portion of the belt show the following sequence from west to east: (1) andalusite + staurolite-out, in metapelites, (2) orthopyroxene-in, in meta-iron formations, (3) sillimanite + K-feldspar-in, in metapelites and semipelites. A roughly NE–SW-trending migmatite-in isograd (4), in semipelites, transects isograd (3) in the south portion of the study area. Equilibrium-assemblage diagrams for representative bulk-compositions were calculated using the Theriak–Domino software to provide pressure and temperature constraints for each of the metamorphic isograds. Isograds developed at the following metamorphic conditions: isograd (1) 585–600°C, 3.7–3.9 kbar, isograd (2) 625–640°C, 3.0–3.9 kbar, and isograd (3) 640–660°C at 3.0–3.6 kbar. The position of isograd (2), orthopyroxene-in for iron formation, occurs down-grade of isograd (3), sillimanite + K-feldspar-in for pelites, roughly 125°C below temperatures of granulite-Facies metamorphism. The appearance of orthopyroxene at the middle Amphibolite-Facies to upper-Amphibolite-Facies transition is favored by the Fe- and Mn-rich bulk composition of the iron formation. The oblique orientation of isograd (4) to isograds (1–3) may be the result of an increasing pressure gradient toward the south, or the infiltration of melt-inducing fluids, or both. The isograd sequence suggests an apparent thermal gradient of at least 65°C over 420 m. The metastable persistence of staurolite and andalusite could result in an actual gradient closer to 30°C.
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on the initiation of metamorphic sulfide anatexis
Journal of Petrology, 2006Co-Authors: David R.m. Pattison, Andrew George Tomkins, Ronald B FrostAbstract:Mineral assemblages in common sulfide ore deposits are examined together with phase relations to (1) investigate the pressure^ temperature conditions required for the onset of metamorphically induced partial melting involving economic minerals, and (2) place constraints on the amount of melt produced. Deposits that contain sulfosalt or telluride minerals may start to melt at conditions ranging from lowest greenschist Facies to Amphibolite Facies. Deposits lacking sulfosalt and/or telluride minerals may begin to melt once P^T conditions reach the upper Amphibolite Facies, if galena is present, or well into the granulite Facies if galena is absent.The result is two broad melting domains: a lowto medium-temperature, low melt volume domain involving melting of volumetrically minor sulfosalt and/or telluride minerals; and a high-temperature, potentially higher melt volume domain involving partial melting of the major sulfide minerals. Epithermal gold deposits, which are especially rich in sulfosalt minerals, are predicted to commence melting at the lowest temperatures of all sulfide deposit types. Massive Pb^Zn (^Cu) deposits may start to melt in the lower to middle Amphibolite Facies if pyrite and arsenopyrite coexist at these conditions, and in the upper Amphibolite Facies if they do not. Excepting sulfosalt-bearing occurrences, massive Ni^Cu^PGE (platinum group element) deposits will show little to no melting under common crustal metamorphic conditions, whereas disseminated Cu deposits are typically incapable of generating melt until the granulite Facies is reached, when partial melting commences in bornite-bearing rocks. The volume of polymetallic melt that can be generated in most deposit types is therefore largely a function of the abundance of sulfosalt minerals. Even at granulite-Facies conditions, this volume is usually less than 0 5%. The exception is massive Pb^Zn deposits, where melt volumes significantly exceeding 0 5 vol. % may be segregated into sulfide magma dykes, allowing mobilization over large distances.
Marty Grove - One of the best experts on this subject based on the ideXlab platform.
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age trends in garnet hosted monazite inclusions from upper Amphibolite Facies schist in the northern grouse creek mountains utah
Geochimica et Cosmochimica Acta, 2008Co-Authors: Thomas D Hoisch, Michael L Wells, Marty GroveAbstract:Abstract We performed in situ Th–Pb dating of monazite in upper Amphibolite Facies pelitic schist from the Grouse Creek Mountains in northwest Utah. Sixty-six ages from inclusions in four garnet grains range from 37 to 72 Ma and decrease with radial distance from garnet cores. The age range of 30 matrix monazite grains overlaps and extends to younger ages than inclusions (25–58 Ma). The monazite grains are not intersected by cracks in the garnets, through which dissolution, reprecipitation or Pb loss might occur, and are generally too small (
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age trends in garnet hosted monazite inclusions from upper Amphibolite Facies schist in the northern grouse creek mountains utah
Geochimica et Cosmochimica Acta, 2008Co-Authors: Thomas D Hoisch, Michael L Wells, Marty GroveAbstract:We performed in situ Th–Pb dating of monazite in upper Amphibolite Facies pelitic schist from the Grouse Creek Mountains in northwest Utah. Sixty-six ages from inclusions in four garnet grains range from 37 to 72 Ma and decrease with radial distance from garnet cores. The age range of 30 matrix monazite grains overlaps and extends to younger ages than inclusions (25– 58 Ma). The monazite grains are not intersected by cracks in the garnets, through which dissolution, reprecipitation or Pb loss might occur, and are generally too small (<20 lm) to allow for more than one age determination on any one grain. Processes that might explain inclusion ages that decrease with radial distance from garnet cores include: (1) Pb diffusion in monazite, (2) dissolution and reprecipitation of monazite, and (3) co-crystallization of monazite and garnet. After consideration of these possibilities, it is concluded that the co-crystallization of monazite and garnet is the most plausible, with monazite neoblasts deriving REEs from the breakdown of muscovite. Garnet ages derived by regression of the inclusion ages and assuming a constant rate of volume increase during garnet growth yield model ages with a maximum difference between core and rim of 22 m.y.
