Cause Of Metamorphism Research Articles

P-T-t PATHS RECONSTRUCTIONS AND TECTONO-MAGMATIC REASONS OF METAMORPHISM FOR COLLISION, EXTENSION, AND SHEAR ZONE GEODYNAMIC SETTINGS OF THE EARTH’S CRUST

An effective method for identifying tectonic-magmatic causes of metamorphism is the reconstruction and analysis of P – T – t paths, which can be used to develop a tectonic model of rock evolution with time. Diagnostic P-T-t paths characteristic of rocks formed in zones of active tectogenesis – under collision, extension and in shear zones of the earth’s crust – have been established. This report discusses some of the ambiguous interpretations of P-T-t paths in areas with simultaneous manifestation of different metamorphic processes.

U–Pb Dating and Hf Isotopes Analysis of Detrital Zircons of the Shanxi Formation in the Otuokeqi Area, Northwestern Ordos Basin

The Paleozoic strata are widely distributed in the northwest of the Ordos Basin, and the provenance attributes of the basin sediments during this period are still controversial. In this paper, the detrital zircon LA-MC-ICPMS U-Pb age test was conducted on the drilling core samples of the Shanxi Formation of the Upper Paleozoic in the Otuokeqi area of the Ordos Basin, and the provenance age and the characteristic of the Shanxi formation in the Otuokeqi area in the northwest were discussed. The cathodoluminescence image shows that the detrital zircon has a clear core-edge structure, and most of the cores have clear oscillatory zonings, which suggests that they are magmatic in origin. Zircons have no oscillatory zoning structure that shows the cause of metamorphism. The age of detrital zircon is dominated by Paleoproterozoic and can be divided into four groups, which are 2500~2300 Ma, 2100~1600 Ma, 470~400 Ma, and 360~260 Ma. The first two groups are the specific manifestations of the Precambrian Fuping Movement (2.5 billion years) and the Luliang Movement (1.8 billion years) of the North China Craton. The third and fourth groups of detrital zircons mainly come from Paleozoic magmatic rocks formed by the subduction and collision of the Siberian plate and the North China plate. The ε Hf t value of zircon ranges from -18.36 to 4.33, and the age of the second-order Hf model T DM 2 ranges from 2491 to 1175 Ma. The source rock reflecting the provenance of the sediments comes from the material recycling of the Paleoproterozoic and Mesoproterozoic in the crust, combined with the Meso-Neoproterozoic detrital zircons discovered this time, indicating that the provenance area has experienced Greenwellian orogeny.

Thermal alteration of CM carbonaceous chondrites: Mineralogical changes and metamorphic temperatures

The CM carbonaceous chondrite meteorites provide a record of low temperature (<150 °C) aqueous reactions in the early solar system. A number of CM chondrites also experienced short-lived, post-hydration thermal metamorphism at temperatures of ∼200 °C to >750 °C. The exact conditions of thermal metamorphism and the relationship between the unheated and heated CM chondrites are not well constrained but are crucial to understanding the formation and evolution of hydrous asteroids. Here we have used position-sensitive-detector X-ray diffraction (PSD-XRD), thermogravimetric analysis (TGA) and transmission infrared (IR) spectroscopy to characterise the mineralogy and water contents of 14 heated CM and ungrouped carbonaceous chondrites. We show that heated CM chondrites underwent the same degree of aqueous alteration as the unheated CMs, however upon thermal metamorphism their mineralogy initially (300–500 °C) changed from hydrated phyllosilicates to a dehydrated amorphous phyllosilicate phase. At higher temperatures (>500 °C) we observe recrystallisation of olivine and Fe-sulphides and the formation of metal. Thermal metamorphism also caused the water contents of heated CM chondrites to decrease from ∼13 wt% to ∼3 wt% and a subsequent reduction in the intensity of the 3 μm feature in IR spectra. We estimate that the heated CM chondrites have lost ∼15 - >65% of the water they contained at the end of aqueous alteration. If impacts were the main cause of metamorphism, this is consistent with shock pressures of ∼20–50 GPa. However, not all heated CM chondrites retain shock features suggesting that some were instead heated by solar radiation. Evidence from the Hayabusa2 and ORSIRS-REx missions suggest that dehydrated materials may be common on the surfaces of primitive asteroids and our results will support upcoming analysis of samples returned from asteroids Ryugu and Bennu.

