Prehnite–pumpellyite facies metamorphism in County Cavan, Ireland

Encyclopedia of Geology, 2021

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Journal of the Geological Society, 1985

Interlayered lithologies of diverse mineralogy enclosing the Middle Dalradian syngenetic Ba-Zn-Pb deposit at Foss, S of Loch Tummel in Perthshire, allow estimates of the P-T conditions of metamorphism using several new geothermometers and geobarometers. Temperatures of 50C-560"C are derived from calcite-dolomite geothermometry and from data on Mg-Fe partitioning within garnet-hornblende and garnet-biotite pairs. Plagioclase-biotitegarnet-muscovite and biotite-muscovite-chlorite equilibria indicate peak metamorphic pressures of 8-10 kbar which are in agreement with the results of sphalerite geobarometry. Textural evidence indicates that the rare platy mineral cymrite (BaA12Si20,~Hz0) was widespread in mineralized lithologies during penetrative deformation, and has since largely been replaced by celsian. The P-T location of the experimentally determined reaction line, cymrite,,=celsian,,+H,O, places further constraints on the peak metamorphic conditions and suggests that the final episode of penetrative deformation (D3) was followed by a period of near-isothermal uplift during which celsian became stable. The cymrite field appears to have been re-entered during late brittle deformation. Peak metamorphic conditions recorded by mineral equilibria at Foss differ from those of areas 10 km to the N and S.

Mineralogical Magazine, 1985

The system Na2O-CaO-MgO Al2O3-SiO2-H2O is proposed to model phase equilibria and mineral parageneses for low-temperature metamorphism of basaltic rocks. Univariant reactions marking the transitions between various sub-greenschist facies are identified and some have been experimentally determined. The introduction of Fe2O3 into the model system at fixed FeO/MgO ratio creates continuous reactions for facies boundaries and discontinuous reactions for invariant points of the model system. Both qualitative and quantitative effects on P-T displacement and phase compositions are discussed. The X Fe3+ isopleths for epidote were plotted to exemplify the transition from the zeolite through prehnite-pumpellyite to prehnite-actinolite facies. T-X Fe 3+ relations were established for continuous and discontinuous reactions relating such facies transitions. Because of the common occurrence of two or three Ca-Al hydrosilicates in low-grade metabasites, an isobaric Al-Ca-Fe3+ projection from chlorit...

METAMORPHISM, METAMORPHIC FACIES AND MINERAL PARAGENESIS, 2014

Metamorphic rocks are rocks whose characters have been changed from its original form by processes operating within the earth. These processes are physical and chemical in nature resulting in a change in mineral makeup of the rock or relationship between the minerals or texture of the rock or both. Temperature and pressure are the most important factors controlling the process of metamorphism. Metamorphism may be local or regional in extent. With the concept of metamorphic facies, it is known that certain mineral sets are at equilibrium under different conditions and thus can be predicted. This paper work covers Metamorphism , types of metamorphism, agents of metamorphism, metamorphic facies, metamorphic reactions and mineral paragenesis.