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u pb isotopic behaviour of zircon during upper Amphibolite Facies fluid infiltration in the napier complex east antarctica
Earth and Planetary Science Letters, 2002Co-Authors: Christopher J Carson, Marty Grove, Jay J Ague, Christopher D Coath, Mark T HarrisonAbstract:Understanding the factors that contribute to U–Pb discordance in zircon is essential for interpreting isotopic data and for assessing the validity of concordia intercept ages. Modification caused by interaction with metamorphic fluids is often cited as a primary means by which significant or even complete isotopic resetting of U–Pb systematics in zircon might be achieved under subsolidus conditions. We present a field example from the Napier Complex, east Antarctica, in which a Palaeoproterozoic (2450–70 Ma) zircon population interacted locally with an Early Palaeozoic (498±1.7 Ma) aqueous fluid at upper-Amphibolite Facies conditions. Conventional ion microprobe analysis of sectioned and polished grain surfaces indicates that fluid interaction resulted in minor disturbance of U and Pb in zircons (both normal and reverse discordance) with limited displacement along a chord with a lower intercept that coincides with the timing of fluid infiltration. In contrast, ion probe ‘drilling’ or depth profiling into unpolished natural zircon crystal surfaces revealed extensive disturbance of U–Pb systematics, to depths of ∼0.2 μm, with near-surface ages consistent with the timing of fluid influx at ∼498 Ma. Although zircon underwent some radiogenic Pb redistribution during fluid interaction, infiltrating fluids resulted in minimal grain-scale isotopic modification of zircon. Based on ion probe depth profiling results, we propose that limited normal discordance observed in the conventional ion microprobe zircon analyses, in this case, is controlled by an analytical mixture of reset and/or recrystallised zircon along penetrative micro-fracture networks with that of adjacent unaffected zircon. We also suggest that the observed reverse discordance is genuine, resulting from localised intra-grain net accumulations of radiogenic Pb. We conclude that the isotopic response of zircon, in this case, is controlled by the interaction of an aqueous metamorphic fluid, of low to moderate salinity, resulting in sub-micrometre depth scale isotopic modification at natural crystal faces and along penetrative micro-fracture networks, and that grain-scale isotopic modification was negligible. Therefore, we urge caution when considering regional chronological interpretations that appeal to significant zircon isotopic resetting caused exclusively by metamorphic fluid interaction at upper-Amphibolite Facies conditions.
Thomas D Hoisch - One of the best experts on this subject based on the ideXlab platform.
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age trends in garnet hosted monazite inclusions from upper Amphibolite Facies schist in the northern grouse creek mountains utah
Geochimica et Cosmochimica Acta, 2008Co-Authors: Thomas D Hoisch, Michael L Wells, Marty GroveAbstract:Abstract We performed in situ Th–Pb dating of monazite in upper Amphibolite Facies pelitic schist from the Grouse Creek Mountains in northwest Utah. Sixty-six ages from inclusions in four garnet grains range from 37 to 72 Ma and decrease with radial distance from garnet cores. The age range of 30 matrix monazite grains overlaps and extends to younger ages than inclusions (25–58 Ma). The monazite grains are not intersected by cracks in the garnets, through which dissolution, reprecipitation or Pb loss might occur, and are generally too small (
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age trends in garnet hosted monazite inclusions from upper Amphibolite Facies schist in the northern grouse creek mountains utah
Geochimica et Cosmochimica Acta, 2008Co-Authors: Thomas D Hoisch, Michael L Wells, Marty GroveAbstract:We performed in situ Th–Pb dating of monazite in upper Amphibolite Facies pelitic schist from the Grouse Creek Mountains in northwest Utah. Sixty-six ages from inclusions in four garnet grains range from 37 to 72 Ma and decrease with radial distance from garnet cores. The age range of 30 matrix monazite grains overlaps and extends to younger ages than inclusions (25– 58 Ma). The monazite grains are not intersected by cracks in the garnets, through which dissolution, reprecipitation or Pb loss might occur, and are generally too small (<20 lm) to allow for more than one age determination on any one grain. Processes that might explain inclusion ages that decrease with radial distance from garnet cores include: (1) Pb diffusion in monazite, (2) dissolution and reprecipitation of monazite, and (3) co-crystallization of monazite and garnet. After consideration of these possibilities, it is concluded that the co-crystallization of monazite and garnet is the most plausible, with monazite neoblasts deriving REEs from the breakdown of muscovite. Garnet ages derived by regression of the inclusion ages and assuming a constant rate of volume increase during garnet growth yield model ages with a maximum difference between core and rim of 22 m.y.
