Geological Processes - Science topic

Rift refers to the wholesale rupture of the lithosphere under extensional strain. It can be a narrow valley such as the most East African rifts and Rhein graben in Europe it can also be very wide and long distributed in large areas such as the Aegean System or Basin & Range province of North America. So, rift refers to the tectonic feature, while rift valley refers to a linear topographic depression, a geomorphological feature, developed due to rifting. Originally, the "Rift Valley" was used for Kenya Rift for the first time by John Walter Gregory (1896) before the advent of Plate Tectonics. This is a common problem; pre-plate tectonic definitions are continued to be used after its advent which has been causing confusion. The term rift is genetic, rift valley is descriptive independent of its origin.

Are you sure it's tourmaline? - it appears to have a cleavage parallel to the elongation and the colour is homogeneous, unlike in most tourmalines.

ليس لديه فكره

This is a good question.

Metamorphism is an isochemical adjustment (be it contactmetamorphic, regional/dynamometamoprhic or burial metamorphic) to changing physical conditions pressure and temperature in the lithosphere. By definition the expulsion of carbon dioxide and water along with an increasing metamorphic grade is not involved in these processes. The lower limit called the very low-grade stage overlaps with the upper part of the diagenesis (around 200°C) and depends on the angle you look at this boundary using siliceous, organic or sulfidic matter. The upper limit is the onset of anatexis between 600 and 800°C which depends on the water content of the system. The metasomatism sensu stricto is a closed system.

Metasomatism is allochemical and an open system where in special zones at a certain P-T level a new mineral partly or wholly different in its chemical composition from the host mineral formed. These mineralizations may be caused by subcritical or supercritical solutions sparked by igneous bodies at different depth (contact -metasomatic / skarn) or in the course of burial or dynamometamorphic processes (see the isochemical analogues above). The replacement of preexisting rock-forming material occurs through chemically active liquids and gases from external sources.

[log (ppm Ti-in-zircon) + logSiO2−logTiO2] = (5.711 ± 0.072) − (4800 ± 86/T(K)) (Ferry and Watson, 2007).

The problem with some of the hydrates, is that you will have to use more energy to dig them out of the ground, than you can get back by using them as fuel. Why should you then use them ????

Dear H-J Förster and Swayoma Bose,

The Nb-Ta composition of your granites is quite interesting. What I can say from my own granite whole-rock compilation is that such Nb-Ta composition (e.g. Nb/Ta ~17.5, Nb~55 ppm, Ta ~3 ppm) is generally only reached by mantle-derived A1-type peralkaline to metaluminous granites.

If you think that this granite is purely crustal in origin. It could suggest that it formed from the melting of either a residual crust or, for example, mantle-derived andesite rocks characterized by high Nb/Ta ratios. If, for example, sediments are submitted to a first event of partial melting at low temperature (until muscovite breakdown), the accumulation of biotite and ilmenite can induce an increase of the Nb/Ta value in the residue. If this residual crust is then submitted to a second partial melting event at high temperature (above biotite breakdown condition), the generated granitic melt can reach a high Nb/Ta value. See Stepanov et al. (2014) for some discussion about such processes.

Regards,

CB

The homogenization T of the clathrate, along with other data can tell you what gas-hydrate you have and what chemical system you are looking at.

Dear Nisarg,

Although I've never used TecDEM, I've calculated the steepness index manually by extracting area and slope data using ArcGIS. I wrote the entire technique in my recent paper, published in Arabian J Geosciences.

In case, if you want to compare steepness index of many streams, you've to normalize the steepness values using a reference concavity. To easily compute the normalized steepness index you may follow the stream profiler tool and the MATLAB codes developed by Kelin Whipple and his team.

Cheers.

Sumit

Fluid inclusions in all minerals should be used with care.  For many minerals the system is open to exchange during formation.  I used cathodoluminescdence to see growth banding in sphalerite and I could see healed incluions that looked primary were aligned along otherwise invisible healed fractures.