Metamorphic Indicators for Collision, Extension, and Shear Zone Geodynamic Settings of the Earth’s Crust

An effective method for identifying the tectonic-magmatic causes of metamorphism is the reconstruction and analysis of P–T–t paths, which can be used to develop a model of rock evolution with time in the “pressure–temperature–time” coordinates. This approach is most widely used in interpreting the development of orogens with a polycyclic history, where different types of metamorphism are combined in contrasting geodynamic settings. The progress in this direction in recent years is due to the improvement of the instrumental base, precision methods for the study of matter – in situ local microanalysis and dating of zonal metamorphic minerals, and the computing apparatus of geothermobarometric procedures using multi-equilibria. The article discusses these issues on the example of geological complexes of various geodynamic nature, which are typical for different types of metamorphism. Reaction microtextures, chemical zoning in minerals, geochronological dating as well as the trajectory of the Р-Т paths provide evidence for the polymetamorphic history. A generalized P-T diagram was built using data on well-studied metamorphic complexes, which are characterized by the simultaneous presence of prograde and retrograde segments of a P-T path. Diagnostic P-T-t paths characteristic of rocks formed in zones of active tectogenesis—under collision, extension and in shear zones of the earth’s crust—have been established. This paper discusses some of the ambiguous interpretations of P-T-t paths in areas with simultaneous manifestation of different metamorphic processes, which requires further research.

High geothermal gradient metamorphism during thermal subsidence

The burial of a basement sequence enriched in heat producing elements during thermal subsidence following rifting produces two concomitant changes in the thermal structure of the crust. Firstly, the burial of the enriched layer produces high geothermal gradients in the overlying sedimentary succession, with the high gradients propagating down into, but not through, the enriched basement sequence. Secondly, the lithospheric thickening that drives thermal subsidence reduces the heat flowing into the deeper crust from the mantle. Because the process of thermal subsidence promotes burial, it naturally increases the depth extent of the high geothermal gradients in the upper crust, potentially inducing significant temperature increases in the mid-upper crust during burial. The lowering of the thermal gradients in the deep crust accompanying burial severely limits the temperature changes affecting the Moho; potentially allowing Moho cooling while the mid-upper crust heats. These effects can promote high geothermal gradient (>40°C/km) metamorphism in the mid-upper crust without inducing significant melting in the lower crust, providing the basement heat production contributes > ∼70 mW m −2 to the surface heat flow and that the horizontal length scale for the basement heat production anomaly is > ∼50 km. These conditions appear to be met in several Australian intermediate- to high-temperature, low-pressure metamorphic terranes where the thermal causes of metamorphism have hitherto remained enigmatic. One of these terrains, the Mt. Painter province in the northern Flinders Ranges, South Australia, is used to illustrate some of the attributes of the model.

The retrograde P–T–t path for low‐pressure granulites from the Reynolds Range, central Australia: petrological constraints and implications for low‐P/high‐T metamorphism

Several aspects of the petrogenesis of low‐pressure granulite facies rocks from the Reynolds Range (central Australia) are contentious, including: (a) the shape of the retrograde P–T –time path, and whether it is an artefact of repeated thermal events at different P–T conditions; (b) the type of regional metamorphism; and (c) the causes of metamorphism. Granulite facies rocks from the Reynolds Range Group experienced three major periods of mineralogical equilibration. Metapelitic rocks underwent dehydration‐melting reactions to form migmatites under peak M2 P–T conditions of c. 5.0–5.3 kbar and c. 750–800 °C. Metapsammitic rocks that did not melt during M2 show spectacular garnet–orthopyroxene intergrowths that developed at c. 3.5–3.7 kbar and c. 700–750 °C after penetrative regional deformation, but prior to amphibolite facies rehydration in discrete strike‐parallel zones. Rehydration occurred within the sillimanite stability field at P–T conditions close to the granite solidus (c. 3.2–3.4 kbar and 650–700 °C). Subsequently the terrane cooled into the andalusite stability field. Geochronological constraints suggest that: (a) peak‐M2 conditions were reached at c. 1594 Ma; (b) the garnet–orthopyroxene intergrowths in unmelted metapsammites probably developed between c. 1594 Ma and c. 1586 Ma; and (c) upper amphibolite facies rehydration occurred between c. 1586 Ma and 1568 Ma. The lack of petrological evidence for multiple dehydration and rehydration of the rocks suggests that the three episodes of mineralogical recrystallization can be linked to yield a single continuous retrograde P–T–t path of minor initial decompression (c. 1.5 kbar) from the M2 peak, followed by cooling (c. 100 °C) to the granite solidus over a period of c. 26 Ma. Late kyanite‐bearing shear zones that dissect the terrane are unrelated to this event and formed during the c. 300–400 Ma Alice Springs Orogeny. The shape of the P–T–t path and the duration of M2 metamorphism suggests that advective heating was not the major cause of high‐grade metamorphism, and that some other, longer lived heat source, such as the burial of anomalously radiogenic, pre‐tectonic granites, is required.

