Dear Damepaia

First ting to check is their contact. Is it disturbed by fault or not? Were they originally close together or their contact is formed during secondary process like tectonic movement during unroffing. Then it would be helpful to check foliations generations in both. are they same or one have more foliation?

Dear Mr. Pdah,

your question finds an answer in the unmetamoprhosed lithological setting as well.

A granite intrusion into country rocks,  effusive rocks overlying basement rocks or dykes and sills are younger than the enclosing country rocks.

With kind regards

H.G.Dill

Absolutely agreed with Dr. Dill! Any intrusive or extrusive body in sediments "a priori" must be younger than their country rocks. Exclusion - the bodies displaced by tectonic forces.

Dear Damepaia,

All previous answers are correct

1. Check the type of contact

2. Small intrusions a. are not always present or easily observed b. are they incorporated within the sedimentary body (metamorphism facies) in a way which are not highly contrasted for example paragneiss VS orthogneiss?

Kind regards

Ioannis

Whether it is  a metamorphosed igneous body or unmetamorphosed.

whether the country rocks are metasediments or not.

In the field an intrusive or younger igneous rocks give clear indication by its disposition.

Any volcanic flow/ body will be stratigraphically above the basement rocks. sills though will be along the bedding/ foliation planes will have textural/mineralogical variations along the contacts. Dykes are clearly show cross cutting relations.

If you have heard about contact metamorphism....

where you can see clear textural and mineralogical variation within the country rock, around the younger/ later igneous intrusion.

The nature of alteration depends on composition of country rock, the size, composition and temperature of igneous intrusion.

Sometimes....you will find chilled margins...at the contact of intrusion and country rock.

Dear Damepaia,

In my study of the provenance of sediments in the New England Orogen (NSW, Australia, 2015) we used zircon U/Pb ages. We found that zircon ages from the sediments and metasediments (Central Tablelands Complex) ranged between 310 and 350 Ma while the the ages of zircon in the intruding granite (Abroi) were around 290 Ma. Although not conclusive on its own, but in the absence of any other evidence, it can be a convincing indicator that the sediments are older than the granite. 

There are many cases of confusing relationships between supposed intrusives bodies and their wallrock. Among other things mentioned in the comments, are possibilities of anatectic derivation of the magma from the metasediments. Metasomatic alteration can also impose some rather complex relationships, obscuring earlier contrasts. Hydrothermal-metamorphic alteration can be very selective in its effect. Some geologists have a more "classical" understanding of contact possibilities. These can unfortunately limit interpretations. Careful, detailing geochronology and isotope studies may be the ultimate way to reveal the sequence of events. A last comment is that depth of intrusion, tectonic condition (pre-, syn-, post-) and fluid activity can make a tremendous difference in the appearance of contacts.

You have plenty of useful suggestions. In summary, field relations are absolutely critical in deciphering the age relationship. It is true for intrusions, as well as migmatitic anatexites (leucosomes being partial melting product of the country rocks during high grade metamorphism). Sills can be identified through baking effect on the contacts, and fine-grained margins, and from flows that would display volcanic structures. The problem becomes complex when the metasediments are in fault contact with metaigneous rocks. There ardiometric dating may be needed.

An addition is if you will see a chilling zone at the contacts with sediments

I have experience at investigating the original nature of metamorphosed igneous rocks at Broken Hill, Australia. It was not easy. Granite gneisses in high grade metamorphic host rocks were originally interpreted as metamorphosed volcanic rocks, because they are mostly conformable with the enclosing metasediments, but studies of relict textures by Vassallo and Vernon (2000 Australian Journal of Earth Sciences) suggested that the rocks were originally granites. This was confirmed by zircon dating that showed the granite gneisses are younger than the enclosing stratigraphy.

A different type of gneiss (quartz-feldspar-biotite-garnet) is also conformable. This gneiss is correlated stratigraphically with a gneiss/schist of slightly different appearance in a lower metamorphic grade area. In that area volcanic textures are preserved (Beta quartz phenocrysts showing resorption, feldspar phenocrysts). Such textures were subsequently found to be preserved in the higher metamorphic grade areas, but only in small patches where early alteration has preserved the bimodal texture.