I stress, also, the issues with glacial ice which is used to determine ancient temperatures by isotope.  The snow on a glacier can go through many freezeing/melting cycles before it is encapsulated.  THe fluid inclusion can lose light isotopes and not reflect what the investigator supposes.  Furthermore, the calibration to ambient temperature at the surface will yield a hockey stick with depth.

Dear Mr. Hosseini,

your question points to "mineralization" sensu lato. Therefore it is very difficult to give an all-embracing answer, because in an arc or active continental margin setting you will find magmatic, sedimentary and even metamorphic subsettings which are quite different as to their lithology , P-T and Eh-pH regimes, all of which are of control  on the individual mineral  deposits. In addition to that the geomorphological and paleoclimatic conditions also play a significant role. Remnants of ophiolites near the ocean with magmatic base metal, Cr,, PGE, umber, ochre, and beach sands of olivine are concentrated in such a great variety of settings that you cannot give a simple answer to your question as it is the case for the setting at the opposite end of the magmatic cycle with felsic and intermetiate volcanic, subvolcanic and plutonic rocks which also are enveloped by sedimentary and covered by supergene mineralizations.

I hope it will give you an idea how complex this issue is.

Best regards

H.G.Dill

I also find it hard to believe there are absoluetly no accessory phases present. Granites are by definition coarse grained rocks, so I suppose it would be possible to cut a single thin section and not see any accessory phases. How many sections have been examined? Phases such as apatite can be very difficult to spot using standard light microscopy, so it might be worth putting a few sections under an SEM and looking at them in backscatter mode....you might be surprised what jumps out at you!

Dear Dr. Camprubi,

there are numerous Cr-bearing phyllosilicates, many of which are supergene in origin and not all of the "green glittering stuff" in the argillaceous alteration zone of mineral deposits qualifies for being named Cr illite or fuchsite. We have studied some argillaceous alteration zones with high Cr contents and micaceous and smectitic material.

I would like to refer to the paper published only recently:

DILL, H.G. (2017)  Residual clay deposits on basement rocks: The impact of climate and the geological setting on supergene argillitization in the Bohemian Massif (Central Europe) and across the globe.- Earth Sciences Reviews 165: 1-58.

There you can find all the common Cr-bearing phyllosilicates and what you can deduce from their presence as to the supergene part of alteration and their parent rocks.

Chromium has favourable sites to be concentrated, the environs of ultramafic rocks and paleosols sensu lato.

Best regards

H.G.Dill

It is hard to understand a positive Eu anomaly in zircon. As suggested by Mr. León and Moscati, Eu anomaly in zircon depends on fractionation of paragenetic mineral which mostly is feldspar. However,  recent experments demonstrate that Eu and Ce anomalies in zircon are mainly controlled by oxygen fugacity (Trail et al., 2012). Terrestrial zircons crystallized from a normal magma should have a negative or no Eu anomaly which is affected by the Eu²⁺/Eu³⁺ratios. The similar geochemistry of  Eu³⁺ to Sm³⁺and Gd³⁺controlled by charge and radius  indicate no Eu anomaly in zircon even under an extermely oxydic condition and/or without feldspar fractionation. Therefore, a positive Eu anomaly in zircon can only be resulted from a nonequilibrium process, such as metamorphism or alteration. However, similar case is rarely reported...

Dear Professor H.G.Dill,

First of all, I am very thankful to the time and concerns that you showed to my question, also all my gratitude for the documents toward which you oriented me,

Well, I am sorry for the lack of explication, I will try to explain what is going on in my mind in words:

we are conducting a metallogenic study for the fluorite ore deposit in morocco, in this aim we used (REE analyses, C/O isotopes, Sr Isotopes, Pb/ S isotopes, Noble Gazes, and Fluid inclusions), and those days I found that there is a method used  to reconstruct the compositions of  REY   of fluorite if it was precipitate from a granitic magma using a theory called ( Lattice-strain), in fact,  Van Hinsberg (2010) states that the trace element partitioning between minerals and aqueous fluids is systematic and obeys lattice-strain theory. It depends strongly on element complexation in the fluid, but this dependence is predictable and can be accommodated. Unlike fluids, minerals with preserved compositions are readily available in the geological record, and the "lattice-strain" approach

therefore provides a powerful and widely applicable tool to reconstruct a quantitative record of fluid composition for the full range of Earth environments and for its earliest history. (end of quote).

