Academic literature on the topic 'Porphyroblasts'
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Journal articles on the topic "Porphyroblasts"
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Bui, Hau Vinh, Hai Thanh Tran, Thanh Xuan Ngo, and Chi Kim Thi Ngo. "Microstructure characteristics of the ganet-bearing schist from Nam Co formation, Son La area, Song Ma suture zone, Northwestern Vietnam." Journal of Mining and Earth Sciences 62, no. 1 (February 28, 2021): 64–72. http://dx.doi.org/10.46326/jmes.2021.62(1).08.
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The garnet-bearing schists of the Nam Co formation have an identical mineral assemblage consisting of garnet, chlorte, albite, quartz and muscovite, together with accessory apatite, zircon, monazite, xenotime, and ilmenite. An aggregate of muscovite and chlorite defines the major foliations (Sn). Both albite and garnet occur as a porphyroblast, ranging in size 0.2÷1 mm and 0.5÷1.2 mm, respectively. Albite porphyroblasts commonly have the curved to sigmoidal inclusion trails defined by graphitic materials (Sn-1). Garnet porphyroblasts in the sample is generally characterized by paucity of inclusions and retrograde corona of bitotite and chlorite. Garnet also occurs as an inclusion within albite porphyroblast. Porphyroblastic garnet shows the compositional zonation typified by a bell-shaped spessartine profile balanced by increasing almandine from core to rim. Whereas, inclusion garnet is homogeneous compositions with rich in almandin and poor in spessatin, pyrop and grossula. All the above microstructures suggest two deformation and metamorphic stages (M1 and M2) that were affected to politic rocks of the Nam Co formation, Song Ma suture zone.
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Mitchell, J. N. "A Scanning Electron Microscopic study of hematite inclusions in cordierite porphyroblasts." Proceedings, annual meeting, Electron Microscopy Society of America 44 (August 1986): 680–981. http://dx.doi.org/10.1017/s0424820100144796.
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Porphyroblasts are monocrystalline to polycrystalline mineral grains in metamorphic rocks that are distinctly larger than the matrix grain surrounding them. Their presence reflects the metamorphic release and rapid diffusion of their chemical components, with subsequent crystallization at limited nucleation sites. Small inclusions of other minerals are often present in porphyroblasts, especially in members of the garnet group, staurolite, and cordierite. Various hypotheses may be suggested to explain such inclusions: (1) they are relics of earlier grains, either of the protolith or of a stage of metamorphism prior to porphyroblast growth; (2) they are reaction residues of the mineral grains that decomposed to form the porphyroblasts; or (3) they are decomposition products produced from the porphyroblasts by secondary or retrograde metamorphism.
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Paudel, L. P., T. Imayama, and K. Arita. "Metabasites petrology and P-T evolution in the Lesser Himalaya, central Nepal." Journal of Nepal Geological Society 42 (September 24, 2011): 21–40. http://dx.doi.org/10.3126/jngs.v42i0.31446.
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Petrological study was carried out for the first time on the metabasites of the Lesser Himalaya in central Nepal. The metabasites are mostly tholeiitic basalts emplaced in the elastic sediments as supracrustal dikes and sills, and later metamorphosed together with the host rocks. They contain almost a constant mineral assemblage of Ca-amphiboles + plagioclase + biotite + quartz ± epidote± chlorite + (Fe-Ti oxides). Amphiboles in the form of porphyroblasts show chemical zonation with actinolite/magnesiohomblende cores, tschermakite/ferro-tschermakite rims, and magnesio-hornblende margins. The cores of porphyroblasts are pre-kinematic and were probably formed prior to the Tertiary Himalayan orogeny. The porphyroblast rims and the matrix amphiboles are syn-kinematic and were formed during the Upper Main Central Thrust activity in the Tertiary period. The compositions of both the porphyroblast rims and matrix amphiboles change from actinolite in the chlorite zone to magnesium hornblende in the biotite zone and totschermakite/ferro-tschermakite in the garnet zone. The systematic changes in amphibole compositions as well as petrographic characteristics of metabasites confirm the classical concept of increasing metamorphic grade structurally upwards to the Upper Main Central Thrust in the Lesser Himalaya. Application of hornblende-plagioclase thermobarometry shows a coherent prograde P-T path in zoned amphiboles. The cores of amphibole porphyroblasts were formed at average peak temperature of ~540"C and at pressure of ~3 kbar. The porphyroblast rims and matrix amphiboles were recrystallized at average peak temperatures of ~570°C in the biotite zone and ~630°C in the garnet zone at pressure of ~6 kbar. The metabasites petrology is in favor of the tectono-metamorphic models that relate the inverted metamorphism with thrusting along the Upper Main Central Thrust and coeval inversion of isoiliem1S. It is suggested that published amphibole cooling ages from the Nepalese Lesser Himalaya based on simples, homogeneous mineralogy should be reinterpreted in view of the presence of polygenetic amphiboles with heterogeneous composition.
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Shchepetova, O. V., A. V. Korsakova, P. S. Zelenovskiy, and D. S. Mikhailenko. "The mechanism of disordered graphite formation in uph diamond-bearing complexes." Доклады Академии наук 484, no. 2 (April 13, 2019): 215–19. http://dx.doi.org/10.31857/s0869-56524842215-219.
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Kyanite gneiss from the "New Barchinsky" locality (Kokchetav Massif) was studied in detail. This rock is characterized by zonal distribution of the C and SiO2 polymorphs in kyanite porphyroblasts: (1) porphyroblast cores with graphite and quartz inclusions; (2) clean overgrowth zone with inclusions of cuboctahedral diamond crystals. The Raman mapping of SiO2 polymorphs originally showed the presence of an association of disordered graphite + coesite “prohibited” in HT diamond-bearing rocks. Graphitization of diamond is the only likely mechanism of the disordered graphite formation in HT diamond-bearing rocks. However, the absence of disordered graphite in association with diamond in kyanite porphyroblasts from kyanite gneiss from the "New Barchinsky" locality eliminates the process of diamond graphitization at the retrograde stage. Most likely, crystallization of disordered graphite occurred at the retrograde stage from the UHP C-O-H fluid.