Introduction Many early reviews of metamorphism considered that true metamorphic conditions are only first encountered at the onset of the greenschist facies, with anything below that, in the subgreenschist realm, not being truly metamorphic, instead being more related to diagenetic or other processes. This led to very confusing concepts such as `spilitization` and even the notion of original spilitic or keratophyric magmas. It was the studies of Coombs and others in the late 1950`s and early 1960`s (eg Coombs et al., 1959; Coombs, 1960) that started to identify a degree of order in rocks that are now accepted as belonging to the zeolite facies, based on intermediate to silicic volcaniclastic rocks in South Island, New Zealand. These rocks contained unstable volcanic glass along with relict high temperature primary igneous minerals and these components, combined with a high porosity, made them highly reactive in the presence of low temperature fluids. The type locality for the zeolite facies is the Taringatura Hills in South Island, New Zealand, and the term `burial metamorphism` was adopted in view of the fact that the rocks have not been deformed, instead appearing to vary in mineralogy with depth of burial. Subsequent studies by Boles and Coombs (1975) in the adjacent Hokonui Hills region demonstrated that the composition of the circulating fluids had an appreciable effect on the secondary minerals present. The presence of prehnite and pumpellyite in some of the highest grade rocks studied by Coombs (1960, 1961) led to definition of the prehnite-pumpellyite metagreywacke facies. Whilst this facies, currently known more simply as the prehnite-pumpellyite facies, is now widely recognised, as late as the early to mid 1970`s there was still debate as to whether such alteration was truly metamorphic in origin (see Amstutz, 1974). It was the upsurge in studies of low-grade metamorphic sequences in the mid to late 1970s that led to the establishment of the various low-grade metamorphic facies that are recognised today. An important study by Kawachi (1975) extended an understanding of the pumpellyiteactinolite facies as defined earlier by Hashimoto (1966), noting also that this is the grade which sees the first development of a rock schistosity. This study also reinforced the relationship between Fe/(Fe+Mg) in whole rock and chlorite, as previously recorded by Horikoshi (1965), a relationship that was further elaborated in the study by Coombs et al. (1976) of pumpellyiteactinolite facies semi-schistose rocks of the Taveyanne Formation at Loèche, Switzerland. In the late 1970`s to early 1980`s metabasites containing mineral assemblages characteristic of the prehnite-pumpellyite facies were recognised widely in the Caledonide sequences of Wales (Bevins, 1978; Bevins and Rowbotham, 1983), Ireland (Oliver, 1978) and Scotland (Oliver and Leggett, 1980). These studies, amongst others, precipitated an upsurge in interest in low-grade metamorphic studies in the 1990's, which led to the establishment of IGCP Project 294 'Very low-grade metamorphism'. Mineral assemblages in low-grade metabasites are dominated by prehnite, pumpellyite, epidote, chlorite and actinolite, along with zeolites, especially laumontite, at slightly lower temperatures and pressures, and stilpnomelane in rocks with high whole rock Fe/Mg ratios.

2017

Plate tectonic forcing leads to changes in the physical conditions that affect the lithosphere. In response to such changes, notably the local temperature ( T ) and pressure ( P ), rocks evolve dynamically. Processes mostly involve mineral transformations, i.e., solid-state reactions, but (hydrous) fluids are often involved, and partial melting may occur in the Earth’s middle and lower crust. While these chemical reactions reflect the tendency of natural systems to reduce their Gibbs free energy, metamorphic rocks commonly preserve textural and mineralogical relics, such as compositionally zoned minerals. Where relics are present, thermodynamic equilibrium clearly was not attained during the evolution of the rock. Petrochronology seeks to establish a temporal framework of petrologic evolution, and for this purpose it is essential to determine the P–T conditions prevailing at several stages. When analyzing a rock sample it is thus critical: 1. to recognize whether several stages of i...

Central European Geology, 2008

Metamorphic mineral assemblages in low-temperature metaclastic rocks often contain paragonite and/or its precursor metastable phase (mixed K-Na-white mica). Relationships between the bulk rock major element chemistries and the formation of paragonite at seven localities from Central and SE-Europe were studied, comparing the bulk chemical characteristics with mineral assemblage, mineral chemical and metamorphic petrological data. Considerable overlaps between the projection fields of bulk chemistries of the Pg-free and Pg-bearing metaclastic rocks indicate significant differences between the actual (as analyzed) and effective bulk chemical compositions. Where inherited, clastic, inert phases/constituents were excluded, it was found that a decrease in Na/(Na+Al*) and in K/(K+Al*) ratios of rocks favors the formation and occurrence of Pg and its precursor phases (Al* denotes here the atomic quantity of aluminum in feldspars, white micas and "pure" hydrous or anhydrous aluminosilicates). In contrast to earlier suggestions, enrichment in Na and/or an increase in Na/K ratio by themselves do not lead to formation of paragonite. Bulk rock chemistries favorable to formation of paragonite and its precursor phases are characterized by enrichment in Al and depletion in Na, K, Ca (and also, Mg and Fe 2+ ). Such bulk rock chemistries are characteristic of chemically "mature" (strongly weathered) source rocks of the pelites and may also be formed by synand post-sedimentary magmatism-related hydrothermal (leaching) activity. What part of the whole rock is active in determining the effective bulk chemistry was investigated by textural examination of diagenetic and anchizone-grade samples. It is hypothesized that although solid phases act as local sources and sinks, transport of elements such as Na through the grain boundaries have much larger communication distances. Sodium-rich white micas nucleate heterogeneously using existing phyllosilicates as templates and are distributed widely on the thin section scale. The results of modeling by THERMOCALC suggest that paragonite preferably forms at higher pressures in low-T metapelites. The stability fields of Pg-bearing assemblages increase, the Pg-in reaction line is shifted Addresses: P. Árkai, towards lower pressures, while the stability field of the Chl-Ms-Ab-Qtz assemblage decreases and is shifted towards higher temperatures with increasing Al* content and decreasing Na/(Na+Al*) and K/(K+Al*) ratios.