Well, this Idea crossed my mind after that i found that an other PhD student ( that I can't get in thouch with) used it in his final presentation to get his PhD dissertation, I will add a figure to this text, to make it more clear,

I Hope that I was able to explain my thoughts,

best regards,

Oussama ZEMRI

P.S: the references cited:

van Hinsberg, V. J., Migdisov, A. A., & Williams-Jones, A. E. (2010). Reading the mineral record of fluid composition from element partitioning. Geology, 38(9), 847-850.

I agree with the answers above, but would add that context can tell you a lot. If the zircon is intergrown with hydrothermal ore and gangue minerals that would favor its interpretation as hydrothermal zircon, but if the zircon occurs in relatively unaltered igneous rock, that would favor its interpretation as igneous zircon.

The Pearce´s diagrams fields were buit up using granitc compositions, ie, high silica contents. 

ETH is the best choice for ya I think 

Dear Dr. Zhang,

Zircon xenocryst refers to zircon that was originated from a complete different source of that of the magma, most likely entrained into the magma during assimilation of the wall rock. However, inherited zircon is more ambiguous since you can have inherited zircon incompletely reabsorb from the source that melted and generated your magma. New terminology it is being apply to avoid this confusion, for example zircon autocryst, antecryst and xenocryst. Find attached a paper where this is explained in detail and may help to respond your questin. 

This is a fascinating question you have asked. Although I am not familiar with your area of interest I see many similarities with other "tectonic controls" as regards orogenic gold deposits of the Archean that I am familiar with.

I see the emplacement of igneous intrusions (due to their volatiles they force themselves upwards) as a reason for the creation of structures where mineralizing fluids travel. In the end minerals get deposited according to many controlling parameters (salinity, temp., isotope, fluid composition and chemistry of wall-rocks among others) . I have followed Rob Kerrich's work on gold deposits, ever since he visited my gold mine (where I worked, it wasn't mine!) to study the nature of the gold deposits. He has explained those parameters that control deposition of metals in a satisfactory way. 

The tectonic control of Sn-W mineralization in Cornwall, England that I am familiar with has many similarities to your mapping experience in Africa. As far as I know, the mineralization there has not been explained fully yet. Tectonics has been mentioned by several investigators to explain the distribution of metals around the granite intrusions.

To answer your question, I agree many factors play a role including the tectonic control.

Hi Annapuma,

Possibly the coal measures of NW England gives a clue.

Barren coal measures are of 2 types:

1/. primary barren, as discussed by most respondents;

2/ secondary barren, due to oxidation of coal measures below the unconformity of the Permo-Triassic red beds that formed in a harsh desert environment. In this montane Permo-Triassic landscape, the coal measures were subjected to oxidation by combustion by air at the surface and underground, and by oxidation by groundwater underground.

As John I Garber pointed out, oscillatory zoning generally  it's considered of magmatic origin. In my short experience I never see metamorphic zircons with oscillatory zoning. Another way to distinguish between magmatic- metamorphic zircons it's with the Th/U ratio (Th/U < 0.1 for metamorphic origin), but not always works as Moller et al 2003 highlights.  

Moller et al 2003 The Use and Abuse of Th-U Ratios in the Interpretation of Zircon

In my knowledge there are two ways to form metamorphic zircon:

1) Precipitation from a melt by anatexis (Roberts & Fingers 1997). Solid crystallization / nucleation occurs due to Zr and Si diffusion processes as a consequence of mineral breakage reactions with sufficient amounts of Zr to favor the crystallization of new zircon (Hoskin & Black, 2000). The most common morphology of this type of zircon is oval, rounded and homogeneous, but with internal zoning. Precipitation from aqueous metamorphic fluids involves the dissolution of part of the zircon and re-precipitation of the dissolved material, nearby in the crystal itself or away from the dissolved zircon, so the habit may change.