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Davis, B. K. "Biotite porphyroblast nucleation and growth: control by microfracture of pre-existing foliations in schists in the Robertson River Metamorphics, Australia." Geological Magazine 133, no. 1 (January 1996): 91–102. http://dx.doi.org/10.1017/s0016756800007275.
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AbstractA macroscopic fold formed during the fourth deformation, D4, in the Robertson River Metamorphics, north Queensland, Australia. Growth of biotite, garnet, staurolite and andalusite porphyroblasts also occurred synchronous with D4. Only biotite porphyroblasts have formed preferred alignments across the fold, and they define two dominant orientations. The more common is parallel to D4 fold axes, which is also parallel to D1 D2 and D4 intersection lineations. The other varies up to 90° from this within the D4 axial plane. Mineral elongation lineations do not reflect the extension direction of the deformation in which they formed (D4), but are a function of pre-existing anisotropics in the rock parallel to intersection lin-eations. Porphyroblast growth is inferred to be a result of microfracturing along favourably oriented linear anisotropics formed by multiple intersection of foliations, as well as parting of the the S1, and S2 foliations.
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Rice, A. H. N., and J. I. Mitchell. "Porphyroblast textural sector-zoning and matrix displacement." Mineralogical Magazine 55, no. 380 (September 1991): 379–96. http://dx.doi.org/10.1180/minmag.1991.055.380.08.
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AbstractThere is an association between the development of cleavage domes, a texture reflecting the displacement of insoluble matrix grains by porphyroblasts growing under a bulk hydrostatic stress, and textural sector-zoning. This has been found in garnet, staurolite, chiastolite, pyrite and possibly emerald porphyroblasts. Sector-zoned porphyroblasts form by lineage growth normal to the crystal faces. This causes several distinctive textures (type 1 inclusions and type 2 intergrowths, inclusion bands, growth prongs), all of which are directly or indirectly related to displacement growth. Graphite or other carbonaceous material is ubiquitous in samples showing textural sector-zoning.
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Carlson, William D. "Competitive diffusion-controlled growth of porphyroblasts." Mineralogical Magazine 55, no. 380 (September 1991): 317–30. http://dx.doi.org/10.1180/minmag.1991.055.380.03.
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AbstractIn a diffusion-controlled process of nucleation and growth, adjacent porphyroblasts compete with one another for nutrients. When the effects of this competition are evaluated quantitatively for garnet porphyroblasts in pelitic rocks from the Picuris Range of New Mexico (U.S.A.), significant correlations arise between crystal sizes and the volumes of the domains from which the crystals drew their nutrients. These correlations strengthen the conclusion drawn from earlier work on spatial dispositions, zoning patterns, and crystal size distributions that the kinetics of intergranular diffusion governed the crystallisation of these porphyroblasts.Computer simulations indicate that competition for nutrients during diffusion-controlled growth may have small but detectable effects on crystal size frequency distributions. Diffusional competition therefore introduces relatively minor inaccuracies into attempts to extract quantitative information on crystallisation processes from size distributions using models for the growth of isolated porphyroblasts. In contrast, the effects of diffusional competition on patterns of compositional zoning may be substantial, especially for porphyroblasts in rocks for which chemical inhomogeneity of the precursor leads to strongly clustered spatial dispositions. In such rocks, clustering may alter the patterns of compositional zoning in ways that obscure evidence for diffusion-controlled growth.
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Pan, Yuanming, Michael E. Fleet, and Neil D. Macrae. "Oriented monazite inclusions in apatite porphyroblasts from the Hemlo gold deposit, Ontario, Canada." Mineralogical Magazine 57, no. 389 (December 1993): 697–707. http://dx.doi.org/10.1180/minmag.1993.057.389.14.
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AbstractOriented inclusions of monazite occur in the dark core of apatite porphyroblasts in a muscovite schist from the Archaean Hemlo gold deposit, Ontario, Canada. The monazite inclusions are elongated along the b-axis and parallel to the c-axis of the apatite host; the complete orientation relationship of the monazite/apatite intergrowth is bMnz//cAp, cMnz//aAp. From analysis by SIMS and EMP, the dark core of the apatite porphyroblasts is depleted in LREE (LaN/YbN = 0.56). The monazite inclusions are correspondingly enriched in LREE, but markedly depleted in HREE, compared with monazite grains in the rock matrix and cross-cutting veins. The monazite inclusions precipitated by oriented reaction through rock-fluid interactions during a late hydrothermal alteration. Their unusual REE composition is probably related to both a preferential leaching of LREE from the dark core and a selective transfer of HREE out of the apatite porphyroblasts.
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Redlińska-Marczyńska, Aleksandra. "Gierałtów versus Śnieżnik gneisses - what is the real difference?" Geologos 17, no. 2 (June 1, 2011): 71–96. http://dx.doi.org/10.2478/v10118-011-0005-6.
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Gierałtów versus Śnieżnik gneisses - what is the real difference?Structural and petrographic study applied to the gneisses from the eastern part of the Orlica-Śnieżnik Dome, indicate that two different types of gneiss are present. The Śnieżnik gneisses are porphyrithic granites, constricted and sheared into L-S tectonites, most commonly with augens; the Gierałtów gneisses are sheared migmatites, porphyroblastic gneisses and banded gneisses, with two sets of metamorphic foliation, intrafolial folds and lensoid leucosome aggregates or metamorphic porphyroblasts. Both lithologies were later zonally sheared and transformed into more or less deformationally advanced mylonites, difficult to be distinguished from one of the two types. Identification of the Śnieżnik and Gierałtów gneisses is possible only between zones of the late (Variscan) shearing, in which the original, pre-kinematic structures are preserved.