A magma-loading model for Barrovian metamorphism in the southeast Coast Plutonic Complex,British Columbia and Washington

Barrovian metamorphism linked to crustal thickening in the southeast Coast Plutonic Complex of British Columbia and Washington is proposed in this paper to be the product of magmatic loading. Relevant observations and arguments are the following. (1) Isotopic ages coupled with structural and fabric relations document that throughout the region Barrovian metamorphism and plutonism are broadly coeval. (2) Baric patterns in country rock indicate steep-sided crustal loads originating within the high-grade parts of the orogen.(3) Patterns of subsidence and uplift are localized and diachronous, precluding a single-event regional thickening mechanism. (4) Country-rock fabrics are dominated by steep, orogen-parallel foliations and shallow, strike-parallel lineations, features not easily reconciled with tectonic thickening mechanisms. (5) Abroad zone of schists flanking the large (25 x 100 km) Scuzzy batholith in British Columbia bears metamorphic isograds and foliations that track the border of the pluton across regional structural trends, implicating pluton emplacement as the cause of metamorphism. (6) The crustal-loading event in the vicinity of the Scuzzy pluton is bracketed by mineral and textural features as being coeval with emplacement of the pluton, which occurred over a period of ∼7 m.y. (7) The regionally developed Chiwaukum Schist in Washington has petrologic and structural features and a metamorphic age that suggest it was formed by, and represents the floor of, an eroded extension of the Scuzzy pluton. The proposed magma-loading model invokes the diapir and ballooning concepts of previous workers in which rising plutons displace country rock downward. This mechanism results in a type of convective crustal overturn, and at least partially solves the space problem related to batholithic emplacement. As a result of this process, relatively broad tracts of country rock acquire Barrovian mineralogy. Following plutonism, magmatically thickened crust is eroded and isostatically up-lifted to expose the plutonic roots and high-grade country-rock basement.

The causes of the high-grade metamorphism of the Scourie complex, NW Scotland

Summary A number of previous studies of the possible causes of Archaean high-grade metamorphism have suggested that the heat source was large bodies of (intermediate) magma emplaced within pre-existing crust. Geochronological data for the Lewisian indicate that there was probably a gap of c. 200 Ma between the cessation of significant igneous activity and the peak of high-grade metamorphism. This is sufficiently long to allow conductive dissipation of magmatic heat and to render a model of the metamorphism relying upon heat transported in by intrusions unsatisfactory. The quantity of deep-derived CO 2 that would be required to make it the primary cause of the metamorphism is larger than petrological considerations allow. Alternative models, based on enhanced basal heat-flow in a static crust or heat derived from the decay of radionuclides resident within the crust coupled with a more normal mantle heat-flow and tectonic crustal-thickening have been erected, and reveal that the P-T conditions and age data recorded by the Scourian granulites can be satisfied by such models. Thermal models of this nature, and a consideration of the available petrological and geochronological data, suggest a very slow cooling rate which would allow considerable intercrystalline diffusion to occur. This could give rise to the observed zoning profiles and other forms of re-equilibration found within the granulite-facies rocks of the Lewisian complex.

Are thermal domes heated by CO 2-rich fluids from the mantle?