2) Dynamic recrystallization of a preexisting zircon crystal (generally igneous) (Pidgeon, 1992; Hoskin & Black, 2000). Recrystallization usually occurs as recrystallization fronts or transgressive lobes cutting the original internal zoning texture. When this process does not occur completely, the crystal acquires domains of oscillatory zoning (of protolite zircon) and domains without zoning (recrystallized homogeneous areas) "phantom zoning ", that is partially recrystallized. The suggested mechanisms for recrystallization correspond with border migration and migration of intracrystalline defects (Hoskin & Black, 2000).

References cited: 

You can also look the review of zircons textures made by Corfu et al 2003. 

I hope this information could help you with your research. 

If you need the cited articles please tell me and I can send them to your email. 

Regards

Cesar

Dear Lu-Lu: for some srange reason I haven't been able to unload the photomicrographs you attached, a message saying "Netwok error" appears all the time. Perhaps the file is too heavy (21 MB) and doesn't come through. Any way, pyroxenes, both ortho and clino, are silica saturated minerals, so they should be stable in a granitic melt, unlike olivine, for example. The possibility that these minerals could be restites is also quite likely, I've found similar examples in Venezuelan and Colombian granitoid rocks. But eclogites don't contain orthopyroxene, only granulites, so the source rock of the magma should have been granulites, not eclogites. Regards. Sebastian. 

P.D.: try to send the pictures one by one, and not as an attached, and heavy, file...

Hello Gaël,

Layer-parallel compaction (LPC) or tectonic compaction is a significant component of shortening. In couple of papers we quantified LPC in sand models it to "lithology", basal friction, stratigraphic location, and amount of bulk shortening. This is easy to quantify in models where the initial dimensions and boundary conditions are better known. We have also compared LPC in models to that in Pyrenees where we measured 16-23% depending in different sections. If you are interested, send me your email address and I will send the PDF of those articles.

Cheers,

Hemin

Hi Hakima,

Such patterns were admired even 100 years ago of visitors and miners underground of the salt mine Marosujvar in Romania (see images attached). The original alternation of flat layers of pure rock salt (light) and clay impurities (dark) was bent and erected steep by salt tectonics. The photographs are printed on historic picture postcards (out of my collection).

Best regards,

Guenter

Dear Mrs. Xinping,

the correlation of metamorphism, excluding shock metamorphism related to impact processes, to orogeny is common sense. I would like to refer to the eclogite and glaucophane facies coming in place as oceanic crust is involved in subduction processes, The alteration of rocks is characterized by high pressure and a wide range of temperature from rather low to very high. You can correlate the various metamoprhic facies types from the classical Abukuma through Barrow types with the architectural elements of plate tectonic (see also paired belts). Textbooks on metamorphism offer a wealth of nice examples on a global basis.

Best regards

H.G.Dill

Dear David Garcia

I exactly do not have the paper this time in which the author tried to describe the precipitation along geological fault lines known as MBT/HFT, MCT AND MMT from north tto south on windward side of great himalayas in Nepal, India and Pakistan. I am in search of papers describing this relationship. Welcome for all for co-operation in this regard.

Please see the journal of geology department of university of peshawar, peshawar, Pakistan.

Regards

Hi Lu Lu,

The subject is also treated in this paper:

Green, T. H. (1995) Significance of Nb/Ta as an indicator of geochemical processes in the crust-mantle system. Chemical Geology 120(3–4):347-359.

Regards,

Antoine Caté

Leonid's comment is very useful. Basalt magmas are commonly believed to originate by partial melting of the upper mantle peridotites. The lower continental crust is generally no more mafic than basalt. It would require unrealistic amount of heat supply to cause high degree of melting to produce basaltic magma. 