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Hirajima, T., R. Zhang, J. Li, and B. Cong. "Petrology of the nyböite-bearing eclogite in the Donghai area, Jiangsu Province, eastern China." Mineralogical Magazine 56, no. 382 (March 1992): 37–46. http://dx.doi.org/10.1180/minmag.1992.056.382.05.
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AbstractNyböite occurs as porphyroblasts in the Jianchang eclogite in the Donghai area, northeastern Jiangsu Province, eastern China. The Jianchang eclogite contains some inclusions of quartz after coesite in clinopyroxene, garnet and epidote. It has colourless to pale-violet pleochroism. A thin rim with violet pleochroism often develops around nyb6ite and is taramitic. It is further retrogressed by the symplectite which is mainly composed of hornblende, aegirine-augite and albite. Nyböite is associated with jadeitic pyroxene in the Jianchang eclogite, although other porphyroblastic amphiboles in other Donghai eclogites are barroisitic to katophoritic and are associated with omphacite.Fe-Mg partitioning between garnet and clinopyroxene and the presence of coesite pseudomorphs indicate P-T conditions in the Jianchang eclogite of about 740 ± 60°C and more than 28 kbar. Similar P-T conditions were estimated for other porphyroblastic amphibole-bearing eclogites in the Donghai area. Nyböite can occur in the Na-Al-Fe-rich local bulk composition under the medium to high temperature and very high-pressure conditions. Retrograde rim amphibole is poorer in NaB, variable in Si content, and richer in NaA variable than the porphyroblastic amphibole in the Donghai area. This roughly implies a P-T path where P decreases without a large decrease of T.
Dissertations / Theses on the topic "Porphyroblasts"
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Rahimi-Chakdel, Aziz. "What controls the inclusion of quartz crystals in porphyroblasts : implications for texture analysis and tectonic interpretations?" Thesis, University of Liverpool, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.272794.
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Osaki, Atsushi. "Cordierite porphyroblast in contact aureole." 京都大学 (Kyoto University), 2000. http://hdl.handle.net/2433/181129.
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Dupee, Matthew E. "Porphyroblast Kinematics and Crenulation Cleavage Development in the Aureole of the Mooselookmeguntic Pluton, Western Maine." Fogler Library, University of Maine, 2005. http://www.library.umaine.edu/theses/pdf/DupeeME2005.pdf.
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Prohoroff, A. "Structural and Metamorphic Conditions of the Lower Burra Group and Callana Group at Arkaroola, Northern Flinders Ranges." Thesis, 2013. http://hdl.handle.net/2440/106297.
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This item is only available electronically.The lowermost Adelaidean sequences exposed to the immediate north of Arkaroola are unusual as they exhibit a localised complexity of deformation and elevated metamorphic grade that is not observed elsewhere in the Adelaide Fold Belt. Deformation and metamorphism in Arkaroola is thought to have formed as part of the Delamerian Orogen approximately 515-490 Ma. The timing of deformation and metamorphism however is poorly constrained in this area. This paper aims to discuss the structural and metamorphic conditions in the area to determine if there was a possibility of a pre or post-Delamerian structural and/or thermal event. A section was mapped to the North-East of the Arkaroola Homestead to gain an insight into the structural and metamorphic conditions of the area. Samples were collected from the field and used for microstructural analysis. An Electron Microprobe, Laser Ablation Inductively Coupled Plasma Mass Spectrometer and an XRF spectrometer were used for geochemical analysis on the samples. Structural and stratigraphic observations combined with microstructural analysis of samples from the field helped the author create an interpreted geological history of the area. Graben formation accommodated an initial period of sediment deposition followed by basalt extrusion. Several phases of localised rifting and deposition followed this initial deposition period due to changing fault geometries. A mineral fabric that occurs parallel to bedding is seen throughout the study area. This fabric is overgrown and included in prominent cordierite porphyroblasts that formed during peak metamorphism of ≥500 °C at a pressure of approximately ~1.30kbars. These pressure and temperature conditions were primarily due to the burial beneath a thick cover of sediments. A number of faults trending in a NE-SW direction have been identified as splays from the Paralana fault system. The strike-slip movement of the Paralana Fault along with the high heat producing basement of the Mount Painter Inlier has controlled the localised structural complexity and elevated metamorphic grade in the Arkaroola area.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2013
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Gavin, Bronwyn Patricia. "The microstructural and metamorphic history preserved within garnet porphyroblasts from southern Vermont and northwestern Massachusetts." Thesis, 2004. https://researchonline.jcu.edu.au/1377/1/01front.pdf.
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Garnet porphyroblasts from southern Vermont and northwestern Massachusetts typically preserve multiple generations of inclusion trails, reflecting a potentially complex deformation and metamorphic growth history. Textural discontinuities, such as inclusion trail truncations or deflection planes, are commonly preserved within these complex inclusion trails and they are dominantly sub-vertically and sub-horizontally oriented. This observation cannot be adequately explained using the rotational model of spiral inclusion trail formation, leading to the conclusion that the trails were formed by the inclusion of multiple sub-vertical and sub-horizontal foliations during episodic garnet growth without porphyroblast rotation. Foliation inflection/intersection axes preserved within porphyroblasts (FIAs) provide important information about the kinematics of deformation, particularly the direction of bulk shortening at the time they formed. Samples from this study preserve a succession of six FIA sets resulting from a progressive change in the direction of bulk shortening through time. The distribution of these FIA sets across the field area indicates that the deformation was heterogeneously partitioned and occurred at different scales throughout orogenesis. Localization of deformation is an important control on garnet growth and repartitioning during successive deformation events resulted in a heterogeneous spatial distribution of garnet growth through time. The episodic nature of garnet growth is reflected in compositional zoning anomalies in garnet porphyroblasts from the Hoosac Formation. Zones of manganese enrichment, accompanied by calcium depletion, reveal pauses in garnet growth that may have been accompanied by fluid infiltration, garnet dissolution-reprecipitation and metasomatism at crystal or greater scales. The metamorphic history of these samples was further investigated using P-T pseudosections constructed via THERMOCALC. The mineral assemblages predicted are in good agreement with the observed mineralogy of the samples and estimates of P-T conditions at the time of garnet core growth were made using compositional isopleths based on microprobe analyses of garnet core composition. The samples do not show a clear relationship between the P-T data and the FIA data, suggesting that the preservation of different FIAs is not simply a function of P-T conditions. In samples where initial garnet appears to have occurred at temperature and/or pressure conditions higher than the minimum P-T conditions predicted for garnet stability, deformation probably played an essential role in garnet nucleation and growth. These “overstepped” samples indicate a progressive increase in pressure through the different phases of orogenesis.