A large part of the fluid present in the thermal dome at Naxos, Greece, during the main stage of metamorphism was CO 2-rich and of deep-seated origin. Samples of this pervasive fluid were trapped in fluid inclusions, which are particularly abundant in syn-metamorphic quartz segregations. These quartz segregations are considered to represent the main passageways of the escaping fluids. Mass-balance calculations based on δ 13C values of fluids and rocks indicate that most fluid/rock mass ratios are between 0.06 and 0.8. The heat added to the metamorphic system by the cooling of these fluids may have been the prime cause of metamorphism and updoming.

The Metamorphism of the Dalradian rocks of Scotland.

Synopsis The metamorphism of the Dalradian rocks of Scotland is reviewed and commented on, attention being paid to the nature and spatial distribution of the zones and their significance in PT modelling. Reactions defining Barrovian and Buchan isograds are listed and figured and the importance of textural and chemical disequilibrium discussed. The effect of rock composition on the isograds is examined and a T-X diagram constructed to indicate the probable metamorphic mineral sequence for the average pelitic Dalradian composition. The history of ideas on the genesis of the migmatites is outlined and it is concluded that they may well be polygenetic, and those in the NE may even be of Pre-Caledonian age. The role of oxidation and reduction reactions is briefly mentioned and the late cooling reactions are described, from which it can be seen that the climatic assemblages have often been markedly modified. The pressures and temperatures of the metamorphism, determined from phase considerations are plotted on a PT diagram and compared to the isotope results. Finally, evidence is used to suggest that although the fundamental cause of metamorphism might be a normal heatflow across the mantle/crust boundary and within the tectonic pile, the evidence at the present level indicates some of the metamorphism is due to high level magma and perhaps heat focussing along major structures.

Problems in Himalayan geology

The Central Crystalline Axis of the Great Himalaya holds clues to the solution of many problems dealing with the crystallines, metamorphics and geological contrasts encountered in the areas to the north and south of the Axis. The problems discussed include criteria for deciphering the stratigraphy in the Himalayan zone to the south of the Crystalline Axis, age of Jutogh and Chail groups, their relationship with each other and with the Crystalline Axis, nature of the Simla Slates, Jaunsar, Blaini, Infra-Krol, and Krol formations, unfossiliferous nature of the Palaeozoic and Mesozoic sediments, relationship between the Lower, Middle and Upper Tertiaries and the age, presence and absence, and also the cause of metamorphism. It has been found that high grade metamorphism is not a Tertiary phenomenon. Very low grade metamorphism in the sedimentaries and the absence of plutonic intrusives from the Palaeozoic, Mesozoic and Tertiary sedimentary rocks indicate that magmatism, regional metamorphism and orogeny are not always mutually associated in space and time. Orogenic and epeirogenic episodes other than Cretaceous-Tertiary hold clues to many problems of Himalayan Geology.

The Nature of the Basement Complex Oil Reservoir, Edison Oil Field, California: ABSTRACT

The completion of the H. H. Magee and Independent Exploration Company's Brockman well No. 3, Sec. 13, T. 30 S., R. 29 E., MDB & M, in the Edison field, Kern County, initiated commercial production from basement complex rocks in the San Joaquin Valley. At the present time about 1,500 acres of these rocks are oil productive in this field. The geologic structure of the field at basement complex depth is a faulted, dome-like fold with extensive south and west flanks, but with the north and east flanks shortened by faulting. The basement complex rocks are divisible into two groups: metamorphic rocks derived from sediments and igneous intrusives, and a younger igneous rock (Sierra Nevada) which was the chief cause of metamorphism in the older rocks. The metamorphic rocks n order of their probable age are (1) McCowan schist, (2) Hershey schist, (3) Dougherty schist, and (4) felsites. The McCowan schists are satiny, lead-gray, laminar rocks of sedimentary origin which occupy the eastern part of the field. The Hershey schists of doubtful sedimentary origin, are pistachio green, poorly laminated rocks. The Dougherty schists are variously metamorphosed fine-grained igneous rocks of probable malachite or diorite-aplite composition which comprise the west half of the basement producing area. The felsites are fresh, fine-grained igneous rocks, mineralogically akin to the Dougherty schists, which occur as segregations throughout the field. The quartz diorite of the Sierra Nevada batholith bounds the east edge of the oil field. Metamorphism is regional, End_Page 2239------------------------------ characteristic of the epizone, and its effects increase as the quartz diorite is approached. The oil originated in the westerly extending Tertiary sedimentary basin and migrated into the pore and fracture spaces of the structurally higher schist. Its accumulation apparently is unaffected by the type of schist, but is governed by the degree of metamorphism, fracturing, and the control of several large faults. End_of_Article - Last_Page 2240------------