Regards,

Qasim

Thank you very much.

Although I assume it's a possibility to have such a high degree of partial melting, I wonder about Sobhi Nasir's answer that one would expect to get melts with over 80% SiO2. What is that number based on, experimental work? It sounds rather high to me.

Probably this article DOI: 10.1016/0899-5362(94)90061-2 and the citing articles will be a good start

check alibaba.com website. i am sure u can get it there.

 I asked about Last glacial maximum...

Hi Mustafa,

Please find enclose paper. The paper is going to help  you for  interpretation to the your analysis results using by some trace elements and isotope compositions.

Best wishes

The complexity of geological conditions must be described in numerous quantitative parameters.

The most important parameters of geological processes (genesis and formation conditions of rocks) are:

Pressure (P)

Temperature (T)

Composition of the fluid phase (X-fluid)

Time (t) (time of the rock formation)

Formation age

Deformation (brittle, ductile) (d)

Relations between crystallization and deformation

Furthermore: all technical properties of the respective rocks of the study area (stress, strain, compressive strength, porosity, permeability, water saturation, tensile stress, etc. etc.).

Dear Sir,

The thickness of the source rock has no relation to hydrocarbon generation. The hydrocarbon generation is depended on TOC, maturity, enough maturity from depth of burial and thermal point of view, and reservoir rock associated with source rock. The thickness of source rock is important from the amount generated and expelled hydrocarbon.

Dear Ronald,

FLDWAV can model single channel or dendritic systems, straight or meandering channels, or divided channels.

You can download it at the following site

 

With my best regards

Prof. Bachir ACHOUR

Dear Nirmalya,

deformation conditions seem to be thus well constrained. Consequently, I think grain size could be a possible explanation to understand the obtained ages. One interesting point that you could evaluate is whether the so called "closure temperatures" are valid, specially for mylonitic rocks (e.g., have a look on Igor Villa´s papers). I´ve been working on this topic for the last year and I think it´s not as easy as it´s thought due to the complexity and interaction of different processes during mylonitization (deformation, metamorphism, magmatism, fluids).

In any case, I am still not pretty sure whether both micas are contemporaneous or not. For example, the biotites >400 microns, do they represent the grain-size reducted biotites or the remnants of "underformed" biotites? Based on what you mention, I would still think that biotite nucleated prior to muscovite, although both could be afterwards homogeneised in terms of 18O. In that case, this event shouldn´t have affected K-Ar isotopy. 

Be careful trying to extrapolate plate boundary offset from basin length.  Pull apart basins are typically short lived features which form in response to bends or step-overs in strike slip systems.  As such they are generally not active for the entire "lifespan" of the fault system and do not record the entire slip history.  

Unfortunately I am not very familiar with the montmorillonite structure. But I had a look to this interesting clay structure and there are two obvious differences to zeolites, as far as I can see.

On the one hand zeolites provide a defined pore/channel opening in every state dry or wet, while the layer distance of the montmorillonite structure is strongly dependent from the swelling ( and so from wetness). Could the small layer distance in your dry clay hinder the solvation?

On the other hand the montmorillonite structure has a neutral surface, while zeolites are acidic. Sodium cations in zeolites are bound to the negatively charged Brønsted-sites of the structure (Zeo- Na+). For montmorillonite sodium cations are intercalated between the layers of the structure. Could this different surrounding of the cations influence the solvation?   

Why don´t you try this website http://app.visiblegeology.com/ It has some structural geology software that you can use for free online

Apparently the difference between the angle of reponse of sand in a subaerial case and in a subaqueous case (sand saturated in water) is given by a difference in the effective stress between the grains.  The effective stress is the force that prevents the movement between particles (related to friction) and in turn is related to the gravitational force of the grains. Once the grains are saturated by water, they "weight less" and both the frictional force and the effective stress between them is smaller. The smaller the effective stress, smaller the angle of repose. For this reason the angle of repose of dry sand is around 34° and of 30° for saturated sand.