This study reveals that garnet porphyroblasts in southern Vermont and northwestern Massachusetts grew during a complex history of deformation involving the production of multiple sub-horizontal and sub-vertical foliations with garnet growth primarily controlled by deformation partitioning, resulting in a heterogeneous spatial distribution through time. FIAs allow the relative age of different phases of garnet growth to be established and provide a framework for interpreting the relationship between deformation and metamorphism. Compositional zoning anomalies reflect the episodic nature of garnet growth and P-T modelling suggests that deformation took place under conditions of increasing pressure.
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Gavin, Bronwyn Patricia. "The microstructural and metamorphic history preserved within garnet porphyroblasts from southern Vermont and northwestern Massachusetts." 2004. http://eprints.jcu.edu.au/1377/1/01front.pdf.
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Garnet porphyroblasts from southern Vermont and northwestern Massachusetts typically preserve multiple generations of inclusion trails, reflecting a potentially complex deformation and metamorphic growth history. Textural discontinuities, such as inclusion trail truncations or deflection planes, are commonly preserved within these complex inclusion trails and they are dominantly sub-vertically and sub-horizontally oriented. This observation cannot be adequately explained using the rotational model of spiral inclusion trail formation, leading to the conclusion that the trails were formed by the inclusion of multiple sub-vertical and sub-horizontal foliations during episodic garnet growth without porphyroblast rotation. Foliation inflection/intersection axes preserved within porphyroblasts (FIAs) provide important information about the kinematics of deformation, particularly the direction of bulk shortening at the time they formed. Samples from this study preserve a succession of six FIA sets resulting from a progressive change in the direction of bulk shortening through time. The distribution of these FIA sets across the field area indicates that the deformation was heterogeneously partitioned and occurred at different scales throughout orogenesis. Localization of deformation is an important control on garnet growth and repartitioning during successive deformation events resulted in a heterogeneous spatial distribution of garnet growth through time. The episodic nature of garnet growth is reflected in compositional zoning anomalies in garnet porphyroblasts from the Hoosac Formation. Zones of manganese enrichment, accompanied by calcium depletion, reveal pauses in garnet growth that may have been accompanied by fluid infiltration, garnet dissolution-reprecipitation and metasomatism at crystal or greater scales. The metamorphic history of these samples was further investigated using P-T pseudosections constructed via THERMOCALC. The mineral assemblages predicted are in good agreement with the observed mineralogy of the samples and estimates of P-T conditions at the time of garnet core growth were made using compositional isopleths based on microprobe analyses of garnet core composition. The samples do not show a clear relationship between the P-T data and the FIA data, suggesting that the preservation of different FIAs is not simply a function of P-T conditions. In samples where initial garnet appears to have occurred at temperature and/or pressure conditions higher than the minimum P-T conditions predicted for garnet stability, deformation probably played an essential role in garnet nucleation and growth. These “overstepped” samples indicate a progressive increase in pressure through the different phases of orogenesis. This study reveals that garnet porphyroblasts in southern Vermont and northwestern Massachusetts grew during a complex history of deformation involving the production of multiple sub-horizontal and sub-vertical foliations with garnet growth primarily controlled by deformation partitioning, resulting in a heterogeneous spatial distribution through time. FIAs allow the relative age of different phases of garnet growth to be established and provide a framework for interpreting the relationship between deformation and metamorphism. Compositional zoning anomalies reflect the episodic nature of garnet growth and P-T modelling suggests that deformation took place under conditions of increasing pressure.
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Hirsch, David Marshall 1969. "Quantitative studies of porphyroblastic textures." Thesis, 2000. http://hdl.handle.net/2152/415.