Natural History of Granite

FOR a century and a half the origin of granite (including granodiorite) has remained one of the most intractable and controversial of the problems geologists have tried to solve. Fouque and Levy's comment in 1882, that "it excites the most lively discussion", is as true to-day as ever before; and indeed might be regarded in some countries as a mild understatement of the passions that are aroused by apparent conflicts of evidence and very real conflicts of opinion. Hutton clearly established the status of granite as a crystalline plutonic rock, but the French geologists and Lyell and his followers soon realized that not all granites are necessarily of igneous origin. Some occurrences were seen to have no sharp contact against the aureole of metamorphic rocks, but to merge into the surrounding schists through a transitional zone of gneisses and felspathized schists so gradually that at no point could it be said that the rock ceased to be metamorphic and became igneous. These granites consequently came to be regarded, not as representing the cause of metamorphism, but as being the extreme products of its action.

Book Reviews

DR. PACKARD has undertaken in this text-book to review and epitomise the vast literature relating to the structure of insects. For such a task special qualifications are necessary; among the rest, unflinching industry, a sound judgment, and a first-hand, practical knowledge of the subject. These qualifications our author exhibits on every page. He has worked long and hard as an investigator; he has a candid mind; and he has spared no pains either upon the collection or the elucidation of his materials. The critic who tries to be wholly impartial may feel compelled to point out a certain slowness to draw general conclusions, which is particularly evident in the concluding section on the causes of metamorphism. This reserve is natural, perhaps laudable, in the writer of an encyclopædic work. Dr. Packard's book will be of the greatest service to students of insect-anatomy, and almost indispensable to future writers on the subject. It is a great store of well-sifted and carefully arranged information, which will guide the naturalist to many a special research which he might easily have passed by in ignorance of its very existence. We must not leave the impression that Dr. Packard has done nothing but condense into a text-book the work of other men. He has made out for himself many interesting and valuable facts, and in no part of this treatise does he find himself altogether remote from his own published researches. The book before us is handsomely printed, profusely illustrated, and furnished with copious bibliographical lists. Together with the very dissimilar treatise by Dr. Sharp in the “Cambridge Natural History,” it puts the student of scientific entomology into a far better position than he occupied a year or two ago. Dr. Packard's book, like Dr. Sharp's, should find a place in every library which includes comparative, anatomy, and both should be the constant companions of all who occupy themselves with the structure and life-histories of insects. A Text-Book of Entomology, including the Anatomy, Embryology and Metamorphoses of Insects, for use in Agricultural and Technical Schools and Colleges. By Prof. Alpheus S. Packard. Pp. xvii + 729. (London: Macmillan and Co., Ltd. New York: The Macmillan Co., 1898.)

On the Quartzites of Shropshire

Introduction T he geology of South Shropshire, though it has received considerable attention from geologists, has been seriously misunderstood. This is owing partly to the disturbing influence of great south-west faults, partly to the obliterating influence of metamorphism, and partly to mistaken views of the causes of metamorphism. In attempting to unravel the complexities of this disturbed area, I have arrived at some definite results, the first of which I communicated to this Society in a paper entitled, “On a new Area of Upper Cambrian Rocks in South Shropshire, with a description of a new Fauna” (See vol. xxxiii. p. 652, of the Quarterly Journal of the Society). In that paper I showed that a part of the so-called Lower Caradoc rocks south of the Wrekin were of the age of the Dictyonema -shales of Malvern and Pedwardine, and that a part of the so-called “quartzite” of the Wrekin consisted of Hollybush Sandstone. I deferred discussion on the true quartzites and on the igneous rocks of the district. The latter I hope to be able to prove in a future paper are only to a slight degree disruptive, and mainly consist of a great bedded series of lavas and tuffs of Precambrian age. My second instalment of information is the announcement that the so-called Caradoc quartzites of the Wrekin and Church-Stretton area are of Cambrian or Precambrian age. In this connexion I shall notice the quartzites of the Stiper Stones, so as to include, so far as I know, all the quartzites of Shropshire.