A final comment: A different scenario comes with sand filled with capillary water! in that case your angle of repose increases! but i think your question was oriented towards the angle of repose of sand in the lee faces of subaqueous and subareal dunes!

Nothing to add to the previous collegues. I do feel you are observing PDFs due to shock in quartz.

Dear Mr. Rurui.

the answer is simply yes. It is only a question of time which lies between the formation  of these very contrasting geodynamic settings. There are numerous crustal sections where you can find interlocked with each other lithologies indicative of both settings.

Best regards

H.G.Dill

Dear all,

what do you think about marks created by rolling ammonidea?

Similar traces where found in the quarries of Solnhofen in a micritic limestone.......

best regards

Andreas

The best way to obtain a numerical age for a sedimentary rock – other than through comparison of fossil content or magnetopolarity reversals with the geological timescale – is through the direct dating of volcanic ash layers (U-Pb and Ar-Ar techniques on mineral separates). The main difficulties with this approach relate to the mixing of grains of different age and significance, and alteration of the dated minerals. An approach that works really well in marine deposits of Oligocene-Miocene age is to measure Sr isotope ratios in microfossils and macrofossils. The seawater ratio is well established for that interval, and more or less linear. Glauconite dating is mostly a mess. Poor resolution.

one can have liquid water at -70 ° C and solid water (ice) to + 80 ° C, and it is the game of pressure that makes the difference !

See perhaps criticism of Lyell's original meaning of uniformitarianism in Steve Gould's book "Time's arrow - time's cycle": Lyell himself included distinct concepts which are not necessarily related, among others:

(1) idea that the same natural laws we observe today were at work in earth history (usually not disputed today, only for the earliest history of the universe exceptions are plausible);

(2) the idea that (only) geological processes observed today shall be taken into account for explaining traces of processes in earth history (= actualism(?); a bit like the geological version of Occam's razor; today only regarded as heuristic principle but not necessarily applicable for most past processes);

(3) the idea that geologically (as well as biological evolutionary) processes run slowly and always at the same rate (problematic in contemporary view);

(4) the idea that certain developmental stages of the earth and life on earth re-occur periodically (only some modern concepts like supercontinent cycles and icehouse-hothouse changes are reminiscient of this idea).

(So discussing Lyell's uniformitarianism it is difficult not to mix up concepts.)

Dear Mrs. Sun,

A mixture of adjacent dolomite ooids and botryoidal lumps are interconnected by an early diagenetic dolomite cement. These connecting 'bridges' consist of a so-called meniscus cement in the form of concave dolomite fiber cement perpendicular to the grain surface. The red-colored calcite forms a late diagenetic granular cement and also fills (includes) the pore spaces in some hollow dolomite grains.

That's my interpretation.

Best regards and greetings to my birthplace Berlin.

Guenter Grundmann

Hi

The vitrinite reflectance shows that the coals may have been formed at a depth and lately uplifted to a shallower depth.

I restrict the term "Backarc" to be followed by "Basin"; a backarc basin is a region of seafloor spreading or at least strong extension. "Reararc" I use for everything else, including rear arc alkaline volcanism, rear arc cross-chains, and rear arc fold-and-thrust belts. For the eastern TMVB I would use term "rear arc"

I salute your dedication to the work, certainly the legacy which has been  left by late Prof. Dr. Antonio de Barros Machado should be maintained. If it losts it will be a great loss to entomologists about evolutionary significance of termites. In future certainly it will provide new directions to the experts in the field of geology and geochemistry and entomology.

Hello Mingdao,

You could compare your cpx data to published analyses that may be slightly more recent than 1962, for instance by using the GeoRoc (http://georoc.mpch-mainz.gwdg.de/georoc/ ) website. However, if I remember correctly (I don't have the paper handy here), Ewart (Ewart, A., 1976. A petrological study of the younger Tongan andesites and dacites, and the olivine tholeiites of Niua Fo'ou Island, S.W. Pacific. Contrib. Mineral. Petrol. 58, 1-21; I hope this is the correct reference) demonstrated that quench crystallisation of pyroxene could increase their Al content to higher values than expected from equilibrium crystallisation. Perhaps something similar is at work here.