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Spatial correlation functions, which quantify spatial relationships among porphyroblasts over a range of length scales, can be used in combination with other techniques of quantitative textural analysis to constrain crystallization mechanisms in metamorphic rocks. The utility, reliability, and robustness of these functions, however, depend critically upon correct methods of calculation and application to geological samples. Application of the L' -function, Pair Correlation Function, and Mark Correlation Function (Stoyan and Stoyan, 1994) to artificial arrangements of crystals yields results consistent with their predetermined ordering and clustering qualities. These results serve as a foundation for the interpretation of more complex simulated and natural crystal arrays. Analysis of artificial and simulated crystal arrays in which ordering signals are obscured in various ways (displacing crystals in an ordered array by increasing amounts, reducing the number of crystals, and increasing the sample's aspect ratio) demonstrates that these scale-dependent functions are robust indicators of effects diagnostic of certain crystallization mechanisms, even in complex circumstances. The effects of clustering of nucleation sites, however, can strongly obscure any underlying signal that might reveal crystallization mechanisms. The L' -function and the Pair Correlation Function are sensitive to short-range ordering of crystals, which may reflect suppression of nucleation in the vicinity of growing porphyroblasts. The Mark Correlation Function is sensitive to size-isolation correlations, which may reflect retardation of growth among crystals competing for nutrients. Interpretation of these functions, however, requires careful attention to proper calculation of Monte Carlo simulations, which are used to identify values of the functions that constitute a null-hypothesis region for comparison to samples with unknown ordering and clustering characteristics. To yield functional values commensurate with those calculated for a particular natural rock specimen, each simulation must match as closely as possible several critical features of the natural rock, including the set of crystal radii, limitations on the observability of crystals, and the shape and size of the bounding surface of the sample. Crystallization mechanisms in seven previously studied garnetiferous rocks from three localities (Carlson et al., 1995; Denison and Carlson, 1997) have been re-assessed using both scale-dependent correlation functions and single-valued spatial statistics, both evaluated by comparison to rigorously computed null-hypothesis regions. The results confirm previous inferences that the nucleation and growth rates of the garnet porphyroblasts in these specimens were governed by rates of diffusion through the intergranular medium.text
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Fay, C. "Modeling, describing, measuring and interpreting porphyroblast inclusion trails to understand inter-relationships between deformation, metamorphism and tectonism." Thesis, 2014. https://researchonline.jcu.edu.au/39976/1/39976-fay-2014-thesis-volume-1.pdf.
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The main objectives of the research presented in this PHD thesis are:
• to demonstrate numerically the relevance of using microstructures encapsulated within porphyroblasts as a quantitative proxy for unravelling lengthy orogenic deformation history;
• applying this quantitative measurements of foliation intersection/inflexion axis (FIAs) method to deciphering the sequence of main shortening directions responsible for most of the deformation features observed in fold interference patterns and the arcuate shape of orogens using the example of the Adelaide Geosyncline (South Australia);
• to describe with great precision the sequence of deformation and metamorphic events and their inter-relationships during one single FIA event affecting the study area;
• to understand the long-lasting stable metamorphic conditions of mid-crustal rocks displaying multiple generations of synchronous staurolite and andalusite using a combination of thermo-dynamic modelling coupled with microstructural analysis.
The first part is based on the data on foliation intersection/inflection axes preserved in porphyroblasts (FIAs) and show that no porphyroblast rotation occurs during ductile deformation relative to spatial coordinates. This contrasts with 99% of investigations of "rigid" objects in non-coaxially deforming media where the objects rotate. When anastomosing shear zone formation around relatively strong objects in a weaker matrix is modelled, no "porphyroblast" rotation occurs. Formation of these anastomosing zones controls the development of this phenomenon, labelled "gyrostasis". If such zones are absent, porphyroblasts rotate. In weak materials the gyrostatic situation arises because the superposition of simple shearing deformation normal to initial coaxial shortening results in only small rotations of principal axes of stress. Since shear zones are controlled by the orientations of principal axes of stress, initial anastomosing zones retain their orientations and positions during subsequent noncoaxial deformation. The porphyroblast is isolated from the embedding non-coaxially deforming material and material close to the porphyroblast continues to deform coaxially; no local rotation occurs. This has major significance since porphyroblasts can be routinely used to access lengthy tectonic histories destroyed in the matrix by reactivation. In particular, changes in relative directions of bulk shortening, associated with orogenesis, can now be determined within ancient orogens.
In the second part of the thesis, porphyroblast microstructures in rocks from a narrow portion of the sigmoidal-shaped Adelaide geosyncline indicate that the Delamerian Orogeny was a product of five changes in the direction of bulk shortening. The progression from NNW-SSE to WSW-ENE to SSW-NNE to WNW-ESE to NNESSW directed bulk shortening, resulted from shifts in the direction of relative plate motion as orogenesis progressed between 522 and 478 Ma. These directions were determined from a succession of 5 FIAs (Foliation Intersection/Inflection Axes preserved within porphyroblasts). The overprinting relationships between the multiple generations of regional folds that developed were resolved using the timing criteria provided by this FIA succession. The S-shape of the orocline has been interpreted as a fold and thrust belt product of either oblique convergence or the development of asymmetric syntaxis zones but this is not the case. Rather the orocline resulted the overprinting of a succession of near orthogonal plan view changes in the direction of convergence. This reflects much of the geodynamic evolution of the Eastern Australian portion of the Gondwana margin during its early Paleozoic history.
In the third part of the thesis, excellent inclusion trails in a staurolite and andalusite-bearing sample preserve 3 main phases of growth of both phases during the early stages of 3 deformation events. Subtle extra periods of growth of both phases occur, being most obvious for andalusite porphyroblasts, which commonly occur as clusters of large crystals that vary from several to tens of degrees in orientation and can encapsulate staurolite grown in an earlier or the same deformation event. All foliations defined by all inclusion trails intersect in a FIA (a foliation intersection axis preserved within porphyroblasts) trending at 25° indicating no change in the direction of horizontal components of bulk shortening while the porphyroblasts grew. Wellpreserved microstructural relationships between successive foliations within porphyroblasts allow a detailed analysis of the approaches to inclusion trail description and interpretation that have resulted from 25 years of quantitative FIA based studies. Spiral-shaped inclusion trails in most porphyroblast clusters contain portions of millipede geometries. The latter clinch the dominant role of bulk shortening in porphyroblast growth even in an environment that is overall non coaxial and which results in the same asymmetry later on in each deformation event. Any role for porphyroblast rotation is strongly refuted by differing stages in the development of these bulk-shortening geometries preserved within staurolite and the andalusite that immediately enclosed them as does such variation in adjacent clusters. They strongly suggest that discrepancy in the orientation of inclusion trails in porphyroblast cores is a function of the early effects of bulk shortening driving porphyroblast growth and cannot be used to imply later porphyroblast rotation. Staurolite and andalusite have grown slightly before, after and synchronously, without reacting with each other, during the early stages of 3 separate deformations. This strongly supports microstructural and more recent metamorphic data that the early stages of bulk shortening start porphyroblast growth; it also indicates that the commencement of the development of a differentiated foliation in the vicinity of a porphyroblast will always stop growth.