The area also contains many amphibolite bodies with compositions of fractionated ferro-tholeiites. Some of these in the high grade metamorphic areas appear to be cross-cutting, although structural transposition of bedding in these areas produces ambiguity. Most of the amphibolite bodies are conformable. It was difficult to determine whether these were intrusive or extrusive. My conclusion was that they were most probably all intrusive. This comes after many years of study, finding no pillow textures, finding very few amygdales, and more recently a study by A.J. Crawford of the University of Tasmania that showed chemical fractionation within several of the amphibolites, a feature found in sills, but not in lava flows. One additional feature is the preservation of a chilled margin on one amphibolite sill at lower sillimanite metamorphic grade.

Dear Madam Asoori Latha,

Yes it is a metamorphosed igneous body. However, it had been noted that the grade of metamorphism varies spatially in the regional area.

These are definitely metasediments, but I doubt that they are the country rocks. I believe they are the younger overlying rocks.

I will remember to note the textural, mineralogical variation and alteration at the contact and margins. But so far, there are no intrusive bodies encountered by me in the field till date.

Thank you.

Dear Ali Akbar Baharifar,

Yes I am working on the contacts. One famous spot taken to be an ideal place for the intrusiveness exhibition has been disturbed by a reverse fault. Foliation development is good in the metasediments but very very poor in the meta-igneous rock. What is unroffing?

Thank you.

Dear Jorge C. de Moura,

The igneous body is not felsic but mafic. The relationship between the meta-igneous and meta-sediments is conformable, that is, simply as two blankets kept together opened on the bed with folds here and there together. This is what I have observed.

Thank you.

Dear Sir Harald G. Dill,

Both the rock types are of Precambrian age. Infact, everything is metamorphosed in my study area. That is why, I do have a problem in distinguishing that rhyolites and tuff when they are deformed and metamorphosed. Kindly help in this regard too, if possible.

Thank you.

Dear Jeffrey Greenberg,

I wish you were here for field work too :). Your suggestions are very new to me. I believe my study area is not that complex. Its simply observed that meta-mafic are overlained by meta-sediments. No intrusive nature observed. I need points on how to prove that the meta-mafic are older and are being overlained by the younger meta-sediments (The reverse is believed by previous workers). However, I will remember your points too.

Thank you.

Dear M. Qasim Jan

Yes at one place (believed to be a intrusive body), it is faulted (Reverse Fault) and to make it worst, there is a younger granitic intrusion that has come into play.

Thank you.

Dear N. L. Alexeev,

Thank you and I got your point. By the way, can you tell how to identify a chilling zone at the contacts with sediments, when the rocks are deformed and metamorphosed of Precambrian times?

Dear Brian P. J. Stevens,

You are the second person to talk about chemical fractionation. I am not able to get the paper on "Chemical fractionation, Amphibolite" by "A.J.Crawford". Can you kindly send me the full name of the papers.

Thank you.

Dear Damepala, There is a continuing disagreement about the nature of a famous exposure at Baraboo, Wisconsin. This is the classic site where Charles Van Hise in the late 1800s worked out the "refraction" of cleavage between phyllite and massive quartzite. On the south range of the major Baraboo fold (plunging syncline), there is a granite in outcrop in very close proximity to the folded metasedimentary (lower-Greenscist facies) package. A PhD thesis in the 1940s interpreted the relationship as intrusive, granite invading the metasediments. Later, some interpreted this as a nonconformity with the quartzite, etc. deposited on a weathered granite surface. To the north, it does appear that the metasediments are on top of rhyolites. My own work at the granite outcrop area leans more logically to the intrusive interpretation for various reasons. Unfortunately, the closer we observe near the granite, the metasediments are highly, hydrothermally deformed. That has been interpreted as a shear contact between lithologies. Without a great amount of research attention, this problematic contact relationship will remain a mystery. 

Hello and my respect to everyone. First of all, thank you so much for giving interest in my question.

My question is very basic on "how a meta-igneous rock can be younger to an overlying meta-sedimentary rock from field observations". I should say that I am happy that none of your kind suggestions and comments were observed by me till date (As for dates, there are none available so far).

No dykes, sills, chilling effect, doming/bulging effect or a clear intrusive disposition, nor my previous two points given in the main question were observed by me in the field.