Marlina

Dear Mr. Sun,

the growth textures of concentric morphology are numerous and run the gamut from pegmatitic zonation through vesicles in basalts like that. They cover the supercritical and subcritical fluids. It is fundamental to know exactly what the different zones are made up mineralogically, and then take a look at the direction of growth (is it unidirectional...). Also keep in mind the size of the rings and zones, it will show you the quantity of the various components involved and last but not least are there breaks or hiatuses in the formation of the concentric shells which may direct your thoughts to different pulses. I know similar textures but feel unable of giving you a good explanation at this stage of investigation. It will only end up in a back- and-fro of speculations. The step "description" has not yet been solved to the extent to allow for a passage into interpretation. A cartoon showing all your information with regard to my recommendations will minimize the field of speculation.

Best regards

Harald G. Dill

Dear Ajay,

As promised, I am sending you a few photomicrographs of shale, tuff and the metamorphosed equivalents from my own thin section collection. In addition, I have included some captions.

If you still have any further questions, do not hesitate to tell me about it.

Best regards,

Guenter

Hi, the following article might be useful for you.

1. Interpreting magmatic processes from accessory phases: titanite—a small-scale recorder of large-scale processes.

2. DISTRIBUTION OF TRACE AND RARE EARTH ELEMENTS IN TITANITE FROM TUNGSTEN AND MOLYBDENUM DEPOSITS IN YUKON AND BRITISH COLUMBIA, CANADA.

Hi Rongqing,

thanks for this valuable information. Rolf Romer is particularly interested in this subject, and I will forward to him your reference.

Have fun,

Hans

SOLVCALC might do the job:

Wen, S.and Nekvasil, H. (1994) SOLVCALC: An interactive graphics program package for calculating the ternary feldspar solvus and for two-feldspar geothermometry. Computers & Geosciences 20, No. 6, 1025-1040.

it is, for example, in ther list of software linked below.

The criteria for using the thermometer are detailed on Henry's website at the link below. Here it states:

"Pressure range of the rocks should be 4-6 kbar - this is the P range of the calibration data set. (Based on evidence from a number of biotite experiments, samples equilibrated at lower P will have higher Ti and yield higher TIB temperatures whereas samples equilibrated at P>6 kbar will have lower Ti and will yield low TIB temperatures.)"

There is no information (there) on what the results will mean if T>800C. Providing the system is saturated in Ti (sounds like you have rutile present) it may be fine.

Stress Pulse Migration by Viscoelastic Process for Long-Distance Delayed Triggering of Shocks in Gujarat, India, After the 2001 Mw 7.7 Bhuj Earthquake,  American Geophysical Union Geophysical Monograph Series 01/2012; DOI: 10.1029/2011GM001061

may be helpful to you.

Ken Towe has indicated the presence of pumice is a way of recognizing air-fall volacanic ash deposits  Although very rare, this phenomenon has been reported in the cinerites of France (see Bouroz et al;. 1983, Review of the formation and evolution of pegtrographic markers in coal basins, Societe Geologique du Norld, Memoires, Tome XVI ( Pl. III,  Fig. 4). Thank you Ken. Best wishes, Paul.

Dear Mr. Ge Songsheng,

I feel fine with the gradual unraveling of the details and it is good to see that you try and constrain the origin of this rock. The basic of all that is, that we have to follow up a clear pathway: (1) Geology (in the field and study) including , in places, geophysics, (2) Mineralogy, (3) Chemistry (4)-if necessary more advanced and more highly-sophisticated methods. And then discussion and interpretation. Otherwise we end up into speculation and the outcome is a matter of conjecture. This is not to complain about your doing; it is my simple attempt to get much of what we are faced with in RG Q&A back on track. Please apologize for that philosophical side leap. I can´t get out of my suit that I had on for 40 years.