In the four[th] part of this thesis, it is revealed that the southern portion of the Adelaide fold belt contains a large region where synchronous to interleaved growth of staurolite and andalusite porphyroblasts has occurred. The truncation and continuity of inclusion trails versus matrix foliations reveal multiple periods of growth of staurolite and andalusite in many samples. The measurement of FIAs (foliation intersection axes preserved within porphyroblasts) revealed a succession of 5 changes in the bulk shortening direction during orogenesis from initially NNW-SSE when the first FIA (I), trending at 75°, formed in garnet. FIAs II through V are present in garnet, staurolite and andalusite. Indeed, staurolite and andalusite grew in the same sample during the development of at least one of FIAs II, III, IV and V. Pseudosections show a remarkably narrow range of PT conditions where this is possible and define a very confined location in PT space where these rocks developed at least 10 foliations over the ~ 30 million years that FIAs II through V developed. Garnet growth occurred early in some samples, but not in others with very similar bulk chemistry, where staurolite and andalusite grew in the same FIA event. This behaviour resulted from slight changes in Mn content and allowed a very tightly constrained PT path to be defined on pseudosections in combination with the minerals that formed early and late in some multi FIA samples. These low-pressure high-temperature rocks remained at the same orogenic level throughout most of the very lengthy deformation history of the Delamerian orogenic cycle once staurolite and andalusite began to grow. They ceased to grow at the commencement of exhumation when retrogressive chlorite growth began in many samples when shortening was directed once again NNW-SSE.
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Chiang, Shu-Fan, and 江曙帆. "Original and metamorphic P-T evolution of porphyroblastic albites of spotted schists in Juisui, Hualien." Thesis, 2003. http://ndltd.ncl.edu.tw/handle/78189273896298242675.
Full textAbstract:
碩士國立中央大學
地球物理研究所
91
Original and metamorphic P-T evolution of porphytroblastic albites of spotted schists in Juisui, Hualien Shu-Fan Chiang ABSTRACT The spotted schists of Juisui area in Hualien are a kind of porphyroblast-bearing black schists, which only occur in the Yuli belt. The general geology of Juisui area is well known. However, the study on metamorphic petrology is very limited in spotted schists. This study on metamorphic petrology in spotted schists is expected to provide more information for understanding the multiple metamorphic histories of the Yuli belt. The petrology show mineral assemblages are garnet+ muscovite+ plagioclase+ chlorite+ qtz± sphene± tourmaline± calcite± rutile. The garnets are major almandine and spessartine and display well bell- shaped chemical zonation. Analytical results show that the growth zoning with ratio of Mn/Fe decreasing from core towards rim. That maybe occurred by prograde metamorphism. However, there are some reverse zoning with ratio of Mn/Fe increasing near the rim of garnet. Moreover, the results of petrological observation show the rim of garnets replace by chlorite. The replacement maybe occurred by retrograde metamorphism after the prograde metamorphism. The P, T estimates of the spotted schists from geothermobarometry for the garnets with growth zoning yield 486℃, 7.4kbar for the core and 546℃, 9kbar for the rim. The P-T path derives by Gibbs method is both increasing in temperature and pressure from core to rim. Study of petrology, mineral chemistry and geothermobarometry suggest that the P-T paths demonstrate systematically increases in pressure and temperature from core to rim. These probably reflect the deeply burial process and the metamorphic grade could be up to epitdote-amphibolite facies. The garnets should grow in epidote-amphibolite facies. Subsequently, some rim of garnets replace by chlorite in retrograde metamorphism. At the same time, the albite porphyroblasts grown in decompression process.
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Welch, Peter W. "Microprobe dating of monazite in relation to porphyroblast growth and thermodynamic modelling P-T paths for rocks affected by prolonged orogenesis." Thesis, 2003. https://researchonline.jcu.edu.au/1178/1/01front.pdf.
Full textAbstract:
Pressure-Temperature-time-deformation paths were modelled using
microstructural, petrologic, geochronologic data for multiply deformed Acadian
metamorphic rocks currently exposed within and mantling the Chester and Athens
Domes in southeastern Vermont, USA. Estimates of pressure, temperature and time
were obtained for samples where the relative timing of distinct periods of garnet growth
has been previously established through detailed microstructural analysis of foliation
intersection axes (FIAs) of inclusion trails in garnet porphyroblast.
Compositional mapping and in-situ microprobe analyses of monazite grains
reveal that monazite growth was the result a series of reactions or growth events.
Numerous monazite grains preserve complex compositional zoning patterns that cannot
be easily explained by a single monazite growth event. Electron microprobe point
analyses of U, Th, Pb and Y were used to determine the ages of monazite grains or
distinct compositional domains within monazite grains. U-Th-Pb age distributions show
monazite growth and subsequent metamorphism occurred over a period at least 80 m.y
from 430 Ma to 350 Ma. A single sample was found to contain distinct populations
when monazites where grouped by microstructural affinities. A single sample yielded
ages of 424 ± 2.4 Ma, 405 ± 6.0 Ma, 386 ± 6.0 Ma, 366 ± 3.8 for monazite populations
analysed in the cores, medians, and rims of garnet porphyroblasts and matrix
respectively. Weighted averages calculated for monazite populations, which lie within
the included/overgrown foliations for all of the samples clustered around 425 Ma, 405,
Ma, 387 Ma, 377, Ma and 365 Ma and 350 Ma. These age distributions are taken to
represent a best estimate of the timing of deformation and accompanying mineral
growth for southeastern Vermont.