Michael L Wells - One of the best experts on this subject based on the ideXlab platform.
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age trends in garnet hosted monazite inclusions from upper Amphibolite Facies schist in the northern grouse creek mountains utah
Geochimica et Cosmochimica Acta, 2008Co-Authors: Thomas D Hoisch, Michael L Wells, Marty GroveAbstract:Abstract We performed in situ Th–Pb dating of monazite in upper Amphibolite Facies pelitic schist from the Grouse Creek Mountains in northwest Utah. Sixty-six ages from inclusions in four garnet grains range from 37 to 72 Ma and decrease with radial distance from garnet cores. The age range of 30 matrix monazite grains overlaps and extends to younger ages than inclusions (25–58 Ma). The monazite grains are not intersected by cracks in the garnets, through which dissolution, reprecipitation or Pb loss might occur, and are generally too small (
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age trends in garnet hosted monazite inclusions from upper Amphibolite Facies schist in the northern grouse creek mountains utah
Geochimica et Cosmochimica Acta, 2008Co-Authors: Thomas D Hoisch, Michael L Wells, Marty GroveAbstract:We performed in situ Th–Pb dating of monazite in upper Amphibolite Facies pelitic schist from the Grouse Creek Mountains in northwest Utah. Sixty-six ages from inclusions in four garnet grains range from 37 to 72 Ma and decrease with radial distance from garnet cores. The age range of 30 matrix monazite grains overlaps and extends to younger ages than inclusions (25– 58 Ma). The monazite grains are not intersected by cracks in the garnets, through which dissolution, reprecipitation or Pb loss might occur, and are generally too small (<20 lm) to allow for more than one age determination on any one grain. Processes that might explain inclusion ages that decrease with radial distance from garnet cores include: (1) Pb diffusion in monazite, (2) dissolution and reprecipitation of monazite, and (3) co-crystallization of monazite and garnet. After consideration of these possibilities, it is concluded that the co-crystallization of monazite and garnet is the most plausible, with monazite neoblasts deriving REEs from the breakdown of muscovite. Garnet ages derived by regression of the inclusion ages and assuming a constant rate of volume increase during garnet growth yield model ages with a maximum difference between core and rim of 22 m.y.
Andrew George Tomkins - One of the best experts on this subject based on the ideXlab platform.
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on the initiation of metamorphic sulfide anatexis
Journal of Petrology, 2006Co-Authors: David R.m. Pattison, Andrew George Tomkins, Ronald B FrostAbstract:Mineral assemblages in common sulfide ore deposits are examined together with phase relations to (1) investigate the pressure^ temperature conditions required for the onset of metamorphically induced partial melting involving economic minerals, and (2) place constraints on the amount of melt produced. Deposits that contain sulfosalt or telluride minerals may start to melt at conditions ranging from lowest greenschist Facies to Amphibolite Facies. Deposits lacking sulfosalt and/or telluride minerals may begin to melt once P^T conditions reach the upper Amphibolite Facies, if galena is present, or well into the granulite Facies if galena is absent.The result is two broad melting domains: a lowto medium-temperature, low melt volume domain involving melting of volumetrically minor sulfosalt and/or telluride minerals; and a high-temperature, potentially higher melt volume domain involving partial melting of the major sulfide minerals. Epithermal gold deposits, which are especially rich in sulfosalt minerals, are predicted to commence melting at the lowest temperatures of all sulfide deposit types. Massive Pb^Zn (^Cu) deposits may start to melt in the lower to middle Amphibolite Facies if pyrite and arsenopyrite coexist at these conditions, and in the upper Amphibolite Facies if they do not. Excepting sulfosalt-bearing occurrences, massive Ni^Cu^PGE (platinum group element) deposits will show little to no melting under common crustal metamorphic conditions, whereas disseminated Cu deposits are typically incapable of generating melt until the granulite Facies is reached, when partial melting commences in bornite-bearing rocks. The volume of polymetallic melt that can be generated in most deposit types is therefore largely a function of the abundance of sulfosalt minerals. Even at granulite-Facies conditions, this volume is usually less than 0 5%. The exception is massive Pb^Zn deposits, where melt volumes significantly exceeding 0 5 vol. % may be segregated into sulfide magma dykes, allowing mobilization over large distances.