Journal of Petrology, 1999

Book Reviews equipment more fully described in other chapters and so Modern Approaches to Ore and there is a chance for the reader to decide which is the Environmental Mineralogy, edited by Louis best technique to choose for a particular problem. Each J. Cabri and David J. Vaughan. Short chapter is written by an acknowledged international Courses Volume 27. Mineralogical leader in their analytical field and it is an impressive line-Association of Canada, Ottawa, Ontario, up of experts. This volume is not the type of book that 1998. 421 pp. ISBN 092129400X. US$48. one should read from cover to cover but it provides a very useful starting point for someone needing to know My expectation on reading the title of this volume was about a particular mineralogical technique because chapthat it would contain a series of case studies on how ters start from basics and progress to cutting edge new mineralogy could be useful in understanding ore and developments in the uses of the techniques and there are environmental issues. Instead, the book is almost entirely abundant references for further information. The book composed of descriptions of modern techniques used in is full of useful tables of properties of minerals and has mineralogy. It contains a collection of papers that formed some spectacular colour plates. The preface states that the basis of a short course held under the auspices of the ore mineralogy is an important subject but is currently International Mineralogical Association's Commission on too little valued. I agree entirely with this statement, as Ore Mineralogy (COM). The content ranges from sample mineralogy is such a fundamental part of all materials preparation, the use of a simple optical microscope and in geology and the environment, and an understanding crystal chemistry of ore minerals to a description of the of the mineralogy is vital to understanding processes advantages and disadvantages of sophisticated pieces of that formed all Earth's materials. This book is full of equipment for particular types of mineralogical analysis. information on how to study the structure and chemistry Thus there are chapters on the electron microprobe of minerals, and it is up to the reader to extract ideas (EPMA) especially its use in trace element analysis, lowfrom this text to help them solve their mineralogically vacuum scanning electron microscopy (SEM) where coatbased problems. ing is not necessary as in ordinary SEM, proton microprobe particle induced X-ray emission (PIXE) for heavy Hazel Prichard elements, secondary ion mass spectrometry (SIMS) for University of Wales, Cardiff all elements and isotopic discrimination, high-resolution SIMS or SHRIMP, cathodoluminescence for distinguishing minerals formed under different conditions, and image analysis with applications in mineral pro

Journal of Metamorphic Geology, 1988

The Tal y Fan Intrusion is a 110 m thick sub-concordant metabasite sheet intruded into volcaniclastic and pyroclastic rocks of Ordovician age in North Wales. Despite low grade metamorphism, primary textural zones resulting from initial cooling of the sheet are preserved and retain primary mineralogical and chemical variations which influenced the nature and extent of metamorphic recrystallization. This has resulted in a vertical sequence of secondary mineral assemblages through the intrusion. During early hydrothermal alteration K-feldspar replaced plagioclase microphenocrysts in the marginal and contact zones, and olivine in the central zone was replaced by saponite. Subsequent regional metamorphism resulted in the development of (metastable) prehnite-pumpellyite+pidote assemblages in two sub-zones characterized by high Fe203. Elsewhere the assemblage prehnite-actinoliteepidote developed except in the contact and marginal zones where activity of C02 suppressed both prehnite and pumpellyite. Both assemblages contain excess albite, quartz and chlorite and, on the basis of uniform mineral compositions over the area of an individual thin section, are considered to represent buffered equilibrium assemblages indicative of prehnite-pumpellyite and prehnite-actinolite facies conditions. A metamorphic temperature of 310" C at 1.85 kbar is obtained using the P-T-X grid of , which implies a field gradient of -44" C km-l.