P-T pseudosections were utilised to map mineral reactions for samples that
contain garnet porphyroblasts with inclusion mineralogies that reveal partial mineral
assemblages prior to garnet growth. Pseudosections were modelled in the system
MnKFMASH for high-Al pelites and MnNCKFMASH for low-Al pelites and calcpelites.
Thermobarometric calculations were compared with P-T pseudosections to
construct P-T paths consistent with observed mineral reactions. P-T estimates for garnet
cores are consistent with the position of garnet-in reactions on MnKFMASH
pseudosections and are generally 25-50°C above the garnet-in reaction on the
MnNCKFMASH pseudosections. P-T estimates for garnet cores reveal that garnet
growth commenced at ª 525°C and moderate pressure (4-9 kbars) and proceeded along
an up-pressure path reaching peak pressures between 13-14 kbars at 600-625°C.
Monazite inclusions in garnet porphyroblasts bracket the timing of the onset of garnet
growth between 425 Ma and 405 Ma. Peak pressure, which is estimated to have
occurred at ª 600°C, was likely reached by 385 Ma. Peak temperatures of ª 650°C are
recorded between 11-12 kbars, which was followed by decompression and cooling.
Decompression occurred between 380 and 350 Ma with very little heating indicating
that initial uplift and subsequently exhumation was rapid. Monazite ages from garnet
rims and matrix mark the end deformation and mineral growth at 350 Ma.
Previous tectonic models for Acadian orogenesis for the region have suggested
that deformation and accompanying metamorphism was a two-stage process. Crustal
thickening was thought to be the result of nappe style thrusting followed by regional
scale doming. The integration of microstructural studies and in-situ monazite dating
along with a detailed evaluation of the thermobarometric history reveals a more
complex deformation and metamorphic history. The peak pressures and relatively low
geothermal gradients associated with the early part of the P-T path are more consistent
with subduction related tectonism than nappe style thrusting. Inclusion trails in garnet
porphyroblasts also reveal that numerous near orthogonal foliations formed during garnet growth that cannot be easily explained by nappe and dome stage deformations. In
the proposed model, foliation development and subsequent mineral growth occurs in
response to crustal thickening related to continental subduction. This resulted in a
greatly over-thickened crust that was unstable and underwent rapid uplift and
exhumation with little heating. Monazite ages that have been linked directly with
microstructures and mineral growth reveal that Acadian Orogenesis may have begun as
early as the earliest Silurian and continued through to the Carboniferous.
Book chapters on the topic "Porphyroblasts"
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Passchier, C. W., and R. A. J. Trouw. "Porphyroblasts and Reaction Rims." In Microtectonics, 153–96. Berlin, Heidelberg: Springer Berlin Heidelberg, 1998. http://dx.doi.org/10.1007/978-3-662-08734-3_7.
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Barker, Andy J. "Porphyroblast—foliation relationships." In Introduction to Metamorphic Textures and Microstructures, 149–62. Boston, MA: Springer US, 1998. http://dx.doi.org/10.1007/978-1-4615-7291-6_9.
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Powell, Roger, and Jon Downes. "Garnet porphyroblast-bearing leucosomes in metapelites: mechanisms, phase diagrams, and an example from Broken Hill, Australia." In High-temperature Metamorphism and Crustal Anatexis, 105–23. Dordrecht: Springer Netherlands, 1990. http://dx.doi.org/10.1007/978-94-015-3929-6_5.
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"Porphyroblasts, porphyroclasts, and augen." In A Pictorial Guide to Metamorphic Rocks in the Field, 191–202. CRC Press, 2014. http://dx.doi.org/10.1201/b17436-23.
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Beam, Eric C. "Modeling growth and rotation of porphyroblasts and inclusion trails." In Structural Geology and Personal Computers, 247–58. Elsevier, 1996. http://dx.doi.org/10.1016/s1874-561x(96)80022-8.
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"porphyroblast." In Dictionary Geotechnical Engineering/Wörterbuch GeoTechnik, 1027. Berlin, Heidelberg: Springer Berlin Heidelberg, 2014. http://dx.doi.org/10.1007/978-3-642-41714-6_163022.
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"porphyroblastic." In Dictionary Geotechnical Engineering/Wörterbuch GeoTechnik, 1027. Berlin, Heidelberg: Springer Berlin Heidelberg, 2014. http://dx.doi.org/10.1007/978-3-642-41714-6_163023.
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"Porphyroblast m." In Wörterbuch GeoTechnik/Dictionary Geotechnical Engineering, 849. Berlin, Heidelberg: Springer Berlin Heidelberg, 2013. http://dx.doi.org/10.1007/978-3-642-33335-4_161442.
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"magnetite porphyroblast." In Dictionary Geotechnical Engineering/Wörterbuch GeoTechnik, 832. Berlin, Heidelberg: Springer Berlin Heidelberg, 2014. http://dx.doi.org/10.1007/978-3-642-41714-6_130187.
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"porphyroblastic texture." In Dictionary Geotechnical Engineering/Wörterbuch GeoTechnik, 1027. Berlin, Heidelberg: Springer Berlin Heidelberg, 2014. http://dx.doi.org/10.1007/978-3-642-41714-6_163024.
Full textConference papers on the topic "Porphyroblasts"
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Backus, Ethan L., and Callum J. Hetherington. "TEXTURAL AND GEOCHEMICAL ANALYSIS OF GARNET PORPHYROBLASTS IN THE CASTNER MABLE, WEST TEXAS, USA." In GSA Annual Meeting in Denver, Colorado, USA - 2016. Geological Society of America, 2016. http://dx.doi.org/10.1130/abs/2016am-287192.
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Okamoto, Atsushi. "Rapid Growth of Garnet within a Metamorphic Vein Inferred from Misorientation Angle Distribution of Garnet Porphyroblasts." In WATER DYANMICS: 3rd International Workshop on Water Dynamics. AIP, 2006. http://dx.doi.org/10.1063/1.2207097.