All and every previous worker up to now had supported and corroborated on the point that "the meta-igneous rock is younger to the meta-sediments". I believe they had had very few field visits and I need to strongly do justice to the actual facts.

Friends, I would like to put another question to you as an extension of help again from your side.

WHAT ARE THE FIELD EVIDENCES THAT I SHOULD LOOK FOR AND RECORD THEM TO CORROBORATE THAT THE METAMORPHOSED IGNEOUS BODY (mafics) ARE OLDER TO THE METASEDIMENTS? THEN, WHAT KIND OF TECTONIC SETTING WOULD BRING SUCH CONFIGURATION?

Thank you.

I have a suggestion that if the intrusive body had intruded in a strong ductile zone it would has been less expected to see dyke or even xenolithes of the country rocks. but if you find dykes it could be suggested that intrusive body have been emplaced into a more likely brittle zone and then the metamorphism would be post-intrusion.   

Nevertheless I believe it is always possible to find enough evidence in the field to answer the question as were mentioned in the other answers.

Dear Jorge C. de Moura,

In some areas, rhyolite were lying on top of the mafic and in other area, tuff were observed. In the later case, I did not see direct contact but is interpretable to be overlying the igneous rocks.

Interestingly, these felsic volcanics are also observed to occur within the meta-sedimentary sequence.

I am not clear with your comments on the tectonic settings. Can you explain a bit more.

From my side, I was thinking LIPs.

Thank you.

DearDamepaia,

I was about to suggest that it is basically a field geology problem. If you have not been able to see any evidence of intrusive relationship, and you do not have mafic flows in the sediments similar to the "igneous body", I would be tempted to suggest that either your metmorphosed mafic rocks are faulted against the metasediments or the sediments have been deposited unconformably over the mafic rocks before they were metamorphosed. Petrographic study would be helpful in comparing the grade of metamorphism in the two.

But your new information on the presence of rhyolites and tuffs is making the problem really complicated. To begin with, you did not mention whether these have also been metamophosed or not. But leaving that aside, if the felsic volcanic material lies on top of the mafic body as well as interbedded with the metasediments, it would probably exclude the possibility of a large-scale fault. I would, hence, think that the (mafic) igneous body is a sole on which sedimentary and felsic volcanic association was laid unconformably. This was followed by metamorphism of the entire package.

Dear Jorge C. de Moura, please see the recommendations of Sir M. Qasim Jan. It is exactly what I wanted to hear. Thank you.

Dear M. Qasim Jan, as I have not seen the thin section yet, I cannot comment now on the metamorphism of the rhyolites. But from field observations, in some areas, they seem to be very fresh, unmetamorphosed and undeformed. Whereas in other places, it looks like clay/metapelite which is underlained by meta-mafic and overlained by meta-sediment. I am assuming the clay/metapelite-looking rocks to be the rhyolite/felsic volcanics (protolith) because I am comparing the same package with the ones that have been seen in other places. In other places, the package is observed to be fresh rhyolites underlained by relatively less metamorphosed mafics (probably are dolerite rocks with pyrites) and overlained by meta-sediment.

Kindly suggest how do I know (that is it metamorphosed?) and check the grade of metamorphism in thin sections. The possible rock types are tuff, rhyolite, dolerite, meta-dolerite, meta-gabbro, amphibolites (I wonder why massive and foliated types are present), quartzites (how do I check & categorize its different varieties), phyllites, etc.

Thank you so much.

Dear Jorge C. de Moura, I have not discussed about the felsic volcanic within the meta-sediments because there are no contradictory observations/ previous reports about them. Thank you.

With due regards, you are providing the field information incrementally. This is the first time I get to know that there are mafic rocks :(dlerite) in the sequence. Look carefully whether the "dolerites" are sills or flows. If they are flows and there also are rhyolites, you should not ignore the possibility of bimodal magmatism. Last, but not least, if the dolerites are sill-form, but similar in petrography to the mafic body, you have to consider that both are younger than the sediments. If the mafics are flows, they may be contemporaneous with the sedimentary rocks.

Présence de métasédiments comme xénolites dans le corps ignée.

Dear Jorge Costa-de-Moura, you asked the right question.

Well, the outcrops here are not that simple. I need more time and more field works. However, I can share some thoughts that has developed from my study area. Please see attached file.

I recommend Jorge's answer