I wish all the best

Harald G. Dill

Hi Gönenç,

You can also have a look at John Winter 'Principles of igneous and metamorphic petrology'. You can download the book for free. Chapter 12 is about Layered mafic intrusions. Publication is 2009, not so old. Have a look.

Cheers.

Michel

The difference lies in the nature of the lithosphere. In breakoff, the subducted oceanic lithosphere is fragmented and falls into the asthenosphere, whereas in orogenic delamination, the base of the continental lithosphere is laminated and falls into the asthenosphere. The first is at the end of the subduction while the latter usually happens at the end of the collision.

see Talbot C.J., 2014b Empirical Paths of Poles to Planes (eppps) in shear zones. Journal of Structural Geology, 66, 309-333

Dear Ioannis,

from your descriptions I assume that the structure under consideration occurs within a normal (i.e. not inverted and not otherwise folded or thrusted) stratigraphic section. Hence I would rather assume that the structure is indeed a slump fold that formed shortly after sedimentation and before the overlying sediments were deposited. Otherwise it would be difficult to explain why only a certain level of the sequence has been subjected to folding. You mention also monomictic limestone breccias; such breccias may also develop by means of slump movements and other kinds of soft-sediment deformation shortly after sedimentation. What is a little bit confusing for me are the boudinaged layers which you indicated on the photograph: are they really boudins (i.e. a tectonically stretched competent layer within a less competent country rock)? If they were indeed boudins, this could be an indication for real tectonic deformation. On the other hand, the supposed boudins could also be just sedimentary concretions.

To sum up: it's difficult to judge from a photo alone, but taking into account your additional comments, I would suggest that the structure is a slump fold.

With best regards from Switzerland

Dominik 

Estimada Mirta,

en las roccas piroclasticas la situacion no es muy diferente del ambiente volcanico, donde se encuentran la esfalerita y la calcopirita como las principales materias de indio. Los contenidos de indio normalmente son muy bajos fuera de las roccas formando en un regimen de alta temperatura.

Deme mas informacion sobre el estado de la alteracion supergeno y el lugar de origin de las roccas piroclasticas considerando en sus estudios. Necesito mas datos para darle a usted una respuesta mas profunda. Pienso tambien en diversos minerales arcillosos.

En espera de su respuesta a proposito de su asunto.

Con saludos cordiales

Harald

Have a look at the paper of M.G. Stamatakis et al. (1998): Mineralogy, origin and commercial value of the zeolithe-rich tuffs in the Petrota-Pentalofos area, Evros County, Greece.-- It might be useful for you.

As briefly mentioned in Zheng and Hermann (2014), both newly grown and relict zircon domains have been found in orogenic peridotites from continental subduction zones. These relict zircons exhibit older U-Pb ages than the subduction-zone metamorphic ages. They were erratically interpreted as xenocrysts in some publications, but substantially they were inherited from the metasomatized mantle wedge peridotites and thus eventually derived from subducted crust-derived fluids/melts. In this process, there are both the chemical transport of dissolved Zr and the physical transport of crustal zircon xenocrysts by either metamorphic fluids or anatectic melts at the slab-mantle interface in subduction channels (Zheng, 2012). While U-Pb ages for the newly grown zircon directly date the metasomatic event, its Lu-Hf isotope composition is primarily inherited from the metasomatic agent and thus has little to do with the composition of the overlying lithosphere. The survival of subducted crust-derived zircon in the orogenic peridotites suggests that the crustal metasomatism beneath the mantle wedge would have proceeded at kinetically limited conditions, and thus it did not achieve the thermodynamic equilibrium between crustal and mantle components.

Zheng Y.-F. (2012) Metamorphic chemical geodynamics in continental subduction zones. Chem. Geol. 328, 5-48.

Zheng Y.-F., Hermann J., 2014. Geochemistry of continental subduction-zone fluids. Earth, Planets and Space 66, 93.