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McFarlane, Chris. "Exploring metal zoning in metapelitic porphyroblasts to reconstruct reaction histories and P-T-t-D paths." In Goldschmidt2021. France: European Association of Geochemistry, 2021. http://dx.doi.org/10.7185/gold2021.7947.
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Backus, Ethan L., and Callum J. Hetherington. "CONTRASTING ELEMENT TRANSPORT MECHANISMS DURING PORPHYROBLAST GROWTH IN HETEROGENEOUS MATRICES." In GSA Annual Meeting in Phoenix, Arizona, USA - 2019. Geological Society of America, 2019. http://dx.doi.org/10.1130/abs/2019am-337905.
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Griffin, Colin G., and Gary S. Solar. "PORPHYROBLAST-MATRIX RELATIONS IN WESTERN CONNECTICUT: INCLUSION TRAILS AND STRAIN-SHADOW TAILS." In 54th Annual GSA Northeastern Section Meeting - 2019. Geological Society of America, 2019. http://dx.doi.org/10.1130/abs/2019ne-328652.
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Chen, Nengsong, Binghan Chen, and Tingting Yang. "Porphyroblastic Andalusite Zoned Microstructures Respond to Multiple Pulse Magma Emplacements?" In Goldschmidt2020. Geochemical Society, 2020. http://dx.doi.org/10.46427/gold2020.392.
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Higgins, Ian Rodger, Tim Lutz, and Howell Bosbyshell. "3D ANALYSIS OF GARNET PORPHYROBLAST ORIENTATION IN THE HONEY BROOK ANORTHOSITE OF PENNSYLVANIA." In Joint 52nd Northeastern Annual Section and 51st North-Central Annual GSA Section Meeting - 2017. Geological Society of America, 2017. http://dx.doi.org/10.1130/abs/2017ne-291601.
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Higgins, Ian. "3-D PHOTOGRAMMETRY OF GARNET PORPHYROBLAST ORIENTATION IN THE HONEY BROOK ANORTHOSITE OF PENNSYLVANIA." In GSA Annual Meeting in Denver, Colorado, USA - 2016. Geological Society of America, 2016. http://dx.doi.org/10.1130/abs/2016am-287934.
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Weiman, Christopher D., David W. Valentino, and Jeffrey Chiarenzelli. "MIGRATION OF THE TACONIC DEFORMATION FRONT ACROSS THE MID-ATLANTIC PIEDMONT: EVIDENCE FROM DETAILED PORPHYROBLAST ANALYSIS." In 54th Annual GSA Northeastern Section Meeting - 2019. Geological Society of America, 2019. http://dx.doi.org/10.1130/abs/2019ne-328374.
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Kopinski, Kayla P., Gary S. Solar, and Paul B. Tomascak. "PORPHYROBLAST-MATRIX RELATIONS IN MYLONITE IN THE NORUMBEGA SHEAR ZONE SYSTEM, HARPSWELL NECK AREA, SOUTHERN COASTAL MAINE." In Joint 69th Annual Southeastern / 55th Annual Northeastern GSA Section Meeting - 2020. Geological Society of America, 2020. http://dx.doi.org/10.1130/abs/2020se-345386.
Full textReports on the topic "Porphyroblasts"
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Padget, C. D. W., D. R. M. Pattison, D. P. Moynihan, and O. Beyssac. Pyrite and pyrrhotite in a prograde metamorphic sequence, Hyland River region, SE Yukon: implications for orogenic gold. Natural Resources Canada/CMSS/Information Management, 2021. http://dx.doi.org/10.4095/328987.
Full textAbstract:
The distribution of pyrite and pyrrhotite is documented within an andalusite-sillimanite type (high-temperature, low-pressure) metasedimentary succession exposed in the Hyland River region of southeastern Yukon, Canada. The following metamorphic zones are recognized: chlorite, biotite, cordierite/staurolite (porphyroblast-in), andalusite, sillimanite, and K-feldspar + sillimanite. Pyrite occurs in the chlorite zone through the biotite zone, while pyrrhotite occurs from the chlorite zone to K-feldspar + sillimanite zone. The pyrite-pyrrhotite transition, therefore, occupies an interval in the chlorite and lower biotite zones that is terminated upgrade by a pyrite-out isograd in the upper part of the biotite zone or lowest grade part of the cordierite/staurolite zone. Pressure and temperature conditions of the rocks were estimated from phase equilibrium modelling and from Raman spectroscopy of carbonaceous material (RSCM) thermometry. Modelling indicates pressures of 3.7-4.1 kbar with temperatures of ~425 °C at the biotite isograd, 560-570 °C for chlorite-out/porphyroblast-in, ~575 °C for andalusite-in, 575-600 °C for the sillimanite isograd, and 645-660 °C at the K-feldspar + sillimanite isograd. RSCM temperatures are greater than or equal to 420 °C in the Chl zone, 500 °C at the Bt isograd, 525-550 °C for porphyroblast-in isograd, ~550 °C at the And isograd, and 580 °C at the Sil isograd. These results suggest the pyrite-pyrrhotite transition occurs from less than or equal to 420°C to ~560 °C. Thermodynamic modelling shows 0.6 wt. % H2O is released during metamorphism over the ~140 °C interval of the pyrite-pyrrhotite transition. The gradual release of fluid in the biotite zone is interpreted to have broadened the pyrite-pyrrhotite transition compared to other studies that predict a small interval of vigorous fluid release associated with volumetric chlorite consumption. Samples from the pyrite-pyrrhotite transition zone contain lower whole rock and pyrite Au values than samples from unmetamorphosed/lower rocks, suggesting that Au was removed from the rock at conditions below the pyrite-pyrrhotite transition (<420 °C). The chlorite zone and higher-grade metamorphic rocks of the Hyland River area do not appear to be a plausible source region for orogenic gold.